CD4 and neoplasm: CD4+ T cells in PBMC HC made up a significantly lower proportion of CTLA-4+ T cells (24.8 ± 10.7%) than in PBMC HNSCC (69.3 ± 14.6%; p < 0.0001), whereas percentages in PBMC HNSCC and tumor tissue (61.4 ± 16.3%) were comparably high.