Immunological studies of malondialdehyde in a murine lupus model.

Abstract

Malondialdehyde (MDA) is a highly reactive three carbon dialdehyde produced as a byproduct of polyunsaturated fatty acid peroxidation and arachidonic acid metabolism. MDA readily forms intermolecular or intramolecular cross-linkages with chemical groups on various molecules and alters both their physicochemical properties and immunogenicity. In vivo these changes may generate neoantigens that consequently can elicit an immune response involving antibody production and immune complex formation. In this study the possible role of an anti-MDA immune response in the pathogenesis of disease in lupus-prone MRL/lpr mice was investigated. Physicochemical alterations to mouse serum albumin (MSA) were characterized and reliable immunological assays were developed to measure this immune response. MDA readily bound to amino groups on MSA, formed epitopes in a dose-dependent manner and altered its antigenicity. The presence of a specific anti-MDA response in MRL/lpr was verified using an enzyme-linked immunosorbent assay (ELISA) and inhibition ELISA. The immune complex capture assay, using MDA-specific polyclonal antiserum as capture antibody, detected significantly higher levels of MDA-containing immune complexes in sera of MRL/lpr mice compared to non-lupus strains. A time course study showed that in comparison to anti-dsDNA antibodies, anti-MDA antibodies were produced earlier and in significantly higher levels. These antibodies were found predominantly in the complement-fixing IgG₂ₐ and IgG₂(b) subclasses. Levels of IgG anti-MDA antibodies and MDA-containing immune complexes were highest at 4 months age. Western blot analysis revealed the presence of a 100 kilodalton MDA-modified protein in sera of MRL/lpr mice at 3 and 5 months of age but not at 1 month. In sera from non-lupus MRL/+ mice the level of MDA-modification of serum proteins was considerably lower. Vitamin E treatment for up to 16 weeks did not significantly affect the anti-MDA response, MDA-containing immune complex levels nor modification of serum proteins by MDA. Immunohistochemical studies demonstrated the presence of MDA-containing immune complexes, IgG and complement deposition in renal glomeruli. This is the first demonstration of circulating MDA-specific antibodies, MDA-containing immune complexes and MDA-modified serum proteins in MRL/lpr mice. This response may contribute to disease pathogenesis in these mice via formation and subsequent tissue deposition of immune complexes containing anti-MDA antibodies and MDA adducts.