Porcine pulmonary collectins show distinct interactions with influenza A viruses: role of the N-linked oligosaccharides in the carbohydrate recognition domain

Multivalent, calcium-independent binding of surfactant protein A and D to sulfated glycosaminoglycans of the alveolar epithelial glycocalyx

Lung surfactant collectins, surfactant protein A (SP-A) and D (SP-D), are oligomeric C-type lectins involved in lung immunity. Through their carbohydrate recognition domain, they recognize carbohydrates at pathogen surfaces and initiate lung innate immune response. Here, we propose that they may also be able to bind to other carbohydrates present in typical cell surfaces, such as the alveolar epithelial glycocalyx. To test this hypothesis, we analyzed and quantified the binding affinity of SP-A and SP-D to different sugars and glycosaminoglycans (GAGs) by microscale thermophoresis (MST). In addition, by changing the calcium concentration, we aimed to characterize any consequences on the binding behavior. Our results show that both oligomeric proteins bind with high affinity (in nanomolar range) to GAGs, such as hyaluronan (HA), heparan sulfate (HS) and chondroitin sulfate (CS). Binding to HS and CS was calcium-independent, as it was not affected by changing calcium concentration in the buffer. Quantification of GAGs in bronchoalveolar lavage (BAL) fluid from animals deficient in either SP-A or SP-D showed changes in GAG composition, and electron micrographs showed differences in alveolar glycocalyx ultrastructure in vivo. Taken together, SP-A and SP-D bind to model sulfated glycosaminoglycans of the alveolar epithelial glycocalyx in a multivalent and calcium-independent way. These findings provide a potential mechanism for SP-A and SP-D as an integral part of the alveolar epithelial glycocalyx binding and interconnecting free GAGs, proteoglycans, and other glycans in glycoproteins, which may influence glycocalyx composition and structure.NEW & NOTEWORTHY SP-A and SP-D function has been related to innate immunity of the lung based on their binding to sugar residues at pathogen surfaces. However, their function in the healthy alveolus was considered as limited to interaction with surfactant lipids. Here, we demonstrated that these proteins bind to glycosaminoglycans present at typical cell surfaces like the alveolar epithelial glycocalyx. We propose a model where these proteins play an important role in interconnecting alveolar epithelial glycocalyx components.

Preparation of eicosavalent triazolylsialoside-conjugated human serum albumin as a dual hemagglutinin/neuraminidase inhibitor and virion adsorbent for the prevention of influenza infection

A triazolylsialoside-human serum albumin conjugate was prepared as a multivalent hemagglutinin and neuraminidase inhibitor using a di-(N-succinimidyl) adipate strategy. Matrix-Assisted Laser Desorption/Ionization-Time of Flight-Mass Spectrometry (MALDI-TOF-MS) indicated that five tetravalent sialyl galactosides were grafted onto the protein backbone resulting in an eicosavalent triazolylsialoside-protein complex. Compared with monomeric sialic acid, molecular interaction studies showed that the synthetic pseudo-glycoprotein bound tightly not only to hemagglutinin (HA)/neuraminidase (NA) but also to mutated drug-resistant NA on the surface of the influenza virus with a dissociation constant (KD) in the 1 μM range, attributed to the cluster effect. Moreover, this glycoconjugate exhibited potent antiviral activity against a broad spectrum of virus strains and showed no cytotoxicity towards Human Umbilical Vein Endothelial Cells (HUVECs) and Madin-Darby canine kidney (MDCK) cells at high concentrations. Further mechanistic studies demonstrated this multivalent sialyl conjugate showed strong capture and trapping of influenza virions, thus disrupting the ability of the influenza virus to infect host cells. This research lays the experimental foundation for the development of new antiviral agents based on multivalent sialic acid-protein conjugates.

Labeled TEMPO-Oxidized Mannan Differentiates Binding Profiles within the Collectin Families

Establishing the rapid and accurate diagnosis of sepsis is a key component to the improvement of clinical outcomes. The ability of analytical platforms to rapidly detect pathogen-associated molecular patterns (PAMP) in blood could provide a powerful host-independent biomarker of sepsis. A novel concept was investigated based on the idea that a pre-bound and fluorescent ligand could be released from lectins in contact with high-affinity ligands (such as PAMPs). To create fluorescent ligands with precise avidity, the kinetically followed TEMPO oxidation of yeast mannan and carbodiimide coupling were used. The chemical modifications led to decreases in avidity between mannan and human collectins, such as the mannan-binding lectin (MBL) and human surfactant protein D (SP-D), but not in porcine SP-D. Despite this effect, these fluorescent derivatives were captured by human lectins using highly concentrated solutions. The resulting fluorescent beads were exposed to different solutions, and the results showed that displacements occur in contact with higher affinity ligands, proving that two-stage competition processes can occur in collectin carbohydrate recognition mechanisms. Moreover, the fluorescence loss depends on the discrepancy between the respective avidities of the recognized ligand and the fluorescent mannan. Chemically modulated fluorescent ligands associated with a diversity of collectins may lead to the creation of diagnostic tools suitable for multiplex array assays and the identification of high-avidity ligands.

Changes in the Hemagglutinin and Internal Gene Segments Were Needed for Human Seasonal H3 Influenza A Virus to Efficiently Infect and Replicate in Swine

The current diversity of influenza A viruses (IAV) circulating in swine is largely a consequence of human-to-swine transmission events and consequent evolution in pigs. However, little is known about the requirements for human IAVs to transmit to and subsequently adapt in pigs. Novel human-like H3 viruses were detected in swine herds in the U.S. in 2012 and have continued to circulate and evolve in swine. We evaluated the contributions of gene segments on the ability of these viruses to infect pigs by using a series of in vitro models. For this purpose, reassortant viruses were generated by reverse genetics (rg) swapping the surface genes (hemagglutinin-HA and neuraminidase-NA) and internal gene segment backbones between a human-like H3N1 isolated from swine and a seasonal human H3N2 virus with common HA ancestry. Virus growth kinetics in porcine intestinal epithelial cells (SD-PJEC) and in ex-vivo porcine trachea explants were significantly reduced by replacing the swine-adapted HA with the human seasonal HA. Unlike the human HA, the swine-adapted HA demonstrated more abundant attachment to epithelial cells throughout the swine respiratory tract by virus histochemistry and increased entry into SD-PJEC swine cells. The human seasonal internal gene segments improved replication of the swine-adapted HA at 33 °C, but decreased replication at 40 °C. Although the HA was crucial for the infectivity in pigs and swine tissues, these results suggest that the adaptation of human seasonal H3 viruses to swine is multigenic and that the swine-adapted HA alone was not sufficient to confer the full phenotype of the wild-type swine-adapted virus.

