Critical role of amino acid position 343 of surfactant protein-D in the selective binding of glycolipids from Mycobacterium tuberculosis

Role of pattern recognition receptors in sensing Mycobacterium tuberculosis

Mycobacterium tuberculosis is one of the major invasive intracellular pathogens causing most deaths by a single infectious agent. The interaction between host immune cells and this pathogen is the focal point of the disease, Tuberculosis. Host immune cells not only mount the protective action against this pathogen but also serve as the primary niche for growth. Thus, recognition of this pathogen by host immune cells and following signaling cascades are key dictators of the disease state. Immune cells, mainly belonging to myeloid cell lineage, recognize a wide variety of Mycobacterium tuberculosis ligands ranging from carbohydrate and lipids to proteins to nucleic acids by different membrane-bound and soluble pattern recognition receptors. Simultaneous interaction between different host receptors and pathogen ligands leads to immune-inflammatory response as well as contributes to virulence. This review summarizes the contribution of pattern recognition receptors of host immune cells in recognizing Mycobacterium tuberculosis and subsequent initiation of signaling pathways to provide the molecular insight of the specific Mtb ligands interacting with specific PRR, key adaptor molecules of the downstream signaling pathways and the resultant effector functions which will aid in identifying novel drug targets, and developing novel drugs and adjuvants.

An evolutionary medicine perspective on the cetacean pulmonary immune system - The first identification of SP-D and LBP in the bottlenose dolphin (Tursiops truncatus)

Evolutionary medicine expresses the present status of biomolecules affected by past evolutionary events. To clarify the whole picture of cetacean pneumonia, which is a major threat to cetaceans, their pulmonary immune system should be studied from the perspective of evolutionary medicine. In this in silico study, we focused on cetacean surfactant protein D (SP-D) and lipopolysaccharide-binding protein (LBP) as two representative molecules of the cetacean pulmonary immune system. Sequencing and analyzing SP-D and LBP in the bottlenose dolphin (Tursiops truncatus) lung and liver tissue collected post-mortem elucidated not only basic physicochemical properties but also their evolutionary background. This is the first study to report the sequences and expression of SP-D and LBP in the bottlenose dolphin. Besides, our findings also suggest the direction of an evolutionary arms race in the cetacean pulmonary immune system. These results have important positive implications for cetacean clinical medicine.

Involvement of Surfactant Protein D in Ebola Virus Infection Enhancement via Glycoprotein Interaction

Since the largest 2014⁻2016 Ebola virus disease outbreak in West Africa, understanding of Ebola virus infection has improved, notably the involvement of innate immune mediators. Amongst them, collectins are important players in the antiviral innate immune defense. A screening of Ebola glycoprotein (GP)-collectins interactions revealed the specific interaction of human surfactant protein D (hSP-D), a lectin expressed in lung and liver, two compartments where Ebola was found in vivo. Further analyses have demonstrated an involvement of hSP-D in the enhancement of virus infection in several in vitro models. Similar effects were observed for porcine SP-D (pSP-D). In addition, both hSP-D and pSP-D interacted with Reston virus (RESTV) GP and enhanced pseudoviral infection in pulmonary cells. Thus, our study reveals a novel partner of Ebola GP that may participate to enhance viral spread.

Functional Analysis of Genetic Variations in Surfactant Protein D in Mycobacterial Infection and Their Association With Tuberculosis

