Innate immune response during Yersinia infection: critical modulation of cell death mechanisms through phagocyte activation

Tracheal epithelial cell-exosome-derived MiR-21-5p inhibits alveolar macrophage pyroptosis to resist pulmonary bacterial infection through PIK3CD-autophagy pathway

AIMS

Structural cells play an important role in regulating immune cells during infection. Our aim was to determine whether structural porcine tracheal epithelial cells (PTECs) can regulate alveolar macrophages (AMs) to prevent bacterial pneumonia, explore the underlying mechanism(s) and therapeutic target.

MATERIALS AND METHODS

Actinobacillus pleuropneumoniae (APP) was used as the model strain for infection studies. Small RNA sequencing was used to identify differentially abundant exosome-derived miRNAs. The role of PTECs exosome-derived miR-21-5p in regulating AMs autophagy, pyroptosis, reactive oxygen species (ROS) was determined using RT-qPCR, western-blotting, flow cytometry, immunohistochemistry. Luciferase reporter assays were conducted to identify potential binding targets of miR-21-5p. The universality of miR-21-5p action on resistance to bacterial pulmonary infection was demonstrated using Klebsiella pneumoniae or Staphylococcus aureus in vitro and in vivo infection models.

KEY FINDINGS

MiR-21-5p was enriched in PETCs-derived exosomes, which protected AMs against pulmonary bacterial infection. Mechanistically, miR-21-5p targeted PIK3CD, to promote autophagy of AMs, which reduced the pyroptosis induced by APP infection via inhibiting the over-production of ROS, which in turn suppressed the over-expression of pro-inflammatory cytokines, and increased bacterial clearance. Importantly, the protective effect and mechanism of miR-21-5p were universal as they also occurred upon challenge with Klebsiella pneumoniae and Staphylococcus aureus.

SIGNIFICANCE

Our data reveals miR-21-5p can promote pulmonary resistance to bacterial infection by inhibiting pyroptosis of alveolar macrophages through the PIK3CD-autophagy-ROS pathway, suggesting PIK3CD may be a potential therapeutic target for bacterial pneumonia.

Streptococcus pyogenes emm98.1 variants activate inflammatory caspases in human neutrophils

ABBREVIATIONS

CovRS, control of virulence regulatory system; GAS, Group A Streptococcus; PMN, polymorphonuclear leukocyte.

Yersinia pestis and Plague: some knowns and unknowns

Since its first identification in 1894 during the third pandemic in Hong Kong, there has been significant progress of understanding the lifestyle of Yersinia pestis, the pathogen that is responsible for plague. Although we now have some understanding of the pathogen's physiology, genetics, genomics, evolution, gene regulation, pathogenesis and immunity, there are many unknown aspects of the pathogen and its disease development. Here, we focus on some of the knowns and unknowns relating to Y. pestis and plague. We notably focus on some key Y. pestis physiological and virulence traits that are important for its mammal-flea-mammal life cycle but also its emergence from the enteropathogen Yersinia pseudotuberculosis. Some aspects of the genetic diversity of Y. pestis, the distribution and ecology of plague as well as the medical countermeasures to protect our population are also provided. Lastly, we present some biosafety and biosecurity information related to Y. pestis and plague.

Role of the Yersinia pseudotuberculosis Virulence Plasmid in Pathogen-Phagocyte Interactions in Mesenteric Lymph Nodes

Yersinia pseudotuberculosis is an Enterobacteriaceae family member that is commonly transmitted by the fecal-oral route to cause infections. From the small intestine, Y. pseudotuberculosis can invade through Peyer's patches and lymph vessels to infect the mesenteric lymph nodes (MLNs). Infection of MLNs by Y. pseudotuberculosis results in the clinical presentation of mesenteric lymphadenitis. MLNs are important for immune responses to intestinal pathogens and microbiota in addition to their clinical relevance to Y. pseudotuberculosis infections. A characteristic of Y. pseudotuberculosis infection in MLNs is the formation of pyogranulomas. Pyogranulomas are composed of neutrophils, inflammatory monocytes, and lymphocytes surrounding extracellular microcolonies of Y. pseudotuberculosis. Key elements of the complex pathogen-host interaction in MLNs have been identified using mouse infection models. Y. pseudotuberculosis requires the virulence plasmid pYV to induce the formation of pyogranulomas in MLNs. The YadA adhesin and the Ysc-Yop type III secretion system (T3SS) are encoded on pYV. YadA mediates bacterial binding to host receptors, which engages the T3SS to preferentially translocate seven Yop effectors into phagocytes. The effectors promote pathogenesis by blocking innate immune defenses such as superoxide production, degranulation, and inflammasome activation, resulting in survival and growth of Y. pseudotuberculosis. On the other hand, certain effectors can trigger immune defenses in phagocytes. For example, YopJ triggers activation of caspase-8 and an apoptotic cell death response in monocytes within pyogranulomas that limits dissemination of Y. pseudotuberculosis from MLNs to the bloodstream. YopE can be processed as an antigen by phagocytes in MLNs, resulting in T and B cell responses to Y. pseudotuberculosis. Immune responses to Y. pseudotuberculosis in MLNs can also be detrimental to the host in the form of chronic lymphadenopathy. This review focuses on interactions between Y. pseudotuberculosis and phagocytes mediated by pYV that concurrently promote pathogenesis and host defense in MLNs. We propose that MLN pyogranulomas are immunological arenas in which opposing pYV-driven forces determine the outcome of infection in favor of the pathogen or host.

