Project Summary Because of their elevated frequency, ability to self-renew and rapid acquisition of effector function following re-activation, memory T cells have an enhanced ability to protect from secondary challenge. The generation of memory T cells is the focal point of numerous vaccine and immunotherapeutic strategies. Most (90-95%) effector T cells die after pathogen clearance, but those fated to become memory cells can be identified during the primary effector response, showing that activated T cells receive differentiation cues during the primary response to infection that influence memory fate differentiation. We have recently shown that T cell receptor (TCR) signals play a key role in driving CD4+ memory T cell differentiation. TCRs that are able to engage MHC Class II-bound antigen (pMHCII) in sustained interactions are biased towards the formation of long-lived memory, while TCRs that engage in short-lived interactions with antigen are biased towards terminal effector cell differentiation. We will build on those studies by using diverse infectious models to define aspects of the TCR-dependent activation and transcriptional program that leads to the formation of lymphoid-resident, circulating and tissue-resident CD4+ memory T cells. We pose three key questions. First, what is the role of TCR signal strength in the formation of memory T cells? We will explore the hypothesis that increasing TCR signal strength in vivo drives terminal effector T cell differentiation, while weaker TCR signal strength allows memory T cell formation. Second, what are the TCR binding parameters associated with memory T cell development? We will measure 2D affinity and bond lifetime with the application of force for TCRs at that are effector-biased or memory-biased. We will test the hypothesis that bond lifetimes will predict TCR-dependent memory differentiation. Third, what are the transcriptional programs that control memory formation? We will test the mechanistic role of molecules that are differentially expressed in memory T cell precursors during the primary effector response, including TCF-1. We anticipate that resolution of the questions posed in this study will provide a framework for determining in greater mechanistic detail how memory T cells form and identify therapeutic approaches for directly modulating CD4+ effector and memory T cell differentiation in vivo.