CD8+ T cells in the tumor immune microenvironment modulate resistance to endocrine therapy in hormone receptor-positive breast cancer

Enhanced immune infiltration in the tumor immune microenvironment (TIME) is associated with better survival outcomes in patients with triple negative and HER2+ breast cancers. In the hormone-dependent hormone receptor-positive (HR+) subtype, previous studies have identified a subgroup with enhanced immune infiltration. However, the biological significance and role of the TIME in modulating the response to anti-estrogen therapy in breast cancer are not well understood. Herein, using cyclic immunofluorescence and spatial transcriptomics (ST), we dissected the TIME in primary breast cancers that were sensitive or resistant to estrogen deprivation (ED) induced by the aromatase inhibitor letrozole. Stromal tumor-infiltrating lymphocytes (TILs) were increased in ED-resistant vs. ED-sensitive tumors. RNA-sequencing from on-treatment tumors showed differentially regulated pathways, including immune-related gene sets, such as upregulation of IFNɣ and IFNα signaling and allograft rejection. Spatial transcriptomics using GeoMx identified increased antigen processing and immune gene expression signatures in both cancer cells and neighboring immune cells in ED-resistant vs. ED-sensitive tumors. Using CIBERSORT, we uncovered a differential distribution of immune cell subtypes, with CD8+ T cells being enriched in ED-resistant tumors and Tregs in ED-sensitive tumors. The expression of chemokine genes involved in CD8+ T cell recruitment, CXCL9, CXCL10, and CXCL11, was upregulated in ED-resistant primary tumors before and after letrozole treatment. Additionally, CXCL11 levels were higher in media conditioned from HR+ breast cancer cells co-cultured with CD8+ T cells. Both recombinant CXCL11 and co-culture with CD8+ T cells stimulated the growth of HR+ breast cancer cells when deprived of estrogen, akin to the scenario of patients treated with letrozole. Taken together, these data suggest that CD8+T cells in the TIME modulate the response of HR+ breast cancer cells to therapeutic estrogen suppression.