Macrophage-independent fungicidal action of the pulmonary collectins

Histoplasmosis in Solid Organ Transplantation

Histoplasma capsulatum, the etiological agent for histoplasmosis, is a dimorphic fungus that grows as a mold in the environment and as a yeast in human tissues. It has a broad global distribution with shifting epidemiology during recent decades. While in immunocompetent individuals infection is usually self-resolving, solid organ transplant recipients are at increased risk of symptomatic disease with dissemination to extrapulmonary tissue. Diagnosis of histoplasmosis relies on direct observation of the pathogen (histopathology, cytopathology, and culture) or detection of antigens, antibodies, or nucleic acids. All transplant recipients with histoplasmosis warrant therapy, though the agent of choice and duration of therapy depends on the severity of disease. In the present article, we describe the pathogenesis, epidemiology, clinical manifestations and management of histoplasmosis in solid organ transplant recipients.

The anti-inflammatory and antiviral properties of anionic pulmonary surfactant phospholipids

The pulmonary surfactant system of the lung is a lipid and protein complex, which regulates the biophysical properties of the alveoli to prevent lung collapse and the innate immune system in the lung. Pulmonary surfactant is a lipoprotein complex consisting of 90% phospholipids and 10% protein, by weight. Two minor components of pulmonary surfactant phospholipids, phosphatidylglycerol (PG) and phosphatidylinositol (PI), exist at very high concentrations in the extracellular alveolar compartments. We have reported that one of the most dominant molecular species of PG, palmitoyl-oleoyl-phosphatidylglycerol (POPG) and PI inhibit inflammatory responses induced by multiple toll-like receptors (TLR2/1, TLR3, TLR4, and TLR2/6) by interacting with subsets of multiprotein receptor components. These lipids also exert potent antiviral effects against RSV and influenza A, in vitro, by inhibiting virus binding to host cells. POPG and PI inhibit these viral infections in vivo, in multiple animal models. Especially noteworthy, these lipids markedly attenuate SARS-CoV-2 infection including its variants. These lipids are natural compounds that already exist in the lung and, thus, are less likely to cause adverse immune responses by hosts. Collectively, these data demonstrate that POPG and PI have strong potential as novel therapeutics for applications as anti-inflammatory compounds and preventatives, as treatments for broad ranges of RNA respiratory viruses.

Pathogenicity & Virulence of Histoplasma capsulatum - a multifaceted organism adapted to intracellular environments

Histoplasmosis is a systemic mycosis caused by the thermally dimorphic fungus Histoplasma capsulatum. Although healthy individuals can develop histoplasmosis, the disease is particularly life-threatening in immunocompromised patients, with a wide range of clinical manifestations depending on the inoculum and virulence of the infecting strain. In this review, we discuss the established virulence factors and pathogenesis traits that make H. capsulatum highly adapted to a wide variety of hosts, including mammals. Understanding and integrating these mechanisms is a key step towards devising new preventative and therapeutic interventions.

New Insights into the Alveolar Epithelium as a Driver of Acute Respiratory Distress Syndrome

The alveolar epithelium serves as a barrier between the body and the external environment. To maintain efficient gas exchange, the alveolar epithelium has evolved to withstand and rapidly respond to an assortment of inhaled, injury-inducing stimuli. However, alveolar damage can lead to loss of alveolar fluid barrier function and exuberant, non-resolving inflammation that manifests clinically as acute respiratory distress syndrome (ARDS). This review discusses recent discoveries related to mechanisms of alveolar homeostasis, injury, repair, and regeneration, with a contemporary emphasis on virus-induced lung injury. In addition, we address new insights into how the alveolar epithelium coordinates injury-induced lung inflammation and review maladaptive lung responses to alveolar damage that drive ARDS and pathologic lung remodeling.

Lung surfactant proteins as potential targets of prallethrin: An in silico approach

Object

Prallethrin is a pyrethroid-based insecticide, commonly used as a liquid vaporizer in household, schools, and offices to repel mosquitoes. Due to worldwide application, human beings are exposed to this compound via inhalation. Inhalation of prallethrin can expose lung surfactant proteins (SPs) to this compound. SPs such as SP-A and SP-D have anti-microbial activities, whereas SP-B and SP-C prevent alveolar collapse during exhalation by reducing surface pressure in alveolar walls. The present study aimed to investigate the binding affinities of prallethrin for the pulmonary SPs and the possible interactions involved in it.

Methods

In this study, molecular docking was performed using prallethrin as ligand and lung SPs as target molecules. The three-dimensional structure of prallethrin (PubChem CID: 9839306) was retrieved from PubChem (https://pubchem.ncbi.nlm.nih.gov/ ), whereas the same for SPs were retrieved from RCSB Protein Data Bank (https://www.rcsb.org/ ). AutoDock 4.2 employing Lamarckian genetic algorithm was used to calculate binding affinities between the target protein and the ligand. Polar and nonpolar interactions between the amino acids of SPs and Prallethrin were studied utilizing Chimera X and Discovery Studio Visualizer.

Results

Results demonstrated that, prallethrin can bind with the four SPs using several interactions such as hydrogen bonds, alkyl bonds, Pi–Pi interaction, Van der Waals interaction and other. Prallethrin interacted with two binding pockets of SP-A and SP-C, whereas the prallethrin interacted with three binding pockets of SP-B and SP-D, respectively.

Conclusion

Findings of the study indicated that prallethrin can bind with the pulmonary SPs employing hydrogen and hydrophobic interactions. Such interactions could impair critical functions of SPs in lungs. This might increase susceptibility of lungs towards a range of respiratory illness, pathogenic infections, as well as malignancy.

Graphical abstract

In silico study reveals binding potential of rotenone at multiple sites of pulmonary surfactant proteins: A matter of concern

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Inhalation of rotenone exposes lung surfactant proteins (SP) to this pesticide.

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SP-A and SP-D provides protection from microbial infection.

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SP-B and SP-C maintain structure and respiratory function of lungs.

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Rotenone has potential to bind SPs at multiple sites.

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Such binding can subvert functions of SPs & may invite respiratory ailments.

Rotenone is a broad-spectrum pesticide employed in various agricultural practices all over the world. Human beings are exposed to this chemical through oral, nasal, and dermal routes. Inhalation of rotenone exposes bio-molecular components of lungs to this chemical. Biophysical activity of lungs is precisely regulated by pulmonary surfactant to facilitate gaseous exchange. Surfactant proteins (SPs) are the fundamental components of pulmonary surfactant. SPs like SP-A and SP-D have antimicrobial activities providing a crucial first line of defense against infections in lungs whereas SP-B and SP-C are mainly involved in respiratory cycle and reduction of surface tension at air–water interface. In this study, molecular docking analysis using AutoDock Vina has been conducted to investigate binding potential of rotenone with the four SPs. Results indicate that, rotenone can bind with carbohydrate recognition domain (CRD) of SP-A, N-, and C- terminal peptide of SP-B, SP-C, and CRD of SP-D at multiples sites via several interaction mediators such as H bonds, C–H bonds, alkyl bonds, pi-pi stacked, Van der Waals interaction, and other. Such interactions of rotenone with SPs can disrupt biophysical and anti-microbial functions of SPs in lungs that may invite respiratory ailments and pathogenic infections.

