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.
Autoimmune diseases like Multiple Sclerosis (MS) result from poorly understood interactions between genetic background of the individual and his/her environment. We find that in mice and humans, genetic and metabolic regulation of a pathway that controls the addition of specific sugars to proteins (i.e. protein glycosylation) controls immune hyperactivity and susceptibility to autoimmunity. Here we further explore the mechanisms by which protein glycosylation regulates immune function and autoimmunity.
