The Old One–Two: Solving the Mystery of Cognate Help

Abstract

T he pivotal findings reported by Noelle et al. (1) in 1992 resolved a puzzle that had been 40 years in the making. In the mid-20th century, seminal studies from Owen (2), Medawar and colleagues (3, 4) and Burnet (5) established the concept of an immunological self, forc-ing re-evaluation of the paradigms driving the field. Indeed, the instructional models that had been forwarded to ex-plain specificity and affinity maturation in humoral re-sponses, already under duress as the rules of molecular biology and protein chemistry advanced, lacked the means to accommodate a mechanism for self versus nonself dis-crimination, producing an intractable conundrum. The key, of course, was to address this problem at a cellular level. Although Ehrlich had enunciated a cell-centric approach to explaining immune responses half a century earlier (discussed in Ref. 6), his side-chain hypothesis had long been aban-doned, and it also lacked a tractable means of dealing with this new but fundamental concept. Within this context, Jerne (7), Talmage (8), and Burnet (9, 10) collectively advanced a paradigm, now termed the clonal selection hypothesis, that would assume prominence during the next 50 years. By pos-iting a clonal distribution of Ag receptor specificities, this model provided a means through which immune repertoires could be purged of unwanted specificities in a quantized man-ner, thus eliminating self-reactive clones but sparing beneficial specificities. By the mid-1980s, most of the potential stum-bling blocks for the clonal selection hypothesis had been cleared: the concept of clonally distributed receptors had been definitively established (11) (reviewed in Ref. 12), and the genetic impasse posed by the need for vast receptor diver-sity had been overcome once the process of somatic recombi-nation involving multiple gene segments had been revealed (13–15). Despite these advances, an ironic caveat to the clonal se-lection hypothesis nonetheless endured: the Ag receptor was required to play both an immunogenic and tolerogenic role, prompting the question of how signals through the same re-ceptor could drive diametrically opposite outcomes. This apparent logical contradiction was recognized early on, and various models were forwarded to deal with it. Among these, the two signal hypothesis advanced by Bretscher and Cohn (16) has proven durable. According to this model, a signal through the Ag receptor, termed signal 1, culminated in cell death unless superseded by a second, Ag receptor–indepen-dent, signal. These overarching ideas laid the conceptual groundwork for the studies that were reported in this issue's Pillars of Immunology article. Observations during the two decades preceding the high-lighted article painted an increasingly complex picture of how such second signals were delivered. For example, classic hapten-conjugate immunization strategies showed that there were multiple ways in which signal 2 could be delivered to B cells. Thus, haptens coupled to mitogens such as bacterial LPS could directly activate hapten-specific B cells. In contrast, when haptens were coupled to protein carriers, signal 2 re-quired and emanated from Th cells. Moreover, this T-depen-dent form of signal 2 was most effective when the epitope recognized by the B cell (the hapten) was physically attached to the protein portion of the Ags recognized by the T cell (the carrier). This so-called " linked recognition " was further refined with the discovery of MHC restriction of Ag recognition by T cells, yielding the realization that it reflected a cognate event: the presentation of processed carrier peptides to the T cell in the context of MHC class II molecules on the B cell. However, in this and other studies it was evident that the TCR–MHC class II interactions per se did not constitute the elusive signal 2 delivered to the B cell by activated Th cells. Within this milieu of increasing conceptual complexity, Noelle and colleagues (17–21) had already invested considerable effort in generating the tools and insights necessary to dissect the requirements and molecular basis for these interactions. These, together with a combination of cellular, biochemical, and molecular approaches, converged to reveal the basis for signal 2 in T-dependent B cell responses, thus resolving the molecular basis of cognate help. Noelle et al. (17) had already shown that membrane preparations from activated CD4 1 T cells could promote activation of B cells, as indicated by an uptick in RNA syn-thesis measured by the incorporation of radiolabeled uridine. Importantly, this was only true for activated T cell membrane preparations, suggesting that T cell activation itself was a requisite step toward acquiring the capacity for signal 2 de-livery. Additionally, prior studies from others had suggested that Abs to CD40 had stimulatory effects on B cells, impli-cating this molecule as a candidate signal 2 target. Armed with this knowledge, and coupled with the emerging technologies that allowed generation of fusion proteins and mAbs, their search ensued for a molecule on these membranes that might be responsible. They reasoned that if CD40 was indeed the target of the elusive signal 2, then the matching ligand on activated T cell membranes should be both blocked by and