Humanized Mice Exhibit Increased Susceptibility to Staphylococcus aureus Pneumonia

Virulence attributes of successful methicillin-resistant Staphylococcus aureus lineages

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of severe and often fatal infections. MRSA epidemics have occurred in waves, whereby a previously successful lineage has been replaced by a more fit and better adapted lineage. Selection pressures in both hospital and community settings are not uniform across the globe, which has resulted in geographically distinct epidemiology. This review focuses on the mechanisms that trigger the establishment and maintenance of current, dominant MRSA lineages across the globe. While the important role of antibiotic resistance will be mentioned throughout, factors which influence the capacity of S. aureus to colonize and cause disease within a host will be the primary focus of this review. We show that while MRSA possesses a diverse arsenal of toxins including alpha-toxin, the success of a lineage involves more than just producing toxins that damage the host. Success is often attributed to the acquisition or loss of genetic elements involved in colonization and niche adaptation such as the arginine catabolic mobile element, as well as the activity of regulatory systems, and shift metabolism accordingly (e.g., the accessory genome regulator, agr). Understanding exactly how specific MRSA clones cause prolonged epidemics may reveal targets for therapies, whereby both core (e.g., the alpha toxin) and acquired virulence factors (e.g., the Panton-Valentine leukocidin) may be nullified using anti-virulence strategies.

Next-generation humanized NSG-SGM3 mice are highly susceptible to Staphylococcus aureus infection

Humanized hemato-lymphoid system mice, or humanized mice, emerged in recent years as a promising model to study the course of infection of human-adapted or human-specific pathogens. Though Staphylococcus aureus infects and colonizes a variety of species, it has nonetheless become one of the most successful human pathogens of our time with a wide armory of human-adapted virulence factors. Humanized mice showed increased vulnerability to S. aureus compared to wild type mice in a variety of clinically relevant disease models. Most of these studies employed humanized NSG (NOD-scid IL2Rgnull) mice which are widely used in the scientific community, but show poor human myeloid cell reconstitution. Since this immune cell compartment plays a decisive role in the defense of the human immune system against S. aureus, we asked whether next-generation humanized mice, like NSG-SGM3 (NOD-scid IL2Rgnull-3/GM/SF) with improved myeloid reconstitution, would prove to be more resistant to infection. To our surprise, we found the contrary when we infected humanized NSG-SGM3 (huSGM3) mice with S. aureus: although they had stronger human immune cell engraftment than humanized NSG mice, particularly in the myeloid compartment, they displayed even more pronounced vulnerability to S. aureus infection. HuSGM3 mice had overall higher numbers of human T cells, B cells, neutrophils and monocytes in the blood and the spleen. This was accompanied by elevated levels of pro-inflammatory human cytokines in the blood of huSGM3 mice. We further identified that the impaired survival of huSGM3 mice was not linked to higher bacterial burden nor to differences in the murine immune cell repertoire. Conversely, we could demonstrate a correlation of the rate of humanization and the severity of infection. Collectively, this study suggests a detrimental effect of the human immune system in humanized mice upon encounter with S. aureus which might help to guide future therapy approaches and analysis of virulence mechanisms.

Characterization of the Secreted Acid Phosphatase SapS Reveals a Novel Virulence Factor of Staphylococcus aureus That Contributes to Survival and Virulence in Mice

Staphylococcus aureus possesses a large arsenal of immune-modulating factors, enabling it to bypass the immune system's response. Here, we demonstrate that the acid phosphatase SapS is secreted during macrophage infection and promotes its intracellular survival in this type of immune cell. In animal models, the SA564 sapS mutant demonstrated a significantly lower bacterial burden in liver and renal tissues of mice at four days post infection in comparison to the wild type, along with lower pathogenicity in a zebrafish infection model. The SA564 sapS mutant elicits a lower inflammatory response in mice than the wild-type strain, while S. aureus cells harbouring a functional sapS induce a chemokine response that favours the recruitment of neutrophils to the infection site. Our in vitro and quantitative transcript analysis show that SapS has an effect on S. aureus capacity to adapt to oxidative stress during growth. SapS is also involved in S. aureus biofilm formation. Thus, this study shows for the first time that SapS plays a significant role during infection, most likely through inhibiting a variety of the host's defence mechanisms.

MRSA Infection in the Thigh Muscle Leads to Systemic Disease, Strong Inflammation, and Loss of Human Monocytes in Humanized Mice

MRSA (Methicillin-resistant Staphylococcus aureus) is the second-leading cause of deaths by antibiotic-resistant bacteria globally, with more than 100,000 attributable deaths annually. Despite the high urgency to develop a vaccine to control this pathogen, all clinical trials with pre-clinically effective candidates failed so far. The recent development of “humanized” mice might help to edge the pre-clinical evaluation closer to the clinical situation and thus close this gap. We infected humanized NSG mice (huNSG: (NOD)-scid IL2R_γ^null mice engrafted with human CD34+ hematopoietic stem cells) locally with S. aureus USA300 LAC* lux into the thigh muscle in order to investigate the human immune response to acute and chronic infection. These mice proved not only to be more susceptible to MRSA infection than wild-type or “murinized” mice, but displayed furthermore inferior survival and signs of systemic infection in an otherwise localized infection model. The rate of humanization correlated directly with the severity of disease and survival of the mice. Human and murine cytokine levels in blood and at the primary site of infection were strongly elevated in huNSG mice compared to all control groups. And importantly, differences in human and murine immune cell lineages surfaced during the infection, with human monocyte and B cell numbers in blood and bone marrow being significantly reduced at the later time point of infection. Murine monocytes in contrast behaved conversely by increasing cell numbers. This study demonstrates significant differences in the in vivo behavior of human and murine cells towards S. aureus infection, which might help to sharpen the translational potential of pre-clinical models for future therapeutic approaches.

