Interaction between Complement Regulators andStreptococcus pyogenes: Binding of C4b-Binding Protein and Factor H/Factor H-Like Protein 1 to M18 Strains Involves Two Different Cell Surface Molecules

Conservation of C4BP-binding sequence patterns in Streptococcus pyogenes M and Enn proteins

Antigenically sequence variable M proteins of the major bacterial pathogen Streptococcus pyogenes (Strep A) are responsible for recruiting human C4b-binding protein (C4BP) to the bacterial surface, which enables Strep A to evade destruction by the immune system. The most sequence divergent portion of M proteins, the hypervariable region (HVR), is responsible for binding C4BP. Structural evidence points to the conservation of two C4BP-binding sequence patterns (M2 and M22) in the HVR of numerous M proteins, with this conservation applicable to vaccine immunogen design. These two patterns, however, only partially explain C4BP binding by Strep A. Here, we identified several M proteins that lack these patterns but still bind C4BP and determined the structures of two, M68 and M87 HVRs, in complex with a C4BP fragment. Mutagenesis of these M proteins led to the identification of amino acids that are crucial for C4BP binding, enabling formulation of new C4BP-binding patterns. Mutagenesis was also carried out on M2 and M22 proteins to refine or generate experimentally grounded C4BP-binding patterns. The M22 pattern was the most prevalent among M proteins, followed by the M87 and M2 patterns, while the M68 pattern was rare. These patterns, except for M68, were also evident in numerous M-like Enn proteins. Binding of C4BP via these patterns to Enn proteins was verified. We conclude that C4BP-binding patterns occur frequently in Strep A strains of differing M types, being present in their M or Enn proteins, or frequently both, providing further impetus for their use as vaccine immunogens.

Conservation of C4BP-binding Sequence Patterns in Streptococcus pyogenes M and Enn Proteins

Antigenically sequence variable M proteins of the major bacterial pathogen Streptococcus pyogenes (Strep A) are responsible for recruiting human C4b-binding protein (C4BP) to the bacterial surface, which enables Strep A to evade destruction by the immune system. The most sequence divergent portion of M proteins, the hypervariable region (HVR), is responsible for binding C4BP. Structural evidence points to the conservation of two C4BP-binding sequence patterns (M2 and M22) in the HVR of numerous M proteins, with this conservation applicable to vaccine immunogen design. These two patterns, however, only partially explain C4BP-binding by Strep A. Here, we identified several M proteins that lack these patterns but still bind C4BP, and determined the structures of two, M68 and M87 HVRs, in complex with a C4BP fragment. Mutagenesis of these M proteins led to identification of amino acids that are crucial for C4BP-binding, enabling formulation of new C4BP-binding patterns. Mutagenesis was also carried out on M2 and M22 proteins to refine or generate experimentally grounded C4BP-binding patterns. The M22 pattern was the most populated among M proteins, followed by the M87 and M2 patterns, while the M68 pattern was rare. These patterns, except for M68, were also evident in numerous M-like Enn proteins. Binding of C4BP via these patterns to Enn proteins was verified. We conclude that C4BP-binding patterns occur frequently in Strep A strains of differing M types, being present in their M or Enn proteins, or frequently both, providing further impetus for their use as vaccine immunogens.

Clinico-Pathogenic Similarities and Differences between Infection-Related Glomerulonephritis and C3 Glomerulopathy

Recently, the comprehensive concept of "infection-related glomerulonephritis (IRGN)" has replaced that of postinfectious glomerulonephritis (PIGN) because of the diverse infection patterns, epidemiology, clinical features, and pathogenesis. In addition to evidence of infection, hypocomplementemia particularly depresses serum complement 3 (C3), with endocapillary proliferative and exudative GN developing into membranoproliferative glomerulonephritis (MPGN); also, C3-dominant or co-dominant glomerular immunofluorescence staining is central for diagnosing IRGN. Moreover, nephritis-associated plasmin receptor (NAPlr), originally isolated from the cytoplasmic fraction of group A Streptococci, is vital as an essential inducer of C3-dominant glomerular injury and is a key diagnostic biomarker for IRGN. Meanwhile, "C3 glomerulopathy (C3G)", also showing a histological pattern of MPGN due to acquired or genetic dysregulation of the complement alternative pathway (AP), mimics C3-dominant IRGN. Initially, C3G was characterized by intensive "isolated C3" deposition on glomeruli. However, updated definitions allow for glomerular deposition of other complement factors or immunoglobulins if C3 positivity is dominant and at least two orders of magnitude greater than any other immunoreactant, which makes it challenging to quickly distinguish pathomorphological findings between IRGN and C3G. As for NAPlr, it was demonstrated to induce complement AP activation directly in vitro, and it aggravates glomerular injury in the development of IRGN. A recent report identified anti-factor B autoantibodies as a contributing factor for complement AP activation in pediatric patients with PIGN. Moreover, C3G with glomerular NAPlr deposition without evidence of infection was reported. Taken together, the clinico-pathogenic features of IRGN overlap considerably with those of C3G. In this review, similarities and differences between the two diseases are highlighted.

Kidney diseases

Dysregulation and accelerated activation of the alternative pathway (AP) of complement is known to cause or accentuate several pathologic conditions in which kidney injury leads to the appearance of hematuria and proteinuria and ultimately to the development of chronic renal failure. Multiple genetic and acquired defects involving plasma- and membrane-associated proteins are probably necessary to impair the protection of host tissues and to confer a significant predisposition to AP-mediated kidney diseases. This review aims to explore how our current understanding will make it possible to identify the mechanisms that underlie AP-mediated kidney diseases and to discuss the available clinical evidence that supports complement-directed therapies. Although the value of limiting uncontrolled complement activation has long been recognized, incorporating complement-targeted treatments into clinical use has proved challenging. Availability of anti-complement therapy has dramatically transformed the outcome of atypical hemolytic uremic syndrome, one of the most severe kidney diseases. Innovative drugs that directly counteract AP dysregulation have also opened new perspectives for the management of other kidney diseases in which complement activation is involved. However, gained experience indicates that the choice of drug should be tailored to each patient's characteristics, including clinical, histologic, genetic, and biochemical parameters. Successfully treating patients requires further research in the field and close collaboration between clinicians and researchers who have special expertise in the complement system.

Acute Post-Streptococcal Glomerulonephritis In Children: A Comprehensive Review

BACKGROUND

Acute post-streptococcal glomerulonephritis (APSGN) is an immune-complex (ICs) mediated glomerular disease triggered by group A β-hemolytic streptococcus (GAS) or Streptococcus pyogenes infections. APSGN represents a major cause of acquired kidney injury in children.