Roles of Glycans and Non-glycans on the Epithelium and in the Immune System in H1-H18 Influenza A Virus Infections

The large variation of influenza A viruses (IAVs) in various susceptible hosts and their rapid evolution, which allows host/tissue switching, host immune escape, vaccine escape, and drug resistance, are difficult challenges for influenza control in all countries worldwide. Access and binding of the IAV to actual receptors at endocytic sites is critical for the establishment of influenza infection. In this chapter, the progress in identification of and roles of glycans and non-glycans on the epithelium and in the immune system in H1-H18 IAV infections are reviewed. The first part of the review is on current knowledge of H1-H16 IAV receptors on the epithelium including sialyl glycans, other negatively charged glycans, and annexins. The second part of the review focuses on H1-H16 IAV receptors in the immune system including acidic surfactant phospholipids, Sia on surfactant proteins, the carbohydrate recognition domain (CRD) of surfactant proteins, Sia on mucins, Sia and C-type lectins on macrophages and dendritic cells, and Sia on NK cells. The third part of the review is about a possible H17-H18 IAV receptor. Binding of these receptors to IAVs may result in inhibition or enhancement of IAV infection depending on their location, host cell type, and IAV strain. Among these receptors, host sialyl glycans are key determinants of viral hemagglutinin (HA) lectins for H1-H16 infections. HA must acquire mutations to bind to sialyl glycans that are dominant on a new target tissue when switching to a new host for efficient transmission and to bind to long sialyl glycans found in the case of seasonal HAs with multiple glycosylation sites as a consequence of immune evasion. Although sialyl receptors/C-type lectins on immune cells are decoy receptors/pathogen recognition receptors for capturing viral HA lectin/glycans protecting HA antigenic sites, some IAV strains do not escape, such as by release with neuraminidase, but hijack these molecules to gain entry and replication in immune cells. An understanding of the virus-host battle tactics at the receptor level might lead to the establishment of novel strategies for effective control of influenza.

Emerging and state of the art hemagglutinin-targeted influenza virus inhibitors

Introduction: Seasonal influenza vaccination, together with FDA-approved neuraminidase (NA) and polymerase acidic (PA) inhibitors, is the most effective way for prophylaxis and treatment of influenza infections. However, the low efficacy of prevailing vaccines to newly emerging influenza strains and increasing resistance to available drugs drives intense research to explore more effective inhibitors. Hemagglutinin (HA), one of the major surface proteins of influenza strains, represents an attractive therapeutic target to develop such new inhibitors.Areas covered: This review summarizes the current progress of HA-based influenza virus inhibitors and their mechanisms of action, which may facilitate further research in developing novel antiviral inhibitors for controlling influenza infections.Expert opinion: HA-mediated entry of influenza virus is an essential step for successful infection of the host, which makes HA a promising target for the development of antiviral drugs. Recent progress in delineating the crystal structures of HA, especially HA-inhibitors complexes, has revealed a number of key residues and conserved binding pockets within HA. This has opened up important insights for developing HA-based antiviral inhibitors that have a high resistance barrier and broad-spectrum activities.

Innate Immunity and Influenza A Virus Pathogenesis: Lessons for COVID-19

There is abundant evidence that the innate immune response to influenza A virus (IAV) is highly complex and plays a key role in protection against IAV induced infection and illness. Unfortunately it also clear that aspects of innate immunity can lead to severe morbidity or mortality from IAV, including inflammatory lung injury, bacterial superinfection, and exacerbation of reactive airways disease. We review broadly the virus and host factors that result in adverse outcomes from IAV and show evidence that inflammatory responses can become damaging even apart from changes in viral replication per se, with special focus on the positive and adverse effects of neutrophils and monocytes. We then evaluate in detail the role of soluble innate inhibitors including surfactant protein D and antimicrobial peptides that have a potential dual capacity for down-regulating viral replication and also inhibiting excessive inflammatory responses and how these innate host factors could possibly be harnessed to treat IAV infection. Where appropriate we draw comparisons and contrasts the SARS-CoV viruses and IAV in an effort to point out where the extensive knowledge existing regarding severe IAV infection could help guide research into severe COVID 19 illness or vice versa.

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.

Enhanced Antiviral Activity of Human Surfactant Protein D by Site-Specific Engineering of the Carbohydrate Recognition Domain

Innate immunity is critical in the early containment of influenza A virus (IAV) infection and surfactant protein D (SP-D) plays a crucial role in innate defense against IAV in the lungs. Multivalent lectin-mediated interactions of SP-D with IAVs result in viral aggregation, reduced epithelial infection, and enhanced IAV clearance by phagocytic cells. Previous studies showed that porcine SP-D (pSP-D) exhibits distinct antiviral activity against IAV as compared to human SP-D (hSP-D), mainly due to key residues in the lectin domain of pSP-D that contribute to its profound neutralizing activity. These observations provided the basis for the design of a full-length recombinant mutant form of hSP-D, designated as “improved SP-D” (iSP-D). Inspired by pSP-D, the lectin domain of iSP-D has 5 amino acids replaced (Asp324Asn, Asp330Asn, Val251Glu, Lys287Gln, Glu289Lys) and 3 amino acids inserted (326Gly-Ser-Ser). Characterization of iSP-D revealed no major differences in protein assembly and saccharide binding selectivity as compared to hSP-D. However, hemagglutination inhibition measurements showed that iSP-D expressed strongly enhanced activity compared to hSP-D against 31 different IAV strains tested, including (pandemic) IAVs that were resistant for neutralization by hSP-D. Furthermore, iSP-D showed increased viral aggregation and enhanced protection of MDCK cells against infection by IAV. Importantly, prophylactic or therapeutic application of iSP-D decreased weight loss and reduced viral lung titers in a murine model of IAV infection using a clinical isolate of H1N1pdm09 virus. These studies demonstrate the potential of iSP-D as a novel human-based antiviral inhalation drug that may provide immediate protection against or recovery from respiratory (pandemic) IAV infections in humans.

Adaptation of Human Influenza Viruses to Swine

A large diversity of influenza A viruses (IAV) within the H1N1/N2 and H3N2 subtypes circulates in pigs globally, with different lineages predominating in specific regions of the globe. A common characteristic of the ecology of IAV in swine in different regions is the periodic spillover of human seasonal viruses. Such human viruses resulted in sustained transmission in swine in several countries, leading to the establishment of novel IAV lineages in the swine host and contributing to the genetic and antigenic diversity of influenza observed in pigs. In this review we discuss the frequent occurrence of reverse-zoonosis of IAV from humans to pigs that have contributed to the global viral diversity in swine in a continuous manner, describe host-range factors that may be related to the adaptation of these human-origin viruses to pigs, and how these events could affect the swine industry.