Microbiome-Independent Effects of Antibiotics in a Murine Model of Nosocomial Infections

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most common causes of hospital-acquired pneumonia. To better manage patients with MRSA pneumonia, we require a greater understanding of the host-pathogen interactions during infection. MRSA research focuses on highly virulent and cytotoxic strains, which demonstrate robust phenotypes in animal models of infection. However, nosocomial infections are often caused by hospital-acquired MRSA (HA-MRSA) isolates that exhibit low cytotoxicity and few or no phenotypes in mice, thereby confounding mechanistic studies of pathogenesis. Consequently, virulence pathways utilized by HA-MRSA in nosocomial pneumonia are largely unknown. Here, we report that conditioning mice with broad-spectrum antibiotics lowers the barrier to pneumonia, thereby transforming otherwise avirulent HA-MRSA isolates into lethal pathogens. HA-MRSA isolates are avirulent in gnotobiotic mice, mimicking results in conventional animals. Thus, the observed enhanced susceptibility to infection in antibiotic-treated mice is not due to depletion of the microbiota. More generally, we found that antibiotic conditioning leads to increased susceptibility to infection by diverse antimicrobial-resistant (AMR) pathogens of low virulence. Treatment with antibiotics leads to dehydration and malnutrition, suggesting a potential role for these clinically relevant and reducible hospital complications in susceptibility to pathogens. In sum, the model described here mitigates the impact of low virulence in immunocompetent mice, providing a convenient model to gain fundamental insight into the pathogenesis of nosocomial pathogens.

VraSR Regulatory System Contributes to the Virulence of Community-Associated Methicillin-Resistant Staphylococcus aureus (CA-MRSA) in a 3D-Skin Model and Skin Infection of Humanized Mouse Model

The vancomycin-resistance associated sensor/regulator, VraSR two-component regulatory-system (VraSR), regulates virulence and the response of Staphylococcus aureus (SA) to environmental stress. To investigate the role of VraSR in SA skin and soft tissue infections (SSTI), we inactivated the VraSR of a clinical CA-MRSA ST30 strain by insertional mutation in vraR gene using the TargeTron-Gene Knockout System. We constructed an organotypic keratinocyte fibroblast co-culture (3D-skin model) and a humanized mouse as SSTI infection models. In the 3D-skin model, inactivation of VraSR in the strains ST30 and USA300 showed 1-log reduction in adhesion and internalization (p < 0.001) compared to the respective wildtype. The mutant strains of ST30 (p < 0.05) and USA300-LAC (p < 0.001) also exhibited reduced apoptosis. The wildtype ST30 infection in the humanized mouse model demonstrated increased skin lesion size and bacterial burden compared to BALB/c mice (p < 0.01). The response of the humanized mouse towards the MRSA infection exhibited human similarity indicating that the humanized mouse SSTI model is more suitable for evaluating the role of virulence determinants. Inactivation of VraSR in ST30 strain resulted in decreased skin lesion size in the humanized mouse SSTI model (p < 0.05) and reduction in apoptotic index (p < 0.01) when compared with the wildtype. Our results reveal that inactivating the VraSR system may be a potent anti-virulence approach to control MRSA infection.

Staphylococcus aureus Vaccine Research and Development: The Past, Present and Future, Including Novel Therapeutic Strategies

Staphylococcus aureus is one of the most important human pathogens worldwide. Its high antibiotic resistance profile reinforces the need for new interventions like vaccines in addition to new antibiotics. Vaccine development efforts against S. aureus have failed so far however, the findings from these human clinical and non-clinical studies provide potential insight for such failures. Currently, research is focusing on identifying novel vaccine formulations able to elicit potent humoral and cellular immune responses. Translational science studies are attempting to discover correlates of protection using animal models as well as in vitro and ex vivo models assessing efficacy of vaccine candidates. Several new vaccine candidates are being tested in human clinical trials in a variety of target populations. In addition to vaccines, bacteriophages, monoclonal antibodies, centyrins and new classes of antibiotics are being developed. Some of these have been tested in humans with encouraging results. The complexity of the diseases and the range of the target populations affected by this pathogen will require a multipronged approach using different interventions, which will be discussed in this review.

Staphylococcus aureus Arsenal To Conquer the Lower Respiratory Tract

Staphylococcus aureus is both a commensal and a pathogenic bacterium for humans. Its ability to induce severe infections is based on a wide range of virulence factors. S. aureus community-acquired pneumonia (SA-CAP) is rare and severe, and the contribution of certain virulence factors in this disease has been recognized over the past 2 decades. First, the factors involved in metabolism adaptation are crucial for S. aureus survival in the lower respiratory tract, and toxins and enzymes are required for it to cross the pulmonary epithelial barrier. S. aureus subsequently faces host defense mechanisms, including the epithelial barrier, but most importantly the immune system. Here, again, S. aureus uses myriad virulence factors to successfully escape from the host’s defenses and takes advantage of them. The impact of S. aureus virulence, combined with the collateral damage caused by an overwhelming immune response, leads to severe tissue damage and adverse clinical outcomes. In this review, we summarize step by step all of the S. aureus factors implicated in CAP and described to date, and we provide an outlook for future research.

Humanized Mice Exhibit Exacerbated Abscess Formation and Osteolysis During the Establishment of Implant-Associated Staphylococcus aureus Osteomyelitis

Staphylococcus aureus is the predominant pathogen causing osteomyelitis. Unfortunately, no immunotherapy exists to treat these very challenging and costly infections despite decades of research, and numerous vaccine failures in clinical trials. This lack of success can partially be attributed to an overreliance on murine models where the immune correlates of protection often diverge from that of humans. Moreover, S. aureus secretes numerous immunotoxins with unique tropism to human leukocytes, which compromises the targeting of immune cells in murine models. To study the response of human immune cells during chronic S. aureus bone infections, we engrafted non-obese diabetic (NOD)-scid IL2Rγ^null (NSG) mice with human hematopoietic stem cells (huNSG) and analyzed protection in an established model of implant-associated osteomyelitis. The results showed that huNSG mice have increases in weight loss, osteolysis, bacterial dissemination to internal organs, and numbers of Staphylococcal abscess communities (SACs), during the establishment of implant-associated MRSA osteomyelitis compared to NSG controls (p < 0.05). Flow cytometry and immunohistochemistry demonstrated greater human T cell numbers in infected versus uninfected huNSG mice (p < 0.05), and that T-bet⁺ human T cells clustered around the SACs, suggesting S. aureus-mediated activation and proliferation of human T cells in the infected bone. Collectively, these proof-of-concept studies underscore the utility of huNSG mice for studying an aggressive form of S. aureus osteomyelitis, which is more akin to that seen in humans. We have also established an experimental system to investigate the contribution of specific human T cells in controlling S. aureus infection and dissemination.

