Anti-Klebsiella K30 phospholipid antibodies in systemic lupus erythematosus: antigen cross-reactions and idiotypic sharing with antibodies to DNA and Klebsiella K30 polysaccharide.

Patients with systemic lupus erythematosus were examined for the presence of serum antibodies reactive with phospholipids (PL) extracted from the membranes of Klebsiella K30 (K30PL). Affinity-purified anti-K30PL antibodies were tested for their ability to cross-react with other PL antigens or DNA and for the presence of idiotypic markers known to be associated with anti-DNA antibodies or antibodies to Klebsiella K30 polysaccharide (K30p). Affinity-purified antibodies to K30PL, phosphatidylserine (PS), or phosphatidylinositol (PL) uniformly cross-reacted with each other. Analysis of the PL preparations by thin-layer chromatography and reversed-phase high-performance liquid chromatography (HPLC) showed the presence of several components. Lupus sera reacted with one component mainly in the HPLC-fractionated K30PL preparation, although this component appeared to be present in the other PL preparations. Direct-binding and inhibition studies showed that affinity-purified antibodies to the K30PL extract, PS, or PI reacted poorly with DNA. However, the anti-K30PL antibodies possessed a prominent anti-DNA idiotypic marker (AM Id) in 61% of the patients and an anti-K30p idiotypic marker (SP Id) in 94% of the patients. The results thus show that anti-K30PL antibodies are idiotypically related to anti-DNA and anti-K30p antibody subpopulations, although they do not share the same antigen-binding characteristics.