Surfactant proteins (SPs)-A and -D are C-type lectins of the collectin family and function in the clearance of infectious particles in the lungs. Some polymorphisms of SPs that give rise to amino acid changes have been found to affect their function. Several SP-A gene polymorphisms have been reported to be associated with respiratory infection diseases, such as tuberculosis (TB). However, the relationship between surfactant proteins D (SP-D) polymorphisms and TB is still unclear. To study the associations between SP-D polymorphisms and TB, the correlations of SP-D polymorphisms with TB were examined in a case-control study, which included 364 patients with TB and 177 control subjects. In addition, we cloned two major SP-D exonic polymorphism C92T (rs721917) and A538G (rs2243639) constructs and used these for in vitro assays. The effects of SP-D polymorphisms on agglutination and other interactions with Mycobacterium bovis bacillus Calmette-Guérin (M. bovis BCG) were evaluated. In comparison with SP-D 92C (amino acid residue 16, Threonine), our results showed that SP-D 92T (amino acid residue 16, Methionine) had a lower binding ability to M. bovis BCG, a lower capacity to inhibit phagocytosis, lesser aggregation, poorer survival of bacillus Calmette-Guérin (BCG)-infected MH-S cells, and less inhibition of intracellular growth of M. bovis BCG. The case-control association study showed that the 92T homozygous genotype was a risk factor for TB. However, a lesser effect was seen for polymorphism A538G. In conclusion, the results of functional and genetic analyses of SP-D variants consistently showed that the SP-D 92T variant increased susceptibility to TB, which further confirmed the role of SP-D in pulmonary innate immunity against mycobacterial infection.

AmpliSeq Screening of Genes Encoding the C-Type Lectin Receptors and Their Signaling Components Reveals a Common Variant in MASP1 Associated with Pulmonary Tuberculosis in an Indian Population

Tuberculosis (TB) is a multifactorial disease governed by bacterial, host and environmental factors. On the host side, growing evidence shows the crucial role that genetic variants play in the susceptibility to Mycobacterium tuberculosis (Mtb) infection. Such polymorphisms have been described in genes encoding for different cytokines and pattern recognition receptors (PRR), including numerous Toll-like receptors (TLRs). In recent years, several members of the C-type lectin receptors (CTLRs) have been identified as key PRRs in TB pathogenesis. Nevertheless, studies to date have only addressed particular genetic polymorphisms in these receptors or their related pathways in relation with TB. In the present study, we screened the main CTLR gene clusters as well as CTLR pathway-related genes for genetic variation associated with pulmonary tuberculosis (PTB). This case-control study comprised 144 newly diagnosed pulmonary TB patients and 181 healthy controls recruited at the Bhagwan Mahavir Medical Research Center (BMMRC), Hyderabad, India. A two-stage study was employed in which an explorative AmpliSeq-based screening was followed by a validation phase using iPLEX MassARRAY. Our results revealed one SNP (rs3774275) in MASP1 significantly associated with PTB in our population (joint analysis p = 0.0028). Furthermore, serum levels of MASP1 were significantly elevated in TB patients when compared to healthy controls. Moreover, in the present study we could observe an impact of increased MASP1 levels on the lectin pathway complement activity in vitro. In conclusion, our results demonstrate a significant association of MASP1 polymorphism rs3774275 and MASP1 serum levels with the development of pulmonary TB. The present work contributes to our understanding of host-Mtb interaction and reinforces the critical significance of mannose-binding lectin and the lectin-complement pathway in Mtb pathogenesis. Moreover, it proposes a MASP1 polymorphism as a potential genetic marker for TB resistance.

[Preoperatiove Airway Bacterial Colonization: the Missing Link between Non-small Cell Lung Cancer Following Lobectomy and Postoperative Pneumonia?]

背景与目的

外科手术是目前治疗肺癌的主要手段，肺癌患者围术期死亡的主要原因仍是术后肺炎。已有的研究结果显示致病性气道定植菌被认为是术后肺部并发症的一个独立危险因素，本研究旨在探讨术前致病性气道定植菌的存在与术后发生肺炎的关系及其危险因素。

方法

横断面调查2014年5月至2015年1月连续收治于成都市6家三级甲等医院胸外科行手术治疗的125例非小细胞肺癌患者，术前经纤维支气管镜取气管及支气管内液细菌学标本，并检测术前血清肺表面活性蛋白D（surfactant protein D, SP-D）水平，术后肺部相关并发症进行分析。

