Complement evasion of pathogens: common strategies are shared by diverse organisms

A Bioluminescence-Based Serum Bactericidal Assay to Detect Bactericidal Antibodies Against Neisseria meningitidis in Human Sera

Serum bactericidal assay (SBA) is a functional assay that evaluates infection- and vaccine-induced neutralizing antibodies representing the serological correlate of protection against Neisseria meningitidis. However, it is time consuming due to its readout using the enumeration of colony-forming units (CFUs), making this conventional SBA (C-SBA) difficult for large-scale use. We developed a new SBA method that takes advantage of a bioluminescence N. meningitidis serogroup B (BioLux-SBA). The assay development steps involved the human complement source validation, the setup of the optimal incubation time, and the assessment of intra-day and inter-day variability. BioLux-SBA was then compared to C-SBA using a serum collection of Norman children vaccinated in 2011 with MenBvac, an OMV meningococcal vaccine. While a conventional approach requests 48 h of work to test 24 sera per day, BioLux-SBA takes only 5 h to test 96 sera per day. The SBA titers (n = 10) correlated with R2 of 0.98 (p-value < 0.0001). The deposition of terminal complement components (C5b-C9) measured by flow cytometry on the bacterial surface well correlated with BioLux SBA titers. This high-throughput method to evaluate the immunogenicity of meningococcal vaccines appears to be a reliable method for an OMV meningococcal B vaccine and requires further assessment in other laboratories and against other meningococcal vaccines.

A potential bimodal interplay between heme and complement factor H 402H in the deregulation of the complement alternative pathway by SARS-CoV-2

The recent discovery that the trimeric SARS-CoV-2 spike S glycoprotein carries heme within an NTD domain pocket of the S1 subunits, suggested that this virus may be cleverly utilizing heme, in addition to the S1 RBD domains, for invading target cells carrying a specific entry receptor like ACE2, TMEM106B and others. Studies during the COVID-19 pandemic revealed that the infectivity of this virus depends on cell surface heparan sulfate and that the infection induces non-canonical activation of the Complement Alternative pathway (AP) on the surface of infected cells. In our recent COVID-19 genomic studies, among the coding SNPs of interest we also detected the presence of the CFH rs1061170, rs800292 and rs1065489 within all the infected patient subgroups examined. The minor C allele of rs1061170 encodes CFH 402H that over the years has been associated with diseases characterized by complement dysregulation namely the age-related macular degeneration (AMD) and the atypical haemolytic uremic syndrome (aHUS). Also, more recently with the diminishment of CD4+ T cell responses with ageing. The rs800292 minor allele A encodes CFH 62I that supports enhanced cofactor activity for Complement factor I (CFI). Also, the rs1065489 minor allele T encodes CFH 936D and is located within the CCP16 domain that influences the affinity of CFH with extracellular laminins. A subsequent computational analysis revealed that the CFH residue 402 is located centrally within a heme-binding motif (HBM) in domain CCP7 (398YNQNYGRKF406). Heme on the viral spike glycoprotein S1 subunit could recruit CFH 402H for masking free viral particles from opsonisation, and when in proximity to cell surface, act as a bait disrupting CFH 402H from the heparan sulphate coat of the target cells. Publicly available genetic data for European populations indicate that the minor C allele of rs1061170 is present only in haplotypes that carry the major alleles of rs800292 and rs1065489. This combination encodes for CFH 402H that exhibits increased biochemical affinity for heme in proximity, without enhanced cofactor activity for CFI and weaker association with the extracellular matrix. In the theatre of infection, this combination can promote heme-mediated viral infection with weaker complement opsonisation and potential AP deregulation. This strategy may be evolutionary conserved among various classes of infectious agents.

Determinants of bacterial survival and proliferation in blood

Bloodstream infection is a major public health concern associated with high mortality and high healthcare costs worldwide. Bacteremia can trigger fatal sepsis whose prevention, diagnosis, and management have been recognized as a global health priority by the World Health Organization. Additionally, infection control is increasingly threatened by antimicrobial resistance, which is the focus of global action plans in the framework of a One Health response. In-depth knowledge of the infection process is needed to develop efficient preventive and therapeutic measures. The pathogenesis of bloodstream infection is a dynamic process resulting from the invasion of the vascular system by bacteria, which finely regulate their metabolic pathways and virulence factors to overcome the blood immune defenses and proliferate. In this review, we highlight our current understanding of determinants of bacterial survival and proliferation in the bloodstream and discuss their interactions with the molecular and cellular components of blood.

Microbial evasion of the complement system: a continuous and evolving story

The complement system is a fundamental part of the innate immune system that plays a key role in the battle of the human body against invading pathogens. Through its three pathways, represented by the classical, alternative, and lectin pathways, the complement system forms a tightly regulated network of soluble proteins, membrane-expressed receptors, and regulators with versatile protective and killing mechanisms. However, ingenious pathogens have developed strategies over the years to protect themselves from this complex part of the immune system. This review briefly discusses the sequence of the complement activation pathways. Then, we present a comprehensive updated overview of how the major four pathogenic groups, namely, bacteria, viruses, fungi, and parasites, control, modulate, and block the complement attacks at different steps of the complement cascade. We shed more light on the ability of those pathogens to deploy more than one mechanism to tackle the complement system in their path to establish infection within the human host.

The choline-binding proteins PspA, PspC, and LytA of Streptococcus pneumoniae and their interaction with human endothelial and red blood cells

Streptococcus pneumoniae is a Gram-positive opportunistic pathogen that can colonize the upper respiratory tract. It is a leading cause of a wide range of infectious diseases, including community-acquired pneumonia and meningitis. Pneumococcal infections cause 1-2 million deaths per year, most of which occur in developing countries. Here, we focused on three choline-binding proteins (CBPs), i.e., PspC, PspA, and LytA. These pneumococcal proteins have different surface-exposed regions but share related choline-binding anchors. These surface-exposed pneumococcal proteins are in direct contact with host cells and have diverse functions. We explored the role of the three CBPs on adhesion and pathogenicity in a human host by performing relevant imaging and functional analyses, such as electron microscopy, confocal laser scanning microscopy, and functional quantitative assays, targeting biofilm formation and the hemolytic capacity of S. pneumoniae. In vitro biofilm formation assays and electron microscopy experiments were used to examine the ability of knockout mutant strains lacking the lytA, pspC, or pspA genes to adhere to surfaces. We found that LytA plays an important role in robust synthesis of the biofilm matrix. PspA and PspC appeared crucial for the hemolytic effects of S. pneumoniae on human red blood cells. Furthermore, all knockout mutants caused less damage to endothelial cells than wild-type bacteria, highlighting the significance of each CPB for the overall pathogenicity of S. pneumoniae. Hence, in addition to their structural function within the cell wall of S. pneumoniae, each of these three surface-exposed CBPs controls or mediates multiple steps during bacterial pathogenesis.

