Additive Value of Rheumatoid Factor Isotypes in Sjögren's Syndrome Patients with Joint Complaints of Different Etiologies-Can Rheumatoid Factor IgA Serve as an Early, Poor Prognostic Biomarker Candidate?

The aim of the paper was to characterize rheumatoid factor IgA, IgG, and IgM isotypes in patients with Sjögren's syndrome (SS) subsets, based on the absence or presence of joint complaints of different etiologies. In total, 164 SS patients were grouped based on whether they had polyarthritis as an extraglandular manifestation (n = 73, SS+pa), rheumatoid arthritis as an associated autoimmune disorder (n = 46, SS+RA), or Sjögren's syndrome without inflammatory joint pain (n = 45, SS). The highest IgA rheumatoid factor isotype levels were detected in SS patients, whereas the lowest levels were found in the SS+RA group, without a significant difference. Neither IgG nor IgM RF differed significantly between the patient subclasses. In addition to other disease-specific markers, seropositive patients who were seropositive for any RF isotype were significantly more frequently anti-Ro/SS-A and anti-La/SS-B positive and had higher ESSDAI levels. In SS and SS+pa patients, a strong negative correlation was observed between IgA RF and age, whereas a strong positive correlation was found between IgA RF and ESSDAI, RF, IgA, IgG, anti-Ro/SS-A, and anti-La/SS-B levels. High total IgG levels together with high IgA RF levels occurred most frequently in SS patients (p = 0.05), whereas the combination of normal IgG and high IgM RF was significantly more frequent in the SS+RA group. The co-occurrence of high total IgG and normal IgM RF did not differ significantly between the patient subsets; however, this was the combination with the highest sensitivity (94.5%) for SS+pa patients. Based on our findings, rheumatoid factor isotypes have an additive value in the differentiation of non-erosive polyarthritis and erosive rheumatoid arthritis during the disease course of patients with Sjögren's syndrome. All rheumatoid factor isotypes predict a more severe disease course, but IgA RF may serve as a candidate for being an early, poor prognostic factor for SS patients.