The relationship between Sm and RNP antigens.

The relationship between the antigenic proteins of Sm and RNP is not clear. To further clarify their relationship, we examined sera found monospecific by counterimmunoelectrophoresis (CIEP) for anti-Sm or anti-RNP with the more sensitive techniques of immunoblotting, radioimmunoprecipitation, or enzyme immunoassay (EIA). The same eight unique problems were precipitated by both anti-Sm and anti-RNP in radioimmunoprecipitation. They had molecular weights (MWs) of 11, 13, 17, 18, 24, 26, 28, and 68 kDa. The 17, 18, and 28 kDa bands were more intense with anti-RNP. Immunoblotting with anti-Sm and anti-RNP also recognized similar proteins with MWs of 14, 17, 25, 28, 29, 30, 36, 38, and 68 kDa. Anti-Sm resulted in more intense 14, 28, 29, and 30 kDa bands, while anti-RNP gave maximum intensity of the 14, 36, 38, and 68 kDa bands. The band intensity pattern differences were more easily appreciated with immunoblotting than with radioimmunoprecipitation. RNase, heat, and urea caused a similar diminution of antigen reactivity with both anti-Sm and anti-RNP on immunoblotting, but eliminated immunoprecipitability only of RNP on immunodiffusion. The great similarities between Sm and RNP suggest several possibilities: Anti-Sm and anti-RNP antibodies coexist in the same patients; and the more sensitive techniques of immunoblotting and radioimmunoprecipitation detect both precipitating and nonprecipitating antibodies while only precipitating antibodies are detected by immunodiffusion. Sm and RNP may represent different determinants on the same macromolecular complex. Sm and RNP may be cross-reacting determinants on distinct molecules.