The Role and Molecular Mechanism of Action of Surfactant Protein D in Innate Host Defense Against Influenza A Virus

Influenza A viruses (IAVs) continue to pose major risks of morbidity and mortality during yearly epidemics and periodic pandemics. The genomic instability of IAV allows it to evade adaptive immune responses developed during prior infection. Of particular concern are pandemics which result from wholesale incorporation of viral genome sections from animal sources. These pandemic strains are radically different from circulating human strains and pose great risk for the human population. For these reasons, innate immunity plays a strong role in the initial containment of IAV infection. Soluble inhibitors present in respiratory lining fluids and blood provide a level of early protection against IAV. In general, these inhibitors act by binding to the viral hemagglutinin (HA). Surfactant protein D (SP-D) and mannose-binding lectin (MBL) attach to mannosylated glycans on the HA in a calcium dependent manner. In contrast, surfactant protein A, ficolins, and other inhibitors present sialic acid rich ligands to which the HA can bind. Among these inhibitors, SP-D seems to be the most potent due to its specific mode of binding to viral carbohydrates and its ability to strongly aggregate viral particles. We have studied specific properties of the N-terminal and collagen domain of SP-D that enable formation of highly multimerized molecules and cooperative binding among the multiple trimeric lectin domains in the protein. In addition, we have studied in depth the lectin activity of SP-D through expression of isolated lectin domains and targeted mutations of the SP-D lectin binding site. Through modifying specific residues around the saccharide binding pocket, antiviral activity of isolated lectin domains of SP-D can be markedly increased for seasonal strains of IAV. Wild-type SP-D causes little inhibition of pandemic IAV, but mutated versions of SP-D were able to inhibit pandemic IAV through enhanced binding to the reduced number of mannosylated glycans present on the HA of these strains. Through collaborative studies involving crystallography of isolated lectin domains of SP-D, glycomics analysis of the HA, and molecular modeling, the mechanism of binding of wild type and mutant forms of SP-D have been determined. These studies could guide investigation of the interactions of SP-D with other pathogens.

Lectin-mediated binding and sialoglycans of porcine surfactant protein D synergistically neutralize influenza A virus

Innate immunity is critical in the early containment of influenza A virus (IAV) infection, and surfactant protein D (SP-D) plays a crucial role in the pulmonary defense against IAV. In pigs, which are important intermediate hosts during the generation of pandemic IAVs, SP-D uses its unique carbohydrate recognition domain (CRD) to interact with IAV. An N-linked CRD glycosylation provides interactions with the sialic acid-binding site of IAV, and a tripeptide loop at the lectin-binding site facilitates enhanced interactions with IAV glycans. Here, to investigate both mechanisms of IAV neutralization in greater detail, we produced an N-glycosylated neck-CRD fragment of porcine SP-D (RpNCRD) in HEK293 cells. X-ray crystallography disclosed that the N-glycan did not alter the CRD backbone structure, including the lectin site conformation, but revealed a potential second nonlectin-binding site for glycans. IAV hemagglutination inhibition, IAV aggregation, and neutralization of IAV infection studies showed that RpNCRD, unlike the human analogue RhNCRD, exhibits potent neutralizing activity against pandemic A/Aichi/68 (H3N2), enabled by both porcine-specific structural features of its CRD. MS analysis revealed an N-glycan site-occupancy of >98% at Asn-303 of RpNCRD with complex-type, heterogeneously branched and predominantly α(2,3)-sialylated oligosaccharides. Glycan-binding array data characterized both RpNCRD and RhNCRD as mannose-type lectins. RpNCRD also bound Lewis^Y structures, whereas RhNCRD bound polylactosamine-containing glycans. The presence of the N-glycan in the CRD increases the glycan-binding specificity of RpNCRD. These insights increase our understanding of porcine-specific innate defense against pandemic IAV and may inform the design of recombinant SP-D-based antiviral drugs.

Neutralizing inhibitors in the airways of naïve ferrets do not play a major role in modulating the virulence of H3 subtype influenza A viruses

Many insights regarding the pathogenesis of human influenza A virus (IAV) infections have come from studies in mice and ferrets. Surfactant protein (SP)-D is the major neutralizing inhibitor of IAV in mouse airway fluids and SP-D-resistant IAV mutants show enhanced virus replication and virulence in mice. Herein, we demonstrate that sialylated glycoproteins, rather than SP-D, represent the major neutralizing inhibitors against H3 subtype viruses in airway fluids from naïve ferrets. Moreover, while resistance to neutralizing inhibitors is a critical factor in modulating virus replication and disease in the mouse model, it does not appear to be so in the ferret model, as H3 mutants resistant to either SP-D or sialylated glycoproteins in ferret airway fluids did not show enhanced virulence in ferrets. These data have important implications for our understanding of pathogenesis and immunity to human IAV infections in these two widely used animal models of infection.

Assessment of the antiviral properties of recombinant surfactant protein D against influenza B virus in vitro

The armamentarium of antiviral drugs against influenza viruses is limited. Furthermore, influenza viruses emerge that are resistant to existing antiviral drugs like the M2 and NA inhibitors. Therefore, there is an urgent need for the development of novel classes of antiviral drugs. Here we investigated the antiviral properties of recombinant porcine surfactant protein D (RpSP-D), an innate defense molecule with lectin properties, against influenza B viruses. We have previously shown that porcine SP-D has more potent neutralizing activity against influenza A viruses than human SP-D. Here we show that RpSP-D neutralizes influenza B viruses efficiently and inhibited the binding of these viruses to epithelial cells of the human trachea.

Recombinant porcine surfactant protein D inhibits influenza A virus replication ex vivo

Influenza is a major burden to public health. Due to high mutation rates and selection pressure, mutant viruses emerge which are resistant to currently used antiviral drugs. Therefore, there is a need for the development of novel classes of antiviral drugs that suffer less from the emergence of resistant viruses. Antiviral drugs based on collectin-like surfactant protein D (SP-D) may fulfil these requirements. Especially porcine SP-D displays strong antiviral activity to influenza A viruses. In the present study the antiviral activity of recombinant porcine SP-D was investigated in ex vivo cultures of respiratory tract tissue infected with human influenza A virus of the H3N2 subtype. Porcine SP-D has antiviral activity in these test systems. It is suggested that porcine SP-D may be used as a venue to develop a novel class of antiviral drugs.

Bitter-sweet symphony: glycan-lectin interactions in virus biology

Glycans are carbohydrate modifications typically found on proteins or lipids, and can act as ligands for glycan-binding proteins called lectins. Glycans and lectins play crucial roles in the function of cells and organs, and in the immune system of animals and humans. Viral pathogens use glycans and lectins that are encoded by their own or the host genome for their replication and spread. Recent advances in glycobiological research indicate that glycans and lectins mediate key interactions at the virus-host interface, controlling viral spread and/or activation of the immune system. This review reflects on glycan–lectin interactions in the context of viral infection and antiviral immunity. A short introduction illustrates the nature of glycans and lectins, and conveys the basic principles of their interactions. Subsequently, examples are discussed highlighting specific glycan–lectin interactions and how they affect the progress of viral infections, either benefiting the host or the virus. Moreover, glycan and lectin variability and their potential biological consequences are discussed. Finally, the review outlines how recent advances in the glycan–lectin field might be transformed into promising new approaches to antiviral therapy.