METHODS

This non-systematic review summarizes recent evidence on APSGN. We discuss the epidemiology, pathogenesis, clinical and laboratory findings, histopathology, treatment and prognosis of the disease.

RESULTS

The median APSGN incidence in children in developing countries is estimated at 24.3/100,000 per year, compared with 6.2/100,000 per year in developed countries. Nephritis-associated plasmin receptor, identified as glyceraldehyde-3-phosphate dehydrogenase, and the cationic cysteine proteinase streptococcal pyrogenic exotoxin B are thought to be two leading streptococcal antigens involved in the pathogenesis of APSGN, which activate the complement system, mainly via the alternative but also the lectin pathway. This process is critical for the generation of inflammation by the ICs deposited in the glomerulus. The classic phenotype is an acute diffuse proliferative glomerulonephritis leading to features of the nephritic syndrome including hematuria, oliguria, hypertension and edema. The histopathology shows that the glomeruli are diffused affected, mostly presenting enlarged glomerular tuffs due to hypercellularity. Proliferative endothelial and mesangial cells and inflammation are also observed. APSGN frequently has spontaneous recovery. There is no specific therapy, but its morbidity and mortality are drastically reduced by the prevention and/or treatment of complications.

CONCLUSION

Despite recent advances, the pathogenesis of APSGN is not fully understood. There is no specific treatment for APSGN. The prognosis is generally good. However some cases may evolve to chronic kidney disease.

Autoimmunity in Acute Poststreptococcal GN: A Neglected Aspect of the Disease

Acute poststreptococcal GN (APSGN) is the prototype of immune complex GN and is associated with manifestations of autoimmune reactivity that have been neglected as epiphenomena. Recently, studies have demonstrated transient antifactor B autoantibodies that activate the alternative complement pathway, bringing self-immunity to a central position in the pathogenesis of APSGN. Therefore, examining other manifestations of autoimmunity that have been reported in association with poststreptococcal GN is of interest. This article reviews the renal and extrarenal manifestations of autoimmune reactivity in APSGN and considers their potential relevance in modifying the usually benign clinical course of the disease. It also discusses related aspects of the nephritogenic antigens, complement activation, and genetic elements associated with immune reactivity and their potential relevance to the familial incidence of the disease.

Drivers and regulators of humoral innate immune responses to infection and cancer

The complement cascade consists of cell bound and serum proteins acting together to protect the host from pathogens, remove cancerous cells and effectively links innate and adaptive immune responses. Despite its usefulness in microbial neutralization and clearance of cancerous cells, excessive complement activation causes an immune imbalance and tissue damage in the host. Hence, a series of complement regulatory proteins present at a higher concentration in blood plasma and on cell surfaces tightly regulate the cascade. The complement cascade can be initiated by B-1 B cell production of natural antibodies. Natural antibodies arise spontaneously without any known exogenous antigenic or microbial stimulus and protect against invading pathogens, clear apoptotic cells, provide tissue homeostasis, and modulate adaptive immune functions. Natural IgM antibodies recognize microbial and cancer antigens and serve as an activator of complement mediated lysis. This review will discuss advances in complement activation and regulation in bacterial and viral infections, and cancer. We will also explore the crosstalk of natural antibodies with bacterial populations and cancer.

A Multicomponent Vaccine Provides Immunity against Local and Systemic Infections by Group A Streptococcus across Serotypes

GAS is among the most common human pathogens and causes a wide variety of diseases, likely more than any other microorganism. The diverse clinical manifestations of GAS may be attributable to its large repertoire of virulence factors that are selectively and synergistically involved in streptococcal pathogenesis. To date, GAS vaccines have not been successful due to multiple serotypes and postinfection sequelae associated with autoimmunity. In this study, five conserved virulence factors that are involved in GAS pathogenesis were used as a combined vaccine. Intranasal immunization with this vaccine induced humoral and cellular immune responses across GAS serotypes and protected against mucosal, systemic, and skin infections. The significance of this work is to demonstrate that the efficacy of GAS vaccines can be achieved by including multiple nonredundant critical virulence factors and inducing local and systemic immunity. The strategy also provides valuable insights for vaccine development against other pathogens.

Group A streptococcus (GAS) species are responsible for a broad spectrum of human diseases, ranging from superficial to invasive infections, and are associated with autoimmune disorders. There is no commercial vaccine against GAS. The clinical manifestations of GAS infection may be attributable to the large repertoire of virulence factors used selectively in different types of GAS disease. Here, we selected five molecules, highly conserved among GAS serotypes, and involved in different pathogenic mechanisms, as a multicomponent vaccine, 5CP. Intranasal (i.n.) immunization with 5CP protected mice against both mucosal and systemic GAS infection across serotypes; the protection lasted at least 6 months. Immunization of mice with 5CP constrained skin lesion development and accelerated lesion recovery. Flow cytometry and enzyme-linked immunosorbent assay analyses revealed that 5CP induced Th17 and antibody responses locally and systemically; however, the Th17 response induced by 5CP resolved more quickly than that to GAS when challenge bacteria were cleared, suggesting that 5CP is less likely to cause autoimmune responses. These findings support that immunization through the i.n. route targeting multiple nonredundant virulence factors can induce immunity against different types of GAS disease and represents an alternative strategy for GAS vaccine development, with favorable efficacy, coverage, duration, and safety.

A quantitative Streptococcus pyogenes-human protein-protein interaction map reveals localization of opsonizing antibodies

A fundamental challenge in medical microbiology is to characterize the dynamic protein–protein interaction networks formed at the host–pathogen interface. Here, we generate a quantitative interaction map between the significant human pathogen, Streptococcus pyogenes, and proteins from human saliva and plasma obtained via complementary affinity-purification and bacterial-surface centered enrichment strategies and quantitative mass spectrometry. Perturbation of the network using immunoglobulin protease cleavage, mixtures of different concentrations of saliva and plasma, and different S. pyogenes serotypes and their isogenic mutants, reveals how changing microenvironments alter the interconnectivity of the interaction map. The importance of host immunoglobulins for the interaction with human complement proteins is demonstrated and potential protective epitopes of importance for phagocytosis of S. pyogenes cells are localized. The interaction map confirms several previously described protein–protein interactions; however, it also reveals a multitude of additional interactions, with possible implications for host–pathogen interactions involving other bacterial species.

Characterizing host-pathogen protein interactions can help elucidate the molecular basis of bacterial infections. Here, the authors use an integrative proteomics approach to generate a quantitative map of protein interactions between Streptococcus pyogenes and human saliva and plasma.

