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Infection and Immunity, September 2004, p. 5315-5321, Vol. 72, No. 9
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.9.5315-5321.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Identification of Mycobacterium tuberculosis Counterimmune (cim) Mutants in Immunodeficient Mice by Differential Screening

Katherine B. Hisert,¹ Meghan A. Kirksey,¹ James E. Gomez,¹ Alexandra O. Sousa,¹ Jeffery S. Cox,² William R. Jacobs Jr.,³ Carl F. Nathan,⁴ and John D. McKinney^1*

Laboratory of Infection Biology, The Rockefeller University,¹ Weill Medical College of Cornell University, New York, New York,⁴ Department of Microbiology and Immunology, University of California, San Francisco, California,² Howard Hughes Medical Institute, Albert Einstein College of Medicine, Bronx, New York³

Received 2 February 2004/ Returned for modification 30 March 2004/ Accepted 1 June 2004

Top Abstract Introduction Materials and Methods Results Discussion References

 
Tuberculosis (TB) is characterized by lifetime persistence of Mycobacterium tuberculosis. Despite the induction of a vigorous host immune response that curtails disease progression in the majority of cases, the organism is not eliminated. Subsequent immunosuppression can lead to reactivation after a prolonged period of clinical latency. Thus, while it is clear that protective immune mechanisms are engaged during M. tuberculosis infection, it also appears that the pathogen has evolved effective countermechanisms. Genetic studies with animal infection models and with patients have revealed a key role for the cytokine gamma interferon (IFN-) in resistance to TB. IFN- activates a large number of antimicrobial pathways. Three of these IFN--dependent mechanisms have been implicated in defense against M. tuberculosis: inducible nitric oxide synthase (iNOS), phagosome oxidase (phox), and the phagosome-associated GTPase LRG-47. In order to identify bacterial genes that provide protection against specific host immune pathways, we have developed the strategy of differential signature-tagged transposon mutagenesis. Using this approach we have identified three M. tuberculosis genes that are essential for progressive M. tuberculosis growth and rapid lethality in iNOS-deficient mice but not in IFN--deficient mice. We propose that these genes are involved in pathways that allow M. tuberculosis to counter IFN--dependent immune mechanisms other than iNOS.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), is capable of long-term survival in vivo despite a vigorous host immune response. Although 90 to 95% of infected individuals initially control bacterial replication and do not manifest clinical signs and symptoms, reactivation of latent TB infection (LTBI) can occur many years after infection (42). Human immunodeficiency virus infection has become the most important risk factor for reactivation of LTBI, increasing the risk of reactivation from 5 to 10% per lifetime to 5% per year (9). Immunosuppressive agents, such as Infliximab, an anti-tumor necrosis factor alpha antibody used to treat chronic inflammatory conditions, have also been associated with reactivation of LTBI (42). Approximately half of all active cases of pulmonary TB are thought to result from endogenous reactivation of LTBI (40).

Exogenous reinfection of asymptomatic individuals also appears to be common in areas where TB is endemic, despite the presence of detectable antigen-specific immune responses established by the previous infection (6, 17, 46). Similarly, vaccination with live-attenuated Mycobacterium bovis BCG has yielded highly variable and generally poor efficacy in prevention of pulmonary TB in adults despite eliciting robust antigen-specific immune responses in the majority of vaccinated individuals (4).

A key component of antimycobacterial immunity is provided by the cytokine gamma interferon (IFN-), as evidenced by infection of IFN--deficient mice (12, 19) and IFN-R1-deficient mice (25) and by naturally occurring mutations in IFN--signaling pathways in humans that are associated with increased susceptibility to mycobacterial infections (8). In the lungs, M. tuberculosis resides within macrophages—phagocytic cells possessing a number of antimicrobial mechanisms, many of which depend on IFN- for their activity. The antimicrobial activities elicited by IFN- include activation of phagocyte oxidase (phox) to produce increased levels of reactive oxygen species and induction of inducible nitric oxide synthase (iNOS), which produces toxic reactive nitrogen species (32). IFN- activation of macrophages also results in the upregulation of LRG-47, a phagosome-associated GTPase that appears to be involved in vesicular trafficking (25). Mice deficient in iNOS (24) or LRG-47 (25) are unable to control replication of M. tuberculosis and succumb rapidly to infection. In contrast, phox-deficient mice show little (1, 13) or no (22, 34) increase in susceptibility to M. tuberculosis.

