Clinical and laboratory characteristics of patients with systemic sclerosis positive for anti-ribonucleoprotein antibodies

Among the patients fulfilling the criteria for systemic sclerosis (SS), there is a subgroup without SS-specific antinuclear antibodies, but positive for anti-ribonucleoprotein (anti-U1 RNP) antibodies. The clinical significance of this type of antinuclear antibodies in SS is not clear. The presence of anti-U1 RNP antibodies is of great interest, since they are not only present in other rheumatic diseases, but are also considered as a marker for mixed connective tissue disease. Objective: to reveal the frequency of anti-U1RNP antibodies in patients with SS and to provide the clinical and laboratory characteristics of patients positive for these antibodies.Subjects and methods. 330 patients who fulfilled the 2013 ACR/EULAR criteria for SS and had been followed at the V.A. Nasonova Research Institute of Rheumatology from 2012 to 2017 were included. Anti-U1 RNP were determined by enzyme immunoassay (reference values: 0–25 U/ml).Results and discussion. Anti-U1RNP were detected in 65 (19.7%) patients with SS (85% of patients were highly positive; 15% were low-positive). The group included 8 men and 57 women; their mean age was 46±14 years. The disease duration was 11±7.9 years. Skin lesions were minimal; 59 (91%) patients had a limited form of the disease with swelling in the hands (scleredema) in 40% of cases and sclerodactyly in 60%. Raynaud's phenomenon was present in all the patients. One-half of the cases were observed to have peripheral ischemic disorders: digital scars and/or sores (43%), as well as necroses and ulcers of other sites, which were relatively rare (8%). Interstitial lung disease (ILD) was identified in 63% of cases. Elevated pulmonary artery systolic pressure (PASP) ≥40 mm Hg, as shown by echocardiography, was detected in 26% of cases and was associated mainly with the presence of ILD; pulmonary arterial hypertension was confirmed in three patients. Esophageal lesions were found in 61% of patients. One-third of patients had signs of scleroderma cardiopathy. The feature of the group was the common involvement of the locomotor system: joints with arthralgia and/or synovitis in 65% and muscles with mild and moderate myopathy in 43%. Erythrocyte sedimentation rate (ESR) and C-reactive protein levels were frequently elevated. The concurrence with Sjö gren's syndrome was common (in one-third of patients). None case of scleroderma renal disease was recorded; the mean values of kidney function were within the normal range; however, the glomerular filtration rate was lower than 80 ml/min/m2 in 17% of patients. All the patients were positive for antinuclear factor (HEp-2); in addition, there was rheumatoid factor (22%), antibodies against Ro/SS-A (41%), La/SS-B (18%), double-stranded DNA (42%), Scl70 (7%), and anticentromere antibodies (9%); 39 out of the 55 (71%) patients who were highly anti-U1 RNP-positive fulfilled the mixed connective tissue disease criteria proposed by R. Kasukawa et al. (1987).Conclusion. The investigation allows one to discuss of the presence of a special phenotype of SS, which is characterized by peculiar clinical manifestations in the presence of anti-U1 RNP overproduction