Increased levels of lactoferrin in synovial fluid but not in serum from patients with rheumatoid arthritis

Lactoferrin is a multifunctional immunoregulatory protein, stored in specific granules of neutrophil granulocytes, from which it is released following cell activation. As activated neutrophils play a crucial role in the destruction of synovial joints in rheumatoid arthritis, we evaluated lactoferrin concentration in synovial fluid and sera from 21 patients with rheumatoid arthritis and 11 patients with osteoarthritis. We also measured lactoferrin levels in sera from 12 healthy controls. Lactoferrin was measured by a solid-phase inhibition immunoassay. Median lactoferrin levels were significantly higher in synovial fluid from rheumatoid arthritis than from osteoarthritis patients (P=0.0002). In contrast, no significant difference was found between serum lactoferrin from patients with rheumatoid arthritis or osteoarthritis compared with normal controls. In patients with rheumatoid arthritis, lactoferrin concentrations were higher in synovial fluid than in sera (P=0.036). In both rheumatoid arthritis and osteoarthritis no correlation was found between serum and synovial fluid lactoferrin (P=0.51 and P=0.5, respectively). In synovial fluid from patients with rheumatoid arthritis, lactoferrin concentrations correlated with neutrophil granulocyte count (P<0.0001), but neither serum nor synovial lactoferrin levels correlated with disease activity (P=0.32 and P=0.25, respectively). In conclusion, lactoferrin is a reliable marker of neutrophil activation at sites of inflammation in rheumatoid synovitis, but does not represent a marker of disease activity.