The phenomena of endogenous reactivation and exogenous reinfection indicate that host immunity is only partially protective against TB. A likely explanation is that M. tuberculosis has evolved highly effective countermechanisms for evading, subverting, or detoxifying the host's antimycobacterial defenses. Examples of immune evasion include the suppression of IFN--induced gene expression in M. tuberculosis-infected macrophages (41) and inhibition of interleukin-12 production by M. tuberculosis-infected macrophages in response to proinflammatory stimuli (33). Immune subversion may be mediated by massive release of the T-cell-suppressive cytokine interleukin-6 from M. tuberculosis-infected macrophages (31, 45), which is triggered by bacterial lipids trafficking through the macrophage endocytic network (2). M. tuberculosis mechanisms for detoxification of specific host defenses have also been described previously. Resistance to the phox-dependent oxidative burst in mice and macrophages is mediated by KatG catalase (34). Resistance to iNOS-generated reactive nitrogen species is mediated by UvrB, FbiC, and subunits of a putative M. tuberculosis proteasome (Mpa and Paf) (15). These proteins are essential for M. tuberculosis virulence in wild-type mice but are less important in mice lacking phox (KatG) or iNOS (UvrB, FbiC, Mpa, and Paf). A number of other genes have been implicated in M. tuberculosis growth and persistence in the lungs of mice (21), but with few exceptions it is not known whether these genes play a role in counteracting the host immune response.

Here we use a variant of signature-tagged transposon mutagenesis (STM) (20) as a screening method to identify M. tuberculosis genes that are involved in countering IFN--dependent immune responses. We report the identification of three M. tuberculosis transposon-induced mutants that are capable of progressive replication and rapid lethality in IFN--deficient mice but not in iNOS-deficient mice, indicating increased susceptibility of the mutants to IFN--dependent immune mechanisms other than iNOS. Identification of the genes disrupted in these mutants suggests that such diverse processes as transmembrane transport, polyketide synthesis, and cell wall biogenesis may be important for adaptation of M. tuberculosis to the in vivo environment as shaped by the host immune response.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Bacterial strains and growth conditions. Wild-type M. tuberculosis (strains H37Rv and Erdman) and derivative strains were propagated at 37°C in Middlebrook 7H9 broth containing 10% oleic acid-albumin-dextrose-catalase (Difco), 0.5% glycerol, and 0.05% Tween 80 or on Middlebrook 7H10 agar containing 10% oleic acid-albumin-dextrose-catalase and 0.5% glycerol. Antibiotics were hygromycin (50 µg/ml) and kanamycin (25 µg/ml). Frozen stocks were prepared by growing broth cultures to mid-log phase (optical density at 600 nm, 0.5), adding glycerol to 15%, and storing in aliquots at –80°C.

Construction of the STM mutant library. A panel of 48 transposons in the temperature-sensitive mycobacteriophage phAE87 was constructed as described previously (14). Each phage contained a unique random 40-mer signature tag within the Tn5370 transposon, which carries the hygromycin resistance marker (hyg). Phages were amplified individually in Mycobacterium smegmatis mc²155 to final titers of 5 x 10¹¹ PFU/ml. Cultures of M. tuberculosis H37Rv were mutagenized with the phage stocks, and transposon insertion mutants were selected on 7H10 agar containing 50 µg of hygromycin/ml. Individual colonies were picked and inoculated into 7H9 broth containing 50 µg of hygromycin/ml dispensed into 48-well microtiter plates; each of the 48 signature tags was represented by one mutant per plate. The microtiter plate cultures were grown to stationary phase at 37°C, and aliquots from each well were combined to generate pools of 48 signature-tagged mutants. The pools were diluted 1:100 in 7H9 broth containing 50 µg of hygromycin/ml and were grown at 37°C until the optical density at 600 nm reached 0.2. The amplified pools were then washed and resuspended in phosphate-buffered saline (PBS) containing 0.05% Tween 80 and 15% glycerol as a cryopreservant, aliquoted in cryovials, and stored at –80°C.

