Immunoglobulin G subclass analysis in psoriatic arthritis.

Objective: The occurrence of monoclonal gammopathy of undetermined significance (MGUS) is common in chronic immune mediated disorders. This increased monoclonal antibody production could result from chronic stimulation of lymphocytes, with the immunoglobulin G (IgG) subtype accounting for the majority of cases in psoriatic arthritis (PsA). We aimed to identify IgG subclass profiles in patients with PsA and to determine association with specific disease characteristics.
Methods: Serum samples from 221 patients with PsA from a single cohort were analyzed for their serum IgG subclass levels. All patients fulfilled the ClASsification for Psoriatic ARthritis (CASPAR) criteria and were followed at 6-month to 12-month intervals according to a standard protocol. MGUS was defined as the occurrence of a discrete band in the gammaglobulin region on at least 2 separate serum protein electrophoresis tests performed 6 months apart. Patients with high abnormal IgG subclass levels were compared to patients with normal levels using descriptive tests.
Results: Elevations of IgG1-4 were common in PsA, with ∼20%-49% of patients having elevations of each subclass, IgG2 being the most common subclass abnormality. However, no clinical-serological correlation was found in the group with abnormal IgG2 levels. Of the 38 patients with MGUS, elevations in IgG1 were most common. Patients with an abnormal IgG1 subclass level were more likely to have a discrete band in the gammaglobulin region, higher prevalence of MGUS, and abnormal erythrocyte sedimentation rate or C-reactive protein levels.
Conclusion: Determination of the IgG subclass concentration in PsA did not seem to add any significant value in identifying specific disease manifestations. However, this study provides insight into the pathological process leading to MGUS in PsA.