HIDDEN ANTIPHOSPHOLIPID ANTIBODIES IN NORMAL HUMAN SERA CIRCULATE AS IMMUNE-COMPLEXES WHOSE ANTIGEN CAN BE REMOVED BY HEAT, ACID, HYPERMOLAR BUFFERS OR PHOSPHOLIPASE TREATMENTS

Heat treatment of normal human serum reveals otherwise masked anti-car diolipin antibodies (aCL). We studied the mechanism of masking and the nature of the inhibitor of these aCL IgG. Other forms of treatment, b esides heating for 30 min at 56 degrees C, can also unmask hidden aCL IgG. These include acid pH, hypermolar buffers and phospholipase diges tion. When unmasked, these aCL recognize other anionic and zwitterioni c phospholipids, but do not react with DNA, cell antigens or IgG. Usin g thin layer chromatography we demonstrate that the heat-labile inhibi tor(s) of these aCL are phosphatidylserine, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylcholine. These antibodies are not beta(2)-glycoprotein-I dependent and actually compete with this prote in for phospholipid binding. The hidden antibodies are comprised of tw o populations of IgG autoantibodies: one reactive with cardiolipin, ph osphatidylserine, phosphatidylinositol, phosphatidylglycerol, phosphat idylethanolamine and sphingomyelin, and the other reactive almost excl usively with phosphatidylcholin and phosphorylcholine on enzyme-linked immunosorbent assay plates or when exposed by bromelain on the erythr ocyte surface. Our data suggest that hidden aCL are natural oligoreact ive IgG anti-phospholipid autoantibodies that circulate masked by thei r antigen.