Mouse infections. Male and female C57BL/6, IFN-^–/–, and iNOS^–/– mice, 6 to 10 weeks of age, were from Jackson Laboratories. gp91^phox–/– iNOS^–/– mice were generated and maintained as described previously (38). M. tuberculosis frozen stocks were thawed, diluted in PBS, and sonicated briefly to disperse clumps. Mice were infected by injection of 0.1 ml (10⁵ to 10⁶ CFU) of the diluted bacterial stock into a lateral tail vein and were euthanized by CO₂ exposure at specified time points (n = 4 to 5 mice per time point unless otherwise noted). Organs were removed aseptically and homogenized in PBS containing 0.05% Tween 80 and 100 µg of ampicillin/ml to prevent contamination. Bacteria in the lungs, spleen, and liver were enumerated by plating diluted organ homogenates on 7H10 agar and counting colonies after 3 to 4 weeks at 37°C.

Isolation of M. tuberculosis genomic DNA and amplification of signature tags. The three STM mutants characterized in this paper were identified as follows. Lung homogenates from mice infected with STM pools were plated at low density for enumeration of CFU and at high density for STM analysis. Genomic DNA was isolated from 10,000 pooled colonies per lung homogenate (recovered pools) and from colonies generated by plating the inocula that were used to infect the mice (input pools). STM tags were PCR amplified from input and recovered pools with primers specific for invariant regions on either side of the STM tags (14), and PCR products were purified with the Qiaex II kit (QIAGEN). Amplified tags were labeled with [³²P]dCTP by using the Megaprime Labeling System (Amersham), and free [³²P]dCTP was removed with Probequant G-50 Microcolumns (Amersham).

Southern hybridizations. The 48 individual signature tags were PCR amplified from purified phage DNA and were cloned individually into pUC19 with HindIII. The 48 plasmid DNAs were arrayed onto 9- by 12-cm Zeta Probe nylon membranes (Bio-Rad) by using a 48-well Bio-Dot SF Microfiltration Apparatus (Bio-Rad). Hybridizations using labeled signature tags from input and output STM pools were performed at 65°C in Rapid-Hyb solution (Amersham), and the hybridized membranes were processed per the manufacturer's instructions. Autoradiographic films (Kodak) were exposed to membranes for at least 2 h and were developed per the manufacturer's instructions.

Identification of transposon insertion sites. Genomic DNA was isolated from individual STM mutants grown in 7H9 broth. Transposon insertion sites in the genome were identified by inverse PCR as described previously (14). Briefly, RsaI-digested genomic DNA was purified with the Qiaex II kit (QIAGEN), self ligated, and PCR amplified with primers o84L-F (5'-GTCATCCGGCTCATCACCAG-3') and o84L-R (5'-AACTGGCGCAGTTCCTCTGG-3') to amplify the genomic DNA fragments flanking the ends of the transposon. PCR products were separated on agarose gels and were purified using the Qiaex II kit (QIAGEN). The nucleotide sequences of the purified amplicons were determined by The Rockefeller University DNA sequencing facility. In some cases, PCR products were cloned using the TopoTA Cloning kit (Invitrogen) and the plasmid DNA was purified using the QIAGEN miniprep kit prior to sequencing.

Statistical analysis. The Student's t test was used to evaluate differences in bacterial CFU in infected organs. One-tailed Student's t tests were applied to comparisons of CFU from H37Rv-infected wild-type mice with CFU from H37Rv-infected IFN-^–/– mice and iNOS^–/– mice (see Fig. 4A). Two-tailed Student's t-Tests were applied to the data shown in Fig. 1A to C, Fig. 4B to D, and Fig. 5. Kaplan-Meier curves were compared by using the Mantel-Haenszel test as applied by the GraphPad Prism 4.0 program from GraphPad Software, Inc. (28). P values of <0.05 were considered statistically significant. Data are represented as means; error bars indicate standard deviations from the means.

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FIG. 4. Pathogenesis of M. tuberculosis cim mutants in mice. Mice were infected intravenously with 0.5 x 106 to 1.0 x 106 CFU of wild-type M. tuberculosis H37Rv (A and E) or one of the following M. tuberculosis cim mutants: Rv0072 (B and F), Rv2958c (C and G), or Rv0405 (D and H). Symbols: C57BL/6 mice, filled triangles; iNOS–/– mice, open circles; and IFN-–/– mice, filled squares. (A to D) Bacterial growth kinetics in the lungs of mice (n = 4 per group). Asterisks indicate P < 0.05 for comparisons of bacterial loads (CFU) in IFN-–/– mice versus C57BL/6 mice at 4 weeks postinfection or iNOS–/– mice versus C57BL/6 mice at 6 weeks postinfection. Results are representative of two experiments. (E to H) Survival of infected mice (n = 4 to 6 per group). In all cases, survival of infected iNOS–/– mice was significantly longer than survival of IFN-–/– mice infected with the same M. tuberculosis strain (P < 0.005 for all comparisons). Also, survival of infected mice of both genotypes (IFN-–/– and iNOS–/–) was significantly longer for mice infected with one of the M. tuberculosis cim mutants than for mice infected with wild-type M. tuberculosis (P < 0.005 for all comparisons).

