RIAM-deficient lymphocytes develop normally, but the MZ B cell population is reduced in RIAM-deficient mice. (A) RIAM^+/+ and RIAM^−/− thymocytes were stained with anti-CD4 and anti-CD8 Abs and analyzed by flow cytometry to reveal double-positive (DP), double-negative (DN), and single-positive (CD4⁺ or CD8⁺) cells. (B) Percentages of thymocyte subsets of animals with the indicated genotypes. Plotted are means ± SD; n = 4. (C) BM lymphocytes from RIAM^+/+ or RIAM^−/− mice were stained with anti-B220 and anti-IgM Abs (left) and analyzed by flow cytometry, depicting pre-pro (P; B220^low, IgM^low), immature (I; B220^high, IgM^low), and mature (M; B220^high, IgM^high) B-cell subsets or stained with anti-IgM and anti-IgD Abs (right) to quantify recirculating mature B cells. (D) Absolute numbers of B220⁺ B cells of the indicated subsets within equal volumes of BM suspensions; n = 4. (E, G) Splenocytes derived from RIAM^+/+ and RIAM^−/− mice were characterized by flow cytometry as total B cells or FO (CD23^high, CD21^low), or MZ (CD23^low, CD21^high) B cells (E) or T1 (IgM^high, CD23^low), T2 (IgM^high, CD23^high), or T3 (IgM^low, CD23^high) transitional B cells (G). (F, H) Absolute numbers of the B cells characterized in E and G. Horizontal lines indicate mean ± SD of the individual animals plotted, *P < .05.