PPARγ activation sensitizes MSS gastrointestinal cancer cells to PD1 blockage.

Antibodies (Abs) against immune checkpoint "programmed-cell-death-1" (PD1) achieve benefit in patients with microsatellite instable (MSI), immunologically "hot" gastrointestinal (GI) cancers, but fail in cases with "cold" microsatellite stable (MSS) tumors. One underlying mechanism of therapy failure are activating KRAS mutations which up-regulate PD1 within an immunosuppressive microenvironment. Peroxisome-proliferator-activated-receptor-gamma (PPARγ) is a drugable anti-diabetic and immune-regulatory transcription factor which inhibits RAS signaling. We therefore assessed if PPARγ attenuates PD1 expression and/or function on immune cells and augments efficacy of PD1-blockage.

Tissue samples of patients with colorectal and gastric cancers, wildtype (WT) and KRASG12V transgenic C57BL6/J mice on a 3-months diet enriched with PPARγ-agonist (rosiglitazone, rosi, ∼25 mg/kg*day) were analysed by PCR and immunohistochemistry. Cocultures of human MSS GI cancer cell lines (AGS, HT29, SW480) with "lymphokine-activated NK/T killer cells" (LAK) from peripheral blood of healthy donors were subjected to PCR microarray, immunoblot, flow cytometry and viability assays. KRASG12V mouse intestinal tissues had more PD1 than WT littermates, and PD1 was co-expressed with PPARγ in human GI cancers. Combination of rosi and interferon-gamma (IFNγ) reduced the viability of human KRAS/BRAF mutant MSS tumor cells in presence of LAK and PD1 blocking Ab (pembrolizumab).

Mechanistically, PD1 protein was down-regulated and tyrosine phosphorylation of PP60 and signal-transducer-and-activator-of-transcription-1 (STAT1) enhanced in LAK. MHCclassI and PD1-ligand (PDL1) were up-regulated on cocultured tumor cells, endowing them with higher immunogenicity. Pharmacological activation of PPARγ enhanced anti-tumor efficacy of PD1-blockage. Thus, metabolic modifiers might be developed as immunosensitizers for clinical checkpoint therapies against MSS tumors.