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FIG. 1. Pathogenesis of M. tuberculosis in iNOS–/–, gp91phox–/– iNOS–/–, and IFN-–/– mice. IFN-–/– mice (filled squares), iNOS–/– mice (open circles), and gp91phox–/– iNOS–/– mice (filled triangles) were infected intravenously with 2 x 105 CFU of wild-type M. tuberculosis Erdman. (A to C) Kinetics of bacterial growth in the lungs (A), spleens (B), and livers (C) of infected mice (n = 4 to 5 mice per group, except day 42 iNOS–/–, where n = 3, and day 42 gp91phox–/–iNOS–/–, where n = 8). Asterisks indicate P < 0.05 for comparison of IFN-–/– mice with iNOS–/– and gp91phox–/–iNOS–/– mice. (D) Survival of infected mice. Mean time to death was 24.5 days for IFN-–/– mice (n = 13), 42.4 days for iNOS–/– mice (n = 13), and 39.4 days for gp91phox–/– iNOS–/– mice (n = 19). Survival times were significantly different between IFN-–/– mice and iNOS–/– mice (P < 0.0001) and between IFN-–/– mice and for gp91phox–/– iNOS–/– mice (P < 0.0001) but not between iNOS–/– mice and gp91phox–/– iNOS–/– mice (P = 0.064). Similar results were obtained in a second experiment using M. tuberculosis H37Rv.

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FIG. 5. Host-specific attenuation of the M. tuberculosis cim mutants' ability to replicate in the lungs. Mice were infected intravenously with 0.5 x 106 to 1.0 x 106 CFU of wild-type M. tuberculosis H37Rv (A to I; closed squares) or one of the following M. tuberculosis cim mutants (open circles): Rv0072 (A, D, and G), Rv2958c (B, E, and H), Rv0405 (C, F, and I). Bacterial loads (CFU) were measured in the lungs of C57BL/6 mice (A to C), IFN-–/– mice (D to F), and iNOS–/– mice (G to I) at the indicated time points postinfection. An asterisk indicates P < 0.05 for comparisons of mice infected with the indicated cim mutant versus mice infected with wild-type M. tuberculosis H37Rv. Results are representative of two experiments.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Growth and lethality of M. tuberculosis in IFN-^–/–, iNOS^–/–, and gp91^phox–/– iNOS^–/– mice. IFN--inducible iNOS is an essential component of host defense against M. tuberculosis (24). However, it is clear that IFN--deficient mice are more susceptible than iNOS-deficient mice, because time to death (Fig. 1D) (25, 27) and bacterial growth in the tissues (Fig. 1A to C) (25) are more rapid in the former. These differences indicate that iNOS is not the only antimycobacterial mechanism controlled by IFN-. Recently, MacMicking et al. (25) demonstrated that the IFN--induced GTPase LRG-47 is also an essential component of murine defense against M. tuberculosis. Although chemical inhibition of iNOS further exacerbates infection in LRG-47-deficient mice, indicating that these mechanisms make independent contributions to host defense, the combined loss of LRG-47 and iNOS does not increase susceptibility to the level observed in IFN--deficient mice (25). We considered the possibility that IFN--activated NADPH oxidase might also contribute to host defense against M. tuberculosis in the absence of iNOS. We found that gp91^phox deficiency did not increase the susceptibility of iNOS-deficient mice in terms of time to death (Fig. 1D) or bacterial growth kinetics in the tissues (Fig. 1A to C), in agreement with results of a previous report (22). These observations suggest the existence of an additional, as-yet unidentified IFN--protective mechanism(s) that is independent of iNOS, LRG-47, and phox.

