Natural autoantibody against apolipoprotein A-I. Detection and characterization of the monoclonal antibody established from normal unimmunized BALB/c mice.

During the course of studying the immunogenicity of soybean lipids, we observed frequent production of the mAbs that bind to apolipoprotein A-I (apoA-I) when spleen cells from unimmunized normal BALB/c mice were employed for fusion. Of the 986 colonies from six fusions, 38 (3.9% of the total) were directed against apoA-I and 13 mAbs (IgM) were established for further analysis. The following lines of evidence indicate that this family of mAb may form a novel family of natural autoantibodies against apoA-I: 1) The mAbs were shown to bind effectively to high density lipoprotein from various species, including BALB/c mouse, and immunoblotting analyses revealed that the mAbs bound specifically to the 28-kDa protein of high density lipoprotein. 2) The 28-kDa protein was purified to homogeneity and identified as apoA-I by amino acid sequence analyses and by its cross-reactivity with a xenogenic anti-apoA-I mAb (clone A/11). 3) Differing from the xenogenic anti-apoA-I mAb, the present mAb did not bind to native apoA-I, whereas an effective binding was observed only when the apoA-I had formed a complex with neutral lipids containing polyunsaturated fatty acids such as trilinoleoylglycerol and 5-cholesten-3 beta-ol 3-linoleate. 4) Sera from unimmunized BALB/c mice had readily detectable Abs against apoA-I and the majority of the serum autoantibodies were of the IgA and IgM isotype. 5) The anti-apoA-I mAbs displayed a functional heterogeneity in their reactivity with polyanionic substances and some of the mAbs established showed an extensive cross-reaction with polyanionic substances such as ssDNA and cardiolipin.