Group A streptococcal M-like proteins: From pathogenesis to vaccine potential

M and M-like surface proteins from group A Streptococcus (GAS) act as virulence factors and have been used in multiple vaccine candidates. While the M protein has been extensively studied, the two genetically and functionally related M-like proteins, Mrp and Enn, although present in most streptococcal strains have been relatively less characterised. We compile the current state of knowledge for these two proteins, from discovery to recent studies on function and immunogenicity, using the M protein for comparison as a prototype of this family of proteins. We focus on the known interactions between M-like proteins and host ligand proteins, and analyse the genetic data supporting these interactions. We discuss known and possible functions of M-like proteins during GAS infections, and highlight knowledge gaps where further investigation is warranted.

Pediatric post-streptococcal glomerulonephritis: Clinical and laboratory data

BACKGROUND

Acute post-streptococcal glomerulonephritis (APSGN) is the most common post-infectious glomerulonephritis in childhood. The aim of this study was therefore to identify the possible risk factor(s) responsible for decreased glomerular filtration rate (GFR) in APSGN.

METHODS

The data of patients followed up with a diagnosis of APSGN in the Pediatric Nephrology Clinic of Gaziantep University Hospital between October 2014 and October 2016 were retrospectively evaluated.

RESULTS

The total number of subjects was 75 (male/female, 42/33) with a mean age of 8.20 ± 3.25 years. The most common presentations were edema (86.7%), macroscopic hematuria (82.7%) and hypertension (73.3%, n = 55). On laboratory examination, 28 children (37.3%) had hypoalbuminemia, 58 (77.3%) had proteinuria, 20 (26.7%) had increased C-reactive protein (CRP), while 74 (98.7%) and 12 (16%) had decreased complement (C)3 and C4, respectively. The number of children with GFR <90 mL/min/1.73 m² was 22 (29.3%). The risk of decreased GFR was significantly higher in patients with increased CRP (P = 0.001; OR, 3.58), hypoalbuminemia (P = 0.006; OR, 4.83), and decreased C4 (P = 0.010; OR, 11.53). Additionally, white blood cell (WBC) count, neutrophil count, and neutrophil/lymphocyte ratio (NLR) were significantly higher (P = 0.02, P = 0.006, P = 0.004, respectively) in patients with low GFR.

CONCLUSIONS

Although the prognosis of APSGN in children is good, severe systemic complications and renal failure may develop during the follow-up period. Decreased C4, presence of hypoalbuminemia, and increased inflammatory markers (WBC, CRP, neutrophil count and NLR) might be possible risk factors for severity of renal involvement. Decreased C4, in particular, may be a risk factor for decreased GFR in those children.

Variation, Indispensability, and Masking in the M protein

The M protein is the major surface-associated virulence factor of group A Streptococcus (GAS) and an antigenically variable target of host immunity. How selection pressures to escape immune recognition, maintain indispensable functions, and mask vulnerabilities have shaped the sequences of the >220M protein types is unclear. Recent experiments have shed light on this question by showing that, hidden within the antigenic variability of many M protein types, are sequence patterns conserved for recruiting human C4b-binding protein (C4BP). Other host factors may be recruited in a similar manner by conserved but hidden sequence patterns in the M protein. The identification of such patterns may be applicable to the development of a GAS vaccine.

Post-infectious Proliferative Glomerulonephritis with Monoclonal Immunoglobulin G Deposits Associated with Complement Factor H Mutation

A 55-year-old man developed rapidly progressive glomerulonephritis and nephrotic syndrome. A kidney biopsy specimen showed diffuse proliferative and crescentic glomerulonephritis with monoclonal IgG1κ, humps, and nephritis-associated plasmin receptor, indicating infection-associated proliferative glomerulonephritis with monoclonal immunoglobulin G deposits (PGNMID). Despite dialysis-dependent renal failure, symptomatic therapy resulted in spontaneous recovery of the renal function, mimicking post-infectious glomerulonephritis (PIGN). A heterozygous complement factor H mutation was detected by comprehensive genetic testing of alternative pathway regulatory genes, which might lead to persistent infection-triggered alternative pathway activation and account for severe glomerulonephritis. Post-infectious PGNMID and PIGN might share common clinical presentations and pathogenesis related to the complement pathway.

Factor H specifically capture novel Factor H-binding proteins of Streptococcus suis and contribute to the virulence of the bacteria

Factor H (FH), a regulatory protein of the complement system, can bind specifically to factor H-binding proteins (FHBPs) of Streptococcus suis serotype 2 (SS2), which contribute to evasion of host innate immune defenses. In the present study, we aimed to identify novel FHBPs and characterize the biological functions of FH in SS2 pathogenesis. Here, a method that combined proteomics and Far-western blotting was developed to identify the surface FHBPs of SS2. With this method, fourteen potential novel FHBPs were identified among SS2 surface proteins. We selected eight newly identified proteins and further confirmed their binding activity to FH. The binding of SS2 to immobilized FH decreased dramatically after pre-incubation with anti-FHBPs polyclonal antibodies. We showed for the first time that SS2 also interact specifically with mouse FH. Furthermore, we found that FH play an important role in adherence and invasion of SS2 to HEp-2 cells. Additionally, using a mouse model of intraperitoneal challenge, we confirmed that SS2 pre-incubated with FH enhanced bacteremia and brain invasion, compared with SS2 not pretreated with FH. Taken together, this study provides a useful method to characterize the host-bacteria interactions. These results first indicated that binding of FH to the cell surface improved the adherence and invasion of SS2 to HEp-2 cells, promoting SS2 to resist killing and leading to enhance virulence.

Streptococcus pyogenes Employs Strain-dependent Mechanisms of C3b Inactivation to Inhibit Phagocytosis and Killing of Bacteria

Evasion of complement-mediated opsonophagocytosis enables group A Streptococcus pyogenes (GAS) to establish infection. Different strain-dependent mechanisms are employed by the host to accomplish this goal. In general, GAS inhibits the amplification of the complement cascade on its cell surface by facilitating the degradation of C3b, an opsonin, to an inactive product, inactivated C3b (iC3b), in a step catalyzed by factor I (FI) and its cofactor, factor H (FH), with or without the participation of human host plasmin (hPm). GAS recruits FH to its cell surface via FH receptors, which are transcriptionally controlled by the two-component cluster of virulence responder-sensor system. The manner in which FI-FH and hPm function together on GAS cells is unknown. Using GAS strain AP53, which strongly binds host human plasminogen/plasmin (hPg/hPm) directly via an hPg/hPm surface receptor (PAM), we show that both FI-FH and hPm sequentially cleave C3b. Whereas FI-FH proteolytically cleaves C3b into iC3b, PAM-bound hPm catalyzes cleavage of iC3b into multiple smaller peptides. Unlike AP53, GAS strain M23ND weakly binds FH and recruits hPg/hPm to its cell surface indirectly via fibrinogen bound to M-protein, M23. In this case, FH-FI cleaves C3b into iC3b, with negligible degradation of iC3b by hPm that is bound to fibrinogen on the cells. AP53 and M23ND display similar resistance to human neutrophil-mediated phagocytosis, which results in a corresponding high lethality in mice after injection of these cells. These results suggest that GAS utilizes diverse mechanisms to degrade C3b and thus to protect bacterial cells from the complement response of the host.