Exploring Virulence Factors and Alternative Therapies against Staphylococcus aureus Pneumonia

Pneumonia is an acute pulmonary infection associated with high mortality and an immense financial burden on healthcare systems. Staphylococcus aureus is an opportunistic pathogen capable of inducing S. aureus pneumonia (SAP), with some lineages also showing multidrug resistance. Given the high level of antibiotic resistance, much research has been focused on targeting S. aureus virulence factors, including toxins and biofilm-associated proteins, in an attempt to develop effective SAP therapeutics. Despite several promising leads, many hurdles still remain for S. aureus vaccine research. Here, we review the state-of-the-art SAP therapeutics, highlight their pitfalls, and discuss alternative approaches of potential significance and future perspectives.

Efficacy of Active Immunization With Attenuated α-Hemolysin and Panton-Valentine Leukocidin in a Rabbit Model of Staphylococcus aureus Necrotizing Pneumonia

Staphylococcus aureus is a common pathogen causing infections in humans with various degrees of severity, with pneumonia being one of the most severe infections. In as much as staphylococcal pneumonia is a disease driven in large part by α-hemolysin (Hla) and Panton-Valentine leukocidin (PVL), we evaluated whether active immunization with attenuated forms of Hla (HlaH35L/H48L) alone, PVL components (LukS-PVT28F/K97A/S209A and LukF-PVK102A) alone, or combination of all 3 toxoids could prevent lethal challenge in a rabbit model of necrotizing pneumonia caused by the USA300 community-associated methicillin-resistant S. aureus (MRSA). Rabbits vaccinated with Hla toxoid alone or PVL components alone were only partially protected against lethal pneumonia, whereas those vaccinated with all 3 toxoids had 100% protection against lethality. Vaccine-mediated protection correlated with induction of polyclonal antibody response that neutralized not only α-hemolysin and PVL, but also other related toxins, produced by USA300 and other epidemic MRSA clones.

Staphylococcus aureus Host Tropism and Its Implications for Murine Infection Models

Staphylococcus aureus (S. aureus) is a pathobiont of humans as well as a multitude of animal species. The high prevalence of multi-resistant and more virulent strains of S. aureus necessitates the development of new prevention and treatment strategies for S. aureus infection. Major advances towards understanding the pathogenesis of S. aureus diseases have been made using conventional mouse models, i.e., by infecting naïve laboratory mice with human-adapted S.aureus strains. However, the failure to transfer certain results obtained in these murine systems to humans highlights the limitations of such models. Indeed, numerous S. aureus vaccine candidates showed promising results in conventional mouse models but failed to offer protection in human clinical trials. These limitations arise not only from the widely discussed physiological differences between mice and humans, but also from the lack of attention that is paid to the specific interactions of S. aureus with its respective host. For instance, animal-derived S. aureus lineages show a high degree of host tropism and carry a repertoire of host-specific virulence and immune evasion factors. Mouse-adapted S.aureus strains, humanized mice, and microbiome-optimized mice are promising approaches to overcome these limitations and could improve transferability of animal experiments to human trials in the future.

Development of a vaccine against Staphylococcus aureus invasive infections: Evidence based on human immunity, genetics and bacterial evasion mechanisms

Invasive Staphylococcus aureus infections are a leading cause of morbidity and mortality in both hospital and community settings, especially with the widespread emergence of virulent and multi-drug resistant methicillin-resistant S. aureus strains. There is an urgent and unmet clinical need for non-antibiotic immune-based approaches to treat these infections as the increasing antibiotic resistance is creating a serious threat to public health. However, all vaccination attempts aimed at preventing S. aureus invasive infections have failed in human trials, especially all vaccines aimed at generating high titers of opsonic antibodies against S. aureus surface antigens to facilitate antibody-mediated bacterial clearance. In this review, we summarize the data from humans regarding the immune responses that protect against invasive S. aureus infections as well as host genetic factors and bacterial evasion mechanisms, which are important to consider for the future development of effective and successful vaccines and immunotherapies against invasive S. aureus infections in humans. The evidence presented form the basis for a hypothesis that staphylococcal toxins (including superantigens and pore-forming toxins) are important virulence factors, and targeting the neutralization of these toxins are more likely to provide a therapeutic benefit in contrast to prior vaccine attempts to generate antibodies to facilitate opsonophagocytosis.

Development of humanized mouse and rat models with full-thickness human skin and autologous immune cells

The human skin is a significant barrier for protection against pathogen transmission. Rodent models used to investigate human-specific pathogens that target the skin are generated by introducing human skin grafts to immunocompromised rodent strains. Infection-induced immunopathogenesis has been separately studied in humanized rodent models developed with human lymphoid tissue and hematopoietic stem cell transplants. Successful co-engraftment of human skin, autologous lymphoid tissues, and autologous immune cells in a rodent model has not yet been achieved, though it could provide a means of studying the human immune response to infection in the human skin. Here, we introduce the human Skin and Immune System (hSIS)-humanized NOD-scid IL2Rγ^null (NSG) mouse and Sprague–Dawley-Rag2^tm2hera Il2rγ^tm1hera (SRG) rat models, co-engrafted with human full-thickness fetal skin, autologous fetal lymphoid tissues, and autologous fetal liver-derived hematopoietic stem cells. hSIS-humanized rodents demonstrate the development of human full-thickness skin, along with autologous lymphoid tissues, and autologous immune cells. These models also support human skin infection following intradermal inoculation with community-associated methicillin-resistant Staphylococcus aureus. The co-engraftment of these human skin and immune system components into a single humanized rodent model could provide a platform for studying human skin infections.

