Surfactant protein A (SP-A)-mediated clearance of Staphylococcus aureus involves binding of SP-A to the staphylococcal adhesin eap and the macrophage receptors SP-A receptor 210 and scavenger receptor class A

Advanced MicroRNA delivery for lung inflammatory therapy: surfactant protein A controls cellular internalisation and degradation of extracellular vesicles

INTRODUCTION

Alveolar macrophages (AMs) are the first line of defence against pathogens that initiate an inflammatory response in the lungs and exhibit a strong affinity for surfactant protein A (SP-A). Extracellular vesicles (EVs) have emerged as a promising drug delivery platform due to their minimal cytotoxicity. However, precise targeting of specific cell types and the rapid lysosomal degradation of EVs within recipient cells remain persistent challenges.

METHOD

In this study, we explored the biological significance of SP-A-EVs as novel drug delivery systems for combating lung inflammation. We first verified that respiratory EVs express SP-A receptor (SP-R210), facilitating the conjugation of SP-A with EVs. The delivery efficiency, cellular internalisation pathways and therapeutic effects were evaluated using an in vivo mouse model.

RESULTS

SP-A-EVs were robustly internalised into AMs both in vitro and in vivo. Furthermore, our investigation revealed that the toll-like receptor 4-mediated endocytosis pathway was employed for the uptake of SP-A-EVs, significantly delaying their degradation compared with natural EVs, which primarily followed the conventional lysosomal degradation pathway within AMs. In a functional study, we successfully loaded anti-inflammatory microRNA (let-7b) into SP-A-EVs, leading to the suppression of AM activation and the alleviation of lung inflammation induced by lipopolysaccharide.

CONCLUSION

These findings underscore the potential of SP-A-EVs as highly effective drug delivery systems for targeted therapeutics in lung-related disorders, capitalising on the strong affinity between AMs and SP-A and the modulation of cellular internalisation.

Adaptation and conservation of CL-10/11 in avian lungs: implications for their role in pulmonary innate immune protection

The common avian origin of many zoonotic infections and epidemics warrants investigation into the mechanism of respiratory surface protection in reservoir species such as birds. Our recent molecular investigations on the evolution and pulmonary expression of an ancient family of proteins, the C-type lectins, have revealed unique molecular adaptations in the surfactant proteins avian SP-A1 (aSP-A1), aSP-A2 and aSP-C coupled with the loss of surfactant protein-D (SP-D) in the avian lineage. As surfactant proteins are members of the collectin family, a subgroup of the C-type lectins, an in silico search for related non-surfactant collectin proteins (Collectin-10 (CL-10) and Collectin-11 (CL-11)) in the NCBI genome database was conducted to understand their evolution in the avian lineage. In addition, both CL-10 and CL-11 gene expression in the lungs and other organs of zebra finches and turkeys was confirmed by PCR. These PCR-confirmed zebra finch and turkey CL-10 and CL-11 sequences were compared with sequenced and in silico-predicted vertebrate homologues to develop a phylogenetic tree. Compared with avian surfactant proteins, CL-10 and CL-11 are highly conserved among vertebrates, suggesting a critical role in development and innate immune protection. The conservation of CL-11 EPN and collagen domain motifs may compensate to some extent for the loss of SP-D in the avian lineage.This article is part of the theme issue 'The biology of the avian respiratory system'.

Unraveling the enigma: The emerging significance of pulmonary surfactant proteins in predicting, diagnosing, and managing COVID-19

BACKGROUND

Severe cases of COVID-19 often lead to the development of acute respiratory syndrome, a critical condition believed to be caused by the harmful effects of SARS-CoV-2 on type II alveolar cells. These cells play a crucial role in producing pulmonary surfactants, which are essential for proper lung function. Specifically focusing on surfactant proteins, including Surfactant protein A (SP-A), Surfactant protein B, Surfactant protein C, and Surfactant protein D (SP-D), changes in the levels of pulmonary surfactants may be a significant factor in the pathological changes seen in COVID-19 infection.

OBJECTIVE

This study aims to gain insights into surfactants, particularly their impacts and changes during COVID-19 infection, through a comprehensive review of current literature. The study focuses on the function of surfactants as prognostic markers, diagnostic factors, and essential components in the management and treatment of COVID-19.

FINDING

In general, pulmonary surfactants serve to reduce the surface tension at the gas-liquid interface, thereby significantly contributing to the regulation of respiratory mechanics. Additionally, these surfactants play a crucial role in the innate immune system within the pulmonary microenvironment. Within the spectrum of COVID-19 infections, a compelling association is observed, characterized by elevated levels of SP-D and SP-A across a range of manifestations from mild to severe pneumonia. The sudden decline in respiratory function observed in COVID-19 patients may be attributed to the decreased synthesis of surfactants by type II alveolar cells.

CONCLUSION

Collectin proteins such as SP-A and SP-D show promise as biomarkers, offering potential avenues for predicting and monitoring pulmonary alveolar injury in the context of COVID-19. This clarification enhances our understanding of the molecular complexities contributing to respiratory complications in severe COVID-19 cases, providing a foundation for targeted therapeutic approaches using surfactants and refined clinical management strategies.

Emerging concepts of myosin 18A isoform mechanobiology in organismal and immune system physiology, development, and function

Alternative and combinatorial splicing of myosin 18A (MYO18A) gene transcripts results in expression of MYO18A protein isoforms and isoform variants with different membrane and subcellular localizations, and functional properties. MYO18A proteins are members of the myosin superfamily consisting of a myosin-like motor domain, an IQ motif, and a coiled-coil domain. MYO18A isoforms, however, lack the ability to hydrolyze ATP and do not perform ATP-dependent motor activity. MYO18A isoforms are distinguished by different amino- and carboxy-terminal extensions and domains. The domain organization and functions of MYO18Aα, MYO18Aβ, and MYO18Aγ have been studied experimentally. MYO18Aα and MYO18Aβ have a common carboxy-terminal extension but differ by the presence or absence of an amino-terminal KE repeat and PDZ domain, respectively. The amino- and carboxy-terminal extensions of MYO18Aγ contain unique proline and serine-rich domains. Computationally predicted MYO18Aε and MYO18Aδ isoforms contain the carboxy-terminal serine-rich extension but differ by the presence or absence of the amino-terminal KE/PDZ extension. Additional isoform variants within each category arise by alternative utilization or inclusion/exclusion of small exons. MYO18Aα variants are expressed in somatic cells and mature immune cells, whereas MYO18Aβ variants occur mainly in myeloid and natural killer cells. MYO18Aγ expression is selective to cardiac and skeletal muscle. In the present review perspective, we discuss current and emerging concepts of the functional specialization of MYO18A proteins in membrane and cytoskeletal dynamics, cellular communication and signaling, endocytic and exocytic organelle movement, viral infection, and as the SP-R210 receptor for surfactant protein A.

Are the class 18 myosins Myo18A and Myo18B specialist sarcomeric proteins?

