Mechanisms of Disease: Molecular Mimicry and Autoimmunity

[Guest guest]

Subject: Mechanisms of Disease: Molecular Mimicry and Autoimmunity

This is a very long article that is trying to explain one reason

(molecular mimicry) for why autoimmunity happens. Lyme disease does

create autoimmunity problems in, what I believe is, the majority of

Lymies. This article often speaks about MS (which many Lymies are

diagnosed with) and mentions Graves hyperthyroidism (which many Lymies

are also diagnosed with). Towards the end is a paragraph about Lyme

disease which I have highlighted for you. Lyme is also mentioned in

the conclusion, which I have also highlighted. I understand if you

can't get through this entire paper, please at least read the

conclusion. It offers some insight into why vaccines (including the

one for Lyme) may be hazardous to our health.

Robynn

The New England Journal of Medicine -- December 30, 1999 -- Vol. 341,

No. 27

Mechanisms of Disease: Molecular Mimicry and Autoimmunity

Lori J. Albert, D. Inman

Autoimmune disease is the consequence of an immune response against

self-antigens that results in the damage and eventual dysfunction of

target organs. Although the triggering event in most autoimmune

diseases is unknown, an infectious cause has long been postulated to

explain the development of autoimmunity. Molecular mimicry is one

mechanism by which infectious agents (or other exogenous substances)

may trigger an immune response against autoantigens.

According to this hypothesis a susceptible host acquires an infection

with an agent that has antigens that are immunologically similar to

the host antigens but differ sufficiently to induce an immune response

when presented to T cells. As a result, the tolerance to autoantigens

breaks down, and the pathogen-specific immune response that is

generated cross-reacts with host structures to cause tissue damage and

disease (Figure 1). This model has persisted for more than three

decades because it offers an attractive conceptual link between a

physiologic response (defense against infection) and a pathologic

process

(autoimmunity). Molecular mimicry has been linked to the pathogenesis

of several important conditions, such as multiple sclerosis and type 1

diabetes mellitus. In the case of multiple sclerosis, it has been

hypothesized that the disease is initiated by an infection early in

life by a virus that shares antigenic structures with the host's

central nervous system tissue. The host's antiviral immune response

cross-reacts with central nervous system self-antigens, such as myelin

basic protein, leading to demyelination.

Subsequent viral infections are thought to cause exacerbations of the

disease by reactivating the immune response against viral antigens and

autoantigens. Although there is some evidence that infectious agents

play a part in the pathogenesis of multiple sclerosis and many other

autoimmune diseases (Table 1), it has not yet been proved that

molecular mimicry is the initiating factor in any of these diseases.

It is thus timely to review the evidence of molecular mimicry as a

valid construct to explain the pathogenesis of autoimmune disease.

The Immunologic Basis of Molecular Mimicry

The fate of a T cell, whether it is productive expansion, tolerance,

or death, lies in the recognition by the T-cell antigen receptor of an

antigenic peptide bound to a major-histocompatibility-complex (MHC)

molecule on the surface of an antigen-presenting cell. This

recognition event is very flexible, giving T cells the potential to

recognize a broad range of foreign antigens. (20,21) However, this

range of specificities also makes it possible for T-cell antigen

receptors to cross-react with autoantigens. Protection against

autoimmunity is provided by negative selection of self-reactive T

cells in the thymus

(resulting in apoptosis) and induction of tolerance in T cells in the

periphery (through clonal deletion or induction of anergy or

nonresponsiveness). (22,23) In the thymus and in the periphery, the

fate of a T cell is determined by the avidity of its antigen

receptor for the peptide-MHC complex. It appears that the affinity of

the receptor for the complex and the number of complexes act in

concert to direct the outcome of the reaction between the peptide-MHC

complex and the T-cell receptor.

(24,25,26)

However, some T cells are not deleted in the thymus or rendered

tolerant to autoantigens or deleted peripherally, possibly because

they are inaccessible to antigen or because the level of antigen is

too low to activate them. (23) Since these T cells do not recognize

their cognate antigens, they are called " ignorant " T cells, and the

antigens for which they are specific are referred to as " cryptic. "

(27)

Antibody responses, as well as cellular responses, are important in

the development of autoimmune disease. The responses of antibodies

reflect the antigenic specificity of the B cells. Diversity exists

among antibodies to maximize the host defense, and mechanisms similar

to those that allow T cells to survive may allow autoreactive B cells

to survive. (23) However, most productive B-cell responses depend on

help from T cells. (28) The development of autoimmunity suggests that

tolerance to autoantigens has broken down, allowing previously

quiescent, potentially autoreactive T and B cells to become activated.

