WASP-deficient MZ B cells are reduced in number independent of MZMs. (A) Schematic representation of generation of BM chimeric mice. WT (expressing CD45.1) and WASP^−/− (CD45.2) BM cells at a 1:3 ratio were injected into lethally irradiated WASP^−/− recipient mice. Mice were analyzed 9 to 13 weeks after transplantation. Control mice received WT or WASP^−/− BM alone. (B) Immunohistochemistry of spleen sections from mice receiving WT BM (left panel), WASP^−/− BM (middle), and mixed WT:WASP^−/− BM (right). Note that the MZ architecture in WT:WASP^−/− BM chimeric mice is fully restored. (C) Splenocytes were labeled with anti-CD21, CD23, and IgM and analyzed by flow cytometry. (Top panel) The percentage of FO and MZ B cells in reconstituted mice is shown for the various conditions as illustrated in panel B. Note that WT:WASP^−/− BM chimeric mice have fully restored MZ B-cell population (right). (Bottom panel) The percentage of WASP^−/− CD45.2-expressing FO and MZ B cells is shown. Note the low proportion of WASP^−/− MZ B cells in the WT:WASP^−/− BM chimeric mice (right). The data are representative of 2 independent experiments, in which n = 7 mice were analyzed (original magnification ×10).