BLOTTING PATTERNS OF IGG ANTI-(U1)RNP ANTIBODIES IN MIXED CONNECTIVE-TISSUE DISEASE

Serum reactivities towards individual U1 snRNP proteins were determine d by immunoblotting in 32 patients with mixed connective tissue diseas e (MCTD). Time persistence of immunoblot profiles and clinical signifi cance of anti-(U1)RNP antibody specificities were also investigated. I gG anti-(U1)RNP antibodies were found in the sera of 29 out of 32 pati ents (90.6%): 21 (65.6%) reacted with the 70-kD protein, 25 (78.1%) wi th A, 23 (71.9%) with C and 20 (62.5%) with B/B' proteins. None were r eactive with the Sm-D peptide. Seventy kilodalton antibody specificity was strongly associated with a higher antinuclear antibody titre (>16 0) and slightly associated with disease activity; anti-B/B' specificit y was associated with lymphadenopathy. Anti-A, -C and -B/B' antibodies were negatively associated with systemic lupus erythematosus (SLE) sk in rashes. Two types of anti-(U1)RNP blotting patterns were selected: ''full spectrum'' (53.1% of cases) and a ''partially/no reactive'' one (46.9%). Such patterns were unchanged over time in 14 out of 16 cases prospectively examined(87.5%), while the pattern shifted from ''full spectrum'' to ''partially/no reactive'' in 2 cases (12.5%): in 1 after a prolonged clinical remission (24 years) and in the other following immunosuppressive therapy. The anti- (U1)RNP antibody immunoblot profi le in MCTD patients consisted of various reactivities and remained unc hanged over time in most cases. Antibody reactivity against the 70-kD protein represented the major U1 snRNP specificity. The various anti-( U1)RNP specific reactivities demonstrated poor clinical significance w ithin MCTD. Thus, MCTD seems to be characterized by a longstanding ser ological heterogeneity whose reactivities do not apparently correspond to distinct features within the broad clinical spectrum of MCTD.