Forward genetic dissection of innate response to infection in inbred mouse strains: selected success stories

Polygenic TB control and the sequence of innate/adaptive immune responses to infection: MHC-II alleles determine the size of the S100A8/9-producing neutrophil population

Among several quantitative trait loci involved in tuberculosis (TB) control in mice, one was mapped within the chromosome 17 segment occupied by the H2 complex and another within the chromosome 3 segment comprising the S100A8/9 genes, which encode neutrophil inflammatory factor S100A8/9. Previously, we developed a panel of H2-congenic mouse strains differing by small segments of the major histocompatibility complex Class II (MHC-II) region from TB-susceptible H2j mice transferred onto the genetic background of the TB-resistant C57BL/6 (H2b) strain. Susceptible B6.I-9.3 mice differ from B6 progenitors by the alleles of their only classical MHC-II H2-Aβ gene. The goals of the present study were to: (i) comprehensively characterise the differences in TB-related phenotypes between mice of the two strains and (ii) decipher interactions between the H2-Aβ and S100A8/9 genes. Here, we describe the dynamics of TB-related phenotypes differentiating B6.I-9.3 and B6 mice (colony forming units counts, histopathology, lung immune cell infiltration and cytokine profiles). We show that disproportionally diminished CD4+ T-cell population, an enlarged S100A8/9-positive neutrophil population and higher S100A8/9 serum levels in B6.I-9.3 mice collectively form the 'susceptible' phenotype before infection. An increase in IL-17 and a decrease in intrferon-gamma production by CD4+ T-cells in these mice provide a mechanistic explanation of this phenotype. Using F2 segregation analysis, we show that the number of S100A8/9-producing neutrophils in lungs and spleens and the proportion of Th17 CD4+ T-cells in lungs are significantly lower in the presence of the MHC-II dominant 'resistant' b allele compared to the recessive 'susceptible' j/j genotype. This provides direct genetic evidence that MHC-II-regulated CD4+ T-cell landscapes determine neutrophil abundance before infection, an important pathogenic factor in TB immunity.

Loss of Interleukin-6 Influences Transcriptional Immune Signatures and Alters Bacterial Colonization in the Skin

The skin functions as a protective barrier to inhibit the entry of foreign pathogens, all the while hosting a diverse milieu of microorganisms. Over time, skin cells, immune cells, cytokines, and microbes interact to integrate the processes of maintaining the skin's physical and immune barrier. In the present study, the basal expression of two immunologically divergent mouse strains C57BL/6 and BALB/c, as well as a strain on the C57 background lacking IL-6, was characterized. Additionally, cutaneous antimicrobial gene expression profiles and skin bacterial microbiome were assessed between strains. Total RNA sequencing was performed on untreated C57BL/6 (control), BALB/c, and IL-6-deficient skin samples and found over 3,400 genes differentially modulated between strains. It was found that each strain modulated its own transcriptional "profile" associated with skin homeostasis and also influenced the overall bacterial colonization as indicated by the differential phyla present on each strain. Together, these data not only provide a comprehensive view of the transcriptional changes in homeostatic skin of different mouse strains but also highlight the possible influence of the strain differences (e.g., Th1/Th2 balance) as well as a role for IL-6 in overall skin immunity and resident microbial populations.

Facing the challenges of multidrug-resistant Acinetobacter baumannii: progress and prospects in the vaccine development

In 2017, the World Health Organization (WHO) named A. baumannii as one of the three antibiotic-resistant bacterial species on its list of global priority pathogens in dire need of novel and effective treatment. With only polymyxin and tigecycline antibiotics left as last-resort treatments, the need for novel alternative approaches to the control of this bacterium becomes imperative. Vaccines against numerous bacteria have had impressive records in reducing the burden of the respective diseases and addressing antimicrobial resistance; as in the case of Haemophilus influenzae type b . A similar approach could be appropriate for A. baumannii. Toward this end, several potentially protective antigens against A. baumannii were identified and evaluated as vaccine antigen candidates. A licensed vaccine for the bacteria, however, is still not in sight. Here we explore and discuss challenges in vaccine development against A. baumannii and the promising approaches for improving the vaccine development process.

