TGFB1 and breast cancer: By carrying high levels of TGF-β, C1q, BC-MSCs-EVs not only enhance the immunosuppressive activity of MDSCs and induce conversion to type M2 macrophages expressing high levels of PD-L1, but also reduce PD-1 expression in infiltrating T cells through upregulation of TGF-β in MDSCs, thus inhibiting the anti-tumor immune response [122].