Cohen: round 3

seminars in IMMUNOLOGY, Vol. 12, 2000: pp. 321–323 doi: 10.1006/smim.2000.0248, available online at http://www.idealibrary.com on

Cohen: round 3

and self-antigens has been demonstrated in many studies. Crossreactive clones can even mediate experimental autoimmune diseases. These findings have led to the idea that parasite-host antigen mimicry is a major factor in the induction of autoimmune disease.4 Of course, one might pose the opposite question: if nonself-self mimicry is so rife, why is autoimmune disease so rare? We mount vigorous immune responses to invaders that mimic many host self epitopes; yet most of us escape autoimmune disease. How do we stay healthy despite ubiquitous mimicry? We shall return to the mimicry paradox later, after we sharpen the question of immune specificity. If individual receptors are indiscrete at the microscopic scale, wherein lies the specificity of the immune response at the macroscopic scale? After all, an individual’s immune responses are demonstrably specific, even if his or her clones are not. We do not reject our own organs (unless we have an autoimmune disease). So where does specificity arise, if not at the level of a clone’s antigen receptor? Where else could specificity come from? Immune specificity is not given gratis in the initial act of clonal activation, but is generated, I propose, by the cooperative interactions of cell populations. In other words, the most reliable specificity, the truest specificity emerges down-stream of initial clonal activation, during the evolution of the actual response. Some examples are as follows. Specificity at the population level is evident in the greater specificity of polyclonal sera compared with the disappointing specificity of individual monoclonal antibodies. Anyone who has ever raised a monoclonal antibody can attest to the junk antigens it can recognize, along with the specific antigen. (The specific antigen is defined, of course, as the antigen that interests the experimenter.) Although the individual antibodies in a polyclonal serum are each degenerate, the degeneracy of each antibody manifests a different spectrum of ‘non-specificity’. Each antibody clone, owing to its unique binding site, carries its own unique cross-reactive ‘noise’. In the aggregate serum,

In the closing round, I wish to respond to some points and discuss briefly the machinery that produces immune specificity.

Specificity Specificity is central to the concept of immune self-nonself discrimination. Classical clonal selection has taught us that the specificity of the immune response can be reduced to the chemical specificity of the binding between the antigen and the antigen receptor of the responding clone. Thus, specificity is seen to launch the response. The immune response begins with discrete specificity built-in, as it were. The Neo-Orthodox also equate immune specificity with antigen-receptor specificity. The Conservatives may feel that ‘foreigness’ alone is not sufficient to push the system into activation; they would add discrete signals of danger or infection to the context of stimuli that evoke clone-specific immunity to specific antigens. Unfortunately, there is a fundamental problem with receptor specificity, a problem intrinsic to all biologic receptors and not just to antigen receptors. The problem is that biologic receptors are intrinsically degenerate; any given receptor can bind (‘recognize’) more than one ligand.1–3 Hence, receptor recognition of ligands can never be absolutely specific; it is only a matter of affinity (avidity). The distinction between self and nonself, at least at the scale of the antigen receptor, is necessarily fuzzy. Indeed, the immune system capitalizes on this intrinsic fuzziness between self and nonself by using self-antigens as templates for the positive selection of T cell clones that it will later use to recognize foreign antigens. The origin of the repertoire through self recognition is apparently not forgotten by the lymphocytes; mimicry between the antigens of infectious agents