The amazing innate immune response to influenza A virus infection

Influenza A viruses (IAVs) remain a major health threat and a prime example of the significance of innate immunity. Our understanding of innate immunity to IAV has grown dramatically, yielding new concepts that change the way we view innate immunity as a whole. Examples include the role of p53, autophagy, microRNA, innate lymphocytes, endothelial cells and gut commensal bacteria in pulmonary innate immunity. Although the innate response is largely beneficial, it also contributes to major complications of IAV, including lung injury, bacterial super-infection and exacerbation of reactive airways disease. Research is beginning to dissect out which components of the innate response are helpful or harmful. IAV uses its limited genetic complement to maximum effect. Several viral proteins are dedicated to combating innate responses, while other viral structural or replication proteins multitask as host immune modulators. Many host innate immune proteins also multitask, having roles in cell cycle, signaling or normal lung biology. We summarize the plethora of new findings and attempt to integrate them into the larger picture of how humans have adapted to the threat posed by this remarkable virus. We explore how our expanded knowledge suggests ways to modulate helpful and harmful inflammatory responses, and develop novel treatments.

Serum amyloid P is a sialylated glycoprotein inhibitor of influenza A viruses

Members of the pentraxin family, including PTX3 and serum amyloid P component (SAP), have been reported to play a role in innate host defence against a range of microbial pathogens, yet little is known regarding their antiviral activities. In this study, we demonstrate that human SAP binds to human influenza A virus (IAV) strains and mediates a range of antiviral activities, including inhibition of IAV-induced hemagglutination (HA), neutralization of virus infectivity and inhibition of the enzymatic activity of the viral neuraminidase (NA). Characterization of the anti-IAV activity of SAP after periodate or bacterial sialidase treatment demonstrated that α(2,6)-linked sialic acid residues on the glycosidic moiety of SAP are critical for recognition by the HA of susceptible IAV strains. Other proteins of the innate immune system, namely human surfactant protein A and porcine surfactant protein D, have been reported to express sialylated glycans which facilitate inhibition of particular IAV strains, yet the specific viral determinants for recognition of these inhibitors have not been defined. Herein, we have selected virus mutants in the presence of human SAP and identified specific residues in the receptor-binding pocket of the viral HA which are critical for recognition and therefore susceptibility to the antiviral activities of SAP. Given the widespread expression of α(2,6)-linked sialic acid in the human respiratory tract, we propose that SAP may act as an effective receptor mimic to limit IAV infection of airway epithelial cells.

Pulmonary surfactant protein D in first-line innate defence against influenza A virus infections

Influenza A viruses (IAV) cause respiratory tract infections annually associated with excess mortality and morbidity. Nonspecific, innate immune mechanisms play a key role in protection against viral invasion at early stages of infection. A soluble protein present in mucosal secretions of the lung, surfactant protein D (SP-D), is an important component of this initial barrier that helps to prevent and limit IAV infections of the respiratory epithelium. This collagenous C-type lectin binds IAVs and thereby inhibits attachment and entry of the virus but also contributes to enhanced clearance of SP-D-opsonized virus via interactions with phagocytic cells. In addition, SP-D modulates the inflammatory response and helps to maintain a balance between effective neutralization/killing of IAV, and protection against alveolar damage resulting from IAV-induced excessive inflammatory responses. The mechanisms of interaction between SP-D and IAV not only depend on the structure and binding properties of SP-D but also on strain-specific features of IAV, and both issues will be discussed. SP-D from pigs exhibits distinct anti-IAV properties and is discussed in more detail. Finally, the potential of SP-D as a prophylactic and/or therapeutic antiviral agent to protect humans against infections by IAV is discussed.

The number and position of N-linked glycosylation sites in the hemagglutinin determine differential recognition of seasonal and 2009 pandemic H1N1 influenza virus by porcine surfactant protein D

C-type lectins are important molecules of the innate immune system. These molecules, like surfactant protein D (SP-D) can recognize glycans on pathogens and neutralize these. Also influenza viruses are recognized by SP-D and their susceptibility to neutralization by SP-D is dependent on the number of N-linked glycosylation sites in the hemagglutinin in particular. Porcine SP-D displayed stronger neutralizing activity to human influenza A viruses than to swine influenza A viruses. Although viruses from these species differ with regard to the number of glycosylation sites in the hemagglutinin, the mechanism underlying the differential recognition by porcine SP-D is poorly understood. Here we investigated the molecular basis for the differential recognition of a seasonal H1N1 and a 2009 pandemic H1N1 virus by porcine SP-D. We demonstrated that the number and position of glycosylation sites determine viral susceptibility to the neutralizing activity of porcine SP-D. However, predicting the effect remains difficult as it was shown to be dependent on the strain and the position of the glycosylation sites.

Human H-ficolin inhibits replication of seasonal and pandemic influenza A viruses

The collectins have been shown to have a role in host defense against influenza A virus (IAV) and other significant viral pathogens (e.g., HIV). The ficolins are a related group of innate immune proteins that are present at relatively high concentrations in serum, but also in respiratory secretions; however, there has been little study of the role of ficolins in viral infection. In this study, we demonstrate that purified recombinant human H-ficolin and H-ficolin in human serum and bronchoalveolar lavage fluid bind to IAV and inhibit viral infectivity and hemagglutination activity in vitro. Removal of ficolins from human serum or bronchoalveolar lavage fluid reduces their antiviral activity. Inhibition of IAV did not involve the calcium-dependent lectin activity of H-ficolin. We demonstrate that H-ficolin is sialylated and that removal of sialic acid abrogates IAV inhibition, while addition of the neuraminidase inhibitor oseltamivir potentiates neutralization, hemagglutinin inhibition, and viral aggregation caused by H-ficolin. Pandemic and mouse-adapted strains of IAV are generally not inhibited by the collectins surfactant protein D or mannose binding lectin because of a paucity of glycan attachments on the hemagglutinin of these strains. In contrast, H-ficolin inhibited both the mouse-adapted PR-8 H1N1 strain and a pandemic H1N1 strain from 2009. H-ficolin also fixed complement to a surface coated with IAV. These findings suggest that H-ficolin contributes to host defense against IAV.

Reduced airway surface pH impairs bacterial killing in the porcine cystic fibrosis lung

Cystic fibrosis (CF) is a life-shortening disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene ¹. Although bacterial lung infection and the resulting inflammation cause most of the morbidity and mortality, how loss of CFTR first disrupts airway host defense has remained uncertain ^2–6. We asked what abnormalities impair eradication when a bacterium lands on the pristine surface of a newborn CF airway? To investigate these defects, we interrogated the viability of individual bacteria immobilized on solid grids and placed on the airway surface. As a model we studied CF pigs, which spontaneously develop hallmark features of CF lung disease ^7,8. At birth, their lungs lack infection and inflammation, but have a reduced ability to eradicate bacteria ⁸. Here we show that in newborn wild-type pigs, the thin layer of airway surface liquid (ASL) rapidly killed bacteria in vivo, when removed from the lung, and in primary epithelial cultures. Lack of CFTR reduced bacterial killing. We found that ASL pH was more acidic in CF, and reducing pH inhibited the antimicrobial activity of ASL. Reducing ASL pH diminished bacterial killing in wild-type pigs, and increasing ASL pH rescued killing in CF pigs. These results directly link the initial host defense defect to loss of CFTR, an anion channel that facilitates HCO₃⁻ transport ^9–13. Without CFTR, airway epithelial HCO₃⁻ secretion is defective, ASL pH falls and inhibits antimicrobial function, and thereby impairs killing of bacteria that enter the newborn lung. These findings suggest that increasing ASL pH might prevent the initial infection in patients with CF and that assaying bacterial killing could report on the benefit of therapeutic interventions.

