Antibodies to small ribonucleoprotein and to 73-kD heat shock protein: two distinct markers of mixed connective tissue disease.

We set out to discover whether antibodies to small ribonucleoprotein antigens (RNP) and to 73-kD heat shock protein (hsp 73) which have been proposed as markers of mixed connective tissue disease (MCTD) recognize different epitopes. MCTD serum was immunoadsorbed on hsp 73-coupled Sepharose and the affinity retained, and non-retained fractions were checked by immunoblotting for recognition of either purified bovine hsp 73 or calf thymus extract RNPs. The hsp 73 affinity-bound serum fraction recognized hsp 73 but not RNP antigens, the reverse being true for the non-retained fraction. We conclude that anti-hsp 73 and anti-RNPs are distinct markers of MCTD.