Regulation of B cell function in demyelinating disease by N-glycan branching

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Abstract Our published work has revealed that deficiencies in Asn (N)-linked protein glycosylation reduce inflammatory demyelination in mice and are associated with Multiple Sclerosis (MS). Deficiency in the branching of N-glycan's attached to proteins, either induced experimentally in mice or via natural genetic variation in humans, promotes T-cell mediated inflammatory demyelination and neurodegeneration. For example, branching deficiency induces a spontaneous and slowly progressive MS-like disease in PL/J mice, characterized by inflammatory demyelination, axonal damage and neuronal death. Mechanistically, the branching and number of N-glycans per protein molecule cooperate to regulate binding to galectins, a 14- member family of sugar binding proteins. Galectin binding to cell surface glycoproteins, via their attached N- glycans, forms a macro-molecular lattice at the cell surface that controls the distribution, clustering and endocytosis of surface glycoproteins in a coordinated and predictable manner. N-glycan branching markedly inhibits T cell activity in mice and humans by reducing T cell receptor clustering/signaling at the immune synapse, promoting surface retention of the growth inhibitor CTLA-4 and inhibiting differentiation into pro- inflammatory TH1 and TH17 cells while promoting anti-inflammatory iTreg and TH2 cell differentiation. Although these T cell phenotypes are important regulators of inflammatory demyelination, it has become increasing clear that B cells also play a critical role in MS. This is best exemplified by the potent activity of B cell depleting therapies in MS, such as the anti-CD20 monoclonal antibody ocrelizumab. B cells are unique in the immune system by having both innate and adaptive immune activity; the former exemplified by activation via Toll-like receptors (TLR) and antigen-presenting cell (APC) functions that trigger T cell responses. The mechanism of action of ocrelizumab appears to primarily result from reduced innate immune activity rather than altering antibody production, as ocrelizumab reduces T cell number but not antibody or plasma cell levels in the cerebral spinal fluid of treated MS patients. Here we test the hypothesis that N-glycan branching serves as a critical negative regulator of pro-inflammatory innate immune activity in B cells to suppress pro- inflammatory T cell responses and inflammatory demyelination. To evaluate this hypothesis, the following Aims are proposed. Aim 1 examines regulation of TLR4 and TLR2 responses by N-glycan branching in B cells. Aim 2 examines regulation of B cell receptor signaling by N-glycan branching. Aim 3 examines whether N- glycan branching in B cells suppresses inflammatory demyelination. Positive results will identify N-glycan branching as a major contributor to B cell mediated regulation of inflammatory demyelination and has implications for understanding the mechanism of action of B cell depleting therapies in MS.