Bile acids restrict functional reprogramming of myeloid-derived suppressor cells in tumor beds

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ABSTRACT The expansion of myeloid-derived suppressor cells (MDSC) in tumor-bearing hosts has emerged as a primary mechanism to limit protective anti-tumor T cell responses and a major obstacle to the success of cancer immunotherapy. It is therefore imperative to develop new effective therapeutic strategies to overcome the immune restricting effects induced by MDSC in tumors. Our recent studies demonstrated that MDSC chronically activate an integrated signaling network known as the unfolded protein responses (UPR) as a reaction to the sustained and pronounced endoplasmic reticulum (ER) stress induced by the precarious conditions of the tumor microenvironment (TME). Also, conditional deletion of the UPR-related PKR-like ER kinase (PERK) functionally reprogrammed MDSC into immunostimulatory cells that partially restore protective anti-tumor immunity in tumor beds. Importantly, our new Preliminary Results demonstrate a compensatory accumulation of highly immune- regulatory primary and secondary bile acids (BAs) in PERK-ablated MDSC in tumors. BAs are synthetized in the liver and intestine as final products of cholesterol metabolism and signal through priming of farnesoid X receptor (FXR) transcription factor, which was found increased in PERK-null MDSC and augmented MDSC suppressive activity. Based on these crucial findings, we hypothesize that the accumulation of intracellular BAs restricts the transformation of PERK-null MDSC in tumors into myeloid cells that promote sustained protective T cell immunity through activation of FXR and a subsequent induction of the compensatory UPR mediators, IRE1→Xbp1. We also propose that inhibition of PERK plus IRE1 will completely overcome MDSC-linked suppression in tumors and synergize with T cell and checkpoint immunotherapies. We propose the following Specific Aims: In Aim 1, we will determine the restrictive action of the accumulated BAs in the functional transformation of PERK-null MDSC into myeloid cells that prime sustained protective anti-tumor T cell immunity. In Aim 2, we will elucidate the mechanisms whereby BAs restrict PERKKO MDSC transformation in tumors. In Aim 3, we will test the prediction that combined inhibition of PERK and IRE1 completely overcomes MDSC- associated immunosuppression and enhances the effectiveness of cancer immunotherapy. The metabolic mediators driving the immunosuppressive functionality of MDSC in tumor beds remain poorly described. Thus, development of the Aims will have a profound impact on the field by substantiating a major metabolic signal whereby tumors restrict the functional transformation of MDSC after targeting the ER stress mediator, PERK. Also, our proposal will provide a mechanistic rationale for the development of novel therapeutic approaches to entirely reprogram immunosuppressive myelopoiesis in tumor-bearing hosts, which is likely to synergize with cancer immunotherapy and provide new biomarkers for tumor-MDSC reprogramming.