Serum opsonin ficolin-A enhances host-fungal interactions and modulates cytokine expression from human monocyte-derived macrophages and neutrophils following Aspergillus fumigatus challenge

Complement-pentraxins synergy: Navigating the immune battlefield and beyond

The complement is a crucial immune defense system that triggers rapid immune responses and offers efficient protection against foreign invaders and unwanted host elements, acting as a sentinel. Activation of the complement system occurs upon the recognition of pathogenic microorganisms or altered self-cells by pattern-recognition molecules (PRMs) such as C1q, collectins, ficolins, and pentraxins. Recent accumulating evidence shows that pentraxins establish a cooperative network with different classes of effector PRMs, resulting in synergistic effects in complement activation. This review describes the complex interaction of pentraxins with the complement system and the implications of this cooperative network for effective host defense during pathogen invasion.

PLB-985 Neutrophil-Like Cells as a Model To Study Aspergillus fumigatus Pathogenesis

Fungal infections remain a major global concern. Emerging fungal pathogens and increasing rates of resistance mean that additional research efforts and resources must be allocated to advancing our understanding of fungal pathogenesis and developing new therapeutic interventions. Neutrophilic granulocytes are a major cell type involved in protection against the important fungal pathogen Aspergillus fumigatus, where they employ numerous defense mechanisms, including production of antimicrobial extracellular vesicles. A major drawback to work with neutrophils is the lack of a suitable cell line system for the study of fungal pathogenesis. To address this problem, we assessed the feasibility of using differentiated PLB-985 neutrophil-like cells as an in vitro model to study A. fumigatus infection. We find that dimethylformamide-differentiated PLB-985 cells provide a useful recapitulation of many aspects of A. fumigatus interactions with primary human polymorphonuclear leukocytes. We show that differentiated PLB-985 cells phagocytose fungal conidia and acidify conidia-containing phagolysosomes similar to primary neutrophils, release neutrophil extracellular traps, and also produce antifungal extracellular vesicles in response to infection. In addition, we provide an improved method for the isolation of extracellular vesicles produced during infection by employing a size exclusion chromatography-based approach. Advanced liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomics revealed an enrichment of extracellular vesicle marker proteins and a decrease of cytoplasmic proteins in extracellular vesicles isolated using this improved method. Ultimately, we find that differentiated PLB-985 cells can serve as a genetically tractable model to study many aspects of A. fumigatus pathogenesis. IMPORTANCE Polymorphonuclear leukocytes are an important defense against human fungal pathogens, yet our model systems to study this group of cells remain very limited in scope. In this study, we established that differentiated PLB-985 cells can serve as a model to recapitulate several important aspects of human polymorphonuclear leukocyte interactions with the important human fungal pathogen Aspergillus fumigatus. The proposed addition of a cultured neutrophil-like cell line to the experimental toolbox to study fungal pathogenesis will allow for a more mechanistic description of neutrophil antifungal biology. In addition, the easier handling of the cell line compared to primary human neutrophils allowed us to use PLB-985 cells to provide an improved method for isolation of neutrophil-derived extracellular vesicles using size exclusion chromatography. Together, these results provide significant tools and a baseline knowledge for the future study of neutrophil-derived extracellular vesicles in the laboratory.

Biochemical and structural characterization of a recombinant fibrinogen-related lectin from Penaeus monodon

Fibrinogen-related lectins are carbohydrate-binding proteins of the innate immune system that recognize glycan structures on microbial surfaces. These innate immune lectins are crucial for invertebrates as they do not rely on adaptive immunity for pathogen clearance. Here, we characterize a recombinant fibrinogen-related lectin PmFREP from the black tiger shrimp Penaeus monodon expressed in the Trichoplusia ni insect cell. Electron microscopy and cross-linking experiments revealed that PmFREP is a disulfide-linked dimer of pentamers distinct from other fibrinogen-related lectins. The full-length protein binds N-acetyl sugars in a Ca²⁺ ion-independent manner. PmFREP recognized and agglutinated Pseudomonas aeruginosa. Weak binding was detected with other bacteria, including Vibrio parahaemolyticus, but no agglutination activity was observed. The biologically active PmFREP will not only be a crucial tool to elucidate the innate immune signaling in P. monodon and other economically important species, but will also aid in detection and prevention of shrimp bacterial infectious diseases.