Susceptibility Analysis in Several Mouse Strains Reveals Robust T-Cell Responses After Mycoplasma pneumoniae Infection in DBA/2 Mice

Mycoplasma pneumoniae (Mp) is a highly contagious respiratory pathogen responsible for human community-acquired pneumonia. The number of antibiotic-resistant Mp strains is increasing; therefore, to develop novel therapeutics, it is crucial to precisely understand the pathogenesis of mycoplasma pneumonia. Herein, we examined the susceptibility and response to Mp among eight inbred mouse strains. Following infection, the bacterial load in the bronchoalveolar lavage fluid (BALF) from DBA/2 mice was higher than that in the other tested strains such as BALB/c mice, which are frequently used in Mp research. In contrast, the numbers of CD45⁺ immune cells and neutrophils in BALF were comparable between BALB/c and DBA/2 mice, with lower numbers observed in C57BL/6J and CBA/N mice than in BALB/c mice. Among the tested strains, the BALF level of interleukin 12 subunit p40 was highest in DBA/2 mice; however, significant differences in other cytokines levels were not observed between BALB/c and DBA/2 mice. After Mp infection, Mp-specific Th1 and Th17 responses were significantly enhanced in DBA/2 mice when compared with BALB/c mice. Furthermore, prior infection with Mp increased the number of neutrophils in BALF after the reinfection of DBA/2 mice through an Mp-specific CD4⁺ T cell-dependent mechanism. Thus, DBA/2 may be an appropriate strain for evaluating Mp infection. Moreover, a comparison of responses revealed by various inbred mouse strains could be useful for elucidating the pathogenesis of Mycoplasma pneumonia.

TLR3 Deficiency Leads to a Dysregulation in the Global Gene-Expression Profile in Murine Oviduct Epithelial Cells Infected with Chlamydia muridarum

Chlamydia trachomatis replicates primarily in the epithelial cells lining the genital tract and induces the innate immune response by triggering cellular pathogen recognition receptors (PRRs). Our previous studies showed that Toll-like receptor 3 (TLR3) is expressed in murine oviduct epithelial (OE) cells, is the primary PRR triggered by C. muridarum (Cm) early during infection to induce IFN-β synthesis, and that TLR3 signaling regulates the chlamydial induced synthesis of a plethora of other innate inflammatory modulators including IL-6, CXCL10, CXCL16 and CCL5. We also showed that the expression of these cytokines induced by Chlamydia was severely diminished during TLR3 deficiency; however, the replication of Chlamydiain TLR3 deficient OE cells was more robust than in WT cells. These data suggested that TLR3 had a biological impact on the inflammatory response to Chlamydia infection; however, the global effects of TLR3 signaling in the cellular response to Chlamydia infection in murine OE cells has not yet been investigated. To determine the impact of TLR3 signaling on Chlamydia infection in OE cell at the transcriptome level, we infected wild-type (OE-WT) and TLR3-deficient (OE-TLR3KO) cells with Cm, and performed transcriptome analyses using microarray. Genome-wide expression and ingenuity pathway analysis (IPA) identified enhanced expression of host genes encoding for components found in multiple cellular processes encompassing: (1) pro-inflammatory, (2) cell adhesion, (3) chemoattraction, (4) cellular matrix and small molecule transport, (5) apoptosis, and (6) antigen-processing and presentation. These results support a role for TLR3 in modulating the host cellular responses to Cm infection that extend beyond inflammation and fibrosis, and shows that TLR3 could serve a potential therapeutic target for drug and/or vaccine development.

The impact of host genetic background in the Pseudomonas aeruginosa respiratory infections

Understanding the significance of human genetic diversity in modulating host susceptibility to opportunistic infections is an emerging challenge in the field of respiratory illnesses. While it is recognized that diverse bacterial strains account for differential disease manifestations, emerging data indicate that host genetic diversity is an important determinant factor that influences the severity of opportunistic infections. With particular regard to respiratory illnesses mediated by the gram-negative bacterium Pseudomonas aeruginosa, diverse genetic background is also emerging as a key contributor. Human-genome-wide association studies are a common approach for determining the inter-individual genetic variation associated with variability of the pulmonary infections. Historically, diverse murine inbred mouse strains and ex-vivo cellular models were considered complementary to human studies for establishing the contribution of genetic background to P. aeruginosa respiratory infections. More recently, the development of a new mouse model of infection, mirroring human airway diseases, combined with innovative murine resource populations, modelling human genetic variation, provides additional insights into the mechanisms of genetic susceptibility. In this review, we cover the recent state of the art of human and animal studies and we discuss future potential challenges in the field of P. aeruginosa respiratory infections.