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nated, or manipulated. This is precisely my point. The cumulative experience of the immune system is what determines the functional specificity of any immune signal, or of any string of immune signals, at any particular moment. Specificity is expressed functionally by the evolving phenotype of the immune response.6 Specificity is down-stream of clonal recognition in the flow of the response. This functional specificity can be attributed to the cooperativity of populations of immune interaction and effector molecules such as the chemokines and cytokines that, like the individual antibodies, are each pleiotropic, redundant, and degenerate. Whether or not one detects a specific response depends greatly on the response phenotype that one measures. Now we can return to mimicry. Why, we asked above, is autoimmune disease so much rarer than nonself-self mimicry? Because the specificity of the response is generated by multiclonal interactions. Degenerate clones that cross-reactive with self do indeed get activated during an infection but, like the monoclones of antibodies we discussed above, the self-reactive clones are diluted out, or regulated out, by the multicellular and multimolecular patterns of interactions that constitute the response. Co-respondence6 provides a much sharper image of the nonself (and also of the self) than can be had by any single antigen receptor. Of course, we shall need to develop precise quantitative tools to study modes of cooperativity. The clonal classicists took specificity for granted. However, now that we have solved the molecular identity of the antigen receptors, to explain immune specificity, immunologists will be obliged henceforth to confront complexity. True, the reduction of immune phenomena to microscopic chemistry is still essential, but that reduction, by itself, is not sufficient to explain macroscopic immune behavior and its specificity. As the century turns, biologists generally, and not just immunologists, are preparing themselves to deal with supra-chemical complexity; intra-cellular signal transduction, embryonic development, neoplastic transformation, and all the other mechanisms of real life emerge through cooperative interactions.

however, the different non-specificities are diluted out by the low frequency of each antibody clone. A common binding that the different antibodies do share is to the nominative antigen. This shared binding results in a high frequency of specific binders; the crossreactivities unique to each antibody clone remain at a relatively low frequency. An isolated monoclonal antibody, in contrast to the polyclonal population, cannot hide its degeneracy. Co-respondence, which I mentioned earlier.5 is a second example of a way in which the immune system can create enhanced specificity by cooperative interactions. Antigens entering the immune system are seen by T cells and by B cells responding simultaneously to different types of epitopes. The context of the antigen is sensed and reported to the lymphocytes by macrophages (or other APC). The immune cells interact and mutually exchange cytokines and other signal molecules between them. These cell interactions modify the magnitude and quality of the aggregate response to the antigen. The aggregate response creates, as it were, a sharper immune image of the antigen. In other words, the degeneracy of isolated receptors is overcome by cooperative interactions. As we know from color vision, the brain can discriminate between a diversity of colors and shades of colors using only three types of degenerate photoreceptors: one red, one green and one blue. Population patterns of cells and molecules in the immune system can also create higher orders of specificity (for a more detailed discussion of patterngenerated specificity, see Reference 6). The immune history of the individual also helps determine the specificity of the immune response. For example, immunization with myelin basic protein (MBP) in complete Freund’s adjuvant (CFA) provides all the signals of ‘danger’ and ‘infection’ needed to induce experimental autoimmune encephalomyelitis (EAE) in a naive Lewis rat. However, these signals do not suffice to induce the disease in a Lewis rat that has already recovered from a previous bout of EAE. The resistant rat has not become clonally tolerant or anergic. The resistant rat makes a strong anti-MBP T cell response; one can even isolate lethally pathogenic anti-MBP T cells from such a resistant rat.7 Moreover, an anti-MBP T cell line may mediate EAE in one Lewis rat, but not in another rat that has been T cell vaccinated.8 Thus, the meaning of the antigen and its attendant signals is conditional. In question to this, you say; the signals are specific. It is the animals that vary. The rats have had different immune histories; they have been immunized, vacci-

Immune decisions Bretscher asked me which discrete signals do I entrust to ‘up-regulate the good’ autoimmunity, and Silverstein and Rose asked me what signals determine physiological autoimmunity instead of autoimmune 322

Self-nonself discrimination

approaches we choose to apply to cancer or to autoimmune disease are a product of a worldview. Faith in the chemical simplicity of discrete nonselfrecognition leads to the implementation of strategies that are markedly different from those based on regulatory networks of positive self-recognition. You can try and cure autoimmune disease by killing or blocking the forbidden clones, or you can try and repair the homunculus by epitope vaccinations or by T cell vaccinations. Our beliefs control our investments.

disease. There is no discrete signal. The immune system, I believe, makes changing responses to changing patterns of signals; to make a decision, the immune system does not depend on any single discrete signal. Indeed, the immune performance of mice with critical genes knocked-out attests to the ability of the system to make decisions in the absence of this or that discrete molecule.