Detection of Cytokines and Collectins in Bronchoalveolar Fluid Samples of Patients Infected with Histoplasma capsulatum and Pneumocystis jirovecii

Histoplasmosis and pneumocystosis co-infections have been reported mainly in immunocompromised humans and in wild animals. The immunological response to each fungal infection has been described primarily using animal models; however, the host response to concomitant infection is unknown. The present work aimed to evaluate the pulmonary immunological response of patients with pneumonia caused either by Histoplasma capsulatum, Pneumocystis jirovecii, or their co-infection. We analyzed the pulmonary collectin and cytokine patterns of 131 bronchoalveolar lavage samples, which included HIV and non-HIV patients infected with H. capsulatum, P. jirovecii, or both fungi, as well as healthy volunteers and HIV patients without the studied fungal infections. Our results showed an increased production of the surfactant protein-A (SP-A) in non-HIV patients with H. capsulatum infection, contrasting with HIV patients (p < 0.05). Significant differences in median values of SP-A, IL-1β, TNF-α, IFN-γ, IL-18, IL-17A, IL-33, IL-13, and CXCL8 were found among all the groups studied, suggesting that these cytokines play a role in the local inflammatory processes of histoplasmosis and pneumocystosis. Interestingly, non-HIV patients with co-infection and pneumocystosis alone showed lower levels of SP-A, IL-1β, TNF-α, IFN-γ, IL-18, IL-17A, and IL-23 than histoplasmosis patients, suggesting an immunomodulatory ability of P. jirovecii over H. capsulatum response.

Surfactant phospholipids act as molecular switches for premature induction of quorum sensing-dependent virulence in Pseudomonas aeruginosa

The virulence behaviors of many Gram-negative bacterial pathogens are governed by quorum-sensing (QS), a hierarchical system of gene regulation that relies on population density by producing and detecting extracellular signaling molecules. Although extensively studied under in vitro conditions, adaptation of QS system to physiologically relevant host environment is not fully understood. In this study, we investigated the influence of lung environment on the regulation of Pseudomonas aeruginosa virulence factors by QS in a mouse model of acute pneumonia. When cultured under laboratory conditions in lysogeny broth, wild-type P. aeruginosa strain PAO1 began to express QS-regulated virulence factors elastase B (LasB) and rhamnolipids (RhlA) during transition from late-exponential into stationary growth phase. In contrast, during acute pneumonia as well as when cultured in mouse bronchial alveolar lavage fluids (BALF), exponential phase PAO1 bacteria at low population density prematurely expressed QS regulatory genes lasI-lasR and rhlI-rhlR and their downstream virulence genes lasB and rhlA. Further analysis indicated that surfactant phospholipids were the primary components within BALF that induced the synthesis of N-(3-oxododecanoyl)-L-homoserine lactone (C12-HSL), which triggered premature expression of LasB and RhlA. Both phenol extraction and phospholipase A2 digestion abolished the ability of mouse BALF to promote LasB and RhlA expression. In contrast, provision of the major surfactant phospholipid dipalmitoylphosphatidylcholine (DPPC) restored the expression of both virulence factors. Collectively, our study demonstrates P. aeruginosa modulates its QS to coordinate the expression of virulence factors during acute pneumonia by recognizing pulmonary surfactant phospholipids.

Transcription Factor HSF1 Suppresses the Expression of Surfactant Protein D in Cells Infected with Aspergillus fumigatus

Aspergillosis is a life-threatening disease in patients with compromised immune systems. The process of fungal invasion is an important step during host cell infection. We investigated the transcription factor and promoter region of SFTPD, which is activated during the infection process in conidia-treated cells. To investigate the promoter activity of SFTPD in fungal-infected cells, we cloned various lengths of the promoter region (−1000 to +1) of SFTPD and examined its activity in A549 cells treated with Aspergillus fumigatus conidia. We determined the location within the promoter region of SFTPD that exhibits a response to conidia infection. AliBaba 2.1 software was used to predict the transcription factor involved as well as the binding sites in the SFTPD promoter region. The results of a decoy assay show that the HSF1 transcription factor is sufficient to decrease the SFTPD expression. Using chromatin immunoprecipitation, we confirmed that HSF1 directly binds to the selected sequence, which is located in the response region (−142 to −134 bp). These findings suggest that inhibiting the binding of HSF1 to the promoter region of SFTPD is an important step to prevent conidia infection.

A microfibril-associated glycoprotein 4 (MFAP4) from Nile tilapia (Oreochromis niloticus) possesses agglutination and opsonization ability to bacterial pathogens

Microfibril-associated glycoprotein 4 (MFAP4), a pattern recognition-like molecule with a fibrinogen-like domain (FBG), has the ability to combine and agglutinate pathogens, playing an essential role in the first line of innate immune defense. In this study, the sequence of Nile tilapia (Oreochromis niloticus) microfibril-associated glycoprotein 4 (OnMFAP4) open reading frame (ORF) was amplified and identified. The ORF of OnMFAP4 is 720 bp of nucleotides and codes for 239 amino acids. Spatial mRNA encoding analysis indicated that OnMFAP4 was highly produced in liver, intestine and head kidney in healthy tilapia, and with the lowest expression in muscle. After challenges with Streptococcus agalactiae (S. agalactiae) and Aeromonas hydrophila (A. hydrophila), the expression of OnMFAP4 mRNA was prominently produced in the liver, spleen and head kidney. The up-regulation of OnMFAP4 expression was also presented in head kidney monocytes/macrophages (MO/MΦ) and hepatocytes. Recombinant OnMFAP4 ((r)OnMFAP4) could bind and agglutinate both bacterial pathogens. Moreover, (r)OnMFAP4 could take part in the modulation of inflammation and phagocytosis. In conclusion, this study revealed that OnMFAP4 might take effect in host defense against bacterial infection in Nile tilapia, with agglutination and opsonization capability to bacterial pathogens.

Surfactant Protein D Recognizes Multiple Fungal Ligands: A Key Step to Initiate and Intensify the Anti-fungal Host Defense

With limited therapeutic options and associated severe adverse effects, fungal infections are a serious threat to human health. Innate immune response mediated by pattern recognition proteins is integral to host defense against fungi. A soluble pattern recognition protein, Surfactant protein D (SP-D), plays an important role in immune surveillance to detect and eliminate human pathogens. SP-D exerts its immunomodulatory activity via direct interaction with several receptors on the epithelial cells lining the mucosal tracts, as well as on innate and adaptive immune cells. Being a C-type lectin, SP-D shows calcium- and sugar-dependent interactions with several glycosylated ligands present on fungal cell walls. The interactome includes cell wall polysaccharides such as 1,3-β-D-glucan, 1,6-β-D-glucan, Galactosaminogalactan Galactomannan, Glucuronoxylomannan, Mannoprotein 1, and glycosylated proteins such as gp45, gp55, major surface glycoprotein complex (gpA). Recently, binding of a recombinant fragment of human SP-D to melanin on the dormant conidia of Aspergillus fumigatus was demonstrated that was not inhibited by sugars, suggesting a likely protein-protein interaction. Interactions of the ligands on the fungal spores with the oligomeric forms of full-length SP-D resulted in formation of spore-aggregates, increased uptake by phagocytes and rapid clearance besides a direct fungicidal effect against C. albicans. Exogenous administration of SP-D showed significant therapeutic potential in murine models of allergic and invasive mycoses. Altered susceptibility of SP-D gene-deficient mice to various fungal infections emphasized relevance of SP-D as an important sentinel of anti-fungal immunity. Levels of SP-D in the serum or lung lavage were significantly altered in the murine models and patients of fungal infections and allergies. Here, we review the cell wall ligands of clinically relevant fungal pathogens and allergens that are recognized by SP-D and their impact on the host defense. Elucidation of the molecular interactions between innate immune humoral such as SP-D and fungal pathogens would facilitate the development of novel therapeutic interventions.

