The presence of autoantibodies to C1-inhibitor (C1-INH-Abs) is a hallmark of acquired C1-inhibitor deficiency. However, only scarce data are available on their prevalence in hereditary angioedema (HAE). In a prospective study performed between 2001 and 2004 in 95 patients with Type I or II HAE, serum samples were taken one to three times a year and clinical status of the patients was registered. Serum samples were tested for total activity of the classical pathway, C1q, C3, C4 and C1-inhibitor (C1-INH) concentration and activity levels, as well as the presence of IgG, IgA and IgM type anti-C1-inhibitor antibodies (C1-INH-Ab). Fifty-four healthy age and gender matched persons served as control.
Significant differences between the patients and controls in the occurrence of elevated (2S.D. higher than mean of control) C1-INH-Abs titers was found only in the case of IgM type C1-INH-Abs. Elevated (>4.22 AU/ml) IgM C1-INH-Abs levels were found in 31 and 4% of the patients and controls, respectively (p < 0.001). Surprisingly, high titer IgM C1-INH-Abs were present with equal frequency in the 41 HAE patients ever treated with C1-INH concentrate and in the 54 C1-INH treatment naïve patients. In the latter group, strong positive correlation between the levels of the IgM C1-INH-Abs and the most severe disease (score 1) (p = 0.0021) and the yearly attack rate (p = 0.0173) were obtained. In addition, the levels of the IgM C1-INH-Abs exhibited strong negative correlation to the C1-inhibitor concentration and functional activity, total classical complement pathway activity, and a positive correlation to total IgM concentration.
Taken together, these data indicate that IgM type C1-INH-Abs are present with highly elevated frequency in HAE patients irrespectively of the previous treatment with C1-INH concentrate. Most probable production of these autoantibodies is the consequence of the activation of complement and other plasma enzyme systems during HAE attacks. Determination of IgM C1-INH-Abs can be used as an activity marker in HAE.