Infection may provide the stimulus for the breakdown of tolerance

through several nonspecific mechanisms unrelated to molecular mimicry.

(29) Tissue damage and local necrosis of cells may uncover cryptic

epitopes of autoantigens that then reactivate resting autoreactive T

cells. (30) Inappropriate activation of T cells may occur as a result

of the up-regulation of MHC molecules and costimulatory molecules on

antigen-presenting cells, the latter being required for the activation

of previously unactivated T cells after recognition and binding of

MHC-peptide complexes. Alternatively, nonspecific T-cell activation

may occur as the result of the binding of bacterial and other proteins

known as superantigens. Molecular mimicry, on the other hand, directly

involves the specificity of the immune response in an unintended

process that results in the

breakdown of tolerance.

Sequence or Structural Homology

CD4+ T cells are the main effectors in most autoimmune diseases. Thus,

molecular mimicry depends on demonstrating that these T cells can be

activated by antigenic determinants of an infectious agent that are

similar to the determinants present in the host. The development of

peptide-sequence data bases has resulted in the identification of many

linear sequences of amino acids shared by organisms and humans, but

many of these sequences lack any clinical correlation. Furthermore, it

has been calculated that, on the basis of chance alone, up to 10

perfect matches can be found in protein-sequence data bases for a

sequence of 5 amino acids. (31) However, antigenic mimicry is probably

more complex than simple amino acid-sequence homology. In a study of

the activation of T-cell clones specific for myelin basic protein (a

central nervous system antigen) from patients

with multiple sclerosis, amino acids predicted to be critical for

peptide binding to MHC molecules and recognition by T-cell antigen

receptors were identified, and possible substitute amino acids were

determined. These structural criteria were used to

search a peptide data base. (32) The 129 peptides identified that

contained one of these so-called mimicry motifs were tested against

seven T-cell clones from two patients with multiple sclerosis. Seven

viral peptides and one bacterial peptide activated three of the

clones, thus mimicking the peptide of myelin basic protein; for all

but one peptide, the cross-reactivity would not have been predicted on

the basis of the linear sequence of amino acids. This study confirms

that some T-cell antigen receptors recognize not just a single peptide

but a spectrum of structurally related peptides, which could be

derived from different antigens. Furthermore, some stimulatory

peptides do not share a single

amino acid with the original peptide, (33) an observation underscoring

the fact that linear-sequence homology is not critical for mimicry.

Additional evidence of the complex process by which T-cell antigen

receptors recognize MHC-peptide complexes comes from studies in which

a randomly generated peptide library was used to identify peptides

capable of stimulating T cells specific for myelin basic protein. (34)

Several peptides derived from both native and foreign proteins were

identified, some of which had a greater ability to activate T cells

from patients with multiple sclerosis than did myelin basic protein

itself. It is not known how many of these peptides might be generated

from natural proteins in vivo and subsequently presented to T cells.

The next level of evidence that molecular mimicry is a mechanism

underlying autoimmune disease would be the demonstration that

quiescent, autoreactive T cells could be activated by infection with

an infectious agent bearing an antigen homologous to one in the host.

In mice, a transgene composed of DNA coding for glycoprotein or

nucleoprotein of the lymphocytic choriomeningitis virus can be

directed to pancreatic beta cells by including DNA from the rat

insulin promoter in the transgene construct. (1,2) Viral proteins thus

become " self-proteins " in these transgenic mice. When the mice are

infected with lymphocytic choriomeningitis virus, more than 95 percent

of them have a cytotoxic T-cell response against viral antigens (i.e.,

autoantigens), resulting in diabetes mellitus. Thus, potentially

autoreactive T cells ignore the cognate antigen expressed in

pancreatic beta cells until they are primed through systemic infection

with lymphocytic choriomeningitis virus. However, in this instance,

the beta-cell-specific antigen is created by the transgene construct

and is the same antigen used to induce

disease (i.e., lymphocytic choriomeningitis virus).

