Loss of Prdm1 synergizes with Myd88^L252P and Cd79b^Y195H to promote outgrowths in spontaneous splenic GCs. (A) Experimental scheme for data in panels B to H. Percentages of CD45.2 follicular B cells (FoB), GCBs, or PCs in unimmunized spleens (B) or ratios of frequency of CD45.2 GCB to CD45.2 follicular B cells (C) in unimmunized spleens, immunized pLNs, mesenteric lymph nodes (mLNs), or PPs of mixed BM chimeras that were generated with a mixture of ∼5% Cr2-cre Myd88^L252P/+Cd79b^Y195H/+Prdm1^f/f (Myd88/Cd79b/Prdm1) or control BM (CD45.2) and ∼95% WT CD45.1/2 eight weeks after irradiation. (D) Frequency of CD45.2 cells among FoB, DZ GCBs, LZ GCBs, pre-memory GCBs, or MBCs in Myd88/Cd79b/Prdm1 mixed chimeras. Example gating strategy is shown in supplemental Figure 3D. Intracellular fluorescence-activated cell sorting for 5-bromo-2′-deoxyuridine (BrdU) incorporation (E) or IRF4 expression (F) in IgM⁺ or IgM^– GCBs from spleens of control or Myd88/Cd79b/Prdm1 mixed BM chimeras that were treated intraperitoneally with BrdU 30 minutes before euthanasia. (G) Frequency of CD45.2 IgM⁺ or IgM^– GCBs in spleens of control or Myd88/Cd79b/Prdm1 mixed chimeras. (H) Percentage of active-Caspase-3⁺ GCBs in control, Myd88/Cd79b, or Myd88/Cd79b/Prdm1 mixed chimeras. Example gating strategy is shown on the left. Data in panels B, C, and E to H are pooled from 3 to 5 independent experiments with a total of at least 11 mice per group. Data in panel D are pooled from 2 experiments with 6 and 9 mice per group, respectively. *P < .05, **P < .01, ****P < .0001 paired two-tailed t test for data in panels E, F, and H. *P < .05, **P<.01, ***P < .001, ****P < .0001, unpaired two-tailed t test for all other data. MFI, mean fluorescence intensity.