Effect of low-density lipoprotein (LDL) antigen source on an enzyme-linked immunosorbent assay for autoantibodies...

We examined the effect of antigen source on an enzyme-linked immunosorbent assay (ELISA) for autoantibodies against oxidized low-density lipoprotein (LDL). Serum samples from 20 subjects with systemic lupus erythematosus (SLE) and from 20 controls were assayed for immunoglobulin G (IgG) and immunoglobulin M (IgM) autoantibodies against oxidized LDL, using either a pooled or individual (n = 3) LDL preparation as antigen. For IgG autoantibodies against oxidized LDL there was a relationship (r approximately 0.5, P < 0.01) between data obtained using individual versus pooled antigen preparations. Bias plots demonstrated consistent inverse, concentration-dependent relationships (r approximately -0.6, P < 0.001). The difference in IgG autoantibodies against oxidized LDL levels between SLE patients and controls was underestimated (39-58%) when assays used individual rather than pooled LDL antigen. For IgM autoantibodies against oxidized LDL the direct relationships were stronger (r approximately 0.8, P < 0.001) and the concentration-dependent relationships weaker (r approximately -0.3, significance variable) than for IgG autoantibodies against oxidized LDL. Variations between LDL preparations suggested that a pooled antigen would give a more stable assay. Thus, LDL antigen source is important in assays for both IgG and IgM autoantibodies against oxidized LDL.