PARP Poly (ADP-ribose) polymerase inhibitors (PARPi) are treatment options in TNBC patients with inherited BRCA1 mutations. Recent studies have shown that immune checkpoint therapies (ICT) in combination with PARPi are effective in other solid cancer types such as melanoma, pancreatic and BRCA deficient ovarian cancer. In early and advanced BRCA1 TNBC, ICT studies are limited and not fully elucidated especially for new ICT that target LAG-3, CTLA-4, ICOS. There is a need for identifying potential prognostic markers suggesting which patients could benefit most from ICT. Therefore, we conducted a detailed single cell analysis of the tumor microenvironment (TME) mainly focused on immune checkpoint expression, CD8 T cell exhaustion and DNA damage associated with BRCA1 TNBC compared to WT TNBC. Combining single cell spatial tissue multiplexing (PhenoCycler) (BRCA1 TNBC n=77, WT TNBC n=55), scRNA seq (BRCA2 n=1, WT n=5), FLOW (PBMCs: BRCA1 n=13, BRCA2 n=17, WT n=62) as well as bulk RNA seq (BRCA1 TNBC n=35, WT TNBC n=11), we performed a detailed analysis of the TME associated with BRCA1 and WT breast cancer. We developed a 32-plex antibody panel that evaluated T-cells and other immune cells with markers of exhaustion, DNA damage and immune checkpoint expression. CD8+ T cell frequency was significantly increased in BRCA1 TNBC compared to WT TNBC (p=0.0015). A detailed CD8 T cell characterization revealed increased dysfunctional/exhausted cells (e.g., TOX+/LAG-3+/PD-1+), T cell DNA damage (CD3e+/CD8+/RAD51+, p=0.0047) and LAG-3 transcript levels in BRCA1 TNBC compared to WT TNBC (p=0.0625). Furthermore, immune checkpoint protein expression in tumor- (PD-L1) and immune cells (PD-1, LAG-3, ICOS) was increased in the TME associated with BRCA1 TNBC compared to WT TNBC. Of note, we also found that PBMCs of healthy tumor-free BRCA mutation carriers contained significantly increased CD8+/PD+1 T cells compared to WT (p=0.012). Our data indicates that BRCA1 haploinsufficiency leads to DNA damaged and dysfunctional CD8+ T cells within the TME compared to WT. Thus, the TME of BRCA1 TNBC patients is more immune suppressive due to increased exhausted CD8+ T cell frequency, their DNA damage as well as immune checkpoint expression of both tumor-/and immune cells compared to WT TNBC. The effect of BRCA1 haploinsufficiency on immune cells is important to consider when trials of ICT options are designed.

Citation Format: Dana Pueschl, Anupma Nayak, Derek A. Oldridge, William Chandler, Bria Fulmer, Sokratis Apostolidis, Danielle Bragen, Bradley Wubbenhorst, John Pluta, kConFab Investigator, Antonis Antoniou, Georgia Chenevix-Trench, E. John Wherry, Susan M. Domchek, Katherine L. Nathanson. CD8+ T cell exhaustion and their DNA damage in BRCA1 mutated TNBC [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr A050.