Diacylglycerol (DAG), a second messenger generated by phospholipase Cγ1 activity upon T cell receptor (TCR) engagement, triggers several signaling cascades that play important roles in T cell development and function. A family of enzymes called diacylglycerol kinases (DGKs) catalyzes the phosphorylation of DAG to phosphatidic acid, acting as a braking mechanism that terminates DAG-mediated signals. Two DGK isoforms, α and ζ, are predominantly expressed in T cells and synergistically regulate the development of both conventional αβ T cells and invariant NKT cells in the thymus. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T cell hyper-activation upon TCR stimulation and by promoting T cell anergy. In CD8 cells, reduced DGK activity is associated with enhanced primary responses against viruses and tumors. Recent work has also established an important role for DGK activity at the immune synapse and identified partners that modulate DGK function. In addition, emerging evidence points to previously unappreciated roles for DGK function in directional secretion and T cell adhesion. In this review, we discuss the multitude of roles played by DGKs in T cell development and function, while emphasizing recent advances in the field.

Activation of the transcription factor NF-κB is critical for cytokine production and T cell survival after T cell receptor (TCR) engagement. The effects of persistent NF-κB activity on T cell function and survival are poorly understood. In this study, using a murine model that expresses a constitutively active form of inhibitor of κB kinase β(caIKKβ) in a T-cell specific manner, we demonstrate that chronic IKKβ signaling promotes T cell apoptosis, attenuates responsiveness to TCR-mediated stimulation in vitro, and impairs T cell responses to bacterial infection in vivo. CaIKKβ T cells showed increased FasL expression and caspase-8 activation, and blocking Fas/FasL interactions enhanced cell survival. T cell unresponsiveness was associated with defects in TCR proximal signaling, and elevated levels of Blimp1, a transcriptional repressor that promotes T cell exhaustion. CaIKKβ T cells also showed a defect in IL-2 production, and addition of exogenous IL-2 enhanced their survival and proliferation. Conditional deletion of Blimp1 partially rescued sensitivity of caIKKβ T cells to TCR triggering. Furthermore, adoptively transferred caIKKβ T cells showed diminished expansion and increased contraction in response to infection with Listeria monocytogenes expressing a cognate antigen. Despite their functional defects, caIKKβ T cells readily produced pro-inflammatory cytokines and mice developed autoimmunity. In contrast to NF-κB's critical role in T cell activation and survival, our study demonstrates that persistent IKK-NF-κB signaling is sufficient to impair both T cell function and survival.

Diacylglycerol (DAG) and phosphatidic acid (PA) are bioactive lipids synthesized when the T cell receptor binds to a cognate peptide-MHC complex. DAG triggers signaling by recruiting Ras guanyl-releasing protein 1, PKCθ, and other effectors, whereas PA binds to effector molecules that include mechanistic target of rapamycin, Src homology region 2 domain-containing phosphatase 1, and Raf1. While DAG-mediated pathways have been shown to play vital roles in T cell development and function, the importance of PA-mediated signals remains less clear. The diacylglycerol kinase (DGK) family of enzymes phosphorylates DAG to produce PA, serving as a molecular switch that regulates the relative levels of these critical second messengers. Two DGK isoforms, α and ζ, are predominantly expressed in T lineage cells and play an important role in conventional αβ T cell development. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T cell hyper-activation and promoting T cell anergy. In this review, we discuss the roles of DAG-mediated pathways, PA-effectors, and DGKs in T cell development and function. We also highlight recent work that has uncovered previously unappreciated roles for DGK activity, for instance in invariant NKT cell development, anti-tumor and anti-viral CD8 responses, and the directional secretion of soluble effectors.