Modulation of antigen presentation and B cell receptor signaling in B cells of beige mice.

Binding of Ag by B cells leads to signal transduction downstream of the BCR and to delivery of the internalized Ag-BCR complex to lysosomes where the Ag is processed and presented on MHC class II molecules. T cells that recognize the peptide-MHC complexes provide cognate help to B cells in the form of costimulatory signals and cytokines. Recruitment of T cell help shapes the Ab response by facilitating isotype switching and somatic hypermutation, and promoting the generation of memory cells and long-lived plasma cells. We have used the beige (Bg) mouse, which is deficient in endosome biogenesis, to evaluate the effect of potentially altered Ag presentation in shaping the humoral response. We show that movement of the endocytosed Ag-BCR complex to lysosomes is delayed in Bg B cells and leads to relatively poorer stimulation of Ag-specific T cells. Nevertheless, this does not affect Bg B cell activation or proliferation when competing with wild-type B cells for limiting T cell help in vitro. Interestingly, Bg B cells show more prolonged phosphorylation of signaling intermediates after BCR ligation and proliferate better to low levels of BCR cross-linking. Primary Ab responses are similar in both strains, but memory responses and plasma cell frequencies in bone marrow are higher in Bg mice. Further, Bg B cells mount a higher primary Ab response when competing with wild-type cells in vivo. Thus, the intensity and duration of BCR signaling may play a more important part in shaping B cell responses than early Ag presentation for T cell help.