Identification of M. tuberculosis counterimmune (cim) mutants by differential screening in IFN-^–/– and iNOS^–/– mice. The substantial difference in susceptibility of IFN-^–/– and iNOS^–/– mice to M. tuberculosis suggested the possibility of identifying mechanisms of bacterial defense against IFN--dependent, iNOS-independent immune mechanisms by differential screening in mice. Our strategy was to passage pools of signature-tagged mutants through iNOS^–/– mice and IFN-^–/– mice in order to identify mutants that were underrepresented in the former but not the latter (Fig. 2). Mice were infected with pools representing 48 independently derived STM mutants and were sacrificed at 2 (IFN-^–/– mice) or 3 weeks (iNOS^–/– mice) postinfection for recovery of bacteria from the lungs. These time points were selected to permit maximal expansion of the bacterial populations in the lungs; by sacrificing animals shortly before they would become moribund, the underrepresentation of attenuated mutants in the recovered pools is maximized. Each pool of mutants was passaged through two iNOS^–/– mice and two IFN-^–/– mice; mutants that showed a consistent phenotype in both mice of each genotype were selected for further analysis. Here we report the identification of three cim mutants that appear to be defective in counterimmune mechanisms directed towards IFN--dependent, iNOS-independent host pathways. These mutants were identified in pools comprising 96 independent transposon-induced mutants, indicating a high frequency of cim genes in the M. tuberculosis chromosome.

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FIG. 2. Differential signature-tagged transposon mutagenesis: a strategy to identify M. tuberculosis cim mutants. Signature-tagged transposon mutants are arrayed in 48-well microtiter dishes (1). Pooled mutants are inoculated into IFN-–/– and iNOS–/– mice, and bacteria are recovered from the lungs by plate culture at the indicated times postinfection. Signature tags in the recovered pools of bacteria are amplified and radiolabeled and are used to probe membranes arrayed with the set of 48 signature tags. Tags that are detected in the input pool (2) but reduced or absent in the recovered pools (3 and 4) indicate mutants that are attenuated in vivo. The signature tag spotted in column 3, row 5, is underrepresented in mutant pools recovered from iNOS–/– mice (4) but is well represented in pools recovered from IFN-–/– mice (3), indicating that the cim gene disrupted in this mutant is required for progressive bacterial growth in the face of an IFN--dependent, iNOS-independent immune mechanism. wk, week.

Identification of genes disrupted in the cim mutants. The transposon insertion sites were identified in each of the three cim mutants by inverse PCR (14). The first two mutants had transposon insertions in Rv2958c and Rv0405 (pks6), neither of which is found in an operon, suggesting that the observed phenotype was not due to polar effects of the transposon insertion on neighboring genes. The third transposon insertion site was located in Rv0072, which might be cotranscribed with the downstream gene, Rv0073. Rv0072 and Rv0073 have been annotated as putative components of an ATP-binding cassette (ABC) transporter for glutamine based on homology to genes of known function in other species (3). Expression of Rv2958c, which is homologous to glycosyl transferase genes in other species, was shown previously to increase survival of nonpathogenic M. smegmatis in macrophages (26). Rv0405 (pks6), encoding a putative polyketide synthase, was identified in a previous STM screen as being essential for full virulence of M. tuberculosis in wild-type BALB/c mice (5); the effect of pks6 deficiency on M. tuberculosis replication in immunodeficient mice was not reported.

Confirmation of the cim mutants' in vivo phenotypes. None of the cim mutants displayed a diminished growth rate in vitro (Fig. 3), suggesting that their in vivo growth deficiencies were due to loss of functions required specifically for adaptation of M. tuberculosis to in vivo environmental conditions. The cim mutants' in vivo phenotypes were confirmed by infection of wild-type (C57BL/6), IFN-^–/–, and iNOS^–/– mice with the individual strains (Fig. 4 and 5). As expected, wild-type M. tuberculosis (H37Rv) demonstrated a marked growth advantage in the lungs of IFN-^–/– and iNOS^–/– mice compared to growth in the lungs of C57BL/6 mice (Fig. 4A). All three cim mutants displayed a significant growth advantage in IFN-^–/– mice compared to growth in C57BL/6 mice; however, the cim mutants showed no significant growth advantage in iNOS^–/– mice compared to that in C57BL/6 mice, thus confirming the phenotype selected in the screen (Fig. 4B to D). Likewise, compared to replication in M. tuberculosis H37Rv, replication of each of the cim mutants was significantly attenuated in iNOS^–/– mice (Fig. 5G to I) but not in IFN-^–/– mice (Fig. 5D to F). Notably, there was little or no difference in replication of the cim mutants in C57BL/6 mice compared to replication of H37Rv (see Discussion) (Fig. 5A to C). These observations suggest that the cim-encoded functions are essential for progressive bacterial growth in the presence, but not the absence, of IFN--dependent immune mechanisms other than iNOS. This interpretation is supported by the survival kinetics of mice infected with the cim mutants. There was a modest but significant delay in time to death for IFN--deficient mice infected with one of the cim mutants compared to that for mice infected with wild-type M. tuberculosis (Table 1 and Fig. 4E to H, solid squares). In contrast, iNOS-deficient mice infected with one of the cim mutants survived markedly longer than mice infected with wild-type M. tuberculosis (Table 1 and Fig. 4E to H, open circles). These observations indicate that the role of the cim genes in M. tuberculosis pathogenesis is contingent on aspects of the in vivo environment that are modulated by the host immune response—specifically, IFN--dependent, iNOS-independent immune mechanisms. Confirmation of this hypothesis will require the elucidation of the molecular mechanisms by which the cim genes contribute to virulence.

