Spring, L.

Background Immune exclusion inhibits antitumor immunity and response to immunotherapy, but its mechanisms remain poorly defined. In triple-negative breast cancer (TNBC), an aggressive and generally immune-rich subtype, an immune-cold microenvironment predicts poor prognosis due to a limited response to chemotherapy and immune checkpoint inhibitors. This study aimed to identify mechanisms regulating immune infiltration in TNBC. Methods We performed spatial transcriptomic analysis comparing immune-enriched versus immune-cold treatment-naïve TNBCs. Functional analyses, including loss-of-function and reconstitution experiments, were conducted to investigate the role of trophoblast cell-surface antigen 2 (TROP2), a key target of anticancer antibody drug conjugates (ADCs), in promoting TNBC progression. A humanized TROP2 syngeneic TNBC model was used to assess the effects of TROP2-targeting in combination with anti-programmed cell death protein 1 (PD-1) therapy. Additionally, data from patients treated with immune checkpoint blockade were used to test hypotheses from the preclinical findings. Results We reveal that TROP2 controls barrier-mediated immune exclusion in TNBC through claudin 7 association and tight junction regulation. TROP2 expression is inversely correlated with T-cell infiltration and predicts poor outcomes in TNBC. We demonstrate that TROP2 is sufficient to drive tumor progression in vivo in a CD8 T cell-dependent manner, while its loss deregulates expression and localization of multiple tight junction proteins, enabling T-cell infiltration. We show that TROP2 targeting via hRS7, the antibody component of the ADC sacituzumab govitecan, enhances the anti-PD-1 response and improves T-cell accessibility and effector function. Correspondingly, TROP2 expression is highly associated with lack of response to anti-PD-1 therapy in human breast cancer. Conclusions This study defines a new mechanism of barrier-mediated immune exclusion in cancer controlled by TROP2-dependent tight junctions. This mechanism drives tumor progression but can be targeted via TROP2-directed therapy to activate antitumor immunity and enhance immunotherapy response.

Abstract Introduction: Hormone receptor positive (HR+), HER2- breast cancer (BC) is the most common subtype of BC. Despite advances in chemotherapy and endocrine therapy optimization, local and systemic recurrences frequently occur, especially for patients with node-positive disease. Recent data suggest a subset of patients with this tumor phenotype may benefit from the addition of anti-Programmed Death-1(aPD1) to neoadjuvant chemotherapy. However, T-cell infiltration (TCI), a biomarker of response to aPD1 and chemotherapy, is typically low in this BC subtype. Preclinical models show that combining preoperative radiation therapy (preop RT) with immune checkpoint inhibitors can enhance TCI and anti-tumor immunity. We conducted the first randomized trial to test whether no-, low- or high-dose RT combined with aPD1 increases TCI and improves responses in non-irradiated, metastatic lymph nodes in HR+/HER2- BC (NCT04443348). Materials & Methods: Eligible patients had ER and/or PR≥10%, HER2 negative invasive BC with clipped and biopsy-proven positive axillary node(s), cT1c-T4c, cN1-3, and at least one of the following criteria: grade 3 or grade 1-2 with cN2-N3, Ki67 >20%, or high genomic assay score. Between 2022-2024, 51 patients from 10 centers were randomized to no (0Gy; control arm), low (9Gy) or high (24Gy) doses of preop RT to the primary tumor with concurrent pembrolizumab followed by a 2-week tumor biopsy. Stratification factors were T1c vs. T2-4 and N1 vs. N2-3. Patients subsequently received pembrolizumab with 12 weeks of paclitaxel, followed by 4 cycles of adriamycin-cyclophosphamide (q2wk or q3wk) with pembrolizumab (200 mg q3 wks or 400 mg q6 wks). Surgery and adjuvant therapy followed standard-of-care. Dual co-primary endpoints were 2wk tumor TCI and nodal pathologic complete response (ypN0) at time of definitive surgery. TCI was assessed by multiplex immunofluorescence (pan-cytokeratin, CD3, CD8), using a rank-based Immunoscore (range 0-1), compared to baseline reference biopsies via two-tailed Mann-Whitney test. ypN0 was evaluated non-comparatively. Secondary endpoints included pCR and Residual Cancer Burden (RCB) 0/1. Results: Among 51 patients enrolled, 49 were evaluable for TCI and 48 for the ypN0 endpoint. Median age was 49.5 years (range 23-78). The cohort had a high-risk clinical profile: 92% had cT2-4, 50% N2-3, and 62% grade 3 tumors. Median ER and PR positivity were 90% (20-100) and 68% (1-98), respectively. Multifocal or multicentric tumors were present in 32%. TCI was assessed at 2 weeks post-treatment. The proportion of tumors with TCI in the upper quartile increased with RT dose escalation: 31% (0Gy), 40% (9Gy), and 53% (24Gy). Median TCI scores at 2 weeks were 0.60 (0Gy), 0.56 (9Gy), and 0.82 (24Gy), compared to a median baseline value of 0.50 from untreated reference samples. A statistically significant increase in TCI was observed only in the 24Gy cohort relative to the untreated group (p=0.027). 70.8% underwent mastectomy, with 39.6% receiving immediate reconstruction. 8% experienced G2/3 wound complications, balanced across arms. Median number of lymph nodes removed was 6.5 (1-42). The overall ypN0 rate was 29%, and 24%(0Gy), 29%(9Gy) and 33%(24Gy) per arm. pCR/RCB 0-1 rates were 18%/27% in the entire cohort and 6%/18%(0Gy), 29%/29%(9Gy), and 19%/33%(24Gy), respectively. Conclusion: The addition of 24Gy preop RT to aPD1 significantly increased tumor TCI in HR+/HER2- early-stage BC. Non-irradiated nodal response rates were promising despite over one-third of the study population with grade 1/2 tumors and high disease burden, laying the foundation for a future randomized trial comparing the efficacy of this novel approach against standard of care for node-positive HR+/HER2- BC. Citation Format: G. Gupta, A. Y. Ho, F. Gharibpoor, Y. Abdou, J. D. Anampa, R. C. Blitzblau, B. C. Calhoun, D. L. Casey, S. F. Dent, L. A. Carey, J. R. Bellon, L. E. Warren, J. Lu, T. J. Hieken, A. G. Taghian, A. Bardia, T. Traina, G. Plitas, K. Gallagher, K. Daugherty, A. Thompson, I. Krop, A. Wolff, E. Mittendorf, J. Fox, H. Garber, E. Hwang, A. J. Khan, J. P. Loene, R. Mutter, S. Patil, C. Santa-Maria, B. V. Vincent, J. L. Wright, L. Spring.Primary Results from the HR+/HER2- Cohort of TBCRC-053 (P-RAD): A Randomized Trial of No, Low, or High Dose Preoperative RADiation with Pembrolizumab and Chemotherapy in Node-Positive, HER2-Negative Breast Cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr GS2-05.

