Dispensability of surfactant proteins A and D in immune control of Mycobacterium tuberculosis infection following aerosol challenge of mice

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.

K. pneumoniae and M. smegmatis infect epithelial cells via different strategies

Background

As the first line of defense, epithelial cells play a vital role in the initiation and control of both innate and adaptive immunity, which participate in the development of disease. Despite its therapeutic significance, little is understood about the specific interaction between pathogenic microorganisms and lung epithelial cells.

Methods

In this study, we performed a head-to-head comparison of the virulence and infection mechanisms of Klebsiella pneumoniae (K. pneumoniae) and Mycobacterium smegmatis (M. smegmatis), which represent Gram-negative/positive respiratory pathogens, respectively, in lung epithelial cell models for the first time.

Results

Through scanning electron microscopy combined with bacterial infection experiments, we confirmed the ability of K. pneumoniae and M. smegmatis strains to form biofilm and cord factor out of the cell wall. M. smegmatis has stronger adhesion and intracellular retention ability, while K. pneumoniae is more likely to induce acute infection. These pathogens could stay and proliferate in lung epithelial cells and stimulate the secretion of specific cytokines and chemokines through a gene transcription regulator. M. smegmatis infection can promote crosstalk among epithelial cells and other immune cells in the lung from a very early stage by prompting the secretion of pro-inflammatory cytokines. Meanwhile, there were significant correlations between K. pneumonia infection and higher levels of interleukin-15 (IL-15), interleukin-1Rα (IL-1Rα), fibroblast growth factor (FGF) basic, and granulocyte colony-stimulating factor (G-CSF). At the same time, K. pneumonia infection also led to changes in the expression of cytoskeletal proteins in epithelial cells.

Conclusions

Our results emphasized the immunoprotection and immunomodulation of lung epithelial cells against exogenous pathogenic microorganisms, indicating that different pathogens damaged the host through different strategies and induced varying innate immune responses. At the same time, they provided important clues and key immune factors for dealing with complicated pulmonary infections.

CD14 signaling mediates lung immunopathology and mice mortality induced by Achromobacter xylosoxidans

Objective and design

Our research aimed to investigate the role of CD14 in pulmonary infection by Achromobacter xylosoxidans in an experimental murine model.

Methods

C57Bl/6 or CD14-deficient mice were infected intratracheally with non-lethal inoculum of A. xylosoxidans. At times 1, 3 and 7 days after infection, lungs, bronchoalveolar lavage and blood were collected. CD14 gene expression was determined by RT-PCR. The bacterial load in the lungs was assessed by counting colony forming units (CFU). Cytokines, chemokines, lipocalin-2 and sCD14 were quantified by the ELISA method. Inflammatory infiltrate was observed on histological sections stained with HE, and leukocyte subtypes were assessed by flow cytometry. In another set of experiments, C57Bl/6 or CD14-deficient mice were inoculated with lethal inoculum and the survival rate determined.

Results

CD14-deficient mice are protected from A. xylosoxidans-induced death, which is unrelated to bacterial load. The lungs of CD14-deficient mice presented a smaller area of tissue damage, less neutrophil and macrophage infiltration, less pulmonary edema, and a lower concentration of IL-6, TNF-α, CXCL1, CCL2 and CCL3 when compared with lungs of C57Bl/6 mice. We also observed that A. xylosoxidans infection increases the number of leukocytes expressing mCD14 and the levels of sCD14 in BALF and serum of C57Bl/6-infected mice.

Conclusions

In summary, our data show that in A. xylosoxidans infection, the activation of CD14 induces intense pulmonary inflammatory response resulting in mice death.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00011-022-01641-8.

Underwhelming or Misunderstood? Genetic Variability of Pattern Recognition Receptors in Immune Responses and Resistance to Mycobacterium tuberculosis

Human genetic control is thought to affect a considerable part of the outcome of infection with Mycobacterium tuberculosis (Mtb). Most of us deal with the pathogen by containment (associated with clinical "latency") or sterilization, but tragically millions each year do not. After decades of studies on host genetic susceptibility to Mtb infection, genetic variation has been discovered to play a role in tuberculous immunoreactivity and tuberculosis (TB) disease. Genes encoding pattern recognition receptors (PRRs) enable a consistent, molecularly direct interaction between humans and Mtb which suggests the potential for co-evolution. In this review, we explore the roles ascribed to PRRs during Mtb infection and ask whether such a longstanding and intimate interface between our immune system and this pathogen plays a critical role in determining the outcome of Mtb infection. The scientific evidence to date suggests that PRR variation is clearly implicated in altered immunity to Mtb but has a more subtle role in limiting the pathogen and pathogenesis. In contrast to 'effectors' like IFN-γ, IL-12, Nitric Oxide and TNF that are critical for Mtb control, 'sensors' like PRRs are less critical for the outcome of Mtb infection. This is potentially due to redundancy of the numerous PRRs in the innate arsenal, such that Mtb rarely goes unnoticed. Genetic association studies investigating PRRs during Mtb infection should therefore be designed to investigate endophenotypes of infection - such as immunological or clinical variation - rather than just TB disease, if we hope to understand the molecular interface between innate immunity and Mtb.

