Roitt's Essential Immunology

appropriate red cell type. If an individual is blood group A, she/he would be
tolerant to antigens closely similar to A and would only form cr0sscceacting

antibodies capable of agglutinating B red cells; similarly an O individual
would make anti-A and anti-B (Table 15.3). On transfusion, mismatched
red cells will be coated by the isohemagglutinins, which will cause severe
complement-mediated intravascular hemolysis.


Figure 15.15. The ABO system.
The allelic genes A and B code for transferases that add either N-
acetylgalactosamine (GalNAc) or galactose (Gal), respectively, to H
substance. The oligosaccharide is anchored to the cell membrane by

coupling to a sphingomyelin called ceramide. Eighty-five percent of the
population secrete blood group substances in the saliva, where the
oligosaccharides are present as soluble polypeptide conjugates formed
under the action of a secretor (se) gene. Fuc, fucose.




















Table 15.3. ABO blood groups and serum antibodies.












Clinical refractoriness to platelet transfusions is frequently due to HLA
alloimmunization, but one can usually circumvent this problem by
depleting the platelets of leukocytes.


Rhesus incompatibility

The rhesus (Rh) blood groups form the other major antigenic system, the
RhD antigen being of the most consequence for isoimmune reactions. A

mother with an RhD–ve blood group (i.e. dd genotype) can readily be
sensitized by red cells from a baby carrying RhD antigens ( DD or Dd
genotype). This occurs most often at the birth of the first child when a
placental bleed can release a large number of the babys erythrocytes into the

mother. The antibodies formed are predominantly of the IgG class and,
unlike the IgM anti-A and anti-B mentioned above for the ABO blood
group system, are therefore able to cross the placenta in any subsequent
pregnancy. Reaction with the D-antigen on the fetal red cells leads to their

destruction through opsonic adherence, giving hemolytic disease of the
newborn (Figure 15.16a and b).
Thiese anti-D antibodies fail to agglutinate RhD +ve red cells in vitro
(“incomplete antibodies”) because the low density of antigenic sites does

not allow sufficient antibody bridges to be formed between the negatively
charged erythrocytes to overcome the electrostatic repulsive forces.
Erythrocytes coated with anti-D can be made to agglutinate by addition of

an antiimmunoglobulin serum (Coombs’ reagent; Figure 15.17).
If a mother has natural isohemagglutinins that can react with any fetal
erythrocytes reaching her circulation, sensitiza-tion to the D-antigens is less
likely due to “deviation” of the red cells away from the antigen-sensitive

cells. For example, a group O RhD–ve mother with a group A RhD +ve
baby would destroy any fetal erythrocytes with her anti)A before they could
immunize to produce anti-D. In an extension of this principle, RhD–ve

mothers are treated prophylactically with IgG anti-D at 28 weeks and 34
weeks gestation and then again at the time of birth if the baby is RhD + ve
(Figure 15.16c). This greatly reduces the risk of sensitization—another
success for immunology.


Figure 15.16. Hemolytic disease of the newborn due to rhesus
incompatibility.
(a) RhD +ve red cells from the first baby sensitize the RhD -ve mother. (b)
The mother’s IgG anti-D crosses the placenta and coats the erythrocytes of
the second RhD +ve baby causing type II hypersensitivity hemolytic

disease. (c) IgG anti-D given prophylactically during the first pregnancy

and birth removes the baby’ s red cells through phagocytosis and prevents
sensitization of the mother.

























Figure 15.17. The Coombs’ test for antibody-coated red cells.

This test is used for detecting rhesus antibodies and in the diagnosis of
autoimmune hemolytic anemia. (Photographs courtesy of A. Cooke.)



















Another disease resulting from transplacental passage of maternal

antibodies is neonatal alloimmune thrombocytope-nia. The fall in platelet
numbers is greatly ameliorated by i.v. injections of pooled human IgG
(IVIg). The efficacy of Fcγ fragments and of anti-FcγR suggests that this

works by blockade of the Fcγ receptors.


Organ transplants

Allografts can evoke humoral antibodies in the host directed against surface

transplantation antigens. These may be directly cytotoxic or cause
adherence of phagocytic cells or attack by ADCC. The antibodies may also

lead to platelet adherence when they bind antigens on the surface of the
vascular endothelium (Figure 16.6 p. 427). Hyperacute rejection is

mediated by preformed antibodies in the graft recipient.


Autoimmune type II hypersensitivity reactions


Autoantibodies to the patients own red cells are produced in autoimmune
hemolytic anemia. They react at 37°C with epitopes on antigens of the
rhesus complex distinct from those that incite transfusion reactions.
Erythrocytes coated with these antibodies have a shortened half-l ife,

largely through their adherence to splenic macrophages. Similar
mechanisms account for the anemia in patients with cold hemagglutinin
disease who have monoclonal anti-I after infection with Mycoplasma
pneumoniae, and in some cases of paroxysmal cold hemoglobinuria

associated with the actively lytic Donath— Landsteiner antibodies specific
for blood group β. These antibodies are primarily of IgM isotype and only
react at temperatures well below 37°C. IgG autoantibodies against platelet
surface glycoproteins are responsible for the depletion of platelets in

idiopathic thrombocytopenic purpura; primarily through Fcγ receptor-
mediated clearance by tissue macrophages in spleen and liver.

Figure 15.18. Glomerulonephritis.
(a) In Goodpasture’s syndrome due to a type II hypersensitivity with linear

deposition of antibody to glomerular basement membrane, here visualized
by staining the human kidney biopsy with a fluorescent anti-IgG; and in
contrast to (b) in systemic lupus erythematosus (SLE, cf. p. 495) where a

type III hypersensitivity is associated with deposition of antigen-antibody
complexes, which can be seen as discrete masses lining the glomerular
basement membrane following immunofluorescent staining with anti-IgG.
Similar patterns to these are obtained with a fluorescent anti-C3. (Courtesy
of S. Thiru.)

Patients with Hashimotos thyroiditis have autoantibodies that, in the
presence of complement, are directly cytotoxic for isolated human thyroid
cells in culture. In Goodpasture’s syndrome, autoantibodies recognize type
IV collagen in kidney glomerular basement membrane. These antibodies,
together with complement components, bind to the basement membranes

where the action of the full complement system leads to serious damage
(Figure 15.18a). One could also include the stripping of acetylcholine
receptors from the muscle endplate by autoantibodies in myasthenia gravis

as a further example of type II hypersensitivity.


Type II drug reactions


Drugs can become coupled to body components and thereby undergo
conversion from a hapten to a full immunogen that may elicit an immune
response in some individuals. If IgE antibodies are produced, anaphylactic

reactions can result. In some circumstances, particularly with topically
applied ointments, cell-mediated hypersensitivity may be induced. In other

cases where coupling to serum proteins occurs, the possibility of type III
immune complex-mediated reactions may arise. In the present context, we
are concerned with those instances in which the drug appears to form an
antigenic complex with the surface of circulating blood cells and evokes the

production of antibodies that are cytotoxic for the cell–drug complex. When
the drug is withdrawn, the sensitivity is no longer evident. Examples of this
mechanism occur in the hemolytic anemia sometimes associated with
continued administration of chlo-rpromazine or phenacetin, in the

agranulocytosis associated with the taking of amidopyrine or of quinidine,
and the now classic situation of thrombocytopenic purpura, which may
be produced by sedormid, a sedative of yesteryear. In the latter case, freshly
drawn serum from the patient will lyse platelets in the presence, but not in

the absence, of sedormid; inactivation of complement by preheating the
serum at 56°C for 30 minutes abrogates this effect.



Immune complex-mediated


hypersensitivity (type III)


The body may be exposed to an excess of antigen over a protracted period
in a number of circumstances: persistent infection, autoimmunity to self-

components and repeated contact with environmental agents. The union of
such antigens and antibodies to form a complex within the body may well
give rise to acute inflammatory reactions through a variety of mechanisms

(Figure 15.19). For a start, intravascular complexes can aggregate platelets
with two consequences: they provide a source of vasoactive amines and
may also form microthrombi that can lead to local ischemia (Figure
15.19a). Immune complexes can also stimulate macrophages, through their
Fcγ receptors, to generate the release of proinflammatory cytokines IL-1

and TNF, reactive oxygen intermediates and nitric oxide (Figure 15.19b).
Complexes that are insoluble often cannot be digested after phagocytosis by
macrophages and so provide a persistent activating stimulus. If complement

is fixed, the C5a chemotactic factor will be generated leading to an influx of
neutrophils (Figure 15.19c), which begin the phagocytosis of the immune

complexes; this in turn results in the extracellular release of the neutrophil
granule contents, particularly when the complex is deposited on a basement

membrane and cannot be phagocytosed (so-called “frustrated
phagocytosis”). The pro-teolytic enzymes (including neutral proteases and
collagenase), kinin-forming enzymes, polycationic proteins and reactive
oxygen and nitrogen intermediates that are released will of course damage

local tissues and intensify the inflammatory responses. The anaphylatoxins
C3a and C5a produced following complement activation will cause release
of mast cell mediators resulting in vascular permeability changes (Figure
15.19 d). Further havoc may be mediated by reactive lysis in which

activated C5, C6 and C7 becomes adventitiously attached to the surface of
nearby cells and subsequently binds C8 and C9. Given all these potential
consequences of immune complex formation the need for the system of
inhibitors present in the body should be absolutely clear.


