Specific dysregulation of IFNγ production by natural killer cells confers susceptibility to viral infection.

Natural Killer (NK) cells contribute to the control of viral infection by directly killing target cells and mediating cytokine release. In C57BL/6 mice, the Ly49H activating NK cell receptor plays a key role in early resistance to mouse cytomegalovirus (MCMV) infection through specific recognition of the MCMV-encoded MHC class I-like molecule m157 expressed on infected cells. Here we show that transgenic expression of Ly49H failed to provide protection against MCMV infection in the naturally susceptible A/J mouse strain. Characterization of Ly49H(+) NK cells from Ly49h-A transgenic animals showed that they were able to mount a robust cytotoxic response and proliferate to high numbers during the course of infection. However, compared to NK cells from C57BL/6 mice, we observed an intrinsic defect in their ability to produce IFNγ when challenged by either m157-expressing target cells, exogenous cytokines or chemical stimulants. This effect was limited to NK cells as T cells from C57BL/6 and Ly49h-A mice produced comparable cytokine levels. Using a panel of recombinant congenic strains derived from A/J and C57BL/6 progenitors, we mapped the genetic basis of defective IFNγ production to a single 6.6 Mb genetic interval overlapping the Ifng gene on chromosome 10. Inspection of the genetic interval failed to reveal molecular differences between A/J and several mouse strains showing normal IFNγ production. The chromosome 10 locus is independent of MAPK signalling or decreased mRNA stability and linked to MCMV susceptibility. This study highlights the existence of a previously uncovered NK cell-specific cis-regulatory mechanism of Ifnγ transcript expression potentially relevant to NK cell function in health and disease.

Host-pathogen specificity in tuberculosis.

The host response to mycobacterial infection including tuberculosis depends on genetically controlled host and bacterial factors and their interaction. A largely unknown aspect of this interaction is whether disease results from an additive and independent effect of host and pathogen or from specific host-pathogen combinations. The preferential association of specific mycobacterial strains with specific ethnic groups provided tentative evidence in favor of host-pathogen specificity in tuberculosis and is consistent with the hypothesis of host-mycobacterial co-adaptation. Substantial evidence for specificity has now been provided by animal models and human case-control association studies. These studies indicate that differences in the host response to infection are at least in part due to specific combinations of host genetic factors and genetic and phenotypic characteristics of the infecting mycobacterial strain.

Strain-specific differences in the genetic control of two closely related mycobacteria.

The host response to mycobacterial infection depends on host and pathogen genetic factors. Recent studies in human populations suggest a strain specific genetic control of tuberculosis. To test for mycobacterial-strain specific genetic control of susceptibility to infection under highly controlled experimental conditions, we performed a comparative genetic analysis using the A/J- and C57BL/6J-derived recombinant congenic (RC) mouse panel infected with the Russia and Pasteur strains of Mycobacterium bovis Bacille Calmette Guérin (BCG). Bacillary counts in the lung and spleen at weeks 1 and 6 post infection were used as a measure of susceptibility. By performing genome-wide linkage analyses of loci that impact on tissue-specific bacillary burden, we were able to show the importance of correcting for strain background effects in the RC panel. When linkage analysis was adjusted on strain background, we detected a single locus on chromosome 11 that impacted on pulmonary counts of BCG Russia but not Pasteur. The same locus also controlled the splenic counts of BCG Russia but not Pasteur. By contrast, a locus on chromosome 1 which was indistinguishable from Nramp1 impacted on splenic bacillary counts of both BCG Russia and Pasteur. Additionally, dependent upon BCG strain, tissue and time post infection, we detected 9 distinct loci associated with bacillary counts. Hence, the ensemble of genetic loci impacting on BCG infection revealed a highly dynamic picture of genetic control that reflected both the course of infection and the infecting strain. This high degree of adaptation of host genetics to strain-specific pathogenesis is expected to provide a suitable framework for the selection of specific host-mycobacteria combinations during co-evolution of mycobacteria with humans.

Joint effects of host genetic background and mycobacterial pathogen on susceptibility to infection.