Initially, the two members of class 18 myosins, Myo18A and Myo18B, appeared to exhibit highly divergent functions, complicating the assignment of class-specific functions. However, the identification of a striated muscle-specific isoform of Myo18A, Myo18Aγ, suggests that class 18 myosins may have evolved to complement the functions of conventional class 2 myosins in sarcomeres. Indeed, both genes, Myo18a and Myo18b, are predominantly expressed in the heart and somites, precursors of skeletal muscle, of developing mouse embryos. Genetic deletion of either gene in mice is embryonic lethal and is associated with the disorganization of cardiac sarcomeres. Moreover, Myo18Aγ and Myo18B localize to sarcomeric A-bands, albeit the motor (head) domains of these unconventional myosins have been both deduced and biochemically demonstrated to exhibit negligible ATPase activity, a hallmark of motor proteins. Instead, Myo18Aγ and Myo18B presumably coassemble with thick filaments and provide structural integrity and/or internal resistance through interactions with F-actin and/or other proteins. In addition, Myo18Aγ and Myo18B may play distinct roles in the assembly of myofibrils, which may arise from actin stress fibers containing the α-isoform of Myo18A, Myo18Aα. The β-isoform of Myo18A, Myo18Aβ, is similar to Myo18Aα, except that it lacks the N-terminal extension, and may serve as a negative regulator through heterodimerization with either Myo18Aα or Myo18Aγ. In this review, we contend that Myo18Aγ and Myo18B are essential for myofibril structure and function in striated muscle cells, while α- and β-isoforms of Myo18A play diverse roles in nonmuscle cells.

Disruption of the SP-A/SP-R210L (MYO18Aα) pathway prolongs gestation and reduces fetal survival during lipopolysaccharide-induced parturition in late gestation

Prolonged labor can lead to infection, fetal distress, asphyxia, and life-threatening harm to both the mother and the baby. Surfactant protein A (SP-A) was shown to contribute to the maintenance of pregnancy and timing of term labor. SP-A modulates the stoichiometric expression of the SP-R210L and SP-R210S isoforms of the SP-R210 receptor on alveolar macrophages (AMs). Lack of SP-R210L dysregulates macrophage inflammatory responses. We asked whether SP-A alters normal and inflammation-induced parturition through SP-R210 using SP-A- and SP-R210L-deficient mice. Labor and delivery of time-pregnant mice were monitored in real time using a time-lapse infrared camera. Intrauterine injection with either vehicle or Escherichia coli lipopolysaccharide (LPS) on embryonic (E) day 18.5 post coitus was used to assess the effect of gene disruption in chorioamnionitis-induced labor. We report that either lack of SP-A or disruption of SP-R210L delays parturition by 0.40 and 0.55 days compared with controls, respectively. LPS induced labor at 0.60, 1.01, 0.40, 1.00, and 1.31 days earlier than PBS controls in wild type (WT), SP-A-deficient, littermate controls, heterozygous, and homozygous SP-R210L-deficient mice, respectively. Lack of SP-A reduced litter size in PBS-treated mice, whereas the total number of pups delivered was similar in all LPS-treated mice. The number of live pups, however, was significantly reduced by 50%-70% in SP-A and SP-R210L-deficient mice compared with controls. Differences in gestational length were not associated with intrauterine growth restriction. The present findings support the novel concept that the SP-A/SP-R210 pathway modulates timely labor and delivery and supports fetal lung barrier integrity during fetal-to-neonatal transition in term pregnancy.NEW & NOTEWORTHY To our knowledge, this study is the first to report that SP-A prevents delay of labor and inflammation-induced stillbirth through the receptor SP-R210L.

SP-R210 isoforms of Myosin18A modulate endosomal sorting and recognition of influenza A virus infection in macrophages

Influenza A virus (IAV) infection causes acute and often lethal inflammation in the lung. The role of macrophages in this adverse inflammation is partially understood. The surfactant protein A receptor 210 (SP-R210) consists of two isoforms, a long (L) SP-R210L and a short (S) SP-R210S isoform encoded by alternative splicing of the myosin 18A gene. We reported that disruption of SP-R210L enhances cytosolic and endosomal antiviral response pathways. Here, we report that SP-R210L antagonizes type I interferon β (IFNβ), as depletion of SP-R210L potentiates IFNβ secretion. SP-R210 antibodies enhance and attenuate IFNβ secretion in SP-R210L replete and deficient macrophages, respectively, indicating that SP-R210 isoform stoichiometry alters macrophage function intrinsically. This reciprocal response is coupled to unopposed and restricted expression of viral genes in control and SP-R210L-deficient macrophages, respectively. Human monocytic cells with sub-stoichiometric expression of SP-R210L resist IAV infection, whereas alveolar macrophages with increased abundance of SP-R210L permit viral gene expression similar to murine macrophages. Uptake and membrane binding studies show that lack of SP-R210 isoforms does not impair IAV binding and internalization. Lack of SP-R210L, however, results in macropinocytic retention of the virus that depends on both SP-R210S and interferon-inducible transmembrane protein-3 (IFITM3). Mass spectrometry and Western blot analyses indicate that SP-R210 isoforms modulate differential recruitment of the Rho-family GTPase RAC1 and guanine nucleotide exchange factors. Our study suggests that SP-R210 isoforms modulate RAC-dependent macropinosomal sorting of IAV to discrete endosomal and lysosomal compartments that either permit or prevent endolysosomal escape and inflammatory sensing of viral genomes in macrophages.

Immunoregulatory function of SP-A

Surfactant protein A (SP-A), a natural immune molecule, plays an important role in lung health. SP-A recognizes and binds microbial surface glycogroups through the C-type carbohydrate recognition domain, and then binds corresponding cell surface receptors (such as C1qRp, CRT-CD91 complex, CD14, SP-R210, Toll-like receptor, SIRP-α, CR3, etc.) through collagen-like region, and subsequently mediates biological effects. SP-A regulates lung innate immunity by promoting surfactant absorption by alveolar type II epithelial cells and phagocytosis of pathogenic microorganisms by alveolar macrophages. SP-A also regulates lung adaptive immunity by inhibiting DC maturation, and T cell proliferation and differentiation. This article reviews latest relationships between SP-A and adaptive and intrinsic immunity.

Collectins and ficolins in neonatal health and disease

The immune system starts to develop early in embryogenesis. However, at birth it is still immature and associated with high susceptibility to infection. Adaptation to extrauterine conditions requires a balance between colonization with normal flora and protection from pathogens. Infections, oxidative stress and invasive therapeutic procedures may lead to transient organ dysfunction or permanent damage and perhaps even death. Newborns are primarily protected by innate immune mechanisms. Collectins (mannose-binding lectin, collectin-10, collectin-11, collectin-12, surfactant protein A, surfactant protein D) and ficolins (ficolin-1, ficolin-2, ficolin-3) are oligomeric, collagen-related defence lectins, involved in innate immune response. In this review, we discuss the structure, specificity, genetics and role of collectins and ficolins in neonatal health and disease. Their clinical associations (protective or pathogenic influence) depend on a variety of variables, including genetic polymorphisms, gestational age, method of delivery, and maternal/environmental microflora.

