Paracellular Pathway-Mediated Mycoplasma hyopneumoniae Migration across Porcine Airway Epithelial Barrier under Air-Liquid Interface Conditions

Upregulation of occludin by cytolethal distending toxin facilitates Glaesserella parasuis adhesion to respiratory tract cells

Virulent Glaesserella parasuis may engender systemic infection characterized by fibrinous polyserositis and pneumonia. G. parasuis causes systemic disease through upper respiratory tract infection, but the mechanism has not been fully characterized. Tight junction (TJ) proteins maintain the integrity and impermeability of the epithelial barriers. In this work, we applied the recombinant cytolethal distending toxin (CDT) holotoxin and cdt-deficient mutants to assess whether CDT interacted with TJ proteins of airway tract cells. Our results indicated that CDT induced the TJ occludin (OCLN) expression in newborn pig tracheal epithelial cells within the first 3 hours of bacterial infection, followed by a significant decrease. Overexpression of OCLN in target cells made them more susceptible to G. parasuis adhesion, whereas ablation of OCLN expression by CRISPR/Cas 9 gene editing technology in target cells decreased their susceptibility to bacterial adhesion. In addition, CDT treatment could upregulate the OCLN levels in the lung tissue of C57/BL6 mice. In summary, highly virulent G. parasuis strain SC1401 stimulated the tight junction expression, resulting in higher bacterial adhesion to respiratory tract cells, and this process is closely related to CDT. Our results may provide novel insights into G. parasuis infection and CDT-mediated pathogenesis.

Unveiling the stealthy tactics: mycoplasma's immune evasion strategies

Mycoplasmas, the smallest known self-replicating organisms, possess a simple structure, lack a cell wall, and have limited metabolic pathways. They are responsible for causing acute or chronic infections in humans and animals, with a significant number of species exhibiting pathogenicity. Although the innate and adaptive immune responses can effectively combat this pathogen, mycoplasmas are capable of persisting in the host, indicating that the immune system fails to eliminate them completely. Recent studies have shed light on the intricate and sophisticated defense mechanisms developed by mycoplasmas during their long-term co-evolution with the host. These evasion strategies encompass various tactics, including invasion, biofilm formation, and modulation of immune responses, such as inhibition of immune cell activity, suppression of immune cell function, and resistance against immune molecules. Additionally, antigen variation and molecular mimicry are also crucial immune evasion strategies. This review comprehensively summarizes the evasion mechanisms employed by mycoplasmas, providing valuable insights into the pathogenesis of mycoplasma infections.

Hijacking of Host Plasminogen by Mesomycoplasma (Mycoplasma) hyopneumoniae via GAPDH: an Important Virulence Mechanism To Promote Adhesion and Extracellular Matrix Degradation

Mesomycoplasma hyopneumoniae is the etiological agent of mycoplasmal pneumonia of swine (MPS), which causes substantial economic losses to the world's swine industry. Moonlighting proteins are increasingly being shown to play a role in the pathogenic process of M. hyopneumoniae. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a key enzyme in glycolysis, displayed a higher abundance in a highly virulent strain of M. hyopneumoniae than in an attenuated strain, suggesting that it may have a role in virulence. The mechanism by which GAPDH exerts its function was explored. Flow cytometry and colony blot analysis showed that GAPDH was partly displayed on the surface of M. hyopneumoniae. Recombinant GAPDH (rGAPDH) was able to bind PK15 cells, while the adherence of a mycoplasma strain to PK15 was significantly blocked by anti-rGAPDH antibody pretreatment. In addition, rGAPDH could interact with plasminogen. The rGAPDH-bound plasminogen was demonstrated to be activated to plasmin, as proven by using a chromogenic substrate, and to further degrade the extracellular matrix (ECM). The critical site for GAPDH binding to plasminogen was K336, as demonstrated by amino acid mutation. The affinity of plasminogen for the rGAPDH C-terminal mutant (K336A) was significantly decreased according to surface plasmon resonance analysis. Collectively, our data suggested that GAPDH might be an important virulence factor that facilitates the dissemination of M. hyopneumoniae by hijacking host plasminogen to degrade the tissue ECM barrier. IMPORTANCE Mesomycoplasma hyopneumoniae is a specific pathogen of pigs that is the etiological agent of mycoplasmal pneumonia of swine (MPS), which is responsible for substantial economic losses to the swine industry worldwide. The pathogenicity mechanism and possible particular virulence determinants of M. hyopneumoniae are not yet completely elucidated. Our data suggest that GAPDH might be an important virulence factor in M. hyopneumoniae that facilitates the dissemination of M. hyopneumoniae by hijacking host plasminogen to degrade the extracellular matrix (ECM) barrier. These findings will provide theoretical support and new ideas for the research and development of live-attenuated or subunit vaccines against M. hyopneumoniae.

