Immune cells constitute an important element of tumor tissue. Accumulating evidence indicates their clinicopathological significance in predicting prognosis and therapeutic efficacy. Nonetheless, the combinations of immune cells forming the immune microenvironment and their association with histological findings remain largely unknown. Moreover, it is unclear which immune cells or immune microenvironments are the most prognostically significant. Here, we comprehensively analyzed the immune microenvironment and its intratumor heterogeneity in 919 regions of 158 hepatocellular carcinomas (HCCs), and the results were compared with the corresponding histological and prognostic data. Consequently, we classified the immune microenvironment of HCC into three distinct immunosubtypes: Immune-high, Immune-mid, and Immune-low. The Immune-high subtype was characterized by increased B-/plasma-cell and T cell infiltration, and the Immune-high subtype and B-cell infiltration were identified as independent positive prognostic factors. Varying degrees of intratumor heterogeneity of the immune microenvironment were observed, some of which reflected the multistep nature of HCC carcinogenesis. However, the predominant pattern of immunosubtype and immune cell infiltration of each tumor was prognostically important. Of note, the Immune-high subtype was associated with poorly differentiated HCC, cytokeratin 19 (CK19)+, and/or Sal-like protein 4 (SALL4)+ high-grade HCC, and Hoshida's S1/Boyault's G2 subclasses. Furthermore, patients with high-grade HCC of the predominant Immune-high subtype had significantly better prognosis. These results provide a rationale for evaluating the immune microenvironment in addition to the usual histological/molecular classification of HCC. Conclusion: The immune microenvironment of HCC can be classified into three immunosubtypes (Immune-high, Immune-mid, and Immune-low) with additional prognostic impact on histological and molecular classification of HCC. (Hepatology 2018).
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