PDCD1 and colorectal carcinoma: There was no difference in the levels of PD-1 expressing CD4+ T cells between TILs and NILs, but when we examined its co-expression with intracellular CTLA-4, another ICR, there was a significant increase in the levels of cells co-expressing PD-1 and CTLA-4 infiltrating TT (12.7 ± 3.7%), compared with their levels in NT (2.8 ± 1.1%) and circulation (0.5 ± 0.1%) in CRC patients (Figures 2A,B).