Pivotal Role for CD16+ Monocytes in Immune Surveillance of the Central Nervous System

Abstract Monocytes represent a heterogeneous population of primary immune effector cells. At least three different subsets can be distinguished based on expression of the low-affinity FcγRIII: CD14++CD16− classical monocytes, CD14++CD16+ intermediate monocytes, and CD14+CD16++ non-classical monocytes. Whereas CD16− classical monocytes are considered key players in multiple sclerosis (MS), little is known on CD16+ monocytes and how they contribute to the disease. In this study, we examined the frequency and phenotype of monocyte subpopulations in peripheral blood, cerebrospinal fluid (CSF), and brain biopsy material derived from MS patients and controls. Furthermore, we addressed a possible monocyte dysfunction in MS and analyzed migratory properties of monocyte subsets using human brain microvascular endothelial cells. Our ex vivo studies demonstrated that CD16+ monocyte subpopulations are functional but numerically reduced in the peripheral blood of MS patients. CD16+ monocytes with an intermediate-like phenotype were found to be enriched in CSF and dominated the CSF monocyte population under noninflammatory conditions. In contrast, an inversed CD16+ to CD16− CSF monocyte ratio was observed in MS patients with relapsing-remitting disease. Newly infiltrating, hematogenous CD16+ monocytes were detected in a perivascular location within active MS lesions, and CD16+ monocytes facilitated CD4+ T cell trafficking in a blood–brain barrier model. Our findings support an important role of CD16+ monocytes in the steady-state immune surveillance of the CNS and suggest that CD16+ monocytes shift to sites of inflammation and contribute to the breakdown of the blood–brain barrier in CNS autoimmune diseases.