Virulence of Group A Streptococci Is Enhanced by Human Complement Inhibitors

Streptococcus pyogenes, also known as Group A Streptococcus (GAS), is an important human bacterial pathogen that can cause invasive infections. Once it colonizes its exclusively human host, GAS needs to surmount numerous innate immune defense mechanisms, including opsonization by complement and consequent phagocytosis. Several strains of GAS bind to human-specific complement inhibitors, C4b-binding protein (C4BP) and/or Factor H (FH), to curtail complement C3 (a critical opsonin) deposition. This results in diminished activation of phagocytes and clearance of GAS that may lead to the host being unable to limit the infection. Herein we describe the course of GAS infection in three human complement inhibitor transgenic (tg) mouse models that examined each inhibitor (human C4BP or FH) alone, or the two inhibitors together (C4BPxFH or 'double' tg). GAS infection with strains that bound C4BP and FH resulted in enhanced mortality in each of the three transgenic mouse models compared to infection in wild type mice. In addition, GAS manifested increased virulence in C4BPxFH mice: higher organism burdens and greater elevations of pro-inflammatory cytokines and they died earlier than single transgenic or wt controls. The effects of hu-C4BP and hu-FH were specific for GAS strains that bound these inhibitors because strains that did not bind the inhibitors showed reduced virulence in the 'double' tg mice compared to strains that did bind; mortality was also similar in wild-type and C4BPxFH mice infected by non-binding GAS. Our findings emphasize the importance of binding of complement inhibitors to GAS that results in impaired opsonization and phagocytic killing, which translates to enhanced virulence in a humanized whole animal model. This novel hu-C4BPxFH tg model may prove invaluable in studies of GAS pathogenesis and for developing vaccines and therapeutics that rely on human complement activation for efficacy.

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-mediated opsonization of invasive group A Streptococcus pyogenes strain AP53 is regulated by the bacterial two-component cluster of virulence responder/sensor (CovRS) system

Group A Streptococcus pyogenes (GAS) strain AP53 is a primary isolate from a patient with necrotizing fasciitis. These AP53 cells contain an inactivating mutation in the sensor component of the cluster of virulence (cov) responder (R)/sensor (S) two-component gene regulatory system (covRS), which enhances the virulence of the primary strain, AP53/covR(+)S(-). However, specific mechanisms by which the covRS system regulates the survival of GAS in humans are incomplete. Here, we show a key role for covRS in the regulation of opsonophagocytosis of AP53 by human neutrophils. AP53/covR(+)S(-) cells displayed potent binding of host complement inhibitors of C3 convertase, viz. Factor H (FH) and C4-binding protein (C4BP), which concomitantly led to minimal C3b deposition on AP53 cells, further showing that these plasma protein inhibitors are active on GAS cells. This resulted in weak killing of the bacteria by human neutrophils and a corresponding high death rate of mice after injection of these cells. After targeted allelic alteration of covS(-) to wild-type covS (covS(+)), a dramatic loss of FH and C4BP binding to the AP53/covR(+)S(+) cells was observed. This resulted in elevated C3b deposition on AP53/covR(+)S(+) cells, a high level of opsonophagocytosis by human neutrophils, and a very low death rate of mice infected with AP53/covR(+)S(+). We show that covRS is a critical transcriptional regulator of genes directing AP53 killing by neutrophils and regulates the levels of the receptors for FH and C4BP, which we identify as the products of the fba and enn genes, respectively.

Complement factor H related proteins (CFHRs)

Factor H related proteins comprise a group of five plasma proteins: CFHR1, CFHR2, CFHR3, CFHR4 and CFHR5, and each member of this group binds to the central complement component C3b. Mutations, genetic deletions, duplications or rearrangements in the individual CFHR genes are associated with a number of diseases including atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathies (C3 glomerulonephritis (C3GN), dense deposit disease (DDD) and CFHR5 nephropathy), IgA nephropathy, age related macular degeneration (AMD) and systemic lupus erythematosus (SLE). Although complement regulatory functions were attributed to most of the members of the CFHR protein family, the precise role of each CFHR protein in complement activation and the exact contribution to disease pathology is still unclear. Recent publications show that CFHR proteins form homo- as well as heterodimers. Genetic abnormalities within the CFHR gene locus can result in hybrid proteins with affected dimerization or recognition domains which cause defective functions. Here we summarize the recent data about CFHR genes and proteins in order to better understand the role of CFHR proteins in complement activation and in complement associated diseases.

Factor H binds to the hypervariable region of many Streptococcus pyogenes M proteins but does not promote phagocytosis resistance or acute virulence

Many pathogens express a surface protein that binds the human complement regulator factor H (FH), as first described for Streptococcus pyogenes and the antiphagocytic M6 protein. It is commonly assumed that FH recruited to an M protein enhances virulence by protecting the bacteria against complement deposition and phagocytosis, but the role of FH-binding in S. pyogenes pathogenesis has remained unclear and controversial. Here, we studied seven purified M proteins for ability to bind FH and found that FH binds to the M5, M6 and M18 proteins but not the M1, M3, M4 and M22 proteins. Extensive immunochemical analysis indicated that FH binds solely to the hypervariable region (HVR) of an M protein, suggesting that selection has favored the ability of certain HVRs to bind FH. These FH-binding HVRs could be studied as isolated polypeptides that retain ability to bind FH, implying that an FH-binding HVR represents a distinct ligand-binding domain. The isolated HVRs specifically interacted with FH among all human serum proteins, interacted with the same region in FH and showed species specificity, but exhibited little or no antigenic cross-reactivity. Although these findings suggested that FH recruited to an M protein promotes virulence, studies in transgenic mice did not demonstrate a role for bound FH during acute infection. Moreover, phagocytosis tests indicated that ability to bind FH is neither sufficient nor necessary for S. pyogenes to resist killing in whole human blood. While these data shed new light on the HVR of M proteins, they suggest that FH-binding may affect S. pyogenes virulence by mechanisms not assessed in currently used model systems.

