Dual binding capabilities of anti-double-stranded DNA antibodies and anti-ribosomal phosphoprotein (P) antibodies.

The aim of this study is to identify distinctive properties of pathogenic anti-double stranded DNA antibodies and anti-ribosomal P antibodies. The binding activity of anti-dsDNA and anti-ribosomal P antibodies to their cognate antigens in 0.15 M and 1.5 M NaCl solutions on ELISA was examined. All anti-dsDNA and anti-ribosomal P antibodies exhibited a loss of their binding activity from 37.5 to 100% and from 2.3 to 97.4% in high ionic strength buffers, respectively. In contrast, anti-U1RNP antibodies and anti-Ro/SSA antibodies lost from 0 to 32.7% and from 0 to 40.1% of their binding activity, respectively. Anti-dsDNA and anti-ribosomal P antibodies from patients with nephropathy showed significantly higher binding activity in high ionic strength buffers than those from patients without nephropathy. Study of paired sera from lupus nephritis patients revealed that anti-dsDNA and anti-ribosomal P antibodies from patients during disease flare show stronger binding activity in high ionic strength buffer than those during remission. Most anti-dsDNA and anti-ribosomal P antibodies bind their antigens by ionic interactions that are sensitive to high salt. Such dual binding capability of anti-dsDNA and anti-ribosomal P antibodies may underlie their multiple cross reactivities to various epitopes and help elucidate the pathogenic potential of autoantibody subsets.

Evaluation of assays for the detection of autoantibodies to the ribosomal P proteins.

Sera from systemic lupus erythematosus patients that had antibodies to the ribosomal P proteins were compared in several different assays. The enzyme-linked immunosorbent assay (ELISA) method was compared to the Western immunoblotting method using either affinity purified human or bovine ribosomal P proteins. All 30 normal sera had no significant reactivity with these antigens. The most sensitive test was the ELISA using the human P protein, where 31/32 patients were positive (97%). The assay with bovine proteins in ELISA yielded 28/32 (88%) positive results. Immunoblotting with either bovine or human P protein was equally effective with 30/32 (94%) positive. An ELISA incorporating human P proteins is a more sensitive assay for clinical diagnosis than an ELISA with the bovine protein. Immunoblotting is a sensitive method, but is less convenient and is not quantitative. The ELISA with the human protein appears to be the method of choice.

Heterogeneity of anti-dsDNA antibodies in their cross-reaction with ribosomal P protein.

We have investigated the possible cross-reaction of anti-dsDNA antibodies with ribosomal P peptide for several reasons. First, the antibodies frequently occur together, and secondly, they vary similarly with disease activity. Human polyclonal anti-dsDNA antibodies were affinity purified from eight patients and anti-ribosomal P antibodies from two patients with systemic Lupus erythematosus (SLE) who had high titers of anti-dsDNA as well as anti-ribosomal P antibodies. Nine of the 10 sera were totally specific in their reactivity with their cognate antigens. In only one patient did we find a subpopulation of antibodies which cross-reacted with both dsDNA and the carboxyl terminal 22 amino acid peptide. Our results indicate that anti-dsDNA antibodies are heterogeneous and usually do not cross-react with the carboxyl terminal P peptide, but on occasion (1/10) a patient will produce anti-dsDNA antibodies cross-reactive with the carboxyl terminal P peptide.