Two distinct amphipathic peptide antibiotics with systemic efficacy

Antimicrobial peptides are important candidates for developing new classes of antibiotics because of their potency against antibiotic-resistant pathogens. Current research focuses on topical applications and it is unclear how to design peptides with systemic efficacy. To address this problem, we designed two potent peptides by combining database-guided discovery with structure-based design. When bound to membranes, these two short peptides with an identical amino acid composition can adopt two distinct amphipathic structures: A classic horizontal helix (horine) and a novel vertical spiral structure (verine). Their horizontal and vertical orientations on membranes were determined by solid-state ¹⁵N NMR data. While horine was potent primarily against gram-positive pathogens, verine showed broad-spectrum antimicrobial activity. Both peptides protected greater than 80% mice from infection-caused deaths. Moreover, horine and verine also displayed significant systemic efficacy in different murine models comparable to conventional antibiotics. In addition, they could eliminate resistant pathogens and preformed biofilms. Significantly, the peptides showed no nephrotoxicity to mice after intraperitoneal or intravenous administration for 1 wk. Our study underscores the significance of horine and verine in fighting drug-resistant pathogens.

Epidemiological and Clinical Evidence for the Role of Toxins in S. aureus Human Disease

Staphylococcus aureus asymptomatically colonizes approximately 30–50% of the population and is a leading cause of bacteremia, bone/joint infections, and skin infections in the US. S. aureus has become a major public health threat due to antibiotic resistance and an increasing number of failed vaccine attempts. To develop new anti-staphylococcal preventive therapies, it will take a more thorough understanding of the current role S. aureus virulence factors play in contributing to human disease. This review focuses on the clinical association of individual toxins with S. aureus infection as well as attempted treatment options. Further understanding of these associations will increase understanding of toxins and their importance to S. aureus pathogenesis.

Structure-based discovery of a small-molecule inhibitor of methicillin-resistant Staphylococcus aureus virulence

The rapid emergence and dissemination of methicillin-resistant Staphylococcus aureus (MRSA) strains poses a major threat to public health. MRSA possesses an arsenal of secreted host-damaging virulence factors that mediate pathogenicity and blunt immune defenses. Panton-Valentine leukocidin (PVL) and α-toxin are exotoxins that create lytic pores in the host cell membrane. They are recognized as being important for the development of invasive MRSA infections and are thus potential targets for antivirulence therapies. Here, we report the high-resolution X-ray crystal structures of both PVL and α-toxin in their soluble, monomeric, and oligomeric membrane-inserted pore states in complex with n-tetradecylphosphocholine (C₁₄PC). The structures revealed two evolutionarily conserved phosphatidylcholine-binding mechanisms and their roles in modulating host cell attachment, oligomer assembly, and membrane perforation. Moreover, we demonstrate that the soluble C₁₄PC compound protects primary human immune cells in vitro against cytolysis by PVL and α-toxin and hence may serve as the basis for the development of an antivirulence agent for managing MRSA infections.

Sepsis therapies: learning from 30 years of failure of translational research to propose new leads

Sepsis has been identified by the World Health Organization (WHO) as a global health priority. There has been a tremendous effort to decipher underlying mechanisms responsible for organ failure and death, and to develop new treatments. Despite saving thousands of animals over the last three decades in multiple preclinical studies, no new effective drug has emerged that has clearly improved patient outcomes. In the present review, we analyze the reasons for this failure, focusing on the inclusion of inappropriate patients and the use of irrelevant animal models. We advocate against repeating the same mistakes and propose changes to the research paradigm. We discuss the long-term consequences of surviving sepsis and, finally, list some putative approaches-both old and new-that could help save lives and improve survivorship.

Inducible Costimulator Contributes to Methicillin-Resistant Staphylococcus aureus Pneumonia

Staphylococcus aureus is a major cause of both community- and healthcare-acquired pneumonias. Inducible costimulator (ICOS) is part of the CD28 family of proteins and is a target for immune checkpoint therapy. We found ICOS highly expressed on activated CD4 cells in response to S. aureus. In the absence of ICOS, mice had improved survival in a pneumonia model with the methicillin-resistant Staphylococcus aureus (MRSA) strain USA300 and significant reductions in bacterial burden in a nonlethal acute pneumonia model. Infected Icos-/- mice had major reductions in several proinflammatory cytokines, neutrophils, inflammatory monocytes, and eosinophils compared to infected wild-type mice, while there was improved expression of CD11c and macrophage receptor with collagenous structure on the surface of alveolar macrophages. Early during infection infected Icos-/- mice had increased numbers of alveolar macrophages and expression of several surface markers on alveolar macrophages and neutrophils. ICOS signaling also contributed to the pathogenesis of the airway pathogens Klebsiella pneumoniae, Pseudomonas aeruginosa, and Streptococcus pneumoniae, and neutralizing antibody to ICOS led to improved clearance of S. aureus from the airway. Our results indicate that ICOS plays a significant role in orchestrating the innate immune response to S. aureus and other airway pathogens, and could be a potential immunomodulatory target to attenuate S. aureus-related immunopathology.

The Fluctuations of Leukocytes and Circulating Cytokines in Septic Humanized Mice Vary With Outcome

Sepsis remains a major challenge in translational research given its heterogeneous pathophysiology and the lack of specific therapeutics. The use of humanized mouse chimeras with transplanted human hematopoietic cells may improve the clinical relevance of pre-clinical studies. However, knowledge of the human immuno-inflammatory response during sepsis in humanized mice is scarce; it is unclear how similar or divergent mouse and human-origin immuno-inflammatory responses in sepsis are. In this study, we evaluated the early outcome-dependent immuno-inflammatory response in humanized mice generated in the NSG strain after cecal ligation and puncture (CLP) sepsis. Mice were observed for 32 h post-CLP and were assigned to either predicted-to-die (P-DIE) or predicted-to-survive (P-SUR) groups for retrospective comparisons. Blood samples were collected at baseline, 6 and 24 h, whereas the bone marrow and spleen were collected between 24 and 32 h post-CLP. In comparison to P-SUR, P-DIE humanized mice had a 3-fold higher frequency of human splenic monocytes and their CD80 expression was reduced by 1.3-fold; there was no difference in the HLA-DR expression. Similarly, the expression of CD80 on the bone marrow monocytes from P-DIE mice was decreased by 32% (p < 0.05). Sepsis induced a generalized up-regulation of both human and murine plasma cytokines (TNFα, IL-6, IL-10, IL-8/KC, MCP-1); it was additionally aggravated in P-DIE vs. P-SUR. Human cytokines were strongly overridden by the murine ones (approx. ratio 1:9) but human TNFα was 7-fold higher than mouse TNFα. Interestingly, transplantation of human cells did not influence murine cytokine response in NSG mice, but humanized NSG mice were more susceptible to sepsis in comparison with NSG mice (79 vs. 33% mortality; p < 0.05). In conclusion, our results show that humanized mice reflect selected aspects of human immune responses in sepsis and therefore may be a feasible alternative in preclinical immunotherapy modeling.

