Transforming growth factor beta orchestrates PD-L1 enrichment in tumor derived exosomes and mediates CD8 T cell dysfunction regulating early phosphorylation of TCR signalome in breast cancer

Tumor cells promote immune evasion through up regulation of PD-L1 that binds with PD1 on cytotoxic T cells and promote dysfunction. Though therapeutic efficacy of anti PD1 antibody has remarkable effects on different type of cancers it is less effective in breast cancer. Hence more details understanding of PD-L1 mediated immune evasion is necessary. Here we report breast cancer cells secrete extra cellular vesicles in form of exosomes carry PD-L1 and is highly immunosuppressive. TGF-β present in tumor micro-environment orchestrates breast cancer cell secreted exosomal PD-L1 load. Circulating exosomal PD-L1 content is highly correlated with tumor TGF-β level. The later also found to be significantly associated with CD8+CD39+, CD8+PD1+ T cell phenotype. Recombinant TGF-β1 dose dependently induces PD-L1 expression in Texos in vitro and blocking of TGF-β dimmed exosomal PD-L1 level. PD-L1 knocked down exosomes failed to suppress effector activity of activated CD8 T cells like tumor exosomes. While understanding its effect on TCR signalling we found siPD-L1 exosomes failed to block phosphorylation of src family proteins, LAT and PLCγ of CD8 T cells more than PD-L1 exosomes. In-vivo inhibition of exosome release and TGF-β synergistically attenuates tumor burden by promoting Granzyme and IFN-γ release in tumor tissue depicting rejuvenation of exhausted T cells. Thus we establish TGF-β as a promoter of exosomal PD-L1 and unveil a mechanism that tumor cells follow to promote CD8 T cell dysfunction.

Keywords: PD-L1; T cell dysfunction; TGF-β; Tumor derived exosomes; breast cancer.