Anti‐DNA autoantibodies from an MRL//Iprmouse

Autoantigen spermatid nuclear transition protein 1 enhances pro-inflammatory cytokine production stimulated by anti-DNA autoantibodies in macrophages

Introduction

Lupus nephritis (LN), a severe manifestation of systemic lupus erythematosus (SLE), is associated with high fatality rate in patients. The pathogenesis of lupus nephritis is complex and has not been fully elucidated. Kidney inflammation, renal cell damage, and accumulation of immune complexes in the glomerular basement membrane often occur in patients with lupus nephritis. Spermatid nuclear transition protein 1 (TNP1) might be a potentially interesting autoantigen in exploring the pathogenesis and therapy of lupus nephritis.

Objective

This study aimed to explore the effect of TNP1 and its complexes with anti-double-stranded DNA antibodies on the levels of interleukin-6 (IL-6) and interferon-α (IFN-α) in vitro.

Methods

We studied the effect of the synthetic peptide of the autoantigen on the pathogenic characteristics of the G2-6 and G5-8 antibodies in mouse macrophages, using enzyme-linked immunosorbent assay, quantitative RT-PCR, western blotting, and flow cytometry.

Results

The antibodies exhibited cross-reactivity to spermatid TNP1 in direct-binding and competitive enzyme-linked immunosorbent assay. Results of quantitative RT-PCR and western blotting revealed that the antibodies alone enhanced the levels of IL-6 and IFN-α transcripts and proteins, respectively. Flow cytometry revealed that treatment with the autoantigen enhanced the cell-penetrating activities of G2-6 and G5-8 and remarkably increased the cytokine levels.

Conclusion

TNP1 enhanced the cell-penetrating activities of anti-dsDNA auto-Abs, G2-6 and G5-8, and remarkably increased the levels of IL-6 and IFN-α in macrophages, suggesting that TNP1 and cell-penetrating pathogenic anti-dsDNA auto-Abs is potential targets for future therapeutic approaches to treat LN/SLE.

Cell-penetrating autoantibody induces caspase-mediated apoptosis through catalytic hydrolysis of DNA

In the present study, we investigated the substrate specificity of catalytic activity of a cytotoxic anti-DNA monoclonal autoantibody, G1-5, which was obtained from an MRL-lpr/lpr mouse by hybridoma technology. The antibody catalyzed hydrolysis of single- and double-stranded DNA with a higher substrate specificity for thymine than adenine by either beta-glycosidic or phosphodiester bond cleavage. The hydrolysis rate (kcat) showed maximum at acidic pH conditions, suggesting that the catalytic site of the antibody contains essential carboxylic group(s). Treatment of cells with the antibody promoted cell death and induced the activation of caspases. The cell death induced by the antibody was inhibited by the pan-caspase inhibitor. Furthermore, the antibody binds to cell membrane and penetrates into the cells. Our results suggest that the cell death is initiated by antibodies penetrating to cells and nucleus, hydrolyzing considerable amount of DNA, and mediating the caspase-dependent apoptotic pathway.

DNA-specific autoantibody cleaves DNA by hydrolysis of phosphodiester and glycosidic bond

The DNA-recognizing autoantibodies were prepared in milligram scale and their catalytic activities were investigated using various standard substrates for hydrolysis of natural biomolecules such as DNA, carbohydrates, and proteins. Only phosphatase and glycosidase activity was found and no peptidase, sulfatase, or esterase activity was detected in most of anti-DNA monoclonal autoantibodies we tested. Antibody G1-2 showed the highest catalytic activities and its enzymatic characteristics were further investigated. The antibody showed phosphatase activity with sub-millimolar substrate specificity and 10(4)-10(5) rate enhancements. However, Ab G1-2 showed low micro-molar specificity with p-nitrophenyl-beta-D-N-acetylglucosamide with 10(4)-10(5) rate enhancements. Both of the catalytic activities showed pH maximum at 4-5, suggesting that the carboxylate(s) in antigen-binding site is involved in the catalytic mechanism. Chemical protection of carboxylate(s) with diazoacetamide showed much reduced activity of the Ab, confirming that the catalytic activity comes from carboxylate(s) in the Ag-binding region. The activities of phosphatase and glycosidase were thoroughly inhibited by DNA with almost identical K(i) values. These data suggest that DNA-binding site(s) is the enzymatic active site of the catalytic Abs. Capabilities of the DNA recognition might make it possible to confer the Ab the catalytic activity of phosphate and glycosidic bond hydrolysis, which can be the main cause of DNA cleavage.

Specific modulation of the anti-DNA autoantibody-nucleic acids interaction by the high affinity RNA aptamer

Anti-DNA autoantibodies are one of the frequently found autoantibodies in systemic lupus erythematosus patient sera. RNA aptamers for the monoclonal G6-9 anti-DNA autoantibody were selected from a random pool of RNA library. Binding affinity of the best aptamer is around 2nM, which is at least 100-fold higher than that of cognate DNA antigen to the autoantibody. Aptamer binds specifically to the G6-9 autoantibody but not to other similar autoantibodies. Minimal binding motif of the aptamer was mapped, providing a hint for a natural epitope of the autoantibody. DNA binding to the G6-9 autoantibody is shown to be efficiently inhibited by the aptamer. Such binding property of the RNA aptamer could be used not only as a modulator for the pathogenic anti-DNA autoantibody, but also as a useful biochemical reagent for elucidating a fine specificity of the autoantibody-nucleic acid interaction.