Tumor cell-intrinsic PD-L1 promotes tumor-initiating cellgeneration and functions in melanoma and ovarian cancer

As tumor PD-L1 provides signals to anti-tumor PD-1^(+)T cells that blunt their functions,αPD-1 andαPD-L1 antibodies have been developed as anti-cancer immunotherapies based on interrupting this signaling axis.However,tumor cell-intrinsic PD-L1 signals also regulate immune-independent tumor cell proliferation and mTOR signals,among other important effects.Tumor-initiating cells(TICs)generate carcinomas,resist treatments and promote relapse.We show here that in murine B16 melanoma and ID8agg ovarian carcinoma cells,TICs express more PD-L1 versus non-TICs.Silencing PD-L1 in B16 and ID8agg cells by shRNA(‘PD-L1lo’)reduced TIC numbers,the canonical TIC genes nanog and pou5f1(oct4),and functions as assessed by tumorosphere development,immune-dependent and immune-independent tumorigenesis,and serial transplantability in vivo.Strikingly,tumor PD-L1 sensitized TIC to interferon-γand rapamycin in vitro.Cell-intrinsic PD-L1 similarly drove functional TIC generation,canonical TIC gene expression and sensitivity to interferon-γand rapamycin in human ES2 ovarian cancer cells.Thus,tumor-intrinsic PD-L1 signals promote TIC generation and virulence,possibly by promoting canonical TIC gene expression,suggesting that PD-L1 has novel signaling effects on cancer pathogenesis and treatment responses.