Genetic basis of murine antibacterial defense to streptococcal lung infection

Development of a Novel Live Attenuated Influenza A Virus Vaccine Encoding the IgA-Inducing Protein

Live attenuated influenza virus (LAIV) vaccines elicit a combination of systemic and mucosal immunity by mimicking a natural infection. To further enhance protective mucosal responses, we incorporated the gene encoding the IgA-inducing protein (IGIP) into the LAIV genomes of the cold-adapted A/Leningrad/134/17/57 (H2N2) strain (caLen) and the experimental attenuated backbone A/turkey/Ohio/313053/04 (H3N2) (OH/04att). Incorporation of IGIP into the caLen background led to a virus that grew poorly in prototypical substrates. In contrast, IGIP in the OH/04att background (IGIP-H1att) virus grew to titers comparable to the isogenic backbone H1att (H1N1) without IGIP. IGIP-H1att- and H1caLen-vaccinated mice were protected against lethal challenge with a homologous virus. The IGIP-H1att vaccine generated robust serum HAI responses in naïve mice against the homologous virus, equal or better than those obtained with the H1caLen vaccine. Analyses of IgG and IgA responses using a protein microarray revealed qualitative differences in humoral and mucosal responses between vaccine groups. Overall, serum and bronchoalveolar lavage samples from the IGIP-H1att group showed trends towards increased stimulation of IgG and IgA responses compared to H1caLen samples. In summary, the introduction of genes encoding immunomodulatory functions into a candidate LAIV can serve as natural adjuvants to improve overall vaccine safety and efficacy.

Cathelicidin preserves intestinal barrier function in polymicrobial sepsis

Objectives

The intestinal epithelium compartmentalizes the sterile bloodstream and the commensal bacteria in the gut. Accumulating evidence suggests that this barrier is impaired in sepsis, aggravating systemic inflammation. Previous studies reported that cathelicidin is differentially expressed in various tissues in sepsis. However, its role in sepsis-induced intestinal barrier dysfunction has not been investigated.

Design

To examine the role of cathelicidin in polymicrobial sepsis, cathelicidin wild-(Cnlp^+/+) and knockout (Cnlp^−/−) mice underwent cecal-ligation and puncture (CLP) followed by the assessment of septic mortality and morbidity as well as histological, biochemical, immunological, and transcriptomic analyses in the ileal tissues. We also evaluated the prophylactic and therapeutic efficacies of vitamin D3 (an inducer of endogenous cathelicidin) in the CLP-induced murine polymicrobial sepsis model.

Results

The ileal expression of cathelicidin was increased by three-fold after CLP, peaking at 4 h. Knockout of Cnlp significantly increased 7-day mortality and was associated with a higher murine sepsis score. Alcian-blue staining revealed a reduced number of mucin-positive goblet cells, accompanied by reduced mucin expression. Increased number of apoptotic cells and cleavage of caspase-3 were observed. Cnlp deletion increased intestinal permeability to 4kD fluorescein-labeled dextran and reduced the expression of tight junction proteins claudin-1 and occludin. Notably, circulating bacterial DNA load increased more than two-fold. Transcriptome analysis revealed upregulation of cytokine/inflammatory pathway. Depletion of Cnlp induced more M1 macrophages and neutrophils compared with the wild-type mice after CLP. Mice pre-treated with cholecalciferol (an inactive form of vitamin D3) or treated with 1alpha, 25-dihydroxyvitamin D3 (an active form of VD3) had decreased 7-day mortality and significantly less severe symptoms. Intriguingly, the administration of cholecalciferol after CLP led to worsened 7-day mortality and the associated symptoms.

Conclusions

Endogenous cathelicidin promotes intestinal barrier integrity accompanied by modulating the infiltration of neutrophils and macrophages in polymicrobial sepsis. Our data suggested that 1alpha, 25-dihydroxyvitamin D3 but not cholecalciferol is a potential therapeutic agent for treating sepsis.

