BAFF-R– and TACI-dependent up-regulation of MHC class II in B cells. (A) Purified wild-type and TACI^−/− B splenocytes were stimulated plus or minus cross-linked APRIL in the presence of IL-4 and IL-6, and surface expression of MHC class II was monitored by FACS. (B) Experiment was performed as in panel A with B cells of the indicated genotype, and with BAFF and APRIL at the indicated concentrations. MFI indicates mean fluorescence intensity.