PKC enzymes play critical roles in the differentiation and proliferation of many cell types, including T cells, and in the response to diverse stimuli. Little is known, however, about the substrate specificity and role of individual PKC isoforms in distinct activation and developmental events in T cells. In 1993, we cloned and identified a novel PKC isoform, PKCq. It is characterized by a unique tissue distribution, i.e., in skeletal muscle, lymphoid organs, and hematopoietic cell lines, in particular T cells. PKCq plays an important role in T cell activation: It selectively activates the transcription factors AP-1 and NF-kB, and integrates TCR and CD28 signals, which lead to activation of the CD28 response element (RE) in the interleukin-2 (IL-2) gene promoter. PKCq also localizes to the center of the T cell immunological synapse (IS) that forms at the contact area between antigen-specific T cells and antigen-presenting cells, and serves as a platform for the transduction of activation signals. Consistent with its important role in T cell activation, mature T cells from PKCq-deficient mice display severely reduced proliferation and IL-2 production, along with impaired activation of NF-kB, AP-1 and NFAT. However, the role of PKCq in T cells is selective since it is required for allergic response mediated by T helper 2 (Th2) cells and autoimmune diseases mediated by Th17 cells, but is largely dispensable for anti-viral immunity. As a result of this selectivity, as well as its relatively specific T cell expression, PKCq has become an attractive drug target.

Other studies on PKCq have recently resolved some enigmas that have dominated the field for a long time, namely, what is the molecular basis for the selective recruitment of this enzyme to the center of the T cell IS, and is this localization essential for the ability of PKCq to activate T cells. Indeed, we found that PKCq‘s unique localization results from its physical association with CD28, a major T cell costimulatory receptor and, furthermore, that a PKCq mutant that does not associate with CD28 fails to activate T cells. These findings form a rational basis for the design of molecular entities that will block the PKCq-CD28 interaction and, therefore, will potentially have the desired effect of inhibiting allergic responses and autoimmune diseases.

We current continue these studies with particular emphasis on the role of PKCq in regulatory T (Treg) cells and in human T cells.

In 2003, we reported the isolation and initial characterization of a novel signaling protein, termed SWAP-70-Like Adaptor of T cells (SLAT; aka Def6), which is selectively expressed in T cells. Upon antigen stimulation, SLAT selectively localizes to the T cell IS, where it plays a key role in initiating cytoskeletal changes and Ca2+ signaling. As a result, T cells that are genetically deficient for SLAT display a severe defect in TCR-induced activation and differentiation into different Th subsets. Our more recent work has elucidated the mechanistic basis for the indispensable role of SLAT in T cell Ca2+ signaling by revealing that , following TCR triggering, SLAT directly binds to the ER-localized inositol triphosphate receptor (IP3R) and facilitates its Ca2+ ion channel function, which initiates Ca2+ signaling in T cells, the latter being required for productive T cell activation and effector functions.

Recent work by others has provided evidence that, in contrast to conventional T cells, PKCq negatively regulates the suppressive activity of Treg cells. Therefore, we initiated efforts to identify other member(s) of the PKC enzyme family, which might positively regulate Treg cell function. This work resulted in the identification of protein kinase C-eta (PKCh) as being required for contact-dependent Treg cell suppressive activity. Furthermore, Treg cell costimulation via the TCR and CTLA4, a major Treg cell-expressed inhibitory receptor required for most suppression pathways, resulted in a direct association between the CTLA4 intracellular domain and PKCh. As a result, strategies that blocked the CTLA4-PKCh interaction impaired the suppressive activity of Treg cells. Interestingly, PKCh-deficient Treg cells lost their ability to inhibit immune response against growing mouse tumors, but retained their ability to inhibit the development of a model autoimmune disease in mice, i.e., adoptive T cell transfer-mediated colitis. Additionally, we found that PKCh-deficient Treg cells display reduced motility and prolonged arrest on antigen-presenting dendritic cells (DC) in vivo, resulting in defective overall depletion of costimulatory DC ligands, an effect that is consistent with reduced suppressive activity and enhanced tumor immunity. We continue to study the mechanistic aspects of this novel signaling pathway, and the applicability of manipulating it in order to enhance tumor-specific immunity.