The hyphal and yeast forms of Candida albicans bind the complement regulator C4b-binding protein

Controlling Candida: immune regulation of commensal fungi in the gut

The intestinal microbiome harbors fungi that pose a significant risk to human health as opportunistic pathogens and drivers of inflammation. Inflammatory and autoimmune diseases are associated with dysbiotic fungal communities and the expansion of potentially pathogenic fungi. The gut is also the main reservoir for disseminated fungal infections. Immune interactions are critical for preventing commensal fungi from becoming pathogenic. Significant strides have been made in defining innate and adaptive immune pathways that regulate intestinal fungi, and these discoveries have coincided with advancements in our understanding of the fungal molecular pathways and effectors involved in both commensal colonization and pathogenesis within the gut. In this review, we will discuss immune interactions important for regulating commensal fungi, with a focus on how specific cell types and effectors interact with fungi to limit their colonization or pathogenic potential. This will include how innate and adaptive immune pathways target fungi and orchestrate antifungal immune responses, in addition to how secreted immune effectors, such as mucus and antimicrobial peptides, regulate fungal colonization and inhibit pathogenic potential. These immune interactions will be framed around our current understanding of the fungal effectors and pathways regulating colonization and pathogenesis within this niche. Finally, we highlight important unexplored mechanisms by which the immune system regulates commensal fungi in the gut.

Diverse Functions of C4b-Binding Protein in Health and Disease

C4b-binding protein (C4BP) is a fluid-phase complement inhibitor that prevents uncontrolled activation of the classical and lectin complement pathways. As a complement inhibitor, C4BP also promotes apoptotic cell death and is hijacked by microbes and tumors for complement evasion. Although initially characterized for its role in complement inhibition, there is an emerging recognition that C4BP functions in a complement-independent manner to promote cell survival, protect against autoimmune damage, and modulate the virulence of microbial pathogens. In this Brief Review, we summarize the structure and functions of human C4BP, with a special focus on activities that extend beyond the canonical role of C4BP in complement inhibition.

Secreted aspartyl proteases family: a perspective review on the regulation of fungal pathogenesis

Secreted aspartyl proteases (SAPs) are important enzymes for fungal pathogenicity, playing a significant role in infection and survival. This article provides insight into how SAPs facilitate the transformation of yeast cells into hyphae and engage in biofilm formation, invasion and degradation of host cells and proteins. SAPs and their isoenzymes are prevalent during fungal infections, making them a potential target for antifungal and antibiofilm therapies. By targeting SAPs, critical stages of fungal pathogenesis such as adhesion, hyphal development, biofilm formation, host invasion and immune evasion can potentially be disrupted. Developing therapies that target SAPs could provide an effective treatment option for a wide range of fungal infections.

Antifungal Potential of Some Herb Decoctions and Essential Oils on Candida Species

(1) Background: Candidiasis is a fungal infectious disease caused by opportunistic Candida species. The incidence of candidiasis has improved, due to prolonged antibiotic therapy and an increased number of immunocompromised patients. The purpose of this study was to evaluate if decoctions and essential oil (EO) of neem (Azadirachta indica, Meliaceae family), coptidis (Coptis chinensis, Ranunculaceae family), magnolia (Magnolia officinalis, Magnoliaceae family), scutellaria (Scutellaria barbata, Lamiaceae family), and the EO of manuka (Leptospermum scoparium, Myrtaceae family), have antifungal activity in vitro against some clinically prevalent species of Candida. (2) Methods: The antifungal activity was studied by the determination of the minimum inhibitory concentration (MIC) and minimum lethal concentration (MLC) against five Candida strains. The effect in dimorphic transition of Candida albicans was also evaluated for the two plants with higher antimicrobial behavior. (3) Results: C. chinensis decoction and EO and L. scoparium EO exhibited antifungal activity in Candida spp. In addition to the fact that both C. chinensis decoction and EO proved strong antifungal activity, L. scoparium EO also displayed a relevant inhibitory effect on the dimorphic transition. (4) Conclusions: The results provided support for the potential use of C. chinensis and L. scoparium in the treatment of infections by Candida spp.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

Anticipatory Stress Responses and Immune Evasion in Fungal Pathogens

In certain niches, microbes encounter environmental challenges that are temporally linked. In such cases, microbial fitness is enhanced by the evolution of anticipatory responses where the initial challenge simultaneously activates pre-emptive protection against the second impending challenge. The accumulation of anticipatory responses in domesticated yeasts, which have been termed 'adaptive prediction', has led to the emergence of 'core stress responses' that provide stress cross-protection. Protective anticipatory responses also seem to be common in fungal pathogens of humans. These responses reflect the selective pressures that these fungi have faced relatively recently in their evolutionary history. Consequently, some pathogens have evolved 'core environmental responses' which exploit host signals to trigger immune evasion strategies that protect them against imminent immune attack.

Detection of Candida albicans Using a Manufactured Electrochemical Sensor

Candida albicans is a commensal fungus that is responsible for a lot of nosocomial infections in immunocompromised people. Cell culture is currently the predominant method for diagnosing candidiasis, but it is time consuming. In this study, we developed a rapid screen procedure by devising a method for detecting C. albicans with the use of electrochemical sensors. Through this experiment, we propose a method for the detection of C. albicans in the system through the use of personal glucose meters. The hemicellulase was used to break down the cell wall of C. albicans to glucose and oligo, which can be detected by a glucose meter. The spiked samples were prepared suspending C. albicans in urine and serum, demonstrating the feasibility of the developed method in a real situation.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

Candida Pathogenicity and Interplay with the Immune System

Candida species are opportunistic fungal pathogens that are part of the normal skin and mucosal microflora. Overgrowth of Candida can cause infections such as thrush or life-threatening invasive candidiasis in immunocompromised patients. Though Candida albicans is highly prevalent, several non-albicans species are also isolated from nosocomial infections. Candida sp. are over presented in the gut of people with Crohn's disease and certain types of neurological disorders, with hyphal form and biofilms being the most virulent states. In addition, Candida uses several secreted and cell surface molecules such as pH related antigen 1, High affinity glucose transporter, Phosphoglycerate mutase 1 and lipases to establish pathogenicity. A strong innate immune response is elicited against Candida via dendritic cells, neutrophils and macrophages. All three complement pathways are also activated. Production of proinflammatory cytokines IL-10 and IL-12 signal differentiation of CD4⁺ cells into Th1 and Th2 cells, whereas IL-6, IL-17 and IL-23 induce Th17 cells. Importance of T-lymphocytes is reflected in depleted T-cell count patients being more prone to Candidiasis. Anti- Candida antibodies also play a role against candidiasis using various mechanisms such as targeting virulent enzymes and exhibiting direct candidacidal activity. However, the significance of antibody response during infection remains controversial. Furthermore, some of the Candida strains have evolved molecular strategies to evade the sophisticated host attack by proteolysis of components of immune system and interfering with immune signalling pathways. Emergence of several non-albicans species that are resistant to current antifungal agents makes treatment more difficult. Therefore, deeper insight into interactions between Candida and the host immune system is required for discovery of novel therapeutic options.