Lipid-Protein and Protein-Protein Interactions in the Pulmonary Surfactant System and Their Role in Lung Homeostasis

Pulmonary surfactant is a lipid/protein complex synthesized by the alveolar epithelium and secreted into the airspaces, where it coats and protects the large respiratory air–liquid interface. Surfactant, assembled as a complex network of membranous structures, integrates elements in charge of reducing surface tension to a minimum along the breathing cycle, thus maintaining a large surface open to gas exchange and also protecting the lung and the body from the entrance of a myriad of potentially pathogenic entities. Different molecules in the surfactant establish a multivalent crosstalk with the epithelium, the immune system and the lung microbiota, constituting a crucial platform to sustain homeostasis, under health and disease. This review summarizes some of the most important molecules and interactions within lung surfactant and how multiple lipid–protein and protein–protein interactions contribute to the proper maintenance of an operative respiratory surface.

Collectins: Innate Immune Pattern Recognition Molecules

Collectins are collagen-containing C-type (calcium-dependent) lectins which are important pathogen pattern recognising innate immune molecules. Their primary structure is characterised by an N-terminal, triple-helical collagenous region made up of Gly-X-Y repeats, an a-helical coiled-coil trimerising neck region, and a C-terminal C-type lectin or carbohydrate recognition domain (CRD). Further oligomerisation of this primary structure can give rise to more complex and multimeric structures that can be seen under electron microscope. Collectins can be found in serum as well as in a range of tissues at the mucosal surfaces. Mannanbinding lectin can activate the complement system while other members of the collectin family are extremely versatile in recognising a diverse range of pathogens via their CRDs and bring about effector functions designed at the clearance of invading pathogens. These mechanisms include opsonisation, enhancement of phagocytosis, triggering superoxidative burst and nitric oxide production. Collectins can also potentiate the adaptive immune response via antigen presenting cells such as macrophages and dendritic cells through modulation of cytokines and chemokines, thus they can act as a link between innate and adaptive immunity. This chapter describes the structure-function relationships of collectins, their diverse functions, and their interaction with viruses, bacteria, fungi and parasites.

Structural and Functional Determinants of Rodent and Human Surfactant Protein A: A Synthesis of Binding and Computational Data

Surfactant protein A (SP-A) provides surfactant stability, first line host defense, and lung homeostasis by binding surfactant phospholipids, pathogens, alveolar macrophages (AMs), and epithelial cells. Non-primates express one SP-A protein whereas humans express two: SP-A1 and SP-A2 with core intra- and inter-species differences in the collagen-like domain. Here, we used macrophages and solid phase binding assays to discern structural correlates of rat (r) and human (h) SP-A function. Binding assays using recombinant rSP-A expressed in insect cells showed that lack of proline hydroxylation, truncations of amino-terminal oligomerization domains, and site-directed serine (S) or alanine (A) mutagenesis of cysteine 6 (C6S), glutamate 195 (E195A), and glutamate 171 (E171A) in the carbohydrate recognition domain (CRD) all impaired SP-A binding. Replacement of arginine 197 with alanine found in hSP-A (R197A), however, restored the binding of hydroxyproline-deficient rSP-A to the SP-A receptor SP-R210 similar to native rat and human SP-A. In silico calculation of Ca⁺⁺ coordination bond length and solvent accessibility surface area revealed that the “humanized” R197A substitution alters topology and solvent accessibility of the Ca⁺⁺ coordination residues of the CRD domain. Binding assays in mouse AMs that were exposed to either endogenous SP-A or hSP-A1 (6A²) and hSP-A2 (1A⁰) isoforms in vivo revealed that mouse SP-A is a functional hybrid of hSP-A1 and hSP-A2 in regulating SP-A receptor occupancy and binding affinity. Binding assays using neonatal and adult human AMs indicates that the interaction of SP-A1 and SP-A2 with AMs is developmentally regulated. Furthermore, our data indicate that the auxiliary ion coordination loop encompassing the conserved E171 residue may comprise a conserved site of interaction with macrophages, and SP-R210 specifically, that merits further investigation to discern conserved and divergent SP-A functions between species. In summary, our findings support the notion that complex structural adaptation of SP-A regulate conserved and species specific AM functions in vertebrates.

TLR4-interacting SPA4 peptide improves host defense and alleviates tissue injury in a mouse model of Pseudomonas aeruginosa lung infection

Interaction between surfactant protein-A (SP-A) and toll-like receptor (TLR)4 plays a critical role in host defense. In this work, we studied the host defense function of SPA4 peptide (amino acids GDFRYSDGTPVNYTNWYRGE), derived from the TLR4-interacting region of SP-A, against Pseudomonas aeruginosa. We determined the binding of SPA4 peptide to live bacteria, and its direct antibacterial activity against P. aeruginosa. Pro-phagocytic and anti-inflammatory effects were investigated in JAWS II dendritic cells and primary alveolar macrophages. The biological relevance of SPA4 peptide was evaluated in a mouse model of acute lung infection induced by intratracheal challenge with P. aeruginosa. Our results demonstrate that the SPA4 peptide does not interact with or kill P. aeruginosa when cultured outside the host. The SPA4 peptide treatment induces the uptake and localization of bacteria in the phagolysosomes of immune cells. At the same time, the secreted amounts of TNF-α are significantly reduced in cell-free supernatants of SPA4 peptide-treated cells. In cells overexpressing TLR4, the TLR4-induced phagocytic response is maintained, but the levels of TLR4-stimulated TNF-α are reduced. Furthermore, our results demonstrate that the therapeutic administration of SPA4 peptide reduces bacterial burden, inflammatory cytokines and chemokines, intracellular signaling, and lactate levels, and alleviates lung edema and tissue damage in P. aeruginosa-infected mice. Together, our results suggest that the treatment with SPA4 peptide can help control the bacterial burden, inflammation, and tissue injury in a P. aeruginosa lung infection model.

Identification and characterization of a B-type mannose-binding lectin from Nile tilapia (Oreochromis niloticus) in response to bacterial infection

Lectins are a group of carbohydrate-binding proteins, which play an important role in innate immune system against pathogen infection. In this study, a B-type mannose-binding lectin (OnBML) was identified from Nile tilapia (Oreochromis niloticus), and characterized at expression patterns against bacterial infection and capability to promote phagocytosis by macrophages. The open reading frame of OnBML is 354 bp of nucleotide sequence encoding polypeptides of 117 amino acids. The deduced protein is highly homologous to other teleost BMLs, containing two repeats of the conserved mannose-binding motif QXDXNXVXY. Expression of OnBML was widely exhibited in all examined tissues, with the most abundance in spleen and following gill, peripheral blood, and head kidney. The OnBML expressions were significantly up-regulated following two major bacterial infections including a Gram-positive bacterium (Streptococcus agalactiae) and a Gram-negative bacterium (Aeromonas hydrophila) in vivo and in vitro. Recombinant OnBML protein possessed capacities of mannose-binding and calcium-dependent agglutination to S. agalactiae and A. hydrophila, and promoted the phagocytosis by macrophages. Taken together, the present study indicated that OnBML is likely to get involved in host defense against bacterial infection in Nile tilapia.

Histoplasma Capsulatum: Mechanisms for Pathogenesis

Histoplasmosis, caused by the dimorphic environmental fungus Histoplasma capsulatum, is a major mycosis on the global stage. Acquisition of the fungus by mammalian hosts can be clinically silent or it can lead to life-threatening systemic disease, which can occur in immunologically intact or deficient hosts, albeit severe disease is more likely in the setting of compromised cellular immunity. H. capsulatum yeast cells are highly adapted to the mammalian host as they can effectively survive within intracellular niches in select phagocytic cells. Understanding the biological response by both the host and H. capsulatum will facilitate improved approaches to prevent and/or modify disease. This review presents our current understanding of the major pathogenic mechanisms involved in histoplasmosis.