CD80/CD86 signaling contributes to the proinflammatory response of Staphylococcus aureus in the airway

It was posited that the initial host response to Staphylococcus aureus is a contributing factor in the pathogenesis of acute pneumonia. Having previously observed that T cells play a negative role in the pathogenesis of acute pneumonia to S. aureus the contribution of the CD80/CD86 pathway in pathogenesis was investigated. Mice lacking CD80 and CD86 had significantly improved survival in a mouse model of acute S. aureus pneumonia. This was accompanied by significant reductions in several proinflammatory cytokines, including TNF, MIP-2, IL-1β, IL-17 and IL-6, as well as increased numbers of viable alveolar macrophages. Early during infection reductions in cytokine production were evident and cytokine production in response to S. aureus in bone marrow derived macrophages showed decreases in TNF, KC, IL-1α and GM-CSF. Our data suggest that CD80/CD86 signaling plays a significant role in the initial inflammatory response to S. aureus in the airway and could be a potential acute target to reduce the initial inflammatory insult.

Infection of Primary Human Alveolar Macrophages Alters Staphylococcus aureus Toxin Production and Activity

Pulmonary pathogens encounter numerous insults, including phagocytic cells designed to degrade bacteria, while establishing infection in the human lung. Staphylococcus aureus is a versatile, opportunistic pathogen that can cause severe pneumonia, and methicillin-resistant isolates are of particular concern. Recent reports present conflicting data regarding the ability of S. aureus to survive and replicate within macrophages. However, due to use of multiple strains and macrophage sources, making comparisons between reports remains difficult. Here, we established a disease-relevant platform to study innate interactions between S. aureus and human lungs. Human precision-cut lung slices (hPCLS) were subjected to infection by S. aureus LAC (methicillin-resistant) or UAMS-1 (methicillin-sensitive) isolates. Additionally, primary human alveolar macrophages (hAMs) were infected with S. aureus, and antibacterial activity was assessed. Although both S. aureus isolates survived within hAM phagosomes, neither strain replicated efficiently in these cells. S. aureus was prevalent within the epithelial and interstitial regions of hPCLS, with limited numbers present in a subset of hAMs, suggesting that the pathogen may not target phagocytic cells for intracellular growth during natural pulmonary infection. S. aureus-infected hAMs mounted a robust inflammatory response that reflected natural human disease. S. aureus LAC was significantly more cytotoxic to hAMs than UAMS-1, potentially due to isolate-specific virulence factors. The bicomponent toxin Panton-Valentine leukocidin was not produced during intracellular infection, while alpha-hemolysin was produced but was not hemolytic, suggesting that hAMs alter toxin activity. Overall, this study defined a new disease-relevant infection platform to study S. aureus interaction with human lungs and to define virulence factors that incapacitate pulmonary cells.

S. aureus Evades Macrophage Killing through NLRP3-Dependent Effects on Mitochondrial Trafficking

Clinical severity of Staphylococcus aureus respiratory infection correlates with alpha toxin (AT) expression. AT activates the NLRP3 inflammasome; deletion of Nlrp3, or AT neutralization, protects mice from lethal S. aureus pneumonia. We tested the hypothesis that this protection is not due to a reduction in inflammasome-dependent cytokines (IL-1β/IL-18) but increased bactericidal function of macrophages. In vivo, neutralization of AT or NLRP3 improved bacterial clearance and survival, while blocking IL-1β/IL-18 did not. Primary human monocytes were used in vitro to determine the mechanism through which NLRP3 alters bacterial killing. In cells treated with small interfering RNA (siRNA) targeting NLRP3 or infected with AT-null S. aureus, mitochondria co-localize with bacterial-containing phagosomes. Mitochondrial engagement activates caspase-1, a process dependent on complex II of the electron transport chain, near the phagosome, promoting its acidification. These data demonstrate a mechanism utilized by S. aureus to sequester itself from antimicrobial processes within the cell.

Host-Pathogen Interactions in Gram-Positive Bacterial Pneumonia

Community-acquired pneumonia (CAP) is a leading cause of morbidity and mortality worldwide. Despite broad literature including basic and translational scientific studies, many gaps in our understanding of host-pathogen interactions remain. In this review, pathogen virulence factors that drive lung infection and injury are discussed in relation to their associated host immune pathways. CAP epidemiology is considered, with a focus on Staphylococcus aureus and Streptococcus pneumoniae as primary pathogens. Bacterial factors involved in nasal colonization and subsequent virulence are illuminated. A particular emphasis is placed on bacterial pore-forming toxins, host cell death, and inflammasome activation. Identified host-pathogen interactions are then examined by linking pathogen factors to aberrant host response pathways in the context of acute lung injury in both primary and secondary infection. While much is known regarding bacterial virulence and host immune responses, CAP management is still limited to mostly supportive care. It is likely that improvements in therapy will be derived from combinatorial targeting of both pathogen virulence factors and host immunomodulation.

Modeling Infectious Diseases in Mice with a "Humanized" Immune System

Human immune system (HIS) mice are created by transplanting human immune cells or their progenitor cells into highly immunodeficient recipient mouse hosts, thereby "humanizing" their immune systems. Over past decades, the field of HIS mice has evolved rapidly, as modifications of existing immunodeficient mouse strains have been developed, resulting in increasing levels of human tissue engraftment as humanization is optimized. Current HIS mouse models not only permit elevated levels of human cell engraftment but also demonstrate graft stability. As such, HIS mice are being extensively used to study the human innate and adaptive immune response against microbial infections in vivo. Compared to nonhumanized animal models, which are frequently infected with surrogate or adapted microbes, the HIS mouse models allow the analysis of interactions between human immune cells and bona fide pathogenic microbes, making them a more clinically relevant model. This article reviews the development of HIS mice and covers the different strategies used to humanize mice, as well as discussing the use of HIS mice for studying bacterial infections that cause human disease.