Figure 15.19. I mmune complex-mediated (type III) hypersensitivity—
underlying pathogenic mechanisms. ROI, reactive oxygen intermediates;
NO, nitric oxide.

The outcome of the formation of immune complexes Cn vivo depends not
only on the absolute amounts of antigen and antibody, which determine the

intensity of the reaction, but also on their relative proportions, which
govern the nature of the complexes (cf. Figure 6.24, p. 164) and hence their
distribution within the body. Between antibody excess and mild antigen
excess, the complexes are rapidly precipitated and tend to be localized to

the site of introduction of antigen, whereas in moderate to gross antigen
excess, soluble complexes are formed.
Covalent attachment of C3b prevents the Fc–Fc interactions required to

form large insoluble aggregates, and these small complexes bind to CR1
complement receptors on the human erythrocyte and are transported to
fixed macrophages in the liver and spleen where they are safely inactivated.
This is an important role of the erythrocyte, a cell often unfairly ignored in

discussion of the immune system. If there are defects in this process, for
example deficiencies in classical pathway components, or perhaps if the

system is overloaded, then widespread disease involving deposition of
complexes in the kidneys, joints, skin and choroid plexus (networks of

capillaries in the walls of the ventricles in the brain) may result.

Figure 15.20. Histology of acute inflammatory reaction in polyarteritis
nodosa associated with immune complex formation with hepatitis B
surface antigen (HBsAg).

(a) A vessel showing thrombus (Thr) formation and fibrinoid necrosis (FN)
is surrounded by a mixed inflammatory infiltrate, largely neutrophils. (b)
High-power view of acute inflammatory response in loose connective tissue
of patient with polyarteritis nodosa—polymorphonuclear neutrophils

(PMN) are prominent. (c) Immunofluorescence studies of immune
complexes in the renal artery of a patient with chronic hepatitis B infection
stained



















Inflammatory lesions due to locally formed

complexes



The Arthus reaction

Maurice Arthus found that injection of soluble antigen intra-dermally into

hyperimmunized rabbits with high levels of precipitating antibody produced
an erythematous and edematous reaction reaching a peak at 3—8 hours that
then usually resolved. The lesion was characterized by an intense
infiltration with neutrophils (cf. Figure 15.20a and b). The injected antigen

precipitates with antibody often within the venule, too fast for the classical
complement system to prevent it; subsequently, the complex is able to bind
complement and, using fluorescent reagents, antigen, immunoglobulin and
complement components can all be demonstrated in this lesion, as

illustrated by the inflammatory response to deposits of immune complexes
containing hepatitis B surface antigen in a patient with peri-arteritis nodosa

(Figure 15.20c). Anaphylatoxin production, mast cell degranulation,
macrophage activation, platelet aggregation and influx of neutrophils all
make their contribution. The Arthus reaction can be attenuated by depletion
of neu-trophils by nitrogen mustard or of complement by anti-C5a; soluble

forms of the complement regulatory proteins CD46 (membrane cofactor
protein) and CD55 (delay accelerating factor) are also inhibitory.
with fluoresceinated antihepatitis B antigen (left) and rhodaminated anti-

IgM (right). The presence of both antigen and antibody in the intima and
media of the arterial wall indicates the deposition of the complexes at this
site. IgG and C3 deposits are also detectable with the same distribution. ((a)
and (b) provided by courtesy of N. Woolf; (c) kindly provided by A.
Nowoslowski.)



Reactions to inhaled antigens

Intrapulmonary Arthus-type reactions to exogenous inhaled antigen are
responsible for a number of hypersensitivity disorders. The severe

respiratory difficulties associated with farmer’s lung occur within 6—8
hours of exposure to the dust from mouldy hay. The patients are found to be
sensitized to ther-mophilic actinomycetes that grow in the mouldy hay, and

extracts of these organisms give precipitin reactions with the subject’s
serum and Arthus reactions on intradermal injection. Inhalation of bacterial
spores in dust from the hay introduces antigen into the lungs and an
immune complex-mediated hypersensitivity reaction occurs. Similar
situations arise in pigeon-fancier)s disease, where the antigen is probably

serum protein present in the dust from dried feces, and in many other
quaintly named cases of extrinsic allergic alveolitis resulting from
continual inhalation of organic particles, e.g. cheese washer’s disease

(Penicillium casei spores), furrier’s lung (fox fur proteins) and maple bark
stripper’s disease (spores of Cryptostroma). Evidence that an immediate
anaphylactic type I response may sometimes be of importance for the
initiation of an Arthus reaction comes from the study of patients with
allergic bronchopulmonary aspergillosis who have high levels of IgE and

precipitating IgG antibodies to Aspergillus species.

Reactions to internal antigens


Type III reactions are often provoked by the local release of antigen from
infectious organisms within the body; for example, living filarial worms,
such as Wuchereria bancrofti, are relatively harmless, but the dead parasite
found in lymphatic vessels initiates an inflammatory reaction thought to be

responsible for the obstruction of lymph flow and the ensuing, rather
monstrous, elephantiasis. Microbial cell death following chemotherapy may
cause an abrupt release of microbial antigens and in individuals with high
antibody levels produce quite dramatic immune complex-mediated

reactions, such as erythema nodosum leprosum in the skin of dapsone-
treated leproma-tous leprosy patients (Figure 15.21) and the Jarisch—
Herxheimer reaction in syphilitics on penicillin.

Figure 15.21. Erythema nodosum leprosum, forearm.

The patient has lepromatous leprosy with superimposed erythema nodosum
leprosum. These acutely inflamed nodules were extremely tender and the
patient was pyrexial. (Photograph kindly provided by G. Levene.)























An interesting variant of the Arthus reaction is seen in rheumatoid
arthritis where complexes are formed locally in the joint due to the

production of self-associating IgG anti-IgG by synovial plasma cells (cf. p.
496).
Complexes could also be generated at a local site by a quite different

mechanism involving nonspecific adherence of an antigen to tissue
structures followed by the binding of soluble antibody—-n other words, the
antigen becomes fixed in the tissue before not after combining with

antibody. Although it is not clear to what extent this mechanism operates in
patients with immune complex disease, let us describe the experimental

observation on which it is based. After injection with bacterial endotoxin,
mice release DNA into their circulation that binds specifically to the
collagen in the basement membrane of the glomerular capillaries; the
endotoxin also polyclonally activates B-cells making anti-DNA that gives

rise to antigen-antibody complexes in the kidney.


Disease resulting from circulating complexes



Immune complex glomerulonephritis


The deposition of complexes is a dynamic affair and long-lasting disease is
only seen when the antigen is persistent, as in chronic infections and
autoimmune diseases. In the glomeruli, the smallest complexes reach the

epithelial side but progressively larger complexes are retained in or on
the endothelial side of the glomerular basement membrane (Figure
15.22). They build up as “lumpy” granules staining for antigen, immu-

noglobulin and complement (C3) by immunofluorescence (Figure 15.18b),
and appear as large amorphous masses in the electron microscope (cf.
Figure 18.18). The inflammatory process damages the basement membrane
through engagement of the complexes with effector cells bearing Fcγ
receptors, as revealed by the absence of glomerulonephritis despite immune

complex deposition in the kidneys of Fc-R-knockout New Zealand (B × W)
F1 hybrids (a murine model of human systemic lupus erythematosus, SLE;
p. 495). Proteinuria results from the leakage of serum proteins through the

damaged membrane and serum albumin, being small, appears in the urine
(Figure 15.23, lane 3).
Many cases of glomerulonephritis are associated with circulating
complexes, and biopsies give a fluorescent staining pattern similar to that of

Figure 15.18b, which depicts DNA/ anti-DNA/complement deposits in the
kidney of a patient with SLE (cf. p. 496). Well known is the disease that can
follow infection with certain strains of so-called “nephritogenic”

streptococci and the nephrotic syndrome associated with malaria, where
complexes with antigens of the infecting organism have been implicated.

Immune complex nephritis can arise in the course of chronic viral
infections; as seen in individuals co-infected with HIV and hepatitis C

virus.


Deposition of immune complexes at other sites

The choroid plexuses in the brain are a major filtration site and therefore

also favored for immune complex deposition. This factor could account for
the frequency of central nervous system disorders in SLE. Neurologically
affected patients tend to have depressed complement component C4 in the
cerebro-spinal fluid (CSF) and, at postmortem, SLE patients with
neurologic disturbances and high-titer anti-DNA were shown to have

scattered deposits of immunoglobulin and DNA in the choroid plexus.
Subacute sclerosing panencephalitis is associated with a high CSF to serum
ratio of measles antibody, and deposits containing lg and measles Ag may

be found in neural tissue.
Vasculitic skin rashes are also characteristic of both systemic and discoid
lupus erythematosus (Figure 15.24), and biopsies of the lesions reveal
amorphous deposits of lg and C3 at the basement membrane of the dermo-

epidermal junction (cf. Figure 18.19).
Another example of immune complex hypersensitivity is the hemorrhagic
shock syndrome found in South-East Asia during a second infection with

dengue virus. There are four types of virus, and antibodies to one type
produced during a first infection may not neutralize a second strain but
rather facilitate its entry into, and replication within, human mono-cytes by
attachment of the complex to Fc receptors. The enhanced production of

virus leads to immune complex formation and a massive intravascular
activation of the classical complement pathway. In some instances drugs
such as penicillin become antigenic after conjugation with body proteins
and form complexes that mediate hypersensitivity reactions.


