Immunogenicity of homologous low density lipoprotein after methylation, ethylation, acetylation, or carbamylation: generation of antibodies specific for derivatized lysine.

We previously showed that immunization of guinea pigs with reductively glucosylated guinea pig low density lipoprotein (LDL) or albumin resulted in the formation of antibodies specific for the glucosylated protein. The present studies were done to determine if modifications of homologous LDL or albumin, other than addition of carbohydrate, would also render these proteins immunogenic. We found that derivatization of lysine residues of guinea pig LDL or albumin by carbamylation, acetylation, ethylation, or even methylation rendered them immunogenic in guinea pigs. In addition, the specificity of the antibodies was strikingly influenced by whether modified homologous LDL or modified homologous albumin was used as the immunogen. Antibodies generated against modified LDL were directed almost exclusively against the derivatized lysine residues (i.e., carbamyllysine, acetyllysine, or methyllysine) and hence reacted equivalently with other modified proteins that contained the same lysine derivative. However, antibodies generated against guinea pig albumin (or fibrinogen) modified in the same ways reacted primarily with the modified protein used as immunogen, and not with the free lysine derivative, or with other similarly modified proteins. Each of the modifications referred to above could potentially occur in vivo. Therefore, the findings presented may be relevant to autoantibody formation and immunopathogenetic mechanisms in certain diseases.