Weil's Disease-Immunopathogenesis, Multiple Organ Failure, and Potential Role of Gut Microbiota

Leptospirosis is an important zoonotic disease, causing about 60,000 deaths annually. In this review, we have described in detail the immunopathogenesis of leptospirosis, the influence of cytokines, genetic susceptibility on the course of the disease, and the evasion of the immune response. These data are combined with information about immunological and pathomorphological changes in the kidneys, liver, and lungs, which are most affected by Weil's disease. The review also suggests a possible role of the gut microbiota in the clinical course of leptospirosis, the main mechanisms of the influence of gut dysbiosis on damage in the liver, kidneys, and lungs through several axes, i.e., gut-liver, gut-kidney, and gut-lungs. Modulation of gut microbiota by probiotics and/or fecal microbiota transplantation in leptospirosis may become an important area of scientific research.

Innate immunity and microbial dysbiosis in hidradenitis suppurativa - vicious cycle of chronic inflammation

Hidradenitis Suppurativa (HS) is a chronic multifactorial inflammatory skin disease with incompletely understood mechanisms of disease pathology. HS is characterized by aberrant activation of the innate immune system, resulting in activation of pathways that aim to protect against pathogenic microorganisms, and also contribute to failure to resolve inflammation. Imbalance in innate immunity is evident in deregulation of host antimicrobial peptides (AMPs) and the complement system associated with the microbiome dysbiosis. The pathology is further complicated by ability of pathogens associated with HS to overcome host immune response. Potential roles of major AMPs, cathelicidin, defensins, dermcidin, S100 proteins, RNAse 7 and complement proteins are discussed. Dysregulated expression pattern of innate immunity components in conjunction with bacterial component of the disease warrants consideration of novel treatment approaches targeting both host immunity and pathogenic microbiome in HS.

Deciphering the Role of Leptospira Surface Protein LigA in Modulating the Host Innate Immune Response

Leptospira, a zoonotic pathogen, is known to infect various hosts and can establish persistent infection. This remarkable ability of bacteria is attributed to its potential to modulate (activate or evade) the host immune response by exploiting its surface proteins. We have identified and characterized the domain of the variable region of Leptospira immunoglobulin-like protein A (LAV) involved in immune modulation. The 11^th domain (A₁₁) of the variable region of LigA (LAV) induces a strong TLR4 dependent innate response leading to subsequent induction of humoral and cellular immune responses in mice. A₁₁ is also involved in acquiring complement regulator FH and binds to host protease Plasminogen (PLG), there by mediating functional activity to escape from complement-mediated killing. The deletion of A₁₁ domain significantly impaired TLR4 signaling and subsequent reduction in the innate and adaptive immune response. It also inhibited the binding of FH and PLG thereby mediating killing of bacteria. Our study discovered an unprecedented role of LAV as a nuclease capable of degrading Neutrophil Extracellular Traps (NETs). This nuclease activity was primarily mediated by A₁₁. These results highlighted the moonlighting function of LigA and demonstrated that a single domain of a surface protein is involved in modulating the host innate immune defenses, which might allow the persistence of Leptospira in different hosts for a long term without clearance.

Transcriptomic responses to air exposure stress in coelomocytes of the sea cucumber, Apostichopus japonicus

During rearing in hatcheries and transportation to restocking sites, sea cucumbers are often exposed to air for several hours, which may depress their non-specific immunity and lead to mass mortality. We performed transcriptome analysis of Apostichopus japonicus coelomocytes after air exposure to identify stress-related genes and pathways. After exposure to air for 1 h, individuals were re-submerged in aerated seawater and coelomocytes were collected at 0, 1, 4, and 16 h (B, H1, H4, and H16, respectively). We identified 6148 differentially expressed genes, of which 3216 were upregulated and 2932 were downregulated. Many genes involved in the immune response, antioxidant defense, and apoptosis were highly induced in response to air exposure. Enrichment analysis of Gene Ontology terms showed that the most abundant terms in the biological process category were oxidation-reduction process, protein folding and phosphorylation, and receptor-mediated endocytosis for the comparison of H1 vs. B, H4 vs. H1, and H16 vs. H4, respectively. Kyoto Eecyclopedia of Genes and Genomes enrichment analysis showed that six pathways related to the metabolism of proteins, fats, and carbohydrates were shared among the three comparisons. These results indicated that sea cucumbers regulate the expression of genes related to the antioxidant system and energy metabolism to resist the negative effects of air exposure stress. These findings may be applied to optimize juvenile sea cucumber production, and facilitate molecular marker-assisted selective breeding of an anoxia-resistant strain.

The impact of episporic modification of Lichtheimia corymbifera on virulence and interaction with phagocytes

Fungal infections caused by the ancient lineage Mucorales are emerging and increasingly reported in humans. Comprehensive surveys on promising attributes from a multitude of possible virulence factors are limited and so far, focused on Mucor and Rhizopus. This study addresses a systematic approach to monitor phagocytosis after physical and enzymatic modification of the outer spore wall of Lichtheimia corymbifera, one of the major causative agents of mucormycosis. Episporic modifications were performed and their consequences on phagocytosis, intracellular survival and virulence by murine alveolar macrophages and in an invertebrate infection model were elucidated. While depletion of lipids did not affect the phagocytosis of both strains, delipidation led to attenuation of LCA strain but appears to be dispensable for infection with LCV strain in the settings used in this study. Combined glucano-proteolytic treatment was necessary to achieve a significant decrease of virulence of the LCV strain in Galleria mellonella during maintenance of the full potential for spore germination as shown by a novel automated germination assay. Proteolytic and glucanolytic treatments largely increased phagocytosis compared to alive resting and swollen spores. Whilst resting spores barely (1–2%) fuse to lysosomes after invagination in to phagosomes, spore trypsinization led to a 10-fold increase of phagolysosomal fusion as measured by intracellular acidification. This is the first report of a polyphasic measurement of the consequences of episporic modification of a mucormycotic pathogen in spore germination, spore surface ultrastructure, phagocytosis, stimulation of Toll-like receptors (TLRs), phagolysosomal fusion and intracellular acidification, apoptosis, generation of reactive oxygen species (ROS) and virulence.

