Increase of Immunoglobulin G₃ Subclass is Related to Brain Autoantibody in Alzheimer's Disease but not in Down's Syndrome

Abstract

The proportions of IgG subclasses (G, G₂, G₃ and G₄) were quantified in sera from Alzheimer's disease (AD) patients, older Down's syndrome (DS) patients and age-matched controls. The levels of IgG₁, IgG₂ and IgG₄ were normal in AD patients, but the proportions of IgG₃ were significantly elevated in 9 of 20 (45%) patients (0.803 ± 0.141 mg/ml; p < 0.001) compared to the level found in age-matched controls (0.471 + 0.161 mg/ml; n = 10). The IgG₃ level in the remaining 11 AD patients was slightly lower than the controls (0.385 ± 0.104 vs. 0.471 ± 0.161 mg/ml), but it did not reach statistical significance (p = 0.149). In contrast, patients with DS displayed imbalance of IgG₂, IgG₃, and IgG₄ subclasses; they had significantly increased IgG₃ but decreased IgG₂ and IgG₄ levels. The IgG₁ level was within normal range. Moreover, a majority of AD sera (8 of 9) with elevated IgG₃ concentration were positive for brain autoantibody. The remaining 11 AD sera without elevated IgG₃ level, all DS sera and all control sera were negative for brain autoantibody. This finding indirectly suggests that brain autoantibody is mainly due to IgG₃ subclass, at least in one subset of AD patients.

Key Words:

• IgG subclasses
• Brain autoantibody
• Alzheimer's disease
• Down's syndrome