Microbes bind complement inhibitor factor H via a common site

There Is a Method to the Madness: Strategies to Study Host Complement Evasion by Lyme Disease and Relapsing Fever Spirochetes

Lyme disease and relapsing fever are caused by various Borrelia species. Lyme disease borreliae, the most common vector-borne pathogens in both the U.S. and Europe, are transmitted by Ixodes ticks and disseminate from the site of tick bites to tissues leading to erythema migrans skin rash, arthritis, carditis, and neuroborreliosis. Relapsing fever borreliae, carried by ticks and lice, trigger reoccurring fever episodes. Following transmission, spirochetes survive in the blood to induce bacteremia at the early stages of infection, which is thought to promote evasion of the host complement system. The complement system acts as an important innate immune defense mechanism in humans and vertebrates. Upon activation, the cleaved complement components form complexes on the pathogen surface to eventually promote bacteriolysis. The complement system is negatively modulated by a number of functionally diverse regulators to avoid tissue damage. To evade and inhibit the complement system, spirochetes are capable of binding complement components and regulators. Complement inhibition results in bacterial survival in serum (serum resistance) and is thought to promote bloodstream survival, which facilitates spirochete dissemination and disease manifestations. In this review, we discuss current methodologies to elucidate the mechanisms of Borrelia spp. that promote serum resistance and bloodstream survival, as well as novel methods to study factors responsible for bloodstream survival of Lyme disease borreliae that can be applied to relapsing fever borreliae. Understanding the mechanisms these pathogens utilize to evade the complement system will ultimately aid in the development of novel therapeutic strategies and disease prevention to improve human health.

Complement Evasion by Pathogenic Leptospira

Leptospirosis is a neglected infectious disease caused by spirochetes from the genus Leptospira. Pathogenic microorganisms, notably those which reach the blood circulation such as Leptospira, have evolved multiple strategies to escape the host complement system, which is important for innate and acquired immunity. Leptospira avoid complement-mediated killing through: (i) recruitment of host complement regulators; (ii) acquisition of host proteases that cleave complement proteins on the bacterial surface; and, (iii) secretion of proteases that inactivate complement proteins in the Leptospira surroundings. The recruitment of host soluble complement regulatory proteins includes the acquisition of Factor H (FH) and FH-like-1 (alternative pathway), C4b-binding protein (C4BP) (classical and lectin pathways), and vitronectin (Vn) (terminal pathway). Once bound to the leptospiral surface, FH and C4BP retain cofactor activity of Factor I in the cleavage of C3b and C4b, respectively. Vn acquisition by leptospires may result in terminal pathway inhibition by blocking C9 polymerization. The second evasion mechanism lies in plasminogen (PLG) binding to the leptospiral surface. In the presence of host activators, PLG is converted to enzymatically active plasmin, which is able to degrade C3b, C4b, and C5 at the surface of the pathogen. A third strategy used by leptospires to escape from complement system is the active secretion of proteases. Pathogenic, but not saprophytic leptospires, are able to secrete metalloproteases that cleave C3 (central complement molecule), Factor B (alternative pathway), and C4 and C2 (classical and lectin pathways). The purpose of this review is to fully explore these complement evasion mechanisms, which act together to favor Leptospira survival and multiplication in the host.

Hijacking Complement Regulatory Proteins for Bacterial Immune Evasion

The human complement system plays an important role in the defense against invading pathogens, inflammation and homeostasis. Invading microbes, such as bacteria, directly activate the complement system resulting in the formation of chemoattractants and in effective labeling of the bacteria for phagocytosis. In addition, formation of the membrane attack complex is responsible for direct killing of Gram-negative bacteria. In turn, bacteria have evolved several ways to evade complement activation on their surface in order to be able to colonize and invade the human host. One important mechanism of bacterial escape is attraction of complement regulatory proteins to the microbial surface. These molecules are present in the human body for tight regulation of the complement system to prevent damage to host self-surfaces. Therefore, recruitment of complement regulatory proteins to the bacterial surface results in decreased complement activation on the microbial surface which favors bacterial survival. This review will discuss recent advances in understanding the binding of complement regulatory proteins to the bacterial surface at the molecular level. This includes, new insights that have become available concerning specific conserved motives on complement regulatory proteins that are favorable for microbial binding. Finally, complement evasion molecules are of high importance for vaccine development due to their dominant role in bacterial survival, high immunogenicity and homology as well as their presence on the bacterial surface. Here, the use of complement evasion molecules for vaccine development will be discussed.

Complement factor H in host defense and immune evasion

Complement is the major humoral component of the innate immune system. It recognizes pathogen- and damage-associated molecular patterns, and initiates the immune response in coordination with innate and adaptive immunity. When activated, the complement system unleashes powerful cytotoxic and inflammatory mechanisms, and thus its tight control is crucial to prevent damage to host tissues and allow restoration of immune homeostasis. Factor H is the major soluble inhibitor of complement, where its binding to self markers (i.e., particular glycan structures) prevents complement activation and amplification on host surfaces. Not surprisingly, mutations and polymorphisms that affect recognition of self by factor H are associated with diseases of complement dysregulation, such as age-related macular degeneration and atypical haemolytic uremic syndrome. In addition, pathogens (i.e., non-self) and cancer cells (i.e., altered-self) can hijack factor H to evade the immune response. Here we review recent (and not so recent) literature on the structure and function of factor H, including the emerging roles of this protein in the pathophysiology of infectious diseases and cancer.

Multifunctional and Redundant Roles of Borrelia burgdorferi Outer Surface Proteins in Tissue Adhesion, Colonization, and Complement Evasion

Borrelia burgdorferi is the causative agent of Lyme disease in the U.S., with at least 25,000 cases reported to the CDC each year. B. burgdorferi is thought to enter and exit the bloodstream to achieve rapid dissemination to distal tissue sites during infection. Travel through the bloodstream requires evasion of immune surveillance and pathogen clearance in the host, a process at which B. burgdorferi is adept. B. burgdorferi encodes greater than 19 adhesive outer surface proteins many of which have been found to bind to host cells or components of the extracellular matrix. Several others bind to host complement regulatory factors, in vitro. Production of many of these adhesive proteins is tightly regulated by environmental cues, and some have been shown to aid in vascular interactions and tissue colonization, as well as survival in the blood, in vivo. Recent work has described multifaceted and redundant roles of B. burgdorferi outer surface proteins in complement component interactions and tissue targeted adhesion and colonization, distinct from their previously identified in vitro binding capabilities. Recent insights into the multifunctional roles of previously well-characterized outer surface proteins such as BBK32, DbpA, CspA, and OspC have changed the way we think about the surface proteome of these organisms during the tick-mammal life cycle. With the combination of new and old in vivo models and in vitro techniques, the field has identified distinct ligand binding domains on BBK32 and DbpA that afford tissue colonization or blood survival to B. burgdorferi. In this review, we describe the multifunctional and redundant roles of many adhesive outer surface proteins of B. burgdorferi in tissue adhesion, colonization, and bloodstream survival that, together, promote the survival of Borrelia spp. throughout maintenance in their multi-host lifestyle.

