As
tumor PD-L1 provides signals to anti-
tumor PD-1+ T cells that blunt their functions, αPD-1 and αPD-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 (
TIC) generate
carcinomas, resist treatments and promote relapse. We show here that in murine
B16 melanoma and ID8agg ovarian
carcinoma cells,
TIC express more PD-L1 versus non-
TIC. 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-γ and
rapamycin in vitro. Cell-intrinsic PD-L1 similarly drove functional
TIC generation, canonical
TIC gene expression, and sensitivity to
interferon-γ and
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.