Bcl6^+/−Bach2^+/−mice display a marked reduction of GCs. WT, Bach2^+/−, Bcl6^+/−, and Bach2^+/−Bcl6^+/− mice (n = 4/group) were immunized intraperitoneally with SRBC and euthanized after 10 days to evaluate GC formation. (A) Representative peanut agglutinin staining of splenic sections from immunized mice. A small GC in the Bcl6^+/−Bach2^+/− splenic section is indicated by the red arrow and shown as inset (original magnification ×20). Bars represent 200 μm (B-C). The size (B) and number (C) of GCs in spleen sections of immunized mice with the indicated genotypes. Individual dots represent each GC. (D) Representative flow cytometric plots of GC B cells (FAS⁺ CD38^lo-neg, boxed) gated on live splenic B220⁺ lymphocytes from immunized mice. (E) μMT chimeras were generated by transferring either 20% WT or Bcl6^+/−Bach2^+/− with 80% μMT bone marrow cells. Seven weeks later, these chimeras were immunized with SRBCs and were euthanized 10 days later for analysis. Flow cytometry plots are shown depicting the percent of FAS⁺ CD38^low/− GC B cells among viable splenic B220⁺ cells from the indicated μMT chimeras. Data are shown as mean ± standard error of the mean from 2 independent experiments. *P < .05 and **P < .01; 2-tailed Student t test.