Anti-complement-factor H-associated glomerulopathies

Atypical haemolytic uraemic syndrome (aHUS), an important cause of acute kidney injury, is characterized by dysregulation of the complement pathway, frequent need for dialysis, and progression to end-stage renal disease. Autoantibodies against complement factor H (FH), the main plasma regulatory protein of the alternative pathway of the complement system, account for a considerable proportion of children with aHUS. The autoantibodies are usually associated with the occurrence of a homozygous deletion in the genes encoding the FH-related proteins FHR1 and FHR3. High levels of autoantibodies, noted at the onset of disease and during relapses, induce functional deficiency of FH, whereas their decline, in response to plasma exchanges and/or immunosuppressive therapy, is associated with disease remission. Management with plasma exchange and immunosuppression is remarkably effective in inducing and maintaining remission in aHUS associated with FH autoantibodies, whereas terminal complement blockade with eculizumab is considered the most effective therapy in other forms of aHUS. Anti-FH autoantibodies are also detected in a small proportion of patients with C3 glomerulopathies, which are characterized by chronic glomerular injury mediated by activation of the alternative complement pathway and predominant C3 deposits on renal histology.

Self-nonself discrimination by the complement system

The alternative pathway (AP) of complement can recognize nonself structures by only two molecules, C3b and factor H. The AP deposits C3b covalently on nonself structures via an amplification system. The actual discrimination is performed by factor H, which has binding sites for polyanions (sialic acids, glycosaminoglycans, phospholipids). This robust recognition of 'self' protects our own intact viable cells and tissues, while activating structures are recognized by default. Foreign targets are opsonized for phagocytosis or killed. Mutations in factor H predispose to severe diseases. In hemolytic uremic syndrome, they promote complement attack against blood cells and vascular endothelial cells and lead, for example, to kidney and brain damage. Even pathogens can exploit factor H. In fact, the ability to bind factor H discriminates most pathogenic microbes from nonpathogenic ones.

Recruitment of Factor H to the Streptococcus suis Cell Surface is Multifactorial

Streptococcus suis is an important bacterial swine pathogen and a zoonotic agent. Recently, two surface proteins of S. suis, Fhb and Fhbp, have been described for their capacity to bind factor H—a soluble complement regulatory protein that protects host cells from complement-mediated damages. Results obtained in this study showed an important role of host factor H in the adhesion of S. suis to epithelial and endothelial cells. Both Fhb and Fhbp play, to a certain extent, a role in such increased factor H-dependent adhesion. The capsular polysaccharide (CPS) of S. suis, independently of the presence of its sialic acid moiety, was also shown to be involved in the recruitment of factor H. However, a triple mutant lacking Fhb, Fhbp and CPS was still able to recruit factor H resulting in the degradation of C3b in the presence of factor I. In the presence of complement factors, the double mutant lacking Fhb and Fhbp was similarly phagocytosed by human macrophages and killed by pig blood when compared to the wild-type strain. In conclusion, this study suggests that recruitment of factor H to the S. suis cell surface is multifactorial and redundant.

Contribution of the outer membrane protein OmpW in Escherichia coli to complement resistance from binding to factor H

The serum complement system is essential for innate immune defense against invading pathogenic bacteria. Some of the 8-stranded β-barrel outer membrane proteins confer bacterial resistance to the innate host immunity. We have previously demonstrated that OmpW, also an 8-stranded β-barrel protein that was identified a decade ago, protects bacteria against host phagocytosis. In this study, we investigated the complement resistance of OmpW. Our results indicate that the upregulation of OmpW is associated with increased survival when bacteria are exposed to normal human sera (NHS). Mutant bacteria lacking OmpW in NHS exhibited significantly lower survival rates in comparison to wild-type and ompW complemented bacteria. Furthermore, the bacterial survival significantly decreased in NHS that was supplemented with EGTA-Mg(2+) compared to that in NHS supplemented with EDTA. These results suggest that OmpW confer resistance to alternative complement pathway-mediated killing. Moreover, the binding of OmpW to factor H, a major inhibitor of alternative pathway, was found, indicating that OmpW recruitment of factor H is a mechanism for bacterial evasion of complement attack.

Travelling between Two Worlds: Complement as a Gatekeeper for an Expanded Host Range of Lyme Disease Spirochetes

Evading innate immunity is a prerequisite for pathogenic microorganisms in order to survive in their respective hosts. Concerning Lyme disease spirochetes belonging to the Borrelia (B.) burgdorferi sensu lato group, a broad range of diverse vertebrates serve as reservoir or even as incidental hosts, including humans. The capability to infect multiple hosts implies that spirochetes have developed sophisticated means to counter the destructive effects of complement of humans and various animals. While the means by which spirochetes overcome the hosts immune defense are far from being completely understood, there is a growing body of evidence suggesting that binding of the key regulator of the alternative pathway, Factor H, plays a pivotal role for immune evasion and that Factor H is an important determinant of host specificity. This review covers (i) the contribution of complement in host-specificity and transmissibility of Lyme disease spirochetes; (ii) the involvement of borrelial-derived determinants to host specificity; (iii) the interplay of human and animal Factor H with complement-acquiring surface proteins of diverse borrelial species; and (iv) the potential role of additional animal complement proteins in the immune evasion of spirochetes.

FHR3 Blocks C3d-Mediated Coactivation of Human B Cells

The autoimmune renal disease deficient for complement factor H-related (CFHR) genes and autoantibody-positive form of hemolytic uremic syndrome is characterized by the presence of autoantibodies specific for the central complement regulator, factor H, combined with a homozygous deficiency, mostly in CFHR3 and CFHR1 Because FHR3 and FHR1 bind to C3d and inactivated C3b, which are ligands for complement receptor type 2 (CR2/CD21), the aim of the current study was to examine whether FHR3-C3d or FHR1-C3d complexes modulate B cell activation. Laser-scanning microscopy and automated image-based analysis showed that FHR3, but not FHR1 or factor H, blocked B cell activation by the BCR coreceptor complex (CD19/CD21/CD81). FHR3 bound to C3d, thereby inhibiting the interaction between C3d and CD21 and preventing colocalization of the coreceptor complex with the BCR. FHR3 neutralized the adjuvant effect of C3d on B cells, as shown by inhibited intracellular CD19 and Akt phosphorylation in Raji cells, as well as Ca(2+) release in peripheral B cells. In cases of CFHR3/CFHR1 deficiency, the FHR3 binding sites on C3d are occupied by factor H, which lacks B cell-inhibitory functions. These data provide evidence that FHR3, which is absent in patients with the autoimmune form of hemolytic uremic syndrome, is involved in B cell regulation.

