Multi-tiered proteome analysis displays the hyper-permeability of the rheumatoid synovial compartment for plasma proteins.

Rheumatoid arthritis (RA) is characterized by synovial hyperplasia and cartilage/bone destruction. RA affects the synovial joints, the synovial lining and the permeability of the synovium. As the latter is of central relevance for the distribution of systemically delivered therapeutics into synovial fluid (SF), we here assessed the protein composition of paired plasma and SF of patients diagnosed with RA at three distinct levels of depth using mass spectrometric approaches: the "total" proteome, the "total" IgG1 antibody repertoire and the RA-specific ACPA IgG1 autoantibody repertoire. The SF proteome was found to be dominated in numbers and in concentration by plasma proteins, although we additionally detected several cartilage- and neutrophil-derived proteins of lower abundance. Strikingly, the plasma proteins were not only qualitatively reflected in SF but also quantitatively, independent of their size and/or other biochemical features. Also, the synovial "total" IgG1 and autoreactive ACPA IgG1 repertoire highly resembled the IgG1 repertoires detected in plasma within the same patient. Our comprehensive multilayer data thus reveals that the proteome, including the dominant, most abundant (auto)antibody clones, present in SF of RA patients is a direct reflection of the proteome present in blood, spiked by the local (immune) processes within the RA joint. We thus conclude that proteins directly pass from blood into SF of these joints without substantial bias. These findings thereby not only exemplify the use of in-depth multilayer proteome analyses to revisit basic concepts underlying RA pathology and to monitor the local (immune) processes destructive to cartilage, but also provide evidence indicating that (protein-based) therapeutics may equally enter SF of swollen joints and that pharmacokinetic analyses of such therapeutics in blood are directly relevant to the synovial compartment.