FCN-A mediates the inflammatory response and the macrophage polarization in Aspergillus fumigatus keratitis of mice by activating the MAPK signaling pathway

Fungal keratitis (FK) is a severe corneal disease that may cause vision loss. Previous studies indicate that the innate immune response produces the most effective anti-Aspergillus immune resistance. Ficolin-A (FCN-A), a soluble pattern-recognition receptor (PRR) family plays an important role in the innate immunity. In this study, we aimed to study the role of FCN-A in the A. fumigatus infected cornea. Here for the first time, we reported that the expression of FCN-A increases after A. fumigatus infection in the cornea of mice. Then, our results showed that the down-regulation of FCN-A reduced the inflammatory response of the cornea infected mice and decreased the expression of the TNF-a, p-p38, p-JNK. We also found that FCN-A can affect the recruitment of macrophages in the cornea of mice with A. fumigatus keratitis. In the mouse model of A. fumigatus keratitis and the A. fumigatus stimulation of RAW 264.7 cells, knocking down of FCN-A expression promoted the macrophage polarization toward M2. Furthermore, we observed that both the p38 and JNK inhibitors pretreatment decreased the proportion of M1/M2 in RAW 264.7 cells. Taken together, our data provide evidence that FCN-A participated in the inflammatory response of A. fumigatus keratitis in mice. Moreover, FCN-A mediates the inflammatory response and the polarization of the macrophages by activating the MAPK signaling pathway in A. fumigatus keratitis.

The complement system in Aspergillus fumigatus infections and its crosstalk with pentraxins

Aspergillosis is a life-threatening infection mostly affecting immunocompromised individuals and primarily caused by the saprophytic fungus Aspergillus fumigatus. At the host-pathogen interface, both cellular and humoral components of the innate immune system are increasingly acknowledged as essential players in the recognition and disposal of this opportunistic mold. Fundamental hereof is the contribution of the complement system, which deploys all three activation pathways in the battle against A. fumigatus, and functionally cooperates with other soluble pattern recognition molecules, including pentraxins. In particular, preclinical and clinical observations point to the long pentraxin PTX3 as a nonredundant and complement-dependent effector with protective functions against A. fumigatus. Based on past and current literature, here we discuss how the complement participates in the immune response to this fungal pathogen, and illustrate its crosstalk with the pentraxins, with a focus on PTX3. Emphasis is placed on the molecular mechanisms underlying such processes, the genetic evidence from human epidemiology, and the translational potential of the currently available knowledge.

Ficolins and the Recognition of Pathogenic Microorganisms: An Overview of the Innate Immune Response and Contribution of Single Nucleotide Polymorphisms

Ficolins are innate pattern recognition receptors (PRR) and play integral roles within the innate immune response to numerous pathogens throughout the circulation, as well as within organs. Pathogens are primarily removed by direct opsonisation following the recognition of cell surface carbohydrates and other immunostimulatory molecules or via the activation of the lectin complement pathway, which results in the deposition of C3b and the recruitment of phagocytes. In recent years, there have been a number of studies implicating ficolins in the recognition and removal of numerous bacterial, viral, fungal, and parasitic pathogens. Moreover, there has been expanding evidence highlighting that mutations within these key immune proteins, or the possession of particular haplotypes, enhance susceptibility to colonization by pathogens and dysfunctional immune responses. This review will therefore encompass previous knowledge on the role of ficolins in the recognition of bacterial and viral pathogens, while acknowledging the recent advances in the immune response to fungal and parasitic infections. Additionally, we will explore the various genetic susceptibility factors that predispose individuals to infection.

