CD274 and neoplasm: Tumor-derived exosomes can also promote the polarization of monocytes to M2 macrophages and the expression of PD-L1 in M2-polarized macrophages through STAT3 phosphorylation, further enhancing the immunosuppressive effects [133]; however, it is not yet clear whether exosomes also activate STAT3 through TLR7 in this case.