Anti-fibrotic strategies and pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) results from the dysregulated process of injury and repair, which promotes scarring of the lung tissue and deposition of collagen-rich extracellular matrix (ECM) components, that make the lung unphysiologically stiff. IPF presents a serious concern as its pathogenesis remains elusive, and current anti-fibrotic treatments are only effective in slowing rather than halting disease progression. The IPF disease pathogenesis is incompletely defined, complex and incorporates interplay between different fibrogenesis signaling pathways. Preclinical IPF experimental models used to validate drug candidates present significant limitations in modeling IPF pathobiology, with their limited time frame, simplicity and inaccurate representation of the disease and the mechanical influences of IPF. Potentially more accurate mimetic disease models that capture the cell-cell and cell-matrix interaction, such as 3D cultures, organoids and precision-cut lung slices (PCLS), may yield more meaningful clinical predictions for drug candidates. Recent advances in developing anti-fibrotic compounds have positioned drug towards targeting components of the fibrogenesis signaling pathway of IPF or the extracellular microenvironment. The major goals in this area of research focus on finding ways to reverse or halt the disease progression by utilizing more disease-relevant experimental models to improve the qualification of potential drug targets for treating pulmonary fibrosis.

Regulatory functional role of NLRP3 inflammasome during Mycoplasma hyopneumoniae infection in swine

Mycoplasma hyopneumoniae causes enzootic pneumonia, a highly contagious respiratory disease in swine that causes significant economic losses worldwide. It is unknown whether the nucleotide oligomerization domain-like receptor (NLR) family pyrin domain containing 3 (NLRP3) inflammasome regulates the immune response in swine during M. hyopneumoniae infection. The current study utilized an in vivo swine model of M. hyopneumoniae infection to investigate the regulatory functional role of the NLRP3 inflammasome during M. hyopneumoniae infection. Notable histopathological alterations were observed in M. hyopneumoniae-infected swine tissues, which were associated with an inflammatory response and disease progression. Swine M. hyopneumoniae infection was associated with an increase in the expression of the NLRP3 inflammasome, which stimulated pro-inflammatory cytokines such as tumor necrosis factor-alpha, interleukin 18, and interleukin 1 beta (IL-1β). The impact of the NLRP3 inhibitor, MCC950 on NLRP3 and pro-inflammatory cytokines in M. hyopneumoniae-infected swine was examined to investigate the relationship between the NLRP3 inflammasome and M. hyopneumoniae infection. Taken together, our findings provide strong evidence that the NLRP3 inflammasome plays a critical regulatory functional role in M. hyopneumoniae infection in swine.

Polymorphisms in Pattern Recognition Receptor Genes Are Associated with Respiratory Disease Severity in Pig Farms

Reduced productivity caused by infections, particularly respiratory diseases, is a serious problem in pig farming. We have previously reported polymorphisms in porcine pattern recognition receptor genes affecting molecular functions and demonstrated that the 2197A/C polymorphism in the nucleotide-binding oligomerization domain containing 2 (NOD2) gene influences porcine circovirus 2-induced mortality. Here, we investigated how these polymorphisms affect respiratory disease-induced lesions, using samples from a slaughterhouse dealing with pigs from two farms. Lung lesions were evaluated using two scoring systems, Goodwin (GW) and slaughterhouse pleuritis evaluation system (SPES), to determine the influence of Mycoplasma hyopneumoniae (Mhp) and Actinobacillus pleuropneumoniae (App), respectively. SPES scores were significantly higher when the 1205T allele of Toll-like receptor 5 (TLR5-1205T), rather than TLR5-1205C, was present. On the farm with more severe Mhp invasion, lower GW lesion scores were significantly associated with the presence of the NOD-like receptor family pyrin domain containing 3 (NLRP3)-2906G allele; where App invasion was worse, lower SPES scores were significantly associated with the presence of the NOD2-2197C allele. Combinations of polymorphisms in pattern recognition receptor genes can therefore be utilized for breeding for resistance against respiratory diseases in pigs. DNA markers of these polymorphisms can thus be used to improve productivity by reducing respiratory diseases due to bacterial pathogens in pig livestock.

Protective efficacy of intranasal inactivated pseudorabies vaccine is improved by combination adjuvant in mice

Pseudorabies virus (PRV) not only causes great economic loss to the pig industry but also seriously threatens the biosafety of other mammals, including humans. Since 2011, PRV mutant strains have emerged widely in China, and the classical Bartha-K61 vaccine cannot confer complete protection for pigs. PRV mainly infects pigs via the respiratory tract. Intranasal immunization with PRV has received more attention because intranasal vaccination elicits systemic and mucosal immune responses. To induce systemic and mucosal immune responses against PRV, we developed a combination adjuvant as a delivery system for intranasal vaccine, which was formulated with MONTANIDE™ Gel 01 and CVCVA5. In comparison to naked antigen of inactivated PRV, single Gel 01 adjuvanted inactivated antigen and single CVCVA5 adjuvanted inactivated antigen, intranasal inactivated PRV vaccine formulated with the combination adjuvant induced greater mucosal IgA immunity and serum antibody responses (IgG, IgG1, and IgG2a). Furthermore, the production of the Th1-type cytokine IFN-γ and the Th2-type cytokine IL-4 indicated that the cellular and humoral responses to the intranasal vaccine were improved by the combination adjuvant. In addition, the intranasal vaccine formulated with the combination adjuvant induced long-term T lymphocyte memory with increased central (CD62L⁺CD44⁺) and effector (CD62L^–CD44⁺) memory subsets of both CD4 and CD8 T cells in nasal-associated lymphoid tissue. Intranasal challenge with virulent PRV in mice showed that the protective efficacy of the intranasal PRV vaccine was improved by the combination adjuvant compared with the other single-adjuvanted vaccines. In summary, these data demonstrated that Gel 01 combined with the CVCVA5 adjuvant induced a synergistic effect to improve mucosal immunity and protective efficacy of the intranasally inactivated PRV vaccine in mice. It represents a promising vaccination approach against PRV infection.

