A theory of the ontogeny of the chicken humoral immune system: the consequences of diversification by gene hyperconversion and its extension to rabbit.

Summary The immune system's repertoire is generated in two stages : Stage I results in a small size high copy number repertoire that is diversified by “mutation” to result in a large size low copy number repertoire referred to as Stage II. The Stage I or high copy number repertoire is derived from information stored directly in the genome by two mechanisms. (a) The copy-cassette mechanism : the Ig-locus has one rearrangeable V gene segment which acts as recipient for controlled gene conversion in cis from a set of donor V gene segments that results in a family of subunits, L and H. This is illustrated by the avian systems. (b) The cassette-exchange mechanism : the Iglocus has many rearrangeable V gene segments which are fused into transcription units, the products of which are a family of L and H subunits identical in function to those resulting from the copy-cassette mechanism. This is illustrated by the murine or human systems. It is possible for a species to use both mechanisms, copy-cassette at one Ig locus and cassetteexchange at the other Ig locus. This seems to obtain in the rabbit system. Further, it is possible to encode the high copy number repertoire directly in the genome as tandemly repeated rearranged transcription units as one sees in shark (a genomic analogue of the cassette-exchange mechanism). We have discussed here and elsewhere ( Cohn and Langman, 1990 ) the consequences of these mechanisms for haplotype exclusion and functional responsiveness to antigen. The Stage I or high copy number repertoire generated by any of the above mechanisms is now a substrate for “mutation” which generates the low copy number or Stage II repertoire. These three species are compared in table V. The high copy number repertoire is small but the response to any antigen that it recognizes is rapid. The low copy number repertoire is large but responsiveness to any antigen it recognizes is slow. Cooperativity between the two repertoires optimizes the overall responsiveness with respect to rapidity of response and range of responsiveness. The use of a copy-cassette mechanism requires that the OB cell undergoing gene conversion have a single rearranged L- and H-chain haplotype (L +/o H +/o ). The reason is that conversion can correct an aberrantly rearranged transcription unit and generate an unacceptable level of doubles. In order to have one chromosome functionally rearranged and the homologue in the germline configuration, a selection mechanism is required. We propose the following : A stem B cell enters the bursal rudiment where it differentiates into an mIg + B cell. At the second rearrangement step, the first mIg + B cells appear and they are of the configuration L +/o H +/o . The interaction between them and the bursal follicular epithelium induces follicle formation. Once formed, the follicle closes to further entry of mIg + B cells. This step selects for mIg + B cells of the L +/o H +/o phenotype. As D H in LRF 3 is chain-terminating, only mIg + B cells expressing D H in LRF 1 and 2 enter follicles. Those in LRF 1 are deleted by tolerance as the D H LRF 1 is functional in signalling. The mIg + B cells expressing D H in LRF 2 now differentiate to ΦB cells, divide and undergo successive rounds of gene conversion. This process converts the D H in LRF 2 to D H in LRF 1 that is functional. A proportion of them exit to the periphery as differentiated iB cells (tolerizable and inducible) to form the functional humoral immune system. The final product, a functional humoral immune system, is the same for the copy-cassette and cassetteexchange pathways and both are determined by the rules of Protecton theory. The role of the reading frame of D H is postulated to be the control of the signal to the antigenresponsive cell upon interaction with ligand. In murine B cells, this plays a role in haplotype exclusion ; in chicken B cells, this controls the differentiation in bursal follicles ; and in all T cells, this is central to the functioning of a dual recognitive-single receptor ( Langman and Cohn, 1985 ). This one function has multiple consequences dependent on the system involved. The difference between the pathways of differentiation of B cells found in chickens and in mice has been driven during evolution by the choice of the mechanism (copy-cassette or cassette-exchange) responsible for extracting the information stored in the genome. The problem has now become, “What was it that prompted the choice?” We are aware of the gross incompleteness of the bibliography. We essentially cited only examples; however, there is no doubt that the other participants will fill this out making our joint effort more complete.