M2 Macrophage Polarization Characterizes an Immunosuppressive Microenvironment in Extracranial Arteriovenous...

Background: Extracranial arteriovenous malformations (eAVMs) are aggressive vascular anomalies consisting of abnormal blood vessels (BVs) and multiple other cell types, including macrophages. Although inflammation and the presence of immune cells are characteristics of eAVMs, the contribution of macrophage polarization to eAVM pathophysiology is unknown. Methods: In this study, pediatric eAVM tissues and adjacent control tissues were analyzed using immunohistochemistry (IHC) and immunofluorescence (IF) to assess M1 and M2 macrophage localization, loss of endothelial CD31 expression, and expression of the immune-regulatory protein PDL-1. In addition, serum samples from eAVM patients were analyzed using a human inflammation antibody array to profile cytokines and other circulating factors associated with M2 macrophage and immunosuppressive microenvironment. Results: eAVM tissues demonstrate accumulation of M2-polarized macrophages around abnormal CD31^-ve BVs, while M1 macrophages were primarily associated with normal appearing CD31^+ve vessels. eAVM tissues demonstrated increased expression of PD-L1 in regions enriched with M2 macrophages, which were absent in paired control tissues. Serum analysis revealed increased levels of circulating factors associated with M2 macrophages and immune suppression, including PDGF-BB, IL-4, and IL-16. Conclusions: These findings suggest that CD31^-ve vessels in eAVMs are associated with enrichment of M2 macrophages and a microenvironment suggestive of localized immune regulation. These observations are hypothesis-generating and warrant validation in larger patient cohorts and future mechanistic studies.