The human complement system may be rapidly activated upon infection and thereby plays a key role in innate immunity. However, activation must be tightly controlled, to avoid attack on self tissues. A key component of this control system is the plasma protein factor H (FH). Many pathogens bind FH, as first described for Streptococcus pyogenes, and it has been proposed that the surface-localized M protein of this bacterium “hijacks” FH to escape phagocytosis. However, it remains unclear whether FH-binding to M protein indeed protects S. pyogenes against phagocytosis and promotes bacterial growth in vivo. Here, we demonstrate that FH binds to some but not all M proteins and solely binds to the hypervariable region (HVR), a part of M protein important for virulence. Nevertheless, several lines of evidence, including studies with transgenic mice, indicated that FH-binding ability did not contribute to acute virulence or phagocytosis resistance. These data shed new light on the HVR of M proteins but underline the difficulty in determining the in vivo role of a ligand-binding region. Binding of FH may contribute to S. pyogenes virulence by mechanisms not assessed in currently used models.

Protease-dependent mechanisms of complement evasion by bacterial pathogens

The human immune system has evolved a variety of mechanisms for the primary task of neutralizing and eliminating microbial intruders. As the first line of defense, the complement system is responsible for rapid recognition and opsonization of bacteria, presentation to phagocytes and bacterial cell killing by direct lysis. All successful human pathogens have mechanisms of circumventing the antibacterial activity of the complement system and escaping this stage of the immune response. One of the ways in which pathogens achieve this is the deployment of proteases. Based on the increasing number of recent publications in this area, it appears that proteolytic inactivation of the antibacterial activities of the complement system is a common strategy of avoiding targeting by this arm of host innate immune defense. In this review, we focus on those bacteria that deploy proteases capable of degrading complement system components into non-functional fragments, thus impairing complement-dependent antibacterial activity and facilitating pathogen survival inside the host.

Fhb, a novel factor H-binding surface protein, contributes to the antiphagocytic ability and virulence of Streptococcus suis

Streptococcus suis serotype 2 is a Gram-positive bacterium that causes sepsis and meningitis in piglets and humans. The mechanisms of S. suis serotype 2 invasive disease are not well understood. The surface proteins of pathogens usually play important roles in infection and bacterium-host interactions. Here, we identified a novel surface protein that contributed significantly to the virulence of S. suis serotype 2 in a piglet infection model. This protein showed little similarity to other reported proteins and exhibited strong binding activity to human factor H (hFH). It was designated Fhb (factor H-binding protein). The fhb genes found in S. suis serotypes 1, 2, 4, 7, and 9 exhibited molecular polymorphism. Fhb possessed two proline-rich repeat sequences and XPZ domains, and one repeat sequence exhibited a high homology to Bac, an IgA-binding protein of Streptococcus agalactiae. Evidence strongly indicated that fhb-deficient mutants had diminished phagocytosis resistance in bactericidal assays. In addition, Fhb plays important roles in complement-mediated immunity by interacting with hFH. These findings indicated that Fhb is a crucial surface protein contributing to the virulence of S. suis, with important functions in evading innate immune defenses by interaction with host complement regulatory factor hFH.

Complete deficiency of the sixth complement component (C6Q0), susceptibility to Neisseria meningitidis infections and analysis of the frequencies of C6Q0 gene defects in South Africans

Complete complement component 6 deficiency (C6Q0) is a co-dominant genetic disease presenting as increased susceptibility to invasive Neisseria meningitidis infections. Affected individuals have two affected alleles which can be homozygous or compound heterozygous for the particular gene defects they carry. This disorder has been diagnosed relatively frequently in Western Cape South Africans. Affected patients are prescribed penicillin prophylaxis. In 2004 we commenced a clinical follow-up study of 46 patients. Of these, 43 had family age-matched C6 sufficient controls. Participants were classified as either (i) well, or (ii) having a serious illness (SI) or died (D). An SI was a long-term illness that did not allow the performance of normal daily activities. Among 43 patients, 21 were well and 22 were SI/D, while among 43 matched controls, 35 were well and eight were SI/D. This difference is highly significant. Among all 46 C6Q0 patients, those who had had recurrent infection had significantly more SI/D than those who had suffered none or one infection. Thus, this work demonstrates the long-term serious outcome of repeated meningococcal disease (MD) episodes. We investigated the frequencies of four C6Q0 pathogenic mutations known to affect Cape patients (828delG, 1138delC, 821delA and 1879delG) in 2250 newborns. A total of 103 defective alleles (2·28%) and three affected C6Q0 individuals were detected. For all defects combined, 5·24 affected subjects (C6Q0) are expected among 10,000 individuals. What is still unknown is the number of C6Q0 individuals who suffer MD or other infectious diseases.

The streptococcal M protein: a highly versatile molecule

Interaction of the M-protein of group A Streptococcus (GAS) with its numerous host binding partners might assist the bacteria in evading host immune responses. Although the extensive diversity of this protein has been highlighted by different GAS typing schemes, most of the structural and functional information has been obtained from a limited number of types. Increasing numbers of epidemiological, clinical and biological reports suggest that the structure and function of the M protein is less conserved than previously thought. This review focuses on the known interactions between M proteins and host ligand proteins, emphasizing that our understanding of this well-studied molecule is fragmented.

Acquisition of factor H by a novel surface protein on group B Streptococcus promotes complement degradation

Binding of the host complement regulator, factor H (FH), by some pathogenic microbes constitutes an important virulence mechanism, whereby complement is broken down to help microbes survive in the host. Although it has been hypothesized for the past two decades that GBS type III binds FH via sialic acid present on its capsule, neither the binding of FH to GBS has been demonstrated nor the mechanism of interaction identified. We observed that FH bound to both wild-type and capsule or sialic acid-deficient GBS that were used as negative controls. Wild-type and acapsular GBS were incubated with serum or pure FH degraded almost 90% of C3b, suggesting that the GBS-bound FH maintained cofactor activity. In addition, dot-blot analysis showed approximately 5-10% of C5 and C9 formation, as compared to an Escherichia coli control, suggesting breakdown at the C3b level. Protease treatment of the bacteria completely abolished binding of FH. Using overlay assays and mass spectroscopic analysis, we identified the FH receptor as the streptococcal histidine triad (SHT) surface protein. The ability of binding FH to SHT was further confirmed by using recombinant SHT. This report describes the identification of the SHT as an FH-binding protein on the surface of GBS type III, revealing a novel mechanism by which the bacterium acquires FH to evade complement opsonization.

