Ipr1 gene mediates innate immunity to tuberculosis

Abstract

An estimated eight million people are infected each year with the pathogen Mycobacterium tuberculosis, and more than two million die annually. Yet only about 10% of those infected develop tuberculosis. Genetic variation within host populations is known to be significant in humans and animals, but the nature of genetic control of host resistance to tuberculosis remains poorly understood. Previously we mapped a new genetic locus on mouse chromosome 1, designated sst1 (for supersusceptibility to tuberculosis 1). Here we show that this locus mediates innate immunity in sst1 congenic mouse strains and identify a candidate gene, Intracellular pathogen resistance 1 (Ipr1), within the sst1 locus. The Ipr1 gene is upregulated in the sst1 resistant macrophages after activation and infection, but it is not expressed in the sst1 susceptible macrophages. Expression of the Ipr1 transgene in the sst1 susceptible macrophages limits the multiplication not only of M. tuberculosis but also of Listeria monocytogenes and switches a cell death pathway of the infected macrophages from necrosis to apoptosis. Our data indicate that the Ipr1 gene product might have a previously undocumented function in integrating signals generated by intracellular pathogens with mechanisms controlling innate immunity, cell death and pathogenesis.

Figure 1. The sst1 locus mediates innate immunity to tuberculosis.

a, b, Survival of C3H, B6, and the C3H.B6-sst1 (sst1^R) mice after i.v. (a) or aerosol (b) infection with MTB; c, MTB bacterial loads in the lungs of the sst1 congenic mice after the aerosol infection; d, e Tuberculosis lung lesions 25 days after i.v. infection (d) and 12 weeks after aerosol infection, H&E, 40X original magnification (e); f, FACS analysis of mechanism of cell death of the sst1 congenic macrophages infected with MTB (top panels) or BCG (bottom panels) in vitro; g, Multiplication of MTB (left panel) or M. bovis BCG (right panel) in the sst1 congenic macrophages in vitro (*p<0.01, **p<0.001). Error bars represent 95% confidence intervals.

Figure 2. Identification of the sst1candidate gene.

a, Physical map of the sst1 minimal region. (#)- number of recombination evens, (M) – polymorphic markers, (C) – chromosome with distances between the markers (kb); (G) – known genes; (RC) - recombinant chromosomes containing the sst1 resistant (R) or susceptible (S) alleles, genotypes for each marker are represented by solid (B6) and opened (C3H) boxes; b, Analysis of the Ifi75-rs expression in the tuberculosis lung lesions of the sst1 congenic mice by RACE. c, Domain structure of the Ipr1 and its human homolog SP110b and location of the PCR primers. d, Ipr1 and Sp100-rs gene expression in the lungs during MTB infection (Northern blot); e, Ipr1 gene expression in sst1^S (S) or sst1^R (R) macrophages infected MTB, BCG or activated with IFN-γ in vitro.

Figure 3. Lack of the Ipr1 gene expression in the C3HeB/FeJ substrain correlates with its extreme susceptibility to MTB infection.

a, Survival after the intravenous infection with MTB; b, MTB bacterial loads three weeks after the infection (4 mice per strain, ** p<0.001, error bars represent standard deviation); c, Analysis of the sst1-encoded candidate gene expression in the tuberculosis lung lesions by RT-PCR three weeks after the infection.

Figure 4. Expression of the Ipr1 transgene in the sst1^S macrophages confers resistance to intracellular pathogens MTB and L. monocytogenes.

a, RT-PCR of Ipr1 and Sp100-rs in macrophages isolated from the sst1^S (S), sst1^R(R) mice, their F1 hybrids (SxR, RxS) and the Ipr1 transgenic (Tg) mice, 1 - IFNγ -stimulated, 2 - MTB-infected b, MTB bacterial loads in the Ipr1 transgenic (Tg+/−) and control (Tg−/−) mice after infection with MTB (7 mice/strain, * p<0.05); c, e growth of MTB (c) and L.monocytogenes (e) in the Ipr1 transgenic and control (sst1^S) macrophages (three experiments were performed in triplicates, *p<0.01, **p<0.001, error bars represent 95% confidence interval).