Allelic variation on murine chromosome 11 modifies host inflammatory responses and resistance to Bacillus anthracis

Pyroptosis and the fight against lung cancer

Pyroptosis, a newly characterized type of inflammatory programmed cell death (PCD), is usually triggered by multiple inflammasomes which can recognize different danger or damage-associated molecular patterns (DAMPs), leading to the activation of caspase-1 and the cleavage of gasdermin D (GSDMD). Gasdermin family pore-forming proteins are the executers of pyroptosis and are normally maintained in an inactive state through auto-inhibition. Upon caspases mediated cleavage of gasdermins, the pro-pyroptotic N-terminal fragment is released from the auto-inhibition of C-terminal fragment and oligomerizes, forming pores in the plasma membrane. This results in the secretion of interleukin (IL)-1β, IL-18, and high-mobility group box 1 (HMGB1), generating osmotic swelling and lysis. Current therapeutic approaches including chemotherapy, radiotherapy, molecularly targeted therapy and immunotherapy for lung cancer treatment efficiently force the cancer cells to undergo pyroptosis, which then generates local and systemic antitumor immunity. Thus, pyroptosis is recognized as a new therapeutic regimen for the treatment of lung cancer. In this review, we briefly describe the signaling pathways involved in pyroptosis, and endeavor to discuss the antitumor effects of pyroptosis and its potential application in lung cancer therapy, focusing on the contribution of pyroptosis to microenvironmental reprogramming and evocation of antitumor immune response.

Multi-omics data elucidate parasite-host-microbiota interactions and resistance to Haemonchus contortus in sheep

BACKGROUND

The integration of molecular data from hosts, parasites, and microbiota can enhance our understanding of the complex biological interactions underlying the resistance of hosts to parasites. Haemonchus contortus, the predominant sheep gastrointestinal parasite species in the tropics, causes significant production and economic losses, which are further compounded by the diminishing efficiency of chemical control owing to anthelmintic resistance. Knowledge of how the host responds to infection and how the parasite, in combination with microbiota, modulates host immunity can guide selection decisions to breed animals with improved parasite resistance. This understanding will help refine management practices and advance the development of new therapeutics for long-term helminth control.

METHODS

Eggs per gram (EPG) of feces were obtained from Morada Nova sheep subjected to two artificial infections with H. contortus and used as a proxy to select animals with high resistance or susceptibility for transcriptome sequencing (RNA-seq) of the abomasum and 50 K single-nucleotide genotyping. Additionally, RNA-seq data for H. contortus were generated, and amplicon sequence variants (ASV) were obtained using polymerase chain reaction amplification and sequencing of bacterial and archaeal 16S ribosomal RNA genes from sheep feces and rumen content.

RESULTS

The heritability estimate for EPG was 0.12. GAST, GNLY, IL13, MGRN1, FGF14, and RORC genes and transcripts were differentially expressed between resistant and susceptible animals. A genome-wide association study identified regions on chromosomes 2 and 11 that harbor candidate genes for resistance, immune response, body weight, and adaptation. Trans-expression quantitative trait loci were found between significant variants and differentially expressed transcripts. Functional co-expression modules based on sheep genes and ASVs correlated with resistance to H. contortus, showing enrichment in pathways of response to bacteria, immune and inflammatory responses, and hub features of the Christensenellaceae, Bacteroides, and Methanobrevibacter genera; Prevotellaceae family; and Verrucomicrobiota phylum. In H. contortus, some mitochondrial, collagen-, and cuticle-related genes were expressed only in parasites isolated from susceptible sheep.

CONCLUSIONS

The present study identified chromosome regions, genes, transcripts, and pathways involved in the elaborate interactions between the sheep host, its gastrointestinal microbiota, and the H. contortus parasite. These findings will assist in the development of animal selection strategies for parasite resistance and interdisciplinary approaches to control H. contortus infection in sheep.