Quantification of Factor H Mediated Self vs. Non-self Discrimination by Mathematical Modeling

The complement system is part of the innate immune system and plays an important role in the host defense against infectious pathogens. One of the main effects is the opsonization of foreign invaders and subsequent uptake by phagocytosis. Due to the continuous default basal level of active complement molecules, a tight regulation is required to protect the body's own cells (self cells) from opsonization and from complement damage. A major complement regulator is Factor H, which is recruited from the fluid phase and attaches to cell surfaces where it effectively controls complement activation. Besides self cells, pathogens also have the ability to bind Factor H; they can thus escape opsonization and phagocytosis causing severe infections. In order to advance our understanding of the opsonization process at a quantitative level, we developed a mathematical model for the dynamics of the complement system-termed DynaCoSys model-that is based on ordinary differential equations for cell surface-bound molecules and on partial differential equations for concentration profiles of the fluid phase molecules in the environment of cells. This hybrid differential equation approach allows to model the complement cascade focusing on the role of active C3b in the fluid phase and on the cell surface as well as on its inactivation on the cell surface. The DynaCoSys model enables us to quantitatively predict the conditions under which Factor H mediated complement evasion occurs. Furthermore, investigating the quantitative impact of model parameters by a sensitivity analysis, we identify the driving processes of complement activation and regulation in both the self and non-self regime. The two regimes are defined by a critical Factor H concentration on the cell surface and we use the model to investigate the differential impact of complement model parameters on this threshold value. The dynamic modeling on the surface of pathogens are further relevant to understand pathophysiological situations where Factor H mutants and defective Factor H binding to target surfaces results in pathophysiology such as renal and retinal disease. In the future, this DynaCoSys model will be extended to also enable evaluating treatment strategies of complement-related diseases.

Identification, characterization, and immunological analysis of complement component 4 from grass carp (Ctenopharyngodon idella)

Complement component 4 (C4) has critical immunological functions in vertebrates. In the current study, a C4 homolog (gcC4) was identified in grass carp (Ctenopharyngodon idella). The full-length 5458 bp gcC4 cDNA contained a 5148 bp open reading frame (ORF) encoding a protein of 1715 amino acids with a signal peptide and eight conservative domains. The gcC4 protein has a high level of identity with other fish C4 counterparts and is phylogenetically clustered with cyprinid fish C4. The gcC4 transcript shows wide tissue distribution and is inducible by Aeromonas hydrophila in vivo and in vitro. Furthermore, its expression also fluctuates upon lipopolysaccharide or flagellin stimulation in vitro. During infection, the gcC4 protein level decreases or increases to varying degrees, and the intrahepatic C4 expression location changes. With gcC4 overexpression, interleukin 1 beta, tumor necrosis factor alpha, and interferon transcripts are all upregulated by A. hydrophila infection. Meanwhile, overexpression of gcC4 reduces bacterial invasion or proliferation. Moreover, gcC4 may activate the NF-κB signaling pathway. These findings demonstrate the vital role of gcC4 in the innate immunity of grass carp.

Complement in malaria: immune evasion strategies and role in protective immunity

The malaria parasite has for long been thought to escape host complement attack as a survival strategy. However, it was only recently that complement evasion mechanisms of the parasite were described. Simultaneously, the role of complement in antibody-mediated naturally acquired and vaccine-induced protection against malaria has also been reported. Such findings should be considered in future vaccine design, given the current need to develop more efficacious vaccines against malaria. Parasite antigens derived from molecules mediating functions crucial for parasite survival, such as complement evasion, or parasite antigens against which antibody responses lead to an efficient complement attack could present new candidates for vaccines. In this review, we discuss recent findings on complement evasion by the malaria parasites and the emerging role of complement in antibody-mediated protection against malaria. We emphasize that immune responses to vaccines based on complement inhibitors should not only induce complement-activating antibodies but also neutralize the escape mechanisms of the parasite.

Transcriptomic signature of rapidly evolving immune genes in a highland fish

Recent genome-wide studies have begun to elucidate the genomic basis of hypoxia, long-term cold and high saline and alkaline adaptation in highland fish, and a number of key genes contributed to its highland adaptation were identified. An increasing number of studies indicated that immune genes of Tibetan endemic fish species underwent positive selection towards functional shift, while the insight into immune gene repertoire of Tibetan highland fishes from genome-wide studies has largely lagged behind. In this study, we performed one of the first comparative genomics study in particular focusing on the signatures of immune genes in a highland fish, Gymnocypris przewalskii based on immune-relevant tissue transcriptome assemblies. We identified seven putative rapidly evolving immune genes with elevated molecular evolutionary rate (dN/dS) relative to lowland fish species. Using tissue-transcriptome data, we found most of rapidly evolving immune genes were broadly expressed in head-kidney, spleen, gills and skin tissues, which significantly enriched for complement activation and inflammatory response processes. In addition, we found a set of complement activation related genes underwent accelerated evolution and showed consistently repressed expression patterns in response to parasite Ichthyophthirius multifiliis infection. Moreover, we detected a number of immune genes involved in adaptive immune system exhibited distinct signature of upregulated expression patterns after parasite infection. Taken together, this study provided putative transcriptomic signatures of rapidly evolving immune genes, and will gain the insight into Schizothoracine fish adaptation to high-altitude extreme aquatic environments including diversified pathogen challenge.

Deciphering Biological Processes at the Tick-Host Interface Opens New Strategies for Treatment of Human Diseases

Ticks are obligatory blood-feeding ectoparasites, causing blood loss and skin damage in their hosts. In addition, ticks also transmit a number of various pathogenic microorganisms that cause serious diseases in humans and animals. Ticks evolved a wide array of salivary bioactive compounds that, upon injection into the host skin, inhibit or modulate host reactions such as hemostasis, inflammation and wound healing. Modulation of the tick attachment site in the host skin involves mainly molecules which affect physiological processes orchestrated by cytokines, chemokines and growth factors. Suppressing host defense reactions is crucial for tick survival and reproduction. Furthermore, pharmacologically active compounds in tick saliva have a promising therapeutic potential for treatment of some human diseases connected with disorders in hemostasis and immune system. These disorders are often associated to alterations in signaling pathways and dysregulation or overexpression of specific cytokines which, in turn, affect mechanisms of angiogenesis, cell motility and cytoskeletal regulation. Moreover, tick salivary molecules were found to exert cytotoxic and cytolytic effects on various tumor cells and have anti-angiogenic properties. Elucidation of the mode of action of tick bioactive molecules on the regulation of cell processes in their mammalian hosts could provide new tools for understanding the complex changes leading to immune disorders and cancer. Tick bioactive molecules may also be exploited as new pharmacological inhibitors of the signaling pathways of cytokines and thus help alleviate patient discomfort and increase patient survival. We review the current knowledge about tick salivary peptides and proteins that have been identified and functionally characterized in in vitro and/or in vivo models and their therapeutic perspective.

