Nuclear protein antigens in autoimmune Rheumatic and Connective Tissue Diseases1

Abstract

Two nuclear antigens, Sm and RNP, that act as markers in Systemic Rheumatic (SRD) and Connective Tissue Diseases (CTD) have been studied with regard to their composition and biochemical properties. RNP antigen appears to be a 10S nuclear RNP particle containing SnRNA. Sm antigen, formerly considered to be a protein, may also contain RNA. Although both antigens are considered to be saline soluble, evidence suggests that they are part of a nuclear structure that is only slowly dissociated under isotonic conditions.

An immunoassay differentiating sera with antibodies to sm alone, antibodies to sm/RNP complex, and antibodies to RNP alone

“…In any case, our data appear to indicate that the majority (perhaps the near total) of Sm-and RNP-reactive sera will have reactivities with either the 68-kd protein or the D protein. Such evidence is supported by data from other publications (6)(7)(8)(9)(10)(19)(20)(21)(22). Another factor also makes it somewhat difficult to more precisely compare our data with those of Reeves et al (19): they did not analyze for the 68-kd protein, and this protein was the single component differentiating the Sm/RNP complex from the D protein of Sm in our study.…”

“…It is possible that free Sm and Sm bound to U1 RNP or to nuclear matrix may exhibit differences in reactivity. In fact, MacGillivray and Carroll have demonstrated such differences in reactivity of anti-Sm antibody with free and bound Sm (22). Anti-Sm sera might have shown the different OD readings recorded in this report for these and perhaps other reasons.…”

“…Another issue concerns observations, made in many studies but emphasized by MacGillivray and Carroll (22), that snRNP particles (including Sm and U1 RNP) exist in the free form and "bound" to nuclear matrix. It is possible that free Sm and Sm bound to U1 RNP or to nuclear matrix may exhibit differences in reactivity.…”