Soluble P-selectin rescues mice from anthrax lethal toxin-induced mortality through PSGL-1 pathway-mediated correction of hemostasis

As one of the virulence factors of Bacillus anthracis, lethal toxin (LT) induces various pathogenic responses including the suppression of the coagulation system. In this study, we observed that LT markedly increased the circulating soluble P-selectin (sP-sel) levels and microparticle (MP) count in wild-type but not P-selectin (P-sel, Selp^-/-) or P-sel ligand-1 (PSGL-1, Selplg^-/-) knockout mice. Because sP-sel induces a hypercoagulable state through PSGL-1 pathway to generate tissue factor-positive MPs, we hypothesized that the increase in plasma sP-sel levels can be a self-rescue response in hosts against the LT-mediated suppression of the coagulation system. In agreement with our hypothesis, our results indicated that compared with wild-type mice, Selp^-/- and Selplg^-/- mice were more sensitive to LT. In addition, the recombinant sP-sel treatment markedly ameliorated LT-mediated pathogenesis and reduced mortality. As a result, elicitation of circulating sP-sel is potentially a self-rescue response, which is beneficial to host recovery from an LT-induced hypocoagulation state. These results suggest that the administration of sP-sel is likely to be useful in the development of a new strategy to treat anthrax.

Animal Models for the Pathogenesis, Treatment, and Prevention of Infection by Bacillus anthracis

This article reviews the characteristics of the major animal models utilized for studies on Bacillus anthracis and highlights their contributions to understanding the pathogenesis and host responses to anthrax and its treatment and prevention. Advantages and drawbacks associated with each model, to include the major models (murine, guinea pig, rabbit, nonhuman primate, and rat), and other less frequently utilized models, are discussed. Although the three principal forms of anthrax are addressed, the main focus of this review is on models for inhalational anthrax. The selection of an animal model for study is often not straightforward and is dependent on the specific aims of the research or test. No single animal species provides complete equivalence to humans; however, each species, when used appropriately, can contribute to a more complete understanding of anthrax and its etiologic agent.

The NLRP1 inflammasomes

Inflammasomes are cytosolic protein complexes that serve as platforms for the recruitment and activation of the pro-inflammatory CASPASE-1 protease. CASPASE-1 activation leads to processing and maturation of the cytokines interleukin-1β and interleukin-18 and a lytic form of cell death termed pyroptosis. Inflammasome assembly is initiated by cytosolic sensors in response to microbial infections. Many of these sensors, including NLRP1 (NLR family, pyrin domain containing 1), are described to form an inflammasome, but until recently, the mechanism of inflammasome activation and its physiological functions in host defense have remained unclear. In the last few years, important advances in our understanding of NLRP1 biology have been achieved. In this review, we discuss the activation of NLRP1 by various stimuli, including Bacillus anthracis lethal toxin, Toxoplasma gondii, muramyl dipeptide, and host intracellular ATP depletion. The role NLRP1 plays in pathogen recognition and resistance during infection is also discussed, as is the regulation of NLRP1 by host and viral proteins. We conclude by discussing the unexpected differences in the mechanism of NLRP1 inflammasome activation, as compared to the activation of other inflammasomes, such as the NAIP (NLR family, apoptosis inhibitory protein)/NLRC4 inflammasomes.

Anthrax lethal factor as an immune target in humans and transgenic mice and the impact of HLA polymorphism on CD4+ T cell immunity

Bacillus anthracis produces a binary toxin composed of protective antigen (PA) and one of two subunits, lethal factor (LF) or edema factor (EF). Most studies have concentrated on induction of toxin-specific antibodies as the correlate of protective immunity, in contrast to which understanding of cellular immunity to these toxins and its impact on infection is limited. We characterized CD4⁺ T cell immunity to LF in a panel of humanized HLA-DR and DQ transgenic mice and in naturally exposed patients. As the variation in antigen presentation governed by HLA polymorphism has a major impact on protective immunity to specific epitopes, we examined relative binding affinities of LF peptides to purified HLA class II molecules, identifying those regions likely to be of broad applicability to human immune studies through their ability to bind multiple alleles. Transgenics differing only in their expression of human HLA class II alleles showed a marked hierarchy of immunity to LF. Immunogenicity in HLA transgenics was primarily restricted to epitopes from domains II and IV of LF and promiscuous, dominant epitopes, common to all HLA types, were identified in domain II. The relevance of this model was further demonstrated by the fact that a number of the immunodominant epitopes identified in mice were recognized by T cells from humans previously infected with cutaneous anthrax and from vaccinated individuals. The ability of the identified epitopes to confer protective immunity was demonstrated by lethal anthrax challenge of HLA transgenic mice immunized with a peptide subunit vaccine comprising the immunodominant epitopes that we identified.