Antibacterial Fusion Proteins Enhance Moraxella catarrhalis Killing

Moraxella catarrhalis is a human-specific commensal of the respiratory tract and an opportunistic pathogen. It is one of the leading cause of otitis media in children and of acute exacerbations in patients with chronic obstructive pulmonary disease, resulting in significant morbidity and economic burden. Vaccines and new immunotherapeutic strategies to treat this emerging pathogen are needed. Complement is a key component of innate immunity that mediates the detection, response, and subsequent elimination of invading pathogens. Many pathogens including M. catarrhalis have evolved complement evasion mechanisms, which include the binding of human complement inhibitors such as C4b-binding protein (C4BP) and Factor H (FH). Inhibiting C4BP and FH acquisition by M. catarrhalis may provide a novel therapeutic avenue to treat infections. To achieve this, we created two chimeric proteins that combined the Moraxella-binding domains of C4BP and FH fused to human immunoglobulin Fcs: C4BP domains 1 and 2 and FH domains 6 and 7 fused to IgM and IgG Fc, respectively. As expected, FH6-7/IgG displaced FH from the bacterial surface while simultaneously activating complement via Fc-C1q interactions, together increasing pathogen elimination. C4BP1-2/IgM also increased serum killing of the bacteria through enhanced complement deposition, but did not displace C4BP from the surface of M. catarrhalis. These Fc fusion proteins could act as anti-infective immunotherapies. Many microbes bind the complement inhibitors C4BP and FH through the same domains as M. catarrhalis, therefore these Fc fusion proteins may be promising candidates as adjunctive therapy against many different drug-resistant pathogens.

Complement Evasion Strategies of Human Pathogenic Bacteria

Human pathogens need to overcome an elaborate network of host defense mechanisms in order to establish their infection, colonization, proliferation and eventual dissemination. The interaction of pathogens with different effector molecules of the immune system results in their neutralization and elimination from the host. The complement system is one such integral component of innate immunity that is critically involved in the early recognition and elimination of the pathogen. Hence, under this immune pressure, all virulent pathogens capable of inducing active infections have evolved immune evasive strategies that primarily target the complement system, which plays an essential and central role for host defense. Recent reports on several bacterial pathogens have elucidated the molecular mechanisms underlying complement evasion, inhibition of opsonic phagocytosis and cell lysis. This review aims to comprehensively summarize the recent findings on the various strategies adopted by pathogenic bacteria to escape complement-mediated clearance.

Podocytes Produce and Secrete Functional Complement C3 and Complement Factor H

Podocytes are an important part of the glomerular filtration barrier and the key player in the development of proteinuria, which is an early feature of complement mediated renal diseases. Complement factors are mainly liver-born and present in circulation. Nevertheless, there is a growing body of evidence for additional sites of complement protein synthesis, including various cell types in the kidney. We hypothesized that podocytes are able to produce complement components and contribute to the local balance of complement activation and regulation. To investigate the relevant balance between inhibiting and activating sides, our studies focused on complement factor H (CFH), an important complement regulator, and on C3, the early key component for complement activation. We characterized human cultured podocytes for the expression and secretion of activating and regulating complement factors, and analyzed the secretion pathway and functional activity. We studied glomerular CFH and C3 expression in puromycin aminonucleoside (PAN) -treated rats, a model for proteinuria, and the physiological mRNA-expression of both factors in murine kidneys. We found, that C3 and CFH were expressed in cultured podocytes and expression levels differed from those in cultivated glomerular endothelial cells. The process of secretion in podocytes was stimulated with interferon gamma and located in the Golgi apparatus. Cultured podocytes could initiate the complement cascade by the splitting of C3, which can be shown by the generation of C3a, a functional C3 split product. C3 contributed to external complement activation. Podocyte-secreted CFH, in conjunction with factor I, was able to split C3b. Podocytes derived from a patient with a CFH mutation displayed impaired cell surface complement regulation. CFH and C3 were synthesized in podocytes of healthy C57Bl/6-mice and were upregulated in podocytes of PAN treated rats. These data show that podocytes produce functionally active complement components, and could therefore influence the local glomerular complement activation and regulation. This modulating effect should therefore be considered in all diseases where glomerular complement activation occurs. Furthermore, our data indicate a potential novel role of podocytes in the innate immune system.

Antivirulence and avirulence genes in human pathogenic fungi

Opportunistic commensal and environmental fungi can cause superficial to systemic diseases in humans. But how did these pathogens adapt to infect us and how does host-pathogen co-evolution shape their virulence potential? During evolution toward pathogenicity, not only do microorganisms gain virulence genes, but they also tend to lose non-adaptive genes in the host niche. Additionally, virulence factors can become detrimental during infection when they trigger host recognition. The loss of non-adaptive genes as well as the loss of the virulence potential of genes by adaptations to the host has been investigated in pathogenic bacteria and phytopathogenic fungi, where they are known as antivirulence and avirulence genes, respectively. However, these concepts are nearly unknown in the field of pathogenic fungi of humans. We think that this unnecessarily limits our view of human-fungal interplay, and that much could be learned if we applied a similar framework to aspects of these interactions. In this review, we, therefore, define and adapt the concepts of antivirulence and avirulence genes for human pathogenic fungi. We provide examples for analogies to antivirulence genes of bacterial pathogens and to avirulence genes of phytopathogenic fungi. Introducing these terms to the field of pathogenic fungi of humans can help to better comprehend the emergence and evolution of fungal virulence and disease.

Moonlighting Proteins at the Candidal Cell Surface

The cell wall in Candida albicans is not only a tight protective envelope but also a point of contact with the human host that provides a dynamic response to the constantly changing environment in infection niches. Particularly important roles are attributed to proteins exposed at the fungal cell surface. These include proteins that are stably and covalently bound to the cell wall or cell membrane and those that are more loosely attached. Interestingly in this regard, numerous loosely attached proteins belong to the class of “moonlighting proteins” that are originally intracellular and that perform essentially different functions in addition to their primary housekeeping roles. These proteins also demonstrate unpredicted interactions with non-canonical partners at an a priori unexpected extracellular location, achieved via non-classical secretion routes. Acting both individually and collectively, the moonlighting proteins contribute to candidal virulence and pathogenicity through their involvement in mechanisms critical for successful host colonization and infection, such as the adhesion to host cells, interactions with plasma homeostatic proteolytic cascades, responses to stress conditions and molecular mimicry. The documented knowledge of the roles of these proteins in C. albicans pathogenicity has utility for assisting the design of new therapeutic, diagnostic and preventive strategies against candidiasis.

