ICAM1+ neutrophils promote chronic inflammation via ASPRV1 in B cell-dependent autoimmune encephalomyelitis.

Neutrophils contribute to demyelinating autoimmune diseases, yet their phenotype and functions have been elusive to date. Here, we demonstrate that ICAM1 surface expression distinguishes extra- from intravascular neutrophils in the mouse CNS during experimental autoimmune encephalomyelitis (EAE). Transcriptomic analysis of these 2 subpopulations indicated that neutrophils, once extravasated, acquire macrophage-like properties, including the potential for immunostimulation and MHC class II-mediated antigen presentation. In corroboration, super-resolution (3D stimulated emission-depletion [STED]) microscopy revealed neutrophils forming synapses with T and B cells in situ. Further, neutrophils specifically express the aspartic retroviral-like protease ASPRV1, which increases in the CNS during EAE and severe cases of multiple sclerosis. Without ASPRV1, mice immunized with a new B cell-dependent myelin antigen (but not with the traditional myelin oligodendrocyte glycoprotein peptide) develop a chronic phase of EAE that is less severe and even completely fades in many individuals. Therefore, ICAM1+ macrophage-like neutrophils can play both shared and nonredundant roles in autoimmune demyelination, among them perpetuating inflammation via ASPRV1.

Neutrophil perversion in demyelinating autoimmune diseases: Mechanisms to medicine.

Neutrophils are essential to a healthy life, yet pose a threat if improperly controlled. Neutrophil perversion is well documented in a variety of inflammatory disorders (e.g. arthritis, lupus, psoriasis), but is only beginning to be demystified in autoimmune demyelination, the most common cause of neurological disability in young adults. Using the animal model experimental autoimmune encephalomyelitis (EAE), several molecules that help neutrophils invade the central nervous system (CNS) have been identified. Mechanisms by which neutrophils may contribute to demyelination have also been proposed (e.g. secretion of endothelial/leukocytic modulators, antigen presentation to T cells, myelin degradation and phagocytosis). In human, neutrophils are seen in the CNS of people with neuromyelitis optica spectrum disorder and other severe variants of autoimmune demyelinating diseases. At the time of autopsy for multiple sclerosis (MS) - often many years after its onset - neutrophils appear to have escaped the scene of the crime. However, new clues implicate neutrophils in MS relapses and progression. This warrants further investigating 1) the differential importance of neutrophils among demyelinating diseases, 2) the largely unknown effects of current MS therapies on neutrophils, and 3) the potential of neutrophil proteins as clinical biomarkers or therapeutic targets.