Expression and functional characterization of collection-K1 from Nile tilapia (Oreochromis niloticus) in host innate immune defense

Collectin-K1 (CL-K1), a multifunctional Ca²⁺-dependent lectin, is able to bind carbohydrates on pathogens and inhibit infection by direct neutralization, agglutination, opsonization and killing, which plays an important role in innate immunity. In this study, a CL-K1 homolog (OnCL-K1) was identified from Nile tilapia (Oreochromis niloticus) and characterized at expression and agglutination functional levels. The open reading frame of OnCL-K1 is 720 bp of nucleotide sequence encoding a polypeptide of 239 amino acids. The deduced amino acid sequence has two characteristic structures, containing a collagen-like region and a carbohydrate recognition domain. Expression analysis revealed that the OnCL-K1 was highly expressed in the liver, and widely exhibited in other tissues including kidney, intestine and spleen. In addition, the OnCL-K1 expression was significantly up-regulated in spleen and anterior kidney following challenges with a Gram-positive bacterial pathogen (Streptococcus agalactiae) and a Gram-negative bacterial pathogen (Aeromonas hydrophila). The up-regulation of OnCL-K1 expression was also demonstrated in hepatocytes and monocytes/macrophages in vitro stimulation with S. agalactiae and A. hydrophila. Recombinant OnCL-K1 protein was able to agglutinate both S. agalactiae and A. hydrophila in vitro, and participate in the regulation of inflammatory, migration reaction and promote the phagocytosis by monocytes/macrophages. Taken together, the results of this study indicated that OnCL-K1, possessing apparent agglutination, opsonization and killing ability to bacterial pathogens and participating in the regulation mechanisms of the non-specific cellular immune, might be involved in host defense of innate immunity against bacterial infection in Nile tilapia.

A C-type lectin (CL11X1-like) from Nile tilapia (Oreochromis niloticus) is involved in host defense against bacterial infection

Collectins, a subfamily of the C-type lectins, are able to bind non-self glycoconjugates on the surface of microorganisms and inhibit infection by direct neutralization, agglutination and/or opsonization, which play important roles in innate immunity. In this study, a CL11X1-like collectin (OnCL11X1) was identified from Nile tilapia (Oreochromis niloticus) and characterized at expression and agglutination functional levels. The open reading frame of OnCL11X1 is 840 bp of nucleotide sequence encoding polypeptides of 279 amino acids. The deduced amino acid sequence is highly homology to teleost and similar to mammalian CL11X1, containing a canonical collagen-like region, a carbohydrate recognition domain and a neck region. Expression analysis revealed that the OnCL11X1 was highly expressed in the liver, and widely exhibited in other tissues including kidney, intestines and spleen. In addition, the OnCL11X1 expression was significantly up-regulated in spleen and anterior kidney following challenges with a Gram-positive bacterial pathogen (Streptococcus agalactiae) and a Gram-negative bacterial pathogen (Aeromonas hydrophila). The up-regulation of OnCL11X1 expression was also demonstrated in hepatocytes and macrophages in vitro stimulation with S. agalactiae and A. hydrophila. Recombinant OnCL11X1 protein was able to agglutinate both S. agalactiae and A. hydrophila in vitro and promote the phagocytosis by macrophages. Taken together, the results of this study indicated that OnCL11X1, possessing apparent agglutination and opsonization ability to bacterial pathogens, might be involved in host defense against bacterial infection in Nile tilapia.

The Interaction of Human Pathogenic Fungi With C-Type Lectin Receptors

Fungi, usually present as commensals, are a major cause of opportunistic infections in immunocompromised patients. Such infections, if not diagnosed or treated properly, can prove fatal. However, in most cases healthy individuals are able to avert the fungal attacks by mounting proper antifungal immune responses. Among the pattern recognition receptors (PRRs), C-type lectin receptors (CLRs) are the major players in antifungal immunity. CLRs can recognize carbohydrate ligands, such as β-glucans and mannans, which are mainly found on fungal cell surfaces. They induce proinflammatory immune reactions, including phagocytosis, oxidative burst, cytokine, and chemokine production from innate effector cells, as well as activation of adaptive immunity via Th17 responses. CLRs such as Dectin-1, Dectin-2, Mincle, mannose receptor (MR), and DC-SIGN can recognize many disease-causing fungi and also collaborate with each other as well as other PRRs in mounting a fungi-specific immune response. Mutations in these receptors affect the host response and have been linked to a higher risk in contracting fungal infections. This review focuses on how CLRs on various immune cells orchestrate the antifungal response and on the contribution of single nucleotide polymorphisms in these receptors toward the risk of developing such infections.

Surfactant Protein D in Respiratory and Non-Respiratory Diseases

Surfactant protein D (SP-D) is a multimeric collectin that is involved in innate immune defense and expressed in pulmonary, as well as non-pulmonary, epithelia. SP-D exerts antimicrobial effects and dampens inflammation through direct microbial interactions and modulation of host cell responses via a series of cellular receptors. However, low protein concentrations, genetic variation, biochemical modification, and proteolytic breakdown can induce decomposition of multimeric SP-D into low-molecular weight forms, which may induce pro-inflammatory SP-D signaling. Multimeric SP-D can decompose into trimeric SP-D, and this process, and total SP-D levels, are partly determined by variation within the SP-D gene, SFTPD. SP-D has been implicated in the development of respiratory diseases including respiratory distress syndrome, bronchopulmonary dysplasia, allergic asthma, and chronic obstructive pulmonary disease. Disease-induced breakdown or modifications of SP-D facilitate its systemic leakage from the lung, and circulatory SP-D is a promising biomarker for lung injury. Moreover, studies in preclinical animal models have demonstrated that local pulmonary treatment with recombinant SP-D is beneficial in these diseases. In recent years, SP-D has been shown to exert antimicrobial and anti-inflammatory effects in various non-pulmonary organs and to have effects on lipid metabolism and pro-inflammatory effects in vessel walls, which enhance the risk of atherosclerosis. A common SFTPD polymorphism is associated with atherosclerosis and diabetes, and SP-D has been associated with metabolic disorders because of its effects in the endothelium and adipocytes and its obesity-dampening properties. This review summarizes and discusses the reported genetic associations of SP-D with disease and the clinical utility of circulating SP-D for respiratory disease prognosis. Moreover, basic research on the mechanistic links between SP-D and respiratory, cardiovascular, and metabolic diseases is summarized. Perspectives on the development of SP-D therapy are addressed.

Modeling pulmonary alveolar microlithiasis by epithelial deletion of the Npt2b sodium phosphate cotransporter reveals putative biomarkers and strategies for treatment

Pulmonary alveolar microlithiasis (PAM) is a rare, autosomal recessive lung disorder associated with progressive accumulation of calcium phosphate microliths. Inactivating mutations in SLC34A2, which encodes the NPT2b sodium-dependent phosphate cotransporter, has been proposed as a cause of PAM. We show that epithelial deletion of Npt2b in mice results in a progressive pulmonary process characterized by diffuse alveolar microlith accumulation, radiographic opacification, restrictive physiology, inflammation, fibrosis, and an unexpected alveolar phospholipidosis. Cytokine and surfactant protein elevations in the alveolar lavage and serum of PAM mice and confirmed in serum from PAM patients identify serum MCP-1 (monocyte chemotactic protein 1) and SP-D (surfactant protein D) as potential biomarkers. Microliths introduced by adoptive transfer into the lungs of wild-type mice produce marked macrophage-rich inflammation and elevation of serum MCP-1 that peaks at 1 week and resolves at 1 month, concomitant with clearance of stones. Microliths isolated by bronchoalveolar lavage readily dissolve in EDTA, and therapeutic whole-lung EDTA lavage reduces the burden of stones in the lungs. A low-phosphate diet prevents microlith formation in young animals and reduces lung injury on the basis of reduction in serum SP-D. The burden of pulmonary calcium deposits in established PAM is also diminished within 4 weeks by a low-phosphate diet challenge. These data support a causative role for Npt2b in the pathogenesis of PAM and the use of the PAM mouse model as a preclinical platform for the development of biomarkers and therapeutic strategies.

