CD4 and esophageal squamous cell carcinoma: Notably, different CD4+ T cell subsets expressed distinct immune checkpoint factors in ESCC tumors compared to nonmalignant samples CD4_1 had upregulated TIGIT expression in tumors, CD4_2 expressed PD-1 (encoded by PDCD1) exclusively in tumor samples, CD4_3 showed tumor-specific TIGIT and CD96 expression23.