结果

肺癌患者术前合并致病性气道定植菌的发生率为15.2%（19/125），以革兰氏阴性菌为主（19/22, 86.36%）；肺癌患者术前合并致病性气道定植菌的高危因素为：高龄（≥75岁）和长期吸烟史（吸烟指数≥400支/年）；术后肺部相关并发症和术后肺炎发生率在肺癌合并致病性气道定植菌组（42.11%, 26.32%）均显著高于非合并组（16.04%, 6.60%）（P=0.021, P=0.019）。术前血清SP-D浓度在肺癌合并致病性气道定植菌（31.25±6.09）显著高于非合并组（28.17±5.23）（P=0.023）。并发术后肺炎患者中气道致病性定值菌发生率为41.67%（5/12），其发生率是无手术后肺炎患者的3.4倍（OR=3.363, 95%CI: 1.467-7.711）。

结论

肺癌患者合并致病性气道定植菌与术后肺炎发生密切相关，且高危险因素是高龄和长期吸烟史。

Mannose-recognition mutant of the galactose/N-acetylgalactosamine-specific C-type lectin CEL-I engineered by site-directed mutagenesis

BACKGROUND

CEL-I is a galactose/N-acetylgalactosamine-specific C-type lectin isolated from the sea cucumber Cucumaria echinata. Its carbohydrate-binding site contains a QPD (Gln-Pro-Asp) motif, which is generally recognized as the galactose specificity-determining motif in the C-type lectins. In our previous study, replacement of the QPD motif by an EPN (Glu-Pro-Asn) motif led to a weak binding affinity for mannose. Therefore, we examined the effects of an additional mutation in the carbohydrate-binding site on the specificity of the lectin.

METHODS

Trp105 of EPN-CEL-I was replaced by a histidine residue using site-directed mutagenesis, and the binding affinity of the resulting mutant, EPNH-CEL-I, was examined by sugar-polyamidoamine dendrimer assay, isothermal titration calorimetry, and glycoconjugate microarray analysis. Tertiary structure of the EPNH-CEL-I/mannose complex was determined by X-ray crystallographic analysis.

RESULTS

Sugar-polyamidoamine dendrimer assay and glycoconjugate microarray analysis revealed a drastic change in the specificity of EPNH-CEL-I from galactose/N-acetylgalactosamine to mannose. The association constant of EPNH-CEL-I for mannose was determined to be 3.17×10(3) M(-1) at 25°C. Mannose specificity of EPNH-CEL-I was achieved by stabilization of the binding of mannose in a correct orientation, in which the EPN motif can form proper hydrogen bonds with 3- and 4-hydroxy groups of the bound mannose.

CONCLUSIONS

Specificity of CEL-I can be engineered by mutating a limited number of amino acid residues in addition to the QPD/EPN motifs.

GENERAL SIGNIFICANCE

Versatility of the C-type carbohydrate-recognition domain structure in the recognition of various carbohydrate chains could become a promising platform to develop novel molecular recognition proteins.

Changes in the major cell envelope components of Mycobacterium tuberculosis during in vitro growth

One-third of the world's population is infected with Mycobacterium tuberculosis (M.tb), which causes tuberculosis. Mycobacterium tuberculosis cell envelope components such as glycolipids, lipoglycans and polysaccharides play important roles in bacteria-host cell interactions that dictate the host immune response. However, little is known about the changes in the amounts and types of these cell envelope components as the bacillus divides during in vitro culture. To shed light on these phenomena, we examined growth-dependent changes over time in major cell envelope components of virulent M.tb by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, thin-layer chromatography, mass spectrometry, immunoblotting and flow cytometry. Our studies provide evidence that major mannosylated glycoconjugates on the M.tb cell envelope change as M.tb grows in vitro on the widely used Middlebrook 7H11 agar. In particular, our compositional analyses show that from Day 9 to 28 the amounts of mannose-containing molecules, such as mannose-capped lipoarabinomannan, lipomannan and phosphatidyl-myo-inositol mannosides, change continuously in both the cell envelope and outer cell surface. Along with these changes, mannan levels on the outer cell surface also increase significantly over time. The implications of these differences in terms of how M.tb is grown for studies performed in vitro and in vivo for assessing M.tb-host recognition and establishment of infection are discussed.

