Exosomal HMGB1 educate PD-L1+-tumor associated macrophages via glycometabolic reprogramming to induce lung-tropic metastasis of triple negative breast cancer.

1090 Background: Lung metastasis occurs in one third of triple negative breast cancer (TNBC), the underlying molecular mechanism remains largely unclear. Mounting evidence suggests that PD-L1 + -tumor associated macrophages (PD-L1 + -TAMs) are closely associated with immune suppression, invasion, and metastasis. Tumor-derived exosomes have been reported to remodel tumor microenvironment and accelerate metastasis through packing and delivering a variety of biologically active molecules. The aim of this study was to investigate whether TNBC-derived exosomes were able to polarize lung macrophages towards PD-L1 + -TAMs and promote a pre-metastatic immunosuppressive niche, therefore discovering the underlying mechanism of lung-tropic metastasis. Methods: Exosomes obtained from EO771 TNBC cell line (EO771/exo) and HC11 normal breast epithelial cell line (HC11/exo) were characterized. Absorption of exosomes by macrophages was observed by confocal microscopy. Mass spectrometry was used to identify the significantly highly expressed HMGB1 protein in EO771/exo, and ligands of HMGB1 on lung macrophages were detected by co-immunoprecipitation. Significance of EO771/exo and HMGB1 in inducing PD-L1 + -TAMs and generating a pre-metastatic immunosuppressive niche were confirmed by both in vitro and in vivo studies. Functions of EO771/exo and HMGB1 on glycolysis and lactate production were evaluated by metabolism assays. Results: EO771/exo could polarize macrophages toward PD-L1 + -TAMs and generate a pre-metastatic immunosuppressive niche. Mass spectrometry, clinical samples, and bioinformatics analysis showed that HMGB1 was significantly expressed in EO771/exo and was involved in cellular communication and metabolism processes. Co-immunoprecipitation and 3-dimensional modeling suggested a strong binding of HMGB1 with Tim-3 ligand on lung macrophages. Macrophages incubated with EO771/exo exhibited an enhanced glycolysis and lactate production. Flow cytometry demonstrated that green fluorescent protein (GFP)-EO771 tumor-bearing mice had more lung micro-metastases than brain and liver. Immunofluorescence and laser confocal microscopy demonstrated that EO771/exo were able to increase lung micro-metastases and escalate PD-L1 expression on lung macrophages. Moreover, elevated PD-L1 + -TAMs and reduced CD3 + CD8 + T lymphocytes were detected in TNBC positive axillary lymph nodes and lung metastasis specimens than TNBC negative axillary lymph nodes. Conclusions: Exosomal HMGB1 from TNBC could target Tim-3 ligand on lung macrophages and accelerate glycolysis and lactate production to induce PD-L1 + -TAMs, thereby suppressing CD3 + CD8 + T lymphocytes immunity and generating a pre-metastasis immune-suppressive niche. Our research would uncover a novel mechanism of lung-tropic metastasis.