Two important Ras-guanyl nucleotide exchange factors, Sos and RasGRP1, control Ras activation in thymocytes. However, the relative contribution of these two exchange factors to Ras/ERK activation and their resulting impact on positive and negative selection is unclear. We have produced two lines of RasGRP1(-/-) TCR transgenic mice to determine the effect of RasGRP1 in T cell development under conditions of defined TCR signaling. Our results demonstrate that RasGRP1 is crucial for thymocytes expressing weakly selecting TCRs whereas those that express stronger selecting TCRs are more effective at utilizing RasGRP1-independent mechanisms for ERK activation and positive selection. Analysis of RasGRP1(-/-) peripheral T cells also revealed hitherto unidentified functions of RasGRP1 in regulating T cell homeostasis and sustaining antigen-induced developmental programming.
Naive T cells require costimulation for robust Ag-driven differentiation and survival. Members of the TNFR family have been shown to provide costimulatory signals conferring survival at distinct phases of the T cell response. In this study, we show that CD4 and CD8 T cells depend on TNFR type 2 (p75) for survival during clonal expansion, allowing larger accumulation of effector cells and conferring protection from apoptosis for a robust memory pool in vivo. We demonstrate using the MHC class I-restricted 2C TCR and MHC class II-restricted AND TCR transgenic systems that TNFR2 regulates the threshold for clonal expansion of CD4 and CD8 T cell subsets in response to cognate Ag. Using a novel recombinant Listeria monocytogenes (rLM) expressing a secreted form of the 2C agonist peptide (SIY) to investigate the role of TNFR2 for T cell immunity in vivo, we found that TNFR2 controls the survival and accumulation of effector cells during the primary response. TNFR2−/− CD8 T cells exhibit loss of protection from apoptosis that is correlated with diminished survivin and Bcl-2 expression. Null mutant mice were more susceptible to rLM-SIY challenge at high doses of primary infection, correlating with impaired LM-specific T cell response in the absence of TNFR2-mediated costimulation. Moreover, the resulting memory pools specific for SIY and listeriolysin O epitopes derived from rLM-SIY were diminished in TNFR2−/− mice. Thus, examination of Ag-driven T cell responses revealed a hitherto unknown costimulatory function for TNFR2 in regulating T cell survival during the differentiation program elicited by intracellular pathogen in vivo.
Apoptotic cell death plays a fundamental role in the maintenance of tissue homeostasis in complex biological systems. It is also a major mechanism for keeping immune reactions in check. Members of the TNF family of receptors and cytokines are implicated in the regulation of apoptotic signals that shape the immune system. In this study, we have examined the role of three members of the TNFR family, Fas (CD95), TNFR1 (p55), and TNFR2 (p75), in inducing cell death in Con A-activated CD4 and CD8 T cells. It was found that Con A-activated p55−/− CD4 or CD8 T cells were highly resistant to TNF-induced cell death. By contrast, although activated p75−/− CD4 or CD8 T cells were killed by TNF, they were more resistant to TNF-induced killing when compared with p75+/+ cells, particularly at higher concentrations of TNF. We also determined whether activated p55−/− and p75−/− T cells differ in their sensitivity to cell death induced by TCR cross-linking. We found that activated p55−/− CD4 or CD8 T cells were equally susceptible to TCR-induced cell death. More interestingly, the loss of the p75 receptor conferred resistance to TCR-induced death in activated CD8, but not CD4 T cells. This resistance to TCR-induced death in activated p75−/− CD8 T cells correlated with the resistance of these cells to Fas/Fas ligand-induced cell death.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.