The survival of transitional and mature B cells requires both the B cell antigen receptor (BCR) and BLyS receptor 3 (BR3), which suggests that these receptors send signals that are nonredundant or that engage in crosstalk with each other. Here we show that BCR signaling induced production of the nonclassical transcription factor NF-κB pathway substrate p100, which is required for transmission of BR3 signals and thus B cell survival. The capacity for sustained p100 production emerged during transitional B cell differentiation, the stage at which BCR signals begin to mediate survival rather than negative selection. Our findings identify a molecular mechanism for the reliance of primary B cells on continuous BR3 and BCR signaling, as well as for the gradual resistance to negative selection that is acquired during B cell maturation.Primary B cells rely on signals from both the B cell antigen receptor (BCR) and B lymphocyte stimulator (BLyS1; also called BAFF2; A000383) receptor 3 (BR3; also called BAFFr; A000374) for survival. Most peripheral B cells die after BCR ablation regardless of BR3 sufficiency, which indicates a need for continuous 'tonic' signals through the BCR3. Conversely, the lack of either BLyS or BR3, both of which are members of the tumor necrosis factor (TNF) family, results in B cell deficiency despite normal BCR function4-6. The requirement for both BCR and BR3 becomes apparent during transitional B cell differentiation and affects survival at the transitional 2 (T2) and T3 differentiation stages, such that the BCR signaling thresholds for negative and positive selection are modulated by BLyS availability7, 8. The molecular mechanism that underlies this codependence on BCR and BR3 is poorly understood.Correspondence should be addressed to M.P.C. (cancro@mail.med.upenn.edu). Accession codes. UCSD-Nature Signaling Gateway (http://www.signaling-gateway.org): A000383, A000374, A002936, A002248, A000374 and A000305.Note: Supplementary information is available on the Nature Immunology website. AUTHOR CONTRIBUTIONS J.E.S., M.K., F.G.K., J.L.S., J.P.M., W.J.Q., R.J.B., L.S.T. and K.A.J. did research, analyzed data and generated key reagents; J.E.S., M.K., J.G.M., R.S. and M.P.C. designed research and analyzed data; and J.E.S., R.S., M.K. and M.P.C. wrote the paper.Published online at http://www.nature.com/natureimmunology/ Reprints and permissions information is available online at

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Multiple studies have established that the ability of CD8؉ T cells to act as cytolytic effectors and produce gamma interferon is important in mediating resistance to the intracellular parasite Toxoplasma gondii. To better understand the generation of the antigen-specific CD8 ؉ T-cell responses induced by T. gondii, mice were immunized with replication-deficient parasites that express the model antigen ovalbumin (OVA). Class I tetramers specific for SIINFEKL were used to track the OVA-specific endogenous CD8 ؉ T cells. The peak CD8 ؉ T-cell response was found at day 10 postimmunization, after which the frequency and numbers of antigen-specific cells declined. Unexpectedly, replication-deficient parasites were found to induce antigenspecific cells with faster kinetics than replicating parasites. The generation of optimal numbers of antigenspecific CD8 ؉ effector T cells was found to require CD4 ؉ T-cell help. At 7 days following immunization, antigen-specific cells were found to be CD62L low , KLRG1 ؉ , and CD127 low , and they maintained this phenotype for more than 70 days. Antigen-specific CD8؉ effector T cells in immunized mice exhibited potent perforindependent OVA-specific cytolytic activity in vivo. Perforin-dependent cytolysis appeared to be the major cytolytic mechanism; however, a perforin-independent pathway that was not mediated via Fas-FasL was also detected. This study provides further insight into vaccine-induced cytotoxic T-lymphocyte responses that correlate with protective immunity to T. gondii and identifies a critical role for CD4 ؉