Characterization of the tumor immune microenvironment in pregnancy-associated breast cancer through multiplex immunohistochemistry and transcriptome analyses.

Background: Pregnancy-associated breast cancer (PABC) is breast cancer diagnosed during pregnancy or within 2 years postpartum. Although relatively rare, it is associated with a poor prognosis, and the underlying mechanisms contributing to this unfavorable condition remain incompletely understood. In this study, we investigated tumor microenvironmental features linked to pregnancy and lactation in an effort to elucidate these mechanisms.

Methods: This retrospective study included 26 patients with PABC, 51 patients with breast cancer diagnosed 2-5 years postpartum (post-weaning breast cancer [PWBC]), and 28 patients with no prior history of pregnancy at the time of breast cancer diagnosis (nulliparous breast cancer [NPBC]). The tumor immune microenvironment in PABC, PWBC, and NPBC cases was profiled using Opal Polaris 7 color immunohistochemistry (IHC) and the NanoString Breast Cancer 360 Gene Expression Panel.

Results: No significant differences in tumor stage or molecular subtype were observed among the PABC, PWBC, and NPBC groups. The age of diagnosis was comparable between NPBC and PABC patients (38.0 vs. 35.4 years), but significantly higher in the PWBC group (42.2 years). Both multiplex IHC and transcriptomic analyses consistently demonstrated that the PABC and PWBC groups exhibited a higher abundance of tumor-infiltrating immune cells than the NPBC group. Specifically, multiplex IHC analysis revealed that PABC and PWBC were associated with increased densities of CD4⁺, CD8⁺, CD20⁺, and CD68⁺CD163⁺ cells. Consistently, transcriptomic analysis indicated that the PABC and PWBC groups exhibited elevated gene expression signatures associated with macrophages, cytotoxic cells, CD8⁺ T cells, and B cells compared with the NPBC group. The primary differences observed between the PABC and NPBC groups were validated using three publicly available datasets from the Gene Expression Omnibus.

Conclusions: Using multiplex IHC and transcriptome analyses, this study demonstrated that PABC was associated with a higher abundance of immune cells, including increased infiltration of T cells, B cells, and macrophages, in the breast tumor microenvironment. Future research is required to focus on the role of immune cells in pregnancy-associated breast cancer patients.