Protective effect of surfactant protein d in pulmonary vaccinia virus infection: implication of A27 viral protein

Vaccinia virus (VACV) was used as a surrogate of variola virus (VARV) (genus Orthopoxvirus), the causative agent of smallpox, to study Orthopoxvirus infection. VARV is principally transmitted between humans by aerosol droplets. Once inhaled, VARV first infects the respiratory tract where it could encounter surfactant components, such as soluble pattern recognition receptors. Surfactant protein D (SP-D), constitutively present in the lining fluids of the respiratory tract, plays important roles in innate host defense against virus infection. We investigated the role of SP-D in VACV infection and studied the A27 viral protein involvement in the interaction with SP-D. Interaction between SP-D and VACV caused viral inhibition in a lung cell model. Interaction of SP-D with VACV was mediated by the A27 viral protein. Binding required Ca2+ and interactions were blocked in the presence of excess of SP-D saccharide ligands. A27, which lacks glycosylation, directly interacted with SP-D. The interaction between SP-D and the viral particle was also observed using electron microscopy. Infection of mice lacking SP-D (SP-D-/-) resulted in increased mortality compared to SP-D+/+ mice. Altogether, our data show that SP-D participates in host defense against the vaccinia virus infection and that the interaction occurs with the viral surface protein A27.

The role of airway epithelial cells in response to mycobacteria infection

Airway epithelial cells (AECs) are part of the frontline defense against infection of pathogens by providing both a physical barrier and immunological function. The role of AECs in the innate and adaptive immune responses, through the production of antimicrobial molecules and proinflammatory factors against a variety of pathogens, has been well established. Tuberculosis (TB), a contagious disease primarily affecting the lungs, is caused by the infection of various strains of mycobacteria. In response to mycobacteria infection, epithelial expression of Toll-like receptors and surfactant proteins plays the most prominent roles in the recognition and binding of the pathogen, as well as the initiation of the immune response. Moreover, the antimicrobial substances, proinflammatory factors secreted by AECs, composed a major part of the innate immune response and mediation of adaptive immunity against the pathogen. Thus, a better understanding of the role and mechanism of AECs in response to mycobacteria will provide insight into the relationship of epithelial cells and lung immunocytes against TB, which may facilitate our understanding of the pathogenesis and immunological mechanism of pulmonary tuberculosis disease.

Optimization of inhaled therapies for tuberculosis: the role of macrophages and dendritic cells

Inhaled therapies in the form of drugs or vaccines for tuberculosis treatment were reported about a decade ago. Experts around the world met to discuss the scientific progress in inhaled therapies at the international symposium "Optimization of inhaled Tuberculosis therapies and implications for host-pathogen interactions" held in New Delhi, India on November 3-5, 2009. The meeting was organized by the Central Drug Research Institute (CDRI) Lucknow, India. The lung is the main route for infection with Mycobacterium tuberculosis bacilli and the primary site of reactivation of latent disease. The only available vaccine BCG is relatively ineffective at preventing tuberculosis disease and current therapy requires prolonged treatment with drugs which results in low patient compliance. Consequently, there is a need to design new vaccines and therapies for this disease. Recently there has been increased interest in the development of inhaled formulations to deliver anti-mycobacterial drugs and vaccines directly to the lung and many of these therapies are designed to target lung macrophages and dendritic cells. However, the development of effective inhaled therapies requires an understanding of the unique function and immunosuppressive environment of the lung which is driven, in part, by alveolar macrophages and dendritic cells. In this review, we will discuss the role of alveolar macrophages and dendritic cells in the host immune response to M. tuberculosis infection and the ways in which inhaled therapies might enhance the anti-microbial response of phagocytes and boost pulmonary immunity.

