Regulatory T cells (Tregs) are abnormally enriched in HCC tissues, where they suppress CTL and natural killer (NK) cell activity through the secretion of interleukin-10 (IL-10) and transforming growth factor-β (TGF-β), while promoting immune tolerance via direct cell-cell contact mediated by CTLA-4 (Sawant et al., 2019; Yang et al., 2006). Here, CTLA4 is linked to hepatocellular carcinoma.