Figure 15.22. Deposition of immune complexes in the kidney
glomerulus.
(1) Complexes induce release of vasoactive mediators from basophils and

platelets that cause (2) separation of endothelial cells. (3) Attachment of
larger complexes to exposed basement membrane, with smaller complexes

passing through to the epithelial side. (4) Complexes induce platelet
aggregation. (5) Chemotactically attracted neutrophils release granule

contents in “frustrated phagocytosis” to damage basement membrane and
cause leakage of serum proteins. Complex deposition is favored in the
glomerular capillary because it is a major filtration site and has a high
hydrodynamic pressure. Deposition is greatly reduced in animals depleted

of platelets or treated with vasoactive amine antagonists.

























Figure 15.23. Proteinuria demonstrated by electrophoresis.
Lane 1: Normal serum as reference. The major band nearest to the cathode
is albumin. Lane 2: Normal urine showing a trace of albumin. Lane 3:
Glomerular proteinuria showing a major albumin component. Lane 4:

Proteinuria resulting from tubular damage with a totally different
electrophoretic pattern. Lane 5: Bence Jones proteinuria representing
excreted paraprotein light chains. Lane 6: Bence Jones proteinuria with a

trace of the intact paraprotein. Some of the samples have been concentrated.
(Electropherograms kindly supplied by T. Heys.)

It should be said that persistence of circulating complexes does not
invariably lead to type III hypersensitivity (e.g. in many cancer patients and
in individuals with idiotype–anti-idiotype reactions). Perhaps in these cases
the complexes lack the ability to initiate the changes required for complex

deposition, but some hold the view that complexes detected in the serum
may sometimes be artifacts released from their in vivo attachment to the
erythrocyte CR1 receptors by the action of factor I during processing of the
blood.



Treatment

The avoidance of exogenous inhaled antigens inducing type III reactions is

obvious. Elimination of microorganisms associated with immune complex
disease by chemotherapy may provoke a further reaction due to copious
release of antigen. Suppression of the accessory factors thought to be

necessary for the deposition of complexes would seem logical. Sodium
cromoglycate, heparin and salicylates are often used, the latter being an
effective platelet stabilizer as well as a potent anti-inflammatory agent.
Corticosteroids are particularly powerful inhibitors of inflammation and are
immunosuppressive. In many cases, particularly those involving

autoimmunity, conventional immunosuppressive agents may be justified.

Figure 15.24. Vasculitic skin rashes due to immune complex deposition.
(a) Facial appearance in systemic lupus erythematosus (SLE). Lesions of
recent onset are symmetrical, red and edematous. They are often most

pronounced on the areas of the face that receive most light exposure, i.e. the

upper cheeks and bridge of the nose, and the prominences of the forehead.
(b) Vasculitic lesions in SLE. Small purpuric macules are seen.

























Figure 15.25. Cell-mediated (type IV) hypersensitivity reactions.
(a) Mantoux test showing cell-mediated hypersensitivity reaction to
tuberculin, characterized by induration and erythema. (b) Chronic type IV
inflammatory lesion in tuberculous lung showing caseous necrosis (CN),

epithelioid cells (E), giant cells (G) and mononuclear inflammatory cells
(M). (c) Diagrammatic representation of a granuloma with central caseous
(“cheesy”) necrosis. (d) Type IV contact hypersensitivity reaction to nickel

caused by the clasp of a necklace. ((a) Kindly provided by J. Brostoff and
(b) by R. Barnetson; (d) reproduced from the British Society for
Immunology teaching materials with permission of the Society and the
Dermatology Department, London Hospital.)

Cell-mediated (delayed-type)


hypersensitivity (type IV)


Delayed-type hypersensitivity (DTH) is encountered in many allergic
reactions to infectious agents, in the contact dermatitis resulting from

sensitization to certain simple chemicals and in transplant rejection. Perhaps
the best known example is the Mantoux reaction obtained by injection of
tuberculin into the skin of an individual in whom previous infection with
the mycobacterium had induced a state of cell-mediated immunity (CMI).

The reaction is characterized by erythema and induration (Figure 15.25a),
which appears only after several hours (hence the term “delayed”) and
reaches a maximum at 24–48 hours, thereafter subsiding. Histologically, the
earliest phase of the reaction is seen as a perivascular cuffing with

mononuclear cells followed by a more extensive exudation of mono-and
polymorphonuclear cells. The latter soon migrate out of the lesion leaving
behind a predominantly mononuclear cell infiltrate consisting of
lymphocytes and cells of the monocyte-macrophage series (Figure 15.25b).

This contrasts with the essentially “neutrophil” character of the Arthus
reaction (Figure 15.20b).

Comparable reactions to soluble proteins are obtained when sensitization
is induced by incorporation of the antigen into complete Freund’s adjuvant

(see p. 365). In some, but not all cases, if animals are primed with antigen
alone or in incomplete Freund’s adjuvant (which lacks the mycobacteria
present in the complete adjuvant), the delayed hypersensitivity state is of
shorter duration and the dermal response more transient. This is known as

“Jones–Mote” sensitivity but has more recently been termed cutaneous
basophil hypersensitivity on account of the high proportion of basophils
infiltrating the skin lesion.

Figure 15.26. The cellular basis of type IV hypersensitivity.

Th1 cells will activate macrophages and cytotoxic T-cells, whereas Th2
cells will recruit eosinophils. ROI, reactive oxygen intermediates; NO,
nitric oxide.

The cellular basis of type IV hypersensitivity


Unlike the other forms of hypersensitivity that we have discussed, delayed-
type reactivity cannot be transferred from a sensitized to a nonsensitized
individual with serum antibody; T-lymphocytes are required. It cannot be
stressed too often that the hypersensitivity lesion results from an

exaggerated interaction between antigen and the normal cell-mediated
immune mechanisms (cf. p. 237)) Following earlier priming, memory T-
cells recognize the antigen peptide together with MHC class II molecules
on an antigen-presenting cell and are activated to undergo proliferation. The

stimulated T-cells release a number of cytokines that mediate the ensuing
hypersensitivity response, particularly by attracting and activating
macrophages if they belong to the Th1 subset, or eosinophils if they are
Th2. Helper

T-cells also assist Tc precursors to become killer cells that can cause
damage to virally infected target cells (Figure 15.26), the CD8 TCRaP
cytotoxic cells being activated by recognition of MHC class I complexes

with processed viral proteins and TCRyS killers operating through binding
to native viral proteins on the surface of the infected cells. It is also thought
that Th17 cells play a role. Thus, IL-17 knockout mice have impaired DTH
responses, and IL-17 is secreted by cutaneous lymphocyte antigen (CLA)+

nickel-specific T-cell clones from patients allergic to this allergen.
Cytokines released by Th1 and Th17 cells cause the activation of NK cells
that then release proinflammatory cytokines and exert killing activity by
inducing apoptosis in keratinocytes. IL-17 is known to synergise with IFNγ

in upregulating ICAM-1 expression and chemokine production by human
keratinocytes, which may hasten their demise.


Tissue damage produced by type IV reactions



Infections


The development of a state of cell-mediated hypersensitivity to bacterial
products is probably responsible for the lesions, such as the cavitation,
caseation and general toxemia, seen in human tuberculosis and the

granulomatous skin lesions found in patients with the borderline form of

leprosy. When the battle between the replicating bacteria and the body
defenses fails to be resolved in favor of the host, persisting antigen

provokes a chronic local delayed hypersensitivity reaction. Continual
release of cytokines from sensitized T-lymphocytes leads to the
accumulation of large numbers of macrophages, many of which give rise to
arrays of epithelioid cells (macrophages that morphologically resemble

epithelial cells), while others fuse to form multinucleated giant cells.
Macrophages presenting pep-tides derived from bacterial antigens using
their surface MHC class I molecules may become targets for cytotoxic T-
cells and be destroyed. Further tissue damage will occur as a result of

indiscriminate cytotoxicity by cytokine-activated macrophages.
Morphologically, this combination of cell types with proliferating
lymphocytes and fibroblasts associated with areas of fibrosis and necrosis is
termed a chronic granuloma and represents an attempt by the body to wall

off a site of persistent infection (Figures 15.25b,c and 15.26). It should be
noted that granulomas can also arise from the persistence of indigestible
antigen-antibody complexes or inorganic materials, such as talc, within
macrophages, although nonimmunological granulomas may be

distinguished by the absence of lymphocytes.
The skin rashes in measles and the lesions associated with herpes simplex
infection may be largely attributed to delayed-type reactions with extensive
Tc-mediated damage to virally infected cells. By the same token, specific

cytotoxic T-cells can cause extensive destruction of liver cells infected with
hepatitis B virus. Cell-mediated hypersensitivity has also been
demonstrated in the fungal diseases candidiasis, dermato-mycosis,

coccidioidomycosis and histoplasmosis, and in the parasitic disease
leishmaniasis.
Crohn’s disease and ulcerative colitis are the two main forms of
inflammatory bowel disease (IBD) and are distinct entities, although both

probably result from dysregulated mucosal immune responses to microbial
antigens in the gut. Crohn’s disease is characterized by transmural
granulomatous inflammation involving the entire bowel wall from mucosa

to serosa, and the development of fibrosis, microperforations and fistulas.
Inflammation can occur throughout the gastrointestinal tract. By contrast, in
ulcerative colitis there is a more superficial inflammation that is confined

to the colon and rectum. Mutations in the NOD2 gene, encoding a
cytoplasmic pattern recognition receptor for the muramyl dipeptide of

bacterial cell wall peptidoglycan, are strongly associated with susceptibility
to Crohn’s disease. IBD can be induced in severe combined
hi
immunodeficient (SCID) mice by the transfer of CD45RB (naive) CD4 T-
cells, but the colitis that develops can be cured by the subsequent transfer of

