Complement activation in factor D-deficient mice

Abstract

To assess the contribution of the alternative pathway in complement activation and host defense and its possible role in the regulation of systemic energy balance in vivo, factor D-deficient mice were generated by gene targeting. The mutant mice have no apparent abnormality in development and their body weights are similar to those of factor D-sufficient littermates. Complement activation could not be initiated in the serum of deficient mice by the alternative pathway activators rabbit erythrocytes and zymosan. Surprisingly, injection of cobra venom factor (CVF) caused a profound and reproducible reduction in serum C3 levels, whereas, as expected, there was no C3 reduction in factor B-deficient mice treated similarly. Studies of C3 and factor B activation in vitro by CVF demonstrated that in factor D-deficient serum the ␣ chain of C3 was cleaved gradually over a period of 60 min without detectable cleavage of factor B. CVF-dependent C3 cleavage in the deficient serum required the presence of Mg 2؉ , whereas in normal mouse serum the presence of divalent cations was not required. These results suggest that in mouse proteolytic cleavage of factor B by factor D is not an absolute requirement for the zymogen to active enzyme conformational transition of CVF-bound factor B. Kinetics of opsonization of Streptococcus pneumoniae by C3 fragments was much slower in factor D-deficient serum, suggesting a significant contribution of the alternative pathway to antibacterial host defense early after infection. T he alternative pathway (AP) of complement activation is a self-amplifying mechanism important for pathogen recognition and elimination in the absence of specific antibodies. The AP also amplifies complement activation initiated by the other two pathways of complement activation, classical and lectin (1). The dual function (recognition and amplification) of the AP underlines its importance for host defense against pathogens. The proximal result of complement activation is the formation of convertases, enzymes that activate C3 and C5, generating biologically active protein fragments and complexes (1). In the AP, assembly of the C3͞C5 convertase requires the initial attachment of the C3b fragment of C3 to the surface of a pathogen and proceeds through the formation of a complex with factor B (C3bB) and the subsequent cleavage of factor B by factor D to form C3bBb, the C3͞C5 convertase of the AP.Mouse factor D was initially recognized as a serine protease encoded by a differentiation-specific message present mainly in adipocytes and cells of the nervous system (2). Because its mRNA was significantly reduced in mouse and rat models of obesity, it was thought that the protein was involved in fat metabolism (3). These observations led to naming the protein adipsin, which soon was identified with mouse factor D (4). Subsequently it was demonstrated that adipocytes also synthesize and secrete C3 and factor B, leading to the formation of a C3 convertase in culture supernatants (5). Other results indicated that C3a desArg , the ...

The classical pathway is the dominant complement pathway required for innate immunity toStreptococcus pneumoniaeinfection in mice

“…As the MBL pathway contributed little toward C3 deposition, the low-intensity C3 deposition on bacteria incubated in Bf Ϫ/Ϫ serum represents the pure effects of classical pathwaydependent C3 activation. These results confirmed that a major role for the alternative pathway is amplification of C3 deposition (25), whereas the proportion of bacteria bound with C3 is influenced mainly by the classical pathway.…”

“…By using mice with specific genetic defects, we have characterized the roles of the classical, alternative, and MBL complement pathways for innate immunity to S. pneumoniae. The results of the C3-binding assays demonstrate that in the absence of specific anti-S. pneumoniae IgG, the proportion of bacteria opsonized by complement depends mainly on the classical pathway, and that the major role of the alternative pathway, as shown by Xu et al (25), is amplification of complement activation leading to denser C3 deposition on the bacteria. The low level of C3 binding to S. pneumoniae in C1qa Ϫ/Ϫ .Bf Ϫ/Ϫ serum suggests that the MBL pathway has only a minor role in complement activation by S. pneumoniae.…”

Mannan-Binding Lectin-Associated Serine Protease (MASP)-1 Is Crucial for Lectin Pathway Activation in Human Serum, whereas neither MASP-1 nor MASP-3 Is Required for Alternative Pathway Function

“…This is in agreement with observations that fDdeficient individuals have no detectable alternative pathway activity (42,43). Notably, fD-deficient mice also are reported to have no appreciable alternative pathway activity (44). In contrast, we find that MASP-1 is crucially involved in the lectin pathway in humans (Fig.…”

Complement C1q and C3 Are Critical for the Innate Immune Response to Streptococcus pneumoniae in the Central Nervous System

“…Combined, these findings demonstrate that the classical and the alternative pathways are both required for efficient opsonophagocytosis of S. pneumoniae (in nonimmunized hosts). The latter pathway amplifies pneumococcal opsonization by C3, which the former pathway initiated (42,43). Second, the results of our reconstitution experiments, when combined with the finding that pneumococcal infection induces complement expression in the CNS, show that the inability of the host immune response to overcome S. pneumoniae infection within the CSF (44) does not seem to be due to the lack of C proteins.…”