Autoantibody-Induced Apoptosis of Neutrophils in Systemic Lupus Erythematosus Patients

Won, Dong Il; Suh, Jang Soo; Kim, Hyon Suk

Abstract

BACKGROUND
In patients with systemic lupus erythematosus (SLE), serum factors play a role in the apoptosis and necrosis of neutrophils. We intended to verify that autoantibodies including anti-dsDNA antibody are one of the factors. We also investigated the potential usefulness of simultaneous flow cytometric measurement of cytotoxicity and autoantibody binding to neutrophils in SLE sera for evaluation of disease activity. METHODS: A total of 228 sera from 48 patients with lupus nephritis (LN), 19 patients with SLE without LN, 35 patients with rheumatoid arthritis (RA) and 40 healthy males were studied. Whole blood from healthy males mixed with test sera was incubated. Autoantibody binding, apoptosis and necrosis of neutrophils were measured by flow cytometry using IgG-FITC and 7-aminoactinomycin D after a 20-hour incubation period or after adjustments of incubation conditions. The results were expressed as the test/healthy control ratio of measured values, and the correlations between these results and anti-dsDNA antibody levels were investigated. RESULTS: IgG mean fluorescence intensity (MFI) ratio was 1.0+/-0.3, 1.6+/-1.6, 2.0+/-1.5 and 4.8+/-7.5 in the healthy, RA, SLE and LN group, respectively, and showed a significant increase in the LN group when compared with the healthy group (N=20 in each group, P<0.05). Apoptosis & necrosis ratio was 1.0+/-0.2, 1.0+/-0.5, 1.6+/-1.0 and 2.6+/-1.9 in each of the above 4 groups, and showed a significant increase in the LN group when compared with the healthy (P<0.005) and RA group (P< 0.01). By immunofluorescence microscopy, increased nuclear reactivities on neutrophils by autoantibody binding were observed in 12 (60%) of 20 LN sera. All three correlations between anti-dsDNA antibody level, apoptosis & necrosis ratio and IgG MFI ratio were significant (P<0.0005). Preincubation with DNA extracts decreased both IgG MFI ratio and apoptosis & necrosis ratio significantly (N=25, P<0.05). CONCLUSIONS: Our findings confirm the previous reports of increased neutrophil apoptosis in the peripheral blood of patients with SLE. This study indicates that anti-dsDNA antibody or other antinuclear antibodies in sera are associated with an active increase in the apoptosis and necrosis of neutrophils as well as simple binding to neutrophils. This may suggest that autoantibodies increase the exposure of autoantigen DNA and exacerbate autoimmunity in the pathogenesis. Although further studies are needed, we suggest that measuring the cytotoxicity and autoantibody binding to neutrophils in SLE serum simultaneously by flow cytometry should be a useful test for evaluation of disease activity.