Apoptosis is associated with the extensive B cell death in the sheep ileal Peyer's patch and the chicken bursa of Fabricius: A possible role in B cell selection

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Abstract
The ileal Peyer's patch (PP) and the bursa of Fabricius have major roles in populating the B cell system in sheep and chickens, respectively. These tissues contain > 90% B cells and possess a massive proliferation index with > 5% of B cells entering mitosis per hour. Paradoxically, almost all of the B cells produced in these sites rapidly die in situ. Here we show that the extensive B cell death occurring in the ileal PP and bursa is associated with apoptosis. Gel electrophoresis of ileal PP cell DNA from 7–14 week‐old lambs and bursal cell DNA from 4‐week‐old chickens demonstrated a laddering of DNA in multiples of approximately 200 bp, a pattern indicative of apoptosis. In sheep, the intensity of the laddering pattern seen after agarose gel electrophoresis was always greater with ileal PP cell DNA compared with thymocyte DNA, and usually greater than jejunal PP cell DNA. Likewise, DNA isolated from chicken bursal cells and mouse PP cells always exhibited a more intense laddering pattern than chicken or mouse thymocytes, respectively. When placed in culture ileal PP cells died rapidly < 40% viable cells were recovered after 24 h. Within 6 h of culture many ileal PP cells exhibited an apoptotic appearance in that they contained condensed chromatin and fragmented nuclei. Moreover, < 55% of total cellular DNA was fragmented. Compared with thymocytes, ileal PP cells underwent DNA fragmentation to a much greater extent and with a faster time course in short‐term culture. We propose that cell death by apoptosis may make an important contribution to B cell development in the lamb ileal PP and the chicken bursa. Apoptosis may provide a mechanism for the diversification of the B cell immune repertoire and/or the selection of non‐self reactive B cells.