CD4-CD25+, CD45RB l o regulatory T-cells. The aggressor cells belong to
the IL-12-driven Th1 population producing TNF and IFNγ, which are
highly toxic for enterocytes, whereas the regulators secrete the suppressor

cytokines TGFP and IL-10. Monoclonal anti-TNF is a very effective
therapy; probiotic treatment with lactobacilli and Streptococcus salivarius
would appear to maintain remission in severe colitis, is less draconian and
is easier on the budget (remember the friendly yoghurt adverts). Clinical

trials to establish the efficacy of such treatments in large cohorts of patients
are underway.
Experimental colitis induced in SJL/J mice by administration of

oxazolone presents as a relatively superficial inflammation resembling
human ulcerative colitis. It is initially mediated by IL-4-producing Th2 cells
but rapidly superceded by an atypical Th2 response involving IL-13-
producing NKT cells. The inflamed tissue in patients with ulcerative colitis

has also been shown to contain increased numbers of IL-13-producing
nonclassical NKT cells (unlike most NKT cells, these do not bear an
invariant TCR) which have the potential to be cyto-toxic for human
epithelial cells.


Sarcoidosis


Sarcoidosis is a disease of unknown etiology affecting lymphoid tissue and
involving the formation of chronic granulomas. A chronic inflammatory
Th1 response to an infectious, environmental or autoantigen is thought to be

responsible. Increased numbers of activated B-cells and
hypergammaglobulinemia are often present. Evidence for atypical
mycobacteria has been obtained, but delayed-ype hypersensitivity is

depressed and the patients are anergic on skin testing with tuberculin,
perhaps due to the presence of increased numbers of regulatory T-cells in

those patients with active disease. Patients develop a granu-lomatous
reaction a few weeks after intradermal injection of spleen extract from

another sarcoid patient—the Kveim reaction.
Contact dermatitis
The epidermal route of inoculation tends to favor the development of a

Th1 response through processing by class II-rich dendritic Langerhans-cells
(cf. Figure 2.7f), which migrate to the lymph nodes and present antigen to
T-lymphocytes. Thus, delayed-type reactions in the skin are often produced

by foreign low molecular weight materials capable of binding to peptides
within the groove of MHC molecules on the surface of Langerhans’ cells, to
form new antigens. The reactions are characterized by a mononuclear cell
infiltrate peaking at 12–15 hours, accompanied by edema of the epidermis
with micro-vesicle formation (Figure 15.27). There is a most unusual twist

to this story, however, possibly because the inciting reagent is a reactive
hapten. The late mononuclear reaction is entirely dependent upon very early
events (1–2 hours) mediated by hapten-specific IgM produced by B-1 cells

that, together with complement, activates local vessels to permit T-cell
recruitment. Contact hypersensitivity can occur in people who become
sensitized while working with chemicals, such as picryl chloride and
chromates, or who repeatedly come into contact with the substance urushiol
from the poison ivy plant. p- Phenylene diamine in certain hair dyes,

neomycin in topically applied ointments, and nickel salts formed from
articles such as nickel jewellery clasps (Figure 15.25d) can provoke similar
reactions. T-cell clones specific for nickel salts isolated from the latter

group produce a Th1-type profile of cytokines (IFNγ, IL-2) on antigen
stimulation (Figure 15.10b). Invariant NKT cells producing both the “Th1”
cytokine IFNγ and the “Th2” cytokine IL-4 are also present in the skin
infiltrate of patients with contact dermatitis.


Figure 15.27. Contact sensitivity.
(a) Perivascular lymphocytic infiltrates (PL) and blister (Bl) formation
characterize a contact sensitivity reaction of the skin. (b) High-power view
to show the lymphocytic nature of the infiltrate in a contact hypersensitivity
reaction. (Photographs kindly provided by N. Woolf.)

Other examples

Excessive responses by Th2 cells can damage tissues through activation of

eosinophils (Figure 15.26). As recounted earlier, T)cells synthesizing IL-5
are largely responsible for the sustained influx of eosinophils in asthma and
atopic dermatitis (cf. p. 401). Th2 cells also account for the liver pathology
in schistosomiasis that has been attributed to a reaction against soluble

enzymes derived from the eggs that lodge in the capillaries (Figure 15.28).
It has been suggested that the relatively mixed Th1—Th2 DTH response
induced by bites from bloodcCeeding insects such as sand flies

(Phlebotomus papatasi) might represent an adaptation of the insect to direct
the host immune response to its own advantage. Thus, it was shown that the
increased blood flow associated with the DTH sites allowed sand flies to
feed twice as fast relative to feeding from normal skin sites.

The contribution of DTH reactions to allograft rejection is covered in
Chapter 16, whereas the potential role of Tc cells for the control of cancer
cells is discussed in Chapter 17. In certain organ-specific autoimmune

diseases, such as type I diabetes, cell-mediated hypersensitivity reactions
undoubtedly provide the major engine for tissue destruction.
The intestinal inflammation in celiac disease, an HLA) DQ2/8-associated
enteropathy, is precipitated by exposure to dietary wheat gliadin. The

disorder involves what is probably a genetically related increased mucosal
activity of transglutami-nase (the main target antigen of anti-endomysium
autoanti-bodies; cf. p. 495). This enzyme deamidates the glutamine residues
in gliadin and creates a new T-cell epitope that binds efficiently to DQ2 and

is recognized by IFNγ-secreting intraepithelial CD4+ Thi1-cells. Local

production of IL-15 also plays a role by increasing expression of
nonclassical MHC class I molecules such as MICA on epithelial cells and

+
receptors for these such as NKG2D on intraepithelial CD8 αβ T-cells, γβ
T-cells and NK cells, leading to cytotoxic killing of the epithelial cells.


Figure 15.28. Th2-mediated response to schistosome egg.
Th2-type hypersensitivity lesion of inflammatory cells (M) around a
schistosome egg (SE) within the liver parenchyma (LP). (Photograph by
courtesy of M. Doenhoff.)























Psoriasis involves marked proliferation of epidermal kerat-inocytes and

accelerated incomplete epidermal differentiation. For reasons that are not
understood, in around 10% of patients the skin manifestations are
associated with psoriatic arthritis involving joint inflammation and
destruction. The skin inflammation involves neutrophils and both CD4 and
CD8 T-cells that are CD45RO) indicating that they are antigen experienced.

The release of IFNγ induces epidermal hyperplasia and, together with TNF,
increases the expression of I CAM 1 on epidermal keratinocytes, thereby
facilitating the adhesion of T))ells. Experiments in a skin xenograft model

of psoriasis involving the transplantation of human skin onto severe
combined immunodeficient (SCID) mice have identified that the activated
form of the -ignal transducer and activator of -ran-scription 3 (STAT3) cell
signaling molecule localizes to the nucleus of epidermal keratinocytes
following the transfer of CD4 but not CD8 T-cells, indicating a central role

for STAT3 signaling in the interactions between activated CD4 cells and
keratinocytes. Biological agents that are effective in the treatment of
psoriasis include etanercept (TNF receptor-IgG fusion protein) and the

monoclonal antibodies adalimumab (human anti-TNF), infliximab
(chimeric anti-TNF) and ustekinumab (human anti-IL-12 and IL-23).

Efalizumab, a humanized IgG1 monoclonal antibody against CD11a (LFA-
1), is also effective but has been withdrawn due to safety concerns.
Recently IL-22 production by Th17 cells has been shown to mediate kerati-
nocyte proliferation and epidermal cell hyperplasia, providing yet another

potential therapeutic target.



An addition to the original classification—


stimulatory hypersensitivity (“type V”)


Although Gell and Coombs only categorized four types of hypersensitivity
reaction, a fifth type (type V) is sometimes added. This is where antibody to
a cell surface receptor acts as an agonist leading to stimulation of the cell.

When thyroid-stimulating hormone (TSH) binds to its receptor on the
thyroid epithelial cells, adenylyl cyclase is activated, and the cAMP “s
econd messenger” is generated to stimulate thyroid hormone production.

Once sufficient levels of the hormones are produced a negative feedback
loop shuts off the production of TSH. The thyroid-stimulating antibody
present in patients with Graves’ disease (cf. p. 493) is an autoantibody
against the TSH receptor and mimics the effect of TSH, except in this case
there is continuous secretion of the autoantibody by plasma cells that

provides a constant stimulation of the thyroid leading to hyperthyroidism.
Agonistic autoantibodies that stimulate the angiotensin II AT1 receptor have
been described in patients with preeclampsia and with hypertension.




“Innate” hypersensitivity reactions


Many infections provoke a “toxic shock syndrome” characterized by
hypotension (low blood pressure), hypoxia (shortage of oxygen), oliguria

(decreased urine output) and microvascu-lar abnormalities and mediated by
elements of the innate immune system independently of the operation of
acquired immune responses.