Galectin-1 Cooperates with Yersinia Outer Protein (Yop) P to Thwart Protective Immunity by Repressing Nitric Oxide Production

Yersinia enterocolitica (Ye) inserts outer proteins (Yops) into cytoplasm to infect host cells. However, in spite of considerable progress, the mechanisms implicated in this process, including the association of Yops with host proteins, remain unclear. Here, we evaluated the functional role of Galectin-1 (Gal1), an endogenous β-galactoside-binding protein, in modulating Yop interactions with host cells. Our results showed that Gal1 binds to Yops in a carbohydrate-dependent manner. Interestingly, Gal1 binding to Yops protects these virulence factors from trypsin digestion. Given that early control of Ye infection involves activation of macrophages, we evaluated the role of Gal1 and YopP in the modulation of macrophage function. Although Gal1 and YopP did not influence production of superoxide anion and/or TNF by Ye-infected macrophages, they coordinately inhibited nitric oxide (NO) production. Notably, recombinant Gal1 (rGal1) did not rescue NO increase observed in Lgals1-/- macrophages infected with the YopP mutant Ye ∆yopP. Whereas NO induced apoptosis in macrophages, no significant differences in cell death were detected between Gal1-deficient macrophages infected with Ye ∆yopP, and WT macrophages infected with Ye wt. Strikingly, increased NO production was found in WT macrophages treated with MAPK inhibitors and infected with Ye wt. Finally, rGal1 administration did not reverse the protective effect in Peyer Patches (PPs) of Lgals1-/- mice infected with Ye ∆yopP. Our study reveals a cooperative role of YopP and endogenous Gal1 during Ye infection.

Paradoxical Temporal Enlargement: An Expansion of Superficial Temporal Fat Pad Following Interfacial Technique for Pterional Craniotomy

OBJECTIVE

Contour irregularities in the temporal region have been reported previously after procedures involving temporal dissection. In this study, we report paradoxical temporal enlargement (PTE) following interfascial pterional craniotomy.

METHODS

A retrospective review of patients who underwent a unilateral transcranial procedure with frontotemporal approach at our institution between September 2013 and December 2017 was performed. Patients with a previous craniotomy or bilateral craniotomy were excluded. Radiological imaging series including computed tomography and magnetic resonance imaging were utilized to calculate temporal soft tissue volumes both preoperatively and postoperatively by using advanced software technology. Relative soft tissue volume differences between the operative side and the contralateral side were calculated at different time-points including preoperative, 3-months follow-up (3M), 12-months (12M) follow-up, and the last follow-up (LFU, over 1-year).

RESULTS

Forty-three patients were included. Mean age was 52.7 ± 4.5 years. Mean follow-up was 27.9 ± 15.8 months. Significant changes of temporal fat pad relative-volume difference were observed between the preoperative and the corresponding 3M (t [82] = -2.8865, P = 0.0050); 12M (t [77] = -4.4321, P < 0.0001), and LFU (t [74] = -4.9862, P < 0.0001) postoperative time points. No significant change of the temporalis muscle was observed between the preoperative and the corresponding 3M (P = 0.3629), 12M (P = 0.1553), or LFU (P = 0.0715). Soft tissue volume showed a significant increase on the operative side between the preoperative and the corresponding LFU (t [74] = -2.5866, P =  0.0117).

CONCLUSIONS

Paradoxical temporal enlargement with more than 10% volumetric change was observed in 24% of the patients at their LFU (>1-year). This change was not due to temporalis muscle changes. Paradoxical temporal enlargement was due to hypertrophy of the superficial temporal fat pad. Before surgical correction of postoperative temporal contour changes, it is important to obtain imaging and characterize the etiology of the deformity.

Comparison of three non-human primate aerosol models for glanders, caused by Burkholderia mallei

Burkholderia mallei is a gram-negative obligate animal pathogen that causes glanders, a highly contagious and potentially fatal disease of solipeds including horses, mules, and donkeys. Humans are also susceptible, and exposure can result in a wide range of clinical forms, i.e., subclinical infection, chronic forms with remission and exacerbation, or acute and potentially lethal septicemia and/or pneumonia. Due to intrinsic antibiotic resistance and the ability of the organisms to survive intracellularly, current treatment regimens are protracted and complicated; and no vaccine is available. As a consequence of these issues, and since B. mallei is infectious by the aerosol route, B. mallei is regarded as a major potential biothreat agent. To develop optimal medical countermeasures and diagnostic tests, well characterized animal models of human glanders are needed. The goal of this study was to perform a head-to-head comparison of models employing three commonly used nonhuman primate (NHP) species, the African green monkey (AGM), Rhesus macaque, and the Cynomolgus macaque. The natural history of infection and in vitro clinical, histopathological, immunochemical, and bacteriological parameters were examined. The AGMs were the most susceptible NHP to B. mallei; five of six expired within 14 days. Although none of the Rhesus or Cynomolgus macaques succumbed, the Rhesus monkeys exhibited abnormal signs and clinical findings associated with B. mallei infection; and the latter may be useful for modeling chronic B. mallei infection. Based on the disease progression observations, gross and histochemical pathology, and humoral and cellular immune response findings, the AGM appears to be the optimal model of acute, lethal glanders infection. AGM models of infection by B. pseudomallei, the etiologic agent of melioidosis, have been characterized recently. Thus, the selection of the AGM species provides the research community with a single NHP model for investigations on acute, severe, inhalational melioidosis and glanders.