A unique sugar-binding site mediates the distinct anti-influenza activity of pig surfactant protein D

Pigs can act as intermediate hosts by which reassorted influenza A virus (IAV) strains can be transmitted to humans and cause pandemic influenza outbreaks. The innate host defense component surfactant protein D (SP-D) interacts with glycans on the hemagglutinin of IAV and contributes to protection against IAV infection in mammals. This study shows that a recombinant trimeric neck lectin fragment derived from porcine SP-D (pSP-D) exhibits profound inhibitory activity against IAV, in contrast to comparable fragments derived from human SP-D. Crystallographic analysis of the pSP-D fragment complexed with a viral sugar component shows that a unique tripeptide loop alters the lectin site conformation of pSP-D. Molecular dynamics simulations highlight the role of this flexible loop, which adopts a more stable conformation upon sugar binding and may facilitate binding to viral glycans through contact with distal portions of the branched mannoside. The combined data demonstrate that porcine-specific structural features of SP-D contribute significantly to its distinct anti-IAV activity. These findings could help explain why pigs serve as important reservoirs for newly emerging pathogenic IAV strains.

Soluble host defense lectins in innate immunity to influenza virus

Host defenses against viral infections depend on a complex interplay of innate (nonspecific) and adaptive (specific) components. In the early stages of infection, innate mechanisms represent the main line of host defense, acting to limit the spread of virus in host tissues prior to the induction of the adaptive immune response. Serum and lung fluids contain a range of lectins capable of recognizing and destroying influenza A viruses (IAV). Herein, we review the mechanisms by which soluble endogenous lectins mediate anti-IAV activity, including their role in modulating IAV-induced inflammation and disease and their potential as prophylactic and/or therapeutic treatments during severe IAV-induced disease.

The role of cell tropism for the pathogenesis of influenza in humans

Seasonal, pandemic and avian influenza viruses are able to infect humans, but the disease outcome often differs, ranging from mild upper respiratory tract disease to fatal pneumonia. The cell tropism of influenza viruses is thought to be an important determinant of these factors. Therefore, this review focuses on the factors that, together, determine the cell tropism of influenza viruses. These include: the receptor specificity of the viral hemagglutinin and the distribution of these receptors in the respiratory tract; the presence of inhibitory factors in the fluid lining the respiratory mucosa; and the requirement for host cell proteases that can cleave the precursor hemagglutinin of influenza viruses. Finally, we will discuss how the route of inoculation influences the cell types infected by influenza viruses and associated pathogenesis.

Assessment of the antiviral properties of recombinant porcine SP-D against various influenza A viruses in vitro

The emergence of influenza viruses resistant to existing classes of antiviral drugs raises concern and there is a need for novel antiviral agents that could be used therapeutically or prophylacticaly. Surfactant protein D (SP-D) belongs to the family of C-type lectins which are important effector molecules of the innate immune system with activity against bacteria and viruses, including influenza viruses. In the present study we evaluated the potential of recombinant porcine SP-D as an antiviral agent against influenza A viruses (IAVs) in vitro. To determine the range of antiviral activity, thirty IAVs of the subtypes H1N1, H3N2 and H5N1 that originated from birds, pigs and humans were selected and tested for their sensitivity to recombinant SP-D. Using these viruses it was shown by hemagglutination inhibition assay, that recombinant porcine SP-D was more potent than recombinant human SP-D and that especially higher order oligomeric forms of SP-D had the strongest antiviral activity. Porcine SP-D was active against a broad range of IAV strains and neutralized a variety of H1N1 and H3N2 IAVs, including 2009 pandemic H1N1 viruses. Using tissue sections of ferret and human trachea, we demonstrated that recombinant porcine SP-D prevented attachment of human seasonal H1N1 and H3N2 virus to receptors on epithelial cells of the upper respiratory tract. It was concluded that recombinant porcine SP-D holds promise as a novel antiviral agent against influenza and further development and evaluation in vivo seems warranted.

GM-CSF modulates pulmonary resistance to influenza A infection

Alveolar type II epithelial or other pulmonary cells secrete GM-CSF that regulates surfactant catabolism and mucosal host defense through its capacity to modulate the maturation and activation of alveolar macrophages. GM-CSF enhances expression of scavenger receptors MARCO and SR-A. The alveolar macrophage SP-R210 receptor binds the surfactant collectin SP-A mediating clearance of respiratory pathogens. The current study determined the effects of epithelial-derived GM-CSF in host resistance to influenza A pneumonia. The results demonstrate that GM-CSF enhanced resistance to infection with 1.9×10(4) ffc of the mouse-adapted influenza A/Puerto Rico/8/34 (PR8) H1N1 strain, as indicated by significant differences in mortality and mean survival of GM-CSF-deficient (GM(-/-)) mice compared to GM(-/-) mice in which GM-CSF is expressed at increased levels. Protective effects of GM-CSF were observed both in mice with constitutive and inducible GM-CSF expression under the control of the pulmonary-specific SFTPC or SCGB1A1 promoters, respectively. Mice that continuously secrete high levels of GM-CSF developed desquamative interstitial pneumonia that impaired long-term recovery from influenza. Conditional expression of optimal GM-CSF levels at the time of infection, however, resulted in alveolar macrophage proliferation and focal lymphocytic inflammation of distal airways. GM-CSF enhanced alveolar macrophage activity as indicated by increased expression of SP-R210 and CD11c. Infection of mice lacking the GM-CSF-regulated SR-A and MARCO receptors revealed that MARCO decreases resistance to influenza in association with increased levels of SP-R210 in MARCO(-/-) alveolar macrophages. In conclusion, GM-CSF enhances early host resistance to influenza. Targeting of MARCO may reinforce GM-CSF-mediated host defense against pathogenic influenza.

Introduction of N-linked glycans in the lectin domain of surfactant protein D: impact on interactions with influenza A viruses

Porcine surfactant protein D (pSP-D) displays distinctively strong, broad-range inhibitory activity against influenza A virus (IAV). N-Linked glycosylation of the carbohydrate recognition domain (CRD) of pSP-D contributes to the high affinity of this collectin for IAV. To investigate the role of the N-linked glycan further, HEK293E protein expression was used to produce recombinant pSP-D (RpSP-D) that has similar structural and antiviral properties as NpSP-D. We introduced an additional N-linked glycan in the CRD of RpSP-D but this modification did not alter the antiviral activity. Human SP-D is unglycosylated in its CRD and less active against IAV compared with pSP-D. In an attempt to modify its antiviral properties, several recombinant human SP-D (RhSP-D) mutants were constructed with N-linked glycans introduced at various locations within its CRD. To retain lectin activity, necessary for the primary interactions between SP-D and IAV, N-linked glycosylation of RhSP-D was shown to be restricted to the corresponding position in the CRD of either pSP-D or surfactant protein A (SP-A). These N-glycosylated RhSP-D mutants, however, did not show increased neutralization activity against IAV. By developing RhSP-D mutants that also have the pSP-D-specific Ser-Gly-Ala loop inserted in the CRD, we could demonstrate that the N-linked glycan-mediated interactions between pSP-D and IAV involves additional structural prerequisites of the pSP-D CRD. Ultimately, these studies will help to develop highly effective SP-D-based therapeutic and prophylactic drugs against IAV.

