Background Emerging evidence indicates that tumor-directed anticancer agents have immunomodulatory effects that contribute to therapeutic outcomes. Targeted therapy by inhibition of poly(ADP-ribose) polymerase (PARPi) has had a profound effect on disease outcomes in ovarian cancer. The cytotoxic effects of PARPi are attributed to inhibition of single-stranded DNA repair pathways resulting in accumulation of DNA-damage in BRCA-deficient cells. While PARP is commonly associated with DNA repair, it has also been linked to a variety of other processes including cell division and inflammation. Published data using PARP1-KO mice suggests that PARP impairs circulating regulatory T cell (Treg) function through modulation of FoxP3, however, the impact on tumor-associated Tregs is unclear. As in many cancers, Tregs accumulate in the ovarian cancer tumor microenvironment (TME) and represent a key mechanism of immune escape. With growing interest in testing combinations of PARPi and immunotherapy, we sought to examine the impact of PARPi on peripheral and tumor-associated Tregs.
Methods To isolate viable Tregs we used FoxP3-eGFP co-expressing transgenic mice. Female mice (8-10 wk) were challenged with BRCA1-deficient ID8 cells (i.p.) and treated daily with ABT-888 (40 mg/kg) for six weeks. Tumor-associated Tregs were sorted from total peritoneal cells pre-enriched for CD4+ T cells. In separate experiments, peripheral Tregs were sorted from CD4+ spleen and lymph node cells and pre-treated with ABT-888 for 24 hours prior to use. Standard proliferation assays using naïve T cells were used to assess Treg suppressive function. Flow cytometry was used to measure cell divisions and expression of FoxP3 and CTLA-4.
Results Ex-vivo treatment of Treg from non-treated, tumor-naïve mice showed that ABT-888 pre-treatment significantly reduced the suppressive capacity of Tregs in a dose-dependent manner (P < 0.05). Similarly, ABT-888 treatment resulted in decreased expression of FoxP3 and CTLA-4 in a dose-dependent manner (P < 0.05). Conversely, tumor-associated Tregs from PARPi-treated mice had superior suppressive capacity compared to those from non-treated mice (P < 0.05). No differences between treated and non-treated groups were observed in Tregs isolated from the spleen.
Conclusions Taken together these data highlight the immunomodulatory effects of PARPi on tumor-associated Tregs. Here we present evidence that PARPi treatment promotes the suppressive capacity of Tregs in the TME and also identify a potential interaction between PARPi and response to immunotherapy in BRCA-deficient ovarian cancer. Future studies will include PARP inhibitors with varying degrees of DNA trapping ability as well as non-BRCA mutated tumors.
Statistics from Altmetric.com
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.