Interestingly, HHLA2 in positive HCC cells co-cultured with macrophages demonstrated higher PD-L1 expression, and this effect was not observed in HHLA2-high cells alone, suggesting that HHLA2 overexpression may enhance PD-L1 expression by promoting the polarization of M2 macrophages [31,32]. Here, HHLA2 is linked to hepatocellular carcinoma.