Intravital imaging of skin infections

Intravital imaging of cutaneous immune responses has revealed intricate links between the skin's structural properties, the immune cells that reside therein, and the carefully orchestrated migratory dynamics that enable rapid sensing and subsequent elimination of skin pathogens. In particular, the development of 2-photon intravital microscopy (2P-IVM), which enables the excitation of fluorescent molecules within deep tissue with minimal light scattering and tissue damage, has proven an invaluable tool in the characterization of different cell subset's roles in skin infection. The ability to visualize cells, tissue structures, pathogens and track migratory dynamics at designated times following infection, or during inflammatory responses has been crucial in defining how immune responses in the skin are coordinated, either locally or in concert with circulating immune cells. Skin pathogens affect millions of people worldwide, and skin infections leading to cutaneous pathology have a considerable impact on the quality of life and longevity of people affected. In contrast, pathogens that infect the skin to later cause systemic illness, such as malaria parasites and a variety of arthropod-borne viruses, or infection in distant anatomical sites are a significant cause of morbidity and mortality worldwide. Here, we review recent advances and seminal studies that employed intravital imaging to characterize key immune response mechanisms in the context of viral, bacterial and parasitic skin infections, and provide insights on skin pathogens of global significance that would benefit from such investigative approaches.

Paradoxical Adipose Hypertrophy (PAH) After Cryolipolysis

BACKGROUND

Cryolipolysis is a minimally invasive technique used to decrease local adipose tissue by thermal cooling. Paradoxical adipose hypertrophy (PAH) is a rare complication of cryolipolysis with important aesthetic consequences.

OBJECTIVES

The objective of this study was to describe four cases of PAH after a cryolipolysis treatment.

METHODS

Between January 2014 and January 2017, all patients who had undergone a cryolipolysis treatment in a single center were reviewed. The device used was a CoolSculpting device and the same operator performed all the cryolipolysis treatments. We retrospectively included all patients who had a suspicion of PAH.

RESULTS

In our study, 398 patients underwent a session of cryolipolysis. Four patients presented with a voluminous painless swelling in the treated area, between 2 and 4 months after the cryolipolysis session. One patient was treated with liposuction. Histological analysis of the adipose tissue in this patient revealed a nonspecific panniculitis. The other three patients did not receive any additional treatment, and their symptoms stabilized after several months.

CONCLUSIONS

Although cryolipolysis generally yields good results, it can be complicated with PAH, which tends to occur a few months after the cryolipolysis treatment. Patients should be informed of the possibility of developing this complication and encouraged to attend regular follow up for at least 6 months, so that this condition can be readily detected. Surgical treatment should be offered if there is no spontaneous improvement of the symptoms.

LEVEL OF EVIDENCE 5

Characterization of V. cholerae T3SS-dependent cytotoxicity in cultured intestinal epithelial cells

AM-19226 is a pathogenic, non-O1/non-O139 serogroup strain of Vibrio cholerae that uses a Type 3 Secretion System (T3SS) mediated mechanism to colonize host tissues and disrupt homeostasis, causing cholera. Co-culturing the Caco2-BBE human intestinal epithelial cell line with AM-19226 in the presence of bile results in rapid mammalian cell death that requires a functional T3SS. We examined the role of bile, sought to identify the mechanism, and evaluated the contributions of T3SS translocated effectors in in vitro cell death. Our results suggest that Caco2-BBE cytotoxicity does not proceed by apoptotic or necrotic mechanisms, but rather displays characteristics consistent with osmotic lysis. Cell death was preceded by disassembly of epithelial junctions and reorganization of the cortical membrane skeleton, although neither cell death nor cell-cell disruption required VopM or VopF, two effectors known to alter actin dynamics. Using deletion strains, we identified a subset of AM-19226 Vops that are required for host cell death, which were previously assigned roles in protein translocation and colonization, suggesting that they function other than to promote cytotoxicity. The collective results therefore suggest that cooperative Vop activities are required to achieve cytotoxicity in vitro, or alternatively, that translocon pores destabilize the membrane in a bile dependent manner.

Evasion and interference: intracellular pathogens modulate caspase-dependent inflammatory responses

Pathogens have evolved to complete the virulence cycle of colonization, replication and dissemination in intimate association with a complex network of extracellular and intracellular surveillance systems that guard tissue spaces. In this Review, we discuss the strategies used by bacteria and viruses to evade or inhibit intracellular detection that is coupled to pro-inflammatory caspase-dependent protective responses. Such strategies include alterations of lipopolysaccharide (LPS) structures, the regulated expression of components of type III secretion systems, and the utilization of proteins that inhibit inflammasome formation, the enzymatic activity of caspases and cytokine signalling. Inflammation is crucial in response to exposure to pathogens, but is potentially damaging and thus tightly regulated. The threshold for the activation of pro-inflammatory caspases is determined by the immediate stimulus in the context of previous signals. Pathogen, genetic and situational factors modulate this threshold, which determines the ability of the host to resist infection while minimizing harm.

Uncovering an Important Role for YopJ in the Inhibition of Caspase-1 in Activated Macrophages and Promoting Yersinia pseudotuberculosis Virulence