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FIG. 3. The cim mutants do not manifest any obvious growth deficiencies in vitro. Stationary-phase cultures of the indicated M. tuberculosis strains were diluted 1:500 in fresh medium and were incubated at 37°C. Aliquots were withdrawn at the indicated times for measurement of the optical density at 600 nm (A) or enumeration of CFU by plate culture (B). Symbols: wild-type M. tuberculosis H37Rv, filled diamonds; Rv0072, open circles; Rv0405, open triangles; Rv2958c, open squares. At time points later than 9 days, clumping of the cultures prevented accurate measurement of CFU (B). Results are representative of two experiments.

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TABLE 1. Time-to-death kinetics of mice infected with M. tuberculosis counterimmune (cim) mutants

Top Abstract Introduction Materials and Methods Results Discussion References

 
Long-term persistence in immunocompetent individuals is a hallmark of M. tuberculosis infection, but little is known about the bacterial mechanisms involved in countering the host immune response. Here we describe a genetic strategy to identify M. tuberculosis genes that are involved in protecting the pathogen against immune mechanisms controlled by IFN-, a key cytokine required for macrophage activation and antimycobacterial defense. IFN- is known to activate a large number of pathways at the level of the macrophage (18); however, with few exceptions, the roles of these pathways in antimycobacterial defense are unknown. The three cim genes described here appear to be involved in adaptation of M. tuberculosis to pressures exerted by IFN--dependent mechanisms other than iNOS, because the corresponding mutants showed little or no defect for growth and lethality in IFN--deficient mice yet were highly attenuated in iNOS-deficient mice. A similar strategy of characterizing virulence in immunodeficient mouse strains was used to demonstrate the role of the M. tuberculosis KatG catalase (34) and the Salmonella pathogenicity island 2 (47) in countering the host NADPH oxidase; in these cases, attenuation of the mutants was reversed in gp91^phox–/– mice.

Of interest, the cim mutants showed little or no growth attenuation in wild-type C57BL/6 mice despite being severely attenuated in iNOS-deficient mice (Fig. 4 and 5). The cim mutants do not appear to be killed in iNOS^–/– mice, but they are apparently incapable of progressive growth; indeed, the growth profiles of the cim mutants are very similar in C57BL/6 and iNOS^–/– mice, in sharp contrast to wild-type M. tuberculosis, which replicates much more extensively in iNOS^–/– mice than in C57BL/6 mice (Fig. 4 and 5). We anticipated this possibility, which is why we chose to screen mutant pools directly in immunodeficient mice. Several explanations are possible. The effect of immune mechanisms that are engaged at later stages of infection, after iNOS has already come into play, might be masked by the dominant effect of iNOS. It is also possible that alternative pathways that are upregulated by IFN- only in the absence of iNOS (18) might exert an antibacterial effect only in that context. Direct screening in immunodeficient mice will be particularly useful for identification of M. tuberculosis genes that permit continued bacterial growth in the face of otherwise bacteriostatic immune mechanisms activated by IFN-; it is unlikely that such mutants would be identified by screening in C57BL/6 mice due to the masking effect of iNOS.

