Detection of autoantibodies to extractable nuclear antigens (ENA): Traditional versus molecular approaches

Autoantibodies to extractable nuclear antigens (ENA) are a clinically important subset of anti-nuclear antibodies which were originally defined by immunodiffusion. Most ENA consist of complexes of a nucleic acid (DNA or RNA) associated with one or more antigenic proteins. Although the autoimmune response is generally directed against the native protein, the autoantibodies occasionally bind denatured protein. These observations are relevant to ELISA and immunoblotting where the form of the antigen may be critical for detection of the ENA. Prokaryotic expression systems in Escherichia coli are useful for producing large amounts of recombinant antigen for ELISA; these proteins may be denatured with consequent loss of the B cell epitopes. Eukaryotic expression systems including the baculovirus system produce post-translationally modified protein in native form but are expensive and too complicated for in-house production. The advantages and disadvantages of immunodiffusion, immunoblotting, immunoprecipitation and ELISA for detection of ENA are discussed. At present, there is no ideal method which can simply and accurately detect all of these autoantibodies, and it is often necessary for laboratories to confirm an ENA with a second or third test. A newer approach is to stably transfect mammalian cells with a CDNA encoding the antigenic protein. Cells hyperexpressing the protein can be used to increase the sensitivity of detection of antibodies to ENA of low abundance such as Ro(SS-A).