Germinal center expansion but not plasmablast differentiation is directly proportional to peptide-MHCII density via CD40-CD40L signaling strength

Abstract

Abstract Although cognate peptide-MHCII (pMHCII) is critical for B cell selection in the germinal center (GC), it is unclear how cell intrinsic differences in peptide levels contribute to selection and cell fate decisions. Here, we applied the a-DEC205-OVA (dec) model system, to deliver different levels of OVA peptide to GC B cells in situ in order to interrogate how intermediate and high levels of pMHCII affect selection and cell fate on a per cell basis. We used Ly75+/− (DEC-het) B cells, which expressed ~50% of surface DEC205 protein compared to WT B cells, and presented proportional amount of OVA peptide after dec treatment. Using competitive co-transfers, we found that WT B cells expanded two-fold more than DEC-het B cells. This 2-fold difference in expansion was maintained at a wide range of dec administration. To study T cell recognition in situ, we developed a novel caged ovalbumin peptide, which is readily detected by OT-II T cells upon photo-uncaging, and revealed that initial T cell recognition in the GC leads to increased B-T serial contacts. Differential CD40 signaling, was both necessary and sufficient to mediate 2-fold differences in GC B cell expansion and also promoted GC-like B cell morphology, suggesting CD40 signaling is a rheostat of pMHCII dose. Surprisingly, we found that while plasmablast numbers were increased upon dec stimulation, both WT and DEC-het GC B cells were equally capable of differentiating into plasma cells, suggesting that pMHCII density does not directly control the output or quality of plasma cells generated. Thus, these results delineate distinct roles pMHCII play in expansion vs. differentiation in the GC. Supported by grants from NIH (R01 HL141491-05)