6P - Tumor cell-released autophagosomes (TRAPs) promote lung metastasis through inducing PD-L1 high expression of pulmonary vascular endothelial cells (PVECs) in breast cancer

Background

Development of an immunosuppressive pre-metastatic microenvironment is a prerequisite for lung metastasis of breast cancer. However, previous mechanisms have focused on immune cells rather than vascular endothelial cells. Our previous research has shown that tumor cell-released autophagosomes (TRAPs) preferentially reaches the lung rather than breast cancer cells in the mouse model. Could the pre-arrival of TRAPs promote the immunosuppressive pre-metastatic microenvironment by acting on endothelial cells?.

Methods

TRAPs-treated ECs were subjected to transcriptome analysis. T cell function was detected by flow cytometry after co-culture of TRAP-treated ECs with T cells in vitro. We injected TRAPs into mice via the tail vein or established endogenous Becn1 knockdown 4T1 tumor cells model which reduced TRAPs release. Flow cytometry will be used to analyze Immunosuppressive function of PVECs and the late lung metastases was monitored. Antibody blocking assay and HMGB1 knockdown cell lines were used to detect key DAMP on the surface of TRAPs. Inhibitor-treated ECs and TLR4 knockout mice were used to detect corresponding functional receptors on the surface of ECs. CD/FillpinⅢ/CPZ -treated ECs to detect the movement of the cytoskeleton. Activation of signal pathways was detected by Western blot.

Results

TRAPs from breast tumor cell lines stimulated the upregulation of PD-L1 in PVECs via a HMGB1-TLR4–MyD88–p38/STAT3 signal cascade and depended on the movement of the cytoskeleton of ECs. TRAPs-treated ECs suppressed IFN-γ secretion and proliferation of CD4+ and CD8+ T cells in vitro and in vivo. Anti-PD-L1 treatment upregulated pulmonary T-cell function and reduced lung metastasis in tumor-bearing mice.

Conclusions

These findings elucidate a novel role and mechanism of TRAPs-induced immunosuppression of vascular endothelial cells in the pre-metastatic microenvironment. TRAPs or their surface HMGB1 represent important therapeutic targets to reverse immunosuppressive formation, while also providing a new theoretical basis for the treatment of early breast cancer with PD-L1 antibodies.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

Y. Wu, X. Zhou, C. Wu.

Funding

National Natural Science Foundation, China.

Disclosure

The author has declared no conflicts of interest.