Abstract 4455: Protein phosphatase 4 inhibition stimulates anti-tumor immunity in ovarian cancer

Background: Increased infiltration of T cells into ovarian tumors has been repeatedly shown to be predictive of enhanced patient survival. However, despite the evidence of an active immune response in OC, immune checkpoint blockade therapy has been ineffective. Recently studies have shown that deficiencies in the DNA damage response (DDR) can drive increased genomic instability and tumor immunogenicity. One target involved in the DDR that has the potential to be of therapeutic value is protein phosphatase 4 (PP4); however, the effect of PP4 deficiency on anti-tumor immunity remain unknown. Results: Our results show that PP4 inhibition combined with carboplatin leads to increased carboplatin sensitivity, DNA damage, and micronuclei formation. Using a panel of ovarian cancer cells, we show that PP4 inhibition triggers inflammatory signaling via NF-κB and STAT1 activation resulting in increased expression of pro-inflammatory cytokines including IFNβ1, CCL5, CXCL10, and IL-6. Moreover, conditioned media from OC cells treated with the combination of PP4 inhibitor and carboplatin significantly increased migration of both CD8 T cell and NK cells over carboplatin treatment alone. Knockdown of STING in OC cells significantly abrogated the increase in CD8 T cell migration induced by PP4 inhibition. PPP4C or PPP4R3B knockdown resulted in strong induction of NK cell IFN-γ, increased degranulation, and increased NK cell-mediated cytotoxicity against OC cells. Stable knockdown of PP4C in a syngeneic, immunocompetent mouse model of ovarian cancer resulted in significant reduction of tumor growth in vivo. PP4C low tumors had increased infiltration of CD161+ NK cells and CD4+ T cells. Addition of low dose carboplatin treatment in vivo led to increased CD8+ T cell infiltration in PP4C low tumors when compared to the untreated groups. Conclusions: Our work has identified a role for PP4 inhibition in promoting anti-tumor immune activation. These findings provide the rationale for combining PP4 inhibitors with immunotherapy as a new approach in ovarian cancer treatment. Citation Format: Remya Raja, Christopher Wu, Esen Y. Bassoy, Thomas Rubino, Emma Utagawa, Paul Magtibay, Kristina Butler, Marion Curtis. Protein phosphatase 4 inhibition stimulates anti-tumor immunity in ovarian cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Can