Novel Innate Immune Genes Regulating the Macrophage Response to Gram Positive Bacteria

Host variation in Toll-like receptors and other innate immune signaling molecules alters infection susceptibility. However, only a portion of the variability observed in the innate immune response is accounted for by known genes in these pathways. Thus, the identification of additional genes that regulate the response to Gram positive bacteria is warranted. Bone marrow-derived macrophages (BMMs) from 43 inbred mouse strains were stimulated with lipotechoic acid (LTA), a major component of the Gram positive bacterial cell wall. Concentrations of the proinflammatory cytokines IL-6, IL-12, and TNF-α were measured. In silico whole genome association (WGA) mapping was performed using cytokine responses followed by network analysis to prioritize candidate genes. To determine which candidate genes could be responsible for regulating the LTA response, candidate genes were inhibited using RNA interference (RNAi) and were overexpressed in RAW264.7 macrophages. BMMs from Bdkrb1-deficient mice were used to assess the effect of Bdkrb1 gene deletion on the response to LTA, heat-killed Streptococcus pneumoniae, and heat-killed Staphylococcus aureus WGA mapping identified 117 loci: IL-6 analysis yielded 20 loci (average locus size = 0.133 Mb; 18 genes), IL-12 analysis produced 5 loci (0.201 Mb average; 7 genes), and TNF-α analysis yielded 92 loci (0.464 Mb average; 186 genes of which 46 were prioritized by network analysis). The follow-up small interfering RNA screen of 71 target genes identified four genes (Bdkrb1, Blnk, Fbxo17, and Nkx6-1) whose inhibition resulted in significantly reduced cytokine production following LTA stimulation. Overexpression of these four genes resulted in significantly increased cytokine production in response to LTA. Bdkrb1-deficient macrophages were less responsive to LTA and heat-killed S. aureus, validating the genetic and RNAi approach to identify novel regulators of the response to LTA. We have identified four innate immune response genes that may contribute to Gram positive bacterial susceptibility.

Genetic factors regulating lung vasculature and immune cell functions associate with resistance to pneumococcal infection

Streptococcus pneumoniae is an important human pathogen responsible for high mortality and morbidity worldwide. The susceptibility to pneumococcal infections is controlled by as yet unknown genetic factors. To elucidate these factors could help to develop new medical treatments and tools to identify those most at risk. In recent years genome wide association studies (GWAS) in mice and humans have proved successful in identification of causal genes involved in many complex diseases for example diabetes, systemic lupus or cholesterol metabolism. In this study a GWAS approach was used to map genetic loci associated with susceptibility to pneumococcal infection in 26 inbred mouse strains. As a result four candidate QTLs were identified on chromosomes 7, 13, 18 and 19. Interestingly, the QTL on chromosome 7 was located within S. pneumoniae resistance QTL (Spir1) identified previously in a linkage study of BALB/cOlaHsd and CBA/CaOlaHsd F2 intercrosses. We showed that only a limited number of genes encoded within the QTLs carried phenotype-associated polymorphisms (22 genes out of several hundred located within the QTLs). These candidate genes are known to regulate TGFβ signalling, smooth muscle and immune cells functions. Interestingly, our pulmonary histopathology and gene expression data demonstrated, lung vasculature plays an important role in resistance to pneumococcal infection. Therefore we concluded that the cumulative effect of these candidate genes on vasculature and immune cells functions as contributory factors in the observed differences in susceptibility to pneumococcal infection. We also propose that TGFβ-mediated regulation of fibroblast differentiation plays an important role in development of invasive pneumococcal disease. Gene expression data submitted to the NCBI Gene Expression Omnibus Accession No: GSE49533

SNP data submitted to NCBI dbSNP Short Genetic Variation http://www.ncbi.nlm.nih.gov/projects/SNP/snp_viewTable.cgi?handle=MUSPNEUMONIA.

