Role of Natural Antibodies Against Oxidation Specific Epitopes in Bone Homeostasis

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Lipid peroxidation forms highly reactive molecules that generate adducts with amino groups of proteins and lipids, known as oxidation specific epitopes (OSEs). OSEs are ubiquitous pro-inflammatory moieties that cause extensive cell damage if not promptly eliminated. Natural antibodies of the innate immune system, recognize OSEs and block their adverse effects. Scavenger receptors (SRs) expressed in macrophages and other cell types recognize OSEs and dispose of oxidatively-modified endogenous molecules. Persistence and/or excessive amounts of OSEs overwhelm these defense mechanisms, and the inflammatory response initiated by scavenger receptor/toll-receptor activation causes tissue damage and the development of disease. Phosphocholine-containing oxidized phospholipids (PC-OxPLs) form OSEs on oxidized low density lipoproteins (OxLDLs) and on the surface of apoptotic cells. The IgM E06 is a natural antibody that recognizes PC-OxPL. Transgenic mice expressing a single chain (scFv) form of the antigen-binding domain of E06 IgM (E06-scFv) are protected against atherosclerosis and have increased lifespan. This effect results from the prevention of OSE binding to macrophages and the subsequent activation of inflammatory pathways. In humans, low levels of natural antibodies are associated with increased incidence of cardiovascular disease. Epidemiologic evidence in humans as well as mechanistic studies in mice indicate that the OSE-rich OxLDL is a pathogenic factor for osteoporosis as well. Remarkably, we found that the E06-scFv transgene prevents the cortical bone loss caused by HFD by increasing endosteal osteoblast number and bone formation. In addition, E06-scFv increases cancellous and cortical bone in male and female C57Bl/6J mice fed a normal diet by increasing bone formation and osteoblast number. Mechanistic studies showed that E06 IgM prevents the negative effects of OxLDL on proliferation, differentiation, and survival of cultured osteoblastic cells suggesting that anti-OSE antibodies promote bone anabolism by preventing the negative effects of OSEs on osteoblasts. Consistently, the production of anti-osteogenic cytokines by macrophages was not affected in E06-scFv transgenic mice. Deletion of the scavenger receptor ScrB1, the most abundant scavenger receptor for PC- OxPL in cells of the osteoblast lineage, prevents the adverse effects of OxLDL on apoptosis and differentiation. Moreover, anti-PC IgM (which includes E06) decline with age in mice, in association with a decline in bone mass. Therefore, we hypothesize that anti PC-OxPLs antibodies play a beneficial role in skeletal homeostasis by protecting against deleterious effects of PC-OxPLs on bone formation, that are transmitted by ScrB1 on osteoblasts. Increased production of OSEs due to accelerated apoptosis, oxidative stress, or chronic inflammation, in concert with declining levels of anti PC-OxPLs antibodies with age, contributes to the pathogenesis of osteoporosis in both mice and humans. Studies proposed in this application will establish whether ScrB1 mediates the negative effects of PC-OxPLs on osteoblasts in vivo by characterizing the bone phenotype of mice in which the ScrB1 gene has been deleted in cells of the osteoblast lineage. We will also determine if E06-scFv prevents the reduction in bone formation caused by old age. Additionally, we will investigate whether endogenous anti-PC IgM levels correlate with bone mineral density in humans by measuring the levels of anti-PC IgM in a cohort of eligible Veterans who had a bone mineral density (BMD) assessment by dual X-ray absorptiometry (DXA). Successful completion of this work may may suggest a novel therapeutic approach for the management of osteoporosis and atherosclerosis simultaneously.