Recombinant chimeric lectins consisting of mannose-binding lectin and L-ficolin are potent inhibitors of influenza A virus compared with mannose-binding lectin

MBL structurally contains a type II-like collagenous domain and a carbohydrate recognition domain (CRD). We have recently generated three novel recombinant chimeric lectins (RCL), in which varying length of collagenous domain of mannose-binding lectin (MBL) is replaced with that of L-ficolin (L-FCN). CRD of MBL is used for target recognition because it has a broad spectrum in pathogen recognition compared with L-FCN. Results of our study demonstrate that these RCLs are potent inhibitors of influenza A virus (IAV). RCLs, against IAV, show dose-dependent activation of the lectin complement pathway, which is significantly higher than that of recombinant human MBL (rMBL). This activity is observed even without MBL-associated serine proteases (MASPs, provided by MBL deficient mouse sera), which have been thought to mediate complement activation. These observations suggest that RCLs are more efficient in associating with MASP-2, which predominantly mediates the activity. Yet, additional serum further increases the activity while RCL-mediated coagulation-like enzyme activities are diminished compared with rMBL, suggesting reduced association with MASP-1, which has been shown to mediate coagulation-like activity. These data suggest that RCLs may interfere less with host coagulation, which is advantageous to be a therapeutic drug. Importantly, these RCLs have surpassed rMBL for anti-viral activities, such as viral aggregation, reduction of viral hemagglutination (HA) and inhibition of virus-mediated HA and neuraminidase (NA) activities. These results are encouraging that novel RCLs could be used as anti-IAV agents with less side effect and that RCLs would be suitable candidates in developing a new anti-IAV therapy.

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.

Transcriptional expression levels of chicken collectins are affected by avian influenza A virus inoculation

Mammalian collectins have been found to play an important role in the defense against influenza A virus H9N2 inoculation, but for chicken collectins this has not yet been clarified. The aim of this study was to determine the effect of avian influenza A virus (AIV) inoculation on collectin gene expression in the respiratory tract of chickens and whether this was affected by age. For this purpose 1- and 4-week-old chickens were inoculated intratracheally with PBS or H9N2 AIV. Chickens were killed at 0, 8, 16 and 24h post-inoculation and trachea and lung were harvested for analysis. Viral RNA expression and mRNA expression of chicken collectins 1 and 2 (cCL-1 and cCL-2), chicken lung lectin (cLL) and chicken surfactant protein A (cSP-A) were determined using real-time quantitative RT-PCR. In lung, a decrease in mRNA expression of cCL-2, cLL and cSP-A after inoculation with H9N2 was seen in both 1- and 4-week-old birds, although at different time points, while in trachea changes were only seen in 4-week-old birds and expression was increased. Moreover, collectin expression correlated with viral RNA expression in lung of 1-week-old birds. These results suggest that both age and location in the respiratory tract affect changes in collectin mRNA expression after inoculation with H9N2 and indicate a possible role for collectins in the host response to AIV in the respiratory tract of chickens.

The porcine lung as a potential model for cystic fibrosis

Airway disease currently causes most of the morbidity and mortality in patients with cystic fibrosis (CF). However, understanding the pathogenesis of CF lung disease and developing novel therapeutic strategies have been hampered by the limitations of current models. Although the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) has been targeted in mice, CF mice fail to develop lung or pancreatic disease like that in humans. In many respects, the anatomy, biochemistry, physiology, size, and genetics of pigs resemble those of humans. Thus pigs with a targeted CFTR gene might provide a good model for CF. Here, we review aspects of porcine airways and lung that are relevant to CF.

N-linked glycosylation attenuates H3N2 influenza viruses

Over the last four decades, H3N2 subtype influenza A viruses have gradually acquired additional potential sites for glycosylation within the globular head of the hemagglutinin (HA) protein. Here, we have examined the biological effect of additional glycosylation on the virulence of H3N2 influenza viruses. We created otherwise isogenic reassortant viruses by site-directed mutagenesis that contain additional potential sites for glycosylation and examined the effect on virulence in naïve BALB/c, C57BL/6, and surfactant protein D (SP-D)-deficient mice. The introduction of additional sites was consistent with the sequence of acquisition in the globular head over the past 40 years, beginning with two sites in 1968 to the seven sites found in contemporary influenza viruses circulating in 2000. Decreased morbidity and mortality, as well as lower viral lung titers, were seen in mice as the level of potential glycosylation of the viruses increased. This correlated with decreased evidence of virus-mediated lung damage and increased in vitro inhibition of hemagglutination by SP-D. SP-D-deficient animals displayed an inverse pattern of disease, such that more highly glycosylated viruses elicited disease equivalent to or exceeding that of the wild type. We conclude from these data that increased glycosylation of influenza viruses results in decreased virulence, which is at least partly mediated by SP-D-induced clearance from the lung. The continued exploration of interactions between highly glycosylated viruses and surfactant proteins may lead to an improved understanding of the biology within the lung and strategies for viral control.

Effects of surfactant protein D on growth, adhesion and epithelial invasion of intestinal Gram-negative bacteria

Surfactant protein D (SP-D) interacts with various different microorganisms and plays an important role in pulmonary innate immunity. SP-D expression has also been detected in extrapulmonary tissues, including the gastro-intestinal tract. However, its function in the intestine is unknown and may differ considerably from SP-D functions in the lung. Therefore, the effects of porcine SP-D (pSP-D) on several strains of intestinal bacteria were studied by means of bacterial growth assays, colony-count assays, radial diffusion assays and differential fluorescent staining. Furthermore, the effect of pSP-D on the adhesion- and invasion-characteristics was investigated. All bacterial strains tested in this study were aggregated by pSP-D, but only Escherichia coli K12 was susceptible to pSP-D-mediated growth inhibition. Bacterial membrane integrity of E. coli K12 was affected by pSP-D, but this did not lead to a reduced bacterial viability. Therefore, it is unlikely that pSP-D has a direct antimicrobial effect, and the observed effects are most likely due to pSP-D-mediated bacterial aggregation. The effects of pSP-D on bacterial adhesion and invasion were studied with the porcine intestinal epithelial cell line IPI-2I. Preincubation with pSP-D results in a several-fold increase in adhesion (E. coli and Salmonella) and invasion (Salmonella), but did not affect the IL-8 production induced by the bacteria. Results obtained in this study suggest that pSP-D promotes uptake of pathogenic bacteria by epithelial cells. This may reflect a scavenger function for pSP-D in the intestine, which enables the host to generate a more rapid response to infectious bacteria.