Pathogenic Yersinia species utilize a type III secretion system to translocate Yop effectors into infected host cells. Yop effectors inhibit innate immune responses in infected macrophages to promote Yersinia pathogenesis. In turn,Yersinia-infected macrophages respond to translocation of Yops by activating caspase-1, but different mechanisms of caspase-1 activation occur, depending on the bacterial genotype and the state of phagocyte activation. In macrophages activated with lipopolysaccharide (LPS) prior to Yersinia pseudotuberculosis infection, caspase-1 is activated by a rapid inflammasome-dependent mechanism that is inhibited by translocated YopM. The possibility that other effectors cooperate with YopM to inhibit caspase-1 activation in LPS-activated macrophages has not been investigated. Toward this aim, epistasis analysis was carried out in which the phenotype of aY. pseudotuberculosis yopM mutant was compared to that of a yopJ yopM, yopE yopM, yopH yopM, yopT yopM, or ypkA yopM mutant. Activation of caspase-1 was measured by cleavage of the enzyme, release of interleukin-1β (IL-1β), and pyroptosis in LPS-activated macrophages infected with wild-type or mutant Y. pseudotuberculosis strains. Results show enhanced activation of caspase-1 after infection with the yopJ yopM mutant relative to infection by any other single or double mutant. Similar results were obtained with the yopJ, yopM, and yopJ yopM mutants ofY ersinia pestis Following intravenous infection of mice, theY. pseudotuberculosis yopJ mutant was as virulent as the wild type, while the yopJ yopM mutant was significantly more attenuated than the yopM mutant. In summary, through epistasis analysis this work uncovered an important role for YopJ in inhibiting caspase-1 in activated macrophages and in promoting Yersinia virulence.

Bacterial recognition pathways that lead to inflammasome activation

Inflammasomes are multi-protein signaling platforms that upon activation trigger the maturation of the pro-inflammatory cytokines, interleukin-1β (IL-1β) and IL-18, and cell death. Inflammasome sensors detect microbial and host-derived molecules. Here, we review the mechanisms of inflammasome activation triggered by bacterial infection, primarily focusing on two model intracellular bacterial pathogens, Francisella novicida and Salmonella typhimurium. We discuss the complex relationship between bacterial recognition through direct and indirect detection by inflammasome sensors. We highlight regulation mechanisms that potentiate or limit inflammasome activation. We discuss the importance of caspase-1 and caspase-11 in host defense, and we examine the downstream consequences of inflammasome activation within the context of bacterial infections.

Group A Streptococcus Modulates Host Inflammation by Manipulating Polymorphonuclear Leukocyte Cell Death Responses

Polymorphonuclear leukocyte (PMN) cell death strongly influences the resolution of inflammatory episodes, and may exacerbate adverse pathologies in response to infection. We investigated PMN cell death mechanisms following infection by virulent group A Streptococcus (GAS). Human PMNs were infected in vitro with a clinical, virulent GAS isolate and an avirulent derivative strain, and compared for phagocytosis, the production of reactive oxygen species (ROS), mitochondrial membrane depolarization and apoptotic markers. C57BL/6J mice were then infected, in order to observe the effects on murine PMNs in vivo. Human PMNs phagocytosed virulent GAS less efficiently, produced less ROS and underwent reduced mitochondrial membrane depolarization compared with phagocytosis of avirulent GAS. Morphological and biochemical analyses revealed that PMNs infected with avirulent GAS exhibited nuclear fragmentation and caspase-3 activation consistent with an anti-inflammatory apoptotic phenotype. Conversely, virulent GAS induced PMN vacuolization and plasma membrane permeabilization, leading to a necrotic form of cell death. Infection of the mice with virulent GAS engendered significantly higher systemic pro-inflammatory cytokine release and localized infiltration of murine PMNs, with cells associated with virulent GAS infection exhibiting reduced apoptotic potential. Avirulent GAS infection was associated with lower levels of proinflammatory cytokines and tissue PMN apoptosis. We propose that the differences in PMN cell death mechanisms influence the inflammatory responses to infection by GAS.

Bacterial secreted effectors and caspase-3 interactions

Apoptosis is a critical process that intrinsically links organism survival to its ability to induce controlled death. Thus, functional apoptosis allows organisms to remove perceived threats to their survival by targeting those cells that it determines pose a direct risk. Central to this process are apoptotic caspases, enzymes that form a signalling cascade, converting danger signals via initiator caspases into activation of the executioner caspase, caspase-3. This enzyme begins disassembly of the cell by activating DNA degrading enzymes and degrading the cellular architecture. Interaction of pathogenic bacteria with caspases, and in particular, caspase-3, can therefore impact both host cell and bacterial survival. With roles outside cell death such as cell differentiation, control of signalling pathways and immunomodulation also being described for caspase-3, bacterial interactions with caspase-3 may be of far more significance in infection than previously recognized. In this review, we highlight the ways in which bacterial pathogens have evolved to subvert caspase-3 both through effector proteins that directly interact with the enzyme or by modulating pathways that influence its activation and activity.

Transcriptomic response to Yersinia pestis: RIG-I like receptor signaling response is detrimental to the host against plague

Bacterial pathogens have evolved various mechanisms to modulate host immune responses for successful infection. In this study, RNA-sequencing technology was used to analyze the responses of human monocytes THP1 to Yersinia pestis infection. Over 6000 genes were differentially expressed over the 12 h infection. Kinetic responses of pathogen recognition receptor signaling pathways, apoptosis, antigen processing, and presentation pathway and coagulation system were analyzed in detail. Among them, RIG-I-like receptor (RLR) signaling pathway, which was established for antiviral defense, was significantly affected. Mice lacking MAVS, the adaptor of the RLR signaling pathway, were less sensitive to infection and exhibited lower IFN-β production, higher Th1-type cytokines IFN-γ and IL-12 production, and lower Th2-type cytokines IL-4 and IL-13 production in the serum compared with wild-type mice. Moreover, infection of pathogenic bacteria other than Y. pestis also altered the expression of the RLR pathway, suggesting that the response of RLR pathway to bacterial infection is a universal mechanism.