ABSTRACT Background Immune exclusion inhibits anti-tumor immunity and response to immunotherapy, but its mechanisms remain poorly defined. In triple-negative breast cancer (TNBC), an aggressive and generally immune-rich subtype, an immune-cold microenvironment predicts poor prognosis due to a limited response to chemotherapy and immune checkpoint inhibitors. This study aimed to identify mechanisms regulating immune infiltration in TNBC. Methods We performed spatial transcriptomic analysis comparing immune-enriched versus immune-cold treatment-naïve TNBCs. Functional analyses, including loss-of-function and reconstitution experiments, were conducted to investigate the role of Trophoblast Cell-Surface Antigen 2 (TROP2), a key target of anti-cancer Antibody Drug Conjugates (ADCs), in promoting TNBC progression. A humanized TROP2 syngeneic TNBC model was used to assess the effects of TROP2-targeting in combination with anti-PD1 therapy. Additionally, data from patients treated with immune checkpoint blockade were used to test hypotheses from the preclinical findings. Results We reveal that TROP2 controls barrier-mediated immune exclusion in TNBC through Claudin 7 association and tight junction regulation. TROP2 expression is inversely correlated with T cell infiltration and predicts poor outcomes in TNBC. We demonstrate that TROP2 is sufficient to drive tumor progression in vivo in a CD8 T cell-dependent manner, while its loss deregulates expression and localization of multiple tight junction proteins, enabling T cell infiltration. We show that TROP2 targeting via hRS7, the antibody component of the ADC Sacituzumab govitecan (SG), enhances the anti-PD1 response and improves T cell accessibility and effector function. Correspondingly, TROP2 expression is highly associated with lack of response to anti-PD1 therapy in human breast cancer. Conclusions This study defines a new mechanism of barrier-mediated immune exclusion in cancer controlled by TROP2-dependent tight junctions. This mechanism drives tumor progression but can be targeted via TROP2-directed therapy to activate anti-tumor immunity and enhance immunotherapy response. What is already known on this topic Barrier-mediated immune exclusion is emerging as a mechanism of immune evasion in cancer such as triple-negative breast cancer (TNBC), but its molecular underpinnings remain poorly understood. TROP2 is a surface glycoprotein targeted by antibody-drug conjugates such as Sacituzumab govitecan, yet its functional role beyond drug delivery has been unclear. What this study adds This study identifies TROP2 as a key regulator of tight junction-mediated immune exclusion in TNBC, independent of its intracellular signaling function. TROP2 promotes an immune-cold tumor microenvironment by enforcing mechanical barriers that limit T cell infiltration. How this study might affect research, practice or policy These findings establish TROP2 as a functional driver of immune evasion and provide a mechanistic rationale for combining TROP2-targeting therapies with immune checkpoint inhibitors to overcome resistance in immune-excluded tumors.