Pentraxins in Complement Activation and Regulation

The complement is the first line of immune defense system involved in elimination of invading pathogens and dying host cells. Its activation is mainly triggered by immune complexes or pattern recognition molecules (PRMs) upon recognition against non-self or altered self-cells, such as C1q, collectins, ficolins, and properdin. Recent findings have interestingly shown that the pentraxins (C-reactive protein, CRP; serum-amyloid P component, SAP; long pentraxin 3, PTX3) are involved in complement activation and amplification via communication with complement initiation PRMs, but also complement regulation via recruitment of complement regulators, for instance C4b binding protein (C4BP) and factor H (fH). This review addresses the potential roles of the pentraxins in the complement system during infection and inflammation, and emphasizes the underlining implications of the pentraxins in the context of complement activation and regulation both under physiological and pathological conditions.

HIF1α-dependent glycolysis promotes macrophage functional activities in protecting against bacterial and fungal infection

Macrophages are important innate immune defense system cells in the fight against bacterial and fungal pathogenic infections. They exhibit significant plasticity, particularly with their ability to undergo functional differentiation. Additionally, HIF1α is critically involved in the functional differentiation of macrophages during inflammation. However, the role of macrophage HIF1α in protecting against different pathogenic infections remains unclear. In this study, we investigated and compared the roles of HIF1α in different macrophage functional effects of bacterial and fungal infections in vitro and in vivo. We found that bacterial and fungal infections produced similar effects on macrophage functional differentiation. HIF1α deficiency inhibited pro-inflammatory macrophage functional activities when cells were stimulated with LPS or curdlan in vitro or when mice were infected with L. monocytogenes or C. albicans in vivo, thus decreasing pro-inflammatory TNFα and IL-6 secretion associated with pathogenic microorganism survival. Alteration of glycolytic pathway activation was required for the functional differentiation of pro-inflammatory macrophages in protecting against bacterial and fungal infections. Thus, the HIF1α-dependent glycolytic pathway is essential for pro-inflammatory macrophage functional differentiation in protecting against bacterial and fungal infections.

Immune Recognition of Fungal Polysaccharides

The incidence of fungal infections has dramatically increased in recent years, in large part due to increased use of immunosuppressive medications, as well as aggressive medical and surgical interventions that compromise natural skin and mucosal barriers. There are relatively few currently licensed antifungal drugs, and rising resistance to these agents has led to interest in the development of novel preventative and therapeutic strategies targeting these devastating infections. One approach to combat fungal infections is to augment the host immune response towards these organisms. The polysaccharide-rich cell wall is the initial point of contact between fungi and the host immune system, and therefore, represents an important target for immunotherapeutic approaches. This review highlights the advances made in our understanding of the mechanisms by which the immune system recognizes and interacts with exopolysaccharides produced by four of the most common fungal pathogens: Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, and Histoplasma capsulatum. Work to date suggests that inner cell wall polysaccharides that play an important structural role are the most conserved across diverse members of the fungal kingdom, and elicit the strongest innate immune responses. The immune system senses these carbohydrates through receptors, such as lectins and complement proteins. In contrast, a greater diversity of polysaccharides is found within the outer cell walls of pathogenic fungi. These glycans play an important role in immune evasion, and can even induce anti-inflammatory host responses. Further study of the complex interactions between the host immune system and the fungal polysaccharides will be necessary to develop more effective therapeutic strategies, as well as to explore the use of immunosuppressive polysaccharides as therapeutic agents to modulate inflammation.

An overview of the synergy and crosstalk between pentraxins and collectins/ficolins: their functional relevance in complement activation

The complement system is an innate immune defense machinery comprising components that deploy rapid immune responses and provide efficient protection against foreign invaders and unwanted host elements. The complement system is activated upon recognition of pathogenic microorganisms or altered self-cells by exclusive pattern recognition molecules (PRMs), such as collectins, ficolins and pentraxins. Recent accumulating evidence shows that the different classes of effector PRMs build up a co-operative network and exert synergistic effects on complement activation. In this review, we describe our updated view of the crosstalk between previously unlinked PRMs in complement activation and the potential pathogenic effects during infection and inflammation.