Autoimmune-Disease-Prone NOD Mice Help To Reveal a New Genetic Locus for Reducing Pulmonary Disease Caused by Mycoplasma pulmonis

Mycoplasmas are bacterial pathogens of a range of animals, including humans, and are a common cause of respiratory disease. However, the host genetic factors that affect resistance to infection or regulate the resulting pulmonary inflammation are not well defined. We and others have previously demonstrated that nonobese diabetic (NOD) mice can be used to investigate disease loci that affect bacterial infection and autoimmune diabetes. Here we show that NOD mice are more susceptible than C57BL/6 (B6) mice to infection with Mycoplasma pulmonis, a natural model of pulmonary mycoplasmosis. The lungs of infected NOD mice had higher loads of M. pulmonis and more severe inflammatory lesions. Moreover, congenic NOD mice that harbored different B6-derived chromosomal intervals enabled identification and localization of a new mycoplasmosis locus, termed Mpr2, on chromosome 13. These congenic NOD mice demonstrated that the B6 allele for Mpr2 reduced the severity of pulmonary inflammation caused by infection with M. pulmonis and that this was associated with altered cytokine and chemokine concentrations in the infected lungs. Mpr2 also colocalizes to the same genomic interval as Listr2 and Idd14, genetic loci linked to listeriosis resistance and autoimmune diabetes susceptibility, respectively, suggesting that allelic variation within these loci may affect the development of both infectious and autoimmune disease.

The host genetic background defines diverse immune-reactivity and susceptibility to chronic Pseudomonas aeruginosa respiratory infection

Patients with P. aeruginosa airways infection show markedly variable clinical phenotypes likely influenced by genetic backgrounds. Here, we investigated the cellular events involved in resistance and susceptibility to P. aeruginosa chronic infection using genetically distinct inbred mouse strains. As for patients, different murine genotypes revealed variable susceptibility to infection. When directly compared, resistant C3H/HeOuJ and susceptible A/J strains revealed distinct immune responsiveness to the pathogen. In C3H/HeOuJ resistant mice, IL17-producing cells rapidly and transiently infiltrated the infected lung, and this was paralleled by the acute accumulation of alveolar macrophages, bacterial clearance and resolution of infection. In contrast, A/J susceptible mice revealed a more delayed and prolonged lung infiltration by IL17⁺ and IFNγ⁺ cells, persistence of innate inflammatory cells and establishment of chronic infection. We conclude that the host genetic background confers diverse immunoreactivity to P. aeruginosa and IL17-producing cells might contribute to the progress of chronic lung infection.

Mapping genetic determinants of host susceptibility to Pseudomonas aeruginosa lung infection in mice

Background

P. aeruginosa is one of the top three causes of opportunistic human bacterial infections. The remarkable variability in the clinical outcomes of this infection is thought to be associated with genetic predisposition. However, the genes underlying host susceptibility to P. aeruginosa infection are still largely unknown.

Results

As a step towards mapping these genes, we applied a genome wide linkage analysis approach to a mouse model. A large F2 intercross population, obtained by mating P. aeruginosa-resistant C3H/HeOuJ, and susceptible A/J mice, was used for quantitative trait locus (QTL) mapping. The F2 progenies were challenged with a P. aeruginosa clinical strain and monitored for the survival time up to 7 days post-infection, as a disease phenotype associated trait. Selected phenotypic extremes of the F2 distribution were genotyped with high-density single nucleotide polymorphic (SNP) markers, and subsequently QTL analysis was performed. A significant locus was mapped on chromosome 6 and was named P. aeruginosa infection resistance locus 1 (Pairl1). The most promising candidate genes, including Dok1, Tacr1, Cd207, Clec4f, Gp9, Gata2, Foxp1, are related to pathogen sensing, neutrophils and macrophages recruitment and inflammatory processes.

