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Reduced systemic IgG levels against peptidoglycan in rheumatoid arthritis (RA) patients

The gut flora is believed to play a role in the pathogenesis of RA. Peptidoglycan, a major cell wall component of Gram‐positive bacteria, is a candidate antigen because of its capability to trigger production of proinflammatory cytokines, to induce arthritis in rodents, and because of its presence in antigen‐presenting cells in RA joints. We investigated whether the systemic and local antibody levels against a peptidoglycan–polysaccharide (PG–PS) are related to the presence and disease activity of RA. Significantly lower levels of systemic IgG directed against PG–PS were found in healthy females compared with healthy males, and systemic IgA levels specific for PG–PS were negatively correlated with age. Levels of systemic IgG directed against PG–PS were significantly reduced in RA patients compared with sex‐ and age‐matched healthy controls. Local (synovial fluid) levels of IgG did not correlate with disease activity whereas synovial fluid levels of IgA correlated positively with disease activity. These data suggest that IgG in healthy people mediates protection against spreading of PG to non‐mucosal sites.