Adaptation of the Staphylococcus aureus leukocidin LukGH for the rabbit host by protein engineering

Host defense against Staphylococcus aureus greatly depends on bacterial clearance by phagocytic cells. LukGH (or LukAB) is the most potent staphylococcal leukocidin towards human phagocytes in vitro, but its role in pathogenesis is obscured by the lack of suitable small animal models because LukGH has limited or no cytotoxicity towards rodent and rabbit compared with human polymorphonuclear cells (PMNs) likely due to an impaired interaction with its cellular receptor, CD11b. We aimed at adapting LukGH for the rabbit host by improving binding to the rabbit homolog of CD11b, specifically its I-domain (CD11b-I). Targeted amino acid substitutions were introduced into the LukH polypeptide to map its receptor interaction site(s). We found that the binding affinity of LukGH variants to the human and rabbit CD11b-I correlated well with their PMN cytotoxicity. Importantly, we identified LukGH variants with significantly improved cytotoxicity towards rabbit PMNs, when expressed recombinantly (10-15-fold) or by engineered S. aureus strains. These findings support the development of small animal models of S. aureus infection with the potential for demonstrating the importance of LukGH in pathogenesis.

Staphylococcus aureus: setting its sights on the human innate immune system

Staphylococcus aureus has colonized humans for at least 10 000 years, and today inhabits roughly a third of the population. In addition, S. aureus is a major pathogen that is responsible for a significant disease burden, ranging in severity from mild skin and soft-tissue infections to life-threatening endocarditis and necrotizing pneumonia, with treatment often hampered by resistance to commonly available antibiotics. Underpinning its versatility as a pathogen is its ability to evade the innate immune system. S. aureus specifically targets innate immunity to establish and sustain infection, utilizing a large repertoire of virulence factors to do so. Using these factors, S. aureus can resist phagosomal killing, impair complement activity, disrupt cytokine signalling and target phagocytes directly using proteolytic enzymes and cytolytic toxins. Although most of these virulence factors are well characterized, their importance during infection is less clear, as many display species-specific activity against humans or against animal hosts, including cows, horses and chickens. Several staphylococcal virulence factors display species specificity for components of the human innate immune system, with as few as two amino acid changes reducing binding affinity by as much as 100-fold. This represents a major issue for studying their roles during infection, which cannot be examined without the use of humanized infection models. This review summarizes the major factors S. aureus uses to impair the innate immune system, and provides an in-depth look into the host specificity of S. aureus and how this problem is being approached.

Staphylococcus aureus Impairs the Function of and Kills Human Dendritic Cells via the LukAB Toxin

Antigen-presenting cells such as dendritic cells (DCs) fulfill an indispensable role in the development of adaptive immunity by producing proinflammatory cytokines and presenting microbial antigens to lymphocytes to trigger a faster, specific, and long-lasting immune response. Here, we studied the effect of Staphylococcus aureus toxins on human DCs. We discovered that the leukocidin LukAB hinders the development of adaptive immunity by targeting human DCs. The ability of S. aureus to blunt the function of DCs could help explain the high frequency of recurrent S. aureus infections. Taken together, the results from this study suggest that therapeutically targeting the S. aureus leukocidins may boost effective innate and adaptive immune responses by protecting innate leukocytes, enabling proper antigen presentation and T cell activation.

Staphylococcus aureus is a human pathogen responsible for high morbidity and mortality worldwide. Recurrent infections with this bacterium are common, suggesting that S. aureus thwarts the development of sterilizing immunity. S. aureus strains that cause disease in humans produce up to five different bicomponent toxins (leukocidins) that target and lyse neutrophils, innate immune cells that represent the first line of defense against S. aureus infections. However, little is known about the role of leukocidins in blunting adaptive immunity. Here, we explored the effects of leukocidins on human dendritic cells (DCs), antigen-presenting cells required for the development of adaptive immunity. Using an ex vivo infection model of primary human monocyte-derived dendritic cells, we found that S. aureus, including strains from different clonal complexes and drug resistance profiles, effectively kills DCs despite efficient phagocytosis. Although all purified leukocidins could kill DCs, infections with live bacteria revealed that S. aureus targets and kills DCs primarily via the activity of leukocidin LukAB. Moreover, using coculture experiments performed with DCs and autologous CD4⁺ T lymphocytes, we found that LukAB inhibits DC-mediated activation and proliferation of primary human T cells. Taken together, the data determined in the study reveal a novel immunosuppressive strategy of S. aureus whereby the bacterium blunts the development of adaptive immunity via LukAB-mediated injury of DCs.

Subinhibitory concentrations of tedizolid potently inhibit extracellular toxin production by methicillin-sensitive and methicillin-resistant Staphylococcus aureus

PURPOSE

Potent extracellular toxins including alpha-haemolysin, Panton-Valentine leukocidin (PVL) and toxic-shock syndrome toxin 1 (TSST-1) significantly contribute to Staphylococcus aureus pathogenesis, thus, toxin suppression is a primary focus in treatment of staphylococcal disease. S. aureus maintains complex strategies to regulate toxin expression and previous data have demonstrated that subinhibitory concentrations of beta-lactam antibiotics can adversely increase S. aureus exotoxin production. The current study evaluates the effects of subinhibitory concentrations of tedizolid, a second-generation oxazolidinone derivative, on expression of staphylococcal exotoxins in both methicillin-resistant and methicillin-sensitive S. aureus.

METHODOLOGY

S. aureus exotoxin expression levels were compared at 12 and 24 h following treatment with tedizolid, linezolid, nafcillin or vehicle control.

RESULTS

Our findings show that the level of antibiotic required to alter toxin production was strain-dependent and corresponds with the quantity of toxin produced, but both tedizolid and linezolid could effectively reduce expression of alpha-haemolysin, PVL and TSST-1 toxin at subinhibitory concentrations. In contrast, nafcillin showed less attenuation and, in some S. aureus strains, led to an increase in toxin expression. Tedizolid consistently inhibited toxin production at a lower overall drug concentration than comparator agents.

CONCLUSION

Together, our data support that tedizolid has the potential to improve outcomes of infection due to its superior ability to inhibit S. aureus growth and attenuate exotoxin production.