Capturing host-pathogen interactions by protein microarrays: identification of novel streptococcal proteins binding to human fibronectin, fibrinogen, and C4BP

Microbial pathogen entry and survival in the host is mediated by a network of molecular interactions between the two partners, which has been the subject of many research efforts. A complex picture is emerging in which host-pathogen crosstalk involves a high number of proteins, often with redundant functions. In the present study, we investigated the potential of protein microarrays to simultaneously scan interactions between surface proteins from two main human streptococcal pathogens, Streptococcus pyogenes and Streptococcus agalactiae, and three human ligands, fibronectin, fibrinogen, and C4 binding protein, known to play an important role in streptococcal pathogenesis. By using this technology, we confirmed interactions described in the literature and detected a novel set of streptococcal proteins with binding capacities for the human ligands. The observations were validated by Western blot and ELISA techniques. Three of the newly identified proteins were isoforms of a group B streptococcus-secreted component named Fib and displayed differential binding capacities for fibronectin, fibrinogen, and C4BP. The protein regions involved in the interaction with each ligand were identified by constructing fragments of one of the Fib variants. The approach proved valuable for the acquisition of novel insights into the complex network of protein-protein interactions occurring during microbial infection.

Binding of complement regulatory proteins to group A Streptococcus

Streptococcus pyogenes or Group A Streptococcus (GAS) is the etiologic agent of important human infections such as acute pharyngitis, impetigo, rheumatic fever and the streptococcal toxic shock syndrome. Binding of the complement regulatory proteins factor H, factor H-like protein 1 (FHL-1), C4b-binding protein (C4BP), or CD46 is a crucial step in the pathogenesis of these infections. M protein is the GAS protein that generally mediates these interactions. However, a detailed analysis of the reports that have investigated the binding of complement regulatory components to GAS indicates that this microorganism has evolved alternative mechanisms for the recruitment of complement regulatory proteins to the bacterial surface. This article summarizes these data to provide a starting point for future research aimed at the characterization of additional mechanisms developed by GAS to evade the immune system.

Complement factor H, a marker of self protects against experimental autoimmune encephalomyelitis

The CNS innate immune response is a "double-edged sword" representing a fine balance between protective antipathogen responses and detrimental neurocytotoxic effects. Hence, it is important to identify the key regulatory mechanisms involved in the control of CNS innate immunity and which could be harnessed to explore novel therapeutic avenues. In analogy to the newly described neuroimmune regulatory proteins also known as "don't eat me" signals (CD200, CD47, CD22, fractalkine, semaphorins), we herein identify the key role of complement regulator factor H (fH) in controlling neuroinflammation initiated in an acute mouse model of Ab-dependent experimental autoimmune encephalomyelitis. Mouse fH was found to be abundantly expressed by primary cultured neurons and neuronal cell lines (N1E115 and Neuro2a) at a level comparable to BV2 microglia and CLTT astrocytes. Mouse neurons expressed other complement regulators crry and low levels of CD55. In the brain, the expression of fH was localized to neuronal bodies and axons, endothelial cells, microglia but not oligodendrocytes and myelin sheaths and was dramatically reduced in inflammatory experimental autoimmune encephalomyelitis settings. When exogenous human fH was administered to disease Ab-dependent experimental autoimmune encephalomyelitis animals, there was a significant decrease in clinical score, inflammation, and demyelination, as compared with PBS-injected animals. We found that the accumulation of human fH in the brain parenchyma protected neurons from complement opsonization, axonal injury, and leukocyte infiltration. Our data argue for a key regulatory activity of fH in neuroprotection and provide novel therapeutic avenues for CNS chronic inflammatory diseases.

Relationship between expression of the family of M proteins and lipoteichoic acid to hydrophobicity and biofilm formation in Streptococcus pyogenes

BACKGROUND

Hydrophobicity is an important attribute of bacteria that contributes to adhesion and biofilm formation. Hydrophobicity of Streptococcus pyogenes is primarily due to lipoteichoic acid (LTA) on the streptococcal surface but the mechanism(s) whereby LTA is retained on the surface is poorly understood. In this study, we sought to determine whether members of the M protein family consisting of Emm (M protein), Mrp (M-related protein), Enn (an M-like protein), and the streptococcal protective antigen (Spa) are involved in anchoring LTA in a manner that contributes to hydrophobicity of the streptococci and its ability to form biofilms.

METHODOLOGY/PRINCIPAL FINDINGS

Isogenic mutants defective in expression of emm, mrp, enn, and/or spa genes of eight different serotypes and their parental strains were tested for differences in LTA bound to surface proteins, LTA released into the culture media, and membrane-bound LTA. The effect of these mutations on the ability of streptococci to form a hydrophobic surface and to generate biofilms was also investigated. A recombinant strain overexpressing Emm1 was also engineered and similarly tested. The serotypes tested ranged from those that express only a single M protein gene to those that express two or three members of the M protein family. Overexpression of Emm1 led to enhanced hydrophobicity and biofilm formation. Inactivation of emm in those serotypes expressing only a single emm gene reduced biofilm formation, and protein-bound LTA on the surface, but did not alter the levels of membrane-bound LTA. The results were more varied in those serotypes that express two to three members of the M protein family.

CONCLUSIONS/SIGNIFICANCE

Our findings suggest that the formation of complexes with members of the M protein family is a common mechanism for anchoring LTA on the surface in a manner that contributes to hydrophobicity and to biofilm formation in S. pyogenes, but these activities in some serotypes are dependent on a trypsin-sensitive protein(s) that remains to be identified. The need for interactions between LTA and M proteins may impose functional constraints that limit variations in the sequence of the M proteins, major virulence factors of S. pyogenes.

Bacterial complement escape

Complement activation is a crucial step in our innate immune defense against invading bacteria. Complement proteins can quickly recognize invading bacteria and subsequently label them for phagocytosis or kill them by direct lysis. In order to survive in the human host, bacterial pathogens have evolved a number of excreted and membrane-bound proteins that interfere with several steps of the complement cascade. In this chapter we summarize the most successful complement-modulating strategies by human bacterial pathogens.

Binding of complement regulators factor H and C4b binding protein to group A streptococcal strains isolated from tonsillar tissue and blood

Group A streptococcus (GAS) is the most common pathogen causing bacterial pharyngitis. We isolated streptococcal strains from tonsils removed from patients with tonsillar disease (n=202) and studied their ability to bind the complement regulators factor H (FH) and C4b binding protein (C4BP) using 125 I-labeled proteins. Blood isolates of GAS (n=10) were obtained from patients with bacteraemia. Streptococci were isolated from 21% of the tonsillitis patients. The emm and T types of the GAS strains were determined. Of the 26 GAS strains studied, only six could bind FH and/or C4BP above the threshold levels. The fraction of the offered radioactive protein bound ranged between 6-12% for FH and 19-56% for C4BP. The clinical course of the tonsillar disease was not related to the binding of FH or C4BP by GAS. The binding strains were mostly of the T4M4 or T28M28 type. From the invasive strains (n=10), three bound FH (binding level: 8-11%) and two C4BP (36-39%). The binding correlated only partially to M-protein (emm) type suggesting that the binding was not exclusively due to M-protein. The results indicate that complement regulator binding by GAS is only partially related to pathogenicity and not a universal property of all group A streptococci.

