Abstract IA16: Role of PI3K signaling in T cell-mediated immune responses
Authors: Turka Laurence 
Published: December 2020

Abstract
A number of receptors are capable of activating the PI3K pathway in T cells, most prominently the T-cell receptor for antigen (TCR), the costimulatory receptor CD28, and the interleukin 2 receptor (IL-2R). PI3K signaling is critical for the initiation of effector T-cell responses, and activated multiple central downstream cascades including mTOR. Consistent with this, drugs that target mTOR, such as rapamycin, are effective inhibitors of T cell-mediated immune responses. Despite the important role of PI3K in T-cell activation, mTOR inhibition has been reported to “spare” regulatory T-cell (Treg) responses. This was of particular interest as Tregs, in contrast to all other T cells, constitutively express the high-affinity IL-2R, and thus should be poised for continual PI3K activation. Tregs, defined as cells that express the lineage-defining transcription factor Foxp3, are required for normal immune homeostasis. Foxp3-deficient mice (scurfy) and humans (patients with IPEX) are characterized by widespread and severe multiorgan autoimmunity and inflammation. As well, deletion of Tregs in adult mice leads to rapid and fulminant autoimmunity and death. In a series of studies conducted by members of my group over the past several years, as well as a variety of other laboratories, we have gained insight into the role of PI3K signals in Tregs, and a complex picture has emerged. We have observed that signals that elicit PI3K pathway activation in stimulated T cells, such as the cytokine IL-2, fail to do so in Tregs, despite the fact that Tregs express the IL-2R. Moreover, we have found that this is a selective “defect” in IL-2R signaling as the other major pathway linked to this receptor, STAT5, is intact. Specific loss of PI3K signaling in Tregs can be linked to expression of the lipid phosphatase PTEN. All T cells express PTEN in their resting state. However, while “conventional” T cells (i.e., non-Tregs) downregulate PTEN expression upon activation, Tregs uniquely do not. To test the importance of maintained PTEN expression in Tregs, we created mice with constitutive and with inducible deletion of PTEN specifically in Foxp3+ Tregs. These mice developed a dramatic phenotype of lupus-like autoimmunity, despite the presence of large numbers of Tregs, indeed more than in control mice. Moreover, we observed that loss of PTEN was followed by lineage instability, i.e., loss of Foxp3, and expression of proinflammatory cytokines such as IL-17 and IFNg. PI3K signals activate pathways important for T-cell metabolism, including c-myc, HIF-1a, and mTOR, and promote aerobic glycolysis. In contrast to most activated T cells, Tregs have been reported to be more reliant on fatty acid oxidation for their energy demands, and indeed loss of PTEN in Tregs dramatically augments aerobic glycolysis in these cells. However, most of the studies of Treg metabolism have focused on cells that were differentiated to become Tregs in vitro. In this system, naïve T cells are activated in the presence of the anti-inflammatory cytokine TGF-b, which induces them to express Foxp3. Such cells, “iTregs,” are easy to generate in large numbers and believed to be induced in vivo when antigen is encountered in noninflammatory settings in visceral organs and tissues. However, most T cells in lymphoid organs are thought to have originated in the thymus, are called tTregs, and are important for the prevention of immune responses to self-antigens. We have recently characterized the metabolism of these two Treg subsets. While, as reported, iTregs make minimal use of glycolysis, tTregs are in fact as glycolytic as effector T cells. We have further found that exposure of tTregs to TGF-b represses PI3K-mediated mTOR signaling, inhibits glucose transporter expression, and reprograms their metabolism to favor oxidative phosphorylation. Conversely, replicating the effects of inflammation via elevation of PI3K signaling has minimal effects on tTregs but dramatically enhances the glycolysis of normally oxidative iTregs, resulting in reduction of Foxp3 expression. Collectively, these findings suggest unique signaling and metabolic demands of Tregs and Treg subsets that may be leveraged in the design of therapeutics, both in vivo agents and adoptive cellular therapies.
Citation Format: Laurence Turka. Role of PI3K signaling in T cell-mediated immune responses [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr IA16.