Transcriptomic features of tumour-infiltrating CD4^lowCD8^high double positive αβ T cells in melanoma

Peripheral CD4⁺CD8⁺ double positive (DP) T cells are a phenotypically and functionally heterogeneous population depending on their origin and pathologic context. We previously identified among tumour infiltrating lymphocytes in melanoma, a tumour-reactive MHC class-I restricted CD4^lowCD8^high DP αβ T-cell subpopulation with CD4-like function. In this study, we used an in-depth comparative transriptomic analysis of intra-melanoma DP T cells and CD4 and CD8 single positive (SP) T cells, to better comprehend the origin of this DP phenotype, and define the transcriptomic signature of activated DP T cells. We observed that intra-melanoma DP T cells were transcriptome-wise closer to their CD8 SP T-cell counterparts in terms of number of genes differentially expressed (97 in common with CD8 SP T cells and 15 with CD4 SP T cells) but presented hallmarks of a transition to a CD4-like functional profile (CD40LG) with a decreased cytotoxic signature (KLRC1) in favour of an increased cytokine-receptor interaction signature (IL4, IL24, IL17A…). This unleashed CD4-like program could be the results of the observed unbalanced expression of the THPOK/Runx3 transcription factors in DP T cells. Overall, this study allow us to speculate that intra-melanoma DP T cells arise from CD8 SP T cells being reprogrammed to a helper function.