The FcμR regulates IgM-BCR internalization

The FcμR is a cell surface receptor that can bind to both secreted and membrane-bound IgM. In B cells it co-localizes with IgM-BCR in Golgi vesicles, where it restrains surface BCR-IgM expression. B cell-expressed FcmR is required also for maximal IgG response development after influenza A virus infection, but the mechanisms are unknown. To explore the effects of FcmR expression we generated FcmR-deficient and sufficient BCR-”SwHEL” transgenic C57BL/6 mice. The mice, containing about 20% HEL-specific B cells were immunized either directly with hen egg lysozyme (HEL) or with HEL-ovalbumin (OVA), or we created bone marrow irradiation chimeras in which about 2% of naïve B cells expressed the SwHEL-BCR. Consistent with the influenza infection studies, IgG responses following subcutaneous HEL-OVA immunization were significantly diminished in both systems when B cells lacked FcmR−/− expression compared to controls. Confocal and STED microscopy studies on B cells from these mice were done to quantify IgM-BCR internalization following in vitro exposure to HEL-bodipy. FcmR−/− SwHEL B cells did not show IgM “capping” and retained high amounts of antigen on the cell surface compared to the HEL− FcmR+/+ cells. This reduction in internalization resulted in reduced MHCII-peptide loading, as shown via flow cytometric quantification of surface pMHCII expression using the YAE-antibody. Ongoing studies are defining the subcellular compartments of antigen-processing in the presence and absence of the FcmR. We conclude that the FcmR acts as a non-redundant chaperone that optimizes IgM-BCR antigen internalization and processing. U19AI109962 R01AI148652