Abstract
Breast cancer is genetically and histologically heterogenous, and is influenced by a variety of factors, including the tumor microenvironment (TME). The PAM50 subtypes; Luminal A, Luminal B, Normal-like, Basal-like and Her2-enriched, are associated with different tumor phenotypes and overall survival. The quantity and quality of immune cell infiltration in breast tumors play a key role in cancer development and progression and are associated with survival and treatment response. We used multiplex immunohistochemistry, single-cell RNA sequencing, and two deconvolution algorithms to explore the immune landscape across PAM50 subtypes. Immunostaining of CD3(+) T cells, tryptase(+) mast cells, CD20(+) B cells, CD68(+) CD163(+) macrophages, and CD66b(+) granulocytes revealed marked differences in tumor immune infiltrates according to breast cancer subtypes. Luminal tumors were relatively deprived of T cells and B cells, while exhibiting sparse to moderate amounts of macrophage infiltration. In contrast, Her2-enriched tumors exhibited a moderate immune presence, with sparse to moderate T cell infiltration and moderate infiltration of B cells and macrophages. At the other end of the spectrum, Basal-like tumors stood out for their strikingly rich immune environment and are heavily infiltrated by T cells, B cells, and macrophages. The results from the single-cell and deconvolution analyses confirmed subtype-specific immune microenvironments, which also allowed us to observe increased levels of natural killer (NK) and CD8(+) T cells in Her2-enriched and Basal-like subtypes. In conclusion, our findings demonstrate significant differences in the immune tumor microenvironment between the established breast cancer molecular subtypes.