Novel Role of PPAR-Gamma in GM-CSF Induced Anti-Tumor Immunity

Granulocyte macrophage colony stimulating factor (GM-CSF) mediates context dependent anti- or pro-inflammatory functions through cells of the myeloid lineage. GM-CSF signaling induces the expression of the transcription factor peroxisome proliferator-activated receptor gamma (PPAR-γ). We examined the role PPAR-γ in myeloid cells in the anti-tumor response to GVAX, a GM-CSF based cancer immunotherapy using the B16 model of murine melanoma. We found that selective loss of PPAR-γ in the myeloid lineage using LysM-Cre reduces the efficacy of GVAX which could not be explained by known mechanisms. RNASeq of GVAX draining lymph node identified an increase in regulatory T-cells markers such as FoxP3 and coinhibitory receptors CTLA-4 and TIGIT in LysM-Cre; PPAR-γ fl mice (PPAR-γ KO). We confirmed by flow cytometry that Treg frequency was indeed increased in PPAR-γ KO lymph node with a strong reduction seen in the ratio of CD8 T-cells to regulatory T cell (CD8:Treg). Treg recruiting chemokines CCL17 and CCL22 were upregulated in the draining lymph node. Importantly, tumors in PPAR-γ KO mice had a reduced CD8:Treg ratio explaining the loss in GVAX efficacy. Pharmacological activation or inactivation of PPAR-γ in GM-CSF treated human PBMC showed conservation of the role of PPAR-γ in regulating T-cell numbers in humans. PPAR-γ agonism in mice, using the FDA-approved small molecule ligand rosiglitazone (Rosi), improved CD8:Treg ratios in the vaccine draining lymph node and tumors. The gain-of-function data suggested the Rosi could be used as an adjunct to immunotherapy. All intratumoral Treg expressed high levels of CTLA-4 and TIGIT. Thus, we tested the impact of Rosi on the response to GVAX and anti-CTLA-4 combination therapy. We found that Rosi improved the tumor incidence and overall survival of tumor bearing mice treated with GVAX and anti-CTLA4. Our data have identified a novel role of PPAR-γ in myeloid cells in regulating Treg numbers. This pathway is conserved in humans as seen in ex-vivo studies of PBMC. Further, we provide preclinical evidence that Rosi can be used to improve immunotherapeutic responses by increasing the ratio between intratumoral effector and regulatory cells