Antigenic Mimicry, Clonal Selection and Autoimmunity
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doi:10.1006/jaut.2000.0481, available online at http://www.idealibrary.com on Journal of Autoimmunity (2001) 16, 337–340 Antigenic Mimicry, Clonal Selection and Autoimmunity Irun R. Cohen Department of Immunology, The triggering of autoimmunity by infection or immunization is often blamed The Weizmann Institute of Science, on antigenic mimicry. But the concept of antigen mimicry impinges on our Rehovot 76100, Israel understanding of adaptive immunity in general, and not only on auto- immunity. Here are some thoughts about the consequences of immune Key words: altered peptide mimicry. © 2001 Academic Press ligands, antigenic mimicry, autoimmunity, receptors, specificity Pathogenic Antigenic Mimicry epitopes, but an infectious agent can circumvent the deletion of anti-self clones and present the host with Antigenic mimicry [1, 2] is an attractive concept for epitopes that mimic host epitopes. Although a natural two reasons: it rests on a molecular truth and it seems self-epitope would never be able to find a fitting to explain the origin of autoimmunity. The antigen receptor on its own, the mimicking epitope selects and receptors of T cells and B cells recognize antigens by activates clones whose receptors are sufficiently the power of non-covalent fittings between their com- degenerate to now see and respond to a host antigen. bining sites and epitopes on antigens. Since more than The immune system may not be allowed by the one epitope can be envisioned (and demonstrated) to CST to engender self-recognition, but pseudo-self- fit any receptor combining site, any given antigen recognition is allowed. Host-mimicry by infectious receptor can recognize more than one antigen; antigen agents can explain the induction of autoimmune dis- receptors, like all biological receptors, are degenerate eases, without seeming to violate the CST. [3, 4]. Receptors are receptors because they can trans- duce signals into the cell; ligands alone have no intrinsic capacity to signal; no receptor, no signal. Altered Peptide Ligands: Medicinal Therefore, from the cell’s viewpoint ligands that acti- Mimicry vate the same receptor mimic one another biologically, no matter how the ligands may differ chemically. The immune system, like the endocrine and other signal- What if pathogenic mimicry, as described above, has ing systems, tends not to fit to chemistry, so antigenic actually triggered pseudo-self recognition leading to mimicry is a molecular fact [5, 6]. an autoimmune disease; how are we to remove the Mimicry also expresses an ideology. The classical offending clones? We can try to out-mimic them. formulation of the Clonal Selection Theory of Altered peptide ligands (APL) are benevolent mimics Acquired Immunity (CST) teaches that the receptor [10]. Autoimmune diseases, according to the CST, are repertoire must be purged of all antigen receptors that diseases caused by self-reactive clones; whether the could possibly recognize self-antigens [7, 8]; autoim- self-reactive clones have been activated by mimicry or munity is a theoretical impossibility according to the arise by mutation, a reasonable cure is to inactivate CST. Nevertheless, autoimmune diseases do occur, the self-reactive clones by inducing tolerance or and they occur with regularity in the wake of certain anergy, or to get them to otherwise change their infections, like group A strep, Treponema pallidum, behaviour. The specific key to any clone is its specific Borrelia burgdorferi, some enteric infections, or various antigen. If the pathogenic clone recognizes a self- viruses. How can this regularity be explained? One peptide, then the way to silence that clone is to tweak reasonable explanation blames autoimmune disease it with a pseudo-self-peptide that will anergize it or on molecular mimicry between infectious agents and signal it to stop destroying the disease target. APL host self-antigens [9]. The host’s repertoire, in keeping peptides are molecular mimics of self-epitopes that with the tenets of the CST, should be deleted of bind with an altered affinity to the clone’s receptor. receptors that could be triggered by genuine self- Varying the time the epitope spends in the T-cell receptor can greatly influence the behaviour of the T cell [11]. An APL with altered affinity might be able to Tel.: +972-8-934 2195. Fax: +972-8-934 4103. E-mail: neutralize the bad clone. APL peptides could thus be [email protected] seen as mimicry medicine for mimicry disease. 