Surfactant protein A alters endosomal trafficking of influenza A virus in macrophages

Influenza A virus infection (IAV) often leads to acute lung injury that impairs breathing and can lead to death, with disproportionate mortality in children and the elderly. Surfactant Protein A (SP-A) is a calcium-dependent opsonin that binds a variety of pathogens to help control pulmonary infections by alveolar macrophages. Alveolar macrophages play critical roles in host resistance and susceptibility to IAV infection. The effect of SP-A on IAV infection and antiviral response of macrophages, however, is not understood. Here, we report that SP-A attenuates IAV infection in a dose-dependent manner at the level of endosomal trafficking, resulting in infection delay in a model macrophage cell line. The ability of SP-A to suppress infection was independent of its glycosylation status. Binding of SP-A to hemagglutinin did not rely on the glycosylation status or sugar binding properties of either protein. Incubation of either macrophages or IAV with SP-A slowed endocytic uptake rate of IAV. SP-A interfered with binding to cell membrane and endosomal exit of the viral genome as indicated by experiments using isolated cell membranes, an antibody recognizing a pH-sensitive conformational epitope on hemagglutinin, and microscopy. Lack of SP-A in mice enhanced IFNβ expression, viral clearance and reduced mortality from IAV infection. These findings support the idea that IAV is an opportunistic pathogen that co-opts SP-A to evade host defense by alveolar macrophages. Our study highlights novel aspects of host-pathogen interactions that may lead to better understanding of the local mechanisms that shape activation of antiviral and inflammatory responses to viral infection in the lung.

Genomic and epigenomic adaptation in SP-R210 (Myo18A) isoform-deficient macrophages

Macrophages play an important role in maintaining tissue homeostasis, from regulating the inflammatory response to pathogens to resolving inflammation and aiding tissue repair. The surfactant protein A (SP-A) receptor SP-R210 (MYO18A) has been shown to affect basal and inflammatory macrophage states. Specifically, disruption of the longer splice isoform SP-R210_L/MYO18Aα renders macrophages hyper-inflammatory, although the mechanism by which this occurs is not well understood. We asked whether disruption of the L isoform led to the hyper-inflammatory state via alteration of global genomic responses. RNA sequencing analysis of L isoform-deficient macrophages (SP-R210_L(DN)) revealed basal and influenza-induced upregulation of genes associated with inflammatory pathways, such as TLR, RIG-I, NOD, and cytoplasmic DNA signaling, whereas knockout of both SP-R210 isoforms (L and S) only resulted in increased RIG-I and NOD signaling. Chromatin immunoprecipitation sequencing (ChIP-seq) analysis showed increased genome-wide deposition of the pioneer transcription factor PU.1 in SP-R210_L(DN) cells, with increased representation around genes relevant to inflammatory pathways. Additional ChIP-seq analysis of histone H3 methylation marks showed decreases in both repressive H3K9me3 and H3K27me3 marks with a commensurate increase in transcriptionally active (H3K4me3) histone marks in the L isoform deficient macrophages. Influenza A virus (IAV) infection, known to stimulate a wide array of anti-viral responses, caused a differential redistribution of PU.1 binding between proximal promoter and distal sites and decoupling from Toll-like receptor regulated gene promoters in SP-R210_L(DN) cells. These finding suggest that the inflammatory differences seen in SP-R210_L-deficient macrophages are a result of transcriptional differences that are mediated by epigenetic changes brought about by differential expression of the SP-R210 isoforms. This provides an avenue to explore how the signaling pathways downstream of the receptor and the ligands can modulate the macrophage inflammatory response.

Human Surfactant Protein SP-A1 and SP-A2 Variants Differentially Affect the Alveolar Microenvironment, Surfactant Structure, Regulation and Function of the Alveolar Macrophage, and Animal and Human Survival Under Various Conditions

The human innate host defense molecules, SP-A1 and SP-A2 variants, differentially affect survival after infection in mice and in lung transplant patients. SP-A interacts with the sentinel innate immune cell in the alveolus, the alveolar macrophage (AM), and modulates its function and regulation. SP-A also plays a role in pulmonary surfactant-related aspects, including surfactant structure and reorganization. For most (if not all) pulmonary diseases there is a dysregulation of host defense and inflammatory processes and/or surfactant dysfunction or deficiency. Because SP-A plays a role in both of these general processes where one or both may become aberrant in pulmonary disease, SP-A stands to be an important molecule in health and disease. In humans (unlike in rodents) SP-A is encoded by two genes (SFTPA1 and SFTPA2) and each has been identified with extensive genetic and epigenetic complexity. In this review, we focus on functional, structural, and regulatory differences between the two SP-A gene-specific products, SP-A1 and SP-A2, and among their corresponding variants. We discuss the differential impact of these variants on the surfactant structure, the alveolar microenvironment, the regulation of epithelial type II miRNome, the regulation and function of the AM, the overall survival of the organism after infection, and others. Although there have been a number of reviews on SP-A, this is the first review that provides such a comprehensive account of the differences between human SP-A1 and SP-A2.

The Role of Macrophages in Staphylococcus aureus Infection

Staphylococcus aureus is a member of the human commensal microflora that exists, apparently benignly, at multiple sites on the host. However, as an opportunist pathogen it can also cause a range of serious diseases. This requires an ability to circumvent the innate immune system to establish an infection. Professional phagocytes, primarily macrophages and neutrophils, are key innate immune cells which interact with S. aureus, acting as gatekeepers to contain and resolve infection. Recent studies have highlighted the important roles of macrophages during S. aureus infections, using a wide array of killing mechanisms. In defense, S. aureus has evolved multiple strategies to survive within, manipulate and escape from macrophages, allowing them to not only subvert but also exploit this key element of our immune system. Macrophage-S. aureus interactions are multifaceted and have direct roles in infection outcome. In depth understanding of these host-pathogen interactions may be useful for future therapeutic developments. This review examines macrophage interactions with S. aureus throughout all stages of infection, with special emphasis on mechanisms that determine infection outcome.

Surfactant Protein-A Function: Knowledge Gained From SP-A Knockout Mice

Pulmonary surfactant proteins have many roles in surfactant- related functions and innate immunity. One of these proteins is the surfactant protein A (SP-A) that plays a role in both surfactant-related processes and host defense and is the focus in this review. SP-A interacts with the sentinel host defense cell in the alveolus, the alveolar macrophage (AM), to modulate its function and expression profile under various conditions, as well as other alveolar epithelial cells such as the Type II cell. Via these interactions, SP-A has an impact on the alveolar microenvironment. SP-A is also important for surfactant structure and function. Much of what is understood of the function of SP-A and its various roles in lung health has been learned from SP-A knockout (KO) mouse experiments, as reviewed here. A vast majority of this work has been done with infection models that are bacterial, viral, and fungal in nature. Other models have also been used, including those of bleomycin-induced lung injury and ozone-induced oxidative stress either alone or in combination with an infectious agent, bone marrow transplantation, and other. In addition, models investigating the effects of SP-A on surfactant components or surfactant structure have contributed important information. SP-A also appears to play a role in pathways involved in sex differences in response to infection and/or oxidative stress, as well as at baseline conditions. To date, this is the first review to provide a comprehensive report of the functions of SP-A as learned through KO mice.

Recent progress on surfactant protein A: cellular function in lung and kidney disease development

Pulmonary surfactant is a heterogeneous active surface complex made up of lipids and proteins. The major glycoprotein in surfactant is surfactant protein A (SP-A), which is released into the alveolar lumen from cytoplasmic lamellar bodies in type II alveolar epithelial cells. SP-A is involved in phospholipid absorption. SP-A together with other surfactant proteins and phospholipids prevent alveolar collapse during respiration by decreasing the surface tension of the air-liquid interface. Additionally, SP-A interacts with pathogens to prevent their propagation and regulate host immune responses. Studies in human and animal models have shown that deficiencies or mutations in surfactant components result in various lung or kidney pathologies, suggesting a role for SP-A in the development of lung and kidney diseases. In this mini-review, we discuss the current understanding of SP-A functions, recent findings of its dysfunction in specific lung and kidney pathologies, and how SP-A has been used as a biomarker to detect the outcome of lung diseases.

