Negative signaling by surface IgM on B cells isolated from ileal Peyer's patch follicles of sheep

Lymphoid follicles from the sheep ileal Peyer's patch (PP) were used to prepare a cell suspension consisting of 98% surface IgM-positive (sIgM+) B cells and 1% T cells. Co-stimulation of follicular cells with pokeweed mitogen and either recombinant bovine interleukin 1 (IL 1) or IL 2 resulted in a marked proliferative response. In contrast, the addition of soluble F(ab')2 rabbit anti-sheep Ig completely inhibited the proliferative response induced by pokeweed mitogen and IL 1 or IL 2 co-stimulation. Anti-Ig inhibition of B cell proliferation was specific for ileal PP follicular cells and was not observed with mesenteric lymph node cells or splenocytes. Furthermore, suppression of ileal PP follicular B cell proliferation required at most divalent cross-linking of sIg was independent of Fc receptors, but was dependent on the concentration of anti-Ig and required 48 h for maximal effect. Negative signaling by sIgM indicates that ileal PP follicular B cells are functionally distinct from B cells in other secondary lymphoid tissues. Also, the present observations are consistent with previous reports indicating that B cell proliferation in ileal PP follicles is antigen independent.