PD-1 and ICOS coexpression identifies tumor-reactive CD4(+) T cells in human solid tumors

CD4(+) Th cells play a key role in orchestrating immune responses, but the identity of the CD4(+) Th cells involved in the antitumor immune response remains to be defined. We analyzed the immune cell infiltrates of head and neck squamous cell carcinoma and colorectal cancers and identified a subset of CD4(+) Th cells distinct from FOXP3(+) Tregs that coexpressed programmed cell death 1 (PD-1) and ICOS. These tumor-infiltrating lymphocyte CD4(+) Th cells (CD4(+) Th TILs) had a tissue-resident memory phenotype, were present in MHC class II–rich areas, and proliferated in the tumor, suggesting local antigen recognition. The T cell receptor repertoire of the PD-1(+)ICOS(+) CD4(+) Th TILs was oligoclonal, with T cell clones expanded in the tumor, but present at low frequencies in the periphery. Finally, these PD-1(+)ICOS(+) CD4(+) Th TILs were shown to recognize both tumor-associated antigens and tumor-specific neoantigens. Our findings provide an approach for isolating tumor-reactive CD4(+) Th TILs directly ex vivo that will help define their role in the antitumor immune response and potentially improve future adoptive T cell therapy approaches.