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.

Staphylococcus aureus Lung Infection Results in Down-Regulation of Surfactant Protein-A Mainly Caused by Pro-Inflammatory Macrophages

Pneumonia is the leading cause of hospitalization worldwide. Besides viruses, bacterial co-infections dramatically exacerbate infection. In general, surfactant protein-A (SP-A) represents a first line of immune defense. In this study, we analyzed whether influenza A virus (IAV) and/or Staphylococcus aureus (S. aureus) infections affect SP-A expression. To closely reflect the situation in the lung, we used a human alveolus-on-a-chip model and a murine pneumonia model. Our results show that S. aureus can reduce extracellular levels of SP-A, most likely attributed to bacterial proteases. Mono-epithelial cell culture experiments reveal that the expression of SP-A is not directly affected by IAV or S. aureus. Yet, the mRNA expression of SP-A is strongly down-regulated by TNF-α, which is highly produced by professional phagocytes in response to bacterial infection. By using the human alveolus-on-a-chip model, we show that the down-regulation of SP-A is strongly dependent on macrophages. In a murine model of pneumonia, we can confirm that S. aureus decreases SP-A levels in vivo. These findings indicate that (I) complex interactions of epithelial and immune cells induce down-regulation of SP-A expression and (II) bacterial mono- and super-infections reduce SP-A expression in the lung, which might contribute to a severe outcome of bacterial pneumonia.

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.

Evaluation of surfactant proteins A, B, C, and D in articular cartilage, synovial membrane and synovial fluid of healthy as well as patients with osteoarthritis and rheumatoid arthritis

OBJECTIVE

Surfactant Proteins (SPs) are well known from lung and form, along with phospholipids, a surface-active-layer at the liquid-air-interface of the alveolar lining. They play a major protective role by lowering surface tension, activating innate and adaptive immune defense at the lung mucosal interface, especially during infection. We analyzed the regulation of SPs in human and mouse articular chondrocytes, synoviocytes, and synovial fluid under healthy and inflammatory conditions, as well as in tissues of patients suffering from osteoarthritis and rheumatoid arthritis.

METHODS

Immunohistochemistry, RT-PCR, qRT-PCR, ELISA, Western blotting were performed in cell cultures and tissue samples to determine localization, regulation, and concentration of SPs.

RESULTS

All four SPs, were expressed by healthy human and mouse articular chondrocytes and synoviocytes and were also present in synovial fluid. Treatment with inflammatory mediators like IL-1β and TNF-α led to short-term upregulation of individual SPs in vitro. In tissues from patients with osteoarthritis and rheumatoid arthritis, protein levels of all four SPs increased significantly compared to the controls used.

CONCLUSION

These results show the distribution and amount of SPs in tissues of articular joints. They are produced by chondrocytes and synoviocytes and occur in measurable amounts in synovial fluid. All four SPs seem to be differently regulated under pathologic conditions. Their physiological functions in lowering surface tension and immune defense need further elucidation and make them potential candidates for therapeutic intervention.

MYO18A: An unusual myosin

MYO18A is a divergent member of the myosin family characterized by the presence of an amino-terminal PDZ domain. MYO18A has been found in a few different complexes involved in intracellular transport processes. MYO18A is found in a complex with LURAP1 and MRCK that functions in retrograde treadmilling of actin. It also has been found in a complex with PAK2, βPIX, and GIT1, functioning to transport that protein complex from focal adhesions to the leading edge. Finally, a high proportion of MYO18A is found in complex with GOLPH3 at the trans Golgi, where it functions to promote vesicle budding for Golgi-to-plasma membrane trafficking. Interestingly, MYO18A has been implicated as a cancer driver, as have other components of the GOLPH3 pathway. It remains uncertain as to whether or not MYO18A has intrinsic motor activity. While many questions remain, MYO18A is a fascinatingly unique myosin that is essential in higher organisms.

Hyperoxia treatment of TREK-1/TREK-2/TRAAK-deficient mice is associated with a reduction in surfactant proteins

We previously proposed a role for the two-pore domain potassium (K2P) channel TREK-1 in hyperoxia (HO)-induced lung injury. To determine whether redundancy among the three TREK isoforms (TREK-1, TREK-2, and TRAAK) could protect from HO-induced injury, we now examined the effect of deletion of all three TREK isoforms in a clinically relevant scenario of prolonged HO exposure and mechanical ventilation (MV). We exposed WT and TREK-1/TREK-2/TRAAK-deficient [triple knockout (KO)] mice to either room air, 72-h HO, MV [high and low tidal volume (TV)], or a combination of HO + MV and measured quasistatic lung compliance, bronchoalveolar lavage (BAL) protein concentration, histologic lung injury scores (LIS), cellular apoptosis, and cytokine levels. We determined surfactant gene and protein expression and attempted to prevent HO-induced lung injury by prophylactically administering an exogenous surfactant (Curosurf). HO treatment increased lung injury in triple KO but not WT mice, including an elevated LIS, BAL protein concentration, and markers of apoptosis, decreased lung compliance, and a more proinflammatory cytokine phenotype. MV alone had no effect on lung injury markers. Exposure to HO + MV (low TV) further decreased lung compliance in triple KO but not WT mice, and HO + MV (high TV) was lethal for triple KO mice. In triple KO mice, the HO-induced lung injury was associated with decreased surfactant protein (SP) A and SPC but not SPB and SPD expression. However, these changes could not be explained by alterations in the transcription factors nuclear factor-1 (NF-1), NKX2.1/thyroid transcription factor-1 (TTF-1) or c-jun, or lamellar body levels. Prophylactic Curosurf administration did not improve lung injury scores or compliance in triple KO mice.

Disruption of the structural and functional features of surfactant protein A by acrolein in cigarette smoke

The extent to which defective innate immune responses contribute to chronic obstructive pulmonary disease (COPD) is not fully understood. Pulmonary surfactant protein A (SP-A) plays an important role in regulating innate immunity in the lungs. In this study, we hypothesised that cigarette smoke (CS) and its component acrolein might influence pulmonary innate immunity by affecting the function of SP-A. Indeed, acrolein-modified SP-A was detected in the lungs of mice exposed to CS for 1 week. To further confirm this finding, recombinant human SP-A (hSP-A) was incubated with CS extract (CSE) or acrolein and then analysed by western blotting and nanoscale liquid chromatography-matrix-assisted laser desorption/ionisation time-of-flight tandem mass spectrometry. These analyses revealed that CSE and acrolein induced hSP-A oligomerisation and that acrolein induced the modification of six residues in hSP-A: His39, His116, Cys155, Lys180, Lys221, and Cys224. These modifications had significant effects on the innate immune functions of hSP-A. CSE- or acrolein-induced modification of hSP-A significantly decreased hSP-A's ability to inhibit bacterial growth and to enhance macrophage phagocytosis. These findings suggest that CS-induced structural and functional defects in SP-A contribute to the dysfunctional innate immune responses observed in the lung during cigarette smoking.

