CD4(-CELL RESPONSES IN MICE LACKING MHC CLASS-II MOLECULES SPECIFICALLY ON B-CELLS() T)

The role of B lymphocytes in initiating and maintaining a CD4(+) T cel l response has been examined using a variety of strategies, but remain s controversial because of weaknesses inherent to each of the approach es. Here, we address this issue by measuring CD4(+) T cell priming bot h in mutant mice devoid of B cells and in chimeric animals lacking maj or histocompatibility complex class II molecules specifically on B cel ls. We find that peptide and some protein antigens do not require B ce lls expressing class II molecules, nor B cells themselves, to efficien tly prime. This could be demonstrated by the usual lymph node prolifer ation assay, a rather indirect in vitro measure of priming, and by a d irect ex vivo assay of population expansion and activation marker expr ession. Interestingly, one protein antigen, conalbumin, could not prim e in the absence of B cells, but could in the presence of B cells devo id of class II molecules. This finding constrains the possible mechani sms whereby B lymphocytes contribute to the initiation of a CD4(+) T c ell response, arguing against the importance of surface immunoglobulin -mediated antigen presentation by B cells.