Strain-specific virulence phenotypes of Streptococcus pneumoniae assessed using the Chinchilla laniger model of otitis media

Background

Streptococcus pneumoniae [Sp] infection is associated with local and systemic disease. Our current understanding of the differential contributions of genetic strain variation, serotype, and host response to disease phenotype is incomplete. Using the chinchilla model of otitis media [OM] we investigated the disease phenotype generated by the laboratory strain TIGR4 and each of thirteen clinical strains (BS68-75, BS290, BS291, BS293, BS436 and BS437); eleven of the thirteen strains have been genomically sequenced.

Methodology/Principal Findings

For each strain 100 colony forming units were injected bilaterally into the tympanic bullae of 6 young adult chinchillas under general anesthesia. All animals were examined daily for local and systemic disease by a blinded observer. Pneumatic otoscopy was used to evaluate local disease, and behavioral assessments served as the measure of systemic disease. Virulence scoring was performed using a 4-point scale to assess four clinical parameters [severity and rapidity of local disease onset; and severity and rapidity of systemic disease onset] during a 10-day evaluation period. Highly significant variation was observed among the strains in their ability to cause disease and moribundity.

Conclusions/Significance

As expected, there was a significant correlation between the rapidity of systemic disease onset and severity of systemic disease; however, there was little correlation between the severity of otoscopic changes and severity of systemic disease. Importantly, it was observed that different strains of the same serotype produced as broad an array of disease phenotypes as did strains of different serotypes. We attribute these phenotypic differences among the strains to the high degree of genomic plasticity that we have previously documented.

Acute poststreptococcal glomerulonephritis: an update

PURPOSE OF REVIEW

Acute poststreptococcal glomerulonephritis, the most common form of acute glomerulonephritis in children, continues to be a major concern worldwide. This review summarizes the recent advances in the pathogenesis, host susceptibility factors, diverse clinical presentations, and treatment of the condition.

RECENT FINDINGS

Several recent advances have been made in identifying streptococcal antigens that may play a pathogenic role in acute poststreptococcal glomerulonephritis. Nephritis-associated streptococcal plasmin receptor and streptococcal pyrogenic exotoxin B are currently considered major putative nephritogens. Host susceptibility factors including HLA-DRB1*03011 have been found at a higher frequency in acute poststreptococcal glomerulonephritis patients than in healthy controls. Reversible posterior leukoencephalopathy and autoimmune hemolytic anemia are newly reported clinical associations with the disease. Studies from developing countries question whether the outcome is always benign. Treatment remains mostly conservative; however, controversy exists over the use of aggressive therapy with poor prognostic factors.

SUMMARY

Severe group A streptococcal disease including acute poststreptococcal glomerulonephritis remains a cause of morbidity and mortality in developing countries and among impoverished populations. Various reports on the diverse clinical manifestations that can be associated with the condition will aid physicians in prompt diagnosis and intervention, while studies focusing on better understanding of immunopathogenesis may facilitate vaccine development and prevention.

Pathogenesis of poststreptococcal glomerulonephritis a century after Clemens von Pirquet

Considerable insight has been gained into the etiopathogenesis of poststreptococcal glomerulonephritis since the landmark theoretical construct of Clemens von Pirquet postulated that disease-causing immune complexes were responsible for the nephritis that followed scarlet fever. Over the years, molecular mimicry between streptococcal products and renal components, autoimmune reactivity and several streptococcal antigens have been extensively studied. Recent investigations assign a critical role to both in situ formation and deposition of circulating immune complexes that would trigger a variety of effector mechanisms. Glomerular plasmin-binding activity of streptococcal glyceraldehyde-3-phosphate-dehydrogenase may play a role in nephritogenicity and streptococcal pyrogenic exotoxin B and its zymogen precursor may be the long-sought nephritogenic antigen.

Membrane cofactor protein (MCP, CD46) binding to clinical isolates of Streptococcus pyogenes: binding to M type 18 strains is independent of Emm or Enn proteins

The complement regulatory protein CD46 (MCP, membrane cofactor protein) is used as a cell receptor by a number of bacterial and viral pathogens, including Streptococcus pyogenes (Group A Streptococci). The highly variable M (Emm) proteins are virulence factors of S. pyogenes, and Emm proteins of serotypes 5, 6 or 22 are able of binding to CD46, thus mediating the binding of Streptococci to human cells. In this work, using a soluble construction encompassing the extracellular domain of human CD46, we have analyzed its binding to clinical isolates of S. pyogenes, including isolates of the M types 1, 3 and 18 that are frequently found in invasive infections or rheumatic fever. Our data show a strong binding of CD46 to bacteria of M types 1, 3, 8, 18, 24, 28, 29, 31 and 78; weak binding to M6 and M29 and no binding to M types 11, 12, M27 or M30. Surprisingly, CD46 bound to isogenic mutants of one clinical M18 isolate lacking the Emm protein or Emm and the Emm-related protein Enn, regardless of having capsule or not. In addition, these isogenic mutants bound to keratinocytes in a CD46-dependent manner, confirming the role of CD46 as one of the cell receptors for Group A Streptococci. Furthermore, CD46 did not bind to a recombinant Emm 18 construct, confirming that Emm is not involved in CD46 binding to M18 bacteria. Emm-dependent and -independent CD46 binding of clinical isolates of Streptococci confirms the importance of CD46 as a cell target that might confer pathogens some biological advantages over the host.

BibA: a novel immunogenic bacterial adhesin contributing to group B Streptococcus survival in human blood

By the analysis of the recently sequenced genomes of Group B Streptococcus (GBS) we have identified a novel immunogenic adhesin with anti-phagocytic activity, named BibA. The bibA gene is present in 100% of the 24 GBS strains analysed. BibA-specific IgG were found in human sera from normal healthy donors. The putative protein product is a polypeptide of 630 amino acids containing a helix-rich N-terminal domain, a proline-rich region and a canonical LPXTG cell wall-anchoring domain. BibA is expressed on the surface of several GBS strains, but is also recovered in GBS culture supernatants. BibA specifically binds to human C4-binding protein, a regulator of the classic complement pathway. Deletion of the bibA gene severely reduced the capacity of GBS to survive in human blood and to resist opsonophagocytic killing by human neutrophils. In addition, BibA expression increased the virulence of GBS in a mouse infection model. The role of BibA in GBS adhesion was demonstrated by the impaired ability of a bibA knockout mutant strain to adhere to both human cervical and lung epithelial cells. Furthermore, we calculated that recombinant BibA bound to human epithelial cells of distinct origin with an affinity constant of approximately 10(-8) M for cervical epithelial cells. Hence BibA is a novel multifunctional protein involved in both resistance to phagocytic killing and adhesion to host cells. The identification of this potential new virulence factor represents an important step in the development of strategies to combat GBS-associated infections.

