Tetraspanins are involved in Burkholderia pseudomallei-induced cell-to-cell fusion of phagocytic and non-phagocytic cells

Genetic variation of hemolysin co-regulated protein 1 affects the immunogenicity and pathogenicity of Burkholderia pseudomallei

Hemolysin co-regulated protein 1 (Hcp1) is a component of the cluster 1 Type VI secretion system (T6SS1) that plays a key role during the intracellular lifecycle of Burkholderia pseudomallei. Hcp1 is recognized as a promising target antigen for developing melioidosis diagnostics and vaccines. While the gene encoding Hcp1 is retained across B. pseudomallei strains, variants of hcp1 have recently been identified. This study aimed to examine the prevalence of hcp1 variants in clinical isolates of B. pseudomallei, assess the antigenicity of the Hcp1 variants, and the ability of strains expressing these variants to stimulate multinucleated giant cell (MNGC) formation in comparison to strains expressing wild-type Hcp1 (Hcp1wt). Sequence analysis of 1,283 primary clinical isolates of B. pseudomallei demonstrated the presence of 8 hcp1 alleles encoding three types of Hcp1 proteins, including Hcp1wt (98.05%), Hcp1variant A (1.87%) and Hcp1variant B (0.08%). Compared to strains expressing Hcp1wt, those expressing the dominant variant, Hcp1variant A, stimulated lower levels of Hcp1variant A-specific antibody responses in melioidosis patients. Interestingly, when Hcp1variant A was expressed in B. pseudomallei K96243, this strain retained the ability to stimulate MNGC formation in A549 cells. In contrast, however, similar experiments with the Hcp1variant B demonstrated a decreased ability of B. pseudomallei to stimulate MNGC formation. Collectively, these results show that B. pseudomallei strains expressing variants of Hcp1 elicit variable antibody responses in melioidosis patients and differ in their ability to promote MNGC formation in cell culture.

Human tetraspanin CD81 facilitates invasion of Salmonella enterica into human epithelial cells

Human CD81 and CD9 are members of the tetraspanin family of proteins characterized by a canonical structure of four transmembrane domains and two extracellular loop domains. Tetraspanins are known as molecular facilitators, which assemble and organize cell surface receptors and partner molecules forming clusters known as tetraspanin-enriched microdomains. They have been implicated to play various biological roles including an involvement in infections with microbial pathogens. Here, we demonstrate an important role of CD81 for the invasion of epithelial cells by Salmonella enterica. We show that the overexpression of CD81 in HepG2 cells enhances invasion of various typhoidal and non-typhoidal Salmonella serovars. Deletion of CD81 by CRISPR/Cas9 in intestinal epithelial cells (C2BBe1 and HT29-MTX-E12) reduces S. Typhimurium invasion. In addition, the effect of human CD81 is species-specific as only human but not rat CD81 facilitates Salmonella invasion. Finally, immunofluorescence microscopy and proximity ligation assay revealed that both human tetraspanins CD81 and CD9 are recruited to the entry site of S. Typhimurium during invasion but not during adhesion to the host cell surface. Overall, we demonstrate that the human tetraspanin CD81 facilitates Salmonella invasion into epithelial host cells.

Cell fusion dynamics: mechanisms of multinucleation in osteoclasts and macrophages

Cell-cell fusion is a vital biological process where the membranes of two or more cells merge to form a syncytium. This phenomenon is critical in various physiological and pathological contexts, including embryonic development, tissue repair, immune responses, and the progression of several diseases. Osteoclasts, which are cells from the monocyte/macrophage lineage responsible for bone resorption, have enhanced functionality due to cell fusion. Additionally, other multinucleated giant cells (MGCs) also arise from the fusion of monocytes and macrophages, typically during chronic inflammation and reactions to foreign materials such as prostheses or medical devices. Foreign body giant cells (FBGCs) and Langhans giant cells (LGCs) emerge only under pathological conditions and are involved in phagocytosis, antigen presentation, and the secretion of inflammatory mediators. This review provides a comprehensive overview of the mechanisms underlying the formation of multinucleated cells, with a particular emphasis on macrophages and osteoclasts. Elucidating the intracellular structures, signaling cascades, and fusion-mediating proteins involved in cell-cell fusion enhances our understanding of this fundamental biological process and helps identify potential therapeutic targets for disorders mediated by cell fusion.