Surfactant proteins, SP-A and SP-D, in respiratory fungal infections: their role in the inflammatory response

Pulmonary surfactant is a complex fluid that comprises phospholipids and four proteins (SP-A, SP-B, SP-C, and SP-D) with different biological functions. SP-B, SP-C, and SP-D are essential for the lungs’ surface tension function and for the organization, stability and metabolism of lung parenchyma. SP-A and SP-D, which are also known as pulmonary collectins, have an important function in the host’s lung immune response; they act as opsonins for different pathogens via a C-terminal carbohydrate recognition domain and enhance the attachment to phagocytic cells or show their own microbicidal activity by increasing the cellular membrane permeability. Interactions between the pulmonary collectins and bacteria or viruses have been extensively studied, but this is not the same for fungal pathogens. SP-A and SP-D bind glucan and mannose residues from fungal cell wall, but there is still a lack of information on their binding to other fungal carbohydrate residues. In addition, both their relation with immune cells for the clearance of these pathogens and the role of surfactant proteins’ regulation during respiratory fungal infections remain unknown. Here we highlight the relevant findings associated with SP-A and SP-D in those respiratory mycoses where the fungal infective propagules reach the lungs by the airways.

Keratinocyte growth factor supports pulmonary innate immune defense through maintenance of alveolar antimicrobial protein levels and macrophage function

Keratinocyte growth factor (KGF) is an epithelial mitogen that has been reported to protect the lungs from a variety of toxic and infectious insults. In prior studies we found that recombinant human KGF accelerates clearance of bacteria from the murine lung by augmenting the function of alveolar macrophages (AM). In this study we tested the hypothesis that endogenous KGF plays a role in the maintenance of innate pulmonary defense against gram-negative bacterial infections. KGF-deficient mice exhibited delayed clearance of Escherichia coli from the lungs, attenuated phagocytosis by AM, and decreased antimicrobial activity in bronchoalveolar lavage (BAL) fluid, due in part to reductions in levels of surfactant protein A, surfactant protein D, and lysozyme. These immune deficits were accompanied by lower alveolar type II epithelial cell counts and reduced alveolar type II epithelial cell expression of collectin and lysozyme genes on a per cell basis. No significant between-group differences were detected in selected inflammatory cytokines or BAL inflammatory cell populations at baseline or after bacterial challenge in the wild-type and KGF-deficient mice. A single intranasal dose of recombinant human KGF reversed defects in bacterial clearance, AM function, and BAL fluid antimicrobial activity. We conclude that KGF supports alveolar innate immune defense through maintenance of alveolar antimicrobial protein levels and functions of AM. Together these data demonstrate a role for endogenous KGF in maintenance of normal pulmonary innate immune function.

The Role of Surfactant in Lung Disease and Host Defense against Pulmonary Infections

Pulmonary surfactant is essential for life as it lines the alveoli to lower surface tension, thereby preventing atelectasis during breathing. Surfactant is enriched with a relatively unique phospholipid, termed dipalmitoylphosphatidylcholine, and four surfactant-associated proteins, SP-A, SP-B, SP-C, and SP-D. The hydrophobic proteins, SP-B and SP-C, together with dipalmitoylphosphatidylcholine, confer surface tension-lowering properties to the material. The more hydrophilic surfactant components, SP-A and SP-D, participate in pulmonary host defense and modify immune responses. Specifically, SP-A and SP-D bind and partake in the clearance of a variety of bacterial, fungal, and viral pathogens and can dampen antigen-induced immune function of effector cells. Emerging data also show immunosuppressive actions of some surfactant-associated lipids, such as phosphatidylglycerol. Conversely, microbial pathogens in preclinical models impair surfactant synthesis and secretion, and microbial proteinases degrade surfactant-associated proteins. Deficiencies of surfactant components are classically observed in the neonatal respiratory distress syndrome, where surfactant replacement therapies have been the mainstay of treatment. However, functional or compositional deficiencies of surfactant are also observed in a variety of acute and chronic lung disorders. Increased surfactant is seen in pulmonary alveolar proteinosis, a disorder characterized by a functional deficiency of the granulocyte-macrophage colony-stimulating factor receptor or development of granulocyte-macrophage colony-stimulating factor antibodies. Genetic polymorphisms of some surfactant proteins such as SP-C are linked to interstitial pulmonary fibrosis. Here, we briefly review the composition, antimicrobial properties, and relevance of pulmonary surfactant to lung disorders and present its therapeutic implications.

Evolution, Development, and Function of the Pulmonary Surfactant System in Normal and Perturbed Environments

Surfactant lipids and proteins form a surface active film at the air-liquid interface of internal gas exchange organs, including swim bladders and lungs. The system is uniquely positioned to meet both the physical challenges associated with a dynamically changing internal air-liquid interface, and the environmental challenges associated with the foreign pathogens and particles to which the internal surface is exposed. Lungs range from simple, transparent, bag-like units to complex, multilobed, compartmentalized structures. Despite this anatomical variability, the surfactant system is remarkably conserved. Here, we discuss the evolutionary origin of the surfactant system, which likely predates lungs. We describe the evolution of surfactant structure and function in invertebrates and vertebrates. We focus on changes in lipid and protein composition and surfactant function from its antiadhesive and innate immune to its alveolar stability and structural integrity functions. We discuss the biochemical, hormonal, autonomic, and mechanical factors that regulate normal surfactant secretion in mature animals. We present an analysis of the ontogeny of surfactant development among the vertebrates and the contribution of different regulatory mechanisms that control this development. We also discuss environmental (oxygen), hormonal and biochemical (glucocorticoids and glucose) and pollutant (maternal smoking, alcohol, and common "recreational" drugs) effects that impact surfactant development. On the adult surfactant system, we focus on environmental variables including temperature, pressure, and hypoxia that have shaped its evolution and we discuss the resultant biochemical, biophysical, and cellular adaptations. Finally, we discuss the effect of major modern gaseous and particulate pollutants on the lung and surfactant system.

Histoplasma capsulatum surmounts obstacles to intracellular pathogenesis

The fungal pathogen Histoplasma capsulatum causes respiratory and disseminated disease, even in immunocompetent hosts. In contrast to opportunistic pathogens, which are readily controlled by phagocytic cells, H. capsulatum yeasts are able to infect macrophages, survive antimicrobial defenses, and proliferate as an intracellular pathogen. In this review, we discuss some of the molecular mechanisms that enable H. capsulatum yeasts to overcome obstacles to intracellular pathogenesis. H. capsulatum yeasts gain refuge from extracellular obstacles such as antimicrobial lung surfactant proteins by engaging the β-integrin family of phagocytic receptors to promote entry into macrophages. In addition, H. capsulatum yeasts conceal immunostimulatory β-glucans to avoid triggering signaling receptors such as the β-glucan receptor Dectin-1. H. capsulatum yeasts counteract phagocyte-produced reactive oxygen species by expression of oxidative stress defense enzymes including an extracellular superoxide dismutase and an extracellular catalase. Within the phagosome, H. capsulatum yeasts block phagosome acidification, acquire essential metals such as iron and zinc, and utilize de novo biosynthesis pathways to overcome nutritional limitations. These mechanisms explain how H. capsulatum yeasts avoid and negate macrophage defense strategies and establish a hospitable intracellular niche, making H. capsulatum a successful intracellular pathogen of macrophages.