Utilizing complement evasion strategies to design complement-based antibacterial immunotherapeutics: Lessons from the pathogenic Neisseriae

Novel therapies are urgently needed to combat the global threat of multidrug-resistant pathogens. Complement forms an important arm of innate defenses against infections. In physiological conditions, complement activation is tightly controlled by soluble and membrane-associated complement inhibitors, but must be selectively activated on invading pathogens to facilitate microbial clearance. Many pathogens, including Neisseria gonorrhoeae and N. meningitidis, express glycans, including N-acetylneuraminic acid (Neu5Ac), that mimic host structures to evade host immunity. Neu5Ac is a negatively charged 9-cabon sugar that inhibits complement, in part by enhancing binding of the complement inhibitor factor H (FH) through C-terminal domains (19 and 20) on FH. Other microbes also bind FH, in most instances through FH domains 6 and 7 or 18-20. Here we describe two strategies to target complement activation on Neisseriae. First, microbial binding domains of FH were fused to IgG Fc to create FH18-20/Fc (binds gonococci) and FH6,7/Fc (binds meningococci). A point mutation in FH domain 19 eliminated hemolysis caused by unmodified FH18-20, but retained binding to gonococci. FH18-20/Fc and FH6,7/Fc mediated complement-dependent killing in vitro and showed efficacy in animal models of gonorrhea and meningococcal bacteremia, respectively. The second strategy utilized CMP-nonulosonate (CMP-NulO) analogs of sialic acid that were incorporated into LOS and prevented complement inhibition by physiologic CMP-Neu5Ac and resulted in attenuated gonococcal infection in mice. While studies to establish the safety of these agents are needed, enhancing complement activation on microbes may represent a promising strategy to treat antimicrobial resistant organisms.

The mammalian complement system as an epitome of host-pathogen genetic conflicts

The complement system is an innate immunity effector mechanism; its action is antagonized by a wide array of pathogens and complement evasion determines the virulence of several infections. We investigated the evolutionary history of the complement system and of bacterial-encoded complement-interacting proteins. Complement components targeted by several pathogens evolved under strong selective pressure in primates, with selection acting on residues at the contact interface with microbial/viral proteins. Positively selected sites in CFH and C4BPA account for the human specificity of gonococcal infection. Bacterial interactors, evolved adaptively as well, with selected sites located at interaction surfaces with primate complement proteins. These results epitomize the expectation under a genetic conflict scenario whereby the host's and the pathogen's genes evolve within binding avoidance-binding seeking dynamics. In silico mutagenesis and protein-protein docking analyses supported this by showing that positively selected sites, both in the host's and in the pathogen's interacting partner, modulate binding.

Defining the Binding Region in Factor H to Develop a Therapeutic Factor H-Fc Fusion Protein against Non-Typeable Haemophilus influenzae

Non-typeable Haemophilus influenzae (NTHi) cause a range of illnesses including otitis media, sinusitis, and exacerbation of chronic obstructive pulmonary disease, infections that contribute to the problem of antibiotic resistance and are themselves often intractable to standard antibiotic treatment regimens. We investigated a strategy to exploit binding of the complement inhibitor Factor H (FH) to NTHi as a functional target for an immunotherapeutic containing the NTHi binding domain of FH fused to the Fc domain of IgG1. Chimeric proteins containing the regions that most FH-binding bacteria use to engage human FH, domains 6 and 7 (FH6,7/Fc) and/or 18 through 20 (FH18-20/Fc), were evaluated for binding to NTHi. FH6,7/Fc bound strongly to each of seven NTHi clinical isolates tested and efficiently promoted complement-mediated killing by normal human serum. FH18-20/Fc bound weakly to three of the strains but did not promote complement dependent killing. Outer-membrane protein P5 has been implicated in FH binding by NTHi, and FH6,7/Fc binding was greatly diminished in five of seven P5 deficient isogenic mutant strains tested, implicating an alternative FH binding protein in some strains. Binding of FH18-20/Fc was decreased in the P5 mutant of one strain. A murine model was used to evaluate potential therapeutic application of FH6,7/Fc. FH6,7/Fc efficiently promoted binding of C3 to NTHi exposed to mouse serum, and intranasal delivery of FH6,7/Fc resulted in significantly enhanced clearance of NTHi from the lung. Moreover, a P5 deficient mutant was attenuated for survival in the lung model, suggesting that escape mutants lacking P5 would be less likely to replace strains susceptible to FH6,7/Fc. These results provide evidence for the potential utility of FH6,7/Fc as a therapeutic against NTHi lung infection. FH binding is a common property of many respiratory tract pathogens and FH/Fc chimeras may represent promising alternative or adjunctive therapeutics against such infections, which are often polymicrobial.

The Plasmodium falciparum blood stages acquire factor H family proteins to evade destruction by human complement

The acquisition of regulatory proteins is a means of blood-borne pathogens to avoid destruction by the human complement. We recently showed that the gametes of the human malaria parasite Plasmodium falciparum bind factor H (FH) from the blood meal of the mosquito vector to assure successful sexual reproduction, which takes places in the mosquito midgut. While these findings provided a first glimpse of a complex mechanism used by Plasmodium to control the host immune attack, it is hitherto not known, how the pathogenic blood stages of the malaria parasite evade destruction by the human complement. We now show that the human complement system represents a severe threat for the replicating blood stages, particularly for the reinvading merozoites, with complement factor C3b accumulating on the surfaces of the intraerythrocytic schizonts as well as of free merozoites. C3b accumulation initiates terminal complement complex formation, in consequence resulting in blood stage lysis. To inactivate C3b, the parasites bind FH as well as related proteins FHL-1 and CFHR-1 to their surface, and FH binding is trypsin-resistant. Schizonts acquire FH via two contact sites, which involve CCP modules 5 and 20. Blockage of FH-mediated protection via anti-FH antibodies results in significantly impaired blood stage replication, pointing to the plasmodial complement evasion machinery as a promising malaria vaccine target.