The present study examined the differential contribution of host genetic background and mycobacterial pathogen variability to biological and mechanistic phenotypes of infection. For this purpose, A/J and C57BL/6J mice were infected intravenously with a low dose of Mycobacterium tuberculosis H37Rv or the Russia, Japan, and Pasteur substrains of Mycobacterium bovis bacille Calmette-Guérin (BCG). The pulmonary bacterial counts (number of CFU) and transcript levels of select cytokines (e.g., Ifng, Il12b, and Il4) at 1, 3, and 6 weeks postinfection were measured as biological and mechanistic phenotypes, respectively. The individual and combined impact of the host and mycobacteria on these phenotypes was assessed using three-way analysis of variance (ANOVA), which partitions phenotypic variation into host, pathogen, time, and interaction effects. All phenotypes, except pulmonary Il4 transcript levels, displayed evidence for host-mycobacterium specificity by means of significant interaction terms. Pulmonary expression profiles of 34 chemokines and chemokine-related genes were compared across the hosts and mycobacteria. The differences in induction of these immune messenger genes between A/J and C57BL/6J mice were modest and generally failed to reach significance. In contrast, the mycobacteria induced significant variance in a subset of the immune messenger genes, which was more evident in A/J mice relative to that in C57BL/6J mice. Overall, the results demonstrated the importance of considering the joint effects of the mycobacterial and host genetic backgrounds on susceptibility to mycobacterial infections.

Genetics of infectious diseases: hidden etiologies and common pathways.

Since the completion of the human genome sequence, the study of common genetic polymorphisms in complex human diseases has become a main activity of human genetics. Employing genome-wide association studies, hundreds of modest genetic risk factors have been identified. In infectious diseases the identification of common risk factors has been varied and as in other common diseases it seems likely that important genetic risk factors remain to be discovered. Nevertheless, the identification of disease-specific genetic risk factors revealed an unexpected overlap in susceptibility genes of diverse inflammatory and infectious diseases. Analysis of the multi-disease susceptibility genes has allowed the definition of shared key pathways of inflammatory dysregulation and suggested unexpected infectious etiologies for other "non-infectious" common diseases.

Mouse models for the genetic study of tuberculosis susceptibility.

Tuberculosis, mainly caused by the pathogenic bacterium Mycobacterium tuberculosis, remains an inestimable public health problem, despite the established use of the Bacillus Calmette-Guerin (BCG) vaccine, multidrug therapy and the existence of global tuberculosis control programmes. Statistics show that nearly 2 billion people (approximately one-third of the world's population) are infected with M. tuberculosis. For unknown reasons, only about 10 per cent of those infected by M. tuberculosis will develop tuberculosis, resulting in 9 million new cases yearly and 2 million deaths. A better understanding of the host--mycobacterial--environmental interplay is central to developing better antituberculosis vaccines and treatments. This review will discuss how a clearer idea of this interplay is emerging with new genomic strategies in mouse models.

Impact of methoxymycolic acid production by Mycobacterium bovis BCG vaccines.

BCG vaccines are a family of closely related daughter strains of an attenuated isolate of Mycobacterium bovis derived by in vitro passage from 1908 to 1921. During subsequent laboratory propagation of the vaccine strain until its lyophilization in 1961, BCG Pasteur underwent at least seven further genomic mutations. The impact of these mutations on the properties of the vaccine is currently unknown. One mutation, a glycine-to-aspartic acid substitution in the mmaA3 gene, occurred between 1927 and 1931 and impairs methoxymycolic acid synthesis in BCG strains obtained from the Pasteur Institute after this period. Mycolic acids of the cell wall are classified into three functional groups (alpha-, methoxy-, and ketomycolic acids), and together these lipids form a highly specialized permeability barrier around the bacterium. To explore the impact of methoxymycolic acid production by BCG strains, we complemented the functional gene of mmaA3 into BCG Denmark and tested a number of in vitro and in vivo phenotypes. Surprisingly, restoration of methoxymycolic acids alone had no effect on cell wall permeability, resistance to antibiotics, or growth in cultured macrophages and C57BL/6 mice. Our results demonstrate that the loss of methoxymycolic acid production did not apparently affect the virulence of BCG strains.