RESUMEN
Properdin (P) is a positive regulatory protein that stabilizes the C3 convertase and C5 convertase of the complement alternative pathway (AP). Several studies have suggested that properdin can bind directly to the surface of certain pathogens regardless of the presence of C3bBb. Saprophytic Leptospira are susceptible to complement-mediated killing, but the interaction of properdin with Leptospira spp. has not been evaluated so far. In this work, we demonstrate that properdin present in normal human serum, purified properdin, as well as properdin oligomers P2, P3, and P4, interact with Leptospira. Properdin can bind directly to the bacterial surface even in the absence of C3b. In line with our previous findings, AP activation was shown to be important for killing non-pathogenic L. biflexa, and properdin plays a key role in this process since this microorganism survives in P-depleted human serum and the addition of purified properdin to P-depleted human serum decreases the number of viable leptospires. A panel of pathogenic L.interrogans recombinant proteins was used to identify putative properdin targets. Lsa30, an outer membrane protein from L. interrogans, binds to unfractionated properdin and to a lesser extent to P2-P4 properdin oligomers. In conclusion, properdin plays an important role in limiting bacterial proliferation of non-pathogenic Leptospira species. Once bound to the leptospiral surface, this positive complement regulatory protein of the AP contributes to the formation of the C3 convertase on the leptospire surface even in the absence of prior addition of C3b.
Asunto(s)
Complemento C3b/metabolismo , Factor B del Complemento/metabolismo , Leptospira interrogans/fisiología , Leptospira/fisiología , Leptospirosis/metabolismo , Properdina/metabolismo , Proteínas de la Membrana Bacteriana Externa/metabolismo , Procesos de Crecimiento Celular , Vía Alternativa del Complemento , Citotoxicidad Inmunológica , Humanos , Leptospira/patogenicidad , Leptospira interrogans/patogenicidad , Leptospirosis/inmunología , Properdina/inmunología , Unión Proteica , VirulenciaRESUMEN
Factor I deficiency causes a permanent, uncontrolled activation of the alternative pathway resulting in an increased turnover of C3 and consumption of factor B, factor H and properdin. Factor I deficiency is clinically associated with recurrent bacterial infections already in early infancy, mainly affecting the upper and lower respiratory tract, or presenting as meningitis or septicemia. We here report on a Brazilian family (n = 9) with known consanguinity, where in 3/7 children, suffering from chronic otitis, meningitis, and respiratory infections, a complete factor I deficiency was recognized. One of the patients died after fulminant sepsis. Hemolytic activity of the alternative pathway was not detectable in the patients' sera due to decreased plasma concentrations of C3, factor B and properdin. As a consequence of factor I deficiency, C3b could not be metabolized with the result that no C3-derived split products (C3dg/C3d) were detectable in the patients' sera. In vitro reconstitution with purified factor I restored the regulatory function in the patients' sera with the subsequent cleavage of C3b to C3c and C3dg. Factor H levels were decreased in all patients' sera and found to be tightly complexed with C3b resulting in a modified electrophoretic mobility. Upon factor I reconstitution, factor H was released from C3b regaining its beta 1 electrophoretic mobility. Complement-mediated biological functions like opsonization of bacteria, chemotactic activity and phagocytosis in these patients were impaired. The parents (cousins, 2nd degree) and 3/4 siblings had significantly reduced factor I plasma levels without further alteration in their complement profile. 3 of these obviously heterozygously deficient family members suffered from recurrent bacterial infections of different frequency and severity.