337 0896–8411/01/030337+04 $35.00/0 © 2001 Academic Press 338 I. R. Cohen The concept of the APL as a clonal anergizer has the serum. Hypersensitivity reactions, immediate and resulted in therapeutic trials designed to tolerize delayed, can discriminate between very closely clones; APL trials, to neutralize every last autoim- related antigens. Graft rejection is specific. Acquired mune cell, have employed prolonged treatment with resistance to infectious agents is reasonably specific. relatively high doses of APL. Unfortunately, such a Specificity emerges in polyclonality at the level of the therapeutic strategy can lead to undesirable side whole system; specificity is a collective effort. But how effects [12, 13]. The concept of antigen mimicry pro- can a population of clones act specifically, if each of vides a way for the classical CST to explain autoim- the clones are intrinsically degenerate, if specificity mune diseases, and antigen mimicry, through the cannot be reduced to the individual clones underlying APL, is proposed as a reasonable, CST-compatible the response? cure. Rather than serving the CST, the concept of In principle the answer is known, but at a deeper molecular mimicry actually undermines the CST. level the answer is a mystery. Immune specificity is not unique in its complexity or in its resistance to a simple analytical strategy. The Specificity Agonistes specificity of signal transduction is even more compli- cated than immune specificity; the path from a specific membrane receptor to the activation of a specific gene Antigenic mimicry was thought to be primarily is mediated by a bewildering network of redundant limited to epitopes that shared a noticeable degree of and degenerate signal transduction molecules. The chemical similarity; one peptide could mimic another specificity of signal transduction, like that of immune only if the two peptides did not differ too much in reactivity, emerges from a pattern of collective inter- their amino acid sequences [14]. Indeed, arguments actions [18]. Emergent properties are precisely those for mimicry are based on amino acid homologies behaviours manifested by a system when it operates retrievable from databases. Antigen agonists and as a whole; an emergent property is not evident in any antagonists were thought to share some familial particular component of the system when we examine chemistry that necessarily limited the promiscuity of each part individually. We can therefore answer the antigen mimicry. But, as we mentioned above, the riddle of immune specificity, despite antigen mimicry, basic chemistry of receptors renders them ignorant of by invoking emergence; we can say that the precision ligand chemistry; what fits, fits, irrespective of of immune specificity is an emergent property of the sequence and irrespective of self or not-self chemistry: immune system, and not an intrinsic property of this affinity is the all. Sugars and peptides and nucleic or that degenerate clone examined on its own. The acids, which share no chemical subunits, can mutually specificity of the immune response is created by the mimic, and can be seen to activate a given receptor immune system downstream of the initial antigen [15]. recognition [17–19]. The CST paradigm is accurate when it teaches that The emergence of specificity by a degenerate and the adaptive repertoire is composed of a differential redundant central nervous system is a manifest frequency of clones and that antigens are critical to the example of the inadequacy of reductionism; any process by which clones, are selected. Unfortunately, colour-sighted person can see the power of emergence the antigen receptors of clones cannot guarantee the with his or her own eyes. Each of us can easily antigen specificity required by the CST worldview. discriminate thousands of different colours, yet our The problem for the CST is that antigen receptors are retinas contain only three types of colour receptors: fundamentally degenerate, and this means that the red, green and blue. How can only three different specificity of the immune response is not granted receptors make thousands of discriminations? They automatically by an act of clonal selection. The degen- can do it because each of the receptors is degenerate; eracy of antigen receptors also means that clonal each responds with high ‘affinity’ to only a few selection cannot be entrusted to absolutely discrimi- wavelengths but also with lesser ‘affinity’ to many nate between self and not-self epitopes [16, 17]. The other wavelengths. The important feature is that the phenomenon of antigen mimicry proclaims this con- degeneracy of the colour receptors overlaps from one clusion loud and clear, the CST notwithstanding. receptor to another; different receptors can be activ- Clonal selection pushes the millstone of immunity, ated by the same coloured light. This redundant and while suffering from a lack of visual acuity. Specificity degenerate arrangement allows the three receptors to is the central pillar of adaptive immunity; how does generate complex patterns of signals, which are sent the immune system keep the edifice from crashing to the brain. The brain, in turn, discriminates between onto the self? thousands of different patterns (all emanating from different combinations of the three colour receptors) and interprets the different patterns as different col- Specificity Emergent ours [18]. Obviously, the immune system does not have a brain to read different clonal patterns, and that Although individual clones can be demonstrated to is the mystery. We simply don’t know how the emer- be less than specific, the immune response, at the gence of immune specificity works in molecular population level, is manifestly specific.