At this period, it was concluded that the immunoconglutinins are convalescence factor, helping the healing process (25)

At this period, it was concluded that the immunoconglutinins are convalescence factor, helping the healing process (25). reported. These Rabbit polyclonal to PLEKHA9 antibodies recognize epitopes, shared between C3(H2O)/C3b/iC3b/C3c and have overt functional activity. They correlate with low plasmatic C3 levels in patients.In vitro, they increase the activity of the alternative pathway C3 convertase, without being C3 nephritic factors. They perturb the binding of the negative regulators Complement Receptor 1 and Factor H. The clear functional consequences and association with disease severity warrant further studies to establish the link between the anti-C3b autoantibodies and tissue injury. Comparative studies with such antibodies, found in patients with infections, may help to uncover their origin and epitopes specificity. Patients with complement overactivation due to presence of anti-C3b antibodies may benefit from therapeutic targeting of C3. Keywords:anti-C3 autoantibodies, anti-C3b autoantibodies, complement, immunoconglutinins, autoimmunity, systemic lupus erythematosus, lupus nephritis, C3 glomerulopathy == Introduction == The complement system is apart of the innate immune defense (1,2). Autoantibodies against complement components and regulators have proven pathogenic effect. These antibodies (Ab) may cause acquired functional deficiencies of the complement cascade or induce amplification of an already activated complement (36). This review is focused on the auto-antibodies, recognizing the activation products of the central complement GSK 4027 component C3. The GSK 4027 history of their discovery, their prevalence in different diseases as well as their functional and clinical relevance are discussed. Multiple unanswered questions open avenues for future studies. == The Complement Component C3Structure and Function == C3 is the central component of the complement system (7). It is the convergent point of the three pathways of the cascade (1,2). C3 is a 185-kDa glycoprotein, which belongs to the 2-macroglobulin family, part of the thioester-containing protein superfamily. C3 consists of two polypeptide chains-chain (110 kDa) and -chain (75 kDa), linked by one disulfide bond and by non-covalent forces (8). It contains a globular thioester domain (TED) with an intrachain thioester bond, capable to attach covalently to surfaces; eight macroglobulin (MG) domains, and CUB domain, which frames and holds the TED domain (Figure 1A). == Figure 1. == Structural organization of C3, its activation forms, localization of the binding epitopes, and proposed mode of action of the anti-C3b auto-antibodies.(A)Structural organization of C3, based on the crystal structure published in Janssen et al. (8). The different domains, corresponding to the different fragments of the molecule are depicted with different color. Of note are the ANA domain, corresponding to C3a (red), the domains corresponding to C3c (green), and the TED domain, corresponding to C3d (blue).(B)Steps of cleavage of C3 by the C3 convertase of the alternative pathway C3bBb and generation of its activation forms. The cofactors needed for the cleavage of C3b by Factor I (FI) are indicated. The activation forms of C3, recognized by the autoantibodies (C3(H2O), C3b, iC3b, C3c) are indicated as complexed with an antibody in orange.(C)Proposed mode of action of the anti-C3b autoantibodies. The Ab will bind to C3b, preventing its interaction with CR1, and in the context of lupus nephritis, with Factor H (FH). This loss of regulation, together with the direct stabilization/enhancement of formation of the alternative pathway C3 convertase C3bBb will result in overactivation of complement, generating inflammation (via C3a), massive deposits of C3 activation fragments and finally, tissue damage. The exact mechanism linking complement overactivation to tissue injury and its contribution to organ injury need further studies. C3 undergoes low-rate spontaneous conformational change, leading to hydrolysis of its thioester bond (9). This generates GSK 4027 an activated form of the protein, called C3(H2O). It does not bind to surfaces (as.