Hippocampal microglia are vulnerable to the effects of ageing, displaying a primed phenotype and hyper-responsiveness to various stimuli. We have previously shown that short-term high fat diet (HFD) significantly impairs hippocampal-and amygdala-based cognitive function in the aged without affecting it in the young. Here we assessed if morphological and functional changes in microglia might be responsible for this. We analysed hippocampus and amygdala from young and ageing rats that had been given three days HFD; a treatment sufficient to cause both hippocampaland amygdala-dependent cognitive and neuroinflammatory differences in the aged. Ageing led to the expected priming of hippocampal microglia in that it increased microglial numbers and reduced branching in this region. Ageing also increased microglial phagocytosis of microbeads in the hippocampus, but the only effect of HFD in this region was to increase the presence of enlarged synaptophysin boutons in the aged, indicative of neurodegeneration. In the amygdala, HFD exacerbated the effects of ageing on microglial priming (morphology) and markedly suppressed phagocytosis without notably affecting synaptophysin. These data reveal that, like the

Epithelial ovarian cancer (EOC) is most frequently diagnosed at an advanced stage, and is characterized by its high recurrence rate and almost-universal acquisition of chemoresistance. Metastasis of EOC is driven by heterogenous spheroids that passively disseminate throughout the peritoneal cavity, promoting tumor spread and recurrence. We previously identified a sub-population of highly motile and invasive “Leader cells” (LCs) that express the basal epithelial protein KRT14 as an absolute determinant of invasive potential. We demonstrated KRT14 knockout in EOC cells does not impact viability or proliferation, however completely ablates their ability to invade through Matrigel in 2D and 3D format in vitro. Although a role for LCs in mesothelial clearance and invasion has been established, these mechanisms remain poorly understood. Here we show a critical role for KRT14+ LCs in immunosuppression and tumor-immune privilege in vivo. In the ID8 syngeneic EOC mouse model, tumor-specific LC (KRT14) loss (LCKO) impaired ovarian intrabursal primary tumor engraftment and subsequent metastatic spread. Immune phenotyping by flow cytometry revealed that mice bearing LCKO tumors displayed decreased proportions of immunosuppressive tumor-adjacent Tregs and M2 macrophages, and improved CD8+ T-cell/Treg ratios. Conversely, forced LC-overexpression (LCOE) in EOC tumors accelerated metastasis, and promoted tumor-immune evasion via upregulated secretion of suppressive chemokines including CCL22 and CCL5. In addition to their critical role in migration and invasion, the current study provides in vivo evidence that leader cells are mediators of immune suppression in ovarian cancer. This work supports the basis that defining novel strategies to target LCs to promote anti-tumor immunity and limit metastatic progression may be a useful strategy for improved epithelial ovarian cancer therapy. Citation Format: Amy L. Wilson, Brittany R. Doran, Bashirah Basri, Laura R. Moffitt, Magdalena Plebanski, Andrew N. Stephens, Maree Bilandzic. Leader cells mediate tumor engraftment and promote immunosuppression to drive ovarian cancer progression in vivo [abstract]. In: Proceedings of the AACR Special Conference: Cancer Metastasis; 2022 Nov 14-17; Portland, OR. Philadelphia (PA): AACR; Cancer Res 2022;83(2 Suppl_2):Abstract nr B040.

Ovarian cancer is the most lethal gynecological malignancy and treatment options have not improved in decades. Whilst ovarian tumors have an immune-mediated component, emerging immunotherapies (e.g. anti-PD-1, anti-PD-L1) have failed to achieve robust clinical response for ovarian cancer patients. There is an urgent need to develop new therapeutic strategies to improve survival rates in women, particularly those with recurrent, chemoresistant disease. In an ongoing study we are investigating the repurposing of sitagliptin, an anti-diabetes drug, as a novel immunomodulatory therapy in ovarian cancer. Ovarian tumors were generated in C57BL/6 wild-type mice by intrabursal implantation of ID8 murine ovarian cancer cells. Following the formation of primary tumors (~2 weeks), mice received a daily oral dose of sitagliptin (50mg/kg) until ethical endpoint. Anti-tumor immune responses were examined by flow cytometry and immunohistochemical staining according to tumor stage. Mice were assessed for overall survival compared to paclitaxel chemotherapy as standard-of-care. Daily sitagliptin treatment increased circulating Teff:Treg cell ratios in the blood, peritoneal fluid and tumor tissue at an early stage, and this was sustained throughout disease. Sitagliptin also enhanced T effector cell activation and proliferation, indicating that increased Teff:Treg ratios were a positive indicator of the anti-tumor immune response. Treatment with sitagliptin alone increased median survival time by 27% compared to both untreated controls and paclitaxel as standard-of-care, suggesting a beneficial effect mediated by anti-tumor immune responses. Of particular interest, sitagliptin treatment specifically increased the percentage of CXCR3+ T effector cells in the peritoneal cavity. Recent data has suggested that CXCR3+ T cells are functionally required for immune checkpoint inhibitors to exert efficacy. Thus, we hypothesize that sitagliptin may enhance the efficacy of immune checkpoint inhibitor therapies in ovarian cancers. Our ongoing studies are investigating the combination of sitagliptin with anti-PD-1 and anti-PD-L1 antibodies, as a rational approach to improving their efficacy in ovarian cancers. These studies will identify a clinically relevant and directly translatable approach to improve prognosis for ovarian cancer patients. Citation Format: Laura R. Moffitt, Amy L. Wilson, Bashirah Basri, Kirsty L. Wilson, Magdalena Plebanski, Andrew N. Stephens. Synergistic action of sitagliptin and checkpoint inhibition for ovarian cancer therapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5547.