The structural requirements for anti-cardiolipin antibody binding in sera from patients with syphilis and SLE

In a search for specific binding patterns of anti-phospholipid (PL) antibody reactivity in sera from patients with systemic lupus erythematosus (SLE), a quantitative assay was used to compare binding curves with those from beef heart cardiolipin (CL) and the following CL analogues: diphosphatidyl propylene glycol (DPPG), which lacks the internal hydroxyl group on the glycerol moiety; acetyl CL (ACL), in which an acetyl group is substituted for the glycerol hydroxyl group; and dimethyl CL (DCL), in which a methyl group is positioned on each phosphate group. In syphilitic sera the plateau level of antibody binding was decreased by 15 and 41% with DPPG and ACL, respectively. Binding to DCL was dramatically suppressed to levels only slightly above baseline. Only the binding curves for CL and DPPG showed saturability, and analysis by Eadie-Scatchard plots showed that the change in binding was primarily due to a more than twofold increase in KD (decreased antibody avidity). Similar patterns were seen with sera from patients with SLE and SLE-like illness, but some uniquely shaped binding curves were observed. Compared to control CL, peak binding levels were 75-88% for DPPG, 16-20% for ACL, and only 1-4% for DCL. These data indicate that the integrity of the CL headgroup, especially at the phosphate moiety, is essential for recognition by anti-CL antibodies from some sources.