Identification and CRISPR/Cas9 Inactivation of the C1s Protease Responsible for Proteolysis of Recombinant Proteins Produced in CHO Cells

Proteolysis associated with recombinant protein expression in Chinese Hamster Ovary (CHO) cells has hindered the development of biologics including HIV vaccines. When expressed in CHO cells, the recombinant HIV envelope protein, gp120, undergoes proteolytic clipping by a serine protease at a key epitope recognized by neutralizing antibodies. The problem is particularly acute for envelope proteins from clade B viruses that represent the major genetic subtype circulating in much of the developed world, including the US and Europe. In this paper, we have identified complement Component 1's (C1s), a serine protease from the complement cascade, as the protease responsible for the proteolysis of gp120 in CHO cells. CRISPR/Cas9 knockout of the C1s protease in a CHO cell line was shown to eliminate the proteolytic activity against the recombinantly expressed gp120. In addition, the C1s^-/- MGAT1^- CHO cell line, with the C1s protease and the MGAT1 glycosyltransferase knocked out, enabled the production of unclipped gp120 from a clade B isolate (BaL-rgp120) and enriched for mannose-5 glycans on gp120 that are required for the binding of multiple broadly neutralizing monoclonal antibodies (bN-mAbs). The availability of this technology will allow for the scale-up and testing of multiple vaccine concepts in regions of the world where clade B viruses are in circulation. Furthermore, the proteolysis issues caused by the C1s protease suggests a broader need for a C1s-deficient CHO cell line to express other recombinant proteins that are susceptible to serine protease activity in CHO cells. Similarly, the workflow described here to identify and knockout C1s in a CHO cell line can be applied to remedy the proteolysis of biologics by other CHO proteases.

Complement component 3 (C3): An important role in grass carp (Ctenopharyngodon idella) experimentally exposed to Aeromonas hydrophila

Complement is traditionally recognized as part of the innate immune system, defending the host against the invasion of foreign pathogens. In complement system, C3 (complement component 3) is a central component. Therefore, research into C3 can help us better understand the functions of fish complement system. In this study, we detected the grass carp C3 (gcC3) mRNA expression in all sample tissues from healthy grass carp, which was highest in the liver, followed by the heart and the spleen, and lowest in the muscle, head kidney, trunk kidney, blood and intestine. After infection with Aeromonas hydrophila, gcC3 mRNA expression levels were significantly upregulated in the gill, liver, spleen, intestine, trunk kidney and head kidney. Interestingly, C3 protein levels were downregulated and subsequently upregulated in the liver and serum. Histologically, C3 protein at 24 h pi was over expressed in necrotic liver sites, and the liver index (LI) at this point was significantly higher than that of the control. These findings are indicated that C3 plays an important role in the immune response of grass carp after A. hydrophila infection, and C3 protein may play an assistant role in repairing liver tissues from A. hydrophila injury.

Integrating Genomic and Morphological Approaches in Fish Pathology Research: The Case of Turbot (Scophthalmus maximus) Enteromyxosis

Enteromyxosis, caused by Enteromyxum scophthalmi, is one of the most devastating diseases stemming from myxozoan parasites in turbot (Scophthalmus maximus L.), being a limiting factor for its production. The disease develops as a cachectic syndrome, associated to catarrhal enteritis and leukocytic depletion, with morbidity and mortality rates usually reaching 100%. To date, no effective treatment exists and there are different unknown issues concerning its pathogenesis. The gross and microscopic lesions associated to enteromyxosis have been thoroughly described, and several morphopathological studies have been carried out to elucidate the mechanisms of this host-parasite interaction. More recently, efforts have been focused on a multidisciplinary approach, combining histopathology and transcriptome analysis, which has provided significant advances in the understanding of the pathogenesis of this parasitosis. RNA-Seq technology was applied at early and advanced stages of the disease on fishes histologically evaluated and classified based on their lesional degree. In the same way, the transcriptomic data were analyzed in relation to the morphopathological picture and the course of the disease. In this paper, a comprehensive review of turbot enteromyxosis is presented, starting from the disease description up to the most novel information extracted by an integrated approach on the infection mechanisms and host response. Further, we discuss ongoing strategies toward a full understanding of host-pathogen interaction and the identification of suitable biomarkers for early diagnosis and disease management strategies.

Complement Activation as a Helping Hand for Inflammophilic Pathogens and Cancer

The complement system, an evolutionarily ancient component of innate immunity, is capable of protecting hosts from invading pathogens, either directly, by lysis of target cells, or indirectly, by mobilization of host immune mechanisms. However, this potentially cytotoxic cascade must be tightly regulated, since improperly controlled complement can damage healthy cells and tissues. The practical importance of this axis is highlighted when impairment of complement regulators or bacterial mechanisms of complement evasion result in pathogenic conditions. Recognition of complement as a "double-edged sword" is widely acknowledged, but another, currently underappreciated aspect of complement function has emerged as an important player in homeostatic balance-the dual outcome of complement-mediated inflammation. In most cases, the proinflammatory properties of complement are beneficial to the host. However, certain pathogens have developed the ability to utilize local inflammation as a source of nutrients and as a way to establish a niche for further colonization. Such a strategy can be illustrated in the example of periodontitis. Interestingly, certain tumors also seem to benefit from complement activation products, which promote a proangiogenic and immunosuppressive microenvironment.

Characterization of a novel protein of Leptospira interrogans exhibiting plasminogen, vitronectin and complement binding properties

Leptospirosis is a severe zoonosis caused by pathogenic species of the genus Leptospira. This work focuses on a hypothetical protein of unknown function, encoded by the gene LIC13259, and predicted to be a surface protein, widely distributed among pathogenic leptospiral strain. The gene was amplified from L. interrogans serovar Copenhageni, strain Fiocruz L1-130, cloned and the protein expressed using Escherichia coli as a host system. Immunofluorescence assay showed that the protein is surface-exposed. The recombinant protein LIC13259 (rLIC13259) has the ability to interact with the extracellular matrix (ECM) laminin, in a dose-dependent manner but saturation was not reach. The rLIC13259 protein is a plasminogen (PLG)-binding protein, generating plasmin, in the presence of urokinase PLG-activator uPA. The recombinant protein is able to mediate the binding to human purified terminal complement route vitronectin, C7, C8 and C9, and to recruit and interact with these components from normal human serum (NHS). These interactions are dose-dependent on NHS increased concentration. The binding of rLIC13259 to C8 and vitronectin was slight and pronounced inhibited in the presence of increasing heparin concentration, respectively, suggesting that the interaction with vitronectin occurs via heparin domain. Most interesting, the interaction of rLIC13259 with C9 protein was capable of preventing C9 polymerization, suggesting that the membrane attack complex (MAC) formation was inhibited. Thus, we tentatively assign the coding sequence (CDS) LIC13259, previously annotated as unknown function, as a novel protein that may play an important role in the host's invasion and immune evasion processes, contributing to the establishment of the leptospiral infection.

