To investigate immunoreactivity of systemic lupus erythematosus (SLE) sera with apolipoprotein Al, (Apo Al), the major lipid-binding protein of high-density lipoprotein (HDL).
Methods. Since early attempts to identify Apo A1 autoantibodies using standard enzyme-linked immunosorbent assay (ELISA) and immunoblotting techniques had been unsuccessful, a mouse complementary DNA lambda phage expression library was screened.
Results. A selected clone (MA1) was found to have 82% DNA sequence homology to a segment of human Apo Al. Since there were nonconservative substitutions in the MA1 protein and lack of a complete sequence, it was possible that the SLE patient's antibodies were binding MA1 epitopes that were shared by the complete human protein but had not been conformationally accessible using the earlier techniques. Thus, gamma-irradiated ELISA plates were used as an alternative antigen-binding surface for intact human Apo A l , and high-titer anti-human Apo A1 autoantibodies were then identified in the sera of 5 more SLE patients.
Conclusion. These findings show that Apo A1 is immunogenic. Apo A1 antibodies may play a role in the decreased HDL levels and Apo A1:Apo B ratios previously reported to occur in subgroups of SLE patients.
Apolipoprotein A1 (Apo Al), which activates the enzyme 1ecithin:cholesterol acyltransferase, is the major protein of high-density lipoprotein (HDL) particles and plays a critical role in lipid transport and Dr. Merrill's work was supported in part by a grant from the SLE Foundation, Inc.