Conserved Patterns of Microbial Immune Escape: Pathogenic Microbes of Diverse Origin Target the Human Terminal Complement Inhibitor Vitronectin via a Single Common Motif

Pathogenicity of many microbes relies on their capacity to resist innate immunity, and to survive and persist in an immunocompetent human host microbes have developed highly efficient and sophisticated complement evasion strategies. Here we show that different human pathogens including Gram-negative and Gram-positive bacteria, as well as the fungal pathogen Candida albicans, acquire the human terminal complement regulator vitronectin to their surface. By using truncated vitronectin fragments we found that all analyzed microbial pathogens (n = 13) bound human vitronectin via the same C-terminal heparin-binding domain (amino acids 352-374). This specific interaction leaves the terminal complement complex (TCC) regulatory region of vitronectin accessible, allowing inhibition of C5b-7 membrane insertion and C9 polymerization. Vitronectin complexed with the various microbes and corresponding proteins was thus functionally active and inhibited complement-mediated C5b-9 deposition. Taken together, diverse microbial pathogens expressing different structurally unrelated vitronectin-binding molecules interact with host vitronectin via the same conserved region to allow versatile control of the host innate immune response.

A Novel Factor H-Fc Chimeric Immunotherapeutic Molecule against Neisseria gonorrhoeae

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection gonorrhea, has developed resistance to almost every conventional antibiotic. There is an urgent need to develop novel therapies against gonorrhea. Many pathogens, including N. gonorrhoeae, bind the complement inhibitor factor H (FH) to evade complement-dependent killing. Sialylation of gonococcal lipooligosaccharide, as occurs in vivo, augments binding of human FH through its domains 18-20 (FH18-20). We explored the use of fusing FH18-20 with IgG Fc (FH18-20/Fc) to create a novel anti-infective immunotherapeutic. FH18-20 also binds to select host glycosaminoglycans to limit unwanted complement activation on host cells. To identify mutation(s) in FH18-20 that eliminated complement activation on host cells, yet maintained binding to N. gonorrhoeae, we created four mutations in domains 19 or 20 described in atypical hemolytic uremic syndrome that prevented binding of mutated fH to human erythrocytes. One of the mutant proteins (D to G at position 1119 in domain 19; FHD1119G/Fc) facilitated complement-dependent killing of gonococci similar to unmodified FH18-20/Fc but, unlike FH18-20/Fc, did not lyse human erythrocytes. FHD1119G/Fc bound to all (100%) of 15 sialylated clinical N. gonorrhoeae isolates tested (including three contemporary ceftriaxone-resistant strains), mediated complement-dependent killing of 10 of 15 (67%) strains, and enhanced C3 deposition (≥10-fold above baseline levels) on each of the five isolates not directly killed by complement. FHD1119G/Fc facilitated opsonophagocytic killing of a serum-resistant strain by human polymorphonuclear neutrophils. FHD1119G/Fc administered intravaginally significantly reduced the duration and burden of gonococcal infection in the mouse vaginal colonization model. FHD1119G/Fc represents a novel immunotherapeutic against multidrug-resistant N. gonorrhoeae.

Recruitment of Factor H as a Novel Complement Evasion Strategy for Blood-Stage Plasmodium falciparum Infection

The human complement system is the frontline defense mechanism against invading pathogens. The coexistence of humans and microbes throughout evolution has produced ingenious molecular mechanisms by which microorganisms escape complement attack. A common evasion strategy used by diverse pathogens is the hijacking of soluble human complement regulators to their surfaces to afford protection from complement activation. One such host regulator is factor H (FH), which acts as a negative regulator of complement to protect host tissues from aberrant complement activation. In this report, we show that Plasmodium falciparum merozoites, the invasive form of the malaria parasites, actively recruit FH and its alternative spliced form FH-like protein 1 when exposed to human serum. We have mapped the binding site in FH that recognizes merozoites and identified Pf92, a member of the six-cysteine family of Plasmodium surface proteins, as its direct interaction partner. When bound to merozoites, FH retains cofactor activity, a key function that allows it to downregulate the alternative pathway of complement. In P. falciparum parasites that lack Pf92, we observed changes in the pattern of C3b cleavage that are consistent with decreased regulation of complement activation. These results also show that recruitment of FH affords P. falciparum merozoites protection from complement-mediated lysis. Our study provides new insights on mechanisms of immune evasion of malaria parasites and highlights the important function of surface coat proteins in the interplay between complement regulation and successful infection of the host.

Streptococcus pneumoniae proteomics: determinants of pathogenesis and vaccine development

Streptococcus pneumoniae is a major pathogen that is responsible for a variety of invasive diseases. The bacteria gain entry initially by establishing a carriage state in the nasopharynx from where they migrate to other sites in the body. The worldwide distribution of the bacteria and the severity of the diseases have led to a significant level of interest in the development of vaccines against the bacteria. Current vaccines, based on the bacterial polysaccharide, have a number of limitations including poor immunogenicity and limited effectiveness against all pneumococcal serotypes. There are many challenges in developing vaccines that will be effective against the diverse range of isolates and serotypes for this highly variable bacterial pathogen. This review considers how proteomic technologies have extended our understanding of the pathogenic mechanisms of nasopharyngeal colonization and disease development as well as the critical areas in developing protein-based vaccines.

Complement Factor H Binds to Human Serum Apolipoprotein E and Mediates Complement Regulation on High Density Lipoprotein Particles

The alternative pathway of complement is an important part of the innate immunity response against foreign particles invading the human body. To avoid damage to host cells, it needs to be efficiently down-regulated by plasma factor H (FH) as exemplified by various diseases caused by mutations in its domains 19-20 (FH19-20) and 5-7 (FH5-7). These regions are also the main interaction sites for microbial pathogens that bind host FH to evade complement attack. We previously showed that inhibition of FH binding by a recombinant FH5-7 construct impairs survival of FH binding pathogens in human blood. In this study we found that upon exposure to full blood, the addition of FH5-7 reduces survival of, surprisingly, also those microbes that are not able to bind FH. This effect was mediated by inhibition of complement regulation and subsequently enhanced neutrophil phagocytosis by FH5-7. We found that although FH5-7 does not reduce complement regulation in the actual fluid phase of plasma, it reduces regulation on HDL particles in plasma. Using affinity chromatography and mass spectrometry we revealed that FH interacts with serum apolipoprotein E (apoE) via FH5-7 domains. Furthermore, binding of FH5-7 to HDL was dependent on the concentration of apoE on the HDL particles. These findings explain why the addition of FH5-7 to plasma leads to excessive complement activation and phagocytosis of microbes in full anticoagulated blood. In conclusion, our data show how FH interacts with apoE molecules via domains 5-7 and regulates alternative pathway activation on plasma HDL particles.

Evasion and interactions of the humoral innate immune response in pathogen invasion, autoimmune disease, and cancer

The humoral innate immune system is composed of three major branches, complement, coagulation, and natural antibodies. To persist in the host, pathogens, such as bacteria, viruses, and cancers must evade parts of the innate humoral immune system. Disruptions in the humoral innate immune system also play a role in the development of autoimmune diseases. This review will examine how Gram positive bacteria, viruses, cancer, and the autoimmune conditions systemic lupus erythematosus and anti-phospholipid syndrome, interact with these immune system components. Through examining evasion techniques it becomes clear that an interplay between these three systems exists. By exploring the interplay and the evasion/disruption of the humoral innate immune system, we can develop a better understanding of pathogenic infections, cancer, and autoimmune disease development.