Candida and Complement: New Aspects in an Old Battle

Candida is a dominant fungal pathogen in immunocompromised hosts, leading to opportunistic infections. Complement with its multifaceted functions is involved in the immune defense against this yeast, and recently several novel aspects have emerged in this old battle. It is clear that Candida can adopt both roles as a colonizer or as a pathogen. In our article, we focus on the molecular mechanisms of the Candida-complement interplay, which occur in disseminated disease as well as locally on skin or on mucous membranes in mouth and vagina; the mechanisms can be supposed to be the same. Activation of the complement system by Candida is facilitated by directly triggering the three dominant pathways, but also indirectly via the coagulation and fibrinolysis systems. The complement-mediated anti-Candida effects induced thereby clearly extend chemotaxis, opsonization, and phagocytosis, and even the membrane attack complex formed on the fungal surface plays a modulatory role, although lysis of the yeast per se cannot be induced due to the thick fungal cell wall. In order to avoid the hostile action of complement, several evasion mechanisms have evolved during co-evolution, comprising the avoidance of recognition, and destruction. The latter comes in many flavors, in particular the cleavage of complement proteins by yeast enzymes and the exploitation of regulatory proteins by recruiting them on the cell wall, such as factor H. The rationale behind that is that the fluid phase regulators on the fungal cell surface down-regulate complement locally. Interestingly, however, evasion protein knockout strains do not necessarily lead to an attenuated disease, so it is likely more complex in vivo than initially thought. The interactions between complement and non-albicans species also deserve attention, especially Candida auris, a recently identified drug-resistant species of medical importance. This is in particular worth investigating, as deciphering of these interactions may lead to alternative anti-fungal therapies directly targeting the molecular mechanisms.

Mechanisms of Pathogenic Candida Species to Evade the Host Complement Attack

Candida species are common colonizers of the human skin, vagina, and the gut. As human commensals, Candida species do not cause any notable damage in healthy individuals; however, in certain conditions they can initiate a wide range of diseases such as chronic disseminated candidiasis, endocarditis, vaginitis, meningitis, and endophthalmitis. The incidence of Candida caused infections has increased worldwide, with mortality rates exceeding 70% in certain patient populations. C. albicans, C. glabrata, C. tropicalis, C. parapsilosis, and C. krusei are responsible for more than 90% of Candida-related infections. Interestingly, the host immune response against these closely related fungi varies. As part of the innate immune system, complement proteins play a crucial role in host defense, protecting the host by lysing pathogens or by increasing their phagocytosis by phagocytes through opsonization. This review summarizes interactions of host complement proteins with pathogenic Candida species, including C. albicans and non-albicans Candida species such as C. parapsilosis. We will also highlight the various ways of complement activation, describe the antifungal effects of complement cascades and explore the mechanisms adopted by members of pathogenic Candida species for evading complement attack.

C4BP-IgM protein as a therapeutic approach to treat Neisseria gonorrhoeae infections

Gonorrhea is a sexually transmitted infection with 87 million new cases per year globally. Increasing antibiotic resistance has severely limited treatment options. A mechanism that Neisseria gonorrhoeae uses to evade complement attack is binding of the complement inhibitor C4b-binding protein (C4BP). We screened 107 porin B1a (PorB1a) and 83 PorB1b clinical isolates randomly selected from a Swedish strain collection over the last 10 years and noted that 96/107 (89.7%) PorB1a and 16/83 (19.3%) PorB1b bound C4BP; C4BP binding substantially correlated with the ability to evade complement-dependent killing (r = 0.78). We designed 2 chimeric proteins that fused C4BP domains to the backbone of IgG or IgM (C4BP-IgG; C4BP-IgM) with the aim of enhancing complement activation and killing of gonococci. Both proteins bound gonococci (KD C4BP-IgM = 2.4 nM; KD C4BP-IgG 980.7 nM), but only hexameric C4BP-IgM efficiently outcompeted heptameric C4BP from the bacterial surface, resulting in enhanced complement deposition and bacterial killing. Furthermore, C4BP-IgM substantially attenuated the duration and burden of colonization of 2 C4BP-binding gonococcal isolates but not a non-C4BP-binding strain in a mouse vaginal colonization model using human factor H/C4BP-transgenic mice. Our preclinical data present C4BP-IgM as an adjunct to conventional antimicrobials for the treatment of gonorrhea.

Immunopathology of Recurrent Vulvovaginal Infections: New Aspects and Research Directions

Recurrent vulvovaginal infections (RVVI), a devastating group of mucosal infection, are severely affecting women's quality of life. Our understanding of the vaginal defense mechanisms have broadened recently with studies uncovering the inflammatory nature of bacterial vaginosis, inflammatory responses against novel virulence factors, innate Type 17 cells/IL-17 axis, neutrophils mediated killing of pathogens by a novel mechanism, and oxidative stress during vaginal infections. However, the pathogens have fine mechanisms to subvert or manipulate the host immune responses, hijack them and use them for their own advantage. The odds of hijacking increases, due to impaired immune responses, the net magnitude of which is the result of numerous genetic variations, present in multiple host genes, detailed in this review. Thus, by underlining the role of the host immune responses in disease etiology, modern research has clarified a major hypothesis shift in the pathophilosophy of RVVI. This knowledge can further be used to develop efficient immune-based diagnosis and treatment strategies for this enigmatic disease conditions. As for instance, plasma-derived MBL replacement, adoptive T-cell, and antibody-based therapies have been reported to be safe and efficacious in infectious diseases. Therefore, these emerging immune-therapies could possibly be the future therapeutic options for RVVI.

The hijackers guide to escaping complement: Lessons learned from pathogens

Pathogens that invade the human host are confronted by a multitude of defence mechanisms aimed at preventing colonization, dissemination and proliferation. The most frequent outcome of this interaction is microbial elimination, in which the complement system plays a major role. Complement, an essential feature of the innate immune machinery, rapidly identifies and marks pathogens for efficient removal. Consequently, this creates a selective pressure for microbes to evolve strategies to combat complement, permitting host colonization and access to resources. All successful pathogens have developed mechanisms to resist complement activity which are intimately aligned with their capacity to cause disease. In this review, we describe the successful methods various pathogens use to evade complement activation, shut down inflammatory signalling through complement, circumvent opsonisation and override terminal pathway lysis. This review summarizes how pathogens undermine innate immunity: 'The Hijackers Guide to Complement'.

