The immune response to oxidized LDL (OxLDL) may play an important role in atherogenesis. Working with apoE-deficient mice, we isolated a panel of OxLDL-specific B-cell lines that secrete IgM Abs that specifically bind to oxidized phospholipids such as 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC). These Abs block uptake of OxLDL by macrophages, recognize similar oxidation-specific epitopes on apoptotic cells, and are deposited in atherosclerotic lesions. The Abs were found to be structurally and functionally identical to classic "natural" T15 anti-PC Abs that are of B-1 cell origin and are reported to provide optimal protection from virulent pneumococcal infection. These findings suggest that there has been natural selection for B-1 cells secreting oxidation-specific/T15 antibodies, both for their role in natural immune defense and for housekeeping roles against oxidation-dependent neodeterminants in health and disease.

We recently cloned monoclonal IgM autoantibodies which bind to epitopes of oxidized low-density lipoprotein (OxLDL) from apoE-deficient mice (EO- autoantibodies). We now demonstrate that those EO- autoantibodies that were originally selected for binding to copper-oxidized low-density lipoproteins (CuOx-LDL), also bound both to the oxidized protein and to the oxidized lipid moieties of CuOx-LDL. The same EO- autoantibodies showed specific binding to products of oxidized 1-palmitoyl-2-arachidonoyl-phosphatidylcholine (OxPAPC) and to the specific oxidized phospholipid, 1-palmitoyl-2-(5-oxovaleroyl)-phosphatidyl-choline (POVPC), whereas oxidation of fatty acids (linoleic or arachidonic acid) or cholesteryl esters (cholesteryl-oleate or cholesteryl-linoleate) did not yield any binding activity. Those EO- autoantibodies that bound to oxidized phospholipids (e.g., EO6) inhibited the binding and degradation of CuOx-LDL by mouse peritoneal macrophages up to 91%, whereas other IgM EO- autoantibodies, selected for binding to malondialdehyde (MDA)-LDL, had no influence on binding of either CuOx-LDL or MDA-LDL by macrophages. F(ab')2 fragments of EO6 were equally effective as the intact EO6 in preventing the binding of CuOx-LDL by macrophages. The molar ratios of IgM to LDL needed to maximally inhibit the binding varied from approximately 8 to 25 with different CuOx-LDL preparations. Finally, a POVPC-bovine serum albumin (BSA) adduct also inhibited CuOx-LDL uptake by macrophages. These data suggest that oxidized phospholipid epitopes, present either as lipids or as lipid-protein adducts, represent one class of ligands involved in the recognition of OxLDL by macrophages, and that apoE-deficient mice have IgM autoantibodies that can bind to these neoepitopes and inhibit OxLDL uptake.

During atherogenesis, LDL is oxidized, generating various oxidation-specific neoepitopes, such as malondialdehyde-modified (MDA-modified) LDL (MDA-LDL) or the phosphorylcholine (PC) headgroup of oxidized phospholipids (OxPLs). These epitopes are recognized by both adaptive T cell-dependent (TD) and innate T cell-independent type 2 (TI-2) immune responses. We previously showed that immunization of mice with MDA-LDL induces a TD response and atheroprotection. In addition, a PC-based immunization strategy that leads to a TI-2 expansion of innate B-1 cells and secretion of T15/EO6 clonotype natural IgM antibodies, which bind the PC of OxPLs within oxidized LDL (OxLDL), also reduces atherogenesis. T15/EO6 antibodies inhibit OxLDL uptake by macrophages. We now report that immunization with MDA-LDL, which does not contain OxPL, unexpectedly led to the expansion of T15/EO6 antibodies. MDA-LDL immunization caused a preferential expansion of MDA-LDL-specific Th2 cells that prominently secreted IL-5. In turn, IL-5 provided noncognate stimulation to innate B-1 cells, leading to increased secretion of T15/EO6 IgM. Using a bone marrow transplant model, we also demonstrated that IL-5 deficiency led to decreased titers of T15/EO6 and accelerated atherosclerosis. Thus, IL-5 links adaptive and natural immunity specific to epitopes of OxLDL and protects from atherosclerosis, in part by stimulating the expansion of atheroprotective natural IgM specific for OxLDL.

The relationships between oxidation-specific epitopes (OSE) and lipoprotein (a) [Lp(a)] and progressive atherosclerosis and plaque rupture have not been determined. Coronary artery sections from sudden death victims and carotid endarterectomy specimens were immunostained for apoB-100, oxidized phospholipids (OxPL), apo(a), malondialdehyde-lysine (MDA), and MDA-related epitopes detected by antibody IK17 and macrophage markers. The presence of OxPL captured in carotid and saphenous vein graft distal protection devices was determined with LC-MS/MS. In coronary arteries, OSE and apo(a) were absent in normal coronary arteries and minimally present in early lesions. As lesions progressed, apoB and MDA epitopes did not increase, whereas macrophage, apo(a), OxPL, and IK17 epitopes increased proportionally, but they differed according to plaque type and plaque components. Apo(a) epitopes were present throughout early and late lesions, especially in macrophages and the necrotic core. IK17 and OxPL epitopes were strongest in late lesions in macrophage-rich areas, lipid pools, and the necrotic core, and they were most specifically associated with unstable and ruptured plaques. Specific OxPL were present in distal protection devices. Human atherosclerotic lesions manifest a differential expression of OSEs and apo(a) as they progress, rupture, and become clinically symptomatic. These findings provide a rationale for targeting OSE for biotheranostic applications in humans.