Clinical Significance of the HHLA2 Protein in Hepatocellular Carcinoma and the Tumor Microenvironment

Background: The protein "human endogenous retrovirus H long terminal repeat-associating 2" (HHLA2), a member of the B7 family, has been linked to cancer progression and immune responses. However, its functional role in hepatocellular carcinoma (HCC) remains unknown. Methods: Bioinformatics was used to examine the potential roles of HHLA2 in HCC and the molecular pathways involved. Expression of HHLA2 and PD-L1 as well as the density of tumor-infiltrating lymphocytes (TILs) in tumoral areas were evaluated by immunohistochem-istry and hematoxylin-eosin staining of 202 resected human HCC samples. Potential correla-tions of HHLA2 expression with pathological characteristics or prognosis of HCC patients were explored. Different types of immune microenvironment in HCC were defined based on HHLA2 expression and TIL density. Results: High HHLA2 levels in HCC correlated with more advanced clinical cancer stage (P = 0.040), multiple tumors (P = 0.044), poor tumor differentiation (P = 0.048), micro-vascular invasion (P = 0.011) and hepatic capsule invasion (P = 0.047). HHLA2 levels correlated significantly with density of TILs, but not with PD-L1 levels. High HHLA2 levels were associated with worse prognosis. Intermediate and high TIL densities were independent predictors of better prognosis. Tumor microenvironments with type I (HHLA2 -high TILs +) or type IV (HHLA2 -low TILs +) were associated with better prognosis. Conclusion: HHLA2 level can independently predict worse prognosis and affect the tumor microenvironment in HCC, which may help guide immunotherapy against the cancer.