Increased serum levels of immunoglobulins, C-reactive protein, type 1 and type 2 cytokines in patients with mixed connective tissue disease.

It is controversial whether mixed connective tissue disease (MCTD) should be regarded as a distinct disease entity. In the present study, we investigated immunological parameters in patients with MCTD by studying serum levels of immunoglobulins, C-reactive protein (CRP) and cytokines and compared the results to the corresponding values in systemic lupus erythematosus (SLE), in rheumatoid arthritis (RA) patients and in healthy controls. Using the ELISA technique, serum levels of the cytokines interferon-gamma (IFN-gamma), interleukin-10 (IL-10) and tumour necrosis factor-alpha (TNF-alpha) were investigated. Cytokine levels in SLE and MCTD were correlated to disease activity as assessed by systemic lupus activity measure (SLAM). They were also correlated to serum levels of CRP, IgG, IgA and IgM in the three patient groups. The MCTD patients had the highest levels of immunoglobulins, followed by the SLE patients. In contrast, the highest CRP levels were observed in RA patients, followed by the MCTD patients. The MCTD patients had the highest serum levels of IL-10, but also had elevated IFN-gamma and TNF-alpha levels similar to the RA patients. There was no correlation between the investigated cytokine levels and disease activity, as assessed by SLAM. We conclude that MCTD patients have high immunoglobulin levels as well as high CRP levels and that this situation is compatible with the observed increase in both type 1 and type 2 cytokine levels. The findings imply that MCTD shares some distinct immunological properties with both RA and SLE and that MCTD may also be considered as a separate disease entity according to these properties. Copyright 1998 Academic Press.