Differentiation in pyroptosis induction by Burkholderia pseudomallei and Burkholderia thailandensis in primary human monocytes, a possible cause of sepsis in acute melioidosis patients

Melioidosis caused by Burkholderia pseudomallei is an infectious disease with a high mortality rate. In acute melioidosis, sepsis is a major cause of death among patients. Once the bacterium enters the bloodstream, immune system dysregulation ensues, leading to cytokine storms. In contrast to B. pseudomallei, a closely related but non-virulent strain B. thailandensis has rarely been reported to cause cytokine storms or death in patients. However, the mechanisms in which the virulent B. pseudomallei causes sepsis are not fully elucidated. It is well-documented that monocytes play an essential role in cytokine production in the bloodstream. The present study, therefore, determined whether there is a difference in the innate immune response to B. pseudomallei and B. thailandensis during infection of primary human monocytes and THP-1 monocytic cells by investigating pyroptosis, an inflammatory death pathway known to play a pivotal role in sepsis. Our results showed that although both bacterial species exhibited a similar ability to invade human monocytes, only B. pseudomallei can significantly increase the release of cytosolic enzyme lactate dehydrogenase (LDH) as well as the increases in caspase-1 and gasdermin D activations in both cell types. The results were consistent with the significant increase in IL-1β and IL-18 production, key cytokines involved in pyroptosis. Interestingly, there was no significant difference in other cytokine secretion, such as IL-1RA, IL-10, IL-12p70, IL-15, IL-8, and IL-23 in cells infected by both bacterial species. Furthermore, we also demonstrated that ROS production played a crucial role in controlling pyroptosis activation during B. pseudomallei infection in primary human monocytes. These findings suggested that pyroptosis induced by B. pseudomallei in the human monocytes may contribute to the pathogenesis of sepsis in acute melioidosis patients.

Identification of virulence-associated factors in Vibrio parahaemolyticus with special reference to moonlighting protein: a secretomics study

Vibrio parahaemolyticus causes seafood-borne gastroenteritis infection in human which can even lead to death. The pathogenic strain of V. parahaemolyticus secretes different types of virulence factors that are directly injected into the host cell by a different type of secretion system which helps bacteria to establish its own ecological niche within the organism. Therefore, the aim of this study was to isolate the extracellular secreted proteins from the trh positive strain of V. parahaemolyticus and identify them using two-dimensional gel electrophoresis and MALDI-TOFMS/MS. Seventeen different cellular proteins viz, Carbamoyl-phosphate synthase, 5-methyltetrahydropteroyltriglutamate, tRNA-dihydrouridine synthase, Glycerol-3-phosphate dehydrogenase, Orotidine 5'-phosphate decarboxylase, Molybdenum import ATP-binding protein, DnaJ, DNA polymerase IV, Ribosomal RNA small subunit methyltransferase G, ATP synthase subunit delta and gamma, Ribosome-recycling factor, 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase, tRNA pseudouridine synthase B, Ditrans, polycis-undecaprenyl-diphosphate synthase, Oxygen-dependent coproporphyrinogen-III oxidase, and Peptide deformylase 2 were identified which are mainly involved in different metabolic and biosynthetic pathways. Furthermore, the molecular function of the identified proteins were associated with catalytic activity, ligase activity, transporter, metal binding, and ATP synthase when they are intercellular. However, to understand the importance of these secreted proteins in the infection and survival of bacteria inside the host cell, pathogen-host protein-protein interactions (PPIs) were carried out which identified the association of eight secreted proteins with 41 human proteins involved in different cellular pathways, including ubiquitination degradation, adhesion, inflammation, immunity, and programmed cell death. The present study provides unreported strategies on host-cell environment's survival and adaptation mechanisms for the successful establishment of infections and intracellular propagation.

Burkholderia thailandensis uses a type VI secretion system to lyse protrusions without triggering host cell responses

To spread within a host, intracellular Burkholderia form actin tails to generate membrane protrusions into neighboring host cells and use type VI secretion system-5 (T6SS-5) to induce cell-cell fusions. Here, we show that B. thailandensis also uses T6SS-5 to lyse protrusions to directly spread from cell to cell. Dynamin-2 recruitment to the membrane near a bacterium was followed by a short burst of T6SS-5 activity. This resulted in the polymerization of the actin of the newly invaded host cell and disruption of the protrusion membrane. Most protrusion lysis events were dependent on dynamin activity, caused no cell-cell fusion, and failed to be recognized by galectin-3. T6SS-5 inactivation decreased protrusion lysis but increased galectin-3, LC3, and LAMP1 accumulation in host cells. Our results indicate that B. thailandensis specifically activates T6SS-5 assembly in membrane protrusions to disrupt host cell membranes and spread without alerting cellular responses, such as autophagy.