Anthrax is of concern with respect to human exposure in endemic regions, concerns about bioterrorism and the considerable global burden of livestock infections. The immunology of this disease remains poorly understood. Vaccination has been based on B. anthracis filtrates or attenuated spore-based vaccines, with more recent trials of next-generation recombinant vaccines. Approaches generally require extensive vaccination regimens and there have been concerns about immunogenicity and adverse reactions. An ongoing need remains for rationally designed, effective and safe anthrax vaccines. The importance of T cell stimulating vaccines is inceasingly recognized. An essential step is an understanding of immunodominant epitopes and their relevance across the diverse HLA immune response genes of human populations. We characterized CD4 T cell immunity to anthrax Lethal Factor (LF), using HLA transgenic mice, as well as testing candidate peptide epitopes for binding to a wide range of HLA alleles. We identified anthrax epitopes, noteworthy in that they elicit exceptionally strong immunity with promiscuous binding across multiple HLA alleles and isotypes. T cell responses in humans exposed to LF through either natural anthrax infection or vaccination were also examined. Epitopes identified as candidates were used to protect HLA transgenic mice from anthrax challenge.

Inflammasome sensor NLRP1 controls rat macrophage susceptibility to Toxoplasma gondii

Toxoplasma gondii is an intracellular parasite that infects a wide range of warm-blooded species. Rats vary in their susceptibility to this parasite. The Toxo1 locus conferring Toxoplasma resistance in rats was previously mapped to a region of chromosome 10 containing Nlrp1. This gene encodes an inflammasome sensor controlling macrophage sensitivity to anthrax lethal toxin (LT) induced rapid cell death (pyroptosis). We show here that rat strain differences in Toxoplasma infected macrophage sensitivity to pyroptosis, IL-1β/IL-18 processing, and inhibition of parasite proliferation are perfectly correlated with NLRP1 sequence, while inversely correlated with sensitivity to anthrax LT-induced cell death. Using recombinant inbred rats, SNP analyses and whole transcriptome gene expression studies, we narrowed the candidate genes for control of Toxoplasma-mediated rat macrophage pyroptosis to four genes, one of which was Nlrp1. Knockdown of Nlrp1 in pyroptosis-sensitive macrophages resulted in higher parasite replication and protection from cell death. Reciprocally, overexpression of the NLRP1 variant from Toxoplasma-sensitive macrophages in pyroptosis-resistant cells led to sensitization of these resistant macrophages. Our findings reveal Toxoplasma as a novel activator of the NLRP1 inflammasome in rat macrophages.

Inflammasomes are multiprotein complexes that are a major component of the innate immune system. They contain “sensor” proteins that are responsible for detecting various microbial and environmental danger signals and function by activating caspase-1, an enzyme that mediates cleavage and release of the pro-inflammatory cytokines, IL-1β and IL-18. Toxoplasma gondii is a highly successful protozoan parasite capable of infecting a wide range of host species that have variable levels of resistance. Rat strains have been previously shown to vary in their susceptibility to this parasite. We report here that rat macrophages from different inbred strains also vary in sensitivity to Toxoplasma induced lysis. We find that NLRP1, an inflammasome sensor whose only known agonist is anthrax LT, is also activated by Toxoplasma infection. In rats there is a perfect correlation between NLRP1 sequence and macrophage sensitivity to Toxoplasma-induced rapid cell death, inhibition of parasite proliferation, and IL-1β/IL-18 processing. Nlrp1 genes from sensitive rat macrophages can confer sensitivity to this rapid cell death when expressed in Toxoplasma resistant rat macrophages. Our findings suggest Toxoplasma is a new activator of the NLRP1 inflammasome.