The cytotoxic necrotizing factor of Yersinia pseudotuberculosis (CNFY) enhances inflammation and Yop delivery during infection by activation of Rho GTPases

Some isolates of Yersinia pseudotuberculosis produce the cytotoxic necrotizing factor (CNF_Y), but the functional consequences of this toxin for host-pathogen interactions during the infection are unknown. In the present study we show that CNF_Y has a strong influence on virulence. We demonstrate that the CNF_Y toxin is thermo-regulated and highly expressed in all colonized lymphatic tissues and organs of orally infected mice. Most strikingly, we found that a cnfY knock-out variant of a naturally toxin-expressing Y. pseudotuberculosis isolate is strongly impaired in its ability to disseminate into the mesenteric lymph nodes, liver and spleen, and has fully lost its lethality. The CNF_Y toxin contributes significantly to the induction of acute inflammatory responses and to the formation of necrotic areas in infected tissues. The analysis of the host immune response demonstrated that presence of CNF_Y leads to a strong reduction of professional phagocytes and natural killer cells in particular in the spleen, whereas loss of the toxin allows efficient tissue infiltration of these immune cells and rapid killing of the pathogen. Addition of purified CNF_Y triggers formation of actin-rich membrane ruffles and filopodia, which correlates with the activation of the Rho GTPases, RhoA, Rac1 and Cdc42. The analysis of type III effector delivery into epithelial and immune cells in vitro and during the course of the infection further demonstrated that CNF_Y enhances the Yop translocation process and supports a role for the toxin in the suppression of the antibacterial host response. In summary, we highlight the importance of CNF_Y for pathogenicity by showing that this toxin modulates inflammatory responses, protects the bacteria from attacks of innate immune effectors and enhances the severity of a Yersinia infection.

Various toxins and effector proteins of bacterial pathogens have been found to manipulate eukaryotic cell machineries to promote persistence and proliferation within their hosts. Many of these virulence factors target small Rho GTPases, but their role in pathogenesis is often unknown. Here, we addressed the expression and functional consequences of the CNF_Y toxin found in some isolates of Y. pseudotuberculosis. We found that CNF_Y besides modulating the cell cytoskeleton by activation of the GTPases RhoA, Rac1 and Cdc42, contributes to increased inflammation and tissue damage. Moreover, CNF_Y increases the ability of Yersinia to prevent the attack of the immune system, by enhancing the delivery of antiphagocytic and cytotoxic effectors into professional phagocytes. Our findings provide the first insights into the multi-functional action and severe consequences of the CNF_Y toxin on the inflammatory response and disease-associated tissue damage during the natural course of the infection.

Macrophages and recently identified forms of cell death

Recent advances in cell death biology have uncovered an ever increasing range of cell death forms. Macrophages have a bidirectional relationship with cell death that modulates the immune response. Thus, macrophages engulf apoptotic cells and secrete cytokines that may promote cell death in parenchymal cells. Furthermore, the presence of apoptotic or necrotic dead cells in the microenvironment elicits differential macrophage responses. Apoptotic cells elicit anti-inflammatory responses in macrophages. By contrast macrophages may undergo a proinflammatory form of cell death (pyroptosis) in response to damage-associated molecular patterns (DAMPs) released from necrotic cells and also in response to pathogen-associated molecular patterns (PAMPs). Pyroptosis is a recently identified form of cell death that occurs predominantly in subsets of inflammatory macrophages and is associated to the release of interleukin-1β (IL-1β) and IL-18. Deregulation of these processes may result in disease. Thus, failure of macrophages to engulf apoptotic cells may be a source of autoantigens in autoimmune diseases, excessive macrophage release of proapoptotic factors or sterile pyroptosis may contribute to tissue injury and failure of pathogen-induced pyroptosis may contribute to pathogen survival. Ongoing research is exploring the therapeutic opportunities resulting this new knowledge.

Early apoptosis of macrophages modulated by injection of Yersinia pestis YopK promotes progression of primary pneumonic plague

Yersinia pestis causes pneumonic plague, a disease characterized by inflammation, necrosis and rapid bacterial growth which together cause acute lung congestion and lethality. The bacterial type III secretion system (T3SS) injects 7 effector proteins into host cells and their combined activities are necessary to establish infection. Y. pestis infection of the lungs proceeds as a biphasic inflammatory response believed to be regulated through the control of apoptosis and pyroptosis by a single, well-conserved T3SS effector protein YopJ. Recently, YopJ-mediated pyroptosis, which proceeds via the NLRP3-inflammasome, was shown to be regulated by a second T3SS effector protein YopK in the related strain Y. pseudotuberculosis. In this work, we show that for Y. pestis, YopK appears to regulate YopJ-mediated apoptosis, rather than pyroptosis, of macrophages. Inhibition of caspase-8 blocked YopK-dependent apoptosis, suggesting the involvement of the extrinsic pathway, and appeared cell-type specific. However, in contrast to yopJ, deletion of yopK caused a large decrease in virulence in a mouse pneumonic plague model. YopK-dependent modulation of macrophage apoptosis was observed at 6 and 24 hours post-infection (HPI). When YopK was absent, decreased populations of macrophages and dendritic cells were seen in the lungs at 24 HPI and correlated with resolution rather than progression of inflammation. Together the data suggest that Y. pestis YopK may coordinate the inflammatory response during pneumonic plague through the regulation of apoptosis of immune cells.

In this work, we studied the mechanism whereby bacteria manipulate innate immune responses by controlling host cell death. Yersinia pestis, the causative agent of plague, requires effector Yops of the Type III Secretion System (T3SS) to evade the innate immune system during infection. We show that Yersinia induces apoptosis of macrophages through two distinct mechanisms, each through the activity of the well-characterized T3SS effector YopJ, yet regulated in an opposing manner through the activity of a second effector protein YopK. In a murine pneumonic plague model, we found evidence that YopK regulates apoptosis of macrophages during the early stage of infection, leading to uncontrolled inflammation and disease. In contrast, the absence of YopK-regulated apoptosis allowed recruitment of lymphocytes and CCR2⁺ immune cells which led to bacterial clearance and resolution of inflammation. Together the data suggest that Yersinia YopK modulates apoptosis of immune cells to control the inflammatory response during plague.

