Abstract/Summary Everyone is chronically infected by pathogens that are able to evade the host adaptive immune responses. Our work focuses on the process of antigen recognition by CD8 T cells during chronic viral and parasitic infections with the goal of determining how these infections avoid T cell immunity. Many chronic infections generate a robust CD8 effector response that ultimately results in compromised responses and exhausted CD8 T cells despite persistent levels of antigen. Antigen recognition is governed by the interaction of the T cell receptor (TCR) with antigenic peptide:MHC (pMHC). These proteins are found in the surface membranes of T cells and antigen presenting cells. Protein interactions that occur between opposing cells happen within two dimensional (2D) constraints that pertinently influence the kinetics of the interaction. Kinetic binding parameters such as 2D TCR affinity and bond lifetime for antigenic pMHC are critical for T cell activation and subsequently dictate T cell activity. Our revised R01 application builds on our previously published work and preliminary data that defines the importance of TCR affinity and bond lifetime for pMHC antigen as key parameters that determine T cell fate. The assessment of the dynamic interaction between the 2D-restricted TCR and pMHC is a novel and exciting area of research because we, and others, have found that T cells apply force through the TCR:pMHC bond resulting in alteration in the length of time of antigen recognition. Importantly, our work shows that an increase in bond lifetime under force is absolutely required for optimal T cell effector functions. Thus, we hypothesize that chronic infection leads to exhausted T cell responses through reduction in the bond lifetime and overall strength of antigen recognition. So far, we have generated compelling preliminary data during chronic viral and parasitic infections. The ensuing work will be undertaken through 3 specific aims that will: 1) Define T cell effector functions, affinity and bond lifetime under force during chronic infection, 2) Dissect the role of the CD8 coreceptor in antigen recognition, and 3) Correlate 2D affinity and bond lifetimes with defined effector functions. The innovative insights generated by our work will provide the base understanding of CD8 T cell activation/inactivation in the presence of persistent encounters with antigen. Thus in this application, we seek to understand how changes in 2D kinetics of antigen recognition distinguish competent effectors from incompetent exhausted CD8 T cells during chronic infections.