Identification and examination of a novel 9-bp insert/deletion polymorphism on porcine SFTPA1 exon 2 associated with acute lung injury using an oleic acid-acute lung injury model

The pulmonary surfactant-associated protein (SFTPA1, SP-A) gene has been studied as a candidate gene for lung disease resistance in humans and livestock. The objective of the present study was to identify polymorphisms of the porcine SFTPA1 gene coding region and its association with acute lung injury (ALI). Through DNA sequencing and the PCR-single-strand conformation polymorphism method, a novel 9-bp nucleotide insertion (+) or deletion (-) was detected on exon 2 of SFTPA1, which causes a change in three amino acids, namely, alanine (Ala), glycine (Gly) and proline (Pro). Individuals of three genotypes (-/-, +/- and +/+) were divided into equal groups from 60 Rongchang pigs that were genotyped. These pigs were selected for participation in the oleic acid (OA)-ALI model by 1-h and 3-h injections of OA, and there were equal numbers of pigs in the control and injection groups. The lung water content, a marker for acute lung injury, was measured in this study; there is a significant correlation between high lung water content and the presence of the 9-bp indel polymorphism (P < 0.01). The lung water content of the OA injection group was markedly higher than that of the control group and lung water content for the +/+ genotype was significantly higher than that of the others in the 1-h group (P < 0.01). No differences in the expression of the SFTPA1 gene were found among individuals with different SFTPA1 genotypes, indicating that the trait is not caused by a linked polymorphism causing altered expression of the gene. The individuals with the -/- genotype showed lower lung water content than the +/+ genotype pigs, which suggests that polymorphism could be a potential marker for lung disease-resistant pig breeding and that pig can be a potential animal model for human lung disease resistance in future studies.

Usefulness of the murine model to study the immune response against Histoplasma capsulatum infection

The present paper is an overview of the primary events that are associated with the histoplasmosis immune response in the murine model. Valuable data that have been recorded in the scientific literature have contributed to an improved understanding of the clinical course of this systemic mycosis, which is caused by the dimorphic fungus Histoplasma capsulatum. Data must be analyzed carefully, given that misinterpretation could be generated because most of the available information is based on experimental host-parasite interactions that used inappropriate proceedings, i.e., the non-natural route of infection with the parasitic and virulent fungal yeast-phase, which is not the usual infective phase of the etiological agent of this mycosis. Thus, due to their versatility, complexity, and similarities with humans, several murine models have played a fundamental role in exploring the host-parasite interaction during H. capsulatum infection.

The detection of surfactant proteins A, B, C and D in the human brain and their regulation in cerebral infarction, autoimmune conditions and infections of the CNS

Surfactant proteins (SP) have been studied intensively in the respiratory system. Surfactant protein A and surfactant protein D are proteins belonging to the family of collectins each playing a major role in the innate immune system. The ability of surfactant protein A and surfactant protein D to bind various pathogens and facilitate their elimination has been described in a vast number of studies. Surfactant proteins are very important in modulating the host's inflammatory response and participate in the clearance of apoptotic cells. Surfactant protein B and surfactant protein C are proteins responsible for lowering the surface tension in the lungs. The aim of this study was an investigation of expression of surfactant proteins in the central nervous system to assess their specific distribution patterns. The second aim was to quantify surfactant proteins in cerebrospinal fluid of healthy subjects compared to patients suffering from different neuropathologies. The expression of mRNA for the surfactant proteins was analyzed with RT-PCR done with samples from different parts of the human brain. The production of the surfactant proteins in the brain was verified using immunohistochemistry and Western blot. The concentrations of the surfactant proteins in cerebrospinal fluid from healthy subjects and patients suffering from neuropathologic conditions were quantified using ELISA. Our results revealed that surfactant proteins are present in the central nervous system and that the concentrations of one or more surfactant proteins in healthy subjects differed significantly from those of patients affected by central autoimmune processes, CNS infections or cerebral infarction. Based on the localization of the surfactant proteins in the brain, their different levels in normal versus pathologic samples of cerebrospinal fluid and their well-known functions in the lungs, it appears that the surfactant proteins may play roles in host defense of the brain, facilitation of cerebrospinal fluid secretion and maintenance of the latter's rheological properties.

Donor surfactant protein D (SP-D) polymorphisms are associated with lung transplant outcome

Chronic lung allograft dysfunction (CLAD) is the major factor limiting long-term success of lung transplantation. Polymorphisms of surfactant protein D (SP-D), an important molecule within lung innate immunity, have been associated with various lung diseases. We investigated the association between donor lung SP-D polymorphisms and posttransplant CLAD and survival in 191 lung transplant recipients consecutively transplanted. Recipients were prospectively followed with routine pulmonary function tests. Donor DNA was assayed by pyrosequencing for SP-D polymorphisms of two single-nucleotide variations altering amino acids in the mature protein N-terminal domain codon 11 (Met(11) Thr), and in codon 160 (Ala(160) Thr) of the C-terminal domain. CLAD was diagnosed in 88/191 patients, and 60/191 patients have died. Recipients of allografts that expressed the homozygous Met(11) Met variant of aa11 had significantly greater freedom from CLAD development and better survival compared to those with the homozygous Thr(11) Th variant of aa11. No significant association was noted for SP-D variants of aa160. Lung allografts with the SP-D polymorphic variant Thr(11) Th of aa11 are associated with development of CLAD and reduced survival. The observed genetic differences of the donor lung, potentially with their effects on innate immunity, may influence the clinical outcomes after lung transplantation.

Surfactant Protein D modulates HIV infection of both T-cells and dendritic cells

Surfactant Protein D (SP-D) is an oligomerized C-type lectin molecule with immunomodulatory properties and involvement in lung surfactant homeostasis in the respiratory tract. SP-D binds to the enveloped viruses, influenza A virus and respiratory syncytial virus and inhibits their replication in vitro and in vivo. SP-D has been shown to bind to HIV via the HIV envelope protein gp120 and inhibit infectivity in vitro. Here we show that SP-D binds to different strains of HIV (BaL and IIIB) and the binding occurs at both pH 7.4 and 5.0 resembling physiological relevant pH values found in the body and the female urogenital tract, respectively. The binding of SP-D to HIV particles and gp120 was inhibited by the presence of several hexoses with mannose found to be the strongest inhibitor. Competition studies showed that soluble CD4 and CVN did not interfere with the interaction between SP-D and gp120. However, soluble recombinant DC-SIGN was shown to inhibit the binding between SP-D and gp120. SP-D agglutinated HIV and gp120 in a calcium dependent manner. SP-D inhibited the infectivity of HIV strains at both pH values of 7.4 and 5.0 in a concentration dependent manner. The inhibition of the infectivity was abolished by the presence of mannose. SP-D enhanced the binding of HIV to immature monocyte derived dendritic cells (iMDDCs) and was also found to enhance HIV capture and transfer to the T-cell like line PM1. These results suggest that SP-D can bind to and inhibit direct infection of T-cells by HIV but also enhance the transfer of infectious HIV particles from DCs to T-cells in vivo.