The effect of colchicine on pyrin and pyrin interacting proteins

MEFV which encodes pyrin, cause familial Mediterranean fever (FMF), the most common auto-inflammatory disease. Pyrin is believed to be a regulator of inflammation, though the nature of this regulatory activity remains to be identified. Prophylactic treatment with colchicine, a microtubule toxin, has had a remarkable effect on disease progression and outcome. It has been thought that, inhibition of microtubule polymerization is the main mechanism of action of colchicine. But, the exact cellular mechanism explaining the efficacy of colchicine in suppressing FMF attacks is still unclear. Given the ability of colchicine treatment to be considered as a differential diagnosis criteria of FMF, we hypothesized that colchicine may have a specific effect on pyrin and pyrin interacting proteins. This study showed that colchicine prevents reticulated fibrils formed by PSTPIP1 filaments and reduces ASC speck rates in transfected cells. We further noted that, colchicine down-regulates MEFV expression in THP-1 cells. We also observed that colchicine causes re-organization of actin cytoskeleton in THP-1 cells. Pyrin is an actin-binding protein that specifically localizes with polymerizing actin filaments. Thus, MEFV expression might be affected by re-organization of actin cytoskeleton. The data presented here reveal an important connection between colchicine and pyrin which might explain the remarkable efficacy of colchicine in preventing FMF attacks.

Yersinia pestis versus Yersinia pseudotuberculosis: effects on host macrophages

Yersinia pestis, the causative agent of plague, is proved to be a recently emerged clone from Y. pseudotuberculosis. However, the diseases they cause and their patterns of transmission are very different. People always focus on the genetic changes between Y. pestis and Y. pseudotuberculosis to reveal their pathogenic differences, and little is known about host defence differences to these two Yersinia. In this study, the effects of Y. pestis and Y. pseudotuberculosis on macrophages were analysed. Cell apoptosis showed significant difference after the macrophages were infected by these two strains, and caspase-3 activity also demonstrated a similar tendency. Further, macrophage function activities were evaluated. We found during the early infection of Y. pestis, several basic functions of macrophages, including phagocytosis, secretion of cytokine tumour necrosis factor-α and nitric oxide, macrophage polarity and antigen presenting, were significantly interrupted. In comparison, Y. pseudotuberculosis infection showed lower inhibition on macrophages. Especially, Y. pestis infection might cause macrophage to polarize to M2 macrophages in the early phase, compared with Y. pseudotuberculosis infection, which was different from the common acute infection. These results clearly indicated even in the early stage of infection, different host macrophage defence patterns could help us to understand the obvious virulence differences between Y. pestis and Y. pseudotuberculosis.

Omics strategies for revealing Yersinia pestis virulence

Omics has remarkably changed the way we investigate and understand life. Omics differs from traditional hypothesis-driven research because it is a discovery-driven approach. Mass datasets produced from omics-based studies require experts from different fields to reveal the salient features behind these data. In this review, we summarize omics-driven studies to reveal the virulence features of Yersinia pestis through genomics, trascriptomics, proteomics, interactomics, etc. These studies serve as foundations for further hypothesis-driven research and help us gain insight into Y. pestis pathogenesis.

Host stress and immune responses during aerosol challenge of Brown Norway rats with Yersinia pestis

Inhalation exposure models are becoming the preferred method for the comparative study of respiratory infectious diseases due to their resemblance to the natural route of infection. To enable precise delivery of pathogen to the lower respiratory tract in a manner that imposes minimal biosafety risk, nose-only exposure systems have been developed. Early inhalation exposure technology for infectious disease research grew out of technology used in asthma research where predominantly the Collison nebulizer is used to generate an aerosol by beating a liquid sample against glass. Although infectious aerosol droplets of 1–5 μm in size can be generated, the Collison often causes loss of viability. In this work, we evaluate a gentler method for aerosolization of living cells and describe the use of the Sparging Liquid Aerosol Generator (SLAG) in a rat pneumonic plague model. The SLAG creates aerosols by continuous dripping of liquid sample on a porous metal disc. We show the generation of 0.5–1 μm Yersinia pestis aerosol particles using the SLAG with spray factors typically ranging from 10⁻⁷ to 10⁻⁸ with no detectable loss of bacterial viability. Delivery of these infectious particles via nose-only exposure led to the rapid development of lethal pneumonic plague. Further, we evaluated the effect of restraint-stress imposed by the nose-only exposure chamber on early inflammatory responses and bacterial deposition. Elevated serum corticosterone which peaked at 2 h post-procedure indicated the animals experienced stress as a result of restraint in the nose-only chamber. However, we observed no correlation between elevated corticosterone and the amount of bacterial deposition or inflammation in the lungs. Together these data demonstrate the utility of the SLAG and the nose-only chamber for aerosol challenge of rodents by Y. pestis.