Complement C3 Affects Rac1 Activity in the Developing Brain

The complement system, which is part of the innate immune response system, has been recently shown to participate in multiple key processes in the developing brain. Here we aimed to elucidate downstream signaling responses linking complement C3, a key molecule of the pathway, to small GTPases, known to affect the cytoskeleton. The expression pattern of the activated small GTPase Rac1 resembled that of complement C3. C3-deficient mice exhibited reduced Rac1 and elevated RhoA activity in comparison with control mice. The most pronounced reduction of Rac1 activity occurred at embryonic day 14. Rac1 has been implicated in neuronal migration as well as neuronal stem cell proliferation and differentiation. Consistent with the reduction in Rac1 activity, the expression of phospho-cofilin, decreased in migrating neurons. Reduced Rac1-GTP was also correlated with a decrease in the expression of progenitor markers (Nestin, Pax6 and Tbr2) and conversely the expression of neuronal markers (Dcx and NeuN) increased in C3 knockout (KO) cortices in comparison with wild-type (WT) cortices. More specifically, C3 deficiency resulted in a reduction in the number of the cells in S-phase and an elevation in the number of cells that precociously exited the cell cycle. Collectively, our findings suggest that C3 impacts the activity of small GTPases resulting in cell cycle defects and premature neuronal differentiation.

Pathogen clearance and immune adherence "revisited": Immuno-regulatory roles for CRIg

Rapid elimination of microbes from the bloodstream, along with the ability to mount an adaptive immune response, are essential for optimal host-defense. Kupffer cells are strategically positioned in the liver sinusoids and efficiently capture circulating microbes from the hepatic artery and portal vein, thus preventing bacterial dissemination. In vivo and in vitro studies have probed how complement receptor of the immunoglobulin superfamily (CRIg), also referred to as Z39Ig and V-set and Ig domain-containing 4 (VSIG4), acts as a critical player in pathogen recognition and clearance. While recent data suggested that CRIg may bind bacterial cell wall components directly, the single transmembrane receptor is best known for its interaction with complement C3 opsonization products on the microbial surface. On Kupffer cells, CRIg must capture opsonized microbes against the shear forces of the blood flow. In vivo work reveals how immune adherence (IA), a process in which blood platelets or erythrocytes associate with circulating bacteria, plays a critical role in regulating pathogen capture by CRIg under flow conditions. In addition to its typical innate immune functions, CRIg was shown to directly and indirectly influence adaptive immune responses. Here, we review our current understanding of the diverse roles of CRIg in pathogen elimination, anti-microbial immunity and autoimmunity. In particular, we will explore how, through selective capturing by CRIg, an important balance is achieved between the immunological and clearance functions of liver and spleen.

Characterization of a teleost membrane-associated protein that is involved in the regulation of complement activation and bacterial infection

In mammals, membrane-associated complement regulatory proteins (MCRP) can protect host cells from the damaging of the activated complement. In teleost, few studies on the function of MCRP have been documented. In the present report, we identified a MCRP (named CsMCRP) from the teleost fish tongue sole Cynoglossus semilaevis and examined its immune function. CsMCRP shares moderate sequence identities with fish DAF-like molecules. CsMCRP was predicted to be a transmembrane protein with three short consensus repeats located in the extracellular region. CsMCRP expression occurred in nine different tissues, especially blood, and in peripheral blood leukocytes (PBL). Recombinant CsMCRP inhibited complement activation and interacted with bacterial pathogen, the latter in a highly selective manner. Antibody blocking the CsMCRP on PBL significantly inhibited bacterial infection of PBL. These results indicate that teleost CsMCRP is both a regulator of complement activation and a cellular receptor involved in bacterial invasion.

Pathogens' toolbox to manipulate human complement

The surveillance and pathogen fighting functions of the complement system have evolved to protect mammals from life-threatening infections. In turn, pathogens have developed complex molecular mechanisms to subvert, divert and evade the effector functions of the complement. The study of complement immunoevasion by pathogens sheds light on their infection drivers, knowledge that is essential to implement therapies. At the same time, complement evasion also acts as a discovery ground that reveals important aspects of how complement works under physiological conditions. In recent years, complex interrelationships between infection insults and the onset of autoimmune and complement dysregulation diseases have led to propose that encounters with pathogens can act as triggering factors for disease. The correct management of these diseases involves the recognition of their triggering factors and the development and administration of complement-associated molecular therapies. Even more recently, unsuspected proteins from pathogens have been shown to possess moonlighting functions as virulence factors, raising the possibility that behind the first line of virulence factors there be many more pathogen proteins playing secondary, helping and supporting roles for the pathogen to successfully establish infections. In an era where antibiotics have a progressively reduced effect on the management and control of infectious diseases worldwide, knowledge on the mechanisms of pathogenic invasion and evasion look more necessary and pressing than ever.

A teleost CD46 is involved in the regulation of complement activation and pathogen infection

In mammals, CD46 is involved in the inactivation of complement by factor I (FI). In teleost, study on the function of CD46 is very limited. In this study, we examined the immunological property of a CD46 molecule (CsCD46) from tongue sole, a teleost species with important economic value. We found that recombinant CsCD46 (rCsCD46) interacted with FI and inhibited complement activation in an FI-dependent manner. rCsCD46 also interacted with bacterial pathogens via a different mechanism to that responsible for the FI interaction, involving different rCsCD46 sites. Cellular study showed that CsCD46 was expressed on peripheral blood leukocytes (PBL) and protected the cells against the killing effect of complement. When the CsCD46 on PBL was blocked by antibody before incubation of the cells with bacterial pathogens, cellular infection was significantly reduced. Consistently, when tongue sole were infected with bacterial pathogens in the presence of rCsCD46, tissue dissemination and survival of the pathogens were significantly inhibited. These results provide the first evidence to indicate that CD46 in teleosts negatively regulates complement activation via FI and protects host cells from complement-induced damage, and that CD46 is required for optimal bacterial infection probably by serving as a receptor for the bacteria.