Tetraspanin CD9-derived peptides inhibit Pseudomonas aeruginosa corneal infection and aid in wound healing of corneal epithelial cells

Pseudomonas aeruginosa is a leading cause of corneal infection both within India and globally, often causing a loss of vision. Increasing antimicrobial resistance among the bacteria is making its treatment more difficult. Preventing initial bacterial adherence to the host membrane has been explored here to reduce infection of the cornea. Synthetic peptides derived from human tetraspanin CD9 have been shown to reduce infection in corneal cells both in vitro, ex vivo and in vivo. We found constitutive expression of CD9 in immortalized human corneal epithelial cells by flow cytometry and immunocytochemistry. The synthetic peptides derived from CD9 significantly reduced bacterial adherence to cultured corneal epithelial cells and ex vivo human cadaveric corneas as determined by colony forming units. The peptides also significantly reduced bacterial burden in a murine model of Pseudomonas keratitis and lowered the cellular infiltration in the corneal stroma. Additionally, the peptides aided corneal wound healing in uninfected C57BL/6 mice compared to control mice. These potential therapeutics had no effect on cell viability or proliferation of corneal epithelial cells and have the potential to be developed as an alternative therapeutic intervention.

Inhibition of macrophage infectivity potentiator in Burkholderia pseudomallei suppresses pro-inflammatory responses in murine macrophages

Introduction

Melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, is a disease endemic in many tropical countries globally. Clinical presentation is highly variable, ranging from asymptomatic to fatal septicemia, and thus the outcome of infection can depend on the host immune responses. The aims of this study were to firstly, characterize the macrophage immune response to B. pseudomallei and secondly, to determine whether the immune response was modified in the presence of novel inhibitors targeting the virulence factor, the macrophage infectivity potentiator (Mip) protein. We hypothesized that inhibition of Mip in B. pseudomallei would disarm the bacteria and result in a host beneficial immune response.

Methods

Murine macrophage J774A.1 cells were infected with B. pseudomallei K96243 in the presence of small-molecule inhibitors targeting the Mip protein. RNA-sequencing was performed on infected cells four hours post-infection. Secreted cytokines and lactose dehydrogenase were measured in cell culture supernatants 24 hours post-infection. Viable, intracellular B. pseudomallei in macrophages were also enumerated 24 hours post-infection.

Results

Global transcriptional profiling of macrophages infected with B. pseudomallei by RNA-seq demonstrated upregulation of immune-associated genes, in particular a significant enrichment of genes in the TNF signaling pathway. Treatment of B. pseudomallei-infected macrophages with the Mip inhibitor, AN_CH_37 resulted in a 5.3-fold reduction of il1b when compared to cells treated with DMSO, which the inhibitors were solubilized in. A statistically significant reduction in IL-1β levels in culture supernatants was seen 24 hours post-infection with AN_CH_37, as well as other pro-inflammatory cytokines, namely IL-6 and TNF-α. Treatment with AN_CH_37 also reduced the survival of B. pseudomallei in macrophages after 24 hours which was accompanied by a significant reduction in B. pseudomallei-induced cytotoxicity as determined by lactate dehydrogenase release.

Discussion

These data highlight the potential to utilize Mip inhibitors in reducing potentially harmful pro-inflammatory responses resulting from B. pseudomallei infection in macrophages. This could be of significance since overstimulation of pro-inflammatory responses can result in immunopathology, tissue damage and septic shock.

