Pparβ/δ Has an Indispensable Role in T Cell Function and Anti-Tumor Responses

The peroxisome proliferator-activated receptors (PPARs) are a group of transcriptional factor of the nuclear hormone receptor superfamily, which promote ligand-dependent transcription of target genes that regulate substrate delivery, oxidative phosphorylation (OXPHOS), and energy balance. The PPAR superfamily consists of three subtypes, PPARalpha (PPARα), PPARgamma (PPARγ), and PPARbeta/delta (PPARβ/δ), with distinct tissue allocations. In addition to their roles in energy homeostasis and carbohydrate and lipid metabolism in metabolic organs, an emerging role of PPARs involves the differentiation and fate of immune cells. PPARβ/δ is ubiquitously expressed and activated by fatty acid ligands. While the effects of PPARα and PPARγ on M2 macrophage polarization and Th1 inhibition are well established, only few studies have investigated the effects of PPARβ/δ on the immune responses. It has been proposed that PPARβ/δ might inhibit T cell differentiation towards T helper 1 cells (Th1) and T helper 17 cells (Th17), and synthetic ligands of PPARβ/δ might alleviate inflammation in autoimmune diseases by downregulating pro-inflammatory mediators. Conversely, other studies have observed that PPARβ/δ agonists might enhance anti-tumor T cell responses by reprogramming immunometabolism toward fatty acid oxidation and increasing T cell longevity. In the present study, we sought to determine the role of PPARβ/δ in T cell function by combining genetic and pharmacologic approaches. We generated mice with conditional targeting of Pparβ/δ gene and crossed them with CD4Cre (Pparβ/δ^f/fCD4Cre) to delete Pparβ/δ in T cells. In vitro stimulation of T cells from Pparβ/δ^f/fCD4Cre mice with anti-CD3/TCR-and-CD28 mediated signaling resulted in diminished expansion of CD4⁺ and CD8⁺ T cells. Consistent with these findings, treatment of antigen-specific CD4⁺ and CD8⁺ T cells from OTII and OTI TCR-transgenic mice with the Pparβ/δ-specific antagonist GSK3787 diminished responses to cognate antigen as determined by DNA synthesis, expression of activation markers and differentiation to T effector cells. Furthermore, under these conditions, GSK 3787 induced a dose-dependent decrease in the production of IL-2, TNF-α, and IFN-γ by OTII antigen-specific CD4⁺ T cells. To investigate the T cell-specific role of Pparβ/δ in anti-tumor responses, we used Pparβ/δ^f/fCD4Cre and control Pparβ/δ^f/f mice and implanted them with MC38 colon adenocarcinoma tumor cells. We found that Pparβ/δ^f/fCD4Cre mice had enhanced tumor growth compared to control. No quantitative differences were detected in tumor infiltrating CD4⁺, CD8⁺ and Treg cells. However, Treg cells from tumor-bearing Pparβ/δ^f/fCD4Cre mice had a diminished activation state as determined by lower expression of CD44 and PD-1. CD4⁺ T cells in tumors and tumor draining lymph nodes also had decreased expression of CD44 and ICOS but no differences in other activation or checkpoint markers, including GITR, TIGIT, CD69 and CD44 compared to their counterparts from tumor-bearing control mice. In contrast, CD8⁺ T cells from tumor-bearing Pparβ/δ^f/fCD4Cre mice had increased expression of Tox and multiple checkpoint inhibitors including CTLA4, PD-1, and Lag3 and decreased expression of IFN-g compared to CD8⁺ cells from control tumor-bearing mice. These results reveal a previously unappreciated indispensable role of PPARβ/δ in regulating anti-tumor immune responses and protecting CD8⁺ tumor infiltrating T cells from developing an exhaustion immunological and functional phenotype. Our findings suggest that pharmacologic PPARβ/δ-specific agonists might prevent tumor-mediated exhaustion and support anti-tumor function of CD8⁺ T effector cells.