Immunization of LDL receptor-deficient mice with homologous malondialdehyde-modified and native LDL reduces progression of atherosclerosis by mechanisms other than induction of high titers of antibodies to oxidative neoepitopes

We and others previously showed that immunization of rabbits with different forms of oxidized low density lipoprotein (LDL) significantly reduced athe rogenesis. We now investigated the effect of continued immunization on athe rosclerosis in LDL receptor-deficient (LDLR-/-) mice to determine whether a similar reduction of atherosclerosis occurred in murine models and whether this was due to humoral immune responses, ie, formation of high titers of antibodies to oxidation-specific epitopes. Three groups of LDLR-/- mice wer e repeatedly immunized with homologous malondialdehyde-modified LDL (MDA-LD L), native LDL, or phosphate-buffered saline (PBS) for 7 weeks. Extensive h ypercholesterolemia and accelerated atherogenesis were then induced by feed ing a cholesterol-rich diet for 17 weeks, during which immunizations were c ontinued. Binding of immunoglobulin (Ig) M and IgG antibodies, as well as I gG1 and IgG2a isotypes, to several epitopes of oxidized LDL were followed t hroughout the study. After 24 weeks of intervention, atherosclerosis in the aortic origin was significantly reduced by 46.3% and 36.9% in mice immuniz ed with MDA-LDL and native LDL, respectively, compared with PBS (133 558 an d 157 141 versus 248 867 mu m(2) per section, respectively). However, the h umoral immune response to oxidative neoepitopes in the MDA-LDL group was ve ry different from that of the LDL or PBS group. IgG antibody binding to MDA -LDL and other epitopes of oxidized LDL, such as oxidized phospholipid (car diolipin), oxidized cholesterol, or oxidized cholesteryl linoleate, but not native LDL, increased markedly in mice immunized with MDA-LDL, but not in mice immunized with native LDL or PBS. In the MDA-LDL group, both T helper cell (Th)2-dependent IgG1 antibody and Th1-dependent 1gG2a antibody binding to oxidative neoepitopes increased significantly over time. The fact that mice immunized with both MDA-LDL and native LDL had a significant reduction in atherosclerosis, whereas only the MDA-LDL group developed very high tit ers of antibodies to oxidation-specific epitopes, suggests that the antiath erogenic effect of immunization is not primarily dependent on very high tit ers of antibodies to oxidation-specific epitopes but is more likely to resu lt from the activation of cellular immune responses.