Nicotinamide nucleotide transhydrogenase (NNT) acts as a novel modulator of macrophage inflammatory responses

Nicotinamide nucleotide transhydrogenase (NNT) is a mitochondrial redox-driven proton pump that couples the production of NADPH to the mitochondrial metabolic rate. In this study, we demonstrated for the first time that NNT has a significant effect in the modulation of the immune response and host defense against pathogens. We found that NNT mRNA is enriched in immune system-related tissues and regulated during macrophage activation. Overexpression of NNT in a macrophage cell-line resulted in decreased levels of reactive oxygen species (ROS) and nitric oxide upon induction of the macrophage inflammatory responses. These cells failed to fully activate MAPK signaling pathways, resulting in defective secretion of proinflammatory cytokines in response to LPS, and were inefficient in clearance of intracellular bacteria. We have shown that C57BL/6J mice, which have a deletion in the Nnt gene, exhibited greater resistance to acute pulmonary infection with Streptococcus pneumoniae. Macrophages from these mice generated more ROS and established a stronger inflammatory response to this pathogen. Our results demonstrate a novel role for NNT as a regulator of macrophage-mediated inflammatory responses.

Genetically determined heterogeneity of lung disease in a mouse model of airway mucus obstruction

Mucus clearance is an important airway innate defense mechanism. Airway-targeted overexpression of the epithelial Na(+) channel β-subunit [encoded by sodium channel nonvoltage gated 1, beta subunit (Scnn1b)] in mice [Scnn1b-transgenic (Tg) mice] increases transepithelial Na(+) absorption and dehydrates the airway surface, which produces key features of human obstructive lung diseases, including mucus obstruction, inflammation, and air-space enlargement. Because the first Scnn1b-Tg mice were generated on a mixed background, the impact of genetic background on disease phenotype in Scnn1b-Tg mice is unknown. To explore this issue, congenic Scnn1b-Tg mice strains were generated on C57BL/6N, C3H/HeN, BALB/cJ, and FVB/NJ backgrounds. All strains exhibited a two- to threefold increase in tracheal epithelial Na(+) absorption, and all developed airway mucus obstruction, inflammation, and air-space enlargement. However, there were striking differences in neonatal survival, ranging from 5 to 80% (FVB/NJ<BALB/cJ<C3H/HeN<C57BL/6N), which correlated with the incidence of upper airway mucus plugging and the levels of Muc5b in bronchoalveolar lavage. The strains also exhibited variable Clara cell necrotic degeneration in neonatal intrapulmonary airways and a variable incidence of pulmonary hemorrhage and lung atelectasis. The spontaneous occurrence of a high surviving BALB/cJ line, which exhibited delayed onset of Na(+) hyperabsorption, provided evidence that: 1) air-space enlargement and postnatal death were only present when Na(+) hyperabsorption occurred early, and 2) inflammation and mucus obstruction developed whenever Na(+) hyperabsorption was expressed. In summary, the genetic context and timing of airway innate immune dysfunction critically determines lung disease phenotype. These mouse strains may be useful to identify key modifier genes and pathways.

Host genetic background strongly influences the response to influenza a virus infections

The genetic make-up of the host has a major influence on its response to combat pathogens. For influenza A virus, several single gene mutations have been described which contribute to survival, the immune response and clearance of the pathogen by the host organism. Here, we have studied the influence of the genetic background to influenza A H1N1 (PR8) and H7N7 (SC35M) viruses. The seven inbred laboratory strains of mice analyzed exhibited different weight loss kinetics and survival rates after infection with PR8. Two strains in particular, DBA/2J and A/J, showed very high susceptibility to viral infections compared to all other strains. The LD₅₀ to the influenza virus PR8 in DBA/2J mice was more than 1000-fold lower than in C57BL/6J mice. High susceptibility in DBA/2J mice was also observed after infection with influenza strain SC35M. In addition, infected DBA/2J mice showed a higher viral load in their lungs, elevated expression of cytokines and chemokines, and a more severe and extended lung pathology compared to infected C57BL/6J mice. These findings indicate a major contribution of the genetic background of the host to influenza A virus infections. The overall response in highly susceptible DBA/2J mice resembled the pathology described for infections with the highly virulent influenza H1N1-1918 and newly emerged H5N1 viruses.