Abstract
Anti-neutrophil cytoplasmic autoantibodies (ANCAs) are directed against lysosomal components of neutrophils. ANCAs directed to proteinase 3 and myeloperoxidase (MPO) in particular are associated with distinct forms of small vessel vasculitides. MPO is an abundant neutrophil-derived heme protein that is part of the antimicrobial defense system. The protein is typically present in the azurophilic granules of neutrophils, but a large portion may also enter the extracellular space. It remains unclear why MPO is frequently the target of antibody-mediated autoimmune responses. MPO is a homodimeric glycoprotein, posttranslationally modified with complex sugars at specific sites. Glycosylation can strongly influence protein function, affecting its folding, receptor interaction, and backbone accessibility. MPO potentially can be heavily modified as it harbors 5 putative N-glycosylation sites (10 in the mature dimer). Although considered important for MPO structure and function, the full scope and relative abundance of the glycans attached to MPO is unknown. Here, combining bottom-up glycoproteomics and native MS approaches, we structurally characterized MPO from neutrophils of healthy human donors. We quantified the relative occupancy levels of the glycans at each of the five sites and observed complex heterogeneity and site-specific glycosylation. In particular, we detected glycosylation phenotypes uncommon for glycoproteins in the extracellular space, such as a high abundance of phosphorylated high-mannose species and severely truncated small glycans having the size of paucimannose or smaller. We hypothesize that the atypical glycosylation pattern found on MPO might contribute to its specific processing and presentation as a self-antigen by antigen-presenting cells.
Highlights
Anti-neutrophil cytoplasmic autoantibodies (ANCAs) are directed against lysosomal components of neutrophils
ANCA can be found in a variety of autoimmune disorders, they are most common in distinct forms of small vessel vasculitis, the term ANCA-associated vasculitis (AAV) which includes granulomatosis with polyangiitis (GPA) and microscopic polyangiitis (MPA) [1, 2]
ANCA that target proteinase 3 are typically found in GPA patients whereas ANCA against myeloperoxidase (MPO) are predominantly observed in MPA patients [3]
Summary
Anti-neutrophil cytoplasmic autoantibodies (ANCAs) are directed against lysosomal components of neutrophils. The protein is typically present in the azurophilic granules of neutrophils, but a large portion may enter the extracellular space. It remains unclear why MPO is frequently the target of antibody-mediated autoimmune responses. MPO, one of the most common ANCA autoantigens, is an important and abundant enzyme within the host defense system [4]. It catalyzes the conversion of hydrogen peroxidase and chloride anions into hypochlorous acid, a toxin with bactericidal properties [5]. The glycosylation of MPO has been indicated to affect the interaction with ANCA as well [9, 10]
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