Heparan sulphate-ELISA gives false positive results for anti-DNA-DNA/histone immune complexes in sera of patients with SLE

Heparan sulphate-reactive antibodies in lupus sera have been suggested to be anti-DNA-DNA/ histone immune complexes and to be associated with lupus nephritis. In this study, 23 anti-DNA- positive lupus sera including 13 active nephritis sera were tested for the presence of circulating anti- DNA-DNA/histone immune complexes by solid phase heparan sulphate-ELISA. Because of high background binding to protamine chloride-linked heparan sulphate plates, poly-L-lysine (PLL) was used as a linker and the remaining active sites of PLL were blocked with poly-L-glutamic acid. The ELISA was capable of detecting small amounts of anti-DNA IgG-DNA/histone immune complexes formed in vitro. However, only three active nephritis sera of the 23 sera tested showed significant binding to heparan sulphate plates. This binding was found to be non-specific, the result of high background binding of IgG to PLL. Anti-heparan sulphate ELISA using positively charged linkers detects non-specific binding when lupus sera are tested. Specific assays need to be developed for DNA/histone-related immune complexes present in lupus sera.