CD8 + PD-1 - ILT2 + T Cells Are an Intratumoral Cytotoxic Population Selectively Inhibited by the Immune-Checkpoint HLA-G.

Only some cancer patients respond to the immune-checkpoint inhibitors being used in the clinic, and other therapeutic targets are sought. Here, we investigated the HLA-G/ILT2 checkpoint in clear-cell renal-cell carcinoma (ccRCC) patients and focused on tumor-infiltrating CD8 ⁺ T lymphocytes (TIL) expressing the HLA-G receptor ILT2. Using transcriptomics and flow cytometry, we characterized both peripheral blood and tumor-infiltrating CD8 ⁺ ILT2 ⁺ T cells from cancer patients as late-differentiated CD27 ^- CD28 ^- CD57 ⁺ cytotoxic effectors. We observed a clear dichotomy between CD8 ⁺ ILT2 ⁺ and CD8 ⁺ PD-1 ⁺ TIL subsets. These subsets, which were sometimes present at comparable frequencies in TIL populations, barely overlapped phenotypically and were distinguished by expression of exclusive sets of surface molecules that included checkpoint molecules and activating and inhibitory receptors. CD8 ⁺ ILT2 ⁺ TILs displayed a more mature phenotype and higher expression of cytotoxic molecules. In ex vivo functional experiments with both peripheral blood T cells and TILs, CD8 ⁺ ILT2 ⁺ T cells displayed significantly higher cytotoxicity and IFNγ production than their ILT2 ^- (peripheral blood mononuclear cells, PBMC) and PD-1 ⁺ (TILs) counterparts. HLA-G expression by target cells specifically inhibited CD8 ⁺ ILT2 ⁺ T-cell cytotoxicity, but not that of their CD8 ⁺ ILT2 ^- (PBMC) or CD8 ⁺ PD-1 ⁺ (TIL) counterparts, an effect counteracted by blocking the HLA-G/ILT2 interaction. CD8 ⁺ ILT2 ⁺ TILs may therefore constitute an untapped reservoir of fully differentiated cytotoxic T cells within the tumor microenvironment, independent of the PD1 ⁺ TILs targeted by immune therapies, and specifically inhibited by HLA-G. These results emphasize the potential of therapeutically targeting the HLA-G/ILT2 checkpoint in HLA-G ⁺ tumors, either concomitantly with anti-PD-1/PD-L1 or in cases of nonresponsiveness to anti-PD-1/PD-L1.
(©2019 American Association for Cancer Research.)