Abstract

Hepatocellular carcinoma (HCC) is the third largest cause of cancer-induced mortality worldwide. The MYC
oncogene is a prevalent driver of human neoplasia, including HCC. Immune checkpoint inhibitors have succeeded in treating many MYC-driven cancers but have failed as first-line monotherapy for HCC. Despite the failure of immune checkpoint inhibitors targeting PDL1 and CTLA4 as monotherapies, their combination has shown promise in treating MYC-driven HCC. The success of this combination blockade has been confirmed via measurements of tumor volume, quantity, and proliferative index. Here, we have quantified the immune cell infiltrates in mice with HCC treated with the combination of anti-PDL1 and anti-CTLA4, in order to determine which immune cells are responsible for the synergistic efficacy of this combination blockade. Immunofluorescence and immunohistochemistry experiments showed that the combination blockade results in
significantly increased macrophage infiltration while also yielding significantly decreased immunosuppressive PDL1+ macrophages. Both of these results are not seen when treating with either monotherapy. In addition, the blockade results in greater recruitment of CD4+ and CD8+ T cells, which was precluded by macrophage depletion. These findings suggest the following model for the efficacy of the treatment: The combined PDL1 and CTLA4 blockade reduces immunosuppressive macrophages and increases inflammatory macrophages, which are causally required. This leads to recruitment of T cells, thus restoring immune activity and overcoming the MYC-induced immune evasion.