Serum IgG levels demonstrate seasonal change in connective tissue diseases: a large-scale, 4-year analysis in Japanese

Hypergammaglobulinemia is often found in patients with autoimmune diseases, such as systemic lupus erythematosus (SLE), and its level may correlate with disease activity. However, it is unclear whether immunoglobulin G (IgG) displays seasonal changes. We analyzed the seasonal change in serum IgG by assessing 450 patients with connective tissue disease. The serum IgG levels in summer were compared with those in winter from 2006 to 2009. Independent samples from 355 patients were analyzed to confirm results in the first set. The differences in the IgG levels between the two seasons were analyzed in each disease and compared with disease activity. 488 patients without connective tissue disease were analyzed as reference instead of healthy people as control. We found that connective tissue disease patients tended to show higher levels of serum IgG in summer than in winter every year from 2006 to 2009, whereas patients without connective tissue disease did not demonstrate such a tendency. We observed this seasonal tendency in each disease. Seasonal changes weakly correlated with those of anti-DNA antibody in SLE patients and those of disease activity score in rheumatoid arthritis (RA) patients. Serum IgG levels of patients with connective tissue diseases display seasonal variations. Biological and clinical significance of these variations should be elucidated.