Septicemia associated with Gram-negative bacteria results in excessive
release of TNF, IL-1 and IL-6 through stimulation of macrophages and

endothelial cells by the lipopolysaccharide (LPS) endotoxin. Normally this
would enhance host defenses, aiding the recruitment of phagocytes by
promoting adherence to endothelium, priming neutrophils for subsequent
release of reactive oxygen intermediates, inducing febrile responses

(immune responses improve steadily from 33 to 44°C), and so on.
Unfortunately, the excess of circulating LPS, and the cytokines released in
response to its presence, lead to unwanted pathophysiology at distant sites.
This occurs in, for example, the adult respiratory distress syndrome

brought about by an overwhelming invasion of the lung by neutrophils.
There is a prolonged pathologically high concentration of nitric oxide but,
additionally, LPS can activate the alternative complement pathway, and this
may be linked to its ability to induce the release of thromboxane A and
2
prostaglandin from platelets leading to disseminated intravascular
coagulation.

Whereas the major culprit in Gram-negative sepsis is LPS, Gram-
positive organisms possess a variety of components that act on host
defense elements to initiate septic shock. Thus adherence of Staphylococcus
aureus to macrophages induces TNF synthesis, and peptidoglycan-mediated

aggregation of platelets by the same organism leads to disseminated
intravas-cular coagulation. The staphylococcal and streptococcal entero-
toxins induce toxic shock syndrome by quite different means. By

functioning as superantigens (cf. p. 136), they react directly with particular
T-cell receptor families and give rise to massive cytokine release, including
TNF and macrophage migration inhibitory factor (MIF), which is detected
in high concentrations in the plasma of patients with septic shock. Various
treatments are under investigation. Pentoxifylline blocks TNF production

by macrophages. Experimental models of septic shock can be blocked by
anti-MIF and by a peptide derived from the natural sequence 150–161 of
staphylococcal entero-toxin B, which is part of a domain crucial for T-cell

activation.
The readers attention has already been drawn to both the tumor necrosis
factor receptor-associated periodic syndrome (TRAPS) (p. 372) and to
paroxysmal nocturnal

hemoglobinuria (p. 373) in the previous chapter. Undue C3 consumption
is associated with mesangiocapillary glomeru-lonephritis and partial

lipodystrophy (degeneration of adipose tissue) in patients with the so-called
C3 nephritic factor, an IgG autoantibody capable of activating the
alternative pathway by combining with and stabilizing the convertase.
In patients with idiopathic pulmonary fibrosis there is a defective

response to tissue damage in the lung with an imbalance between wound
repair and fibrinolysis. TGFβ and TNF production by epithelial cells and
macrophages cause fibroblasts to proliferate and overproduce extracellular

matrix. Antiinflammatory agents have not generally proved of benefit in
this disease, indeed there is some indication that IFNγ may have some
therapeutic potential by acting as an anti-fibrotic agent.
The neuropathological hallmarks of Alzheimer’s disease are extracellular

plaques and intracellular neurofibrillary tangles. The senile plaques contain
4kDa β-amyloid hydrophobic peptides derived from β-amyloid precursor
protein (APP). Normally APP is cleaved by an α-secretase into soluble

products that cannot form the Alzheimer’-β-amyloid fragment. However, in
individuals with this neurodegenerative disease the pathogenic 4 kDa
peptides are produced following sequential proteolytic processing of APP
by β-secretase (BACE, β-site APP cleavage enzyme) and γ-secretase
(composed of presenilin-1 and -2). Aggregated β-amyloid peptides

produced by this pathway are thought to trigger apoptosis in neurons. The
APOE4 variant of the gene encoding apolipoprotein E, a cholesterol
transporter, is the only established susceptibility gene, although other genes

are also thought to be involved. Following the observation that individuals
treated with high doses of nonsteroidal antiinflammatory drugs (NSAIDs)
for conditions such as rheumatoid arthritis appear to have a reduced
likelihood of developing Alzheimer’s disease, clinical trials are underway to
test the ability of these immunomodulatory agents to delay or prevent the

onset of Alzheimer’s disease.


SUMMARY


Excessive stimulation of the normal effector mechanisms of the
immune system can lead to tissue damage and we speak of

hypersensitivity reactions, several types of which can be distinguished.

Anaphylactic hypersensitivity (type I)

Anaphylaxis involves contraction of smooth muscle and dilatation of
capillaries.
This depends upon the reaction of antigen with specific IgE antibody
bound through its Fc to the mast cell high affinity receptor FceRI.

Cross-linking and clustering of the IgE receptors activates the Lyn
protein tyrosine kinase, recruits other kinases and leads to release from
the granules of mediators including histamine, leukotrienes and

platelet activating factor, plus eosinophil and neutrophil chemotactic
factors and numerous other cytokines.
Atopic allergy

Atopy stems from an excessive IgE response to extrinsic antigens
(allergens) that leads to local anaphylactic reactions at sites of contact
with allergen.

Hay fever and extrinsic asthma represent the most common atopic
allergic disorders resulting from exposure to inhaled allergens. Atopic
dermatitis is also extremely common.

Whereas the immediate reaction to extrinsic allergen (maximum at
30 minutes) is due to mast cell triggering, a late phase reaction peaking
at 5 hours, involving eosinophil infiltration, is initiated by the
activation of alveolar and other macrophages through surface-bound

IgE; secreted TNF and IL-1β now act upon epithelial cells and
fibroblasts to release powerful eosinophil chemoattractants such as
CCL5 and CCL11.

In asthma, serious prolongation of the response to allergen is caused
by Th2 cells that sustain the recruitment of tissue-damaging
eosinophils through the release of IL-5. The soup of powerful
bronchoconstrictors, the injurious effect of eosinophil major basic

protein and the mucus hypersecretion stimulated by IL-13 and IL-4, all
contribute to the airway damage characteristic of chronic asthma.
Many food allergies involve type I hypersensitivity.

Genetic factors include linkage to the propensity to make the IgE
isotype and to genes encoding a number of pattern-recognition
receptors.

Exposure to Th1-stimulating infections may strongly influence the
“immunostat” setting of the tendency to either Th1 or Th2 responses,

the latter increasing the risk of allergy through promotion of IgE
synthesis and eosinophil recruitment.
The offending antigen is identified by intradermal prick tests, giving
immediate weal and erythema reactions, by provocation testing and by

RAST.
Where possible, allergen avoidance is the best treatment.

A monoclonal antibody directed to the receptor-binding domain of
IgE dramatically reduces IgE levels and synthesis, and decreases mast
cell responsiveness.

Symptomatic treatment involves the use of long-acting β -agonists
2
and leukotriene antagonists. Sodium cromoglycate blocks chloride

channel activity thereby stabilizing mast cells and inhibiting
bronchoconstriction. Theophylline is a phosphodiesterase inhibitor that
raises intracellular cAMP causing bronchodilatation.
Chronic asthma is dominated by activated Th2 cells and is treated

with inhaled steroids that display a wide range of anti-inflammatory
actions, including the ability to block the production of mediators by
stimulated macrophages or Th2 cells. These are supplemented where

necessary by long-acting β -agonists.
2
Courses of antigen injection or sublingual administration can

desensitize by the formation of blocking or regulatory IgG antibodies,
or through T-cell regulation. T-cell epitope peptides may be
manipulated to modulate the atopic state.

Antibody-dependent cytotoxic hypersensitivity (type II)
This involves the death of cells bearing antibody attached to a
surface antigen.

The cells may be taken up by phagocytic cells to which they adhere
through their coating of IgG or C3b, lysed by complement or killed by
ADCC effectors.

Examples are: transfusion reactions, hemolytic disease of the
newborn through rhesus incompatibility, antibody-mediated graft

destruction, autoimmune reactions directed against the formed
elements of the blood and kidney glomerular basement membranes,
and hypersensitivity resulting from the coating of erythrocytes or
platelets by a drug.

Complex-mediated hypersensitivity (type III)
This results from the effects of antigen-antibody complexes through:

(i) activation of complement resulting in mast cell degranulation and
the attraction of neutrophils, which release tissue-damaging mediators
on contact with the complex; (ii) stimulation of macrophages to
release proinflammatory cytokines; and (iii) aggregation of platelets to
cause microthrombi and vasoactive amine release.

Where circulating antibody levels are high, the antigen is
precipitated near the site of entry into the body. The reaction in the
skin is characterized by neutrophil infiltration, edema and erythema

maximal at 3–8 hours (Arthus reaction).
Examples are farmer’s lung, pigeon-fancier’s disease and pulmonary
aspergillosis where inhaled antigens provoke high antibody levels,

reactions to an abrupt increase in antigen caused by microbial cell
death during chemotherapy for leprosy or syphilis, polyarteritis nodosa
linked to complexes with hepatitis B virus and an element of the
synovial lesion in rheumatoid arthritis.

In relative antigen excess, soluble complexes are formed that are
removed by binding to the CR1 C3b receptors on red cells. If this
system is overloaded or if the classical complement components are
deficient, the complexes circulate in the free state and are deposited

under circumstances of increased vascular permeability at certain
preferred sites: the kidney glomerulus, the joints, the skin and the
choroid plexus.

Examples are: glomerulonephritis associated with systemic lupus
erythematosus (SLE) or infections with streptococci, malaria and co-
infection with HIV and hepatitis C virus; neurological disturbances in
SLE and subacute sclerosing panencephalitis; and hemorrhagic shock

in dengue viral infection.