Conclusions

We propose a set of genes involved in the pathogenesis of P. aeruginosa infection that may be explored to complement human studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2676-4) contains supplementary material, which is available to authorized users.

Host genetic background influences the response to the opportunistic Pseudomonas aeruginosa infection altering cell-mediated immunity and bacterial replication

Pseudomonas aeruginosa is a common cause of healthcare-associated infections including pneumonia, bloodstream, urinary tract, and surgical site infections. The clinical outcome of P. aeruginosa infections may be extremely variable among individuals at risk and patients affected by cystic fibrosis. However, risk factors for P. aeruginosa infection remain largely unknown. To identify and track the host factors influencing P. aeruginosa lung infections, inbred immunocompetent mouse strains were screened in a pneumonia model system. A/J, BALB/cJ, BALB/cAnNCrl, BALB/cByJ, C3H/HeOuJ, C57BL/6J, C57BL/6NCrl, DBA/2J, and 129S2/SvPasCRL mice were infected with P. aeruginosa clinical strain and monitored for body weight and mortality up to seven days. The most deviant survival phenotypes were observed for A/J, 129S2/SvPasCRL and DBA/2J showing high susceptibility while BALB/cAnNCrl and C3H/HeOuJ showing more resistance to P. aeruginosa infection. Next, one of the most susceptible and resistant mouse strains were characterized for their deviant clinical and immunological phenotype by scoring bacterial count, cell-mediated immunity, cytokines and chemokines profile and lung pathology in an early time course. Susceptible A/J mice showed significantly higher bacterial burden, higher cytokines and chemokines levels but lower leukocyte recruitment, particularly neutrophils, when compared to C3H/HeOuJ resistant mice. Pathologic scores showed lower inflammatory severity, reduced intraluminal and interstitial inflammation extent, bronchial and parenchymal involvement and diminished alveolar damage in the lungs of A/J when compared to C3H/HeOuJ. Our findings indicate that during an early phase of infection a prompt inflammatory response in the airways set the conditions for a non-permissive environment to P. aeruginosa replication and lock the spread to other organs. Host gene(s) may have a role in the reduction of cell-mediated immunity playing a critical role in the control of P. aeruginosa infection. These results now provide a basis for mapping genomic regions underlying host susceptibility to P. aeruginosa infection.

A highly conserved Toxo1 haplotype directs resistance to toxoplasmosis and its associated caspase-1 dependent killing of parasite and host macrophage

Natural immunity or resistance to pathogens most often relies on the genetic make-up of the host. In a LEW rat model of refractoriness to toxoplasmosis, we previously identified on chromosome 10 the Toxo1 locus that directs toxoplasmosis outcome and controls parasite spreading by a macrophage-dependent mechanism. Now, we narrowed down Toxo1 to a 891 kb interval containing 29 genes syntenic to human 17p13 region. Strikingly, Toxo1 is included in a haplotype block strictly conserved among all refractory rat strains. The sequencing of Toxo1 in nine rat strains (5 refractory and 4 susceptible) revealed resistant-restricted conserved polymorphisms displaying a distribution gradient that peaks at the bottom border of Toxo1, and highlighting the NOD-like receptor, Nlrp1a, as a major candidate. The Nlrp1 inflammasome is known to trigger, upon pathogen intracellular sensing, pyroptosis programmed-cell death involving caspase-1 activation and cleavage of IL-1β. Functional studies demonstrated that the Toxo1-dependent refractoriness in vivo correlated with both the ability of macrophages to restrict T. gondii growth and a T. gondii-induced death of intracellular parasites and its host macrophages. The parasite-induced cell death of infected macrophages bearing the LEW-Toxo1 alleles was found to exhibit pyroptosis-like features with ROS production, the activation of caspase-1 and IL1-β secretion. The pharmacological inactivation of caspase-1 using YVAD and Z-VAD inhibitors prevented the death of both intravacuolar parasites and host non-permissive macrophages but failed to restore parasite proliferation. These findings demonstrated that the Toxo1-dependent response of rat macrophages to T. gondii infection may trigger two pathways leading to the control of parasite proliferation and the death of parasites and host macrophages. The NOD-like receptor NLRP1a/Caspase-1 pathway is the best candidate to mediate the parasite-induced cell death. These data represent new insights towards the identification of a major pathway of innate resistance to toxoplasmosis and the prediction of individual resistance.

