Antibody reactivity to bacterial lipopolysaccharide in rheumatoid arthritis: insights from Proteus mirabilis O3.

Lipopolysaccharides and their O-polysaccharide are structurally diverse bacterial carbohydrates polymers that strongly influence immune recognition. Here, we investigated human affinity-purified IgG antibodies recognizing the structurally defined lysine containing Proteus mirabilis O3 LPS, focusing on the contribution of glycan-amino acid motifs to antibody binding specificity. Antibodies from rheumatoid arthritis patients and healthy donors displayed comparable affinity and avidity toward Proteus mirabilis O3 LPS, although significantly higher amounts of LPS-binding IgG were isolated from patients' sera. Using synthetic glycan-amino acid haptens mimicking O-polysaccharide fragments, we demonstrated that antibody recognition depended strongly on glycan composition and spatial arrangement. Lysine-containing motifs, including the proposed Lys-GalA epitope, showed weak reactivity, whereas serine- and threonine-containing glycoconjugates displayed stronger binding. Purified antibodies also reacted with type I collagen and human fibroblasts, indicating broad polyspecificity toward glycan-rich structures. Increased IgG2 and IgG4 reactivity was observed in rheumatoid arthritis patients, and antibody complexes formed on fibroblasts efficiently recruited C1q. Together, the data suggest that antibodies isolated on P. mirabilis O3 LPS recognize shared structural and physicochemical features of glycan-rich antigens rather than a single dominant glycan epitope.