T helper responses are maintained by basal-like breast cancer cells and confer to immune modulation via upregulation of PD-1 ligands

A conspicuous T cell infiltration is frequently observed in triple-negative and/or basal-like breast cancers. Since the immunological course of breast cancer is explicitly directed by helper T cells, this study aims to determine the influence of basal-like breast cancer (BLBC) cells on CD4(+) T cell responses. Co-cultures were established with breast cancer cell lines and CD4(+) T cells under stimulatory conditions. Helper T cell activation, proliferation, cytokine secretion, and differentiation were assessed. Protein and mRNA expression of PD-1 ligands were determined on breast cancer cell lines. Blockade assays were performed in order to determine the functional assets of PD-1 ligation. In contrast to luminal breast cancer cells, BLBC cells allowed CD4(+) T cell activation, proliferation, and IFN-gamma secretion, but only to a certain extent. A substantial population of CD25(+)CD127(low/-) regulatory T (Treg) cells was also induced in BLBC co-cultures. In return, IFN-gamma stimulated the upregulation of PD-L1 (B7-H1) and/or PD-L2 (B7-DC) inhibitory molecules on the basal-like cells. In prolonged periods of co-culturing, blockade of PD-1 ligands on BLBC cell lines impaired Treg differentiation, restored IL-2 secretion, and increased CD8(+) T cell activation. In conclusion, T helper responses were maintained by BLBC cells. On the other hand, IFN-gamma secreted from Th1 and other immune cells upregulated the expression of PD-1 ligands on BLBC cells and modulated the immune reactions. Our results indicate the capacity of BLBCs to adapt to IFN-gamma-mediated anti-tumor immune responses and to evade immunity via upregulation of PD-1 ligands.