Candida albicans Elicits Pro-Inflammatory Differential Gene Expression in Intestinal Peyer's Patches

The details of how gut-associated lymphoid tissues such as Peyer's patches (PPs) in the small intestine play a role in immune surveillance, microbial differentiation and the mucosal barrier protection in response to fungal organisms such as Candida albicans are still unclear. We particularly focus on PPs as they are the immune sensors and inductive sites of the gut that influence inflammation and tolerance. We have previously demonstrated that CD11c⁺ phagocytes that include dendritic cells and macrophages are located in the sub-epithelial dome within PPs sample C. albicans. To gain insight on how specific cells within PPs sense and respond to the sampling of fungi, we gavaged naïve mice with C. albicans strains ATCC 18804 and SC5314 as well as Saccharomyces cerevisiae. We measured the differential gene expression of sorted CD45⁺ B220⁺ B-cells, CD3⁺ T-cells and CD11c⁺ DCs within the first 24 h post-gavage using nanostring nCounter^® technology. The results reveal that at 24 h, PP phagocytes were the cell type that displayed differential gene expression. These phagocytes were able to sample C. albicans and discriminate between strains. In particular, strain ATCC 18804 upregulated fungal-specific pro-inflammatory genes pertaining to innate and adaptive immune responses. Interestingly, PP CD11c⁺ phagocytes also differentially expressed genes in response to C. albicans that were important in the protection of the mucosal barrier. These results highlight that the mucosal barrier not only responds to C. albicans, but also aids in the protection of the host.

Streptococcus pneumoniae From Patients With Hemolytic Uremic Syndrome Binds Human Plasminogen via the Surface Protein PspC and Uses Plasmin to Damage Human Endothelial Cells

Pneumococcal hemolytic uremic syndrome (HUS) in children is caused by infections with Streptococcus pneumoniae. Because endothelial cell damage is a hallmark of HUS, we studied how HUS-inducing pneumococci derived from infant HUS patients during the acute phase disrupt the endothelial layer. HUS pneumococci efficiently bound human plasminogen. These clinical isolates of HUS pneumococci efficiently bound human plasminogen via the bacterial surface proteins Tuf and PspC. When activated to plasmin at the bacterial surface, the active protease degraded fibrinogen and cleaved C3b. Here, we show that PspC is a pneumococcal plasminogen receptor and that plasmin generated on the surface of HUS pneumococci damages endothelial cells, causing endothelial retraction and exposure of the underlying matrix. Thus, HUS pneumococci damage endothelial cells in the blood vessels and disturb local complement homeostasis. Thereby, HUS pneumococci promote a thrombogenic state that drives HUS pathology.

Candida albicans Factor H Binding Molecule Hgt1p - A Low Glucose-Induced Transmembrane Protein Is Trafficked to the Cell Wall and Impairs Phagocytosis and Killing by Human Neutrophils

Complement is a tightly controlled arm of the innate immune system, facilitating phagocytosis and killing of invading pathogens. Factor H (FH) is the main fluid-phase inhibitor of the alternative pathway. Many pathogens can hijack FH from the host and protect themselves from complement-dependent killing. Candida albicans is a clinically important opportunistic yeast, expressing different FH binding molecules on its cell surface, which allow complement evasion. One such FH binding molecule is the transmembrane protein "High affinity glucose transporter 1" (Hgt1p), involved in glucose metabolism. This study demonstrated that Hgt1p transcription and expression is induced and highest at the low, but physiological glucose concentration of 0.1%. Thus, this concentration was used throughout the study. We also demonstrated the transport of Hgt1p to the fungal cell wall surface by vesicle trafficking and its release by exosomes containing Hgt1p integrated in the vesicular membrane. We corroborated Hgt1p as FH binding molecule. A polyclonal anti-Hgt1p antibody was created which interfered with the binding of FH, present in normal human serum to the fungal cell wall. A chimeric molecule consisting of FH domains 6 and 7 fused to human IgG1 Fc (FH6.7/Fc) even more comprehensively blocked FH binding, likely because FH6.7/Fc diverted FH away from fungal FH ligands other than Hgt1p. Reduced FH binding to the yeast was associated with a concomitant increase in C3b/iC3b deposition and resulted in significantly increased in vitro phagocytosis and killing by human neutrophils. In conclusion, Hgt1p also exhibits non-canonical functions such as binding FH after its export to the cell wall. Blocking Hgt1p-FH interactions may represent a tool to enhance complement activation on the fungal surface to promote phagocytosis and killing of C. albicans.

The Pneumococcal Surface Proteins PspA and PspC Sequester Host C4-Binding Protein To Inactivate Complement C4b on the Bacterial Surface

Complement is a critical component of antimicrobial immunity. Various complement regulatory proteins prevent host cells from being attacked. Many pathogens have acquired the ability to sequester complement regulators from host plasma to evade complement attack. We describe here how Streptococcus pneumoniae adopts a strategy to prevent the formation of the C3 convertase C4bC2a by the rapid conversion of surface bound C4b and iC4b into C4dg, which remains bound to the bacterial surface but no longer forms a convertase complex. Noncapsular virulence factors on the pneumococcus are thought to facilitate this process by sequestering C4b-binding protein (C4BP) from host plasma. When S. pneumoniae D39 was opsonized with human serum, the larger C4 activation products C4b and iC4b were undetectable, but the bacteria were liberally decorated with C4dg and C4BP. With targeted deletions of either PspA or PspC, C4BP deposition was markedly reduced, and there was a corresponding reduction in C4dg and an increase in the deposition of C4b and iC4b. The effect was greatest when PspA and PspC were both knocked out. Infection experiments in mice indicated that the deletion of PspA and/or PspC resulted in the loss of bacterial pathogenicity. Recombinant PspA and PspC both bound serum C4BP, and both led to increased C4b and reduced C4dg deposition on S. pneumoniae D39. We conclude that PspA and PspC help the pneumococcus to evade complement attack by binding C4BP and so inactivating C4b.

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.

Anti-Immune Strategies of Pathogenic Fungi

Pathogenic fungi have developed many strategies to evade the host immune system. Multiple escape mechanisms appear to function together to inhibit attack by the various stages of both the adaptive and the innate immune response. Thus, after entering the host, such pathogens fight to overcome the immune system to allow their survival, colonization and spread to different sites of infection. Consequently, the establishment of a successful infectious process is closely related to the ability of the pathogen to modulate attack by the immune system. Most strategies employed to subvert or exploit the immune system are shared among different species of fungi. In this review, we summarize the main strategies employed for immune evasion by some of the major pathogenic fungi.

