Surface abundance of MHCII in GC B cells is determined by March1-mediated ubiquitination and CD83 regulation. (a) Mixed BM chimeric mice containing WT CD45.1⁺ cells and I-A^b(K>R) CD45.2⁺ cells were generated and immunized with SRBCs. Surface levels of MHCII and CXCR4 were determined in IgD^low CD95⁺ GL7⁺ GC B cells of both origins. (b) Frequencies of CXCR4^high CD23^low centroblasts and CXCR4^low CD23^high centrocytes were determined in the BM chimeras. (c) The same analyses as in panel a were performed using BM chimeric mice containing March1^−/− cells. (d) CD86 levels on GC B cells were determined. (e) The same analysis as in panel a was performed using BM chimeric mice containing Cd83^−/− cells. (f) Comparisons of surface MHCII level (mean fluorescence intensity [MFI]) on CXCR4^high and CXCR4^low GC cells of the various genotypes. (g and h) Summary of MHCII staining data (g) and I-A^b–clip staining data (h) from multiple mice. a and c–e are representative plots. Points in b and f–h represent individual mice pooled from two to three experiments. Horizontal lines in b and f–h indicate means. Analysis was performed using an unpaired two-tailed Student’s t test. **, P < 0.01; ***, P < 0.001.