Cell-mediated or delayed-type hypersensitivity (type IV)

This is based upon the interaction of antigen with primed T-cells and
represents tissue damage resulting from inappropriate cell-mediated
immunity reactions.
Cytokines, including IFNγ, are released that activate macrophages

and account for the events that occur in a typical delayed
hypersensitivity response such as the Mantoux reaction to tuberculin,
that is, the delayed appearance of an indurated and erythematous

reaction that reaches a maximum at 24–48 hours and is characterized
histologically by infiltration with mononuclear phagocytes and
lymphocytes.
Continuing provocation of delayed hypersensitivity by persisting

antigen leads to formation of chronic granulomas.
Th2-type cells producing IL-5 can also produce tissue damage
through their ability to recruit eosinophils.

CD8 T-cells are activated by class I MHC antigens to become
directly cytotoxic to target cells bearing the appropriate antigen.
IL-22 production by Th17 cells in patients with psoriasis results in

keratinocyte proliferation and epidermal cell hyperplasia.
Examples are: tissue damage occurring in bacterial (tuberculosis,
leprosy), viral (measles, herpes), fungal (candidiasis, histoplasmosis)

and parasitic (leishmaniasis, schistosomiasis) infections, contact
dermatitis from exposure to chromates and poison ivy, insect bites and
psoriasis. Inflammatory bowel disease can result from Th1-type
(Crohn’s disease) or “Th2-like” NKT (ulcerative colitis) reactions to

intestinal bacteria. Celiac disease is an aberrant response to wheat
gliadin.
Stimulatory hypersensitivity (type V)

The antibody reacts with a key surface component such as a
hormone receptor and “switches on” the cell.
An example is the hyperthyroidism in Graves’ disease due to a

stimulatory anti-TSHR autoantibody. Features of these five types of
acquired hypersensitivity are compared in Table 15.4.
“Innate” hypersensitivity reactions

Some infections provoke a “toxic shock syndrome” involving
excessive release of TNF, IL-1 and IL-6 and activation of the alternative

complement pathway.

Table 15.4. Comparison of types of hypersensitivity involving
acquired responses.






















Acute respiratory distress syndrome associated with Gram-negative
bacteria is primarily due to the lipopolysaccharide (LPS) endotoxin
provoking a massive invasion of the lung by neutrophils.

Gram-positive organisms cause release of TNF and macrophage
migration inhibitory factor (MIF) through direct action on
macrophages and stimulation of selected T-cell families by the
enterotoxin superantigens.

Aberration of innate mechanisms may underlie idiopathic pulmonary
fibrosis and contribute to the β-amyloid plaques in Alzheimer’s
disease.



FURTHER READING


Alcorn J.F., Crowe C.R. & Kolls J.K. (2010) T 17 cells in asthma and
H
COPD. Annual Review of Physiology 72, 495–516.

Broide D.H. (2009) Immunomodulation of allergic disease. AnnualReview

ofMedicine 60, 279–291.
Chapel H., Haeney M., Misbah S. & Snowden N. (2006) Essentials of
Clinical Immunology, 5th edn. Blackwell Publishing, Oxford.

Frew A.J. (2008) Sublingual immunotherapy. New England Journal of
Medicine 358, 2259–2264.

Holgate S.T., Church M.K. & Lichtenstein L.M. (2006) Allergy, 3rd edn.
Mosby, London.

Medoff B.D., Thomas S.Y. & Luster A.D. (2008) T cell trafficking in
allergic asthma: the ins and outs. Annual Review of Immunology 26, 205–

232.
Meyer E.H., DeKruyff R.H., & Umetsu D.T. (2008) T cells and NKT cells
in the pathogenesis of asthma. Annual Review of Medicine 59, 281—292.

Palomares O. et al. (2010) Role of T regulatory cells in immune regulation

of allergic diseases. European Journal of Immunology 40, 1232—1240.
Rothenberg M.E. & Hogon S.P. (2006) The eosinophil. Annual Review of
Immunology 24, 147—174.

Sicherer S.H. & Sampson H.A. (2009) Food allergy: recent advances in
pathophysiology and treatment. Annual Review of Medicine 60, 261—277.

Valenta R. et al. (2010) From allergen genes to allergy vaccines. Annual
Review of Immunology 28, 211–241.




Now visit www.roitt.com to test yourself on this chapter.

CHAPTER 16

Transplantation






Key Topics

Types of graft

Genetic control of transplantation antigens
Some other consequences of MHC incompatibility

Mechanisms of graft rejection
The prevention of graft rejection

Is xenografting a practical proposition?
Stem cell therapy

Clinical experience in grafting
The fetus is a potential allograft



Just to Recap ...


Whilst the cells of the innate response, and the B-lymphocytes of the
adaptive response recognize intact antigen, T-lymphocytes recognize

processed antigens in the form of peptides presented by cell surface MHC
molecules. At the population level there is an incredible diversity of MHC
genes that is thought to have evolved in response to pathogen diversity The
V(D)J recombination mechanisms associated with antibodies and T-cell

receptors have the potential to generate responses again virtually any
foreign antigen, including allogeneic MHC molecules.



Introduction


The replacement of diseased organs by a transplant of healthy tissue has
long been an objective in medicine but has been frustrated to no mean
degree by the uncooperative attempts by the body to reject grafts from other

individuals. Unfortunately a relatively high percentage of T-cells have T-
cell receptors specific for “allo-MHC,” i.e. the MHC variants of other

individuals. Antibodies can also be produced against nonself antigens on
transplanted tissues or organs. These constraints necessitate both tissue type

matching and immunosuppression in most cases of transplantation from
genetically nonidentical individuals.




Types of graft


We first need to define the terms used for transplants between individuals
and species:
Autograft—tissue grafted back on to the original donor.

Isograft—graft between syngeneic individuals (i.e. of identical genetic
constitution) such as identical twins or mice of the same pure inbred
strain.

Allograft—graft between allogeneic individuals (i.e. members of the
same species but different genetic constitution), e.g. human to human and
one mouse strain to another.

Xenograft—graft between xenogeneic individuals (i.e. of different
species), e.g. pig to human.
Most cases of clinical transplantation involve allografts, although there is

now a serious interest in the use of grafts from other species. The most
common allografting procedure is blood transfusion where the unfortunate
consequences of mismatching include hemolysis (lysis of red cells),
intravascular coagulation, chills and nausea. However, such events are rare
because infused blood would of course normally have been cross-matched

for the ABO and rhesus (Rh) blood groups.
Considerable attention has been paid to the rejection of solid grafts such

as skin and the sequence of events is worth describing. In mice, for
example, the skin allograft settles down and becomes vascularized within a
few days. Between 3 and 9 days the circulation gradually diminishes and
there is increasing infiltration of the graft bed with lymphocytes and
monocytes. Necrosis begins to be visible macroscopically and within a day

or so the graft is sloughed completely (Figure M16.1.1). Rejection is an
immunological phenomenon, showing both memory and specificity
(Milestone 16.1). Furthermore, the recipient of T cells from a donor who

has already rejected a graft will give accelerated rejection of a further graft

of the same type (Figure 16.1), showing that the lymphoid cells are primed
and retain memory of the first contact with graft antigens.




Genetic control of transplantation antigens


The specificity of the antigens involved in graft rejection is under genetic
control. Genetically identical individuals, such as mice of a pure strain or

monozygotic twins, have identical transplantation antigens and grafts can
be freely exchanged between them. The mendelian segregation of the genes
controlling these antigens has been revealed by interbreeding experiments



Milestone 16.1—The Immunological Basis

of Graft Rejection


The field of transplantation owes a tremendous debt to Sir Peter Medawar, the outstanding
scientist who kick-started and inspired its development. Even at the turn of the twentieth
century it was an accepted paradigm that grafts between unrelated members of a species
would be unceremoniously rejected after a brief initial period of acceptance (Figure
M16.1.1). That there was an underlying genetic basis for rejection became apparent from
Padgett’s observations in Kansas City in 1932 that skin allografts between family members
tended to survive for longer than those between unrelated individuals and J.B. Brown’s
critical demonstration in St. Louis in 1937 that monozygotic (i.e. genetically identical) twins
accepted skin grafts from each other. However, it was not until Medawar’s research in the
early part of the Second World War, motivated by the need to treat aircrew with appalling
burns, that rejection was laid at immunology’s door. He showed that a second graft from a
given donor was rejected more rapidly and more vigorously than the first and, further, that
an unrelated graft was rejected with the kinetics of a first set reaction (Figure M16.1.2). This
second set rejection

Figure M16.1.1. Appearance of skin grafts in mice.
(a) Graft undergoing rejection; (b) complete rejection (scab); and, for
comparison, (c) a completely healed skin graft without evidence of
rejection. (Reproduced from McFarland H.I. & Rosenberg A.S. Current

Protocols in Immunology. Unit Number: UNIT 4.4. DOI:
10.1002/0471142735.im0404s84).