Transcriptomic profiling of Tibetan highland fish (Gymnocypris przewalskii) in response to the infection of parasite ciliate Ichthyophthirius multifiliis

Gymnocypris przewalskii is a native cyprinid in the Lake Qinghai of the Qinghai-Tibetan Plateau. G. przewalskii is highly susceptible to the infection of a parasite, Ichthyophthirius multifiliis, in the artificial propagation and breeding. To better understand the host immune reaction to I. multifiliis infection, we characterize the gene expression profiles in the spleen of healthy and I. multifiliis infected G. przewalskii by RNA-seq. Totally, the transcriptomic analysis produces 463,031,110 high quality reads, which are assembled to 213,538 genes with N50 of 1918 bp and the average length of 1205 bp. Of assembled genes, 90.52% are annotated by public databases. The expression analysis shows 744 genes are significantly changed by the infection of I. multifiliis, which are validated by qRT-PCR with the correlation coefficient of 0.896. The differentially expressed genes are classified into 689 GO terms and 230 KEGG pathways, highlighting the promoted innate immunity in I. multifiliis infected G. przewalskii at 2 days post infection. Our results pinpoint that the up-regulated genes are enriched in TLR signaling pathway, inflammatory response and activation of immune cell migration. On the contrary, complement genes are down-regulated, indicating the evasion of host complement cascades by I. multifiliis. The repressed genes are also enriched in the pathways related to metabolism and endocrine, suggesting the metabolic disturbance in I. multifiliis treated G. przewalskii. In summary, the present study profiles the gene expression signature of G. przewalskii in the responses to I. multifiliis infection, and improves our understanding on molecular mechanisms of host-parasite interaction in G. przewalskii, which focuses the crucial function of TLRs, cytokines and complement components in the host defense against I. multifiliis.

Fungal Strategies to Evade the Host Immune Recognition

The recognition of fungal cells by the host immune system is key during the establishment of a protective anti-fungal response. Even though the immune system has evolved a vast number of processes to control these organisms, they have developed strategies to fight back, avoiding the proper recognition by immune components and thus interfering with the host protective mechanisms. Therefore, the strategies to evade the immune system are as important as the virulence factors and attributes that damage the host tissues and cells. Here, we performed a thorough revision of the main fungal tactics to escape from the host immunosurveillance processes. These include the composition and organization of the cell wall, the fungal capsule, the formation of titan cells, biofilms, and asteroid bodies; the ability to undergo dimorphism; and the escape from nutritional immunity, extracellular traps, phagocytosis, and the action of humoral immune effectors.

The secreted Candida albicans protein Pra1 disrupts host defense by broadly targeting and blocking complement C3 and C3 activation fragments

Candida albicans the most frequently isolated clinical fungal pathogen can cause local as well as systemic and life-threatening infections particularly in immune-compromised individuals. A better and more detailed understanding how C. albicans evades human immune attack is therefore needed for identifying fungal immune-evasive proteins and develop new therapies. Here, we identified Pra1, the pH-regulated C. albicans antigen as a hierarchical complement inhibitor that targets C3, the central human complement component. Pra1 cleaved C3 at a unique site and further inhibited effector function of the activation fragments. The newly formed C3a-like peptide lacked the C-terminal arginine residue needed for C3a-receptor binding and activation. Moreover, Pra1 also blocked C3a-like antifungal activity as shown in survival assays, and the C3b-like molecule formed by Pra1 was degraded by the host protease Factor I. Pra1 also bound to C3a and C3b generated by human convertases and blocked their effector functions, like C3a antifungal activity shown by fungal survival, blocked C3a binding to human C3a receptor-expressing HEK cells, activation of Fura2-AM loaded cells, intracellular Ca²⁺ signaling, IL-8 release, C3b deposition, as well as opsonophagocytosis and killing by human neutrophils. Thus, upon infection C. albicans uses Pra1 to destroy C3 and to disrupt host complement attack. In conclusion, candida Pra1 represents the first fungal C3-cleaving protease identified and functions as a fungal master regulator of innate immunity and as a central fungal immune-escape protein.

Host immune responses to the itch mite, Sarcoptes scabiei, in humans

Scabies is a parasitic disease due to infestation of skin by the burrowing mite Sarcoptes scabiei. Scabies is a major public health problem and endemic in resource poor communities worldwide affecting over 100 million people. Associated bacterial infections cause substantial morbidity, and in severe cases can lead to renal and cardiac diseases. Mite infestation of the skin causes localised cutaneous inflammation, pruritus, skin lesions, and allergic and inflammatory responses are mounted by the host against the mite and its products. Our current understanding of the immune and inflammatory responses associated with the clinical manifestations in scabies is far outweighed by the significant global impact of the disease. This review aims to provide a better understanding of human immune responses to S. scabiei in ordinary and crusted scabies phenotypes.

Evolution and Function of Thioester-Containing Proteins and the Complement System in the Innate Immune Response

The innate immune response is evolutionary conserved among organisms. The complement system forms an important and efficient immune defense mechanism. It consists of plasma proteins that participate in microbial detection, which ultimately results in the production of various molecules with antimicrobial activity. Thioester-containing proteins (TEPs) are a superfamily of secreted effector proteins. In vertebrates, certain TEPs act in the innate immune response by promoting recruitment of immune cells, phagocytosis, and direct lysis of microbial invaders. Insects are excellent models for dissecting the molecular basis of innate immune recognition and response to a wide range of microbial infections. Impressive progress in recent years has generated crucial information on the role of TEPs in the antibacterial and antiparasite response of the tractable model insect Drosophila melanogaster and the mosquito malaria vector Anopheles gambiae. This knowledge is critical for better understanding the evolution of TEPs and their involvement in the regulation of the host innate immune system.

Staphylococcus aureus VraX specifically inhibits the classical pathway of complement by binding to C1q

VraX is a protein secreted by Staphylococcus aureus, an important human pathogen. A dramatic over expression of VraX is observed when S. aureus is exposed to several antimicrobial agents; however, its function remains unclear. Here, we aimed to reveal the function of this protein and the mechanism by which it affects the immune system to enhance the pathogenesis of the bacterium. Our results showed that VraX specifically inhibited the classical pathway of the complement system. In particular, VraX could bind to the C1q protein and block the formation of the C1 complex. Deletion of VraX decreased the pathogenesis of S. aureus. Our findings indicate that over expression of VraX might be a protective response for S. aureus survival.

