Exploratory Analysis of the Impact of a Single Dose of Trastuzumab on the Immune Microenvironment in HER2-Positive Early-Stage Breast Cancer

<b>Background:</b> How the tumor microenvironment (TME) influences treatment response in HER2+ breast cancer following HER2-directed therapy is crucial for individualizing therapies and is currently understudied. The purpose of this exploratory analysis was to elucidate changes in the TME following treatment with trastuzumab. <b>Methods:</b> Fourteen HER2+ early-stage breast cancer patients underwent tissue biopsies before and after a dose of trastuzumab. Samples were evaluated for stromal tumor-infiltrating lymphocytes (TILs) and RNA-based cell and gene expression signatures. Tumor inflammation signature scores were generated to measure whether an adaptive immune response developed to trastuzumab within the tumor. Patients were also stratified as immune responders or non-responders based on changes in TILs. <b>Results:</b> Of the 14 enrolled patients, 13 had samples available for analysis, and 7 had an immune response as assessed by changes in TILs compared to 6 non-responders. Trastuzumab treatment decreased PD-L1 and TGF-Beta signatures and increased CTLA4 gene signatures, although results were not statistically significant, and increased <i>DUSP1</i> expression. In the TIL responder group, there was increased expression of dendritic cells as well as <i>MARCO</i> expression. <b>Conclusions:</b> These findings, although exploratory in nature, highlight trastuzumab's ability to induce an immune response and suggest that some patients may be more primed to mount an immune response following treatment than others. Patients without a robust response in TILs may benefit from additional agents to favorably modulate the TME for optimized responses to HER2-directed therapy, an area of research which warrants further study.