Complement System Part II: Role in Immunity

The complement system has been considered for a long time as a simple lytic cascade, aimed to kill bacteria infecting the host organism. Nowadays, this vision has changed and it is well accepted that complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. This review discusses recent advances in the understanding of the role of complement in physiology and pathology. It starts with a description of complement contribution to the normal physiology (homeostasis) of a healthy organism, including the silent clearance of apoptotic cells and maintenance of cell survival. In pathology, complement can be a friend or a foe. It acts as a friend in the defense against pathogens, by inducing opsonization and a direct killing by C5b–9 membrane attack complex and by triggering inflammatory responses with the anaphylatoxins C3a and C5a. Opsonization plays also a major role in the mounting of an adaptive immune response, involving antigen presenting cells, T-, and B-lymphocytes. Nevertheless, it can be also an enemy, when pathogens hijack complement regulators to protect themselves from the immune system. Inadequate complement activation becomes a disease cause, as in atypical hemolytic uremic syndrome, C3 glomerulopathies, and systemic lupus erythematosus. Age-related macular degeneration and cancer will be described as examples showing that complement contributes to a large variety of conditions, far exceeding the classical examples of diseases associated with complement deficiencies. Finally, we discuss complement as a therapeutic target.

Mechanisms of Bone Resorption in Periodontitis

Alveolar bone loss is a hallmark of periodontitis progression and its prevention is a key clinical challenge in periodontal disease treatment. Bone destruction is mediated by the host immune and inflammatory response to the microbial challenge. However, the mechanisms by which the local immune response against periodontopathic bacteria disturbs the homeostatic balance of bone formation and resorption in favour of bone loss remain to be established. The osteoclast, the principal bone resorptive cell, differentiates from monocyte/macrophage precursors under the regulation of the critical cytokines macrophage colony-stimulating factor, RANK ligand, and osteoprotegerin. TNF-α, IL-1, and PGE₂ also promote osteoclast activity, particularly in states of inflammatory osteolysis such as those found in periodontitis. The pathogenic processes of destructive inflammatory periodontal diseases are instigated by subgingival plaque microflora and factors such as lipopolysaccharides derived from specific pathogens. These are propagated by host inflammatory and immune cell influences, and the activation of T and B cells initiates the adaptive immune response via regulation of the Th1-Th2-Th17 regulatory axis. In summary, Th1-type T lymphocytes, B cell macrophages, and neutrophils promote bone loss through upregulated production of proinflammatory mediators and activation of the RANK-L expression pathways.

Edwardsiella tarda evades serum killing by preventing complement activation via the alternative pathway

Edwardsiella tarda is a Gram-negative bacterium with a broad host range that includes a wide variety of farmed fish as well as humans. E. tarda has long been known to be able to survive in host serum, but the relevant mechanism is unclear. In this study, we investigated the fundamental question, i.e. whether E. tarda activated serum complement or not. We found that (i) when incubated with flounder serum, E. tarda exhibited a high survival rate (87.6%), which was slightly but significantly reduced in the presence of Mg(2+); (ii) E. tarda-incubated serum possessed strong hemolytic activity and bactericidal activity, (iii) compared to the serum incubated with a complement-sensitive laboratory Escherichia coli strain, E. tarda-incubated serum exhibited much less chemotactic activity, (iv) in contrast to the serum incubated with live E. tarda, the serum incubated with heat-inactivated E. tarda exhibited no apparent hemolytic capacity. Taken together, these results indicate for the first time that E. tarda circumvents serum attack by preventing, to a large extent, complement activation via the alternative pathway, and that heat-labile surface structures likely play an essential role in the complement evasion of E. tarda.

More than just immune evasion: Hijacking complement by Plasmodium falciparum

Malaria remains one of the world's deadliest diseases. Plasmodium falciparum is responsible for the most severe and lethal form of human malaria. P. falciparum's life cycle involves two obligate hosts: human and mosquito. From initial entry into these hosts, malaria parasites face the onslaught of the first line of host defence, the complement system. In this review, we discuss the complex interaction between complement and malaria infection in terms of hosts immune responses, parasite survival and pathogenesis of severe forms of malaria. We will focus on the role of complement receptor 1 and its associated polymorphisms in malaria immune complex clearance, as a mediator of parasite rosetting and as an entry receptor for P. falciparum invasion. Complement evasion strategies of P. falciparum parasites will also be highlighted. The sexual forms of the malaria parasites recruit the soluble human complement regulator Factor H to evade complement-mediated killing within the mosquito host. A novel evasion strategy is the deployment of parasite organelles to divert complement attack from infective blood stage parasites. Finally we outline the future challenge to understand the implications of these exploitation mechanisms in the interplay between successful infection of the host and pathogenesis observed in severe malaria.

Complement evasion by Bordetella pertussis: implications for improving current vaccines

Bordetella pertussis causes whooping cough or pertussis, a highly contagious disease of the respiratory tract. Despite high vaccination coverage, reported cases of pertussis are rising worldwide and it has become clear that the current vaccines must be improved. In addition to the well-known protective role of antibodies and T cells during B. pertussis infection, innate immune responses such as the complement system play an essential role in B. pertussis killing. In order to evade this complement activation and colonize the human host, B. pertussis expresses several molecules that inhibit complement activation. Interestingly, one of the known complement evasion proteins, autotransporter Vag8, is highly expressed in the recently emerged B. pertussis isolates. Here, we describe the current knowledge on how B. pertussis evades complement-mediated killing. In addition, we compare this to complement evasion strategies used by other bacterial species. Finally, we discuss the consequences of complement evasion by B. pertussis on adaptive immunity and how identification of the bacterial molecules and the mechanisms involved in complement evasion might help improve pertussis vaccines.