Innate Immune Pattern Recognition Receptors of Mycobacterium tuberculosis: Nature and Consequences for Pathogenesis of Tuberculosis

Innate immunity against Mycobacterium tuberculosis is a critical early response to prevent the establishment of the infection. Despite recent advances in understanding the host-pathogen dialogue in the early stages of tuberculosis (TB), much has yet to be learnt. The nature and consequences of this dialogue ultimately determine the path of infection: namely, either early clearance of M. tuberculosis, or establishment of M. tuberculosis infection leading to active TB disease and/or latent TB infection. On the frontline in innate immunity are pattern recognition receptors (PRRs), with soluble factors (e.g. collectins and complement) and cell surface factors (e.g. Toll-like receptors and other C-type lectin receptors (Dectin 1/2, Nod-like receptors, DC-SIGN, Mincle, mannose receptor, and MCL) that play a central role in recognising M. tuberculosis and facilitating its clearance. However, in a 'double-edged sword' scenario, these factors can also be involved in enhancement of pathogenesis as well. Furthermore, innate immunity is also a critical bridge in establishing the subsequent adaptive immune response, which is also responsible for granuloma formation that cordons off M. tuberculosis infection, establishing latency and acting as a reservoir for bacterial persistence and dissemination of future disease. This chapter discusses the current understanding of pattern recognition of M. tuberculosis by innate immunity and the role this plays in the pathogenesis and protection against TB.

Selected factors of the innate immunity in Polish patients suffering from pulmonary tuberculosis

We conducted a prospective study of 453 Polish patients suffering from pulmonary tuberculosis and 267 healthy controls. Selected polymorphisms of the genes encoding for collectins, ficolins and MBL-associated serine protease 2 were investigated as were serum concentrations of mannose-binding lectin, surfactant protein D, ficolin-1 and ficolin-3. The number of MBL2 gene exon 1 variant allele carriers was significantly higher in patients, compared with controls. The homozygosity for SFTPA2 +26 C > A SNP variant allele occurred less commonly within TB, while homozygosity for the FCN1 -542 G > A major allele was less frequent within the control group. Two patients were found MASP-2-deficient. Serum concentrations of MBL, SP-D and ficolin-1 were higher amongst patients while the converse was found for ficolin-3. Ficolin-1 had high specificity to differentiate between individuals with tuberculosis and healthy persons and therefore may be considered potential disease marker.

Collectins: Innate Immune Pattern Recognition Molecules

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

Surfactant Protein D in Respiratory and Non-Respiratory Diseases

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

Leukotriene B4 is essential for lung host defence and alpha-defensin-1 production during Achromobacter xylosoxidans infection

Leukotriene B₄ (LTB₄) is essential for host immune defence. It increases neutrophil recruitment, phagocytosis and pathogen clearance, and decreases oedema and inflammasome activation. The host response and the role of LTB₄ during Achromobacter xylosoxidans infection remain unexplored. Wild-type (129sv) and LTB₄ deficient (Alox5 ^-/-) mice were intratracheally infected with A. xylosoxidans. Wild-type 129sv infected mice survived beyond the 8^th day post-infection, exhibited increased levels of LTB₄ in the lung on the 1^st day, while levels of PGE₂ increased on the 7^th day post-infection. Infected Alox5 ^-/- mice showed impaired bacterial clearance, increased lung inflammation, and succumbed to the infection by the 7^th day. We found that exogenous LTB₄ does not affect the phagocytosis of A. xylosoxidans by alveolar macrophages in vitro. However, treatment of infected animals with LTB₄ protected from mortality, by reducing the bacterial load and inflammation via BLT₁ signalling, the high affinity receptor for LTB₄. Of importance, we uncovered that LTB₄ induces gene and protein expression of α-defensin-1 during the infection. This molecule is essential for bacterial clearance and exhibits potent antimicrobial activity by disrupting A. xylosoxidans cell wall. Taken together, our data demonstrate a major role for LTB₄ on the control of A. xylosoxidans infection.

The Importance of First Impressions: Early Events in Mycobacterium tuberculosis Infection Influence Outcome

Tuberculosis remains a major health threat in much of the world. New vaccines against Mycobacterium tuberculosis are essential for preventing infection, disease, and transmission. However, the host immune responses that need to be induced by an effective vaccine remain unclear. Increasingly, it has become clear that early events in infection are of major importance in the eventual outcome of the infection. Studying such events in humans is challenging, as they occur within the lung and thoracic lymph nodes, and any clinical signs of early infection are relatively nonspecific. Nonetheless, clinical studies and animal models of tuberculosis have provided new insights into the local events that occur in the first few weeks of tuberculosis. Development of an effective vaccine requires a clear understanding of the successful (and detrimental) early host responses against M. tuberculosis, with the goal to improve upon natural immune responses and prevent infection or disease.

Pentraxins and Collectins: Friend or Foe during Pathogen Invasion?