Kinetics of innate immune response to Yersinia pestis after intradermal infection in a mouse model

A hallmark of Yersinia pestis infection is a delayed inflammatory response early in infection. In this study, we use an intradermal model of infection to study early innate immune cell recruitment. Mice were injected intradermally in the ear with wild-type (WT) or attenuated Y. pestis lacking the pYV virulence plasmid (pYV(-)). The inflammatory responses in ear and draining lymph node samples were evaluated by flow cytometry and immunohistochemistry. As measured by flow cytometry, total neutrophil and macrophage recruitment to the ear in WT-infected mice did not differ from phosphate-buffered saline (PBS) controls or mice infected with pYV(-), except for a transient increase in macrophages at 6 h compared to the PBS control. Limited inflammation was apparent even in animals with high bacterial loads (10(5) to 10(6) CFU). In addition, activation of inflammatory cells was significantly reduced in WT-infected mice as measured by CD11b and major histocompatibility complex class II (MHC-II) expression. When mice infected with WT were injected 12 h later at the same intradermal site with purified LPS, Y. pestis did not prevent recruitment of neutrophils. However, significant reduction in neutrophil activation remained compared to that of PBS and pYV(-) controls. Immunohistochemistry revealed qualitative differences in neutrophil recruitment to the skin and draining lymph node, with WT-infected mice producing a diffuse inflammatory response. In contrast, focal sites of neutrophil recruitment were sustained through 48 h postinfection in pYV(-)-infected mice. Thus, an important feature of Y. pestis infection is reduced activation and organization of inflammatory cells that is at least partially dependent on the pYV virulence plasmid.

Roles of chaperone/usher pathways of Yersinia pestis in a murine model of plague and adhesion to host cells

Yersinia pestis and many other Gram-negative pathogenic bacteria use the chaperone/usher (CU) pathway to assemble virulence-associated surface fibers termed pili or fimbriae. Y. pestis has two well-characterized CU pathways: the caf genes coding for the F1 capsule and the psa genes coding for the pH 6 antigen. The Y. pestis genome contains additional CU pathways that are capable of assembling pilus fibers, but the roles of these pathways in the pathogenesis of plague are not understood. We constructed deletion mutations in the usher genes for six of the additional Y. pestis CU pathways. The wild-type (WT) and usher deletion strains were compared in the murine bubonic (subcutaneous) and pneumonic (intranasal) plague infection models. Y. pestis strains containing deletions in CU pathways y0348-0352, y1858-1862, and y1869-1873 were attenuated for virulence compared to the WT strain by the intranasal, but not subcutaneous, routes of infection, suggesting specific roles for these pathways during pneumonic plague. We examined binding of the Y. pestis WT and usher deletion strains to A549 human lung epithelial cells, HEp-2 human cervical epithelial cells, and primary human and murine macrophages. Y. pestis CU pathways y0348-0352 and y1858-1862 were found to contribute to adhesion to all host cells tested, whereas pathway y1869-1873 was specific for binding to macrophages. The correlation between the virulence attenuation and host cell binding phenotypes of the usher deletion mutants identifies three of the additional CU pathways of Y. pestis as mediating interactions with host cells that are important for the pathogenesis of plague.

Acetylcholine and an acetylcholinesterase inhibitor neostigmine can aggravate tularemia progress in BALB/c mice

The present experiment was aimed at assessing the application of neostigmine, an acetylcholinesterase (AChE) pseudo-irreversible inhibitor with poor penetration through the hematoencephalitic barrier, and the neurotransmitter acetylcholine (ACh). The experiment was done to evaluate their ability to modulate an infectious disease: tularemia. Mice infected with Franciselle tularensis and exposed to either ACh or neostigmine had a higher mortality and spleen bacterial burden when compared to infected mice exposed to saline solution only. The activated cholinergic anti-inflammatory pathway suppressed pathways necessary for tularemia resolution. Administration of AChE inhibitors to the individuals suffering from tularemia is contra-indicatory. Drugs based on AChE inhibition should be restricted when tularemia or disease with a similar pathogenesis is suspected.

The in vivo extracellular life of facultative intracellular bacterial parasites: role in pathogenesis

Classically labeled facultative intracellular pathogens are characterized by the ability to have an intracellular phase in the host, which is required for pathogenicity, while capable of extracellular growth in vitro. The ability of these bacteria to replicate in cell-free conditions is usually assessed by culture in artificial bacteriological media. However, the extracellular growth ability of these pathogens may also be expressed by a phase of extracellular infection in the natural setting of the host with pathologic consequences, an ability that adds to the pathogenic potential of the infectious agent. This infective capability to grow in the extracellular sites of the host represents an additional virulence attribute of those pathogens which may lead to severe outcomes. Here we discuss examples of infectious diseases where the in vivo infective extracellular life is well documented, including infections by Francisella tularensis, Yersinia pestis, Burkholderia pseudomallei, Burkholderia cenocepacia, Salmonella enterica serovar Typhimurium and Edwardsiella tarda. The occurrence of a phase of systemic dissemination with extracellular multiplication during progressive infections by facultative intracellular bacterial pathogens has been underappreciated, with most studies exclusively centered on the intracellular phase of the infections. The investigation of the occurrence of a dual lifestyle in the host among bacterial pathogens in general should be extended and likely will reveal more cases of infectious diseases with a dual infective intracellular/extracellular pattern.

Cluster analysis of host cytokine responses to biodefense pathogens in a whole blood ex vivo exposure model (WEEM)

Background

Rapid detection and therapeutic intervention for infectious and emerging diseases is a major scientific goal in biodefense and public health. Toward this end, cytokine profiles in human blood were investigated using a human whole blood ex vivo exposure model, called WEEM.

Results

Samples of whole blood from healthy volunteers were incubated with seven pathogens including Yersinia pseudotuberculosis, Yersinia enterocolitica, Bacillus anthracis, and multiple strains of Yersinia pestis, and multiplexed protein expression profiling was conducted on supernatants of these cultures with an antibody array to detect 30 cytokines simultaneously. Levels of 8 cytokines, IL-1α, IL-1β, IL-6, IL-8, IL-10, IP-10, MCP-1 and TNFα, were significantly up-regulated in plasma after bacterial exposures of 4 hours. Statistical clustering was applied to group the pathogens based on the host response protein expression profiles. The nearest phylogenetic neighbors clustered more closely than the more distant pathogens, and all seven pathogens were clearly differentiated from the unexposed control. In addition, the Y. pestis and Yersinia near neighbors were differentiated from the B. anthracis strains.

