Folate receptor ß performs a checkpoint function in activated macrophages

Macrophages and monocytes are essential components of the immune system that distinguish themselves from all other cells in the body by expressing a beta isoform of the folate receptor (FRß). Because FRß does not bind folate until the macrophage/monocyte is exposed to immunosuppressive cytokines, the question naturally arose whether FRß might perform an immune-related function. We report here that FRß knockout mice display autoimmune symptoms that can include alopecia, enlarged spleens, and dermatitis. We further show that bone marrow-derived macrophages from KO mice exhibit strong upregulation of pro-inflammatory genes and downregulation of anti-inflammatory genes. Exogenous inflammatory stimuli are also more intense in FRß KO mice, and syngeneic tumors (TRAMP C2, MC38) grow much slower in FRß-depleted mice. Analysis of these tumors further reveals that CD69+ T-cell populations are increased while PD1+ T-cells, PD-L1+ TAMs and PD-L1+ MDSCs are decreased in the malignant lesions. Because FRß deletion or blockade of FRß with a monoclonal antibody impairs a macrophages' ability to suppress T-cell activation, and since FRß expression correlates inversely with human cancer patient survival, we conclude that FRß performs a checkpoint function specifically in macrophages and MDSCs. Overall design: To investigate the role of FRß, we generated a FRß knock out (KO) mouse model that lacks functional FRß and then compared the FRß KO macrophages with the wild type (WT) ones. Next, we isolated bone marrow derived macrophages from WT and FRß KO mice and performed a total RNA-sequencing. Finally, the differential gene expression and the relative changes in gene expression were analyzed in the WT and FRß KO macrophages.