B cell activation defects in the absence of MALT1 and IRAK4. (A) Flow cytometric analysis of splenic MALT1 KO and MALT1/IRAK4 DKO lymphocytes. Upper panel: Percentages of B220+ lymphocytes; the data were analyzed using the dependent (paired samples) t-test (n = 5). Lower Panel: Percentages of IgM single or IgM/IgD double positive B220+ splenocytes. (B) Splenic MALT1 KO and IRAK4/MALT1 DKO B cells from RAG1-/- mice reconstituted with MALT1 KO or MALT1/IRAK4 DKO BM cells were stimulated as in Figure 1A and the proliferative response was measured accordingly. Results are presented as the mean [3H]thymidine incorporation ± S.D. for triplicate samples after an 8 h pulse and are 1 trial representative of at least 3 independent experiments. (C) Splenic MALT1 KO and MALT1/IRAK4 DKO B cells from reconstituted RAG1-/- mice were stimulated and analyzed by Western Blotting as in Figure 1B. (D) Splenic MALT1 KO and MALT1/IRAK4 DKO B cells from reconstituted RAG1-/- mice were stimulated with soluble anti-IgM or LPS for 6 hours and AP-1 activation was determined by Electrophoretic Mobility Shift Assay (EMSA).