Vitamin D (1α,25(OH)2D3) supplementation minimized multinucleated giant cells formation and inflammatory response during Burkholderia pseudomallei infection in human lung epithelial cells

Melioidosis is an infectious disease with high mortality rates in human, caused by the bacterium Burkholderia pseudomallei. As an intracellular pathogen, B. pseudomallei can escape from the phagosome and induce multinucleated giant cells (MNGCs) formation resulting in antibiotic resistance and immune evasion. A novel strategy to modulate host response against B. pseudomallei pathogenesis is required. In this study, an active metabolite of vitamin D3 (1α,25-dihydroxyvitamin D3 or 1α,25(OH)2D3) was selected to interrupt pathogenesis of B. pseudomallei in a human lung epithelium cell line, A549. The results demonstrated that pretreatment with 10-6 M 1α,25(OH)2D3 could reduce B. pseudomallei internalization to A549 cells at 4 h post infection (P < 0.05). Interestingly, the presence of 1α,25(OH)2D3 gradually reduced MNGC formation at 8, 10 and 12 h compared to that of the untreated cells (P < 0.05). Furthermore, pretreatment with 10-6 M 1α,25(OH)2D3 considerably increased hCAP-18/LL-37 mRNA expression (P < 0.001). Additionally, pro-inflammatory cytokines, including MIF, PAI-1, IL-18, CXCL1, CXCL12 and IL-8, were statistically decreased (P < 0.05) in 10-6 M 1α,25(OH)2D3-pretreated A549 cells by 12 h post-infection. Taken together, this study indicates that pretreatment with 10-6 M 1α,25(OH)2D3 has the potential to reduce the internalization of B. pseudomallei into host cells, decrease MNGC formation and modulate host response during B. pseudomallei infection by minimizing the excessive inflammatory response. Therefore, 1α,25(OH)2D3 supplement may provide an effective supportive treatment for melioidosis patients to combat B. pseudomallei infection and reduce inflammation in these patients.

Phenotypic and genetic alterations of Burkholderia pseudomallei in patients during relapse and persistent infections

The bacterium Burkholderia pseudomallei is the causative agent of melioidosis, a severe tropical disease associated with high mortality and relapse and persistent infections. Treatment of melioidosis requires prolonged antibiotic therapy; however, little is known about relapse and persistent infections, particularly the phenotypic and genetic alterations of B. pseudomallei in patients. In this study, we performed pulsed-field gel electrophoresis (PFGE) to compare the bacterial genotype between the initial isolate and the subsequent isolate from each of 23 suspected recurrent and persistent melioidosis patients in Northeast Thailand. We used whole-genome sequencing (WGS) to investigate multilocus sequence types and genetic alterations of within-host strain pairs. We also investigated the bacterial phenotypes associated with relapse and persistent infections, including multinucleated giant cell (MNGC) formation efficiency and intracellular multiplication. We first identified 13 (1.2%) relapse, 7 (0.7%) persistent, and 3 (0.3%) reinfection patients from 1,046 survivors. Each of the 20 within-host strain pairs from patients with relapse and persistent infections shared the same genotype, suggesting that the subsequent isolates arise from the infecting isolate. Logistic regression analysis of clinical data revealed regimen and duration of oral antibiotic therapies as risk factors associated with relapse and persistent infections. WGS analysis demonstrated 17 within-host genetic alteration events in 6 of 20 paired isolates, including a relatively large deletion and 16 single-nucleotide polymorphism (stocktickerSNP) mutations distributed across 12 genes. In 1 of 20 paired isolates, we observed significantly increased cell-to-cell fusion and intracellular replication in the second isolate compared with the initial isolate from a patient with persistent infection. WGS analysis suggested that a non-synonymous mutation in the tssB-5 gene, which encoded an essential component of the type VI secretion system, may be associated with the increased intracellular replication and MNGC formation efficiency of the second isolate of the patient. This information provides insights into genetic and phenotypic alterations in B. pseudomallei in human melioidosis, which may represent a bacterial strategy for persistent and relapse infections.

Functional Activities of O-Polysaccharide and Hemolysin Coregulated Protein 1 Specific Antibodies Isolated from Melioidosis Patients