Figure M16.1.2. Memory and specificity in skin allograft rejection
in rabbits.
(a) Skin autografts and allografts from two unrelated donors B and C are

applied to rabbit A that has already rejected a first graft from B (B ).
1
While the autograft A remains intact, graft C seen for the first time
1
undergoes first set rejection, whereas a second graft from B (B ) is
2
sloughed off very rapidly.
(b) Median survival times of first and second set skin allografts showing
faster second set rejection with a median 50% graft survival of 6 days

compared with 10 days for a first set rejection. (From Medawar PB.
(1944) Journal of Anatomy 78, 176.)





















is characterized by memory and specificity and thereby bears the hallmarks of an
immunological response. This was later confirmed by transferring the ability to express a
second set reaction with lymphocytes.
The message was clear: to achieve successful transplantation of tissues and organs in the
human, it would be necessary to overcome this immunogenetic barrier. Limited success was
obtained by Murray at the Peter Bent Brigham Hospital (Boston) and Hamburger in Paris,
who grafted kidneys between dizygotic twins using sublethal X-irradiation. The key
breakthrough came when Schwartz and Damashek’s report on the immunosuppressive
effects of the antimitotic drug 6-mercaptopurine was applied independently by Calne and
Zukowski in 1960 to the prolongation of renal allografts in dogs. This finding was followed

very rapidly by Murray’ s successful grafting in 1962 of an unrelated cadaveric kidney
under the immunosuppressive umbrella of azathioprine, the more effective derivative of 6-
mercaptopurine devised by Hutchings and Elion.

This story is studded with Nobel Prize winners and readers of a historical bent will gain
further insight into the development of this field and the minds of the scientists who gave
medicine this wonderful prize in Hakim N.S. and Papalois V. (eds.) (2003) History of Organ
and Cell Transplantation, Imperial College Press, London and in Brent L. (1996) A History
of Transplantation Immunology, Academic Press, London.



between mice of different pure strains. As these mice breed true within a
given strain and always accept grafts from each other, they must be
homozygous for the “transplantation” genes. Consider two such strains A
and B with allelic genes differing at one locus. In each case paternal and

maternal genes will be identical and they will have a genetic constitution of,
say, A/A and B/B respectively. Crossing strains A and B gives a first familial
generation (F1) of constitution A/B. Now, all F1 mice accept grafts from
either parent showing that they are immunologically tolerant to both A and

B due to the fact that the transplantation antigens from each parent are
codominantly expressed (see Figure 4.26). By intercrossing the F1
generation, one would expect an average distribution of genotypes for the
F2s as shown in Figure 16.2; only one in four would have no A genes and

would therefore reject an A graft because of lack of tolerance, and one in
four would reject B grafts for the same reason. Thus, for each locus, three
out of four of the F2 generation will accept parental strain grafts.

In the mouse around 40 such loci have been established but, as we have
seen earlier, the complex set of loci termed H-2 (HLA in the human)
predominates in the sense that it controls the “strong” transplantation
antigens that provoke intense allograft reactions. We have looked at the
structure (cf. Figure 4.17) and biology of this major histocompatibility

complex (MHC) in some detail in previous chapters (see Milestone 4.2, p.
100). Given the mendelian segregation and codominant expression of these
genes, it should be evident that in outbred populations siblings have a 1 : 4

chance of identity with respect to MHC. The non-H-2 or “minor”
transplantation antigens, such as the male H-Y, are recognized by T-cells as
processed peptides in association with the MHC molecules. One should not
be misled by the term “minor” into thinking that these antigens cannot give

rise to serious rejection problems; they do, albeit more slowly than the
MHC.


Figure 16.1. Graft rejection induces memory that is specific and can be
transferred by T-cells.
In experiment 1, an A strain recipient of T-cells from another A strain
mouse, which had previously rejected a graft from strain B, will give

accelerated (i.e. 2nd set) rejection of a B graft. This occurs even though the
mouse that has received the graft has not itself previously been grafted.
Experiments 2 and 3 show the specificity of the phenomenon with respect
to the genetically unrelated third party strain C.




















Figure 16.2. Inheritance of genes controlling transplantation antigens.
A represents a gene expressing the A antigen and B the corresponding
allelic gene at the same genetic locus. The pure strains are homozygous for
A/A and B/B respectively. As the genes are codominant, an animal with A/B
genome will express both antigens, become tolerant to them and therefore

accept grafts from either A or B donors. The illustration shows that, for
each gene controlling a transplantation antigen specificity, three-quarters of
the F2 generation will accept a graft of parental skin. For n genes the

n
fraction is (3/4) . If F1 A/B animals are back-crossed with an A/A parent,
half the progeny will be A/A and half A/B; only the latter will accept B

grafts.

Some other consequences of MHC

incompatibility




Class II MHC differences produce a mixed

lymphocyte reaction (MLR)


When peripheral blood mononuclear cells (PBMCs) from individuals of
different class II haplotype are cultured together, lymphocyte activation and
proliferation occurs (MLR), the T-cells of each population reacting against
MHC class II determinants on the surface of the cells of the other

+
population. The responding cells are predominantly CD4 T-cells and are
stimulated by the class II determinants present mostly on B-cells,
macrophages and especially dendritic cells. Thus, the MLR is inhibited by
antisera to class II determinants on the stimulator cells.



The graft-versus-host (GVH) reaction

When competent T-cells are transferred from a donor to a recipient who is
incapable of rejecting them, the grafted cells survive and have time to

recognize the host antigens and react immunologically against them.
Instead of the normal transplantation reaction of host against graft, we have
the reverse, a graft-versus-host (GVH) reaction. In the young rodent there
can be inhibition of growth (runting), spleen enlargement and hemolytic

anemia (due to the production of red cell antibodies). In the human, fever,
anemia, weight loss, rash, diarrhea and splenomegaly are observed, with
cytokines, especially tumor necrosis factor (TNF), being major mediators of

pathology. The “stronger” the transplantation antigen difference, the more
severe the reaction. Where donor and recipient differ at HLA or H-2 loci,

the consequences can be fatal, although it should be noted that reactions to
dominant minor transplantation antigens, or combinations of them, may be
equally difficult to control.
Two possible situations leading to GVH reactions are illustrated in Figure

16.3) In humans this may arise in immunologically compromised subjects
receiving hematopoietic stem cell grafts, e.g. for severe combined
immunodeficiency (see p. 378) or as a form of cancer therapy. Competent

T-cells in blood or present in grafted organs given to immunosuppressed
patients may also mediate g.v.h. reactions.



Mechanisms of graft rejection



Various immune system components can mediate an attack upon the foreign
organ or tissue and thereby contribute towards hyperacute, acute or
chronic rejection (Table 16.1).

Table 16.1. The various types of graft rejection.


















Figure 16.3. Graft-versus-host reaction.

When competent T-cells are inoculated into a host incapable of reacting
against them, the grafted cells are free to react against the antigens on the
host’s cells that they recognize as foreign. The ensuing reaction may be
fatal. Two of several possible situations are illustrated: (a) the hybrid AB

receives cells from one parent (BB) that are tolerated but react against the A
antigen on host cells; (b) an X-irradiated AA recipient restored
immunologically with BB cells cannot react against the graft and a graft-
versus-host (GVH) reaction will result.

Lymphocytes can mediate rejection


A primary role of lymphocytes in first set rejection is consistent with the
histology of the early reaction showing infiltration by mononuclear cells
with very few polymorphonuclear cells or plasma cells (Figure 16.4). The

dramatic effect of neonatal thymectomy on prolonging skin transplants, and
the long survival of grafts in children with thymic deficiencies, implicate
the T-cells in these reactions. In the chicken, allograft rejection and GVH
reactivity are influenced by neonatal thymectomy but not bursectomy. More
direct evidence has come from in vitro studies showing that T-cells taken

from mice rejecting an allograft could kill target cells bearing the graft
antigens in vitro. Although CD8 cytotoxic T-cells play a major role in
allograft rejection, a number of murine models have indicated that in the

absence of CD4 T-cells allografts can be accepted indefinitely. Indeed,
rejection can be mediated by CD4 T-cells in the absence of CD8 T-cells,
perhaps because the CD4 cells sometimes have cytotoxic potential for class
II targets. However, in intact animals, cytokine secretion from CD4 T-cells
will recruit and activate CD8 T-cells, B-cells, NKT cells and macrophages

that all have the potential to contribute to the rejection process.
Furthermore, γ-interferon (IFNγ) upregulates MHC expression on the target
graft cell, so increasing its vulnerability to CD8 cytotoxic cells.

Figure 16.4. Acute rejection of human renal allograft showing dense

cellular infiltration of interstitium by mononuclear cells. (Photograph
courtesy of M. Thompson and A. Dorling.)

Recognition of allogeneic MHC by the recipient’s

T-cells


Remember, we defined the MHC by its ability to provoke the most
powerful rejection of grafts between members of the same species. This
intensity of MHC mismatched rejection is a consequence of the very high
frequency of alloreactive T-cells (i.e. cells that react with allografts)

present in normal individuals. Whereas merely a fraction of a percent of
the normal T-cell population is specific for a given single peptide, upwards
of 10% of T-cells react with alloantigens. Two main pathways of
recognition have been described. In the direct pathway large numbers of

recipient alloreactive T-cells recognize allo- (i.e. graft) MHC on the
surface of donor cells, whereas in the indirect pathway a smaller number
of recipient T-cells recognize peptides derived from allo-MHC (and allo-
minor transplantation antigens) presented by self MHC molecules on the

recipient’s own antigen-presenting cells (Figure 16.5a and c). A third, semi-
direct, pathway has also been proposed, in which intact MHC molecules
are acquired from the donor cells by the dendritic cells of the recipient
(Figure 16.5b).