Novel role of surfactant protein A in bacterial sinusitis

BACKGROUND

Chronic rhinosinusitis (CRS) is a common inflammatory disorder of the upper airway characterized by chronic inflammation and significant sinonasal remodeling. CRS is comprised of 2 major subgroups, based on whether polyps are present or absent. In some cases, it is characterized by colonization with opportunistic pathogens such as Pseudomonas aeruginosa (PA), Staphylococcus aureus, and other bacteria. The innate immune system of the sinonasal epithelium is the first line of defense against inhaled pathogens. Surfactant protein A (SP-A) is a member of the collectin family secreted by the airway epithelia and plays a critical role in airway innate immunity, as it can aggregate bacteria. We hypothesized that SP-A plays a role in bacterial CRS.

METHODS

Air-liquid interface (ALI) cultures of nasal epithelial cells were derived from human ex-vivo healthy and CRS sinus tissues (n = 26) and challenged with PA. SP-A levels were measured with western blot and quantitative reverse transcript-polymerase chain reaction (qRT-PCR) in ALI and sinus tissues.

RESULTS

We determined that SP-A: (i) mRNA and protein levels are increased significantly in CRS tissues compared with healthy sinuses; (ii) although primarily expressed in the lung, it is also synthesized and expressed in sinonasal epithelia; (ii) is expressed in the sinuses of an SP-A humanized transgenic mouse but not in SP-A knockout mice; (iv) mRNA levels are upregulated significantly during PA challenge, but protein levels are downregulated 4 hours postchallenge and upregulated at 12 hours.

CONCLUSION

Our data suggest that SP-A is expressed in the sinuses and that it plays a role in the sinus innate immune responses during bacterial infections.

Structure, genetics and function of the pulmonary associated surfactant proteins A and D: The extra-pulmonary role of these C type lectins

The collectins family encompasses several collagenous Ca²⁺-dependent defense lectins that are described as pathogen recognition molecules. They play an important role in both adaptive and innate immunity. Surfactant proteins A and D are two of these proteins which were initially discovered in association with surfactant in the pulmonary system. The structure, immune and inflammatory functions, and genetic variations have been well described in relation to their roles, function and pathophysiology in the pulmonary system. Subsequently, these proteins have been discovered in a wide range of other organs and organ systems. The role of these proteins outside the pulmonary system is currently an active area of research. This review intends to provide a current overview of the genetics, structure and extra-pulmonary functions of the surfactant collectin proteins.

Phagocytosis of live versus killed or fluorescently labeled bacteria by macrophages differ in both magnitude and receptor specificity

Scavenger receptor (SR)-mediated opsonin-independent phagocytosis of bacteria by macrophages has been suggested to represent an important, early mechanism of anti-bacterial host defense. However, although the ability to bind bacteria has been demonstrated to be a shared feature of all types of SRs, in many cases the evidence is limited to the demonstration of increased binding of killed, fluorescently labeled bacteria to non-phagocytic cells transfected with these receptors. We sought to verify the ability of SRs to mediate non-opsonic phagocytosis of live Escherichia coli (Ec) and Staphylococcus aureus (Sa), model species of Gram-negative and -positive bacteria, respectively, and to assess the relative contributions of different SRs expressed on murine macrophages in this process. We found that the class A SR SR-A/CD204 was the major receptor mediating phagocytosis of fluorescently labeled Sa, whereas different SRs had highly redundant roles in the phagocytosis of live Sa. Conversely, different SRs contributed to the phagocytosis of fluorescently labeled Ec. In comparison, phagocytosis of live Ec was of much lower magnitude and was selectively mediated by SR-A. These results question the use of fluorescently labeled bacteria as valid replacements for live bacteria. The low magnitude of opsonin-independent phagocytosis of Ec and unimpaired phagocytosis of Sa in SR-A- or CD36-deficient macrophages indicate that the defect in this process might not be responsible for the reported impaired bacteria clearance in mice deficient in these receptors. We postulate that this impairment might result to a larger extent from inhibition of intracellular bacteria killing caused by pro-inflammatory cytokines, produced in excessive amounts by SR-deficient cells in response to bacterial products.

The importance of surfactant proteins-New aspects on macrophage phagocytosis

Surfactant and its components have multiple functions. The so called collectins are surfactant proteins which opsonize bacteria and improve pulmonary host defense via the phagocytosis and clearance of microorganisms and particles. In this special issue of the Annals of Anatomy a new surfactant protein, Surfactant Associated 3, is highlighted. As outlined in this mini review Surfactant Associated 3 is regarded as an enhancer of phagocytosis. In addition, the role played by SP-A is updated and open research questions raised.

Serum Lipoproteins Are Critical for Pulmonary Innate Defense against Staphylococcus aureus Quorum Sensing

Hyperlipidemia has been extensively studied in the context of atherosclerosis, whereas the potential health consequences of the opposite extreme, hypolipidemia, remain largely uninvestigated. Circulating lipoproteins are essential carriers of insoluble lipid molecules and are increasingly recognized as innate immune effectors. Importantly, severe hypolipidemia, which may occur with trauma or critical illness, is clinically associated with bacterial pneumonia. To test the hypothesis that circulating lipoproteins are essential for optimal host innate defense in the lung, we used lipoprotein-deficient mice and a mouse model of Staphylococcus aureus pneumonia in which invasive infection requires virulence factor expression controlled by the accessory gene regulator (agr) operon. Activation of agr and subsequent virulence factor expression is inhibited by apolipoprotein B, the structural protein of low-density lipoprotein, which binds and sequesters the secreted agr-signaling peptide (AIP). In this article, we report that lipoprotein deficiency impairs early pulmonary innate defense against S. aureus quorum-sensing-dependent pathogenesis. Specifically, apolipoprotein B levels in the lung early postinfection are significantly reduced with lipoprotein deficiency, coinciding with impaired host control of S. aureus agr-signaling and increased agr-dependent morbidity (weight loss) and inflammation. Given that lipoproteins also inhibit LTA- and LPS-mediated inflammation, these results suggest that hypolipidemia may broadly impact posttrauma pneumonia susceptibility to both Gram-positive and -negative pathogens. Together with previous reports demonstrating that hyperlipidemia also impairs lung innate defense, these results suggest that maintenance of normal serum lipoprotein levels is necessary for optimal host innate defense in the lung.

SP-R210 (Myo18A) Isoforms as Intrinsic Modulators of Macrophage Priming and Activation

The surfactant protein (SP-A) receptor SP-R210 has been shown to increase phagocytosis of SP-A-bound pathogens and to modulate cytokine secretion by immune cells. SP-A plays an important role in pulmonary immunity by enhancing opsonization and clearance of pathogens and by modulating macrophage inflammatory responses. Alternative splicing of the Myo18A gene results in two isoforms: SP-R210S and SP-R210L, with the latter predominantly expressed in alveolar macrophages. In this study we show that SP-A is required for optimal expression of SP-R210L on alveolar macrophages. Interestingly, pre-treatment with SP-A prepared by different methods either enhances or suppresses responsiveness to LPS, possibly due to differential co-isolation of SP-B or other proteins. We also report that dominant negative disruption of SP-R210L augments expression of receptors including SR-A, CD14, and CD36, and enhances macrophages' inflammatory response to TLR stimulation. Finally, because SP-A is known to modulate CD14, we used a variety of techniques to investigate how SP-R210 mediates the effect of SP-A on CD14. These studies revealed a novel physical association between SP-R210S, CD14, and SR-A leading to an enhanced response to LPS, and found that SP-R210L and SP-R210S regulate internalization of CD14 via distinct macropinocytosis-like mechanisms. Together, our findings support a model in which SP-R210 isoforms differentially regulate trafficking, expression, and activation of innate immune receptors on macrophages.