Staphylococcus aureus SdrE captures complement factor H's C-terminus via a novel 'close, dock, lock and latch' mechanism for complement evasion

Complement factor H (CFH) is a soluble complement regulatory protein essential for the down-regulation of the alternative pathway on interaction with specific markers on the host cell surface. It recognizes the complement component 3b (C3b) and 3d (C3d) fragments in addition to self cell markers (i.e. glycosaminoglycans, sialic acid) to distinguish host cells that deserve protection from pathogens that should be eliminated. The Staphylococcus aureus surface protein serine-aspartate repeat protein E (SdrE) was previously reported to bind human CFH as an immune-evasion tactic. However, the molecular mechanism underlying SdrE-CFH-mediated immune evasion remains unknown. In the present study, we identified a novel region at CFH's C-terminus (CFH1206-1226), which binds SdrE N2 and N3 domains (SdrEN2N3) with high affinity, and determined the crystal structures of apo-SdrEN2N3 and the SdrEN2N3-CFH1206-1226 complex. Comparison of the structure of the CFH-SdrE complex with other CFH structures reveals that CFH's C-terminal tail flips from the main body to insert into the ligand-binding groove of SdrE. In addition, SdrEN2N3 adopts a 'close' state in the absence of CFH, which undergoes a large conformational change on CFH binding, suggesting a novel 'close, dock, lock and latch' (CDLL) mechanism for SdrE to recognize its ligand. Our findings imply that SdrE functions as a 'clamp' to capture CFH's C-terminal tail via a unique CDLL mechanism and sequesters CFH on the surface of S. aureus for complement evasion.

Tongue sole (Cynoglossus semilaevis) CD59: A complement inhibitor that binds bacterial cells and promotes bacterial escape from the killing of fish serum

CD59 is a complement regulatory protein that inhibits the formation of membrane attack complex of complement. In this study, we examined the expression and activity of tongue sole (Cynoglossus semilaevis) CD59 (CsCD59). CsCD59 possesses the conserved structural features of CD59 and shares 33%-46% sequence identities with other fish CD59. Expression of CsCD59 was high in liver, spleen, and muscle, and was stimulated by infection of bacterial pathogens. Recombinant CsCD59 (rCsCD59) exhibited an apparent inhibition effect on the activation of tongue sole serum complement. ELISA and microscopy detected binding of rCsCD59 to a number of Gram-negative and Gram-positive bacteria. Interaction with rCsCD59 did not affect bacterial viability but significantly enhanced bacterial resistance against the killing effect of fish serum. Together these results indicate that fish CD59 may to some degrees facilitate a general escape of bacteria from complement-mediated immunity.

RNA-seq analysis of early enteromyxosis in turbot (Scophthalmus maximus): new insights into parasite invasion and immune evasion strategies

Enteromyxum scophthalmi, an intestinal myxozoan parasite, is the causative agent of a threatening disease for turbot (Scophthalmus maximus, L.) aquaculture. The colonisation of the digestive tract by this parasite leads to a cachectic syndrome associated with high morbidity and mortality rates. This myxosporidiosis has a long pre-patent period and the first detectable clinical and histopathological changes are subtle. The pathogenic mechanisms acting in the early stages of infection are still far from being fully understood. Further information on the host-parasite interaction is needed to assist in finding efficient preventive and therapeutic measures. Here, a RNA-seq-based transcriptome analysis of head kidney, spleen and pyloric caeca from experimentally-infected and control turbot was performed. Only infected fish with early signs of infection, determined by histopathology and immunohistochemical detection of E. scophthalmi, were selected. The RNA-seq analysis revealed, as expected, less intense transcriptomic changes than those previously found during later stages of the disease. Several genes involved in IFN-related pathways were up-regulated in the three organs, suggesting that the IFN-mediated immune response plays a main role in this phase of the disease. Interestingly, an opposite expression pattern had been found in a previous study on severely infected turbot. In addition, possible strategies for immune system evasion were suggested by the down-regulation of different genes encoding complement components and acute phase proteins. At the site of infection (pyloric caeca), modulation of genes related to different structural proteins was detected and the expression profile indicated the inhibition of cell proliferation and differentiation. These transcriptomic changes provide indications regarding the mechanisms of parasite attachment to and invasion of the host. The current results contribute to a better knowledge of the events that characterise the early stages of turbot enteromyxosis and provide valuable information to identify molecular markers for early detection and control of this important parasitosis.

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.

Systemic Lupus Erythematosus and Deficiencies of Early Components of the Complement Classical Pathway

The complement system plays an important role in the innate and acquired immune response against pathogens. It consists of more than 30 proteins found in soluble form or attached to cell membranes. Most complement proteins circulate in inactive forms and can be sequentially activated by the classical, alternative, or lectin pathways. Biological functions, such as opsonization, removal of apoptotic cells, adjuvant function, activation of B lymphocytes, degranulation of mast cells and basophils, and solubilization and clearance of immune complex and cell lysis, are dependent on complement activation. Although the activation of the complement system is important to avoid infections, it also can contribute to the inflammatory response triggered by immune complex deposition in tissues in autoimmune diseases. Paradoxically, the deficiency of early complement proteins from the classical pathway (CP) is strongly associated with development of systemic lupus erythematous (SLE) - mainly C1q deficiency (93%) and C4 deficiency (75%). The aim of this review is to focus on the deficiencies of early components of the CP (C1q, C1r, C1s, C4, and C2) proteins in SLE patients.

The Plasmodium bottleneck: malaria parasite losses in the mosquito vector

Nearly one million people are killed every year by the malaria parasite Plasmodium. Although the disease-causing forms of the parasite exist only in the human blood, mosquitoes of the genus Anopheles are the obligate vector for transmission. Here, we review the parasite life cycle in the vector and highlight the human and mosquito contributions that limit malaria parasite development in the mosquito host. We address parasite killing in its mosquito host and bottlenecks in parasite numbers that might guide intervention strategies to prevent transmission.

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.

Interaction of spirochetes with the host fibrinolytic system and potential roles in pathogenesis

The pathogenic spirochetes Borrelia burgdorferi, B. hermsii, B. recurrentis, Treponema denticola and Leptospira spp. are the etiologic agents of Lyme disease, relapsing fever, periodontitis and leptospirosis, respectively. Lyme borreliosis is a multi-systemic disorder and the most prevalent tick-borne disease in the northern hemisphere. Tick-borne relapsing fever is persistent in endemic areas worldwide, representing a significant burden in some African regions. Periodontal disease, a chronic inflammatory disorder that often leads to tooth loss, is caused by several potential pathogens found in the oral cavity including T. denticola. Leptospirosis is considered the most widespread zoonosis, and the predominant human disease in tropical, undeveloped regions. What these diseases have in common is that they are a significant burden to healthcare costs in the absence of prophylactic measures. This review addresses the interaction of these spirochetes with the fibrinolytic system, plasminogen (Plg) binding to the surface of bacteria and the generation of plasmin (Pla) on their surface. The consequences on host-pathogen interactions when the spirochetes are endowed with this proteolytic activity are discussed on the basis of the results reported in the literature. Spirochetes equipped with Pla activity have been shown to degrade extracellular matrix (ECM) components, in addition to digesting fibrin, facilitating bacterial invasion and dissemination. Pla generation triggers the induction of matrix metalloproteases (MMPs) in a cascade of events that enhances the proteolytic capacity of the spirochetes. These activities in concert with the interference exerted by the Plg/Pla on the complement system - helping the bacteria to evade the immune system - should illuminate our understanding of the mechanisms involved in host infection.

