HHLA2 and nonpapillary renal cell carcinoma: Zhou et al. determined the gene expression level by immunohistochemistry of CCRCC cancer tissues and obtained that HHLA2 gene expression was higher than PD-1 in CCRCC, and HHLA2/PD-1 co-expression was significantly associated with high density CD8 + and CD4 + [49].