Scavenger receptor for lipoteichoic acid is involved in the potent ability of Lactobacillus plantarum strain L-137 to stimulate production of interleukin-12p40

Heat-killed Lactobacillus plantarum strain L-137 (HK L-137) is a more potent inducer of interleukin (IL)-12 than other heat-killed Lactobacillus strains. To elucidate the mechanism involved in this IL-12p40 induction, we compared HK L-137 with heat-killed L. plantarum strain JCM1149 (HK JCM1149) by nuclear magnetic resonance and mass spectrometry. Results showed that HK L-137 contained lipoteichoic acid (LTA) with a chemical structure similar to that of JCM1149, except for a lower degree of glucosyl substitution in the poly(glycerol phosphate) backbone. Lysozyme sensitivity and electrophoretic moiety analysis revealed that HK L-137 exposed more LTA on its cell surface than HK JCM1149. Phagocytosis of HK L-137 by splenic adherent cells was significantly greater than that of HK JCM1149. Anti-LTA antibody and anti-scavenger receptor-A (SR-A) antibody selectively inhibited phagocytosis of HK L-137, as well as IL-12p40 production, by splenic adherent cells. Thus, a higher efficiency of phagocytosis of HK L-137 via SR-A for LTA is responsible for the potent IL-12p40 induction.

Housing conditions modulate the severity of Mycoplasma pulmonis infection in mice deficient in class A scavenger receptor

Mycoplasmosis is a frequent causative microbial agent of community-acquired pneumonia and has been linked to exacerbation of chronic obstructive pulmonary disease. The macrophage class A scavenger receptor (SRA) facilitates the clearance of noxious particles, oxidants, and infectious organisms by alveolar macrophages. We examined wildtype and SRA(-/-) mice, housed in either individually ventilated or static filter-top cages that were cycled with fresh bedding every 14 d, as a model of gene-environment interaction on the outcome of pulmonary Mycoplasma pulmonis infection. Intracage NH3 gas measurements were recorded daily prior to infection. Mice were intranasally infected with 1 × 10(7) cfu M. pulmonis UAB CT and evaluated at 3, 7, and 14 d after inoculation. Wildtype mice cleared 99.5% of pulmonary M. pulmonis by 3 d after infection but remained chronically infected through the study. SRA (-/-) mice were chronically infected with 40-fold higher mycoplasma numbers than were wildtype mice. M. pulmonis caused a chronic mixed inflammatory response that was accompanied with high levels of IL1β, KC, MCP1, and TNFα in SRA(-/-) mice, whereas pulmonary inflammation in WT mice was represented by a monocytosis with elevation of IL1β. Housing had a prominent influence on the severity and persistence of mycoplasmosis in SRA(-/-) mice. SRA(-/-) mice housed in static cages had an improved recovery and significant changes in surfactant proteins SPA and SPD compared with baseline levels. These results indicate that SRA is required to prevent chronic mycoplasma infection of the lung. Furthermore, environmental conditions may exacerbate chronic inflammation in M. pulmonis-infected SRA(-/-) mice.

The role of macrophages in the innate immune response to Streptococcus pneumoniae and Staphylococcus aureus: mechanisms and contrasts

Macrophages are critical mediators of innate immune responses against bacteria. The Gram-positive bacteria Streptococcus pneumoniae and Staphylococcus aureus express a range of virulence factors, which challenge macrophages' immune competence. We review how macrophages respond to this challenge. Macrophages employ a range of strategies to phagocytose and kill each pathogen. When the macrophages capacity to clear bacteria is overwhelmed macrophages play important roles in orchestrating the inflammatory response through pattern recognition receptor-mediated responses. Macrophages also ensure the inflammatory response is tightly constrained, to avoid tissue damage, and play an important role in downregulating the inflammatory response once initial bacterial replication is controlled.

Transmigration and phagocytosis of macrophages in an airway infection model using four-dimensional techniques

During infection, recruited phagocytes transmigrate across the epithelium to remove the pathogens deposited on the airway surface. However, it is difficult to directly observe cellular behaviors (e.g., transmigration) in single-cell layer cultures or in live animals. Combining a three-dimensional (3D) cell coculture model mimicking airway infection with time-lapse confocal imaging as a four-dimensional technique allowed us to image the behaviors of macrophages in 3D over time. The airway infection model was moved to a glass-bottomed dish for live-cell imaging by confocal laser scanning microscopy. Using time-lapse confocal imaging, we recorded macrophages transmigrating across the polyethylene terephthalate (PET) membrane of the inserts through the 5-μm pores in the PET membrane. Macrophages on the apical side of the insert exhibited essentially three types of movements, one of which was transmigrating across the epithelial cell monolayer and arriving at the surface of monolayer. We found that adding Staphylococcus aureus to the model increased the transmigration index but not the transmigration time of the macrophages. Only in the presence of S. aureus were the macrophages able to transmigrate across the epithelial cell monolayer. Apical-to-basal transmigration of macrophages was visualized dynamically. We also imaged the macrophages phagocytizing S. aureus deposited on the surface of the monolayer in the airway infection model. This work provides a useful tool to study the cellular behaviors of immune cells spatially and temporally during infection.

BDCA1-positive dendritic cells (DCs) represent a unique human myeloid DC subset that induces innate and adaptive immune responses to Staphylococcus aureus Infection

Staphylococcus aureus bloodstream infection (bacteremia) is a major cause of morbidity and mortality and places substantial cost burdens on health care systems. The role of peripheral blood dendritic cells (PBDCs) in the immune responses against S. aureus infection has not been well characterized. In this study, we demonstrated that BDCA1(+) myeloid DCs (mDCs) represent a unique PBDC subset that can induce immune responses against S. aureus infection. BDCA1(+) mDCs could engulf S. aureus and strongly upregulated the expression of costimulatory molecules and production of proinflammatory cytokines. Furthermore, BDCA1(+) mDCs expressed high levels of major histocompatibility complex (MHC) class I and II molecules in response to S. aureus and greatly promoted proliferation and gamma interferon (IFN-γ) production in CD4 and CD8 T cells. Moreover, BDCA1(+) mDCs expressed higher levels of Toll-like receptor 2 (TLR-2) and scavenger receptor A (SR-A) than those on CD16(+) and BDCA3(+) mDCs, and these two receptors were both required for the recognition of S. aureus and the subsequent activation of BDCA1(+) mDCs. Finally, BDCA1(+) mDC-mediated immune responses against S. aureus were dependent on MyD88 signaling pathways. These results demonstrate that human BDCA1(+) mDCs represent a unique subset of mDCs that can respond to S. aureus to undergo maturation and activation and to induce Th1 and Tc1 immune responses.

Surfactant protein A mediates pulmonary clearance of Staphylococcus aureus

Surfactant protein A has been shown to enhance opsonization and clearance of Staphylococcus aureus in vitro. Here, the phagocytosis of alveolar S. aureus was investigated in vivo using intravital microscopy. Fluorescence labelled S. aureus Newman cells were intratracheally administered to anesthetized mice and the alveolar surface was observed for fifteen minutes. Confirming previously reported in vitro data, surfactant protein A-deficient mice showed a significantly reduced uptake of bacteria compared to wild-type mice.