Melioidosis is a fatal tropical disease caused by the environmental Gram-negative bacterium, Burkholderia pseudomallei. This bacterium is intrinsically resistant to several antibiotics and treatment of melioidosis requires prolonged antibiotic administration. To date, there are no vaccines available for melioidosis. Previous studies have shown that humoral immunity is critical for surviving melioidosis and that O-polysaccharide (OPS) and hemolysin coregulated protein 1 (Hcp1) are important protective antigens in animal models of melioidosis. Our previous studies revealed that melioidosis patients had high levels of OPS- and Hcp1-specific antibodies and that IgG against OPS (IgG-OPS) and Hcp1 (IgG-Hcp1) were associated with patient survival. In this study, we characterized the potential function(s) of IgG-OPS and IgG-Hcp1 from melioidosis patients. IgG-OPS and IgG-Hcp1 were purified from pooled serum obtained from melioidosis patients using immuno-affinity chromatography. Antibody-dependent cellular phagocytosis assays were performed with pooled serum from melioidosis patients and compared with serum obtained from healthy controls. Serum from melioidosis patients significantly enhanced B. pseudomallei uptake into the human monocytic cell line THP-1 compared with pooled serum from healthy donors. Enhanced opsonization was observed with IgG-OPS and IgG-Hcp1 in a dose-dependent manner. Antibody-dependent complement deposition assays were performed with IgG-OPS and IgG-Hcp1 using flow cytometry and showed that there was enhanced C3b deposition on the surface of B. pseudomallei treated with IgG-OPS but to a lesser degree with IgG-Hcp1. This study provides insight into the function of IgG-OPS and IgG-Hcp1 in human melioidosis and supports that OPS and Hcp1 are potential vaccine antigens for immunization against melioidosis.

Multinucleated Giant Cells: Current Insights in Phenotype, Biological Activities, and Mechanism of Formation

Monocytes and macrophages are innate immune cells with diverse functions ranging from phagocytosis of microorganisms to forming a bridge with the adaptive immune system. A lesser-known attribute of macrophages is their ability to fuse with each other to form multinucleated giant cells. Based on their morphology and functional characteristics, there are in general three types of multinucleated giant cells including osteoclasts, foreign body giant cells and Langhans giant cells. Osteoclasts are bone resorbing cells and under physiological conditions they participate in bone remodeling. However, under pathological conditions such as rheumatoid arthritis and osteoporosis, osteoclasts are responsible for bone destruction and bone loss. Foreign body giant cells and Langhans giant cells appear only under pathological conditions. While foreign body giant cells are found in immune reactions against foreign material, including implants, Langhans giant cells are associated with granulomas in infectious and non-infectious diseases. The functionality and fusion mechanism of osteoclasts are being elucidated, however, our knowledge on the functions of foreign body giant cells and Langhans giant cells is limited. In this review, we describe and compare the phenotypic aspects, biological and functional activities of the three types of multinucleated giant cells. Furthermore, we provide an overview of the multinucleation process and highlight key molecules in the different phases of macrophage fusion.

The cGAS-STING Pathway in Bacterial Infection and Bacterial Immunity

Cyclic guanosine monophosphate (GMP)-adenosine monophosphate (AMP) (cGAMP) synthase (cGAS), along with the adaptor stimulator of interferon genes (STING), are crucial components of the innate immune system, and their study has become a research hotspot in recent years. Many biochemical and structural studies that have collectively elucidated the mechanism of activation of the cGAS-STING pathway with atomic resolution have provided insights into the roles of the cGAS-STING pathway in innate immunity and clues to the origin and evolution of the modern cGAS-STING signaling pathway. The cGAS-STING pathway has been identified to protect the host against viral infection. After detecting viral dsDNA, cGAS synthesizes a second messenger to activate STING, eliciting antiviral immune responses by promoting the expression of interferons (IFNs) and hundreds of IFN-stimulated genes (ISGs). Recently, the cGAS-STING pathway has also been found to be involved in response to bacterial infections, including bacterial pneumonia, melioidosis, tuberculosis, and sepsis. However, compared with its functions in viral infection, the cGAS-STING signaling pathway in bacterial infection is more complex and diverse since the protective and detrimental effects of type I IFN (IFN-I) on the host depend on the bacterial species and infection mode. Besides, STING activation can also affect infection prognosis through other mechanisms in different bacterial infections, independent of the IFN-I response. Interestingly, the core protein components of the mammalian cGAS-STING signaling pathway have been found in the bacterial defense system, suggesting that this widespread signaling pathway may have originated in bacteria. Here, we review recent findings related to the structures of major molecules involved in the cGAS-STING pathway and the effects of the cGAS-STING pathway in various bacterial infections and bacterial immunity, which may pave the way for the development of new antibacterial drugs that specifically kill bacteria without harmful effects on the host.