Mutagenesis of surfactant protein D informed by evolution and x-ray crystallography enhances defenses against influenza A virus in vivo

The recognition of influenza A virus (IAV) by surfactant protein D (SP-D) is mediated by interactions between the SP-D carbohydrate recognition domains (CRD) and glycans displayed on envelope glycoproteins. Although native human SP-D shows potent antiviral and aggregating activity, trimeric recombinant neck+CRDs (NCRDs) show little or no capacity to influence IAV infection. A mutant trimeric NCRD, D325A/R343V, showed marked hemagglutination inhibition and viral neutralization, with viral aggregation and aggregation-dependent viral uptake by neutrophils. D325A/R343V exhibited glucose-sensitive binding to Phil82 hemagglutinin trimer (HA) by surface plasmon resonance. By contrast, there was very low binding to the HA trimer from another virus (PR8) that lacks glycans on the HA head. Mass spectrometry demonstrated the presence of high mannose glycans on the Phil82 HA at positions known to contribute to IAV binding. Molecular modeling predicted an enhanced capacity for bridging interactions between HA glycans and D325A/R343V. Finally, the trimeric D325A/R343V NCRD decreased morbidity and increased viral clearance in a murine model of IAV infection using a reassortant A/WSN/33 virus with a more heavily glycosylated HA. The combined data support a model in which altered binding by a truncated mutant SP-D to IAV HA glycans facilitates viral aggregation, leading to significant viral neutralization in vitro and in vivo. These studies demonstrate the potential utility of homology modeling and protein structure analysis for engineering effective collectin antivirals as in vivo therapeutics.

Synthesis, biological studies of linear and branched arabinofuranoside-containing glycolipids and their interaction with surfactant protein A

Oligoarabinofuranoside-containing glycolipids relevant to mycobacterial cell wall components were synthesized in order to understand the functional roles of such glycolipids. A series of linear tetra-, hexa-, octa- and a branched heptasaccharide oligoarabinofuranosides, with 1 → 2 and 1 → 5 α-linkages between the furanoside residues, were synthesized by chemical methods from readily available monomer building blocks. Upon the synthesis of glycolipids, constituted with a double alkyl chain-substituted sn-glycerol core and oligosaccharide fragments, biological studies were performed to identify the effect of synthetic glycolipids on the biofilm formation and sliding motilities of Mycobacterium smegmatis. Synthetic glycolipids and arabinofuranosides displayed an inhibitory effect on the growth profile, but mostly on the biofilm formation and maturation. Similarly, synthetic compounds also influenced the sliding motility of the bacteria. Further, biophysical studies were undertaken, so as to identify the interactions of the glycolipids with a pulmonary surfactant protein, namely surfactant protein A (SP-A), with the aid of the surface plasmon resonance technique. Specificities of each glycolipid interacting with SP-A were thus evaluated. From this study, glycolipids were found to exhibit higher apparent association constants than the corresponding oligosaccharide portion alone, without the double alkyl group-substituted glycerol core.

C-type lectins with a sweet spot for Mycobacterium tuberculosis

The pattern of receptors sensing pathogens onto host cells is a key factor that can determine the outcome of the infection. This is particularly true when such receptors belong to the family of pattern recognition receptors involved in immunity. Mycobacterium tuberculosis, the etiologic agent of tuberculosis interacts with a wide range of pattern-recognition receptors present on phagocytes and belonging to the Toll-like, Nod-like, scavenger and C-type lectin receptor families. A complex scenario where those receptors can establish cross-talks in recognizing pathogens or microbial determinants including mycobacterial components in different spatial and temporal context starts to emerge as a key event in the outcome of the immune response, and thus, the control of the infection. In this review, we will focus our attention on the family of calcium-dependent carbohydrate receptors, the C-type lectin receptors, that is of growing importance in the context of microbial infections. Members of this family appear to be key innate immune receptors of mycobacteria, capable of cross-talk with other pattern recognition receptors to induce or modulate the inflammatory context upon mycobacterial infection.

Mycobacterium Tuberculosis Infection and Inflammation: what is Beneficial for the Host and for the Bacterium?

Tuberculosis is still a major health problem in the world. Initial interactions between Mycobacterium tuberculosis and the host mark the pathway of infection and the subsequent host inflammatory response. This inflammatory response is tightly regulated by both the host and the bacterium during different stages of infection. As infection progresses, the initial intense pro-inflammatory response observed is regulated by suppressive mediators balancing inflammation. In this environment, M. tuberculosis battles to survive interfering with the host inflammatory response. In this review we discuss the major effector molecules involved in inflammation in relation to the different stages of M. tuberculosis infection.

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.