Or Lm-gp61 and the endogenous and bim2/2 SMARTA responses to GP61?0 were assessed. A, Graphs display fold expansion of SMARTA cells or IFNc-producing polyclonal endogenous CD4+ T cells 10781694 at the indicated time points post-rechallenge. B, Representative flow plots display the cytokine production profile of polyclonal endogenous CD4+ T cells and bim2/2 SMARTA cells following ex vivo peptide restimulation at day 5 post-rechallenge. C, Bar graph indicates the percent of IFNc-producing bim2/2 SMARTA cells and polyclonal endogenous IFNc-producing cells that also make TNFa and IL-2 (“triple producers”). Results are representative of 4? mice per group per time point and two independent experiments. Error bars indicate the SEM. doi:10.1371/journal.pone.0067363.g“doomed to die” SMARTA cells cannot be recovered by a Title Loaded From File subsequent LCMV infection [14]. Thus, merely increasing the presence of antigen in a context that normally stimulates SMARTA memory formation does not rescue the survival or functionality of Lm-gp61-induced SMARTA effector cells. Instead, the decision to enter a Bim-dependent Title Loaded From File apoptotic pathway likely occurs early in the priming phase, well before the observed up-regulation of Bim expression. We did not observe significant up-regulation of Bim expression in SMARTA cells until the peak of the effector response to Lm-gp61 16985061 (day 7 post-infection). We hypothesize that the up-regulation of Bim is a consequence of the qualitative nature of TCR activation signals received early during the priming phase, such that Bim expression serves as a sensor of the fitness of a CD4+ T cell clone to enter the memory pool but not a key mediator of functionally defective CD4+ effector T cell responses. Importantly, Bim has been shown to promote death of functionally fit Th1 effector cells as well [22,24], indicating that Bim activity and subsequent memory T cell differentiation can be influenced by both T cell-intrinsic (i.e. TCR-mediated activation) and extrinsic signals. Additionally, others have shown that Bim can promote the death of functionally protective responders in settings of chronic infection, reflecting the complex nature of the magnitude and duration of signaling in dictating T cell fate specification [23,24]. Although TCR signaling can regulate Bim expression in immature thymocytes, the factors upstream of Bim that may connect its expression to TCR and inflammatory environmentsignaling are not well understood [33]. One possible candidate is Foxo3a, a transcription factor that regulates the expression of several cell cycle inhibitors and proapoptotic factors, including Bim, and is upregulated in Lm-gp61-induced SMARTA cells [14]. Foxo3a-deficient mice have increased T cell accumulation and magnitude of expanded antigen specific T cells following LCMV infection, but it is debatable whether this is dependent upon T cell intrinsic defects, or extrinsic defects in dendritic cell IL-6 signaling that allows increased T cell viability [34?6]. Studies have shown that Foxo3a degradation is important for the survival of human memory CD4+ T cells [37,38], but the effect of Foxo3a deficiency exclusively in antigen specific CD4+ T cells is largely unresolved and is complicated by the diverse biological pathways in which Foxo3a is an important master regulator. Our observation that the role of Bim varied depending on the infection model indicates that a variety of factors may influence Bim activity in these settings. Our previous studies show that the survi.Or Lm-gp61 and the endogenous and bim2/2 SMARTA responses to GP61?0 were assessed. A, Graphs display fold expansion of SMARTA cells or IFNc-producing polyclonal endogenous CD4+ T cells 10781694 at the indicated time points post-rechallenge. B, Representative flow plots display the cytokine production profile of polyclonal endogenous CD4+ T cells and bim2/2 SMARTA cells following ex vivo peptide restimulation at day 5 post-rechallenge. C, Bar graph indicates the percent of IFNc-producing bim2/2 SMARTA cells and polyclonal endogenous IFNc-producing cells that also make TNFa and IL-2 (“triple producers”). Results are representative of 4? mice per group per time point and two independent experiments. Error bars indicate the SEM. doi:10.1371/journal.pone.0067363.g“doomed to die” SMARTA cells cannot be recovered by a subsequent LCMV infection [14]. Thus, merely increasing the presence of antigen in a context that normally stimulates SMARTA memory formation does not rescue the survival or functionality of Lm-gp61-induced SMARTA effector cells. Instead, the decision to enter a Bim-dependent apoptotic pathway likely occurs early in the priming phase, well before the observed up-regulation of Bim expression. We did not observe significant up-regulation of Bim expression in SMARTA cells until the peak of the effector response to Lm-gp61 16985061 (day 7 post-infection). We hypothesize that the up-regulation of Bim is a consequence of the qualitative nature of TCR activation signals received early during the priming phase, such that Bim expression serves as a sensor of the fitness of a CD4+ T cell clone to enter the memory pool but not a key mediator of functionally defective CD4+ effector T cell responses. Importantly, Bim has been shown to promote death of functionally fit Th1 effector cells as well [22,24], indicating that Bim activity and subsequent memory T cell differentiation can be influenced by both T cell-intrinsic (i.e. TCR-mediated activation) and extrinsic signals. Additionally, others have shown that Bim can promote the death of functionally protective responders in settings of chronic infection, reflecting the complex nature of the magnitude and duration of signaling in dictating T cell fate specification [23,24]. Although TCR signaling can regulate Bim expression in immature thymocytes, the factors upstream of Bim that may connect its expression to TCR and inflammatory environmentsignaling are not well understood [33]. One possible candidate is Foxo3a, a transcription factor that regulates the expression of several cell cycle inhibitors and proapoptotic factors, including Bim, and is upregulated in Lm-gp61-induced SMARTA cells [14]. Foxo3a-deficient mice have increased T cell accumulation and magnitude of expanded antigen specific T cells following LCMV infection, but it is debatable whether this is dependent upon T cell intrinsic defects, or extrinsic defects in dendritic cell IL-6 signaling that allows increased T cell viability [34?6]. Studies have shown that Foxo3a degradation is important for the survival of human memory CD4+ T cells [37,38], but the effect of Foxo3a deficiency exclusively in antigen specific CD4+ T cells is largely unresolved and is complicated by the diverse biological pathways in which Foxo3a is an important master regulator. Our observation that the role of Bim varied depending on the infection model indicates that a variety of factors may influence Bim activity in these settings. Our previous studies show that the survi.
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