Autoantibodies against C-reactive protein: clinical associations in systemic lupus erythematosus and primary antiphospholipid syndrome. 
To investigate the prevalence of anti-C-reactive protein (CRP) autoantibodies in patients with systemic lupus erythematosus (SLE) and non-SLE patients with persistent antiphospholipid antibodies (aPL) and their association with clinical manifestations.  Sera of 137 patients with SLE, 127 with persistent aPL and 30 with idiopathic venous thromboembolic disease, were assayed for the presence of anti-CRP reactivity by ELISA.  Associations of anti-CRP reactivity with clinical features, with other autoantibodies, and with serum concentrations of C3 and CRP were assessed.  Antibodies against CRP were seen in 51% (n = 137) of patients with SLE and in 54% (n = 127) of patients with aPL.  SLE patients with anti-CRP antibodies showed increased frequencies of anti-dsDNA and aPL antibodies compared to those without anti-CRP (52% vs 26% and 68% vs 31%, respectively).  Mean serum C3 levels were lower in the subgroup of patients with SLE positive for anti-CRP antibodies (79 +/- 25 vs 92 +/- 25 mg/dl; p = 0.004 ) and mean serum CRP levels were significantly higher (13 +/- 17 vs 5 +/- 8 mg/l; p = 0.01 ).  The frequency of nephritis was higher in SLE patients with anti-CRP antibodies, than in those without (27% vs 13%; p = 0.058).  In patients with clinical and serological evidence of antiphospholipid syndrome (APS) the frequency of anti-CRP antibodies was significantly higher than in asymptomatic aPL carriers, in both SLE patients [85% (23 of 27) vs 59% (19 of 32); p = 0.021] and non-SLE patients [76% (38 of 50) vs 19% (9 of 47); p < 0.001].  Among patients with APS with or without SLE, 26 had arterial events, 31 had venous events, 6 had combined arterial and venous events, and 14 had fetal loss.  Mean titers of IgG anti-CRP (29 +/- 21, 30 +/- 19, 60 +/- 37, and 26 +/- 12 AU/ml) and frequencies of anti-CRP antibodies (88%, 71%, 50%, and 71%) in these subgroups of patients were comparable.  We confirmed the high prevalence of anti-CRP autoantibodies both in patients with SLE and in non-SLE and aPL-positive patients.  We observed that the presence of these antibodies was associated with lupus nephritis and with clinical features of the APS in patients with lupus and non-lupus patients.