Bioreactor-based mass production of human iPSC-derived macrophages enables immunotherapies against bacterial airway infections

The increasing number of severe infections with multi-drug-resistant pathogens worldwide highlights the need for alternative treatment options. Given the pivotal role of phagocytes and especially alveolar macrophages in pulmonary immunity, we introduce a new, cell-based treatment strategy to target bacterial airway infections. Here we show that the mass production of therapeutic phagocytes from induced pluripotent stem cells (iPSC) in industry-compatible, stirred-tank bioreactors is feasible. Bioreactor-derived iPSC-macrophages (iPSC-Mac) represent a highly pure population of CD45⁺CD11b⁺CD14⁺CD163⁺ cells, and share important phenotypic, functional and transcriptional hallmarks with professional phagocytes, however with a distinct transcriptome signature similar to primitive macrophages. Most importantly, bioreactor-derived iPSC-Mac rescue mice from Pseudomonas aeruginosa-mediated acute infections of the lower respiratory tract within 4-8 h post intra-pulmonary transplantation and reduce bacterial load. Generation of specific immune-cells from iPSC-sources in scalable stirred-tank bioreactors can extend the field of immunotherapy towards bacterial infections, and may allow for further innovative cell-based treatment strategies.

Pulmonary infections constitute a substantial health problem worldwide. Here the authors show that phagocytes similar to primitive macrophages can be generated from human induced pluripotent stem cells, by the use of industry-compatible, stirred-tank bioreactors, and applied as a cell-based therapy to treat acute bacterial infections in mice.

Human immunodeficiency virus infection induces lymphoid fibrosis in the BM-liver-thymus-spleen humanized mouse model

A major pathogenic feature associated with HIV infection is lymphoid fibrosis, which persists during antiretroviral therapy (ART). Lymphoid tissues play critical roles in the generation of antigen-specific immune response, and fibrosis disrupts the stromal network of lymphoid tissues, resulting in impaired immune cell trafficking and function, as well as immunodeficiency. Developing an animal model for investigating the impact of HIV infection-induced lymphoid tissue fibrosis on immunodeficiency and immune cell impairment is critical for therapeutics development and clinical translation. Said model will enable in vivo mechanistic studies, thus complementing the well-established surrogate model of SIV infection-induced lymphoid tissue fibrosis in macaques. We developed a potentially novel human immune system-humanized mouse model by coengrafting autologous fetal thymus, spleen, and liver organoids under the kidney capsule, along with i.v. injection of autologous fetal liver-derived hematopoietic stem cells, thus termed the BM-liver-thymus-spleen (BLTS) humanized mouse model. BLTS humanized mouse model supports development of human immune cells and human lymphoid organoids (human thymus and spleen organoids). HIV infection in BLTS humanized mice results in progressive fibrosis in human lymphoid tissues, which was associated with immunodeficiency in the lymphoid tissues, and lymphoid tissue fibrosis persists during ART, thus recapitulating clinical outcomes.

Potential Pitfalls of the Humanized Mice in Modeling Sepsis

Humanized mice are a state-of-the-art tool used to study several diseases, helping to close the gap between mice and human immunology. This review focuses on the potential obstacles in the analysis of immune system performance between humans and humanized mice in the context of severe acute inflammation as seen in sepsis or other critical care illnesses. The extent to which the reconstituted human immune system in mice adequately compares to the performance of the human immune system in human hosts is still an evolving question. Although certain viral and protozoan infections can be replicated in humanized mice, whether a highly complex and dynamic systemic inflammation like sepsis can be accurately represented by current humanized mouse models in a clinically translatable manner is unclear. Humanized mice are xenotransplant animals in the most general terms. Several organs (e.g., bone marrow mesenchymal cells, endothelium) cannot interact with the grafted human leukocytes effectively due to species specificity. Also the interaction between mice gut flora and the human immune system may be paradoxical. Often, grafting is performed utilizing an identical batch of stem cells in highly inbred animals which fails to account for human heterogeneity. Limiting factors include the substantial cost and restricting supply of animals. Finally, humanized mice offer an opportunity to gain knowledge of human-like conditions, requiring careful data interpretation just as in nonhumanized animals.

Deciphering the Pathological Role of Staphylococcal α-Toxin and Panton-Valentine Leukocidin Using a Novel Ex Vivo Human Skin Model

Staphylococcus aureus alpha-toxin and Panton-Valentine leukocidin (PVL) have been reported to play critical roles in different animal models of skin infection. These models, however, do not completely recapitulate the human disease due to the host specificity of these toxins as well as the intrinsic anatomical and immunological differences between animals and humans. Human skin explants represent a valid alternative to animal models for studying skin infections. Herein, we developed a human skin explant wound model to study the pathogenic role of alpha-toxin and PVL; inflammatory responses elicited by these toxins; and the neutralizing ability of antibodies to mitigate skin damage. Different concentrations of alpha-toxin and/PVL were applied to superficial wounds on human skin explants. Treatment with alpha-toxin resulted in high tissue toxicity and loss of skin epithelial integrity. PVL induced a milder but significant toxicity with no loss of skin structural integrity. The combination of both toxins resulted in increased tissue toxicity as compared with the individual toxins alone. Treatment of the skin with these toxins also resulted in a decrease of CD45-positive cells in the epidermis. In addition, both toxins induced the release of pro-inflammatory cytokines and chemokines. Finally, antibodies raised against alpha-toxin were able to mitigate tissue toxicity in a concentration-dependent manner. Results from this study confirm the key role of α-toxin in staphylococcal infection of the human skin and suggest a possible cooperation of the two toxins in tissue pathology.

Human CD45 is an F-component-specific receptor for the staphylococcal toxin Panton-Valentine leukocidin

The staphylococcal bi-component leukocidins Panton-Valentine leukocidin (PVL) and γ-haemolysin CB (HlgCB) target human phagocytes. Binding of the toxins' S-components to human complement C5a receptor 1 (C5aR1) contributes to cellular tropism and human specificity of PVL and HlgCB. To investigate the role of both leukocidins during infection, we developed a human C5aR1 knock-in (hC5aR1^KI) mouse model. HlgCB, but unexpectedly not PVL, contributed to increased bacterial loads in tissues of hC5aR1^KI mice. Compared to humans, murine hC5aR1^KI neutrophils showed a reduced sensitivity to PVL, which was mediated by the toxin's F-component LukF-PV. By performing a genome-wide CRISPR-Cas9 screen, we identified CD45 as a receptor for LukF-PV. The human-specific interaction between LukF-PV and CD45 provides a molecular explanation for resistance of hC5aR1^KI mouse neutrophils to PVL and probably contributes to the lack of a PVL-mediated phenotype during infection in these mice. This study demonstrates an unsuspected role of the F-component in driving the sensitivity of human phagocytes to PVL.

