HDAC6 Inhibition Synergizes with Anti-PD-L1 Therapy in ARID1A-Inactivated Ovarian Cancer

ARID1A, encoding a subunit of the SWI/SNF complex, is the most frequently mutated epigenetic regulator in human cancers and is mutated in more than 50% of ovarian clear cell carcinomas (OCCC), a disease that currently has no effective therapy. Inhibition of histone deacetylase 6 (HDAC6) suppresses the growth of ARID1A-mutated tumors and modulates tumor immune microenvironment. Here, we show that inhibition of HDAC6 synergizes with anti-PD-L1 immune checkpoint blockade in ARID1A-inactivated ovarian cancer. ARID1A directly repressed transcription of CD274, the gene encoding PD-L1. Reduced tumor burden and improved survival were observed in ARID1A(flox/flox)/PIK3CA(H1047R) OCCC mice treated with the HDAC6 inhibitor ACY1215 and anti-PD-L1immune checkpoint blockade as a result of activation and increased presence of IFN gamma-positive CD8 T cells. We confirmed that the combined treatment limited tumor progression in a cytotoxic T-cell-dependent manner, as depletion of CD8(+) T cells abrogated these antitumor effects. Together, these findings indicate that combined HDAC6 inhibition and immune checkpoint blockade represents a potential treatment strategy for ARID1A-mutated cancers. Significance: These findings offer a mechanistic rationale for combining epigenetic modulators and existing immunotherapeutic interventions against a disease that has been so far resistant to checkpoint blockade as a monotherapy.