Prognostic Significance of Tumor-Infiltrating Lymphocytes and the Tertiary Lymphoid Structures in HER2-Positive Breast Cancer Treated With Adjuvant Trastuzumab

Objectives: Tumor-infiltrating lymphocytes (TILs) have prognostic significance in breast cancer. The tertiary lymphoid structure (TLS) is related to the influx of TILs, and expression of major histocompatibility complex (MHC) I in tumor cells is necessary for the effective action of TILs. Methods: We retrospectively evaluated the relationship of TILs and TLS and the expression of MHC I in 447 HER2-positive breast cancers treated with chemotherapy and 1 year of trastuzumab. Results: TILs were more abundant in hormone receptor (HR)–/HER2+ tumors than in HR+/HER2+ tumors. HR–/ HER2+ breast cancers with abundant TILs showed a higher histologic grade, the absence of lymphovascular invasion, the presence of peritumoral lymphocytic infiltration, moderat