Darwinism and antibody diversity: a historical perspective

EVOLUTIONARY

ORIGINS OF Igs AND T-CELL RECEPTORS

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Darwinism and antibody diversity : a historical perspective S.H. Podolsky

and A.I. Tauber

Boston University, 745 Commonwealth

A historical orientation Immunology was born as a daughter of Darwinism. This is a crucial foundation for understanding the dominant themes of immune theorizing that have characterized this most modern discipline. In the 187Os, establishing microbes as the aetiological agents of infectious diseases converged with new discoveries in pathology and comparative embryology to form a unique field of investigation, immunology. We believe these historical antecedents in evolutionary problematics have continued to inform current debate concerning the nature of immunity and the organization of the immune system, and here we will briefly summarize that history and its present status. We must begin with Elie Metchnikoff, whose early career as an embryologist sought to establish genealogical linkages to support Darwinian evolutionary theory (Tauber and Chemyak, 1991). It was in his work with primordial animals that phagocytic cells were identified as purveyors of organismal identity. In this view, the struggle between species was turned within the developing organism, and some agent was required to mediate the development of competing cell lineages as the animal matured. (Despite its eclipse by modem genetic findings, this view has been recently reexamined by Buss (1987).) Beyond its particular functions in embryonic growth (e.g. eating the tadpole’s tail in metamorphosis), the phagocyte in the adult assumed a similar functional role in the broadest sense: it again was to define what in later times was explicitly referred to as self and nonself. As the constituent of the simplest immune system, the phagocyte was “responsible” for maintaining the integrity of the organism, not only against destructive pathogens, but serving as the agent of defence and repair arising from insults of all varieties, whether trauma, disease or

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Avenue, Boston, MA 02215 (USA)

aging. In this view, the immune system - from its simplest inception to its most complex structure reflects the problem of this evolutionary struggle: first, define identity, and second, preserve and protect it (Tauber and Chernyak, 1991)‘. On Met&n&off’s conceptual foundation, Macfarlane Bumet, by focusing generally upon the ecological aspects of host defence, and specifically upon the differences between the primary and secondary response, re-introduced the notion of the evolutionary development of the immune apparatus (Tauber and Podolsky, 1994). Focusing historically upon the development of ideas concerning the elaboration of the antibody repertoire, however, reveals the many forms of “evolution” scientists have envisioned for such an apparatus. One obvious division between such forms extends between strictly Lamar&an and strictly “dogmatic” interpretations of the molecular origins of the repertoire. Bumet himself, from 1941 to 1956, envisioned such origins to be Lamar&an in

’ Subsequent histories have focused on how Metchnikoff’s theory was soon challenged by the immunochemists, a Germandominated group that emerged in the late 1880s. Their famous polemic illustrates the clash of divergent strategies of biology. These descendents of the German reductionists sought chemical specificity as the sine qua non of the new discipline, and this specificity was to be ascertained by chemical principles. Led by Paul Ebrlich, and later Karl Landsteiner, the fast half of 20th century immunology was preoccupied with this chemical agenda as opposed to Metchnikoff s more global, organismic approach to immune function, which originated as a problem in evolutionary biology. Regardless of its present scientific standing, Metchnikoff s theory, drawn from a developmental perspective, newly defined the phagocyte and assigned it the role of mediator of identity, which became the intellectual basis of later evolutionary theories in immunology that emerged after World War II (Tauber, 1994).

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character, placing his adaptive enzyme model in this respect essentially on the same ground as the template models of Haurowitz, Breinl, and Pauling (Burnet, 1941; Breinl and Haurowitz, 1930; Pauling, 1940). Only the rise of the clonal selection theory (CST; Bumet, 1957) would represent the strict rejection of the Lamar&an nature of the intraorganismic evolution of the antibody repertoire, a “dogmatic” stance from which the likes of Haurowitz would continue to wriggle throughout the 196Os, and which would inform the collective quest to locate the origins of antibody diversity in the genome. Immune

