MHCII down-regulation in centroblasts is not determined by affinity and occurs at late times after T cell mediated selection. (a) CXCR4^high MHCII^high and CXCR4^high MHCII^low NP-binding GC B cells, gated as in Fig. 5 a, were sorted on days 11 or 12 of the response to NP-CGG (blue circles) or NP-KLH (white circles) from WT C57BL/6 mice, and the frequency of W33L mutations in the VH186.2 heavy chain was determined. Linked pairs indicate populations from individual mice. (b) Dec205^−/− mice were preimmunized with 50 µg OVA in alum (i.p.) 2 wk before the transfer of Dec205^−/− and Dec205^+/+ NP-specific B1-8 B cells, mixed at a 85:15 ratio. Mice were subsequently reimmunized with 25 µg NP-OVA via s.c. footpad injection. Mice then received a single injection of 5 µg anti–Dec205-OVA at 30- or 60-h time points before analysis to promote interactions with T cells by the Dec205^+/+ GC B cells. (c) CD38^low CD95⁺ GC B cells were gated. The surface levels of MHCII on CXCR4^high CD83^low centroblasts and CXCR4^low CD83^high centrocytes were determined for each population. Representative histograms are shown. (d) Summary of data from multiple mice and experiments, showing the MHCII geometric mean fluorescent intensity (gMFI) for each population, relative to follicular B cells. (e) The frequency of CXCR4^high CD83^low centroblasts for each population and time point. Data in d and e are compiled from two experiments with a total of three to four mice per condition (±SD). Analysis was performed using unpaired two-tailed Student’s t test. *, P < 0.05.