Sequence variations and protein expression levels of the two immune evasion proteins Gpm1 and Pra1 influence virulence of clinical Candida albicans isolates

Candida albicans, the important human fungal pathogen uses multiple evasion strategies to control, modulate and inhibit host complement and innate immune attack. Clinical C. albicans strains vary in pathogenicity and in serum resistance, in this work we analyzed sequence polymorphisms and variations in the expression levels of two central fungal complement evasion proteins, Gpm1 (phosphoglycerate mutase 1) and Pra1 (pH-regulated antigen 1) in thirteen clinical C. albicans isolates. Four nucleotide (nt) exchanges, all representing synonymous exchanges, were identified within the 747-nt long GPM1 gene. For the 900-nt long PRA1 gene, sixteen nucleotide exchanges were identified, which represented synonymous, as well as non-synonymous exchanges. All thirteen clinical isolates had a homozygous exchange (A to G) at position 73 of the PRA1 gene. Surface levels of Gpm1 varied by 8.2, and Pra1 levels by 3.3 fold in thirteen tested isolates and these differences influenced fungal immune fitness. The high Gpm1/Pra1 expressing candida strains bound the three human immune regulators more efficiently, than the low expression strains. The difference was 44% for Factor H binding, 51% for C4BP binding and 23% for plasminogen binding. This higher Gpm1/Pra1 expressing strains result in enhanced survival upon challenge with complement active, Factor H depleted human serum (difference 40%). In addition adhesion to and infection of human endothelial cells was increased (difference 60%), and C3b surface deposition was less effective (difference 27%). Thus, variable expression levels of central immune evasion protein influences immune fitness of the human fungal pathogen C. albicans and thus contribute to fungal virulence.

Pasteurella pneumotropica evades the human complement system by acquisition of the complement regulators factor H and C4BP

Pasteurella pneumotropica is an opportunist Gram negative bacterium responsible for rodent pasteurellosis that affects upper respiratory, reproductive and digestive tracts of mammals. In animal care facilities the presence of P. pneumotropica causes severe to lethal infection in immunodeficient mice, being also a potential source for human contamination. Indeed, occupational exposure is one of the main causes of human infection by P. pneumotropica. The clinical presentation of the disease includes subcutaneous abscesses, respiratory tract colonization and systemic infections. Given the ability of P. pneumotropica to fully disseminate in the organism, it is quite relevant to study the role of the complement system to control the infection as well as the possible evasion mechanisms involved in bacterial survival. Here, we show for the first time that P. pneumotropica is able to survive the bactericidal activity of the human complement system. We observed that host regulatory complement C4BP and Factor H bind to the surface of P. pneumotropica, controlling the activation pathways regulating the formation and maintenance of C3-convertases. These results show that P. pneumotropica has evolved mechanisms to evade the human complement system that may increase the efficiency by which this pathogen is able to gain access to and colonize inner tissues where it may cause severe infections.

Analysis of the complement sensitivity of oral treponemes and the potential influence of FH binding, FH cleavage and dentilisin activity on the pathogenesis of periodontal disease

Treponema denticola, a periopathogen, evades complement-mediated killing by binding the negative complement regulatory protein factor H (FH) to its surface via the FhbB protein. Paradoxically, bound FH is cleaved by T. denticola's dentilisin protease, a process hypothesized to trigger localized dysregulation of complement activation in periodontal pockets. The ability of other oral treponemes to evade complement-mediated killing and bind and cleave FH has not been assessed. In this report, we demonstrate that representative isolates of Treponema socranskii, Treponema medium, Treponema pectinovorum and Treponema maltophilum are also serum resistant, whereas Treponema vincentii and Treponema amylovorum are serum sensitive. Although T. denticola's ability to evade complement-mediated killing is strictly dependent on FH binding, other serum-resistant treponemal species lack FhbB and do not bind FH, indicating an FH-independent mechanism of complement evasion. To assess the influence of FhbB sequence variation on FH binding and cleavage by T. denticola, fhbB sequences were determined for 30 isolates. Three distinct phyletic types were identified. All T. denticola strains bound FH and were serum resistant, but differences in binding kinetics, dentilisin activity and FH cleavage ability were observed. Based on these analyses, we hypothesize that the composition of the T. denticola population is a determining factor that influences the progression and severity of periodontal disease.

American Society of Nephrology clinical pathological conference

A 13-year-old girl presented with proteinuria and acute kidney failure. She was born at full term via cesarean delivery (due to nuchal cord), but there were no other prenatal or perinatal complications. In early childhood the patient had two hospitalizations at ages 4.5 and 9 years, respectively, the latter for pneumonia. She had no history of symptoms of kidney disease. She came to the hospital at age 12 years for routine bilateral molar extractions. She was treated with oral antibiotics and discharged after the procedure without complications. At age 13 years, 10 months after the molar extraction, she was seen by a pediatrician because of puffiness and increased BP. She had had respiratory symptoms 2 weeks before presentation. The pediatrician prescribed furosemide and amlodipine. A few days later, the patient returned to the pediatrician's office because of hand, ankle, and facial swelling and malaise. The pediatrician recommended hospitalization and the patient was admitted at this time.

Staphylococcus aureus: the multi headed hydra resists and controls human complement response in multiple ways

The Gram positive human pathogen Staphylococcus aureus causes a spectrum of human diseases including pneumonia, tissue and skin infections, endocarditis, pneumonia and sepsis. The increasing number of resistant bacteria and the threat of methicillin resistant S. aureus (MRSA) urge for the need to develop new antibacterial compounds. A prerequisite for development of such anti microbial compounds is a better understanding of the complex immune crosstalk between the pathogenic bacterium and its human host. To this end proteins staphylococcal proteins that contribute to innate immune evasion especially to complement control need to be identified and their mode of action needs to be analyzed in order to provide new targets for immune interference.

The complex pathogenesis of bacteremia: from antimicrobial clearance mechanisms to the genetic background of the host

Bacteremia develops when bacteria manage to escape the host immune mechanisms or when the otherwise well-orchestrated immune response fails to control bacterial spread due to inherent or acquired immune defects that are associated with susceptibility to infection. The pathogenesis of bacteremia has some characteristic features that are influenced by the genetic signature of the host. In this review, the host defense mechanisms that help prevent bacteremia will be described and the populations who are at risk because of congenital or acquired deficiencies in such mechanisms will be defined. A special mention will be made to novel insights regarding host immune defense against the most commonly isolated organisms from patients with community-acquired bloodstream infections.