diversity

and the problem

of selection

However, while CST represented the unchallenged application of Darwinian selectionism to the lymphocytes of the immune system, the level of such selection remains debated to this day. At one level, such debate approximates that between Skinner’s behaviourism and Chomsky’s relatively more hardwired linguistic models, which reflects the weight we might assign “genetic determinism” versus “epigenetic freedom” in biological function. At the level of the origins of the molecular repertoire itself, aspects of the historical germline/somatic debate may seem to stratify the models absolutely along such hardwired/behaviourist lines. After all, the original germline models of Dreyer, Bennett, Hood, and Talmage argued that all antibody specificities would be encoded within the germline as the result of their prior usefulness in evolutionary history (Dreyer and Bennett, 1965; Hood and Talmage, 1970). Meanwhile, the stepwise selection models espoused by Bumet in 1964 (Bumet, 1964), by Cohn and Cunningham in the early 1970s (Cohn, 1971; Cunningham, 1974) and espoused by Tonegawa himself in his Nobel Prize acceptance speech of 1987 (Tonegawa, 1988) argue that not only are all antibodies not encoded in the germline, but that novel specificities arise through mutation throughout the active life of the organism, with particular cells bearing such novel specificities selected upon by antigen to divide further and thus permit their antibodies to mutate*.

’ Such a facile division obfuscates several issues under the seemingly uniform somatic umbrella. For example, if the definition of “somatic” signifies that not all antibody specificities are encoded in the germline, then it remains logically possible to have an absolutely hardwired somatic model of antibody diversification. In fact, in 1976, Norman Klinman and his colleagues, in noting both that the murine adult repertoire exceeded the neonatal repertoire by the three orders of magnitude, and that the evolution of such a repertoire yet appeared relatively invariant from individual to individual, argued for a “predetermined permutation” model of somatic diversification which would both be antigen-indepcndent and allow for the temporal elaboration of the repertoire throughout the life of the organism (Klinman et al., 1976).

However, with the acceptance of the notion of the germline’s encoding hundreds of antibody genes as a foundation of a larger, somatically diversified system, the history of models positing a priori versus process-dependent origins of the repertoire points to a relative behaviourist/hardwired division among the somatic models themselves. From the earliest selectionist writings and discussions of Jerne, Burnet, and Lederberg, differences have emerged concerning when and to what extent the antibody repertoire becomes fully elaborated (Pauling, 1940; Jerne, 1955; Lederberg, 1959, 1988). Burnet’s 1957 formulation of the “random scrambling” of the genome to present the full repertoire ontogenetically in a way parallels the modern notions of gene shuffling to produce millions of irmnunoglobulin and T-cell receptor combinations, in that both describe what Melvin Cohn has termed “big bang” models of diversification (Cohn, 1994). Specifically, nearly all specificities are encoded before the organism ever faces a “foreign” antigen, with the lymphocytes equally dividing up the enormous range of potential specificities. Critically, emphasis is placed upon the evolutionary selection not of particular antibody specificities, but of the recombination process itself, since so few resultant specificities will be germline-encoded. An adaptive system is thus established in which the exceedingly fluid (since exceedingly diverse) lymphocyte population can be charmelled in the direction of a contingent antigenic history. In contrast to such an adaptive system, a relatively more viscous somatic system is advocated by Cohn and Langman through their espousal of the Protecton (Cohn and Langman, 1990). In their system, the specificities of the original population of lymphocytes of a given organsim are divided among a relatively small number (e.g., 104) of evolutionarily selected specificities directed against expected pathogens. According to their model, only such a restricted original repertoire will provide an ample number of cells possessing a given specificity to find and recognize a given pathogen’s antigens in time to prevent a lethal infection in an emerging organism. From such an original population, somatic mutation may then occur throughout the life of the organism, molded by the antigenic selection of spontaneous mutants. Thus, within the somatic version of the Darwinian framework of immunological thinking, we still see significant divisions, depending upon where, when - and most critically, why - Darwinian selection appears to be operating. In the early days of the germline/somatic debate, such answers appeared quite clear-cut. In 1967, arch-germliner Lee Hood appeared to look to evolutionary selection operating upon individual antibody specificities (Hood et (II., 1967) while arch-somaticist Mel Cohn