Conclusions

Cluster analysis, based on host response cytokine profiles, indicates that distinct patterns of immunomodulatory proteins are induced by the different pathogen exposures and these patterns may enable further development into biomarkers for diagnosing pathogen exposure.

Resistance to plague among black-tailed prairie dog populations

In some rodent species frequently exposed to plague outbreaks caused by Yersinia pestis, resistance to the disease has evolved as a population trait. As a first step in determining if plague resistance has developed in black-tailed prairie dogs (Cynomys ludovicianus), animals captured from colonies in a plague-free region (South Dakota) and two plague-endemic regions (Colorado and Texas) were challenged with Y. pestis at one of three doses (2.5, 250, or 2500 mouse LD50s). South Dakota prairie dogs were far more susceptible to plague than Colorado and Texas prairie dogs (p<0.001), with a mortality rate of nearly 100% over all doses. Colorado and Texas prairie dogs were quite similar in their response, with overall survival rates of 50% and 60%, respectively. Prairie dogs from these states were heterogeneous in their response, with some animals dying at the lowest dose (37% and 20%, respectively) and some surviving even at the highest dose (29% and 40%, respectively). Microsatellite analysis revealed that all three groups were distinct genetically, but further studies are needed to establish a genetic basis for the observed differences in plague resistance.

Pathogenesis of Y. enterocolitica and Y. pseudotuberculosis in Human Yersiniosis

Yersiniosis is a food-borne illness that has become more prevalent in recent years due to human transmission via the fecal-oral route and prevalence in farm animals. Yersiniosis is primarily caused by Yersinia enterocolitica and less frequently by Yersinia pseudotuberculosis. Infection is usually characterized by a self-limiting acute infection beginning in the intestine and spreading to the mesenteric lymph nodes. However, more serious infections and chronic conditions can also occur, particularly in immunocompromised individuals. Y. enterocolitica and Y. pseudotuberculosis are both heterogeneous organisms that vary considerably in their degrees of pathogenicity, although some generalizations can be ascribed to pathogenic variants. Adhesion molecules and a type III secretion system are critical for the establishment and progression of infection. Additionally, host innate and adaptive immune responses are both required for yersiniae clearance. Despite the ubiquity of enteric Yersinia species and their association as important causes of food poisoning world-wide, few national enteric pathogen surveillance programs include the yersiniae as notifiable pathogens. Moreover, no standard exists whereby identification and reporting systems can be effectively compared and global trends developed. This review discusses yersinial virulence factors, mechanisms of infection, and host responses in addition to the current state of surveillance, detection, and prevention of yersiniosis.

Transcriptomic and innate immune responses to Yersinia pestis in the lymph node during bubonic plague

A delayed inflammatory response is a prominent feature of infection with Yersinia pestis, the agent of bubonic and pneumonic plague. Using a rat model of bubonic plague, we examined lymph node histopathology, transcriptome, and extracellular cytokine levels to broadly characterize the kinetics and extent of the host response to Y. pestis and how it is influenced by the Yersinia virulence plasmid (pYV). Remarkably, dissemination and multiplication of wild-type Y. pestis during the bubonic stage of disease did not induce any detectable gene expression or cytokine response by host lymph node cells in the developing bubo. Only after systemic spread had led to terminal septicemic plague was a transcriptomic response detected, which included upregulation of several cytokine, chemokine, and other immune response genes. Although an initial intracellular phase of Y. pestis infection has been postulated, a Th1-type cytokine response associated with classical activation of macrophages was not observed during the bubonic stage of disease. However, elevated levels of interleukin-17 (IL-17) were present in infected lymph nodes. In the absence of pYV, sustained recruitment to the lymph node of polymorphonuclear leukocytes (PMN, or neutrophils), the major IL-17 effector cells, correlated with clearance of infection. Thus, the ability to counteract a PMN response in the lymph node appears to be a major in vivo function of the Y. pestis virulence plasmid.

Proteasome inhibitors prevent caspase-1-mediated disease in rodents challenged with anthrax lethal toxin

NOD-like receptors (NLRs) and caspase-1 are critical components of innate immunity, yet their over-activation has been linked to a long list of microbial and inflammatory diseases, including anthrax. The Bacillus anthracis lethal toxin (LT) has been shown to activate the NLR Nalp1b and caspase-1 and to induce many symptoms of the anthrax disease in susceptible murine strains. In this study we tested whether it is possible to prevent LT-mediated disease by pharmacological inhibition of caspase-1. We found that caspase-1 and proteasome inhibitors blocked LT-mediated caspase-1 activation and cytolysis of LT-sensitive (Fischer and Brown-Norway) rat macrophages. The proteasome inhibitor NPI-0052 also prevented disease progression and death in susceptible Fischer rats and increased survival in BALB/c mice after LT challenge. In addition, NPI-0052 blocked rapid disease progression and death in susceptible Fischer rats and BALB/c mice challenged with LT. In contrast, Lewis rats, which harbor LT-resistant macrophages, showed no signs of caspase-1 activation after LT injection and did not exhibit rapid disease progression. Taken together, our findings indicate that caspase-1 activation is critical for rapid disease progression in rodents challenged with LT. Our studies indicate that pharmacological inhibition of NLR signaling and caspase-1 can be used to treat inflammatory diseases.