IFNγ producing CD8+ T cells modified to resist major immune checkpoints induce regression of MHC class I-deficient melanomas

Tumors with reduced expression of MHC class I (MHC-I) molecules may be unrecognized by tumor antigen-specific CD8 + T cells and thus constitute a challenge for cancer immunotherapy. Here we monitored development of autochthonous melanomas in TiRP mice that develop tumors expressing a known tumor antigen as well as a red fluorescent protein (RFP) reporter knock in gene. The latter permits non-invasive monitoring of tumor growth by biofluorescence. One developing melanoma was deficient in cell surface expression of MHC-I, but MHC-I expression could be rescued by exposure of these cells to IFNγ. We show that CD8 + T cells specific for tumor antigen/MHC-I were efficient at inducing regression of the MHC-I-deficient melanoma, provided that the T cells were endowed with properties permitting their migration into the tumor and their efficient production of IFNγ. This was the case for CD8 + T cells transfected to express an active form of STAT5 (STAT5CA). The amount of IFNγ produced ex vivo from T cells present in tumors after adoptive transfer of the CD8 + T cells was correlated with an increase in surface expression of MHC-I molecules by the tumor cells. We also show that these CD8 + T cells expressed PD-1 and upregulated its ligand PDL-1 on melanoma cells within the tumor. Despite upregulation of this immunosuppressive pathway, efficient IFNγ production in the melanoma microenvironment was found associated with resistance of STAT5CA-expressing CD8 + T cells to inhibition both by PD-1/PDL-1 engagement and by TGFβ1, two main immune regulatory mechanisms hampering the efficiency of immunotherapy in patients.