981P - Reactive stroma mediates CD8+ T cell spatial distribution and function in ovarian cancer

Abstract

Background: Close proximity between cytotoxic T cells and tumor cells is key to effective immunotherapy. Ovarian cancer exhibits diverse immune phenotypes with distinct prevalence and spatial localization of CD8+ T cells. This study is aimed to characterize the molecular mechanisms orchestrating the localization and function of CD8+ T Cells in ovarian cancer. Methods: CD8 IHC and RNAseq were performed on 277 ovarian tumor tissues from ICON7 phase 3 trial. CD8 T-cells in tumor vs. stromal area was assessed by digital pathology. A Random Forest regression model was constructed to identify molecular features associated with enumeration or spatial localization of CD8+ T cells. In situ validation was performed by MHCI IHC and FAP RNAish. Functional role of ovarian fibroblasts was characterized by ex vivo T cell function assays. Results: We identified three main immune phenotypes, including T-cell infiltrated, T-cell excluded, and immune desert. The immune phenotypes are highly associated with prognosis and the molecular subtypes of ovarian cancer. The T-cell infiltrated phenotype is denoted by high expression of T-effector signatures and antigen presentation machinery. The T-cell excluded phenotype showed similar expression of T-effector signatures as the T-cell infiltrated phenotype, however, most of the CD8+ T-cells were excluded from the tumor bed. The T-cell excluded phenotype showed high expression of the reactive stroma signatures (i.e. FAP), and low expression of class I antigen presentation genes. Lastly, the immune desert phenotype featured low prevalence of CD8+ T-cells, and high expression of neuroendocrine and metabolic pathways. In situ analysis confirmed the two key molecular features associated with the T-cell excluded phenotype: 1) loss of the MHC I expression in the tumor compartment, and 2) high FAP expression in CAFs. Co-culturing of ovarian fibroblast cells with T-cells resulted in reduced T-cell activation and proliferation. Conclusions: Our study uncovered key molecular mechanisms mediating the interplay between CD8+ T cell localization and function in ovarian cancer. Our findings underscore the potential of targeting reactive stroma as a novel therapeutic strategy to optimize immunotherapy for ovarian cancer patients. Legal entity responsible for the study: Genentech Funding: Genentech Disclosure: All authors have declared no conflicts of interest.