EVOLUTIONARY

ORIGINS

OF Igs AND T-CELL RECEPTORS

appeared to look to evolutionary selection operating upon the generator of antibody diversity (GOD) while antigenic selection operated upon the lymphocytes of a given organism (Lennox and Cohn, 1967). However, such terms as “germline” and “somatic” have in 1996 lost their original meaning, and we are left with little heuristic insight from such a division. Do such terms refer, in their traditional sense, to the proportion of genetically encoded variable regions to the total number of eventual variable regions ? If so, then no true germliners remain, and Melvin Cohn [sic] may be said to be the closest to a remaining germline holdout ! Or do such terms now refer to the percentage of eventual specificities an organism begins its foreign-challenged life with? If so, then Cohn moves to the somatic forefront, all the while terming Tonegawa and his supporters “neogermliners” (Cohn, 1994). Clearly, the time has come to shift the question from one of efficient cause to one of the post-Darwinian equivalent of final cause, namely function. The “germline” versus “somatic” polemic was an explicit debate over the efficient origins of the repertoire, engendered to save the Darwinian concept of the genetic encoding of the repertoire ; its solution was made largely without reference to functional concerns themselves, the supposed linchpin of Darwinian analysis. Tonegawa and Cohn differ both in their respective molecular versus cellular approaches to the antibody diversity question, as well as fundamentally differing in how functionality might be incorporated in an evolutionary-cognizant construction of a theory of the immune repertoire. Cohn has been discussing the ability of the immune system to “learn” for nearly three decades, driven by such concerns to formulate his own classic two-signal model. Yet for all his love of learning, Cohn bases his humoral immune system around the presence of relatively few critical pathogens, which a relatively hard-headed immune system must get to in time to eradicate. Thus, few specificities are necessitated to rid these critical pathogens, and evolution can operate principally upon the retention of those necessary specificity-encoding genes in the germline and upon their distribution to an adequate number of cells. Tonegawa, however, bases his humoral immune system upon what Cohn terms the “Landsteiner legacy” (Cohn, 1994) or the need to recognize and defend against a seemingly vast antigenic universe. Therefore, many specificities must somehow be encoded in order to learn this vast universe, and evolution can operate principally upon the formation and retention of an appropriate generator of diversity. Thus, in 1996, the germline/somatic debate has left its quest to defend Darwinism itself as applied to CST and has been transformed into a truly Darwin-

ian debate asking truly Darwinian questions. What is the purpose of the humoral immune system? How wide is its functional purview? Is it more restricted like a non-specific defence system, or is it essentially limitless in defining and recognizing the universe as the nervous system? Such questions form the foundation of the present antibody diversity debate, and the answers to such questions will be critically interrelated to the age-old Metchnikoflian question: at what level is evolution operating ? Immunology,

an evolutionary

science

We began with a historical setting, and we close on a philosophical note. Immunology is a prominent progeny of Darwinism. Here we are referring to Imre Lakatos’ formulation of the “research program” (Lakatos, 1978). He regarded the body of scientific knowledge as consisting of complex levels with a stable core, which remains unperturbed by the conceptual and social whirlwinds rushing through its more peripheral activities (disciplines), the sites where new knowledge is being fashioned. These outlying sites are appreciated as being more subject to various theoretical interpretations, but eventually the conceptual basis is “hardened” by further testing, so that at some point in the trials of scientific theory construction, knowledge coalesces around its pragmatic achievements, freeing itself from its own theoretical vageries and uncertainties. The “protective” peripheral sciences, constructed on their own particular concerns of a more limited nature, mature and eventually support the core theory. No longer subject to radical development, a research program matures, and its core content is defended and codified. On this view, the Darwinian core theory is in the continued process of being validated, and immunology is one of the daughter sciences engaged in that project. In short, there is an acknowledged hierarchy of validation applied to scientific knowledge, and the theoretical core has a standing quite different from those arenas where the principles of the deep theory are applied. It is at one of these less firmly situated peripheral locales that immunology seems to be contributing to the overall Darwinian project as it wrestles with the problematits outlined in this short exposition of the immune diversity issue.

References Breinl,

F. & Haurowitz, F. (1930), Chemische Untersuchung des Prlzipitates aus HImoglobin und AntiHlmoglobin-Serum und Bemerkungen uber die Natur der Antikorper. Hoppe-Seyler’s Zeitschrif fur Physiologische Chemie, 192, 45-57. Bumet, F.M. (1941), The production of antibodies. Macmillan and Company, New York.