Toxoplasmosis is a ubiquitous parasitic infection causing a wide spectrum of diseases. It is usually asymptomatic but can lead to severe ocular and neurological disorders. The host factors that determine natural resistance to toxoplasmosis are yet poorly characterized. Among the animal models to study susceptibility to toxoplasmosis, rats develop like humans a subclinical chronic infection. The finding of a total resistance in the LEW rat strain has allowed genetic studies leading to the identification of Toxo1, a unique locus that controls the outcome of toxoplasmosis. In this report, a panel of recombinant inbred rat strains was used to genetically reduce the Toxo1 locus, on chromosome 10, to a limited region containing 29 genes. This locus is highly conserved among five resistant, by comparison to four susceptible, rat strains, indicating that refractoriness to toxoplasmosis could be predicted. The Toxo1-controlled refractoriness depends on the ability of macrophages to restrict parasite proliferation and the rapid death of both T. gondii and host macrophages in vitro. The NOD-like receptor NLRP1a/Caspase-1 pathway is the best candidate to mediate the parasite-induced cell death. Our data represent new insights towards the identification of a major pathway of innate immunity that protects from toxoplasmosis.

R-spondin 2 signalling mediates susceptibility to fatal infectious diarrhoea

Citrobacter rodentium is a natural mouse pathogen widely used as a model for enteropathogenic and enterohemorrhagic Escherichia coli infections in humans. While C. rodentium causes self-limiting colitis in most inbred mouse strains, it induces fatal diarrhea in susceptible strains. The physiological pathways as well as the genetic determinants leading to susceptibility have remained largely uncharacterized. Here we use a forward genetic approach to identify the R-spondin2 gene (Rspo2) as a major determinant of susceptibility to C. rodentium infection. Robust induction of Rspo2 expression during infection in susceptible mouse strains causes a potent Wnt-mediated proliferative response of colonic crypt cells, leading to the generation of an immature and poorly differentiated colonic epithelium with deficiencies in ion-transport components. Our data demonstrate a previously unknown role of R spondins and Wnt signaling in susceptibility to infectious diarrhea and identify Rspo2 as a key molecular link between infection and intestinal homeostasis.

Fecal transplantation does not transfer either susceptibility or resistance to food borne listeriosis in C57BL/6 and BALB/c/By mice

The composition of the intestinal microbiota has wide reaching effects on the health of an individual, including the development of protective innate immune responses.  In this report, a fecal transplantation approach was used to determine whether resistance to food borne listeriosis was dependent on the murine gut microbiota.  Transplantation of BALB/c/By feces did not increase the susceptibility of C57BL/6 mice to Listeria monocytogenes infection.   Likewise, transplantation of C57BL/6 fecal matter did not enhance the resistance of BALB/c/By mice.  Thus, intestinal microbiota composition is not a key factor that confers either susceptibility or resistance to food borne listeriosis in mice.

Host genetic background impacts modulation of the TLR4 pathway by RON in tissue-associated macrophages

Toll-like receptors (TLRs) enable metazoans to mount effective innate immune responses to microbial and viral pathogens, as well as to endogenous host-derived ligands. It is understood that genetic background of the host can influence TLR responsiveness, altering susceptibility to pathogen infection, autoimmunity and cancer. Macrophage stimulatory protein (MSP), which activates the receptor tyrosine kinase recepteur d'origine nantais (RON), promotes key macrophage functions such as motility and phagocytic activity. MSP also acts via RON to modulate signaling by TLR4, which recognizes a range of pathogen or endogenous host-derived molecules. Here, we show that RON exerts divergent control over TLR4 activity in macrophages from different mouse genetic backgrounds. RON potently modulated the TLR4 response in macrophages from M2-prone FVB mice, as compared with M1-skewed C57Bl6 mice. Moreover, global expression analysis revealed that RON suppresses the TLR4-dependent type-I interferon gene signature only in FVB macrophages. This leads to attenuated production of the potent inflammatory mediator, tumor necrosis factor-α. Eliminating RON kinase activity markedly decreased carcinogen-mediated tumorigenesis in M2/Th2-biased FVB mice. We propose that host genetic background influences RON function, thereby contributing to the variability in TLR4 responsiveness in rodents and, potentially, in humans. These findings provide novel insight into the complex interplay between genetic context and immune function.