Salivary agglutinin and lung scavenger receptor cysteine-rich glycoprotein 340 have broad anti-influenza activities and interactions with surfactant protein D that vary according to donor source and sialylation

We previously found that scavenger receptor cysteine-rich gp-340 (glycoprotein-340), isolated from lung or saliva, directly inhibits human IAVs (influenza A viruses). We now show that salivary gp-340 has broad antiviral activity against human, equine and porcine IAV strains. Although lung and salivary gp-340 are identical in protein sequence, salivary gp-340 from one donor had significantly greater antiviral activity against avian-like IAV strains which preferentially bind sialic acids in alpha(2,3) linkage. A greater density of alpha(2,3)-linked sialic acids was present on the salivary gp-340 from this donor as compared with salivary gp-340 from another donor or several preparations of lung gp-340. Hence, the specificity of sialic acid linkages on gp-340 is an important determinant of anti-IAV activity. Gp-340 binds to SP-D (surfactant protein D), and we previously showed that lung gp-340 has co-operative interactions with SP-D in viral neutralization and aggregation assays. We now report that salivary gp-340 can, in some cases, strongly antagonize certain antiviral activities of SP-D. This effect was associated with greater binding of salivary gp-340 to the carbohydrate recognition domain of SP-D as compared with the binding of lung gp-340. These findings may relate to inter-individual variations in innate defence against highly pathogenic IAV and to effects of aspiration of oral contents on SP-D-mediated lung functions.

Species differences in the carbohydrate binding preferences of surfactant protein D

Interactions of surfactant protein D (SP-D) with micro-organisms and organic antigens involve binding to the trimeric neck plus carbohydrate recognition domain (neck+CRD). In these studies, we compared the ligand binding of homologous human, rat, and mouse trimeric neck+CRD fusion proteins, each with identical N-terminal tags remote from the ligand-binding surface. Although rat and mouse showed similar affinities for saccharide competitors, both differed markedly from the human protein. The human neck+CRD preferentially recognized N-acetyl-mannosamine, whereas the rat and mouse proteins showed greater affinity for myoinositol, maltose, and glucose. Although human neck+CRDs bound to maltosyl-agarose and fungal mannan, only rat and mouse neck+CRDs showed significant binding to maltosyl-Toyopearl beads, solid-phase maltosyl-albumin neo-glycoprotein, or the Phil82 strain of influenza A virus. Likewise, human SP-D dodecamers and trimeric subunits of full-length rat, but not full-length human SP-D trimers, bound to maltosyl-Toyopearl. Site-directed mutagenesis of the human neck+CRD demonstrated an important role of Asp324-Asp325 in the recognition of N-acetyl-mannosamine, and substitution of the corresponding murine sequence (Asn324-Asn325) conferred a capacity to interact with immobilized maltose. Thus, ligand recognition by human SP-D involves a complex interplay between saccharide presentation, the valency of trimeric subunits, and species-specific residues that flank the primary carbohydrate binding site.

Surfactant protein D binds to human immunodeficiency virus (HIV) envelope protein gp120 and inhibits HIV replication

The envelope protein (gp120) of human immunodeficiency virus (HIV) contains highly conserved mannosylated oligosaccharides. These glycoconjugates contribute to resistance to antibody neutralization, and binding to cell surface lectins on macrophages and dendritic cells. Mannose-binding lectin (MBL) binds to gp120 and plays a role in defence against the virus. In this study it is demonstrated that surfactant protein D (SP-D) binds to gp120 and inhibits HIV infectivity at significantly lower concentrations than MBL. The binding of SP-D was mediated by its calcium-dependent carbohydrate-binding activity and was dependent on glycosylation of gp120. Native dodecameric SP-D bound to HIV gp120 more strongly than native trimeric SP-D. Since one common polymorphic form of SP-D is predominantly expressed as trimers and associated with lower blood levels, these individuals may have less effective innate defence against HIV. A chimeric protein containing the N-terminal and collagen domains of SP-D linked to the neck and carbohydrate-recognition domains of MBL (called SP-D/MBLneck+CRD) had greater ability to bind to gp120 and inhibit virus replication than either SP-D or MBL. The enhanced binding of SP-D/MBLneck+CRDwas dependent on assembly into higher molecular mass multimers (i.e. a trimeric form of the chimera did not bind to a greater extent than MBL). Hence, the enhanced binding of SP-D compared with MBL results from distinctive properties of its N-terminal and/or collagen domains. SP-D is present in lung and airway fluids, as well as in blood and various mucosal locations, and could, like MBL, play a role in restricting HIV transmission or replicationin vivo.

Pulmonary infections in swine induce altered porcine surfactant protein D expression and localization to dendritic cells in bronchial-associated lymphoid tissue

Surfactant protein D (SP-D) is a pattern-recognition molecule of the innate immune system that recognizes various microbial surface-specific carbohydrate and lipid patterns. In vitro data has suggested that this binding may lead to increased microbial association with macrophages and dendritic cells. The aim of the present in vivo study was to study the expression of porcine SP-D (pSP-D) in the lung during different pulmonary bacterial infections, and the effect of the routes of infection on this expression was elucidated. Furthermore, the aim was to study the in vivo spatial relationship among pSP-D, pathogens, phagocytic cells and dendritic cells. Lung tissue was collected from experimental and natural bronchopneumonias caused by Actinobacillus pleuropneumoniae or Staphylococcus aureus, and from embolic and diffuse interstitial pneumonia, caused by Staph. aureus or Arcanobacterium pyogenes and Streptococcus suis serotype 2, respectively. By comparing normal and diseased lung tissue from the same lungs, increased diffuse pSP-D immunoreactivity was seen in the surfactant in both acute and chronic bronchopneumonias, while such increased expression of pSP-D was generally not present in the interstitial pneumonias. Co-localization of pSP-D, alveolar macrophages and bacteria was demonstrated, and pSP-D showed a patchy distribution on the membranes of alveolar macrophages. SP-D immunoreactivity was intracellular in dendritic cells. The dendritic cells were identified by their morphology, the absence of macrophage marker immunoreactivity and the presence of dendritic cell marker immunoreactivity. Increased expression of pSP-D in the surfactant coincided with presence of pSP-D-positive dendritic cells in bronchus-associated lymphoid tissue (BALT), indicating a possible transport of pSP-D through the specialized M cells overlying (BALT). In conclusion, we have shown that pSP-D expression in the lung surfactant is induced by bacterial infection by an aerogenous route rather than by a haematogenous route, and that the protein interacts specifically with alveolar macrophages and with dendritic cells in microbial-induced BALT. The function of the interaction between pSP-D and dendritic cells in BALT remain unclear, but pSP-D could represent a link between the innate and adaptive immune system, facilitating the bacterial antigen presentation by dendritic cells in BALT.