Targeting fundamental pathways to disrupt Staphylococcus aureus survival: clinical implications of recent discoveries

The emergence of community-associated methicillin-resistant Staphylococcus aureus during the past decade along with an impending shortage of effective antistaphylococcal antibiotics have fueled impressive advances in our understanding of how S. aureus overcomes the host environment to establish infection. Backed by recent technologic advances, studies have uncovered elaborate metabolic, nutritional, and virulence strategies deployed by S. aureus to survive the restrictive and hostile environment imposed by the host, leading to a plethora of promising antimicrobial approaches that have potential to remedy the antibiotic resistance crisis. In this Review, we highlight some of the critical and recently elucidated bacterial strategies that are potentially amenable to intervention, discuss their relevance to human diseases, and address the translational challenges posed by current animal models.

Panton-Valentine Leukocidin Colocalizes with Retinal Ganglion and Amacrine Cells and Activates Glial Reactions and Microglial Apoptosis

Experimental models have established Panton-Valentine leukocidin (PVL) as a potential critical virulence factor during Staphylococcus aureus endophthalmitis. In the present study, we aimed to identify retinal cell targets for PVL and to analyze early retinal changes during infection. After the intravitreous injection of PVL, adult rabbits were euthanized at different time points (30 min, 1, 2, 4 and 8 h). PVL location in the retina, expression of its binding receptor C5a receptor (C5aR), and changes in Müller and microglial cells were analyzed using immunohistochemistry, Western blotting and RT-qPCR. In this model of PVL eye intoxication, only retinal ganglion cells (RGCs) expressed C5aR, and PVL was identified on the surface of two kinds of retinal neural cells. PVL-linked fluorescence increased in RGCs over time, reaching 98% of all RGCs 2 h after PVL injection. However, displaced amacrine cells (DACs) transiently colocalized with PVL. Müller and microglial cells were increasingly activated after injection over time. IL-6 expression in retina increased and some microglial cells underwent apoptosis 4 h and 8 h after PVL infection, probably because of abnormal nitrotyrosine production in the retina.

Humanized Mouse Models of Staphylococcus aureus Infection

Staphylococcus aureus is a successful human pathogen that has adapted itself in response to selection pressure by the human immune system. A commensal of the human skin and nose, it is a leading cause of several conditions: skin and soft tissue infection, pneumonia, septicemia, peritonitis, bacteremia, and endocarditis. Mice have been used extensively in all these conditions to identify virulence factors and host components important for pathogenesis. Although significant effort has gone toward development of an anti-staphylococcal vaccine, antibodies have proven ineffective in preventing infection in humans after successful studies in mice. These results have raised questions as to the utility of mice to predict patient outcome and suggest that humanized mice might prove useful in modeling infection. The development of humanized mouse models of S. aureus infection will allow us to assess the contribution of several human-specific virulence factors, in addition to exploring components of the human immune system in protection against S. aureus infection. Their use is discussed in light of several recently reported studies.

Staphylococcus aureus pore-forming toxins: The interface of pathogen and host complexity

Staphylococcus aureus is a prominent human pathogen capable of infecting a variety of host species and tissue sites. This versatility stems from the pathogen's ability to secrete diverse host-damaging virulence factors. Among these factors, the S. aureus pore-forming toxins (PFTs) α-toxin and the bicomponent leukocidins, have garnered much attention for their ability to lyse cells at low concentrations and modulate disease severity. Although many of these toxins were discovered nearly a century ago, their host cell specificities have only been elucidated over the past five to six years, starting with the discovery of the eukaryotic receptor for α-toxin and rapidly followed by identification of the leukocidin receptors. The identification of these receptors has revealed the species- and cell type-specificity of toxin binding, and provided insight into non-lytic effects of PFT intoxication that contribute to disease pathogenesis.

Leukocidins: staphylococcal bi-component pore-forming toxins find their receptors

Staphylococcus aureus is a major bacterial pathogen that causes disease worldwide. The emergence of strains that are resistant to commonly used antibiotics and the failure of vaccine development have resulted in a renewed interest in the pathophysiology of this bacterium. Staphylococcal leukocidins are a family of bi-component pore-forming toxins that are important virulence factors. During the past five years, cellular receptors have been identified for all of the bi-component leukocidins. The identification of the leukocidin receptors explains the cellular tropism and species specificity that is exhibited by these toxins, which has important biological consequences. In this Review, we summarize the recent discoveries that have reignited interest in these toxins and provide an outlook for future research.

Animal models of hospital-acquired pneumonia: current practices and future perspectives

Lower respiratory tract infections are amongst the leading causes of mortality and morbidity worldwide. Especially in hospital settings and more particularly in critically ill ventilated patients, nosocomial pneumonia is one of the most serious infectious complications frequently caused by opportunistic pathogens. Pseudomonas aeruginosa is one of the most important causes of ventilator-associated pneumonia as well as the major cause of chronic pneumonia in cystic fibrosis patients. Animal models of pneumonia allow us to investigate distinct types of pneumonia at various disease stages, studies that are not possible in patients. Different animal models of pneumonia such as one-hit acute pneumonia models, ventilator-associated pneumonia models and biofilm pneumonia models associated with cystic fibrosis have been extensively studied and have considerably aided our understanding of disease pathogenesis and testing and developing new treatment strategies. The present review aims to guide investigators in choosing appropriate animal pneumonia models by describing and comparing the relevant characteristics of each model using P. aeruginosa as a model etiology for hospital-acquired pneumonia. Key to establishing and studying these animal models of infection are well-defined end-points that allow precise monitoring and characterization of disease development that could ultimately aid in translating these findings to patient populations in order to guide therapy. In this respect, and discussed here, is the development of humanized animal models of bacterial pneumonia that could offer unique advantages to study bacterial virulence factor expression and host cytokine production for translational purposes.