Differential effects of β-catenin and NF-κB interplay in the regulation of cell proliferation, inflammation and tumorigenesis in response to bacterial infection

Both β-catenin and NF-κB have been implicated in our laboratory as candidate factors in driving proliferation in an in vivo model of Citrobacter rodentium (CR)-induced colonic crypt hyper-proliferation and hyperplasia. Herein, we test the hypothesis that β-catenin and not necessarily NF-κB regulates colonic crypt hyperplasia or tumorigenesis in response to CR infection. When C57Bl/6 wild type (WT) mice were infected with CR, sequential increases in proliferation at days 9 and 12 plateaued off at day 19 and paralleled increases in NF-κB signaling. In Tlr4^−/− (KO) mice, a sequential but sustained proliferation which tapered off only marginally at day 19, was associated with TLR4-dependent and independent increases in NF-κB signaling. Similarly, increases in either activated or total β-catenin in the colonic crypts of WT mice as early as day 3 post-infection coincided with cyclinD1 and c-myc expression and associated crypt hyperplasia. In KO mice, a delayed kinetics associated predominantly with increases in non-phosphorylated (active) β-catenin coincided with increases in cyclinD1, c-myc and crypt hyperplasia. Interestingly, PKCζ-catalyzed Ser-9 phosphorylation and inactivation of GSK-3β and not loss of wild type APC protein accounted for β-catenin accumulation and nuclear translocation in either strain. In vitro studies with Wnt2b and Wnt5a further validated the interplay between the Wnt/β-catenin and NF-κB pathways, respectively. When WT or KO mice were treated with nanoparticle-encapsulated siRNA to β-catenin (si- β-Cat), almost complete loss of nuclear β-catenin coincided with concomitant decreases in CD44 and crypt hyperplasia without defects in NF-κB signaling. si-β-Cat treatment to Apc^Min/+ mice attenuated CR-induced increases in β-catenin and CD44 that halted the growth of mutated crypts without affecting NF-κB signaling. The predominant β-catenin-induced crypt proliferation was further validated in a Castaneus strain (B6.CAST.11M) that exhibited significant crypt hyperplasia despite an attenuated NF-κB signaling. Thus, β-catenin and not necessarily NF-κB regulates crypt hyperplasia in response to bacterial infection.

The systems genetics resource: a web application to mine global data for complex disease traits

The Systems Genetics Resource (SGR) (http://systems.genetics.ucla.edu) is a new open-access web application and database that contains genotypes and clinical and intermediate phenotypes from both human and mouse studies. The mouse data include studies using crosses between specific inbred strains and studies using the Hybrid Mouse Diversity Panel. SGR is designed to assist researchers studying genes and pathways contributing to complex disease traits, including obesity, diabetes, atherosclerosis, heart failure, osteoporosis, and lipoprotein metabolism. Over the next few years, we hope to add data relevant to deafness, addiction, hepatic steatosis, toxin responses, and vascular injury. The intermediate phenotypes include expression array data for a variety of tissues and cultured cells, metabolite levels, and protein levels. Pre-computed tables of genetic loci controlling intermediate and clinical phenotypes, as well as phenotype correlations, are accessed via a user-friendly web interface. The web site includes detailed protocols for all of the studies. Data from published studies are freely available; unpublished studies have restricted access during their embargo period.

Transcriptional analysis of the three Nlrp1 paralogs in mice

Background

Signals of danger and damage in the cytosol of cells are sensed by NOD-like receptors (NLRs), which are components of multiprotein complexes called inflammasomes. Inflammasomes activate caspase-1, resulting in IL-1-beta and IL-18 secretion and an inflammatory response. To date, the only known activator of rodent Nlrp1 is anthrax lethal toxin (LT), a protease secreted by the bacterial pathogen Bacillus anthracis. Although susceptibility of mouse macrophages to LT has been genetically linked to Nlrp1b, mice harbor two additional Nlrp1 paralogs in their genomes (Nlrp1a and Nlrp1c). However, little is known about their expression profile and sequence in different mouse strains. Furthermore, simultaneous expression of these paralogs may lead to competitional binding of Nlrp1b interaction partners needed for inflammasome activation, thus influencing macrophages susceptibility to LT. To more completely understand the role(s) of Nlrp1 paralogs in mice, we surveyed for their expression in a large set of LT-resistant and sensitive mouse macrophages. In addition, we provide sequence comparisons for Nlrp1a and report on previously unrecognized splice variants of Nlrp1b.

