Elevated serum level of soluble HLA class I antigens in patients with systemic lupus erythematosus

Objective. To examine the clinical significance of serum soluble HLA class I antigens (sHLA class I) in patients with systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Methods. Serum levels of sHLA class I were measured by enzyme-linked immunosorbent assay, using a monoclonal antibody against monomorphic determinant of HLA class I (W6/32) and an enzyme-labeled polyclonal antibody to human beta(2)-microglobulin. Results. The serum sHLA class I concentration was 1.85 +/- 1.15 mu g/ml (mean +/- SD) in 27 patients with SLE (P < 0.0001 versus normal controls, P = 0.0001 versus RA), 0.61 +/- 0.34 mu g/ml in 16 patients with RA (P = 0.02 versus normal controls), and 0.41 +/- 0.20 mu g/ml in normal controls. The HLA class I levels were significantly correlated with the SLE Disease Activity Index (r = 0.62, P = 0.0004) and with a reduction of CH50 levels (r = -0.60, P = 0.0007). A longitudinal analysis of patients with SLE indicated that serum sHLA class I levels fluctuated in conjunction with other disease activity markers. Conclusion. Serum sHLA class I may be useful as a disease activity marker of SLE. The mechanism of secretion and the physiologic role of sHLA class I require further study.