Supplementary Material for: Combined heterozygous genetic variations in complement C2 and C8B - an explanation for multi-dimensional immune imbalance?

The complement system plays a crucial role in host defence, homeostasis, tissue regeneration and bridges the innate and the adaptive immune system. Although the genetic variants in complement C2 (c.839_849+17del; p.(Met280Asnfs*5)) and C8B (c.1625C>T; p.(Thr542Ile)) are known individually, here we report on a patient carrying their combination in heterozygous form. The patient presented with a reduced general condition and suffers from a wide variety of autoimmune diseases. While no autoimmune disease-specific autoantibodies could be detected, genetic analysis revealed abnormalities in the two complement genes C2 and C8B. Therefore, we performed a comprehensive investigation of the innate immune system on a cellular and humoral level to define the functional consequences. We found slightly impaired functionality of neutrophils and monocytes regarding phagocytosis and ROS generation and a diminished expression of the C5aR1. An extensive complement analysis revealed a declined activation potential for the alternative and classical pathway. Reconstitution with purified C2 and C8 into patient serum failed to normalize the dysfunction whereas the addition of C3 improved the hemolytic activity. In clinical transfer, in vitro supplementation of the patient’s plasma with FFP as complement source could fully restore full complement functionality. This study describes a combined heterozygous genetic variation in complement C2 and C8B which cannot explain the overall dysfunctions and calls for further complement deficiency research and corresponding therapies.