The Expanding Landscape of Moonlighting Proteins in Yeasts

Moonlighting proteins are multifunctional proteins that participate in unrelated biological processes and that are not the result of gene fusion. A certain number of these proteins have been characterized in yeasts, and the easy genetic manipulation of these microorganisms has been useful for a thorough analysis of some cases of moonlighting. As the awareness of the moonlighting phenomenon has increased, a growing number of these proteins are being uncovered. In this review, we present a crop of newly identified moonlighting proteins from yeasts and discuss the experimental evidence that qualifies them to be classified as such. The variety of moonlighting functions encompassed by the proteins considered extends from control of transcription to DNA repair or binding to plasminogen. We also discuss several questions pertaining to the moonlighting condition in general. The cases presented show that yeasts are important organisms to be used as tools to understand different aspects of moonlighting proteins.

Candida albicans Shaving to Profile Human Serum Proteins on Hyphal Surface

Candida albicans is a human opportunistic fungus and it is responsible for a wide variety of infections, either superficial or systemic. C. albicans is a polymorphic fungus and its ability to switch between yeast and hyphae is essential for its virulence. Once C. albicans obtains access to the human body, the host serum constitutes a complex environment of interaction with C. albicans cell surface in bloodstream. To draw a comprehensive picture of this relevant step in host-pathogen interaction during invasive candidiasis, we have optimized a gel-free shaving proteomic strategy to identify both, human serum proteins coating C. albicans cells and fungi surface proteins simultaneously. This approach was carried out with normal serum (NS) and heat inactivated serum (HIS). We identified 214 human and 372 C. albicans unique proteins. Proteins identified in C. albicans included 147 which were described as located at the cell surface and 52 that were described as immunogenic. Interestingly, among these C. albicans proteins, we identified 23 GPI-anchored proteins, Gpd2 and Pra1, which are involved in complement system evasion and 7 other proteins that are able to attach plasminogen to C. albicans surface (Adh1, Eno1, Fba1, Pgk1, Tdh3, Tef1, and Tsa1). Furthermore, 12 proteins identified at the C. albicans hyphae surface induced with 10% human serum were not detected in other hypha-induced conditions. The most abundant human proteins identified are involved in complement and coagulation pathways. Remarkably, with this strategy, all main proteins belonging to complement cascades were identified on the C. albicans surface. Moreover, we identified immunoglobulins, cytoskeletal proteins, metabolic proteins such as apolipoproteins and others. Additionally, we identified more inhibitors of complement and coagulation pathways, some of them serpin proteins (serine protease inhibitors), in HIS vs. NS. On the other hand, we detected a higher amount of C3 at the C. albicans surface in NS than in HIS, as validated by immunofluorescence.

C4b-binding protein: The good, the bad and the deadly. Novel functions of an old friend

C4b-binding protein (C4BP) is best known as a potent soluble inhibitor of the classical and lectin pathways of the complement system. This large 500 kDa multimeric plasma glycoprotein is expressed mainly in the liver but also in lung and pancreas. It consists of several identical 75 kDa α-chains and often also one 40 kDa β-chain, both of which are mainly composed of complement control protein (CCP) domains. Structure-function studies revealed that one crucial binding site responsible for inhibition of complement is located to CCP1-3 of the α-chain. Binding of anticoagulant protein S to the CCP1 of the β-chain provides C4BP with the ability to strongly bind apoptotic and necrotic cells in order to prevent inflammation arising from activation of complement by these cells. Further, C4BP interacts strongly with various types of amyloid and enhances fibrillation of islet amyloid polypeptide secreted from pancreatic beta cells, which may attenuate pro-inflammatory and cytotoxic effects of this amyloid. Full deficiency of C4BP has not been identified but non-synonymous alterations in its sequence have been found in haemolytic uremic syndrome and recurrent pregnancy loss. Furthermore, C4BP is bound by several bacterial pathogens, notably Streptococcus pyogenes, which due to inhibition of complement and enhancement of bacterial adhesion to endothelial cells provides these bacteria with a survival advantage in the host. Thus, depending on the context, C4BP has a protective or detrimental role in the organism.

Fine Mapping of the Interaction between C4b-Binding Protein and Outer Membrane Proteins LigA and LigB of Pathogenic Leptospira interrogans

The complement system consists of more than 40 proteins that participate in the inflammatory response and in pathogen killing. Complement inhibitors are necessary to avoid the excessive consumption and activation of this system on host cells. Leptospirosis is a worldwide zoonosis caused by spirochetes from the genus Leptospira. Pathogenic leptospires are able to escape from complement activation by binding to host complement inhibitors Factor H [FH] and C4b-binding protein (C4BP) while non-pathogenic leptospires are rapidly killed in the presence of fresh serum. In this study, we demonstrate that complement control protein domains (CCP) 7 and 8 of C4BP α-chain interact with the outer membrane proteins LcpA, LigA and LigB from the pathogenic leptospire L. interrogans. The interaction between C4BP and LcpA, LigA and LigB is sensitive to ionic strength and inhibited by heparin. We fine mapped the LigA and LigB domains involved in its binding to C4BP and heparin and found that both interactions are mediated through the bacterial immunoglobulin-like (Big) domains 7 and 8 (LigA7-8 and LigB7-8) of both LigA and LigB and also through LigB9-10. Therefore, C4BP and heparin may share the same binding sites on Lig proteins.

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.

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.

Candida Immunity

The human pathogenic fungus Candida albicans is the predominant cause of both superficial and invasive forms of candidiasis. C. albicans primarily infects immunocompromised individuals as a result of either immunodeficiency or intervention therapy, which highlights the importance of host immune defences in preventing fungal infections. The host defence system utilises a vast communication network of cells, proteins, and chemical signals distributed in blood and tissues, which constitute innate and adaptive immunity. Over the last decade the identity of many key molecules mediating host defence against C. albicans has been identified. This review will discuss how the host recognises this fungus, the events induced by fungal cells, and the host innate and adaptive immune defences that ultimately resolve C. albicans infections during health.