This model suggests that T cells capable of reacting with beta-cell

autoantigens indeed exist but are quiescent in uninfected mice because

they do not receive appropriate activation signals from the beta

cells. Although proof of principle is established

by this study, the mimicry is too exact to prove the existence of

molecular mimicry in general. Furthermore, other studies in these

transgenic mice point to the importance of cytokines and costimulatory

molecules for the activation of T cells. (35) For

example, in mice that have coexpression of the costimulatory molecule

B7-1 with viral glycoprotein in the islets, spontaneous diabetes

develops without an exogenous viral challenge, independently of any

mimicry trigger. (35)

Key Issues in Molecular Mimicry

There are difficulties inherent in proving that infection is the cause

of autoimmune disease. The infectious process may have resolved long

before the disease becomes clinically evident, thus making it

difficult to identify a reliable temporal

association between a particular organism and a particular disease.

Alternatively, the infection may itself be occult. Thus, it is

difficult to exclude the possibility that a particular autoimmune

disease is the result of an immune response against

persistent viable or even nonviable infectious agents, with mimicry

occurring as an epiphenomenon. Mimicry might also arise as a secondary

phenomenon caused by an alteration of host antigenic determinants

through tissue injury and the creation of

neoepitopes; the T-cell responses and autoantibodies subsequently

formed might then be only the consequence of tissue injury, not the

cause of it. Tissue injury also contributes to the development of

self-reactive T cells by uncovering previously cryptic epitopes. These

mechanisms lead to an expanded T-cell response over time, known as " epitope

spreading. " (36)

In view of these considerations, the experimental and clinical

evidence purported to establish a mechanism for molecular mimicry in

the pathogenesis of autoimmune diseases needs to be examined

critically, with the recognition of several important

principles First, antigenic mimicry alone may not be sufficient for

pathologic tissue cross-reactivity (in the absence of adequate

costimulation and necessary cytokines). Second, neither antigen

homology nor T-cell proliferation in response to complexes of MHC

molecules and peptides can alone be considered a specific indicator of

pathogenic

mimicry. Third, the demonstration of a tissue-specific antibody

response is likewise not, in itself, a specific indicator of

pathogenic mimicry and may be due to tissue injury. Finally,

antigenic mimicry may elicit tolerance of the host immune response

rather than autoimmunity. (37)

Experimental Models

A model has been developed to test the role of molecular mimicry in

the pathogenesis of autoimmune central nervous system disease. In this

model, a transgene composed of DNA coding for nucleoprotein or

glycoprotein of lymphocytic choriomeningitis

virus is expressed exclusively in oligodendrocytes in mice. Infection

with the virus results in acute peripheral infection, but the central

nervous system is spared. (11) After the virus has been cleared,

however, chronic inflammation develops in the

central nervous system, accompanied by local up-regulation of MHC

class I molecules on oligodendrocytes and of MHC class II molecules on

microglia and other cells. Subsequent infection with lymphocytic

choriomeningitis virus leads to further damage in

the central nervous system, with loss of myelin and motor dysfunction. In

addition, subsequent infection with unrelated viruses that cross-react

with T cells specific for lymphocytic choriomeningitis virus leads to

a similar exacerbation of the central nervous system disease. Thus,

infection in the periphery leads to the

development of central nervous system inflammation.

The cross-activation of T cells specific for a neural autoantigen by

subsequent viral infections provides support for the clinical

impression that exacerbations of multiple sclerosis follow infection

with several different viruses. (38) This model

suggests that cross-reactive T cells alone may be sufficient to induce

disease, because the viral infections precipitating disease are

excluded from the central nervous system. However, it should also be

recognized that in certain mouse models of

multiple sclerosis apparently mediated by an immune mechanism (e.g.,

Theiler's encephalomyelitis due to viral infection (12)),

demyelination depends on primary infection of the central nervous

system by virus.

Herpes stromal keratitis is an autoimmune disease of the eye in humans

that is triggered by herpes simplex virus type 1 and is characterized

by T-cell-dependent destruction of corneal tissue. (39) A similar

keratitis can be induced experimentally by

infecting mice with herpes simplex virus type 1. Mice that are

naturally resistant to herpes simplex virus type 1 infection have a

particular variant of a gene coding for antibodies of the IgG2a class

that contains a peptide sequence with similarities

to a protein expressed in corneal cells. These mice are tolerant of

the corneal protein, and keratitis does not develop after they have

been infected with herpes simplex virus type 1. Herpes protein UL6 has

been identified as the viral protein that

resembles the corneal protein, and studies have shown that keratitis

develops at a much lower rate in ordinary mice infected with a mutant

virus lacking UL6 than in those infected with wild-type virus. (40)

The transfer of T cells from mice infected

with wild-type virus, but not T cells from mice infected with UL6

mutant virus, into " nude " mice (i.e., those lacking mature T cells),

followed immediately by viral infection, results in the development of

keratitis. These studies suggest that mimicry

can induce tolerance as well as disease in the same experimental

system (Figure 2) but that mimicry alone is insufficient to cause

disease, because the transfer of T cells caused disease only when

there was a concurrent infection with the virus.

