Abstract PARP inhibitors combined with immune checkpoint blockade (ICB) benefit subsets of patients with BRCA1/2-mutated breast cancer, yet the immune mechanisms driving differential responses remain unclear. Leveraging the neoadjuvant window in a prospective phase II cohort, we performed single-cell RNA sequencing with paired TCR profiling on longitudinal tumor, peripheral blood, and tumor-draining lymph node samples to map systemic and intratumoral T cell dynamics. We observed marked remodeling of the CD8⁺ compartment after therapy, with the most pronounced expansion occurring in a GZMA⁺ effector memory CD8⁺ T cell (Tem) population strongly enriched in patients achieving pathological complete response (pCR) and reduced in non-pCR tumors. This striking enrichment raised the question of how these Tem cells accumulate during treatment. TCR analysis revealed Tem expansion composed of both transitioning and emergent clonotypes. Shared TCRs with other intratumoral CD8⁺ subsets supported contributions from state transitions, whereas many Tem-associated clonotypes absent at baseline indicated a substantial treatment-emergent component and prompted investigation of their extratumoral sources. Clonal tracing identified two major reservoirs for these emergent Tem cells. Progenitor-exhausted CD8⁺ T cells (Tpex) in draining lymph nodes were enriched in pCR patients and shared TCR clonotypes with intratumoral Tem, consistent with differentiation from lymph-node precursors. CX3CR1⁺ effector CD8⁺ T cells (Teff) in peripheral blood also contributed clonally related Tem cells, and baseline Teff abundance was higher in pCR patients, supporting its potential as a predictive biomarker for PARPi plus anti-PD-1 therapy. The emergence and selective expansion of these Tem clones in responders led us to test whether they mediate tumor control and whether this function depends on GZMA. Using a BRCA1-deficient breast cancer model, we found that GZMA loss or mutation impaired tumor control, demonstrating that GZMA is required for the full anti-tumor activity of this Tem population. Overall, our study identifies GZMA⁺ Tem cells as a key effector population associated with therapeutic response in BRCA-mutated breast cancer, shows that their pool is shaped by both intratumoral transitions and treatment-emergent clones, and uncovers Tpex and Teff as major extratumoral reservoirs. Importantly, by demonstrating that their effector function critically depends on GZMA, our work highlights a coordinated regional and systemic immune program that supports tumor clearance and suggests new opportunities for biomarker development and therapeutic optimization in PARPi-ICB combinations. Citation Format: Songhan Li, Bo Liu, Fei Xie, Yuan Peng, Shu Wang, Hai Qi. GZMA+ CD8+ T cells underlie the pathological complete response of BRCA-mutated breast cancers to neoadjuvant PARPi plus anti-PD-1 therapy abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (8Suppl): Abstract nr LB424.
Li et al. (Fri,) studied this question.