246 Background: The transcriptional heterogeneity and immune evasion mechanisms of CTCs during systemic circulation are not well defined. Methods: Blood was drawn from 4 different vascular sites, including hepatic vein (HV), peripheral artery (PA), peripheral vein (PV) and portal vein (PoV) of 10 localized HCC patients. Single CTCs were isolated by negative enrichment and robotic micromanipulator, followed by single-cell RNA-sequencing (sc-RNAseq). After filtering, 113 CTCs with qualified data remained were subjected to further bioinformatics analysis. The scRNA-seq results were further validated in three independent cohorts of HCC patients. Results: Our scRNA-seq data revealed remarkable intra- and inter-vascular heterogeneity among CTCs from four vascular sites. We determined CTC transcriptional dynamics during transportation through consecutive vascular compartments and revealed their adaptation mechanisms under biomechanical stress during circulation. We further classified CTCs from different vascular sites into two subsets, namely dormant CTCs and activated CTCs. Dormant CTCs were associated with a non-cycling state and upregulation of EMT/angiogenic signatures and showed stronger prognostic ability for early recurrence than activated CTCs did. Furthermore, we discovered an immune escape mechanism by which CTCs recruited regulatory T cells (Tregs) via expression of CCL5, consequently promoting the formation of an immunosuppressive microenvironment favorable for their survival in the bloodstream and seeding in secondary organs. We proved that MAX, activated through the p38 pathway, was the key transcriptional factor regulating CCL5 overexpression, which was validated by ChIP, luciferase reporter gene and in vitro/vivo knockdown assays. And we further determined that Tregs-derived TGF-β1 can heighten MAX expression, thus amplifying the CCL5 expression. Conclusions: Collectively, our results reveal a previously unappreciated spatial heterogeneity of CTCs and a CTC immune-escape mechanism, which may aid in designing new anti-metastasis therapeutic strategies in HCC.
TMIC-23. ENDOTHELIN B RECEPTOR EXPRESSION IN MALIGNANT GLIOMAS: THE PERIVASCULAR IMMUNE ESCAPE MECHANISM OF GLIOMAS
