Internalization of myelin debris by neutrophils fuels inflammation

Progressive neurodegeneration in the central nervous system (CNS) in multiple sclerosis (MS) is driven by chronic inflammatory demyelination. Neutrophils are increasingly recognized as versatile innate immune cells with potentially underappreciated roles in CNS inflammation, but their contribution to MS pathology remains poorly understood. Interestingly, we observed foamy neutrophils in active CNS lesions of MS patients. Therefore, we investigated the ability of human neutrophils to internalize myelin debris and assessed how this impacts their functional phenotype. Neutrophils exhibited efficient myelin uptake, peaking between 3 and 6 hours, predominantly through complement opsonization and internalization via complement receptor 3. Prolonged exposure to high concentrations of myelin induced a pro-inflammatory phenotype, marked by increased production of reactive oxygen species, neutrophil extracellular traps, and inflammatory mediators such as CXCL8 and CCL3. Gene expression analysis revealed a dose-dependent inflammatory signature after myelin uptake, characterized by gradual upregulation of CXCL8 and decreased ARG1 expression, suggesting a shift toward a pro-inflammatory neutrophil phenotype. These findings provide novel insights into the role of neutrophils in myelin clearance and inflammation in the CNS, highlighting complement receptor 3-mediated uptake and downstream pro-inflammatory activation as key mechanisms.