Anopheles midgut epithelium evades human complement activity by capturing factor H from the blood meal

Hematophagous vectors strictly require ingesting blood from their hosts to complete their life cycles. Exposure of the alimentary canal of these vectors to the host immune effectors necessitates efficient counteractive measures by hematophagous vectors. The Anopheles mosquito transmitting the malaria parasite is an example of hematophagous vectors that within seconds can ingest human blood double its weight. The innate immune defense mechanisms, like the complement system, in the human blood should thereby immediately react against foreign cells in the mosquito midgut. A prerequisite for complement activation is that the target cells lack complement regulators on their surfaces. In this work, we analyzed whether human complement is active in the mosquito midgut, and how the mosquito midgut cells protect themselves against complement attack. We found that complement remained active for a considerable time and was able to kill microbes within the mosquito midgut. However, the Anopheles mosquito midgut cells were not injured. These cells were found to protect themselves by capturing factor H, the main soluble inhibitor of the alternative complement pathway. Factor H inhibited complement on the midgut cells by promoting inactivation of C3b to iC3b and preventing the activity of the alternative pathway amplification C3 convertase enzyme. An interference of the FH regulatory activity by monoclonal antibodies, carried to the midgut via blood, resulted in increased mosquito mortality and reduced fecundity. By using a ligand blotting assay, a putative mosquito midgut FH receptor could be detected. Thereby, we have identified a novel mechanism whereby mosquitoes can tolerate human blood.

Mosquitoes are important vectors in the transmission of many human diseases. Their life cycle requires a blood meal to be completed. Ingested blood contains bioactive molecules belonging to the innate immune defense mechanisms against microbes, like the complement system, that can damage foreign cells. We have identified in this study a mechanism whereby mosquitoes can escape the damaging activity of the complement system in the ingested human blood. The mosquito midgut epithelial cell surface captured factor H, a natural regulator of the alternative pathway of complement activation, from the ingested blood. Consequently, the deposition of C3b, a key complement component, on the epithelial cell surface was impaired and cell death was avoided. Interfering with the complement regulatory activity of factor H by monoclonal antibodies, carried to the midgut via blood feeding, increased mosquito mortality and reduced fecundity. The putative Anopheles mosquito factor H binding proteins could be transmission blocking vaccine candidates targeting the malaria parasite carrying vectors.

The outer membrane protease PgtE of Salmonella enterica interferes with the alternative complement pathway by cleaving factors B and H

The virulence factor PgtE is an outer membrane protease (omptin) of the zoonotic pathogen Salmonella enterica that causes diseases ranging from gastroenteritis to severe enteric fever. It is surface exposed in bacteria that have a short-chain, i.e., rough LPS, as observed e.g., in bacteria residing inside macrophages or just emerging from them. We investigated whether PgtE cleaves the complement factors B (B) and H (H), key proteins controlling formation and inactivation of the complement protein C3b and thereby the activity of the complement system. S. enterica serovar Typhimurium or omptin-expressing recombinant E. coli bacteria were incubated with purified human complement proteins or recombinant H fragments. PgtE cleaved both B and H, whereas its close homolog Pla of Yersinia pestis cleaved only H. H was cleaved at both N- and C-termini, while the central region resisted proteolysis. Because of multiple effects of PgtE on complement components (cleavage of C3, C3b, B, and H) we assessed its effect on the opsonophagocytosis of Salmonella. In human serum, C3 cleavage was dependent on proteolytically active PgtE. Human neutrophils interacted less with serum-opsonized FITC-stained S. enterica 14028R than with the isogenic ΔpgtE strain, as analyzed by flow cytometry. In conclusion, cleavage of B and H by PgtE, together with C3 cleavage, affects the C3-mediated recognition of S. enterica by human neutrophils, thus thwarting the immune protection against Salmonella.

The major autoantibody epitope on factor H in atypical hemolytic uremic syndrome is structurally different from its homologous site in factor H-related protein 1, supporting a novel model for induction of autoimmunity in this disease

Atypical hemolytic uremic syndrome (aHUS) is characterized by complement attack against host cells due to mutations in complement proteins or autoantibodies against complement factor H (CFH). It is unknown why nearly all patients with autoimmune aHUS lack CFHR1 (CFH-related protein-1). These patients have autoantibodies against CFH domains 19 and 20 (CFH_19–20), which are nearly identical to CFHR1 domains 4 and 5 (CFHR1_4–5). Here, binding site mapping of autoantibodies from 17 patients using mutant CFH_19–20 constructs revealed an autoantibody epitope cluster within a loop on domain 20, next to the two buried residues that are different in CFH_19–20 and CFHR1_4–5. The crystal structure of CFHR1_4–5 revealed a difference in conformation of the autoantigenic loop in the C-terminal domains of CFH and CFHR1, explaining the variation in binding of autoantibodies from some aHUS patients to CFH_19–20 and CFHR1_4–5. The autoantigenic loop on CFH seems to be generally flexible, as its conformation in previously published structures of CFH_19–20 bound to the microbial protein OspE and a sialic acid glycan is somewhat altered. Cumulatively, our data suggest that association of CFHR1 deficiency with autoimmune aHUS could be due to the structural difference between CFHR1 and the autoantigenic CFH epitope, suggesting a novel explanation for CFHR1 deficiency in the pathogenesis of autoimmune aHUS.

Delineating the importance of serum opsonins and the bacterial capsule in affecting the uptake and killing of Burkholderia pseudomallei by murine neutrophils and macrophages

Infection of susceptible hosts by the encapsulated Gram-negative bacterium Burkholderia pseudomallei (Bp) causes melioidosis, with septic patients attaining mortality rates ≥ 40%. Due to its high infectivity through inhalation and limited effective therapies, Bp is considered a potential bioweapon. Thus, there is great interest in identifying immune effectors that effectively kill Bp. Our goal is to compare the relative abilities of murine macrophages and neutrophils to clear Bp, as well as determine the importance of serum opsonins and bacterial capsule. Our findings indicate that murine macrophages and neutrophils are inherently unable to clear either unopsonized Bp or the relatively-avirulent acapsular bacterium B. thailandensis (Bt). Opsonization of Bp and Bt with complement or pathogen-specific antibodies increases macrophage-uptake, but does not promote clearance, although antibody-binding enhances complement deposition. In contrast, complement opsonization of Bp and Bt causes enhanced uptake and killing by neutrophils, which is linked with rapid ROS induction against bacteria exhibiting a threshold level of complement deposition. Addition of bacteria-specific antibodies enhances complement deposition, but antibody-binding alone cannot elicit neutrophil clearance. Bp capsule provides some resistance to complement deposition, but is not anti-phagocytic or protective against reactive oxygen species (ROS)-killing. Macrophages were observed to efficiently clear Bp only after pre-activation with IFNγ, which is independent of serum- and/or antibody-opsonization. These studies indicate that antibody-enhanced complement activation is sufficient for neutrophil-clearance of Bp, whereas macrophages are ineffective at clearing serum-opsonized Bp unless pre-activated with IFNγ. This suggests that effective immune therapies would need to elicit both antibodies and Th1-adaptive responses for successful prevention/eradication of melioidosis.