Cardiac disease has been linked to infection with chlamydia species

through antigenic mimicry. (41) In a study in which protein-sequence

data bases were screened for peptides having homology with the

immunodominant peptide of the heavy chain of

cardiac-specific (alpha) myosin, known as M7A(alpha), several

chlamydia peptides were identified. Immunization of mice with these

peptides induced myocarditis, although it was less severe than in mice

immunized with native M7A(alpha). The transfer of T cells from mice

immunized with chlamydia peptides caused myocarditis in the

recipients, and the T cells reacted with M7A(alpha). Although

infection of normal mice with viable Chlamydia trachomatis organisms

resulted in the development of an antibody response against myosin,

myocarditis was not documented in these mice. The relation of

chlamydia mimicry to the pathogenesis of atherosclerotic coronary

artery disease (suggested by epidemiologic data) remains to be

determined.

Clinical Candidates for Molecular Mimicry

For many autoimmune diseases, a connection with microbial infection

through a mimicry mechanism has been proposed (Table 1). Only a few of

these diseases will be considered here. Autoimmunity mediated by a

mimicry mechanism might not be restricted to a destructive process,

such as the destruction of pancreatic (beta)-cells in diabetes

mellitus, but might also lead to a functional abnormality, as with

thyrotropin-receptor-stimulating antibodies in Graves'

hyperthyroidism. The impetus to prove a mimicry mechanism is the hope

that antimicrobial therapy will provide effective prophylaxis and

treatment in some cases of these often devastating diseases.

The classic clinical paradigm for molecular mimicry has been acute

rheumatic fever after infection with group A (beta)-hemolytic

streptococci. Serum from patients with acute rheumatic fever contains

antibodies to an antigen in the membrane of the organism, the type 5

streptococcal M protein, (42,43) which cross-reacts with myocardial

tissue. (44) In mice, peptides of this M protein that are structurally

similar to cardiac myosin are capable of inducing inflammatory

infiltration of the myocardium and tolerance to coxsackievirus-induced

myocarditis. (45) Monoclonal antibodies that react with group A

streptococcal carbohydrate and human cardiac myosin destroy rat heart

cells in vitro. (46) Although these findings suggest that M proteins

break down tolerance to epitopes of cardiac myosin and could be

important in the pathogenesis of rheumatic carditis, T cells that

cross-react with M protein and myosin peptides have not been

identified in patients with rheumatic fever.

In some patients infected with the spirochete Borrelia burgdorferi,

Lyme arthritis develops as a late sequela. In about 10 percent of

these patients, the arthritis is resistant to antibiotic therapy and

becomes chronic. There is usually no detectable

spirochetal DNA in the affected joints. Human

leukocyte-function-associated antigen 1 (LFA-1, CD11a/CD18, or

integrin (alpha)L(beta)2) has been proposed as an autoantigen in these

patients because it contains a peptide sequence that is homologous to

one in the outer-surface protein A (OspA) of B. burgdorferi. (47)

Synovial-fluid T cells from some patients with chronic Lyme arthritis,

but not from patients with other forms of arthritis, react in vitro

with the peptide, as well as with the intact LFA-1 and OspA proteins.

In patients with the appropriate genetic background associated with

Lyme arthritis,

priming by B. burgdorferi infection is apparently still required for

the development of an autoimmune response to LFA-1. The mechanistic

link between this autoreactivity against LFA-1 and the synovitis of B.

burgdorferi infection awaits further definition.

In the spondyloarthropathies, particularly Reiter's syndrome and

reactive arthritis, there is a clear temporal relation between

arthritis and antecedent bacterial infection, combined with a strong

host genetic susceptibility (HLA-B27). Early studies provided support

for the concept of microbial mimicry of B27 based on

monoclonal-antibody cross-reactivity (13) and sequence homologies.

(14) A systematic search of a sequence data base showed that B27, to a

greater degree than other B alleles, shares an unexpected number of

hexapeptides and pentapeptides with gram-negative bacterial proteins.

