Patients, who are mainly children, suffer from bloody diarrhea, recurrences are uncommon and their prognoses are often good 1. The other form, called atypical hemolytic uremic syndrome (aHUS), occurs at any age, may be sporadic or familial and has a poor prognosis as approximately 50% of the patients progress to end-stage renal disease PF-02341066 nmr and 25% die during the acute phase of the disease. The sporadic form of aHUS may be triggered by non-enteric infections, viruses, pregnancy, drugs, malignancies or transplantation

2. The familial form of aHUS has now been shown to be associated with genetic abnormalities in complement regulators like factor H (FH) 3–6, factor I (FI) 4, 7–10, membrane cofactor protein (MCP) 4, 11–14, C4b-binding protein (C4BP) 15, factor B (FB) 16 and C3 17 or autoantibodies against FH 18, 19. The mutations and polymorphisms in these proteins are mostly found in heterozygous form and can affect both the secretion and function of the proteins, leading to impaired regulation of the alternative pathway of the complement system 2. Since many of the patients carry several HDAC activity assay heterozygous mutations or polymorphisms in different genes, it has been suggested that a combination

of several simultaneous hits strongly predisposes to aHUS 20. The complement system, which is a part of the innate immune system, can be activated through three different pathways, the classical, lectin and alternative pathways. The classical pathway is initiated through the interaction of C1 with ligands such as immune complexes. The lectin pathway is initiated when mannose-binding lectin binds to carbohydrate structures on bacteria, whereas the alternative pathway is constantly activated through auto-hydrolysis of

C3 molecules in the fluid phase. Furthermore, the alternative pathway serves as the amplification loop to the other two pathways. All three pathways generate C3 convertases (C4b2a or C3bBb), which cleave C3 to C3a and C3b 21. To prevent activation by self-tissue, complement has to be tightly regulated by membrane-bound (MCP, decay-accelerating factor, complement receptor 1 (CR1)) and fluid-phase inhibitors (C4BP, FH, FI). Among these during inhibitors, the serine protease (SP) FI is special since it degrades C4b and C3b in the presence of specific cofactors like C4BP 22, FH 23, MCP 24 or CR1 25. FI is a unique protease since it has no natural inhibitors and works only together with its cofactors. The fully processed FI protein consists of a heavy chain (50 kDa) and a light chain (38 kDa), which are connected covalently via a disulfide bond 26. The heavy chain is composed of five domains; the factor I membrane attack complex (FIMAC), CD5-like domain, the low-density lipoprotein receptor 1 and 2 domains (LDLr1 and 2) and a region of unknown homology. The light chain comprises the SP domain 27.