Binding of human factor H to outer membrane protein P5 of non-typeable Haemophilus influenzae contributes to complement resistance

Non-typeable Haemophilus influenzae is an opportunistic pathogen of the human upper respiratory tract and is often found to cause inflammatory diseases that include sinusitis, otitis media and exacerbations of chronic obstructive pulmonary disease. To persist in the inflammatory milieu during infection, non-typeable H. influenzae must resist the antimicrobial activity of the human complement system. Here, we used Tn-seq to identify genes important for resistance to complement-mediated killing. This screen identified outer membrane protein P5 in evasion of the alternative pathway of complement activation. Outer membrane protein P5 was shown to bind human complement regulatory protein factor H directly, thereby, preventing complement factor C3 deposition on the surface of the bacterium. Furthermore, we show that amino acid variation within surface-exposed regions within outer membrane P5 affected the level of factor H binding between individual strains.

Importance of (antibody-dependent) complement-mediated serum killing in protection against Bordetella pertussis

Pertussis is a highly contagious respiratory disease that is caused by Bordetella pertussis. Despite being vaccine preventable, pertussis rates have been rising steadily over the last decades, even in areas with high vaccine uptake. Recently, experiments with infant baboons indicated that although vaccination with acellular pertussis vaccines prevented disease, no apparent effect was observed on infection and transmission. One explanation may be that current acellular pertussis vaccines do not induce high levels of opsonophagocytic and/or bactericidal activity, implying that engineering of vaccines that promote bacterial killing may improve efficacy. Here, we discuss the importance of complement-mediated killing in vaccine-induced protection against B. pertussis. We first examine how B. pertussis may have evolved different complement evasion strategies. Second, we explore the benefits of opsonophagocytic and/or bactericidal killing in vaccine-induced protection and discuss whether or not inclusion of new opsonophagocytic or bactericidal target antigens in pertussis vaccines may benefit efficacy.

The Borrelia hermsii factor H binding protein FhbA is not required for infectivity in mice or for resistance to human complement in vitro

The primary causative agent of tick-borne relapsing fever in North America is Borrelia hermsii. It has been hypothesized that B. hermsii evades complement-mediated destruction by binding factor H (FH), a host-derived negative regulator of complement. In vitro, B. hermsii produces a single FH binding protein designated FhbA (FH binding protein A). The properties and ligand binding activity of FhbA suggest that it plays multiple roles in pathogenesis. It binds plasminogen and has been identified as a significant target of a B1b B cell-mediated IgM response in mice. FhbA has also been explored as a potential diagnostic antigen for B. hermsii infection in humans. The ability to test the hypothesis that FhbA is a critical virulence factor in vivo has been hampered by the lack of well-developed systems for the genetic manipulation of the relapsing fever spirochetes. In this report, we have successfully generated a B. hermsii fhbA deletion mutant (the B. hermsii YORΔfhbA strain) through allelic exchange mutagenesis. Deletion of fhbA abolished FH binding by the YORΔfhbA strain and eliminated cleavage of C3b on the cell surface. However, the YORΔfhbA strain remained infectious in mice and retained resistance to killing in vitro by human complement. Collectively, these results indicate that B. hermsii employs an FhbA/FH-independent mechanism of complement evasion that allows for resistance to killing by human complement and persistence in mice.

Identification of a Haemophilus influenzae factor H-Binding lipoprotein involved in serum resistance

Haemophilus influenzae is a Gram-negative human pathogen that resides in the upper respiratory tract. Encapsulated H. influenzae type b (Hib) and type f (Hif) are the most common serotypes associated with invasive disease. H. influenzae displays various strategies to circumvent the host innate immune response, including the bactericidal effect of the complement system. In this study, we identified an H. influenzae lipoprotein having the ability to bind factor H (FH), the major regulator of the alternative pathway of complement activation. This protein, named protein H (PH), was surface exposed and was found in all clinical Hib and Hif isolates tested. Deletion of the gene encoding for PH (lph) in Hib and Hif significantly reduced the interaction between bacteria and FH. When Hib and Hif PH variants were separately expressed in nontypeable (unencapsulated) H. influenzae, which did not bind FH, an increased FH affinity was observed. We recombinantly expressed the two PH variants in Escherichia coli, and despite sharing only 56% identical amino acids, both FH-binding Haemophilus proteins similarly interacted with the complement regulator FH short consensus repeats 7 and 18-20. Importantly, Hib and Hif resistance against the bactericidal effect of human serum was significantly reduced when bacterial mutants devoid of PH were tested. In conclusion, we have characterized a hitherto unknown bacterial protein that is crucial for mediating an interaction between the human pathogen H. influenzae and FH. This novel interaction is important for H. influenzae resistance against complement activation and will consequently promote bacterial pathogenesis.

Outer membrane protein OlpA contributes to Moraxella catarrhalis serum resistance via interaction with factor H and the alternative pathway

Factor H is an important complement regulator of the alternative pathway commonly recruited by pathogens to achieve increased rates of survival in the human host. The respiratory pathogen Moraxella catarrhalis, which resides in the mucosa, is highly resistant to the bactericidal activity of serum and causes otitis media in children and respiratory tract infections in individuals with underlying diseases. In this study, we show that M. catarrhalis binds factor H via the outer membrane protein OlpA. M. catarrhalis serum resistance was dramatically decreased in the absence of either OlpA or factor H, demonstrating that this inhibition of the alternative pathway significantly contributes to the virulence of M. catarrhalis.

That's my story, and I'm sticking to it--an update on B. burgdorferi adhesins

Adhesion is the initial event in the establishment of any infection. Borrelia burgdorferi, the etiological agent of Lyme disease, possesses myriad proteins termed adhesins that facilitate contact with its vertebrate hosts. B. burgdorferi adheres to host tissues through interactions with host cells and extracellular matrix, as well as other molecules present in serum and extracellular fluids. These interactions, both general and specific, are critical in the establishment of infection. Modulation of borrelial adhesion to host tissues affects the microorganisms's ability to colonize, disseminate, and persist. In this review, we update the current knowledge on structure, function, and role in pathogenesis of these “sticky” B. burgdorferi infection-associated proteins.

Molecular basis of host specificity in human pathogenic bacteria

Pathogenic bacteria display various levels of host specificity or tropism. While many bacteria can infect a wide range of hosts, certain bacteria have strict host selectivity for humans as obligate human pathogens. Understanding the genetic and molecular basis of host specificity in pathogenic bacteria is important for understanding pathogenic mechanisms, developing better animal models and designing new strategies and therapeutics for the control of microbial diseases. The molecular mechanisms of bacterial host specificity are much less understood than those of viral pathogens, in part due to the complexity of the molecular composition and cellular structure of bacterial cells. However, important progress has been made in identifying and characterizing molecular determinants of bacterial host specificity in the last two decades. It is now clear that the host specificity of bacterial pathogens is determined by multiple molecular interactions between the pathogens and their hosts. Furthermore, certain basic principles regarding the host specificity of bacterial pathogens have emerged from the existing literature. This review focuses on selected human pathogenic bacteria and our current understanding of their host specificity.