Protective or deleterious role of scavenger receptors SR-A and CD36 on host resistance to Staphylococcus aureus depends on the site of infection

Staphylococcus aureus is a major human opportunistic pathogen responsible for a broad spectrum of infections ranging from benign skin infection to more severe life threatening disorders (e.g. pneumonia, sepsis), particularly in intensive care patients. Scavenger receptors (SR-A and CD36) are known to be involved in S. aureus recognition by immune cells in addition to MARCO, TLR2, NOD2 and α5β1 integrin. In the present study, we further deciphered the contribution of SR-A and CD36 scavenger receptors in the control of infection of mice by S. aureus. Using double SR-A/CD36 knockout mice (S/C-KO) and S. aureus strain HG001, a clinically relevant non-mutagenized strain, we showed that the absence of these two scavenger receptors was protective in peritoneal infection. In contrast, the deletion of these two receptors was detrimental in pulmonary infection following intranasal instillation. For pulmonary infection, susceptible mice (S/C-KO) had more colony-forming units (CFU) in their broncho-alveolar lavages fluids, associated with increased recruitment of macrophages and neutrophils. For peritoneal infection, susceptible mice (wild-type) had more CFU in their blood, but recruited less macrophages and neutrophils in the peritoneal cavity than resistant mice. Exacerbated cytokine levels were often observed in the susceptible mice in the infected compartment as well as in the plasma. The exception was the enhanced compartmentalized expression of IL-1β for the resistant mice (S/C-KO) after peritoneal infection. A similar mirrored susceptibility to S. aureus infection was also observed for MARCO and TLR2. Marco and tlr2 -/- mice were more resistant to peritoneal infection but more susceptible to pulmonary infection than wild type mice. In conclusion, our results show that innate immune receptors can play distinct and opposite roles depending on the site of infection. Their presence is protective for local pulmonary infection, whereas it becomes detrimental in the peritoneal infection.

TLR2 mediates phagocytosis and autophagy through JNK signaling pathway in Staphylococcus aureus-stimulated RAW264.7 cells

Toll-like receptor 2 (TLR2) is involved in phagocytosis and autophagy to enhance host innate immune response to bacterial infection. TLR2 has been reported to participate in the recognition of Staphylococcus aureus (S. aureus). However, the role of TLR2 in phagocytosis and autophagy in S. aureus-stimulated macrophages and the underlying mechanisms as yet remain unclear. In the present study, stimulation of mouse macrophage cell line RAW264.7 with S. aureus activated multiple signaling pathways including mitogen-activated protein kinases (MAPKs), myeloid differentiation factor 88 (MyD88), phosphatidylinositide 3-kinase (PI3K) and Rac1 and triggered autophagy process. Knockdown of TLR2 by siRNA significantly reduced phagocytosis and autophagy of macrophages upon S. aureus infection. Interestingly, TLR2 siRNA markedly attenuated S. aureus-induced phosphorylation of c-Jun N-terminal kinase (JNK) but not p38 or extracellular regulated protein kinase (ERK) in macrophages. Similarly, SP600125, a JNK inhibitor, also down-regulated phagocytosis and autophagy in S. aureus-stimulated macrophages. Furthermore, TLR2 siRNA and SP600125 simultaneous treatment showed similar phagocytosis and autophagy compared to that in TLR2 siRNA treatment alone. Collectively, our results indicate that TLR2 may be critical for phagocytosis and autophagy through JNK signaling pathway, and provide an underlying mechanistic link between innate immune receptor and induction of phagocytosis and autophagy in S. aureus-stimulated macrophages.

Surfactant protein A binds flagellin enhancing phagocytosis and IL-1β production

Surfactant protein A (SP-A), a pulmonary collectin, plays a role in lung innate immune host defense. In this study the role of SP-A in regulating the inflammatory response to the flagella of Pseudomonas aeruginosa (PA) was examined. Intra-tracheal infection of SP-A deficient (SP-A-/-) C57BL/6 mice with wild type flagellated PA (PAK) resulted in an increase in inflammatory cell recruitment and increase in pro-inflammatory cytokines IL-6 and TNF-α, which was not observed with a mutant pseudomonas lacking flagella (fliC). SP-A directly bound flagellin, via the N-linked carbohydrate moieties and collagen-like domain, in a concentration dependent manner and enhanced macrophage phagocytosis of flagellin and wild type PAK. IL-1β was reduced in the lungs of SP-A-/- mice following PAK infection. MH-s cells, a macrophage cell line, generated greater IL-1β when stimulated with flagellin and SP-A. Historically flagella stimulate IL-1β production through the toll-like receptor 5 (TLR-5) pathway and through a caspase-1 activating inflammasome pathway. IL-1β expression became non-detectable in SP-A and flagellin stimulated MH-s cells in which caspase-1 was silenced, suggesting SP-A induction of IL-1β appears to be occurring through the inflammasome pathway. SP-A plays an important role in the pathogenesis of PA infection in the lung by binding flagellin, enhancing its phagocytosis and modifying the macrophage inflammatory response.

The class A scavenger receptor SR-A/CD204 and the class B scavenger receptor CD36 regulate immune functions of macrophages differently

SR-A/CD204 and CD36 are major receptors responsible for oxidized lipoproteins uptake by macrophages in atherosclerotic plaques. Both receptors also share the role as receptors for different pathogens, but studies on their signaling have been hampered by the lack of selective ligands. We report that, upon specific ligation by Ab, SR-A does not induce cytokine production, but mediates inhibition of LPS-stimulated production of IL-6 and IL-12/23p40, enhancement of IL-10 release, and has no effect on TNF-α and RANTES production in murine macrophages. In contrast, anti-CD36 Ab alone stimulated production of all these cytokines, with IL-10 production being exceptionally high. Effects of anti-CD36 Ab, except of IL-10 production, were mediated by CD14 and TLR2, whereas those of SR-A ligation by heterotrimeric Gi/o proteins and by phosphatidylinositol 3-kinases. Surprisingly, we found that LPS uptake by macrophages was mediated in part by CD36 cooperating with CD14, whereas SR-A was not involved in this process. Finely, during in vitro Ag presentation to naïve CD4(+) lymphocytes, pre-incubation of macrophages with anti-CD36 Ab enhanced IFN-γ production in the co-culture, but exerted the opposite effect under conditions enabling IL-10 accumulation. In contrast, anti-SR-A Ab was ineffective alone, but reversed the Th1-polarizing effect of LPS.

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.

Type IV pilus of Pseudomonas aeruginosa confers resistance to antimicrobial activities of the pulmonary surfactant protein-A

Pseudomonas aeruginosa(PA) is a Gram-negative bacterial pathogen commonly associated with chronic lung infections. Previously, we have identified several PA virulence factors that are important for resistance to the surfactant protein-A (SP-A), a pulmonary innate immunity protein that mediates bacterial opsonization and membrane permeabilization. In this study, we demonstrate that the type IV pilus (Tfp) is important in the resistance of PA to the antibacterial effects of SP-A. The Tfp-deficient mutant ΔpilA is severely attenuated in an acute pneumonia model of infection in the lungs of wild-type mice, but is virulent in the lungs of SP-A(-/-) mice. The ΔpilA bacteria are more susceptible to SP-A-mediated aggregation and opsonization. In addition, the integrity of the outer membranes of ΔpilA bacteria is compromised, rendering them more susceptible to SP-A-mediated membrane permeabilization. By comparing Tfp extension and retraction mutants, we demonstrate that the increased susceptibility of ΔpilA to SP-A-mediated opsonization requires the total absence of Tfp from PA cells. Finally, we provide evidence of increased expression of nonpilus adhesin OprH that may serve as an SP-A ligand, resulting in increased phagocytosis and preferential pulmonary clearance of ΔpilA.