Surfactant protein-A concentration in sera from dogs with pulmonary parenchymal diseases

Pulmonary surfactant protein A (SP-A) is used as a biomarker to understand the clinical features of pulmonary diseases and associated prognostic indices in human medicine. This study was conducted to investigate whether or not serum SP-A concentration can be used as a biomarker for identifying pulmonary parenchymal diseases in dogs. Thirty-two dogs with pulmonary parenchymal diseases, 34 with nonrespiratory diseases and 57 healthy dogs were included. Serum SP-A concentration was measured in all dogs using sandwich enzyme linked immunosorbent assay with an anti-dog SP-A polyclonal antibody. Median serum SP-A concentration in healthy dogs was <2.0 ng/ml, whereas that in dogs with aspiration pneumonia (n=11), primary lung tumors (n=9) and blunt traumatic lung injury (BTLI; n=12) was 3.1, 7.2 and 2.6 ng/ml, respectively; these values were significantly higher than those in healthy dogs. The serum SP-A concentration in dogs with nonrespiratory diseases was comparable with that in healthy dogs. No correlation was observed between the serum SP-A and plasma C-reactive protein concentrations in dogs with aspiration pneumonia and BTLI. There was a significant correlation between the serum SP-A concentration and thoracic radiographic changes in dogs with BTLI. These findings suggest that the serum SP-A concentration may be a useful clinical biomarker of alveolar damage that can be used for differential diagnosis of pulmonary parenchymal diseases and nonrespiratory diseases in dogs.

Surfactant protein D as a biomarker for chronic obstructive pulmonary disease

Clinical research in chronic obstructive pulmonary disease (COPD) has been hampered by the lack of validated blood biomarkers. The ideal COPD biomarker would have the following characteristics: (1) it would be a lung specific protein that could be assayed in blood; (2) it would change with disease severity or during exacerbations; (3) it would be specific for COPD; and would be responsive to change with effective treatments. One such candidate is the lung specific protein surfactant protein D (SP-D). In this review, we discuss the evidence supporting SP-D as a COPD biomarker.

An Insight into the Diverse Roles of Surfactant Proteins, SP-A and SP-D in Innate and Adaptive Immunity

Surfactant proteins SP-A and SP-D are hydrophilic, collagen-containing calcium-dependent lectins, which appear to have a range of innate immune functions at pulmonary as well as extrapulmonary sites. These proteins bind to target ligands on pathogens, allergens, and apoptotic cells, via C-terminal homotrimeric carbohydrate recognition domains, while the collagen region brings about the effector functions via its interaction with cell surface receptors. SP-A and SP-D deal with various pathogens, using a range of innate immune mechanisms such as agglutination/aggregation, enhancement of phagocytosis, and killing mechanisms by phagocytic cells and direct growth inhibition. SP-A and SP-D have also been shown to be involved in the control of pulmonary inflammation including allergy and asthma. Emerging evidence suggest that SP-A and SP-D are capable of linking innate immunity with adaptive immunity that includes modulation of dendritic cell function and helper T cell polarization. This review enumerates immunological properties of SP-A and SP-D inside and outside lungs and discusses their importance in human health and disease.

Diverse functions of pulmonary collectins in host defense of the lung

Pulmonary surfactant is a mixture of lipids and proteins that covers alveolar surfaces and keeps alveoli from collapsing. Four specific proteins have been identified in surfactant. Among them, two C-type lectins, surfactant proteins A and D (SP-A and SP-D), are known to be implicated in host defense and regulation of inflammatory responses of the lung. These host defense lectins are structurally characterized by N-terminal collagen-like domains and lectin domains and are called pulmonary collectins. They prevent dissemination of infectious microbes by their biological activities including agglutination and growth inhibition. They also promote clearance of microbes by enhancing phagocytosis in macrophages. In addition, they interact with the other pattern-recognition molecules, including Toll-like receptors (TLRs) and TLR-associated molecules, CD14 and MD-2, and regulate inflammatory responses. Furthermore, recent studies have demonstrated that these collectins modulate functions of neutrophil-derived innate immune molecules by interacting with them. These findings indicate that pulmonary collectins play critical roles in host defense of the lung.

Pseudomonas aeruginosa elastase provides an escape from phagocytosis by degrading the pulmonary surfactant protein-A

Pseudomonas aeruginosa is an opportunistic pathogen that causes both acute pneumonitis in immunocompromised patients and chronic lung infections in individuals with cystic fibrosis and other bronchiectasis. Over 75% of clinical isolates of P. aeruginosa secrete elastase B (LasB), an elastolytic metalloproteinase that is encoded by the lasB gene. Previously, in vitro studies have demonstrated that LasB degrades a number of components in both the innate and adaptive immune systems. These include surfactant proteins, antibacterial peptides, cytokines, chemokines and immunoglobulins. However, the contribution of LasB to lung infection by P. aeruginosa and to inactivation of pulmonary innate immunity in vivo needs more clarification. In this study, we examined the mechanisms underlying enhanced clearance of the ΔlasB mutant in mouse lungs. The ΔlasB mutant was attenuated in virulence when compared to the wild-type strain PAO1 during lung infection in SP-A+/+ mice. However, the ΔlasB mutant was as virulent as PAO1 in the lungs of SP-A⁻/⁻ mice. Detailed analysis showed that the ΔlasB mutant was more susceptible to SP-A-mediated opsonization but not membrane permeabilization. In vitro and in vivo phagocytosis experiments revealed that SP-A augmented the phagocytosis of ΔlasB mutant bacteria more efficiently than the isogenic wild-type PAO1. The ΔlasB mutant was found to have a severely reduced ability to degrade SP-A, consequently making it unable to evade opsonization by the collectin during phagocytosis. These results suggest that P. aeruginosa LasB protects against SP-A-mediated opsonization by degrading the collectin.

Histoplasma mechanisms of pathogenesis--one portfolio doesn't fit all

Histoplasma capsulatum is the leading cause of endemic mycosis in the world. Analyses of clinical isolates from different endemic regions show important diversity within the species. Recent molecular studies of two isolates, the Chemotype I NAm2 strain G217B and the Chemotype II Panamanian strain G186A, reveal significant genetic, structural, and molecular differences between these representative Histoplasma strains. Some of these variations have functional consequences, representing distinct molecular mechanisms that facilitate Histoplasma pathogenesis. The realization of Histoplasma strain diversity highlights the importance of characterizing Histoplasma virulence factors in the context of specific clinical strain isolates.

Chronic mucocutaneous candidiasis and congenital susceptibility to Candida

PURPOSE OF REVIEW

To give an overview on the clinical spectrum and the molecular background of host defence against Candida.

RECENT FINDINGS

For many decades the molecular causes and the pathogenesis for an increased susceptibility to Candida - and fungal infections in general - have been elusive. In 2009 and 2010 interesting reports on the genetic background and the pathomechanisms involved in chronic mucocutaneous candidiasis (CMC) have been published.

SUMMARY

The susceptibility to recurrent Candida infections can be a monogenetic Mendelian trait. The sensing of Candida cell wall components and the consecutive intracellular signalling in myeloid cells via CARD9, but also the role of Th17 cells and their cytokines take centre stage in the human host defence against Candida.

The Pseudomonas aeruginosa flagellum confers resistance to pulmonary surfactant protein-A by impacting the production of exoproteases through quorum-sensing

Surfactant protein-A (SP-A) is an important antimicrobial protein that opsonizes and permeabilizes membranes of microbial pathogens in mammalian lungs. Previously, we have shown that Pseudomonas aeruginosa flagellum-deficient mutants are preferentially cleared in the lungs of wild-type mice by SP-A-mediated membrane permeabilization, and not by opsonization. In this study, we report a flagellum-mediated mechanism of P. aeruginosa resistance to SP-A. We discovered that flagellum-deficient (ΔfliC) bacteria are unable to produce adequate amounts of exoproteases to degrade SP-A in vitro and in vivo, leading to its preferential clearance in the lungs of SP-A(+/+) mice. In addition, ΔfliC bacteria failed to degrade another important lung antimicrobial protein lysozyme. Detailed analyses showed that ΔfliC bacteria are unable to upregulate the transcription of lasI and rhlI genes, impairing the production of homoserine lactones necessary for quorum-sensing, an important virulence process that regulates the production of multiple exoproteases. Thus, reduced ability of ΔfliC bacteria to quorum-sense attenuates production of exoproteases and limits degradation of SP-A, thereby conferring susceptibility to this major pulmonary host defence protein.