Innate Lung Defense during Invasive Aspergillosis: New Mechanisms

Invasive aspergillosis (IA) is one of the most difficult to treat and, consequently, one of the most lethal fungal infections known to man. Continued use of immunosuppressive agents during chemotherapy and organ transplantation often leads to the development of neutropenia, the primary risk factor for IA. However, IA is also becoming more appreciated in chronic diseases associated with corticosteroid therapy. The innate immune response to Aspergillus fumigatus, the primary agent in IA, plays a pivotal role in the recognition and elimination of organisms from the pulmonary system. This review highlights recent findings about innate host defense mechanisms, including novel aspects of innate cellular immunity and pathogen recognition, and the inflammatory mediators that control infection with A. fumigatus.

Ficolins Promote Fungal Clearance in vivo and Modulate the Inflammatory Cytokine Response in Host Defense against Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic fungal pathogen that causes severe invasive infections in immunocompromised patients. Innate immunity plays a major role in protection against A. fumigatus. The ficolins are a family of soluble pattern recognition receptors that are capable of activating the lectin pathway of complement. Previous in vitro studies reported that ficolins bind to A. fumigatus, but their part in host defense against fungal infections in vivo is unknown. In this study, we used ficolin-deficient mice to investigate the role of ficolins during lung infection with A. fumigatus. Ficolin knockout mice showed significantly higher fungal loads in the lungs 24 h postinfection compared to wild-type mice. The delayed clearance of A. fumigatus in ficolin knockout mice could not be attributed to a compromised recruitment of inflammatory cells. However, it was revealed that ficolin knockout mice exhibited a decreased production of proinflammatory cytokines in the lungs compared to wild-type mice following A. fumigatus infection. The impaired clearance and cytokine production in ficolin knockout mice was independent of complement, as shown by equivalent levels of A. fumigatus-mediated complement activation in ficolin knockout mice and wild-type mice. In conclusion, this study demonstrates that ficolins are important in initial innate host defense against A. fumigatus infections in vivo.

Fluid phase recognition molecules in neutrophil-dependent immune responses

The innate immune system comprises both a cellular and a humoral arm. Neutrophils are key effector cells of the immune and inflammatory responses and have emerged as a major source of humoral pattern recognition molecules (PRMs). These molecules, which include collectins, ficolins, and pentraxins, are specialised in the discrimination of self versus non-self and modified-self and share basic multifunctional properties including recognition and opsonisation of pathogens and apoptotic cells, activation and regulation of the complement cascade and tuning of inflammation. Neutrophils act as a reservoir of ready-made soluble PRMs, such as the long pentraxin PTX3, the peptidoglycan recognition protein PGRP-S, properdin and M-ficolin, which are stored in neutrophil granules and are involved in neutrophil effector functions. In addition, other soluble PRMs, such as members of the collectin family, are not expressed in neutrophils but can modulate neutrophil-dependent immune responses. Therefore, soluble PRMs are an essential part of the innate immune response and retain antibody-like effector functions. Here, we will review the expression and general function of soluble PRMs, focusing our attention on molecules involved in neutrophil effector functions.

Identification of FAM3D as a novel endogenous chemotaxis agonist for the FPRs (formyl peptide receptors)

The family with sequence similarity 3 (FAM3) gene family is a cytokine-like gene family with four members FAM3A, FAM3B, FAM3C, and FAM3D. In this study, we found that FAM3D strongly chemoattracted human peripheral blood neutrophils and monocytes. To identify FAM3D receptor, we used chemotaxis, receptor internalization, calcium flux and radioligand-binding assays in FAM3D-stimulated HEK293 cells that transiently expressed FPR1 or FPR2 to show that FAM3D was a high affinity ligand of formyl peptide receptors (FPR1 and FPR2), both of which were highly expressed on the surface of neutrophils and monocytes/macrophages. After being injected into the mouse peritoneal cavity, FAM3D chemoattracted CD11b+Ly6G+ neutrophils in a short time. In response to FAM3D stimulation, p-ERK and p-p38 were up-regulated in the mouse neutrophils, which could be inhibited by an inhibitor of FPR1 or FPR2. FAM3D was reported to be constitutively expressed in the gastrointestinal tract. We found that FAM3D expression increased significantly in dextran sulfate sodium-induced colitis. Taken together, we propose that FAM3D plays a role in gastrointestinal homeostasis and inflammation through its receptors FPR1 and FPR2.