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Bumet, F. (1957), A modification of Jerne’s theory of antibody production using the concept of clonal selection. Aust. J. Sci., 20, 67-68. Bumet, F.M. (1971), A Darwinian approach to immunity. Nature (Lond.), 203, 451-454. Buss, L.W. (1987), The Evolution of Individuality. Princeton University Press,Princeton. Cohn, M. (1971), Selection under a somatic model. Cell. Immunol., 1,461-467. Cohn, M. (1994), The wisdom of hindsight. Annu. Rev. Immunol., 12, l-62. Cohn, M. & Langman, R.E. (1990), The protecton : the evolutionarily selected unit of humoral immunity. Immunol. Rev., 115, 1-131. Cunningham, A.J. (1974), The generation of antibody diversity : its dependence on antigenic stimulation. Contemp. Top. Mol. Immunol.,

3, 1-26.

Dreyer, W.J. & Bennett, J.C. (1965), The molecular basis of antibody formation: a paradox. Proc. Natl. Acad. Sci. (USA), 54, 864-869.

Hood, L. & Talmage, D. (1970) Mechanism of antibody diversity: germ line basis for variability. Science, 168, 325-334. Hood, L., Gray, W.R., Sanders, B.G. & Dreyer, W.J. (1967), Light chain evolution. Cold Spring Harbor Symp. Quant. Biol., 32, 133-146. Jeme, N.K. (1955), The natural selection theory of antibody formation. Proc. Natl. Acad. Sci. (USA), 41, 849-857.

Klinman, N.H., Sigal, E.S., Metcalf, E.S., Pierce, S.K. & Gearhart, P.J. (1976), The interplay of evolution and environment in B-cell diversification. Cold Spring Harbor Symp. Quant. Biol., 41, 165-174. Lakatos, I. (1978), The Methodology of Scientific Research Programmes. Cambridge University Press, Cambridge. Lederberg, J. (1959), Genes and antibodies. Science, 129, 1649-1653. Lederberg, J. (1988), Ontogeny of the clonal selection tbeory of antibody formation: reflections on Darwin and Ehrhch. Ann. N.Y. Acad. Sci., 546, 175-182. Lennox, ES. & Cohn, M. (1967), Immunoglobulins. Annu. Rev. Biochem.,

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Pauling, L. (1940), A theory of the structure and process of formation of antibodies. J. Am. Chem. Sot., 62, 26432657. Tauber, A.I. & Chemyak, L. (1991), Met&n&off and the Origins of Immunology: From Theory to Metaphor. Oxford University Press,New York. Tauber, A.I. (1994), The Immune Self: Theory or Metaphor? Cambridge University Press, New York and Cambridge. Tauber, A.I. & Podolsky, S.H. (1994), Frank MacFarlane Burnet and the declaration of the immune self. J. Hist. Biol.. 27, 531-573.

Tonegawa, S. (1988), Somatic generation of immune diversity. In Vitro Cell. Dev. Biol., 2, 253-265.

Evolution and somatic learning in V-region genes A.S. Perelson

(I), R. Hightower

(*) and S. Forrest

(*)

(I) Theoretical Division Los Alamos National Laboratory, Los Alamos, NM 87545 (USA), and (2) Dept. of Compute; Science, University of New Mexico, Albuquerque, NM 87131 (USA)

An interesting and still unanswered question about the evolution of the immune system’s variable(V)-region genes is what are the selective forces that operate to maintain them. In a large multigene family with hundreds of genes that have overlapping function it is difficult to imagine how selection can operate on any particular gene, because its deletion or creation is expected to have little effect on the overall fitness of the animal. An extreme example is in the antibody V-region gene segments in which one individual expresses only a small fraction of the potential antibody repertoire, and the function

Received March 29, 1996.

of a gene segment can only be seen when that segment is joined to others to construct one of a large number of possible antibody molecules. Hood and Prahl (1971) speculate that selection for multigene systems must occur at the level of the whole organism and not at the level of individual germline genes. This has been difficult to show analytically or experimentally. Here we show by computer simulation that selection operating at the organismic level can provide the selection pressure needed to generate and maintain diversity in V-region gene families.