Heterogeneity of respiratory dendritic cell subsets and lymphocyte populations in inbred mouse strains

BACKGROUND

Inbred mouse strains are used in different models of respiratory diseases but the variation of critical respiratory leukocyte subpopulations across different strains is unknown.

METHODS

By using multiparameter flow cytometry we have quantitated respiratory leukocyte subsets including dendritic cells subpopulations, macrophages, classical T and B cells, natural killer cells, γδTCR+ T cells and lineage-negative leukocytes in the five most common inbred mouse strains BALB/c, C57BL/6, DBA/2, 129SV and C3H. To minimize confounding environmental factors, age-matched animals were received from the same provider and were housed under identical specific-pathogen-free conditions.

RESULTS

Results revealed significant strain differences with respect to respiratory neutrophils (p=0.005; up to 1.4 fold differences versus C57BL/6 mice), eosinophils (p=0.029; up to 2.7 fold), certain dendritic cell subsets (p≤0.0003; up to 3.4 fold), T (p<0.001; up to 1.6 fold) and B lymphocyte subsets (p=0.005; up to 0.4 fold), γδ T lymphocytes (p=0.003; up to 1.6 fold), natural killer cells (p<0.0001; up to 0.6 fold) and lineage-negative innate leukocytes (p≤0.007; up to 3.6 fold). In contrast, total respiratory leukocytes, macrophages, total dendritic cells and bronchoalveolar lavage leukocytes did not differ significantly. Stimulation of respiratory leukocytes via Toll-like receptor 4 and 9 as well as CD3/CD28 revealed significant strain differences of TNF-α and IL-10 production.

CONCLUSION

Our study demonstrates significant strain heterogeneity of respiratory leukocyte subsets that may impact respiratory immunity in different disease models. Additionally, the results may help identification of optimal strains for purification of rare respiratory leukocyte subsets for ex vivo analyses.

Living with the enemy or uninvited guests: functional genomics approaches to investigating host resistance or tolerance traits to a protozoan parasite, Theileria annulata, in cattle

Many breeds of cattle with long histories of living in areas of endemic disease have evolved mechanisms that enable them to co-exist with specific pathogens. Understanding the genes that control tolerance and resistance could provide new strategies to improve the health and welfare of livestock. Around one sixth of the world cattle population is estimated to be at risk from one of the most debilitating tick-borne diseases of cattle, caused by the protozoan parasite, Theileria annulata. The parasite mainly infects cells of the myeloid lineage which are also the main producers of inflammatory cytokines. If an infectious or inflammatory insult is sufficiently great, inflammatory cytokines produced by macrophages enter the circulation and induce an acute phase proteins (APP) response. The Bos taurus Holstein breed produces higher and more prolonged levels of inflammatory cytokine induced APP than the Bos indicus Sahiwal breed in response to experimental infection with T. annulata. The Sahiwal exhibits significantly less pathology and survives infection, unlike the Holstein breed. Therefore, we hypothesised that the causal genes were likely to be expressed in macrophages and control the production of inflammatory cytokines. A functional genomics approach revealed that the transcriptome profile of the B. taurus macrophages was more associated with an inflammatory programme than the B. indicus macrophages. In particular the most differentially expressed gene was a member of the signal regulatory protein (SIRP) family. These are mainly expressed on myeloid cell surfaces and control inflammatory responses. Other differentially expressed genes included bovine major histocompatibility complex (MHC) (BoLA) class II genes, particularly BoLA DQ, and transforming growth factor (TGF)B2. We are now exploring whether sequence and functional differences in the bovine SIRP family may underlie the resistance or tolerance to T. annulata between the breeds. Potentially, our research may also have more general implications for the control of inflammatory processes against other pathogens. Genes controlling the balance between pathology and protection may determine how livestock can survive in the face of infectious onslaught. Next generation sequencing and RNAi methodologies for livestock species will bring new opportunities to link diversity at the genome level to functional differences in health traits in livestock species.