Innate immunity serves as the frontline defence against invading pathogens. Despite decades of research, new insights are constantly challenging our understanding of host-elicited immunity during microbial infections. Recently, two families of humoral innate immune proteins, pentraxins and collectins, have become a major focus of research in the field of innate immunity. Pentraxins and collectins are key players in activating the humoral arm of innate immunity, taking centre stage in immunoregulation and disease modulation. However, increasing evidence suggests that pentraxins and collectins can also mediate pathogenic effects during some infections. Herein, we discuss the protective and pathogenic effects of pentraxins and collectins, as well as their therapeutic significance.

Collectin CL-LK Is a Novel Soluble Pattern Recognition Receptor for Mycobacterium tuberculosis

Understanding the molecular components of immune recognition of the tuberculosis (TB) bacillus, Mycobacterium tuberculosis, can help designing novel strategies to combat TB. Here, we identify collectin CL-LK as a novel soluble C-type lectin able to bind M. tuberculosis, and characterize mycobacterial mannose-capped lipoarabinomannan as a primary ligand for CL-LK. Mice deficient in CL-K1, one of the CL-LK subunits, do not display altered susceptibility to M. tuberculosis. However, we found that the amount of CL-LK in the serum of patients with active TB is reduced, compared to that in controls, and correlates inversely to the magnitude of the immune response to the pathogen. These findings indicate that CL-LK might be of interest for future diagnostic and treatment monitoring purposes.

Host genome polymorphisms and tuberculosis infection: What we have to say?

Several epidemiology studies suggest that host genetic factors play important roles in susceptibility, protection and progression of tuberculosis infection. Here we have reviewed the implications of some genetic polymorphisms in pathways related to tuberculosis susceptibility, severity and development. Large case-control studies examining single-nucleotide polymorphisms (SNPs) in genes have been performed in tuberculosis patients in some countries. Polymorphisms in natural resistance-associated macrophage protein 1 (NRAMP1), toll-like receptor 2 (TLR2), interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-α), interleukin-1 receptor antagonist (IL-1RA), IL-10, vitamin D receptor (VDR), dendritic cell-specific ICAM-3-grabbing non-integrin (DC-SIGN), monocyte chemoattractant protein-1 (MCP-1), nucleotide oligomerization binding domain 2 (NOD2), interferon-gamma (IFN-γ), inducible nitric oxide synthase (iNOS), mannose-binding lectin (MBL) and surfactant proteins A (SP-A) have been reviewed. These genes have been variably associated with tuberculosis infection and there is strong evidence indicating that host genetic factors play critical roles in tuberculosis susceptibility, severity and development.

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.

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.

The role of surfactant in respiratory distress syndrome

The key feature of respiratory distress syndrome (RDS) is the insufficient production of surfactant in the lungs of preterm infants. As a result, researchers have looked into the possibility of surfactant replacement therapy as a means of preventing and treating RDS. We sought to identify the role of surfactant in the prevention and management of RDS, comparing the various types, doses, and modes of administration, and the recent development. A PubMed search was carried out up to March 2012 using phrases: surfactant, respiratory distress syndrome, protein-containing surfactant, protein-free surfactant, natural surfactant, animal-derived surfactant, synthetic surfactant, lucinactant, surfaxin, surfactant protein-B, surfactant protein-C.Natural, or animal-derived, surfactant is currently the surfactant of choice in comparison to protein-free synthetic surfactant. However, it is hoped that the development of protein-containing synthetic surfactant, such as lucinactant, will rival the efficacy of natural surfactants, but without the risks of their possible side effects. Administration techniques have also been developed with nasal continuous positive airway pressure (nCPAP) and selective surfactant administration now recommended; multiple surfactant doses have also reported better outcomes. An aerosolised form of surfactant is being trialled in the hope that surfactant can be administered in a non-invasive way. Overall, the advancement, concerning the structure of surfactant and its mode of administration, offers an encouraging future in the management of RDS.

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.

Expression of the leptin receptor outside of bone marrow-derived cells regulates tuberculosis control and lung macrophage MHC expression

Leptin is a pleiotropic hormone proposed to link nutritional status to the development of strong Th1 immunity. Because Mycobacterium tuberculosis control is affected by starvation and diabetes, we studied the role of the leptin receptor in regulating distinct immune cells during chronic infection. Infected db/db mice, bearing a natural mutation in the leptin receptor, have a markedly increased bacterial load in their lungs when compared with that of their wild-type counterparts. In response to M. tuberculosis infection, db/db mice exhibited disorganized granulomas, neutrophilia, and reduced B cell migration to the lungs, correlating with dysfunctional lung chemokine responses that include XCL1, CCL2, CXCL1, CXCL2, and CXCL13. In a db/db lung, myeloid cells were delayed in their production of inducible NO synthase and had reduced expression of MHC I and II. Although the Th1 cell response developed normally in the absence of leptin signaling, production of pulmonary IFN-γ was delayed and ineffective. Surprisingly, a proper immune response took place in bone marrow (BM) chimeras lacking leptin receptor exclusively in BM-derived cells, indicating that leptin acts indirectly on immune cells to modulate the antituberculosis response and bacterial control. Together, these findings suggest that the pulmonary response to M. tuberculosis is affected by the host's nutritional status via the regulation of non-BM-derived cells, not through direct action of leptin on Th1 immunity.