CD4 and head and neck squamous cell carcinoma: The GITR expression level was significantly higher on all analyzed T cell subsets of HNSCC patients compared to healthy donors, with the largest difference for CD4+CD39+ Treg (mean value 36.7 ± 11.1% versus 22.5 ± 11.2%, unpaired T test, p < 0.0001; Figure 2B).