The related kinases FAK and Pyk2 serve distinct functions in TCR-mediated T cell activation

T cells are central regulators of adaptive immunity in infectious and pathophysiological diseases. The activation of T cells is regulated by the T cell antigen receptor (TCR) and co-stimulatory receptors like toll-like receptor 2 (TLR2). These receptors activate distinct and overlapping intracellular signaling pathways that ultimately shape T cell responses. Therefore, studies that elucidate the molecular mechanisms of signal transduction downstream of receptors like the TCR and TLR2 will highlight key pathways required for T cell activation. These pathways could then be clinically targeted to alter dysfunctional T cell responses that promote many human diseases.

Focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) are two tyrosine kinases activated by multiple surface receptors expressed on T cells. FAK and Pyk2 signaling regulate cell morphology, migration, adhesion, proliferation, and survival in other cell types. However, their functions in T cells are not well-described. Because FAK and Pyk2 functions are dysregulated in many disease states, it is important to understand their function in human T cells so that clinicians can safely target these kinases to treat various disorders. The studies described in this dissertation aim to more fully elucidate how FAK and Pyk2 control T cell activation mediated by the TCR.

We used recombinant microRNAs or kinase inhibitors to transiently suppress FAK and Pyk2's expression or enzymatic function in human T cells. In doing so, several novel functions of FAK and Pyk2 were uncovered. In Chapter III, we revealed that FAK is a negative regulator of TCR signal transduction and function. Interestingly, in contrast to its function in other immune cell lineages, the work described in Chapters III and IV demonstrates that FAK is not required to regulate actin cytoskeletal responses downstream of the TCR. The data presented in Chapter IV demonstrate that Pyk2 regulates TCR-mediated actin cytoskeleton reorganization. This function appears to be linked to Pyk2's scaffolding function and not its enzymatic activity. In Chapter V, we demonstrated that the catalytic function of Pyk2 controls phosphatidylinositol-3-kinase (PI3K) activation in human T cells. Together, these data revealed that FAK and Pyk2 serve distinct functions in TCR signal transduction, actin cytoskeletal rearrangement, and effector responses.

TCR-driven cytokine production and proliferation are enhanced when T cells are concurrently activated via TLR2 ligands. In Chapter VI, we describe the signaling pathways that TLR2 activates in human T cells, and we characterize how TCR and TLR2 signals converge to augment T cell responses. In contrast to studies performed using isolated murine T cells, we demonstrated that TLR2 does not activate nuclear-factor kappa B in human T cells. Instead, we found that TCR and TLR2 co-ligation selectively augments extracellular signal-related kinase 1 (Erk1)/Erk2 and Akt activation in human T cells. Thus, TLR2 co-stimulates murine and human T cells via distinct signaling mechanisms.