Candida albicans uses the surface protein Gpm1 to attach to human endothelial cells and to keratinocytes via the adhesive protein vitronectin

Candida albicans is a major cause of invasive fungal infections worldwide. Upon infection and when in contact with human plasma as well as body fluids the fungus is challenged by the activated complement system a central part of the human innate immune response. C. albicans controls and evades host complement attack by binding several human complement regulators like Factor H, Factor H-like protein 1 and C4BP to the surface. Gpm1 (Phosphoglycerate mutase 1) is one fungal Factor H/FHL1 -binding protein. As Gpm1 is surface exposed, we asked whether Gpm1 also contributes to host cell attachment. Here, we show by flow cytometry and by laser scanning microscopy that candida Gpm1 binds to human umbilical vein endothelial cells (HUVEC) to keratinocytes (HaCaT), and also to monocytic U937 cells. Wild type candida did bind, but the candida gpm1Δ/Δ knock-out mutant did not bind to these human cells. In addition Gpm1when attached to latex beads also conferred attachment to human endothelial cells. When analyzing Gpm1-binding to a panel of extracellular matrix proteins, the human glycoprotein vitronectin was identified as a new Gpm1 ligand. Vitronectin is a component of the extracellular matrix and also a regulator of the terminal complement pathway. Vitronectin is present on the surface of HUVEC and keratinocytes and acts as a surface ligand for fungal Gpm1. Gpm1 and vitronectin colocalize on the surface of HUVEC and HaCaT as revealed by laser scanning microscopy. The Gpm1 vitronectin interaction is inhibited by heparin and the interaction is also ionic strength dependent. Taken together, Gpm1 the candida surface protein binds to vitronectin and mediates fungal adhesion to human endothelial cells. Thus fungal Gpm1 and human vitronectin represent a new set of proteins that are relevant for fungal attachment to human cells interaction. Blockade of the Gpm1 vitronectin interaction might provide a new target for therapy.

Binding of complement inhibitor C4b-binding protein to a highly virulent Streptococcus pyogenes M1 strain is mediated by protein H and enhances adhesion to and invasion of endothelial cells

Streptococcus pyogenes AP1, a strain of the highly virulent M1 serotype, uses exclusively protein H to bind the complement inhibitor C4b-binding protein (C4BP). We found a strong correlation between the ability of AP1 and its isogenic mutants lacking protein H to inhibit opsonization with complement C3b and binding of C4BP. C4BP bound to immobilized protein H or AP1 bacteria retained its cofactor activity for degradation of (125)I-C4b. Furthermore, C4b deposited from serum onto AP1 bacterial surfaces was processed into C4c/C4d fragments, which did not occur on strains unable to bind C4BP. Recombinant C4BP mutants, which (i) lack certain CCP domains or (ii) have mutations in single aa as well as (iii) mutants with additional aa between different CCP domains were used to determine that the binding is mainly mediated by a patch of positively charged amino acid residues at the interface of domains CCP1 and CCP2. Using recombinant protein H fragments, we narrowed down the binding site to the N-terminal domain A. With a peptide microarray, we identified one single 18-amino acid-long peptide comprising residues 92-109, which specifically bound C4BP. Biacore was used to determine KD = 6 × 10(-7) M between protein H and a single subunit of C4BP. C4BP binding also correlated with elevated levels of adhesion and invasion to endothelial cells. Taken together, we identified the molecular basis of C4BP-protein H interaction and found that it is not only important for decreased opsonization but also for invasion of endothelial cells by S. pyogenes.

Identification of human plasma proteins associated with the cell wall of the pathogenic fungus Paracoccidioides brasiliensis

Paracoccidioides brasiliensis and Paracoccidioides lutzii are thermodimorphic species that cause paracoccidioidomycosis. The cell wall is the outermost fungal organelle to form an interface with the host. A number of host effector compounds, including immunologically active molecules, circulate in the plasma. In the present work, we extracted cell-wall-associated proteins from the yeast pathogenic phase of P. brasiliensis, isolate Pb3, grown in the presence of human plasma and analyzed bound plasma proteins by liquid chromatography-tandem mass spectrometry. Transport, complement activation/regulation, and coagulation pathway were the most abundant functional groups identified. Proteins related to iron/copper acquisition, immunoglobulins, and protease inhibitors were also detected. Several human plasma proteins described here have not been previously reported as interacting with fungal components, specifically, clusterin, hemopexin, transthyretin, ceruloplasmin, alpha-1-antitrypsin, apolipoprotein A-I, and apolipoprotein B-100. Additionally, we observed increased phagocytosis by J774.16 macrophages of Pb3 grown in plasma, suggesting that plasma proteins interacting with P. brasiliensis cell wall might be interfering in the fungal relationship with the host.

Glycerol-3-phosphate dehydrogenase 2 is a novel factor H-, factor H-like protein 1-, and plasminogen-binding surface protein of Candida albicans

Candida albicans uses human complement regulators such as factor H and factor H-like protein 1 (FHL-1) for immune evasion. To define the whole panel of fungal complement-evasion molecules, C. albicans cell extract was absorbed to a factor H-coupled matrix. One 52-kDa protein was eluted and identified by mass spectrometry as glycerol-3-phosphate dehydrogenase 2 (Gpd2). Consequently, Gpd2 was recombinantly expressed and purified. Recombinant Gpd2 binds factor Hand and FHL-1, mainly via short consensus repeat 7; and binds plasminogen, via lysine residues. The 3 human complement regulators, when attached to candida Gpd2, became functionally active, and the attached host proteins assist in inactivation of the complement cascade or cleave fibrinogen in the extracellular matrix component fibrinogen. Polyclonal Gpd2 antiserum was generated and localized Gpd2 at the surface of C. albicans. In addition, candida Gpd2 bound to human nonphagocytic cells but not to phagocytic U937 cells. Thus, candida Gpd2 is a novel fungal immune evasion protein that binds several human complement regulators and, in addition, binds human cells.

Enolase of Streptococcus pneumoniae binds human complement inhibitor C4b-binding protein and contributes to complement evasion

Streptococcus pneumoniae (pneumococcus) is a pathogen that causes severe local and life-threatening invasive diseases, which are associated with high mortality rates. Pneumococci have evolved several strategies to evade the host immune system, including complement to disseminate and to survive in various host niches. Thus, pneumococci bind complement inhibitors such as C4b-binding protein (C4BP) and factor H via pneumococcal surface protein C, thereby inhibiting the classical and alternative complement pathways. In this study, we identified the pneumococcal glycolytic enzyme enolase, a nonclassical cell surface and plasminogen-binding protein, as an additional pneumococcal C4BP-binding protein. Furthermore, we demonstrated that human, but not mouse, C4BP bound pneumococci. Recombinant enolase bound in a dose-dependent manner C4BP purified from plasma, and the interaction was reduced by increasing ionic strength. Enolase recruited C4BP and plasminogen, but not factor H, from human serum. Moreover, C4BP and plasminogen bound to different domains of enolase as they did not compete for the interaction with enolase. In direct binding assays with recombinant C4BP mutants lacking individual domains, two binding sites for enolase were identified on the complement control protein (CCP) domain 1/CCP2 and CCP8 of the C4BP α-chains. C4BP bound to the enolase retained its cofactor activity as determined by C4b degradation. Furthermore, in the presence of exogenously added enolase, an increased C4BP binding to and subsequently decreased C3b deposition on pneumococci was observed. Taken together, pneumococci specifically interact with human C4BP via enolase, which represents an additional mechanism of human complement control by this versatile pathogen.