A zebrafish high throughput screening system used for Staphylococcus epidermidis infection marker discovery

BACKGROUND

Staphylococcus epidermidis bacteria are a major cause of biomaterial-associated infections in modern medicine. Yet there is little known about the host responses against this normally innocent bacterium in the context of infection of biomaterials. In order to better understand the factors involved in this process, a whole animal model with high throughput screening possibilities and markers for studying the host response to S. epidermidis infection are required.

RESULTS

We have used a zebrafish yolk injection system to study bacterial proliferation and the host response in a time course experiment of S. epidermidis infection. By combining an automated microinjection system with complex object parametric analysis and sorting (COPAS) technology we have quantified bacterial proliferation. This system was used together with transcriptome analysis at several time points during the infection period. We show that bacterial colony forming unit (CFU) counting can be replaced by high throughput flow-based fluorescence analysis of embryos enabling high throughput readout. Comparison of the host transcriptome response to S. epidermidis and Mycobacterium marinum infection in the same system showed that M. marinum has a far stronger effect on host gene regulation than S. epidermidis. However, multiple genes responded differently to S. epidermidis infection than to M. marinum, including a cell adhesion gene linked to specific infection by staphylococci in mammals.

CONCLUSIONS

Our zebrafish embryo infection model allowed (i) quantitative assessment of bacterial proliferation, (ii) identification of zebrafish genes serving as markers for infection with the opportunistic pathogen S. epidermidis, and (iii) comparison of the transcriptome response of infection with S. epidermidis and with the pathogen M. marinum. As a result we have identified markers that can be used to distinguish common and specific responses to S. epidermidis. These markers enable the future integration of our high throughput screening technology with functional analyses of immune response genes and immune modulating factors.

A TLR4-interacting SPA4 peptide inhibits LPS-induced lung inflammation

The interaction between surfactant protein-A (SP-A) and TLR4 is important for host defense. We have recently identified an SPA4 peptide region from the interface of SP-A-TLR4 complex. Here, we studied the involvement of the SPA4 peptide region in SP-A-TLR4 interaction using a two-hybrid system, and biological effects of SPA4 peptide in cell systems and a mouse model. HEK293 cells were transfected with plasmid DNAs encoding SP-A or a SP-A-mutant lacking SPA4 peptide region and TLR4. Luciferase activity was measured as the end-point of SP-A-TLR4 interaction. NF-κB activity was also assessed simultaneously. Next, the dendritic cells or mice were challenged with Escherichia coli-derived LPS and treated with SPA4 peptide. Endotoxic shock-like symptoms and inflammatory parameters (TNF-α, NF-κB, leukocyte influx) were assessed. Our results reveal that the SPA4 peptide region contributes to the SP-A-TLR4 interaction and inhibits the LPS-induced NF-κB activity and TNF-α. We also observed that the SPA4 peptide inhibits LPS-induced expression of TNF-α, nuclear localization of NF-κB-p65 and cell influx, and alleviates the endotoxic shock-like symptoms in a mouse model. Our results suggest that the anti-inflammatory activity of the SPA4 peptide through its binding to TLR4 can be of therapeutic benefit.

Staphylococcus aureus and Pseudomonas aeruginosa express and secrete human surfactant proteins

Surfactant proteins (SP), originally known from human lung surfactant, are essential to proper respiratory function in that they lower the surface tension of the alveoli. They are also important components of the innate immune system. The functional significance of these proteins is currently reflected by a very large and growing number of publications. The objective goal of this study was to elucidate whether Staphylococcus aureus and Pseudomonas aeruginosa is able to express surfactant proteins. 10 different strains of S. aureus and P. aeruginosa were analyzed by means of RT-PCR, Western blot analysis, ELISA, immunofluorescence microscopy and immunoelectron microscopy. The unexpected and surprising finding revealed in this study is that different strains of S. aureus and P. aeruginosa express and secrete proteins that react with currently commercially available antibodies to known human surfactant proteins. Our results strongly suggest that the bacteria are either able to express ‘human-like’ surfactant proteins on their own or that commercially available primers and antibodies to human surfactant proteins detect identical bacterial proteins and genes. The results may reflect the existence of a new group of bacterial surfactant proteins and DNA currently lacking in the relevant sequence and structure databases. At any rate, our knowledge of human surfactant proteins obtained from immunological and molecular biological studies may have been falsified by the presence of bacterial proteins and DNA and therefore requires critical reassessment.

Staphylococcus aureus proteases degrade lung surfactant protein A potentially impairing innate immunity of the lung

The pulmonary surfactant is a complex mixture of lipids and proteins that is important for respiratory lung functions, which also provides the first line of innate immune defense. Pulmonary surfactant protein-A (SP-A) is a major surfactant component with immune functions with importance during Staphylococcus aureus infections that has been demonstrated in numerous studies. The current study showed that S. aureus can efficiently cleave the SP-A protein using its arsenal of proteolytic enzymes. This degradation appears to be mediated by cysteine proteases, in particular staphopain A (ScpA). The staphopain-mediated proteolysis of SP-A resulted in a decrease or complete abolishment of SP-A biological activity, including the promotion of S. aureus phagocytosis by neutrophils, aggregation of Gram-negative bacteria and bacterial cell adherence to epithelium. Significantly, ScpA has also efficiently degraded SP-A in complete bronchi-alveolar lavage fluid from human lungs. This indicates that staphopain activity in the lungs is resistant to protease inhibitors, thus suggesting that SP-A can be cleaved in vivo. Collectively, this study showed that the S. aureus protease ScpA is an important virulence factor that may impair innate immunity of the lungs.

Alveolar epithelial cells are critical in protection of the respiratory tract by secretion of factors able to modulate the activity of pulmonary macrophages and directly control bacterial growth

The respiratory epithelium is a physical and functional barrier actively involved in the clearance of environmental agents. The alveolar compartment is lined with membranous pneumocytes, known as type I alveolar epithelial cells (AEC I), and granular pneumocytes, type II alveolar epithelial cells (AEC II). AEC II are responsible for epithelial reparation upon injury and ion transport and are very active immunologically, contributing to lung defense by secreting antimicrobial factors. AEC II also secrete a broad variety of factors, such as cytokines and chemokines, involved in activation and differentiation of immune cells and are able to present antigen to specific T cells. Another cell type important in lung defense is the pulmonary macrophage (PuM). Considering the architecture of the alveoli, a good communication between the external and the internal compartments is crucial to mount effective responses. Our hypothesis is that being in the interface, AEC may play an important role in transmitting signals from the external to the internal compartment and in modulating the activity of PuM. For this, we collected supernatants from AEC unstimulated or stimulated in vitro with lipopolysaccharide (LPS). These AEC-conditioned media were used in various setups to test for the effects on a number of macrophage functions: (i) migration, (ii) phagocytosis and intracellular control of bacterial growth, and (iii) phenotypic changes and morphology. Finally, we tested the direct effect of AEC-conditioned media on bacterial growth. We found that AEC-secreted factors had a dual effect, on one hand controlling bacterial growth and on the other hand increasing macrophage activity.