Mechanisms of resistance of Porphyromonas gingivalis to killing by serum complement

The complement system plays an important role in the host defense against infection, and the formation of the terminal complement complex on the bacterial surface has been shown to be particularly important in killing of gram-negative bacteria. The gram-negative periodontal pathogen Porphyromonas gingivalis is resistant to complement killing, and possible mechanisms suggested for this resistance include protease production and capsule formation. In this study, P. gingivalis Arg- and Lys-gingipain deletion mutants and polysaccharide synthesis deletion mutants have been used to investigate these hypotheses. When Arg- and Lys-gingipain protease mutants were incubated in 20% normal human serum, deposition of complement components on the cell surface was significantly increased compared to that for the wild-type organism. However, despite the increased deposition, the protease mutants maintained resistance to killing and their viability was equal to that seen with heat-inactivated serum. Similar data were obtained when the wild-type organism was treated with gingipain protease inhibitors. K-antigen expression mutants were also resistant to killing. However, mutants which no longer synthesized a surface anionic polysaccharide (APS) (a phosphorylated branched mannan) were extremely sensitive to serum killing. These mutants lack the organized dense glycan surface layer present on the parent strain on the basis of electron microscopy. We conclude that the production of APS at the surface of P. gingivalis rather than Arg- and Lys-gingipain synthesis is the principal mechanism of serum resistance in P. gingivalis.

Extreme sequence divergence but conserved ligand-binding specificity in Streptococcus pyogenes M protein

Many pathogenic microorganisms evade host immunity through extensive sequence variability in a protein region targeted by protective antibodies. In spite of the sequence variability, a variable region commonly retains an important ligand-binding function, reflected in the presence of a highly conserved sequence motif. Here, we analyze the limits of sequence divergence in a ligand-binding region by characterizing the hypervariable region (HVR) of Streptococcus pyogenes M protein. Our studies were focused on HVRs that bind the human complement regulator C4b-binding protein (C4BP), a ligand that confers phagocytosis resistance. A previous comparison of C4BP-binding HVRs identified residue identities that could be part of a binding motif, but the extended analysis reported here shows that no residue identities remain when additional C4BP-binding HVRs are included. Characterization of the HVR in the M22 protein indicated that two relatively conserved Leu residues are essential for C4BP binding, but these residues are probably core residues in a coiled-coil, implying that they do not directly contribute to binding. In contrast, substitution of either of two relatively conserved Glu residues, predicted to be solvent-exposed, had no effect on C4BP binding, although each of these changes had a major effect on the antigenic properties of the HVR. Together, these findings show that HVRs of M proteins have an extraordinary capacity for sequence divergence and antigenic variability while retaining a specific ligand-binding function.

Many pathogens have evolved mechanisms to evade host immunity. In one such mechanism, the sequence of a surface protein varies among different strains of a pathogen. This sequence variability represents an apparent paradox, because the variable protein must retain an important function. The authors studied this problem in Streptococcus pyogenes, a major human pathogen. The surface-localized M protein of this bacterium varies extensively in sequence between bacterial strains, allowing immune escape. Nevertheless, the most variable part of the M protein commonly binds a human plasma protein. By hijacking this human protein the bacteria evade attack by complement an important part of the innate immune system. Comparison of the ligand-binding region in different M proteins showed that these regions lack a shared amino acid sequence motif. Thus, a variable protein can retain a ligand-binding function in the absence of a conserved binding motif. Evidence is also presented that a single amino acid change in the variable region may cause a major antigenic change, providing a selective advantage for the bacteria. Together, these data bear witness to the extraordinary ability of pathogens to escape host immunity, without losing ability to cause disease.

Anti-phagocytic mechanisms of Streptococcus pyogenes: binding of fibrinogen to M-related protein

A key attribute of invasive Streptococcus pyogenes is their ability to resist phagocytosis and multiply in human blood. M-related protein (Mrp) is a major anti-phagocytic factor but the mechanism whereby it helps streptococci to evade phagocytosis has not been demonstrated. We investigated phagocytosis resistance in a strain of serotype M4 by inactivating the mrp gene and also the emm, enn, sof and sfbX genes and by analysing the effect on streptococcal growth in blood and on complement deposition on the bacterial surface. Inactivation of enn4 and sfbX4 had little impact on growth in blood, but ablation of mrp4, emm4 or sof4 reduced streptococcal growth in human blood, confirming that Mrp and Emm are required for optimal resistance to phagocytosis and providing the first indication that Sof may be an anti-phagocytic factor. Moreover, antisera against Mrp4, Emm4 and Sof4 promoted the killing of S. pyogenes, but anti-SfbX serum had no effect. Growth of S. pyogenes in blood was dependent on the presence of fibrinogen and in the absence of fibrinogen there was a twofold increase in complement deposition. Inactivation of mrp4 resulted in a loss of fibrinogen-binding and caused a twofold increase in the binding of C3b that was inhibited by Mg-EGTA. Mrp contained two fibrinogen-binding sites, one of which is within a highly conserved region. These findings indicate that Mrp-fibrinogen interactions prevent surface deposition of complement via the classical pathway, thereby contributing to the ability of these streptococci to resist phagocytosis. This may be a common mechanism for evasion of phagocytosis because Mrp is expressed by approximately half of the clinical isolates of S. pyogenes.

Complement escape of human pathogenic bacteria by acquisition of complement regulators

Pathogenic micro-organisms employ a broad range of strategies to survive in and to persistently infect the human host. Far from being completely understood by which highly sophisticated means invading pathogens overcome the host's destructive immune defence, there is a growing body of evidence on particular mechanisms which play a pivotal role for immune evasion. This review focuses on evasion of medically and scientifically important bacteria by acquisition of host derived fluid-phase complement regulatory proteins, in particular factor H, FHL-1, and C4b binding protein. Expression of microbial surface molecules binding to human complement regulators and thus fixing them in a functionally active state allows pathogens to inhibit and finely regulate complement activation directly on their surface. Further studies on the utilization of host complement regulatory proteins will likely have a marked impact on a more efficient and specific clinical treatment.