Studies on hereditary C2 deficiency: Frequent occurrence of severe infections, atherosclerosis and rheumatological manifestations

University dissertation from Department of Medical Microbiology, Lund University

Abstract: The complement system is a part of the innate immunity and is essential in the defence against microorganisms. Hereditary C2 deficiency (C2D) is one of the most common complement deficiency states with an estimated prevalence of 1:20,000 in persons of Western descent. In the present investigation, the identification of more than 40 C2D persons at a single centre combined with long observation periods provided a unique basis for assessment of C2D-associated manifestations and diseases. The predominant clinical manifestation was severe bacterial infections. The infections were mainly caused by Streptococcus pneumoniae. Repeated infections occurred primarily during infancy and childhood. On the other hand, about 25-30 % of the C2D persons remained healthy during the observation period. Immunological factors as IgG subclass levels, GM allotypes, complement proteins, and Fc receptors were assessed to explain this difference. Homozygosity for the G2M'n allele was strongly associated with protection against severe infections (p<0.001). This indicated that an efficient antibody response to polysaccharide antigens is of great importance in C2D. Mannan-binding lectin deficiency also contributed to the susceptibility to infection. The association between C2D and systemic lupus erythematosus (SLE) was confirmed, but notably the severity of SLE in patients with C2D was similar to that of other SLE patients. Another novel finding was a high occurrence of anti-cardiolipin antibodies (aCL) and antibodies to the collagen-like region of C1q. Both autoantibodies have a pro-atherosclerotic effect that might explain the high occurrence of cardiovascular disease found in the cohort. Interestingly, anti-phospholipid syndrome was not observed despite the high occurrence of aCL. Vaccination in 25 C2D persons resulted in antibody responses which show that C2D persons benefit from vaccination against infections caused by encapsulated bacteria such as pneumococci.