Chloride channel accessory 1 gene deficiency causes selective loss of mucus production in a new pig model

Morbidity and mortality of respiratory diseases are linked to airway obstruction by mucus but there are still no specific, safe, and effective drugs to correct this phenotype. The need for better treatment requires a new understanding of the basis for mucus production. In that regard, studies of human airway epithelial cells in primary culture show that a mucin granule constituent known as chloride channel accessory 1 (CLCA1) is required for inducible expression of the inflammatory mucin MUC5AC in response to potent type 2 cytokines. However, it remained uncertain whether CLCLA1 is necessary for mucus production in vivo. Conventional approaches to functional biology using targeted gene knockout were difficult due to the functional redundancy of additional Clca genes in mice not found in humans. We reasoned that CLCA1 function might be better addressed in pigs that maintain the same four-member CLCA gene locus and the corresponding mucosal and submucosal populations of mucous cells found in humans. Here we develop to our knowledge the first CLCA1-gene-deficient (CLCA1-/-) pig and show that these animals exhibit loss of MUC5AC+ mucous cells throughout the airway mucosa of the lung without affecting comparable cells in the tracheal mucosa or MUC5B+ mucous cells in submucosal glands. Similarly, CLCA1-/- pigs exhibit loss of MUC5AC+ mucous cells in the intestinal mucosa without affecting MUC2+ mucous cells. These data establish CLCA1 function for controlling MUC5AC expression as a marker of mucus production and provide a new animal model to study mucus production at respiratory and intestinal sites.

Glaesserella parasuis serotype 4 HPS4-YC disrupts the integrity of the swine tracheal epithelial barrier and facilitates bacterial translocation

Glaesserella parasuis (G. parasuis) is a commensal bacterium in the upper respiratory tract of pigs that can also cause the swine Glässer disease, which induces an intensive inflammatory response and results in significant economic losses to the swine industry worldwide. G. parasuis can cause disease through infection of the respiratory tract, resulting in systemic infection, but the mechanism is largely unknown. Recently we showed that Glaesserella parasuis serotype 4 (GPS4) increased swine tracheal epithelial barrier permeability, resulting in easier bacterial translocation. Tight junction proteins (TJ) play a crucial role in maintaining the integrity and impermeability of the epithelial barrier. GPS4 decreased the expression of the TJ ZO-1 and occludin in swine tracheal epithelial cells (STEC). Furthermore, the proinflammatory cytokines IL-6, IL-8 and TNF-α were significantly upregulated in GPS4-infected STEC, and both the MAPK and NF-κB signaling pathways were activated and contributed to the expression of TNF-α. We demonstrate that the production of proinflammatory cytokines, especially TNF-α, during GPS4 infection was involved in barrier dysfunction. Additionally, animal challenge experiments confirmed that GPS4 infection downregulated TJ in the lungs of piglets and induced a severe inflammatory response. In general, G. parasuis infection downregulated the expression of TJ and induced massive secretion of proinflammatory cytokines, resulting in epithelial barrier disruption and favoring bacterial infection. This study allowed us to better understand the mechanism by which G. parasuis crosses the respiratory tract of pigs.

Pathogenicity & virulence of Mycoplasma hyopneumoniae

Mycoplasma hyopneumoniae: is the etiological agent of porcine enzootic pneumonia (EP), a disease that impacts the swine industry worldwide. Pathogen-induced damage, as well as the elicited host-response, contribute to disease. Here, we provide an overview of EP epidemiology, control and prevention, and a more in-depth review of M. hyopneumoniae pathogenicity determinants, highlighting some molecular mechanisms of pathogen-host interactions relevant for pathogenesis. Based on recent functional, immunological, and comparative “omics” results, we discuss the roles of many known or putative M. hyopneumoniae virulence factors, along with host molecules involved in EP. Moreover, the known molecular bases of pathogenicity mechanisms, including M. hyopneumoniae adhesion to host respiratory epithelium, protein secretion, cell damage, host microbicidal response and its modulation, and maintenance of M. hyopneumoniae homeostasis during infection are described. Recent findings regarding M. hyopneumoniae pathogenicity determinants also contribute to the development of novel diagnostic tests, vaccines, and treatments for EP.