Ligand specificity of human surfactant protein D: expression of a mutant trimeric collectin that shows enhanced interactions with influenza A virus

Surfactant protein D is a pattern recognition molecule that plays diverse roles in immune regulation and anti-microbial host defense. Its interactions with known ligands are calcium-dependent and involve binding to the trimeric, C-type carbohydrate recognition domain. Surfactant protein D preferentially binds to glucose and related sugars. However, CL-43, a bovine serum lectin, which evolved through duplication of the surfactant protein D gene in ruminants, prefers mannose and mannose-rich polysaccharides. Surfactant protein D is characterized by two relatively conserved motifs at the binding face, along the edges of the shallow carbohydrate-binding groove. For CL-43, sequence alignments demonstrate a basic insertion, Arg-Ala-Lys (RAK), immediately N-terminal to the first motif. We hypothesized that this insertion contributes to the differences in saccharide selectivity and host defense function and compared the activities of recombinant trimeric neck + carbohydrate recognition domains of human surfactant protein D (NCRD) with CL-43 (RCL-43-NCRD) and selected NCRD mutants. Insertion of the CL-43 RAK sequence or a control Ala-Ala-Ala sequence (AAA) into the corresponding position in NCRD increased the efficiency of binding to mannan and changed the inhibitory potencies of competing saccharides to more closely resemble those of CL-43. In addition, RAK resembled CL-43 in its greater capacity to inhibit the infectivity of influenza A virus and to increase uptake of influenza by neutrophils.

Purification, characterization and immunolocalization of porcine surfactant protein D

Surfactant protein D (SP-D) is a collectin believed to play an important role in innate immunity. SP-D is characterized by having a collagen-like domain and a carbohydrate recognition domain (CRD), which has a specific Ca(2+)-dependent specificity for saccharides and thus the ability to bind complex glycoconjugates on micro-organisms. This paper describes the tissue immunolocalization of porcine SP-D (pSP-D) in normal slaughter pigs using a monoclonal antibody raised against purified pSP-D. Porcine SP-D was purified from porcine bronchoalveolar lavage (BAL) by maltose-agarose and immunoglobulin M affinity chromatography. The purified protein appeared on sodium dodecyl sulphate-polyacrylamide gel electrophoresis as a band of approximately 53,000 MW in the reduced state and approximately 138,000 MW in the unreduced state. Porcine SP-D was sensitive to collagenase digestion and N-deglycosylation, which reduced the molecular mass to approximately 24,000 MW and approximately 48,000 MW respectively, in the reduced state. N-deglycosylation of the collagen-resistant fragment, reduced the molecular mass to approximately 21,000 MW showing the presence of an N-glycosylation site located in the CRD. Porcine SP-D bound to solid-phase mannan in a dose and Ca(2+)-dependent manner with a saccharide specificity similar to rat and human SP-D. The purified protein was used for the production of a monoclonal anti-pSP-D antibody. The antibody reacted specifically with pSP-D in the reduced and unreduced state when analysed by Western blotting. Immunohistochemical evaluation of normal porcine tissues showed pSP-D immunoreactivity predominantly in Clara cells and serous cells of the bronchial submucosal glands, and to a lesser extent in alveolar type II cells, epithelial cells of the intestinal glands (crypts of Lieberkuhn) in the duodenum, jejunum and ileum and serous cells of the dorsolateral lacrimal gland.

Cooperative anti-influenza activities of respiratory innate immune proteins and neuraminidase inhibitor

The surfactant collectins, surfactant proteins A and D (SP-A and D), and scavenger receptor-rich glycoprotein 340 (gp340) inhibit influenza A virus (IAV) in the following order of potency: SP-D>gp340>SP-A. SP-D binds in a calcium-dependent manner to carbohydrate attachments on the viral hemagglutinin (HA) and neuraminidase (NA). By contrast, gp340 and SP-A act like mucins in that they provide sialic acid ligands that bind to the viral HA. In this study, SP-D, SP-A, and gp340 showed cooperative antiviral interactions. These cooperative effects were most evident in viral aggregation but were also observed in at least some hemagglutination inhibition and viral neutralization assays. The mechanism of binding between gp340 and SP-D was further characterized using monoclonal antibodies. Although gp340 can bind to SP-D at a site distinct from the mannan-binding site, binding of gp340 to SP-D did not contribute to cooperative antiviral interactions. SP-D and mucin showed cooperative interactions, apparently dependent on NA inhibition by SP-D. The commercial NA inhibitor oseltamivir had a similar effect and also enhanced the neutralizing activity of SP-A and bronchoalveolar lavage fluid. Hence, oseltamivir collaborates with innate immune proteins in inhibiting the initial infection of epithelial cells.

Collectins

Collectins are a family of collagenous calcium-dependent defense lectins in animals. Their polypeptide chains consist of four regions: a cysteine-rich N-terminal domain, a collagen-like region, an alpha-helical coiled-coil neck domain and a C-terminal lectin or carbohydrate-recognition domain. These polypeptide chains form trimers that may assemble into larger oligomers. The best studied family members are the mannan-binding lectin, which is secreted into the blood by the liver, and the surfactant proteins A and D, which are secreted into the pulmonary alveolar and airway lining fluid. The collectins represent an important group of pattern recognition molecules, which bind to oligosaccharide structures and/or lipid moities on the surface of microorganisms. They bind preferentially to monosaccharide units of the mannose type, which present two vicinal hydroxyl groups in an equatorial position. High-affinity interactions between collectins and microorganisms depend, on the one hand, on the high density of the carbohydrate ligands on the microbial surface, and on the other, on the degree of oligomerization of the collectin. Apart from binding to microorganisms, the collectins can interact with receptors on host cells. Binding of collectins to microorganisms may facilitate microbial clearance through aggregation, complement activation, opsonization and activation of phagocytosis, and inhibition of microbial growth. In addition, the collectins can modulate inflammatory and allergic responses, affect apoptotic cell clearance and modulate the adaptive immune system.

Interactions of influenza A virus with sialic acids present on porcine surfactant protein D

Pigs can be infected with both human and avian influenza A virus (IAV) strains and are therefore considered to be important intermediates in the emergence of new IAV strains due to mixing of viral genes derived from human, avian, or porcine influenza viruses. These reassortant strains may have potential to cause pandemic influenza outbreaks in humans. The innate immune response against IAV plays a significant role in containment of IAV in the airways. We studied the interactions of IAV with porcine surfactant protein D (pSP-D), an important component of this first line defense system. Hemagglutination inhibition analysis shows that the distinct interactions of pSP-D with IAV mediated by the N-linked carbohydrate moiety in the carbohydrate recognition domain of pSP-D depend on the terminal sialic acids (SAs) present on this carbohydrate. Analysis by both lectin staining and by cleavage with linkage-specific sialidases shows that the carbohydrate of pSP-D is exclusively sialylated with alpha(2,6)-linked SAs, in contrast to surfactant protein A, which contains both alpha(2,3)- and alpha(2,6)-linked SAs on its N-linked carbohydrate. Enzymatic modification of the SA-linkages present on pSP-D demonstrates that the type of SA-linkage is important for its hemagglutination-inhibitory activity, and correlates with receptor-binding specificity of the IAV strains. The SAs present on pSP-D appear especially important for interactions with poorly glycosylated IAV strains. It remains to be elucidated to what extent the unique sialylation profile of pSP-D is involved in host range control of IAV in pigs, and whether it facilitates adaptation of avian or human IAV strains that can contribute to the production of reassortant strains in pigs.