Significance of Serum TRACP in Rheumatoid Arthritis

Human serum contains two related isoforms of TRACP: TRACP 5a and TRACP 5b. Serum TRACP 5a protein is increased in about one third of rheumatoid arthritis (RA) sera. This study was undertaken to examine the significance of serum TRACP isoforms 5a and 5b as disease markers of inflammation and bone destruction in RA. One hundred eighteen patients were recruited including 50 with RA (25 with nodules), 26 with osteoarthritis (OA), and 42 with other rheumatic diseases. Twenty-six healthy adults served as controls. Serum TRACP 5a activity, TRACP 5a protein, and TRACP 5b activity were determined by in-house immunoassays. C-reactive protein (CRP) was determined by in-house immunoassay using commercial antibodies and CRP. Other commercial markers included bone-specific alkaline phosphatase (BALP), C-telopeptides of type-I collagen (ICTP), cartilage glycoprotein-39 (YKL-40), and IgM rheumatoid factors (IgM-RF). Mean TRACP 5a protein was significantly elevated only in RA compared with healthy controls and other disease groups. TRACP 5a protein correlated significantly only with IgM-RF in RA. Among RA patients, mean TRACP 5a protein and IgM RF were significantly higher in nodule formers. In contrast, TRACP 5b activity was slightly elevated in RA and correlated with BALP, ICTP, and YKL-40 but not with IgM-RF or CRP. Mean TRACP 5b activity was no different in RA patients with or without nodules. TRACP isoforms could be useful disease markers in RA; TRACP 5a protein may be a measure of systemic inflammatory macrophage burden and disease severity. TRACP 5b activity is a marker for osteoclast number and perhaps local or systemic bone destruction.