One of the cim genes, Rv0405, was previously identified as an M. tuberculosis virulence gene (5). Rv0405 is predicted to encode polyketide synthase 6 (Pks6); it is 1 of 16 genes in the M. tuberculosis genome that have been described as encoding putative polyketide synthases (10), although other polyketide synthases continue to be identified (36). To date, only two of the M. tuberculosis polyketide synthases have been assigned biochemical functions: Pks2 (39) and Pks15/1 (11), involved in the biosynthesis of cell wall sulfolipids and phenolglycolipids, respectively. Although the polyketide synthesized by Pks6 has not been identified, a number of polyketides produced by other organisms, such as FK506, rapamycin, and cyclosporine, are potent immunosuppressants (7). We speculate that Pks6 might be involved in synthesis of a polyketide product(s) that interferes with the IFN--dependent arm of the antimycobacterial immune response, which would explain why pks6 is apparently dispensable for bacterial virulence in the absence of IFN- (Fig. 4 and 5).

Rv2958c and its Mycobacterium leprae homolog were identified previously in screens for genes that enhanced the survival of the nonpathogenic saprophyte M. smegmatis following phagocytosis by macrophages (26, 29). Rv2958c is one of several genes in the M. tuberculosis genome that are predicted to encode putative glycosyl transferases (10); these enzymes might be involved in the glycosylation of cell surface lipids or proteins that interact with host cells. Defects in glycosylation of secreted and surface proteins have been shown to affect the immune response to these M. tuberculosis antigens (37). Rv2958c is located within the genomic region that includes pks15/1. This gene cluster is conserved in order and sequence between M. tuberculosis and M. leprae, suggesting that these genes may function in a common pathway for the production of cell wall phenolglycolipids (11).

Rv0072 is predicted to encode a component of a putative ABC transporter. ABC transporters are large multisubunit permeases which function in eukaryotic cells as exporters and in prokaryotes as both importers and exporters (3). ABC transporters consist of two membrane-spanning domains (MSDs) and two nucleotide-binding domains. Prokaryotic ABC transporters often include an additional high-affinity substrate-binding protein that specifies the cargo. Rv0072 encodes one of two annotated MSD subunits for a putative glutamine transporter. The transposon insertion in Rv0072 may also exert polar effects on the downstream gene, Rv0073, which is predicted to encode the glutamine transporter's nucleotide-binding domain subunit. M. tuberculosis is a glutamine prototroph, and previous studies demonstrated that the M. tuberculosis glutamine synthetase, GlnA1, is important for survival in macrophages and in guinea pigs (26, 43), indicating that the phagosomal concentration of glutamine is too low to support bacterial metabolism. Thus, it is unclear what role glutamine transport might play in M. tuberculosis pathogenesis. In other species of bacteria, ABC transporters have been implicated in the transduction of information from the extracellular milieu, leading to adaptive changes in bacterial gene expression (16, 23, 35, 44). We speculate that the very low levels of glutamine in the macrophage phagosome (26, 43) might serve as an environmental cue that is transduced by the glutamine transporter to the appropriate cellular response pathways, resulting in adaptation of the organism to IFN--dependent changes in the intracellular environment.

The interplay between the immune response and persistent pathogens like M. tuberculosis is a fine-tuned balance of assaults by the host and bacterial countermeasures (30). Over thousands of years of coexistence with mammalian hosts, there has presumably been strong evolutionary pressure for pathogenic mycobacteria to evolve countermeasures against each of the potentially bactericidal elements of the host immune response. The counterimmune mechanisms of M. tuberculosis might present a potentially interesting target for therapy. Inhibitors targeting these pathways would promote the clearance of infection not by killing the organism directly but by increasing its vulnerability to the host immune response.

 
We thank Peter Giannakas for expert technical assistance with animal experiments.

K.H. was supported by NIH MSTP grant GM07739 and the William Randolph Hearst Endowed Scholarship Fund for the Tri-Institutional MD/PhD Program. J.D.M. gratefully acknowledges support from the Sequella Global Tuberculosis Foundation, the Ellison Medical Foundation, the Sinsheimer Fund, and the Irma T. Hirschl Trust. This work was supported by National Institutes of Health grant AI051702 (J.D.M.).

 
* Corresponding author. Mailing address: Laboratory of Infection Biology, The Rockefeller University, 1230 York Ave., New York, NY 10021. Phone: (212) 327-7081. Fax: (212) 327-7083. E-mail: mckinney{at}rockefeller.edu.

Editor: S. H. E. Kaufmann

Top Abstract Introduction Materials and Methods Results Discussion References

 

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Infection and Immunity, September 2004, p. 5315-5321, Vol. 72, No. 9
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.9.5315-5321.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

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