The two distinguishing features of memory T cells that enable them to mediate efficacious responses are their rapid recall following TCR engagement, and their ability to persist longterm by perpetual homeostatic turnover.  The biochemical signaling pathways regulating the functional responses and maintenance of memory T cells remain undefined.  My laboratory has taken novel approaches to dissect the pathways and mechanisms by which TCR-coupled signaling pathways trigger rapid effector responses in memory CD4 T cells, and also to define how both TCR and cytokine signaling pathways contribute and regulate memory T cell survival and homeostasis.  These studies have been funded for the past 10 years by the NIH and the renewal is currently in submission.  

To dissect how TCR-coupled signaling events are coupled to rapid recall, we recently implemented multiparameter analysis of signaling and function on the single cell level using flow cytometry. From these analyses, we found that resting memory CD4 T cells exhibit a strikingly elevated expression of the ZAP-70 tyrosine kinase, and that there are two pathways controlling IFN-γ production at rapid and later times, respectively after TCR triggering (7).  By analysis of transcription factor expression, IFN-γ production and also chromatin immunoprecipitation studies, we found that the early IFN-γ production is independent of the T-bet transcription factor and requires NFκB, which is engaged on the IFN-γ promoter at early times after activation.  Based on these findings we have proposed a model whereby memory CD4 T cells use an innate signaling pathway for NFκB-mediated control of rapid effector function (8).  In future studies, we will trace the TCR signaling pathways to rapid activation of NFκB by analysis of the Carma/Bcl10/MALT1 intermediates, and we will use conditional knockout models to analyze transcriptional requirements for rapid recall and their coupling to the TCR in memory CD4 T cells. In addition, we will also determine whether innate immune signals such as inflammatory cytokines including IL-12, or Toll-like receptor agonists can likewise trigger effector function by memory CD4 T cells through similar signaling pathways.  

We have also used novel mouse models for conditional ablation of the TCR-coupled adaptor molecule SLP-76 to investigate TCR signaling requirements for memory CD4 T cell generation and maintenance/homeostasis. We found that conditional ablation of SLP-76 following CD4 T-cell priming, enabled the generation of memory CD4 T cells, yet significantly inhibited their homeostatic turnover and persistence in vivo.  SLP-76-deficient memory CD4 T cells were further impaired in their ability to proliferate in response to IL-7 in vivo and to phosphorylate STAT5 in response to stimulation with multiple γc cytokines. Our results reveal a dominant requirement for TCR signals in the homeostatic maintenance of memory CD4 T cells through cross-regulation with the γc cytokine signaling pathway (9).  Our future plans are to identify the points of intersection between TCR and cytokine signaling pathways, and whether SLP-76 is directly involved in JAK-STAT signaling through the γc subunit.  We also are studying memory CD4 T cell persistence and homeostasis using a mouse model with conditional deletion of STAT5, to dissect the contribution of TCR and cytokine signaling in memory homeostasis.  These studies have important implications for promoting memory T cell persistence in vaccines and understanding mechanisms for altered T cell functions in aging.