Allogeneic MHC differs from the recipient essentially in the groove
residues that contact processed peptide, but much less so in the more
conserved helical regions that are recognized by the TCR. Having a

different groove structure, the allo-MHC will be able to bind a number of
peptides derived from proteins common to donor and host that might be
unable to fit the groove in the host MHC and therefore fail to induce self-

tolerance. Thus the host T-cells that recognize allo-MHC plus common
peptides will not have been eliminated, and will be available to react with

the large number of different peptides binding to the allo-groove of the
donor antigen-presenting cells (APCs) that migrate to the secondary
lymphoid tissue of the graft recipient. In some cases, the polymorphic
residues may lie within the regions of the MHC helices that contact TCR

directly and, by chance, a proportion of the T-cell repertoire cross-reacts
and binds to the donor MHC with high affinity. Attachment of the T-cell to
the APC will be particularly strong as the TCRs will bind to all the donor
MHC molecules on the APC, whereas in the case of normal MHC—peptide

recognition, only a small proportion of the MHC grooves will be filled by
the specific peptide in question. These direct pathways of immunization by
the allograft MHC that are usually initiated by the most powerful APC, the
dendritic cell, dominate the early sensitization events, as this acute phase of

rejection (see below) can be blocked by antibodies to the allo-MHC class II.

Figure 16.5. Recognition of graft antigens by alloreactive T-cells.
(a) Direct pathway. T-cell receptors (TCR) on the recipient’s T-cells
directly recognize allogeneic MHC (brown) on the surface of donor

antigen-presenting cells. Polymorphic differences between MHC allotypes
largely affect peptide binding rather than TCR contact by the donor MHC.
Under these circumstances, the donor allogeneic MHC molecule will be
seen as similar to “self” MHC by the recipient’s T-cells but, unlike the self-

MHC, the donor MHC groove on graft antigen-presenting cells will bind
large numbers of processed peptides common to graft and recipient to
which the responder host T-cells have not been rendered tolerant and that
can therefore provoke a reaction in up to 10% of these host T-cells. This

provides the intensity of the allograft response. This explanation for the
high frequency of alloreactive T-cells is given further credibility by the
isolation of individual T-cell clones that react with self- and allo-MHC,
each binding a different peptide sequence. Direct recognition of donor

MHC by recipient T-cells can also occur if the limited polymorphism in the
α-helix adventitiously allows binding of TCRs to the allo-MHC
independently of the associated peptide. Multiple bonds of this nature
between the APC and T-cell may give rise to a strong enough interaction to

permit T-cell activation. (b) Semi-direct pathway. It has been proposed

that recipient dendritic cells may acquire intact MHC molecules from donor
cells and then show these intact MHC molecules to the recipient’s T-cells.

Again, “nontolerant” peptides derived from antigens common to donor and
recipient are presented. (c) Indirect pathway. The recipient’s APCs
(antigen-presenting cells) process donor MHC (brown) and donor minor
histocompatibility molecules and, just as they would any protein molecule,

then present the generated allogeneic peptides (brown) using their own, i.e.
self, MHC (green). The initially small population of T-cells that are
stimulated by the indirect pathway will expand with time.



























However, with time, as the donor APCs in the graft are replaced by
recipient cells, another rejection mechanism based on an indirect pathway
of sensitization involving the presentation of processed allogeneic peptides

by host MHC (Figure 16.5c) becomes possible. Although T-cells
recognizing peptides derived from polymorphic graft proteins would be
expected to be present in low frequency comparable to that observed with

any foreign antigen, a graft that has been in place for an extended period
will have the time to expand this small population significantly so that later
rejection may depend progressively on this indirect pathway. In these
circumstances, antirecipient MHC class II can now be shown to prolong
renal allografts in rats.



The role of antibody

Allogeneic cells can be destroyed by antibody-mediated cytotoxic (type II
hypersensitivity) reactions (p. 406). Consideration of the different ways in

which kidney allografts can be rejected illustrates the contribution of
antibody to the rejection process.
Hyperacute rejection occurs within minutes of transplantation and is the
result of pre-existing anti-donor antibodies in the recipient binding to blood

vessel endothelium in the donated kidney. The antibodies activate the
classical pathway of complement and initiate the blood clotting cascade.
The blood vessels become blocked with aggregated platelets, and

neutrophils are also rapidly recruited as a result of the complement
activation.
Acute rejection is characterized by dense cellular infiltration (Figure
+
16.4) and rupture of peritubular capillaries. CD8 cytotoxic T-cells attack
the graft cells whose MHC antigen expression has been upregulated by γ-

+
interferon. CD4 T-cells are also present, including cells of the Th17
phenotype. Upregulated expression of the CD80 and CD86 co-stimulatory

molecules occurs on tubular epithelial cells thereby promoting activation of
these cell-mediated responses, further aided by the local production of a
number of chemokines. Although some T-cells may become sensitized

within the graft itself, antigen presentation by dendritic cells of both donor
and recipient origin occurs predominantly in the draining lymph nodes
Acute humoral rejection involving anti-donor MHC contributes to acute
rejection episodes in around 25% of kidney transplant patients. Binding of
graft-specific antibody leads to the deposition of substantial amounts of

complement component C4d in the peritubular capillaries. Immunoglobulin
deposits on the vessel walls induce platelet aggregation in the glomerular
capillaries leading to acute renal shutdown (Figure 16.6). The possibility of

damage to antibody-coated cells through antibody-dependent cellular
cytotoxicity must also be considered.

Figure 16.6. Acute late rejection of human renal allograft showing
platelet aggregation in a glomerular capillary induced by deposition of

antibody on the vessel wall.
Electron micrograph. gbm, glomerular basement membrane; P, platelet.
(Photograph courtesy of K. Porter.)

Chronic rejection involves glomerular and tubular fibrosis and is often

associated with subendothelial deposits of immunoglobulin and C4d in the
glomerular and peritubular capillaries. This may sometimes be an
expression of an ongoing immune complex disorder (causing the renal

pathology that originally resulted in the necessity to replace a damaged
kidney) or possibly of complex formation with soluble antigens derived
from the grafted kidney.
The complexity of the action and interaction of cellular and humoral

factors in graft rejection is therefore considerable and an attempt to
summarize the postulated mechanisms involved is presented in Figure 16.7.
There are also circumstances when antibodies may actually protect a graft

from destruction, a phenomenon termed enhancement.



The prevention of graft rejection




Matching tissue types on graft donor and

recipient


Given the fact that demand for transplantation far outstrips the supply of
available organs (Figure 16.8) it is essential to maximize the chances that
the graft will be immunologically accepted by the recipient. As MHC
differences provoke the most vicious rejection of grafts, a prodigious

amount of effort has gone into defining these antigen specificities, in an
attempt to minimize rejection by matching graft and recipient in much the

same way that individuals are cross-matched for blood transfusions
(incidentally, the ABO group provides strong transplantation antigens).


Figure 16.7. Mechanisms of target cell destruction.
(a) Direct killing by Tc cells and indirect tissue damage through release of
cytokines such as IFNγ and TNF from Th1-cells. (b) Direct killing by NK
cells (see p. 26) enhanced by interferon. (c) Attack by antibody-dependent

cellular cytotoxicity (d) Phagocytosis of target coated with antibody
(heightened by bound C3b). (e) Sticking of platelets to antibody bound to
the surface of graft vascular endothelium leading to formation of
microthrombi. (f) Complement-mediated cytotoxicity. (g) Macrophages

activated nonspecifically by agents such as IFNγ and possibly C3b can be
cytotoxic for graft cells, perhaps through extracellular action of TNF and

O − 2 radicals generated at the cell surface (see p. 13). IFN, interferon; Mφ,
macrophage; NK, natural killer cell; P, polymorphonuclear leukocyte; TNF,

tumor necrosis factor.

Figure 16.8. The unmet demand for kidney transplants.
Dynamics of the Eurotransplant kidney transplant waiting list and

transplants between 1969 and 2008. The curved line indicates the number
of patients awaiting a kidney transplant, and underneath the far fewer
number of transplants that have taken place is shown by the histogram, with
deceased donor transplants indicated by the orange bars and living donor

transplants by the black bars.

HLA tissue typing


HLA alleles are defined by their gene sequences and individuals can be
typed by the polymerase chain reaction (PCR) using discriminating pairs of
primers. Molecules encoded by the class II HLA-DP, -DQ and -DR loci

provoke CD4 T-cell responses, whereas HLA-A, -B and -C gene products
are targets for allo-reactive CD8 T-cells.


The polymorphism of the human HLA system


With so many alleles at each locus and several loci in each individual
(Figure 16.9), it will readily be appreciated that this gives rise to an
exceptional degree of polymorphism. This is of great potential value to the
species, as the need for T-cells to recognize their own individual

specificities provides a defense against microbial molecular mimicry in
which a whole species might be put at risk by its inability to recognize as
foreign an organism that generates MHC–peptide complexes similar to self.
It is also possible that in some way the existence of a high degree of

polymorphism helps to maintain the diversity of antigenic recognition
within the lymphoid system of a given species and also ensures
heterozygosity (hybrid vigor).


The value of matching tissue types


Improvements in surgical techniques and the use of drugs such as
cyclosporine have diminished the effects of mismatching HLA specificities
on solid graft survival but, nevertheless, most transplanters favor a
reasonable degree of matching (see Figure 16.18). Tissue typing can be