(15) However, the pathogenic importance of this mimicry has not been

established. Numerous bacteria that

have sequence homologies with B27 (e.g., Escherichia coli) do not

appear on clinical grounds to be pathogens that cause arthritis, and

T-cell reactivity against microbial peptides recognized by anti-B27

antibodies has not been established. (16) In a

study of B27-transgenic mice immunized with a shared peptide of B27

and klebsiella, the B27 genotype, instead of predisposing the mice to

arthritis, actually rendered them tolerant to this shared peptide.

(17) This finding supports the idea that antigenic

mimicry may induce tolerance rather than autoimmunity.

In patients with type 1 diabetes mellitus, mimicry related to viral

infection has been proposed on the basis of sequence homology between

glutamate decarboxylase (GAD65), an enzyme concentrated in pancreatic

beta cells, (3) and coxsackievirus P2-C, an enzyme involved in the

replication of coxsackievirus B. (4) Although cross-reactivity between

coxsackievirus P2-C and GAD65 has been demonstrated in mice, and an

immune response to the homologous peptides is generated by

immunization of mice with full-length proteins, (5) cross-reactivity

between GAD and coxsackievirus P2-C has not been consistently found in

studies of T cells or serum antibodies from patients with diabetes.

(6,7,8)

Furthermore, a search o0f data bases identified 17 viruses with some

homology to various fragments of GAD65, (48) indicating that

cross-reactivity between GAD65 and coxsackieviruses is not unique.

Moreover, peripheral-blood mononuclear cells from

patients with diabetes mellitus can be stimulated to proliferate by

insulin and several islet-cell antigens as well as GAD65. (49) In view

of this reactivity to several antigens, which could represent epitope

spreading, caution is warranted in drawing conclusions about the role

of cross-reactivity between GAD65 and coxsackievirus peptides

specifically, and that of molecular mimicry in general, in the

pathogenesis of diabetes mellitus.

Conclusions

The experimental and clinical models discussed above fall short of

resolving the key issues in the demonstration of molecular mimicry as

a pathogenic mechanism in autoimmune disease. For researchers in this

area, challenges remain. Infection is common;

autoimmunity is not. The frequency of shared peptide sequences and the

flexibility inherent in immune recognition suggest that mimicry may be

ubiquitous in biologic systems. Defining the default pathways that

normally protect the host from the dangers of an autoreactive response

during or after infection remains an important area of research.

From the clinical perspective, work in this area has not led to any

treatments with proven efficacy. Antibiotic treatment has not been

proved to alter the course of rheumatic fever or postdysenteric

reactive arthritis. (50,51) On the other hand, in a recent trial

comparing immunosuppressive therapy (glucocorticoids plus either

azathioprine or cyclosporine) with conventional therapy (diuretics and

vasodilators) in patients with viral myocarditis, there was no

difference in outcome between the treatment groups. (52) The patients

with markers of immune activation, either humoral or cellular, had a better

outcome, regardless of the treatment they received. Thus, in some

disorders, the immune response, instead of having the deleterious role

suggested by the concept of mimicry, may actually contribute to a

successful host defense and to healing.

If molecular mimicry is a biologically important phenomenon, it has

important implications for vaccination. It is possible that

vaccination against infectious diseases activates pathways of

molecular mimicry in genetically susceptible hosts, and this may

be the basis of adverse reactions to vaccines. The data discussed

above suggest an important caveat in the performance of large-scale

trials of vaccination, such as those involving vaccination against

Lyme disease. (53,54) By the same token, the concept of molecular

mimicry suggests that nonresponsiveness to vaccines could be a

function of tolerance.

Molecular mimicry has remained an attractive explanation for

autoimmune diseases for three decades, mainly on the basis of

circumstantial evidence. No data convincingly demonstrate that mimicry

is an important mechanism in the development of autoimmune disease in

humans. Nonetheless, molecular mimicry retains an intrinsic appeal,

because it links current concepts of the role of the immune system in

the host defense with concepts of autoimmunity.

Source Information

From the Division of Rheumatology, Department of Medicine,

Toronto Western Hospital,

University Health Network (L.J.A., R.D.I.); and the

Immunology (R.D.I.), University of Toronto -- both in

Toronto. Address reprint requests to Dr.

Inman at the Arthritis Center of Excellence, Toronto

Western Hospital, FP 1-221, 399 Bathurst St.,

Toronto, ON M5T 2S8, Canada, or at

rinman@....

Copyright © 1999 by the Massachusetts Medical Society. All rights

reserved.

--

Kiana Rossi

bornfree@...