Smoking reduces surfactant protein D and phospholipids in patients with and without chronic obstructive pulmonary disease

Background

Pulmonary surfactant D (SP-D) has important regulatory functions for innate immunity and has been implicated as a biomarker for chronic obstructive pulmonary disease (COPD). We hypothesized that COPD patients would have reduced bronchoalveolar lavage (BAL) fluid SP-D levels compared to healthy smoking and non-smoking controls.

Methods

BAL SP-D and phospholipids were quantified and corrected for dilution in 110 subjects (65 healthy never smokers, 23 smokers with normal spirometry, and 22 smokers with COPD).

Results

BAL SP-D was highest in never smokers (mean 51.9 μg/mL ± 7.1 μg/mL standard error) compared to both smokers with normal spirometry (16.0 μg/mL ± 11.8 μg/mL) and subjects with COPD (19.1 μg/mL ± 12.9 μg/mL; P < 0.0001). Among smokers with COPD, BAL SP-D correlated significantly with FEV₁% predicted (R = 0.43; P < 0.05); however, the strongest predictor of BAL SP-D was smoking status. BAL SP-D levels were lowest in current smokers (12.8 μg/mL ± 11.0 μg/mL), intermediate in former smokers (25.2 μg/mL ± 14.2 μg/mL; P < 0.008), and highest in never smokers. BAL phospholipids were also lowest in current smokers (6.5 nmol ± 1.5 nmol), intermediate in former smokers (13.1 nmol ± 2.1 nmol), and highest in never smokers (14.8 nmol ± 1.1 nmol; P < 0.0001).

Conclusions

These data suggest that smokers, and especially current smokers, exhibit significantly reduced BAL SP-D and phospholipids compared to nonsmokers. Our findings may help better explain the mechanism that leads to the rapid progression of disease and increased incidence of infection in smokers.

Review: Structural determinants of pattern recognition by lung collectins

Host defense roles for the lung collectins, surfactant protein A (SP-A) and surfactant protein D (SP-D), were first suspected in the 1980s when molecular characterization revealed their sequence homology to the acute phase reactant of serum, mannose-binding lectin. Surfactant protein A and SP-D have since been shown to play diverse and important roles in innate immunity and pulmonary homeostasis. Their location in surfactant ideally positions them to interact with air-space pathogens. Despite extensive structural similarity, the two proteins show many functional differences and considerable divergence in their interactions with microbial surface components, surfactant lipids, and other ligands. Recent crystallographic studies have provided many new insights relating to these observed differences. Although both proteins can participate in calcium-dependent interactions with sugars and other polyols, they display significant differences in the spatial orientation, charge, and hydrophobicity of their binding surfaces. Surfactant protein D appears particularly adapted to interactions with complex carbohydrates and anionic phospholipids, such as phosphatidylinositol. By contrast, SP-A shows features consistent with its preference for lipid ligands, including lipid A and the major surfactant lipid, dipalmitoylphosphatidylcholine. Current research suggests that structural biology approaches will help to elucidate the molecular basis of pulmonary collectin-ligand recognition and facilitate development of new therapeutics based upon SP-A and SP-D.

Current understanding of Pneumocystis immunology

Pneumocystis jirovecii is the opportunistic fungal organism that causes Pneumocystis pneumonia (PCP) in humans. Similar to other opportunistic pathogens, Pneumocystis causes disease in individuals who are immunocompromised, particularly those infected with HIV. PCP remains the most common opportunistic infection in patients with AIDS. Incidence has decreased greatly with the advent of HAART. However, an increase in the non-HIV immunocompromised population, noncompliance with current treatments, emergence of drug-resistant strains and rise in HIV(+) cases in developing countries makes Pneumocystis a pathogen of continued interest and a public health threat. A great deal of research interest has addressed therapeutic interventions to boost waning immunity in the host to prevent or treat PCP. This article focuses on research conducted during the previous 5 years regarding the host immune response to Pneumocystis, including innate, cell-mediated and humoral immunity, and associated immunotherapies tested against PCP.

Pulmonary collectins protect macrophages against pore-forming activity of Legionella pneumophila and suppress its intracellular growth

Pulmonary collectins, surfactant proteins A (SP-A) and D (SP-D), play important roles in innate immunity of the lung. Legionella pneumophila is a bacterial respiratory pathogen that can replicate within macrophages and causes opportunistic infections. L. pneumophila possesses cytolytic activity, resulting from insertion of pores in the macrophage membrane upon contact. We examined whether pulmonary collectins play protective roles against L. pneumophila infection. SP-A and SP-D bound to L. pneumophila and its lipopolysaccharide (LPS) and inhibited the bacterial growth in a Ca(2+)-dependent manner. The addition of LPS in the culture blocked the inhibitory effects on L. pneumophila growth by the collectins, indicating the importance of LPS-collectin interaction. When differentiated THP-1 cells were infected with L. pneumophila in the presence of SP-A and SP-D, the number of permeable cells was significantly decreased, indicating that pulmonary collectins inhibit pore-forming activity of L. pneumophila. The number of live bacteria within the macrophages on days 1-4 after infection was significantly decreased when infection was performed in the presence of pulmonary collectins. The phagocytosis experiments with the pH-sensitive dye-labeled bacteria revealed that pulmonary collectins promoted bacterial localization to an acidic compartment. In addition, SP-A and SP-D significantly increased the number of L. pneumophila co-localized with LAMP-1. These results indicate that pulmonary collectins protect macrophages against contact-dependent cytolytic activity of L. pneumophila and suppress intracellular growth of the phagocytosed bacteria. The promotion of lysosomal fusion with Legionella-containing phagosomes constitutes a likely mechanism of L. pneumophila growth suppression by the collectins.

Pulmonary surfactant: an immunological perspective

Pulmonary surfactant has two crucial roles in respiratory function; first, as a biophysical entity it reduces surface tension at the air water interface, facilitating gas exchange and alveolar stability during breathing, and, second, as an innate component of the lung's immune system it helps maintain sterility and balance immune reactions in the distal airways. Pulmonary surfactant consists of 90% lipids and 10% protein. There are four surfactant proteins named SP-A, SP-B, SP-C, and SP-D; their distinct interactions with surfactant phospholipids are necessary for the ultra-structural organization, stability, metabolism, and lowering of surface tension. In addition, SP-A and SP-D bind pathogens, inflict damage to microbial membranes, and regulate microbial phagocytosis and activation or deactivation of inflammatory responses by alveolar macrophages. SP-A and SP-D, also known as pulmonary collectins, mediate microbial phagocytosis via SP-A and SP-D receptors and the coordinated induction of other innate receptors. Several receptors (SP-R210, CD91/calreticulin, SIRPalpha, and toll-like receptors) mediate the immunological functions of SP-A and SP-D. However, accumulating evidence indicate that SP-B and SP-C and one or more lipid constituents of surfactant share similar immuno-regulatory properties as SP-A and SP-D. The present review discusses current knowledge on the interaction of surfactant with lung innate host defense.