Antilipoprotein autoimmune hyperlipidemia. The Ig-Lp test

In the antilipoprotein type of autoimmune hyperlipidemia (AIH), the immunoglobulins (Ig) are bound to lipoproteins by their antibody site and circulate as immune Ig-Lp complexes. In the earlier studies, the specific antibody activities were demonstrated in vitro by specific but rather sophisticated methods which were not suitable for the screening of antilipoprotein AIH in large populations. In the Ig-Lp test described here, the immunoglobulins bound to the low density lipoproteins (Ig-Lp) are detected by floating the complexes at D 1.10 in the ultracentrifuge in a physiological saline sucrose density gradient; delipidating them by ether in the presence of 0.2 M urea, and assaying the protein by radial immunodiffusion and laser immunonephelometry with antisera specific for IgG, IgA, IgM, low density lipoproteins and albumin. Radial immunodioffusion and immunonephelometry gave similar results. This Ig-Lp test was positive in 5 myelomas associated with hyperlipidemia, which were previously classified as AIH with specific methods. And the test was specific for the Ig type of the monoclonal antibody involved in each case (3 IgA, 1 IgG and 1 IgM). It was negative in 6 normolipidemic myelomas and also in 40 sera from healthy blood donors and one normal serum taken 4 hours after a fat meal. Although the Ig-Lp test is not specific for antilipoprotein antibodies, the results of this study allow to use if for the screening for AIH.