HDAC inhibition suppresses primary immune responses, enhances secondary immune responses, and abrogates autoimmunity during tumor immunotherapy.

Histone deacetylase inhibitors (HDACi) can modulate innate antiviral responses and render tumors more susceptible to oncolytic viruses (OVs); however, their effects on adaptive immunity in this context are largely unknown. Our present study reveals an unexpected property of the HDACi MS-275 that enhances viral vector-induced lymphopenia leading to selective depletion of bystander lymphocytes and regulatory T cells while allowing expansion of antigen-specific secondary responses. Coadministration of vaccine plus drug during the boosting phase focuses the immune response on the tumor by suppressing the primary immune response against the vaccine vector and enhancing the secondary response against the tumor antigen. Furthermore, improvement of T cell functionality was evident suggesting that MS-275 can orchestrate a complex array of effects that synergize immunotherapy and viral oncolysis. Surprisingly, while MS-275 dramatically enhanced efficacy, it suppressed autoimmune pathology, profoundly improving the therapeutic index.