Selfhood Howes implies that I might think that self-nonself discrimination is ‘irrelevant’. I am sorry if I have not been clearer on the issue of selfhood. Discrete self-nonself discrimination, the view inherent in the classical formulation of the self, is what I reject. In fact, I agree essentially with Howes’ description of the non-discrete self and have written about such a self.9 Howes ends by asking, ‘Is there a system of selfrepresentation in the immune system or not?’ There certainly is; it has been termed the immunological homunculus.10 Mitchison writes, ‘It is true that a T cell network can occasionally be found to operate, as Kumar and Sercarz have shown, but only during an exceptionally powerful and restricted T cell response’. Should we really put aside demonstrable T cell networks as ‘occasional’ expressions of ‘exceptionally powerful’ immune responses? Rather than shelving such observations, I recommend taking note. Powerful immune responses attended by T cell networks are directed specifically to particular self-antigens such as MBP, thyroglobulin, insulin, the 60 kDa heat shock protein, the acetylcholine receptor, the tumor suppressor p53, and others. I have termed these self antigens the homuncular self antigens; they constitute the system’s self representation.10 Irrespective of the terminology, it is certainly worth knowing how and why these self antigens are powerful, restricted and bound by T networks. This knowledge will reward.

Summary The exchange of views in this seminar has been useful, at least to me, in clarifying present issues in immunology, and the credit belongs to Langman and Cohn for bringing it off so successfully. Of course, successful clarification does not mean that any of us are likely to change his or her point of view. I suspect, however, that each of us is likely to do what we do with a wider perspective. Perhaps this record of what some immunologists were thinking at the turn of the century might even be of future antiquarian interest.

References 1. Nanda NK, Arzoo KK, Geysen HM, Sercarz EE (1995) Recognition of multiple peptide cores by a single T cell receptor. J Exp Med 182:531–539 2. Ausubel LJ, Kwan CK, Sette A, Kuchroo V, Hafler DA (1996) Complementary mutations in an antigenic peptide allow for crossreactivity of autoreactive T-cell clones. Proc Natl Acad Sci USA 93:15317–15322 3. Mason D (1998) A very high level of crossreactivity is an essential feature of the T-cell receptor. Immunol Today 19:395–404 4. Oldstone MBA (1998) Molecular mimicry and immunemediated diseases. FASEB J 12:1255–1265 5. Cohen IR (2000) Discrimination and dialogue in the immune system. Seminars in Immunology (in press). 6. Cohen IR (2000) Tending Adam’s Garden: Evolving the Cognitive Immune Self, Academic Press, San Diego, CA 7. Ben-Nun A, Cohen IR (1982) Spontaneous remission and acquired resistance to autoimmune encephalomyelitis (EAE) are associated with suppression of T cell reactivity: suppressed EAE effector T cells recovered as T cell lines. J Immunol 128:1450–1457 8. Ben-Nun A, Welerle H, Cohen IR (1981) Vaccination against autoimmune encephalomyelitis with T lymphocyte line cells reactive against myelin basic protein. Nature 292:60–61 9. Atlan H, Cohen IR (1998) Immune information, selforganization and meaning. Int Immunol 10:711–717 10. Cohen IR (1992) The cognitive paradigm and the immunological homunculus. Immunol Today 13:490–494

Programs for research Scientists, biologists in particular, can go about their work productively without wasting so much as a single thought on a worldview of their subject matter; there is no end of facts to gather and sort. A worldview of the immune system, however, is not mere sport; our worldview determines our research programs and our research programs count. The experimental

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