Soluble HER2 promotes chronic antigen-mediated CD8⁺ T-cell dysfunction and limits immunotherapy response in HER2-low triple-negative breast cancer.

HER2-low triple-negative breast cancer (TNBC) has emerged as a clinically relevant subgroup with distinct therapeutic implications. However, the biological mechanisms underlying its heterogeneous response to immunotherapy remain poorly defined. In this manuscript, we conducted a retrospective translational analysis of patients with HER2-zero and HER2-low TNBC. Peripheral blood mononuclear cells (PBMCs) were analyzed by flow cytometry to characterize CD8⁺ T-cell differentiation and functional states. Circulating levels of soluble HER2 extracellular domain (HER2 ECD) were quantified by ELISA, and serum cytokine profiles were assessed using Luminex assays. In vitro assays using tumor-derived conditioned media and purified recombinant HER2 ECD were performed to evaluate the immunomodulatory effects of HER2 antigen exposure. Clinical responses to neoadjuvant chemotherapy with or without PD-1-based immunotherapy were compared between subgroups. In vivo validation was conducted using a syngeneic mouse model. Although the overall proportions of peripheral T cells were comparable between groups, HER2-low TNBC was associated with a shift in CD8⁺ T-cell differentiation toward effector and effector-memory phenotypes, accompanied by increased expression of exhaustion markers. Patients with HER2-low disease exhibited significantly elevated circulating HER2 ECD levels. In vitro, purified recombinant HER2 ECD was sufficient to induce time-dependent activation and subsequent exhaustion-like phenotypes in CD8⁺ T cells, while HER2 blockade in conditioned media partially attenuated these effects. Clinically, HER2-low tumors showed a significantly reduced pathological complete response rate following PD-1-based immunotherapy. Consistently, HER2-targeted therapy enhanced antitumor efficacy and improved CD8⁺ T-cell function when combined with PD-1 blockade in vivo. In summary, HER2-low TNBC is associated with elevated circulating HER2 antigen and a peripheral CD8⁺ T-cell state characterized by progressive differentiation and exhaustion. These findings support a model in which tumor antigen shedding contributes to systemic immune modulation and may limit responsiveness to immunotherapy. Targeting HER2-associated antigen dynamics may represent a potential strategy to improve immunotherapeutic outcomes in HER2-low TNBC.