Multiple activities of LigB potentiate virulence of Leptospira interrogans: inhibition of alternative and classical pathways of complement

Microbial pathogens acquire the immediate imperative to avoid or counteract the formidable defense of innate immunity as soon as they overcome the initial physical barriers of the host. Many have adopted the strategy of directly disrupting the complement system through the capture of its components, using proteins on the pathogen's surface. In leptospirosis, pathogenic Leptospira spp. are resistant to complement-mediated killing, in contrast to the highly vulnerable non-pathogenic strains. Pathogenic L. interrogans uses LenA/LfhA and LcpA to respectively sequester and commandeer the function of two regulators, factor H and C4BP, which in turn bind C3b or C4b to interrupt the alternative or classical pathways of complement activation. LigB, another surface-proximal protein originally characterized as an adhesin binding multiple host proteins, has other activities suggesting its importance early in infection, including binding extracellular matrix, plasma, and cutaneous repair proteins and inhibiting hemostasis. In this study, we used a recent model of ectopic expression of LigB in the saprophyte, L. biflexa, to test the hypothesis that LigB also interacts with complement proteins C3b and C4b to promote the virulence of L. interrogans. The surface expression of LigB partially rescued the non-pathogen from killing by 5% normal human serum, showing 1.3- to 48-fold greater survival 4 to 6 d following exposure to complement than cultures of the non-expressing parental strain. Recombinant LigB7'-12 comprising the LigB-specific immunoglobulin repeats binds directly to human complement proteins, C3b and C4b, with respective K(d)s of 43±26 nM and 69±18 nM. Repeats 9 to 11, previously shown to contain the binding domain for fibronectin and fibrinogen, are also important in LigB-complement interactions, which interfere with the alternative and classical pathways measured by complement-mediated hemolysis of erythrocytes. Thus, LigB is an adaptable interface for L. interrogans to efficiently counteract the multiple homeostatic processes of the host.

Interplay between Candida albicans and the mammalian innate host defense

Candida albicans is both the most common fungal commensal microorganism in healthy individuals and the major fungal pathogen causing high mortality in at-risk populations, especially immunocompromised patients. In this review, we summarize the interplay between the host innate system and C. albicans, ranging from how the host recognizes, responds, and clears C. albicans infection to how C. albicans evades, dampens, and escapes from host innate immunity.

Candida albicans Hgt1p, a multifunctional evasion molecule: complement inhibitor, CR3 analogue, and human immunodeficiency virus-binding molecule

BACKGROUND

The complement system is tightly controlled by several regulators. Two of these, factor H (FH) and C4b-binding protein (C4BP), can be acquired by pathogens conveying resistance to complement attack. The aim of the study was to characterize the FH binding molecule of Candida albicans, a potentially life-threatening yeast.

METHODS

The gene coding for this molecule was identified by probing an expression library and homozygous deletion mutants of the respective gene were constructed. Binding and functional assays were undertaken to compare wild-type and knockout strains.

RESULTS

The high-affinity glucose transporter 1 (CaHgt1p) was identified as an FH-binding molecule. Homozygous hgt1Δ/Δ deletion mutants, but not the restored strain in which HGT1 was reintegrated, showed a decreased binding of FH and even of C4BP, demonstrating its function as an FH- and C4BP-binding protein. This led to an enhanced terminal complement complex deposition after incubation with human serum; CaHgt1p thus functions as complement inhibitor. hgt1Δ/Δ mutants failed to form rosettes with complement-coated sheep erythrocytes, and show reduced binding to HIV-gp160, implying that a complement receptor 3 (CR3) moiety, known as fungal HIV binding molecule is lacking.

CONCLUSIONS

CaHgt1p is a multifunctional evasion molecule, as complement inhibitor, CR3 analogue and HIV receptor.

Mechanisms of immune evasion in fungal pathogens

The incidence of life-threatening fungal infections has continued to increase in recent years, predominantly in patients debilitated by iatrogenic interventions or immunological dysfunctions. While the picture of the immunology of fungal infections grows increasingly complex, it is clear that the phagocyte-pathogen interaction is a critical determinant of establishing an infection. About 10 years ago, genome-scale approaches began to elucidate the intricate and extensive fungal response to phagocytosis and in the last few years it has become clear that some of this response actively modulates immune cell function. Fungal pathogens avoid detection by masking pathogen-associated molecular patterns, such as cell wall carbohydrates, and by downregulating the complement cascade. Once detected, various species interfere with phagocytosis and intracellular trafficking, and can repress production of antimicrobials like nitric oxide (NO). For the most part, the molecular mechanisms behind these behaviors are not yet known. This review discusses recent discoveries and insights into how fungi manipulate the host-pathogen interaction.

Ligation of CD46 to CD40 inhibits CD40 signaling in B cells

CD40 induces B cells to switch to IgE in the presence of IL-4 and up-regulates their expression of the low-affinity receptor for IgE, CD23, which promotes the immune response to allergen complexed with IgE antibody. CD40 binds to CD40L and to the C4b-binding protein (C4BP) using distinct sites. CD46 is a receptor for the product of activated complement C4b. Some microbial antigens bind both C4BP and CD46, potentially bridging CD40 to CD46. In addition, immune complexes containing both C4b and C4BP may cross-link CD40 to CD46. We demonstrate that cross-linking CD46 to CD40 on B cells inhibits CD40-mediated up-regulation of surface CD23 expression and induction of IL-4-dependent IgE isotype switching. This was associated with inhibition of induction of Cε germ line transcripts and of activation-induced cytidine deaminase mRNA expression. Furthermore, co-ligation of CD46 to CD40 blocked CD40-mediated NF-κB activation. These observations suggest that complement components may play an important role in regulating CD40 activation of B cells and the allergic response.

