SUBEPITHELIAL B-CELLS IN THE HUMAN PALATINE TONSIL .2. FUNCTIONAL-CHARACTERIZATION

This study investigates the main functional features of subepithelial (SE) B cells and compares them with those of purified germinal center (GC) and follicular mantle (FM)B cells isolated from the same tonsils. Unlike FM B cells, SE B cells failed to produce polyspecific antibodi es in vitro; unlike GC B cells, SE B cells expressed high levels of Bc l-2 and failed to undergo spontaneous apoptosis in vitro. The most str iking function of SE B cells was their ability to produce IgM antibodi es to T cell-independent type-2 (TI-2) (but not to TI-1) antigens (Ag) . These antibodies could not be detected when both FM and GC B cells w ere stimulated with TI-2 Ag in vitro. Moreover, B cells isolated from peripheral blood were unable to mount a response to TI-2 Ag. The latte r finding is consistent with the observation that B cells with the phe notypic features of SE B cells were virtually absent in the peripheral blood and emphasizes the notion that SE B cells belong to a subset of non-recirculating B cells. SE B cells were by far superior to FM B ce lls in mixed lymphocyte reaction (MLR) stimulation of allogeneic T cel ls in vitro, although they were not as efficient as dendritic cells (D C). In order to stimulate T cells efficiently, SE B cells had to be ex posed to anti-mu antibody, a treatment which induced expression of act ivation markers such as CD80, CD86, CD69 and CD39, usually absent in r esting SE B cells. CD80 and CD86 molecules expressed by SE B cells par ticipated in the chain of events required to promote the proliferation of allogeneic T cells as demonstrated by inhibition tests with the ap propriate mAb. The expression of CD80 and CD86 by anti-mu-treated SE B cells was not, however, the sole explanation for their good antigen p resenting capacities since the exposure of FM B cells to anti-mu antib ody also induced expression of these surface structures. Nevertheless, these cells failed to become good MLR stimulators. Collectively, the above data contribute further to the characterization of a distinct su bset of tonsillar B cells which resemble, both phenotypically and func tionally, the B cells of the splenic marginal zone.