The human immune system has developed an elaborate network of cascades for dealing with microbial intruders. Owing to its ability to rapidly recognize and eliminate microorganisms, the complement system is an essential and efficient component of this machinery. However, many pathogenic organisms have found ways to escape the attack of complement through a range of different mechanisms. Recent discoveries in this field have provided important insights into these processes on a molecular level. These vital developments could augment our knowledge of the pathology and treatment of infectious and inflammatory diseases.Evading the manifold attacks of the immune system is a key determinant for the survival of pathogens within their hosts. It is not surprising, therefore, that the coexistence and coevolution of humans and microorganisms has produced a multitude of microbial mechanisms for attenuating or escaping these attacks. As a first line of defence against pathogenic intruders, and a mediator between the innate and adaptive immune responses, the complement system is a particular focus of these evasion strategies. Although this carefully regulated cascade of enzymes, protein complexes and receptors ensures the rapid recognition and elimination of foreign structures, it also offers many sites of interference that can disrupt this balanced network of protein interactions. A detailed understanding of the individual processes and the underlying interactions on a molecular level is essential for describing the mechanisms of infectious diseases and the development of new therapies. Recent discoveries of complement-targeting proteins, the availability of complete microbial genome sequences and advances in experimental methods have propelled this area of research, and provide fascinating insights into complement attack and evasion. Many pathogens seem to have developed parallel routes for escaping complement, and several evasion principles are shared not only among members of the same genus but even among diverse organisms, such as bacteria, viruses, fungi and parasites.In this Review, we will provide a comprehensive over-view and update of the exciting recent developments in this field. After a short introduction that will discuss the diverse role of the complement system in defence, disease and infection, the emphasis will be on the functional and structural aspects of the evasion strategies of human pathogens. Rather than separating them by organism, we classify distinct and common mechanisms for all pathogens based on their mode of action. In light of recent findings, the unique evasion strategies of Staphylococcus

To provide insight into bacterial suppression of complement-mediated immunity, we present here structures of a bacterial complement inhibitory protein, both free and bound to its complement target. The 1.25-A structure of the complement component C3-inhibitory domain of Staphylococcus aureus extracellular fibrinogen-binding protein (Efb-C) demonstrated a helical motif involved in complement regulation, whereas the 2.2-A structure of Efb-C bound to the C3d domain of human C3 allowed insight into the recognition of complement proteins by invading pathogens. Our structure-function studies provided evidence for a previously unrecognized mode of complement inhibition whereby Efb-C binds to native C3 and alters the solution conformation of C3 in a way that renders it unable to participate in successful 'downstream' activation of the complement response.

Neutrophils are indispensable for clearing infections with the prominent human pathogen Staphylococcus aureus. Here, we report that S. aureus secretes a family of proteins that potently inhibits the activity of neutrophil serine proteases (NSPs): neutrophil elastase (NE), proteinase 3, and cathepsin G. The NSPs, but not related serine proteases, are specifically blocked by the extracellular adherence protein (Eap) and the functionally orphan Eap homologs EapH1 and EapH2, with inhibitory-constant values in the low-nanomolar range. Eap proteins are together essential for NSP inhibition by S. aureus in vitro and promote staphylococcal infection in vivo. The crystal structure of the EapH1/NE complex showed that Eap molecules constitute a unique class of noncovalent protease inhibitors that occlude the catalytic cleft of NSPs. These findings increase our insights into the complex pathogenesis of S. aureus infections and create opportunities to design novel treatment strategies for inflammatory conditions related to excessive NSP activity.immune evasion | bacteria | phagocytes

H edgehog (Hh) is a secreted signaling molecule that mediates key tissue-patterning events during both vertebrate and invertebrate development (1-3). Hh also plays an important role in the maintenance and regulation of stem cells in adult organisms (4, 5). Abnormal activation of the Hh signaling pathway has been implicated in the initiation and growth of many human tumors (6), and drugs targeting the Hh pathway are under development (7).Hh is secreted but undergoes two lipid modifications that restrict its free diffusion and facilitate transport to appropriate target sites (8,9). Hh binding to Ptc, a 12-pass integral membrane protein with homology to bacterial resistance-nodulationdivision (RND) transporters (10) is a central event in Hh signaling and blocks the ability of Ptc to inhibit the seven-pass integral membrane protein Smoothened (Smo), a positive regulator of Hh responses (10). Recent genetic and RNAi experiments implicate the membrane-associated proteins Ihog and dally-like protein (Dlp) in Hh responsiveness (11)(12)(13)(14). Dlp, a member of the glypican family of heparan sulfate proteoglycans (HSPGs) (15), has dual roles in mediating Hh responsiveness and in transport of the Hh signal to distant cells (12)(13)(14). Heparan sulfate glycosaminoglycan chains, in particular, have also been implicated in Hh movement by the observation that tout velu, which encodes a heparan sulfate copolymerase (16, 17) that acts on Dlp (12), is required for normal Hh transport.Ihog, a Drosophila protein previously known as CG9211, is a type I transmembrane protein with four Ig domains followed by two FNIII domains, a membrane-spanning region, and a cytoplasmic region of no known function (11). Ihog is homologous to another Drosophila protein, CG32796 or brother of Ihog (BOI), and two mammalian proteins, CDO and BOC (18,19), that are also components of the Hh signaling pathway (11,20,21). Several observations indicate that Ihog may function as a coreceptor for Hh: (i) reduction of Ihog expression results in diminished Hh binding and responsiveness, (ii) epistasis experiments place Ihog function upstream or at the level of Ptc, (iii) the Ihog extracellular region is able to pull down HhN from conditioned medium, and (iv) coexpression of Ptc and Ihog results in a synergistic increase in Hh binding to the cell surface (11).The first FNIII domain of Ihog (IhogFn1) is necessary and sufficient to pull down HhN, but both FNIII domains are needed to synergize with Ptc and reconstitute Ihog function in cell-based signaling assays (11). Complicating interpretation of Ihog function is the observation that purified HhN and Ihog extracellular regions do not appreciably interact (see Fig. 2 A), indicating that a binary interaction between Ihog and Hh cannot explain Ihog function. To investigate the role of Ihog in Hh signaling, we initiated structural and biophysical studies of functional fragments of Drosophila melanogaster Ihog and Hh. Results and DiscussionStructure of Ihog FNIII Domains. The crystal structure of IhogFn1 was determ...

In subcellular fractionation experiments, we found PICD in both peroxisomes and cytoplasm of human and rat liver cells, with approximately 27% of total PICD protein associated with peroxisomes. The presence of PICD in mammalian peroxisomes suggests roles in the regeneration of NADPH for intraperoxisomal reductions, such as the conversion of 2,4-dienoylCoAs to 3-enoyl-CoAs, as well as in peroxisomal reactions that consume 2-oxoglutarate, namely the ␣-hydroxylation of phytanic acid. As for cytoplasmic PICD, the phenotypes of patients with glucose-6-phosphate dehydrogenase deficiency