Background - Treatment of ovarian cancer remains very challenging, with 80-85% of the cases still dying after relapse to standard chemotherapy, and alternative treatments are urgently needed. Expansion of regulatory T cells (Tregs) is considered the major factor limiting spontaneous immune responses to ovarian cancer. Agonist antibodies against the co-stimulatory receptor OX40 have recently demonstrated to abrogate Treg functions and are under clinical evaluation. We thus studied whether OX40 constituted a valid target of ovarian cancer-associated Tregs.

Methods -Treg immunophenotypic analyses were performed by flow cytometry in ascites and ovarian cancer specimens and studied in association with patients' outcome.

Results - CD4+CD25+Foxp3+ Treg% and OX40 expression were measured in 50 diagnostic ovarian cancer cases (36, high grade serous carcinomas; 14, other histotypes). Treg% was significantly higher in ovary (47 evaluable cases) relative to ascites (39 evaluable cases) (p<0.0001). Treg % in ascites and peripheral blood from ovarian cancer patients or healthy donors were not significantly different. The fraction of Tregs expressing OX40, instead, dramatically increased from peripheral blood to ascites (p<0.01), and was even higher within the tumor (p<0.001). In 24 patients (stage IIC-IV; 19, high grade serous carcinoma; 5, other histotypes) with ≥12-month follow-up after surgery and carboplatin-paclitaxel, we analyzed OX40 expression on intratumor Tregs in function of the outcome. Patients with Treg% or OX40 MFI above the mean had a significantly reduced OS (p=0.004, p=0.02). Interestingly, having both high Treg% and OX40 MFI trended toward the worst OS, followed by high Treg% or OX40 MFI alone, whereas the lack of these parameters was associated with a significantly improved survival (mean: 28, 36, 41 and 53 months). OX40+ Tregs in both ascites and ovarian cancer tissue included CD45RA-, Foxp3hi, ICOS+ IFNγ- effector Tregs, thus indicating that OX40 may define a subset of highly suppressive Tregs in ovarian cancer.

Conclusions - Our study underscores a negative prognostic impact of OX40 expression on ovarian cancer-infiltrating Tregs and indicates that OX40 may constitute a rational target to counteract effector Treg functions and unleash potentially effective immune responses against ovarian cancer.

Citation Format: Massimo Di Nicola, Roberta Zappasodi, Alessia Burocci, Giusi Ruggiero, Fabio Martinelli, Claudio Tripodo, Domenica Lorusso, Filippo De Braud, Francesco Raspagliesi, Mario P. Colombo. OX40 expression in tumor-associated Tregs as a potential prognostic biomarker and immunotherapeutic target in ovarian cancer. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr B157.