Results

Our results show that macrophages from some inbred mouse strains simultaneously express different splice variants of Nlrp1b. In contrast to the highly polymorphic Nlrp1b splice variants, sequencing of expressed Nlrp1a showed the protein to be highly conserved across all mouse strains. We found that Nlrp1a was expressed only in toxin-resistant macrophages, with the sole exception of expression in LT-sensitive CAST/EiJ macrophages.

Conclusions

Our data present a complex picture of Nlrp1 protein variations and provide a basis for elucidating their roles in murine macrophage function. Furthermore, the high conservation of Nlrp1a implies that it might be an important inflammasome sensor in mice.

Anthrax lethal toxin and the induction of CD4 T cell immunity

Bacillus anthracis secretes exotoxins which act through several mechanisms including those that can subvert adaptive immunity with respect both to antigen presenting cell and T cell function. The combination of Protective Antigen (PA) and Lethal Factor (LF) forming Lethal Toxin (LT), acts within host cells to down-regulate the mitogen activated protein kinase (MAPK) signaling cascade. Until recently the MAPK kinases were the only known substrate for LT; over the past few years it has become evident that LT also cleaves Nlrp1, leading to inflammasome activation and macrophage death. The predicted downstream consequences of subverting these important cellular pathways are impaired antigen presentation and adaptive immunity. In contrast to this, recent work has indicated that robust memory T cell responses to B. anthracis antigens can be identified following natural anthrax infection. We discuss how LT affects the adaptive immune response and specifically the identification of B. anthracis epitopes that are both immunogenic and protective with the potential for inclusion in protein sub-unit based vaccines.

Rapid vascular responses to anthrax lethal toxin in mice containing a segment of chromosome 11 from the CAST/Ei strain on a C57BL/6 genetic background

Host allelic variation controls the response to B. anthracis and the disease course of anthrax. Mouse strains with macrophages that are responsive to anthrax lethal toxin (LT) show resistance to infection while mouse strains with LT non-responsive macrophages succumb more readily. B6.CAST.11M mice have a region of chromosome 11 from the CAST/Ei strain (a LT responsive strain) introgressed onto a LT non-responsive C57BL/6J genetic background. Previously, B6.CAST.11M mice were found to exhibit a rapid inflammatory reaction to LT termed the early response phenotype (ERP), and displayed greater resistance to B. anthracis infection compared to C57BL/6J mice. Several ERP features (e.g., bloat, hypothermia, labored breathing, dilated pinnae vessels) suggested vascular involvement. To test this, Evan’s blue was used to assess vessel leakage and intravital microscopy was used to monitor microvascular blood flow. Increased vascular leakage was observed in lungs of B6.CAST.11M mice compared to C57BL/6J mice 1 hour after systemic administration of LT. Capillary blood flow was reduced in the small intestine mesentery without concomitant leukocyte emigration following systemic or topical application of LT, the latter suggesting a localized tissue mechanism in this response. Since LT activates the Nlrp1b inflammasome in B6.CAST.11M mice, the roles of inflammasome products, IL-1β and IL-18, were examined. Topical application to the mesentery of IL-1β but not IL-18 revealed pronounced slowing of blood flow in B6.CAST.11M mice that was not present in C57BL/6J mice. A neutralizing anti-IL-1β antibody suppressed the slowing of blood flow induced by LT, indicating a role for IL-1β in the response. Besides allelic differences controlling Nlrp1b inflammasome activation by LT observed previously, evidence presented here suggests that an additional genetic determinant(s) could regulate the vascular response to IL-1β. These results demonstrate that vessel leakage and alterations to blood flow are part of the rapid response in mice resistant to B. anthracis infection.

Bacillus anthracis factors for phagosomal escape

The mechanism of phagosome escape by intracellular pathogens is an important step in the infectious cycle. During the establishment of anthrax, Bacillus anthracis undergoes a transient intracellular phase in which spores are engulfed by local phagocytes. Spores germinate inside phagosomes and grow to vegetative bacilli, which emerge from their resident intracellular compartments, replicate and eventually exit from the plasma membrane. During germination, B. anthracis secretes multiple factors that can help its resistance to the phagocytes. Here the possible role of B. anthracis toxins, phospholipases, antioxidant enzymes and capsules in the phagosomal escape and survival, is analyzed and compared with that of factors of other microbial pathogens involved in the same type of process.