Low-dose recombinant properdin provides substantial protection against Streptococcus pneumoniae and Neisseria meningitidis infection

Modern medicine has established three central antimicrobial therapeutic concepts: vaccination, antibiotics, and, recently, the use of active immunotherapy to enhance the immune response toward specific pathogens. The efficacy of vaccination and antibiotics is limited by the emergence of new pathogen strains and the increased incidence of antibiotic resistance. To date, immunotherapy development has focused mainly on cytokines. Here we report the successful therapeutic application of a complement component, a recombinant form of properdin (Pn), with significantly higher activity than native properdin, which promotes complement activation via the alternative pathway, affording protection against N. menigitidis and S. pneumoniae. In a mouse model of infection, we challenged C57BL/6 WT mice with N. menigitidis B-MC58 6 h after i.p. administration of Pn (100 µg/mouse) or buffer alone. Twelve hours later, all control mice showed clear symptoms of infectious disease while the Pn treated group looked healthy. After 16 hours, all control mice developed sepsis and had to be culled, while only 10% of Pn treated mice presented with sepsis and recoverable levels of live Meningococci. In a parallel experiment, mice were challenged intranasally with a lethal dose of S. pneumoniae D39. Mice that received a single i.p. dose of Pn at the time of infection showed no signs of bacteremia at 12 h postinfection and had prolonged survival times compared with the saline-treated control group (P < 0.0001). Our findings show a significant therapeutic benefit of Pn administration and suggest that its antimicrobial activity could open new avenues for fighting infections caused by multidrug-resistant neisserial or streptococcal strains.

The Bordetella pertussis Bps polysaccharide enhances lung colonization by conferring protection from complement-mediated killing

Bordetella pertussis is a human-restricted Gram-negative bacterial pathogen that causes whooping cough or pertussis. Pertussis is the leading vaccine preventable disease that is resurging in the USA and other parts of the developed world. There is an incomplete understanding of the mechanisms by which B. pertussis evades killing and clearance by the complement system, a first line of host innate immune defence. The present study examined the role of the Bps polysaccharide to resist complement activity in vitro and in the mouse respiratory tract. The isogenic bps mutant strain containing a large non-polar in-frame deletion of the bpsA-D locus was more sensitive to serum and complement mediated killing than the WT strain. As determined by Western blotting, flow cytometry and electron microscopic studies, the heightened sensitivity of the mutant strain was due to enhanced deposition of complement proteins and the formation of membrane attack complex, the end-product of complement activation. Bps was sufficient to confer complement resistance as evidenced by a Bps-expressing Escherichia coli being protected by serum killing. Additionally, Western blotting and flow cytometry assays revealed that Bps inhibited the deposition of complement proteins independent of other B. pertussis factors. The bps mutant strain colonized the lungs of complement-deficient mice at higher levels than that observed in C57Bl/6 mice. These results reveal a previously unknown interaction between Bps and the complement system in controlling B. pertussis colonization of the respiratory tract. These findings also make Bps a potential target for the prevention and therapy of whooping cough.

Interactions of Candida albicans with host epithelial surfaces

Candida albicans is an opportunistic, fungal pathogen of humans that frequently causes superficial infections of oral and vaginal mucosal surfaces of debilitated and susceptible individuals. The organism is however, commonly encountered as a commensal in healthy individuals where it is a component of the normal microflora. The key determinant in the type of relationship that Candida has with its host is how it interacts with the epithelial surface it colonises. A delicate balance clearly exists between the potentially damaging effects of Candida virulence factors and the nature of the immune response elicited by the host. Frequently, it is changes in host factors that lead to Candida seemingly changing from a commensal to pathogenic existence. However, given the often reported heterogeneity in morphological and biochemical factors that exist between Candida species and indeed strains of C. albicans, it may also be the fact that colonising strains differ in the way they exploit resources to allow persistence at mucosal surfaces and as a consequence this too may affect the way Candida interacts with epithelial cells. The aim of this review is to provide an overview of some of the possible interactions that may occur between C. albicans and host epithelial surfaces that may in turn dictate whether Candida removal, its commensal persistence or infection follows.

Staphylococcal Ecb protein and host complement regulator factor H enhance functions of each other in bacterial immune evasion

Staphylococcus aureus is a major human pathogen causing more than a tenth of all septicemia cases and often superficial and deep infections in various tissues. One of the immune evasion strategies of S. aureus is to secrete proteins that bind to the central complement opsonin C3b. One of these, extracellular complement binding protein (Ecb), is known to interfere directly with functions of C3b. Because C3b is also the target of the physiological plasma complement regulator, factor H (FH), we studied the effect of Ecb on the complement regulatory functions of FH. We show that Ecb enhances acquisition of FH from serum onto staphylococcal surfaces. Ecb and FH enhance mutual binding to C3b and also the function of each other in downregulating complement activation. Both Ecb and the C-terminal domains 19-20 of FH bind to the C3d part of C3b. We show that the mutual enhancing effect of Ecb and FH on binding to C3b depends on binding of the FH domain 19 to the C3d part of C3b next to the binding site of Ecb on C3d. Our results show that Ecb, FH, and C3b form a tripartite complex. Upon exposure of serum-sensitive Haemophilus influenzae to human serum, Ecb protected the bacteria, and this effect was enhanced by the addition of the C-terminal domains 19-20 of FH. This finding indicates that the tripartite complex formation could give additional protection to bacteria and that S. aureus is thereby able to use host FH and bacterial Ecb in a concerted action to eliminate C3b at the site of infection.

Structural basis for complement evasion by Lyme disease pathogen Borrelia burgdorferi

Borrelia burgdorferi spirochetes that cause Lyme borreliosis survive for a long time in human serum because they successfully evade the complement system, an important arm of innate immunity. The outer surface protein E (OspE) of B. burgdorferi is needed for this because it recruits complement regulator factor H (FH) onto the bacterial surface to evade complement-mediated cell lysis. To understand this process at the molecular level, we used a structural approach. First, we solved the solution structure of OspE by NMR, revealing a fold that has not been seen before in proteins involved in complement regulation. Next, we solved the x-ray structure of the complex between OspE and the FH C-terminal domains 19 and 20 (FH19-20) at 2.83 Å resolution. The structure shows that OspE binds FH19-20 in a way similar to, but not identical with, that used by endothelial cells to bind FH via glycosaminoglycans. The observed interaction of OspE with FH19-20 allows the full function of FH in down-regulation of complement activation on the bacteria. This reveals the molecular basis for how B. burgdorferi evades innate immunity and suggests how OspE could be used as a potential vaccine antigen.