A computational and experimental study of the regulatory mechanisms of the complement system

The complement system is key to innate immunity and its activation is necessary for the clearance of bacteria and apoptotic cells. However, insufficient or excessive complement activation will lead to immune-related diseases. It is so far unknown how the complement activity is up- or down- regulated and what the associated pathophysiological mechanisms are. To quantitatively understand the modulatory mechanisms of the complement system, we built a computational model involving the enhancement and suppression mechanisms that regulate complement activity. Our model consists of a large system of Ordinary Differential Equations (ODEs) accompanied by a dynamic Bayesian network as a probabilistic approximation of the ODE dynamics. Applying Bayesian inference techniques, this approximation was used to perform parameter estimation and sensitivity analysis. Our combined computational and experimental study showed that the antimicrobial response is sensitive to changes in pH and calcium levels, which determines the strength of the crosstalk between CRP and L-ficolin. Our study also revealed differential regulatory effects of C4BP. While C4BP delays but does not decrease the classical complement activation, it attenuates but does not significantly delay the lectin pathway activation. We also found that the major inhibitory role of C4BP is to facilitate the decay of C3 convertase. In summary, the present work elucidates the regulatory mechanisms of the complement system and demonstrates how the bio-pathway machinery maintains the balance between activation and inhibition. The insights we have gained could contribute to the development of therapies targeting the complement system.

The complement system, which is the frontline immune defense, constitutes proteins that flow freely in the blood. It quickly detects invading microbes and alerts the host by sending signals into immune responsive cells to eliminate the hostile substances. Inadequate or excessive complement activities harm the host and may lead to immune-related diseases. Thus, it is crucial to understand how the host boosts the complement activity to protect itself and simultaneously establishes tight surveillance to attain homeostasis. Towards this goal, we developed a detailed computational model of the human complement system. To overcome the challenges resulting from the large model size, we applied probabilistic approximation and inference techniques to train the model on experimental data and explored the key network features of the model. Our model-based study highlights the importance of infection-mediated microenvironmental perturbations, which alter the pH and calcium levels. It also reveals that the inhibitor, C4BP induces differential inhibition on the classical and lectin complement pathways and acts mainly by facilitating the decay of the C3 convertase. These predictions were validated empirically. Thus, our results help to elucidate the regulatory mechanisms of the complement system and potentially contribute to the development of complement-based immunomodulation therapies.

The pH-regulated antigen 1 of Candida albicans binds the human complement inhibitor C4b-binding protein and mediates fungal complement evasion

Candida albicans binds and utilizes human complement inhibitors, such as C4b-binding protein (C4BP), Factor H, and FHL-1 for immune evasion. Here, we identify Candida pH-regulated antigen 1 (Pra1) as the first fungal C4BP-binding protein. Recombinant Pra1 binds C4BP, as shown by ELISA and isothermal titration calorimetry, and the Pra1-C4BP interaction is ionic in nature. The Pra1 binding domains within C4BP were localized to the complement control protein domain 4 (CCP4), CCP7, and CCP8. C4BP bound to Pra1 maintains complement-inhibitory activity. C4BP and Factor H bind simultaneously to Candida Pra1 and do not compete for binding at physiological levels. A Pra1-overexpressing C. albicans strain, which had about 2-fold Pra1 levels at the surface acquired also about 2-fold C4BP to the surface, compared with the wild type strain CAI4. A Pra1 knock-out strain showed ∼22% reduced C4BP binding. C4BP captured by C. albicans from human serum inhibits C4b and C3b surface deposition and also maintains cofactor activity. In summary, Candida Pra1 represents the first fungal C4BP-binding surface protein. Pra1, via binding to C4BP, mediates human complement control, thereby favoring the immune and complement evasion of C. albicans.

Binding of the human complement regulators CFHR1 and factor H by streptococcal collagen-like protein 1 (Scl1) via their conserved C termini allows control of the complement cascade at multiple levels

Group A streptococci (GAS) utilize soluble human complement regulators to evade host complement attack. Here, we characterized the binding of the terminal complement complex inhibitor complement Factor H-related protein 1 (CFHR1) and of the C3 convertase regulator Factor H to the streptococcal collagen-like proteins (Scl). CFHR1 and Factor H, but no other member of the Factor H protein family (CFHR2, CFHR3, or CFHR4A), bound to the two streptococcal proteins Scl1.6 and Scl1.55, which are expressed by GAS serotypes M6 and M55. The two human regulators bound to the Scl1 proteins via their conserved C-terminal attachment region, i.e. CFHR1 short consensus repeats 3-5 (SCR3-5) and Factor H SCR18-20. Binding was affected by ionic strength and by heparin. CFHR1 and the C-terminal attachment region of Factor H did not bind to Scl1.1 and Scl2.28 proteins but did bind to intact M1-type and M28-type GAS, which express Scl1.1 and Scl2.28, respectively, thus arguing for the presence of an additional binding mechanism to CFHR1 and Factor H. Furthermore mutations within the C-terminal heparin-binding region and Factor H mutations that are associated with the acute renal disease atypical hemolytic uremic syndrome blocked the interaction with the two streptococcal proteins. Binding of CFHR1 affected the complement regulatory functions of Factor H on the level of the C3 convertase. Apparently, streptococci utilize two types of complement regulator-acquiring surface proteins; type A proteins, as represented by Scl1.6 and Scl1.55, bind to CFHR1 and Factor H via their conserved C-terminal region and do not bind the Factor H-like protein 1 (FHL-1). On the contrary, type B proteins, represented by M-, M-like, and the fibronectin-binding protein Fba proteins, bind Factor H and FHL-1 via domain SCR7 and do not bind CFHR1. In conclusion, binding of CFHR1 is at the expense of Factor H-mediated regulatory function at the level of C3 convertase and at the gain of a regulator that controls complement at the level of the C5 convertase and formation of the terminal complement complex.

Secreted Aspergillus fumigatus protease Alp1 degrades human complement proteins C3, C4, and C5

The opportunistic human pathogenic fungus Aspergillus fumigatus is a major cause of fungal infections in immunocompromised patients. Innate immunity plays an important role in the defense against infections. The complement system represents an essential part of the innate immune system. This cascade system is activated on the surface of A. fumigatus conidia and hyphae and enhances phagocytosis of conidia. A. fumigatus conidia but not hyphae bind to their surface host complement regulators factor H, FHL-1, and CFHR1, which control complement activation. Here, we show that A. fumigatus hyphae possess an additional endogenous activity to control complement activation. A. fumigatus culture supernatant efficiently cleaved complement components C3, C4, C5, and C1q as well as immunoglobulin G. Secretome analysis and protease inhibitor studies identified the secreted alkaline protease Alp1, which is present in large amounts in the culture supernatant, as the central molecule responsible for this cleavage. An alp1 deletion strain was generated, and the culture supernatant possessed minimal complement-degrading activity. Moreover, protein extract derived from an Escherichia coli strain overproducing Alp1 cleaved C3b, C4b, and C5. Thus, the protease Alp1 is responsible for the observed cleavage and degrades a broad range of different substrates. In summary, we identified a novel mechanism in A. fumigatus that contributes to evasion from the host complement attack.