Table 1_Folate receptor β performs an immune checkpoint function in activated macrophages.docx

Monocytes and macrophages are sentinels of the immune system that distinguish themselves from other cells by expressing the beta isoform of the folate receptor (FRβ). Because FRβ does not bind folate until the monocyte/macrophage is exposed to immunosuppressive cytokines, the question naturally arose whether FRβ might also perform an immune-related function. To examine this matter, we compared the properties of wild type (WT) and FRβ knockout mice. We observe that FRβ knockout (KO) mice display autoimmune symptoms that can include alopecia, enlarged spleens, and dermatitis, despite having normal cellular folate levels. We further demonstrate that syngeneic tumors (TRAMP C2, MC38) grow much slower in FRβ KO mice than wildtype mice. Comparison of cells extracted from syngeneic tumors of KO mice further reveal that CD69+ T cells are increased while PD1+ T cells and PD-L1+ myeloid cells are decreased in KO tumors. More detailed comparison of the bone marrow-derived macrophages from KO and WT mice demonstrates that KO mice have upregulated pro-inflammatory genes and downregulated anti-inflammatory genes. Because blockade of FRβ with a monoclonal antibody or deletion of FRβ impairs direct macrophage suppression of T cell activation in vitro, we conclude that FRβ performs a checkpoint function that regulates the immunologic properties of tumor myeloid cells. Since FRβ expression in human cancers is shown to correlate inversely with overall survival, we further posit that FRβ similarly performs an immunosuppressive function in human tumors.

单核细胞与巨噬细胞是免疫系统的哨兵细胞，它们通过表达叶酸受体β亚型（folate receptor beta, FRβ）与其他细胞群区分开来。由于FRβ需在单核细胞/巨噬细胞接触免疫抑制性细胞因子后才会结合叶酸，学界自然产生了疑问：FRβ是否亦具备免疫相关功能？为探究这一问题，本研究对比了野生型（wild type, WT）与FRβ基因敲除（knockout, KO）小鼠的各项表型特征。研究观察到，即便细胞叶酸水平正常，FRβ基因敲除（KO）小鼠仍会出现脱发、脾脏肿大、皮炎等自身免疫症状。本研究进一步证实，同基因肿瘤（TRAMP C2、MC38）在FRβ KO小鼠体内的生长速度显著慢于野生型小鼠。对从KO小鼠同基因肿瘤中分离得到的细胞进行分析后发现，FRβ KO肿瘤内的CD69阳性T细胞比例升高，而PD1阳性T细胞与PD-L1阳性髓系细胞比例则明显降低。对KO与WT小鼠骨髓来源巨噬细胞的更深入对比研究显示，KO小鼠体内促炎基因表达上调，抗炎基因表达下调。由于使用单克隆抗体阻断FRβ或敲除FRβ基因均可在体外削弱巨噬细胞对T细胞活化的直接抑制作用，本研究得出结论：FRβ发挥免疫检查点功能，可调控肿瘤髓系细胞的免疫特性。鉴于已有研究证实人类癌症中FRβ的表达水平与总生存期呈负相关，本研究进一步推测：FRβ在人类肿瘤中同样发挥免疫抑制功能。