• SLE
• diagnostic assays
• differential diagnosis
• anti-dsDNA autoantibodies
• histone
• plasmid DNA

Anti-dsDNA IgG are marker antibodies of systemic lupus erythematosus (SLE) essential for the differential diagnosis of this disease. Actually the three main categories of SLE diagnostic assays suffer from important limitations: indirect immunofluorescence (CLIFT) is 98%-100% specific but low sensitive and it gives only semi-quantitative results. Solid phase tests (included ELISAs) are more sensitive, quantitative, but they have unsatisfactory specificity and exhibit variable diagnostic performances depending on a number of factors. Farr assay, the “gold standard, requires radioactive reagents and it has short shelf- life. Taking into account the structural features dominating the interaction of anti-dsDNA antibodies with bent DNA, a feature present in the CLIFT substrate but also in nucleosomes along with the DNA/histone interactions, we selected a plasmid bearing a highly bent fragment of 211 bp from the minicircles of the protozoan Crithidia Luciliae and a positively charged trait of H4 histone to prepare a complex to be used in a solid phase assay to detect specific IgG in a large number of patients and control’s sera. This assay detects anti- pDNA/H4 (14-34) antibodies in 56% of SLE sera and 5.9% in disease controls. Inhibition assays show that epitopes recognized by sera are exclusively located on DNA or on the complex. Antibody titer correlates positively with disease activity and anti- C1q antibodies, negatively with C3 levels. Anti- pPK201/CAT/H4 (14-34) ELISA shows moderate concordance with CLIFT and a commercial solid phase dsDNA assays. Thus, pDNA/H4 assay can be a valid complement of available assays, concurring to better define the pattern of pathogenic antibodies in SLE.