Rubino, Thomas E.

Abstract 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; Cancer Res 2023;83(7_Suppl):Abstract nr 4455.

Abstract 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 NFκB and STAT1 activation resulting in increased expression of pro-inflammatory cytokines including IFNβ1, CCL5, CXCL10, and IL-6. We further show that PP4 inhibition and carboplatin-induced STAT1 activation is significantly blunted upon STING knockdown in ovarian cancer cells. Moreover, using mouse ID8 ovarian cancer cell line and OT-I cells, we demonstrate that the combination of PP4 inhibition and carboplatin significantly increased CD8 T cell migration over carboplatin treatment alone. In addition, the same treatment resulted in enhanced NK cell-mediated tumor cell killing and T cell killing in an autologous human organoid model. 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. Supported by DOD W81XWH2110489

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 ovarian cancer (OC), the frequency of responses to immune checkpoint blockade (ICB) therapy in OC is much lower than other cancer types. Recent studies have highlighted that deficiencies in the DNA damage response (DDR) can drive increased genomic instability and tumor immunogenicity, which leads to enhanced responses to ICB. Protein phosphatase 4 (PP4) is a critical regulator of the DDR; however, its potential role in antitumor immunity is currently unknown. Results Our results show that the PP4 inhibitor, fostriecin, combined with carboplatin leads to increased carboplatin sensitivity, DNA damage, and micronuclei formation. Using multiple OC cell lines, we show that PP4 inhibition or PPP4C knockdown combined with carboplatin triggers inflammatory signaling via Nuclear factor kappa B (NF-κB) and signal transducer and activator of transcription 1 (STAT1) activation. This resulted in increased expression of the pro-inflammatory cytokines and chemokines: CCL5 , CXCL10 , and IL-6 . In addition, IFNB1 expression was increased suggesting activation of the type I interferon response. Conditioned media from OC cells treated with the combination of PP4 inhibitor and carboplatin significantly increased migration of both CD8 T cell and natural killer (NK) cells over carboplatin treatment alone. Knockdown of stimulator of interferon genes (STING) in OC cells significantly abrogated the increase in CD8 T-cell migration induced by PP4 inhibition. Co-culture of NK-92 cells and OC cells with PPP4C or PPP4R3B knockdown resulted in strong induction of NK cell interferon-γ, increased degranulation, and increased NK cell-mediated cytotoxicity against OC cells. Stable knockdown of PP4C in a syngeneic, immunocompetent mouse model of OC resulted in significantly reduced tumor growth in vivo . Tumors with PP4C knockdown had increased infiltration of NK cells, NK T cells, and CD4 + T cells. Addition of low dose carboplatin treatment led to increased CD8 + T-cell infiltration in PP4C knockdown tumors as compared with the untreated PP4C knockdown tumors. Conclusions Our work has identified a role for PP4 inhibition in promoting inflammatory signaling and enhanced immune cell effector function. These findings support the further investigation of PP4 inhibitors to enhance chemo-immunotherapy for OC treatment.