Phase variation in Mycobacterium tuberculosis glpK produces transiently heritable drug tolerance.

The length and complexity of tuberculosis (TB) therapy, as well as the propensity of Mycobacterium tuberculosis to develop drug resistance, are major barriers to global TB control efforts. M. tuberculosis is known to have the ability to enter into a drug-tolerant state, which may explain many of these impediments to TB treatment. We have identified a mechanism of genetically encoded but rapidly reversible drug tolerance in M. tuberculosis caused by transient frameshift mutations in a homopolymeric tract (HT) of 7 cytosines (7C) in the glpK gene. Inactivating frameshift mutations associated with the 7C HT in glpK produce small colonies that exhibit heritable multidrug increases in minimal inhibitory concentrations and decreases in drug-dependent killing; however, reversion back to a fully drug-susceptible large-colony phenotype occurs rapidly through the introduction of additional insertions or deletions in the same glpK HT region. These reversible frameshift mutations in the 7C HT of M. tuberculosis glpK occur in clinical isolates, accumulate in M. tuberculosis-infected mice with further accumulation during drug treatment, and exhibit a reversible transcriptional profile including induction of dosR and sigH and repression of kstR regulons, similar to that observed in other in vitro models of M. tuberculosis tolerance. These results suggest that GlpK phase variation may contribute to drug tolerance, treatment failure, and relapse in human TB. Drugs effective against phase-variant M. tuberculosis may hasten TB treatment and improve cure rates.

Rapidly Correcting Frameshift Mutations in the Mycobacterium tuberculosis orn Gene Produce Reversible Ethambutol Resistance and Small-Colony-Variant Morphology.

We have identified a previously unknown mechanism of reversible high-level ethambutol (EMB) resistance in Mycobacterium tuberculosis that is caused by a reversible frameshift mutation in the M. tuberculosisorn gene. A frameshift mutation in orn produces the small-colony-variant (SCV) phenotype, but this mutation does not change the MICs of any drug for wild-type M. tuberculosis However, the same orn mutation in a low-level EMB-resistant double embB-aftA mutant (MIC = 8 µg/ml) produces an SCV with an EMB MIC of 32 µg/ml. Reversible resistance is indistinguishable from a drug-persistent phenotype, because further culture of these orn-embB-aftA SCV mutants results in rapid reversion of the orn frameshifts, reestablishing the correct orn open reading frame, returning the culture to normal colony size, and reversing the EMB MIC back to that (8 µg/ml) of the parental strain. Transcriptomic analysis of orn-embB-aftA mutants compared to wild-type M. tuberculosis identified a 27-fold relative increase in the expression of embC, which is a cellular target for EMB. Expression of embC in orn-embB-aftA mutants was also increased 5-fold compared to that in the parental embB-aftA mutant, whereas large-colony orn frameshift revertants of the orn-embB-aftA mutant had levels of embC expression similar to that of the parental embB-aftA strain. Reversible frameshift mutants may contribute to a reversible form of microbiological drug resistance in human tuberculosis.

High Systemic Exposure of Pyrazinoic Acid Has Limited Antituberculosis Activity in Murine and Rabbit Models of Tuberculosis.

Pyrazinamide (PZA) is a prodrug requiring conversion to pyrazinoic acid (POA) by an amidase encoded by pncA for in vitro activity. Mutation of pncA is the most common cause of PZA resistance in clinical isolates. To determine whether the systemic delivery of POA or host-mediated conversion of PZA to POA could circumvent such resistance, we evaluated the efficacy of orally administered and host-derived POA in vivo Dose-ranging plasma and intrapulmonary POA pharmacokinetics and the efficacy of oral POA or PZA treatment against PZA-susceptible tuberculosis were determined in BALB/c and C3HeB/FeJ mice. The activity of host-derived POA was assessed in rabbits infected with a pncA-null mutant and treated with PZA. Median plasma POA values for the area under the concentration-time curve from 0 h to infinity (AUC0-∞) were 139 to 222 µg·h/ml and 178 to 287 µg·h/ml after doses of PZA and POA of 150 mg/kg of body weight, respectively, in mice. Epithelial lining fluid POA concentrations in infected mice were comparable after POA and PZA administration. In chronically infected BALB/c mice, PZA at 150 mg/kg reduced lung CFU counts by >2 log10 after 4 weeks. POA was effective only at 450 mg/kg, which reduced lung CFU counts by ∼0.7 log10 POA had no demonstrable bactericidal activity in C3HeB/FeJ mice, nor did PZA administered to rabbits infected with a PZA-resistant mutant. Oral POA administration and host-mediated conversion of PZA to POA producing plasma POA exposures comparable to PZA administration was significantly less effective than PZA. These results suggest that the intrabacillary delivery of POA and that producing higher POA concentrations at the site of infection will be more effective strategies for maximizing POA efficacy.

Geographic Differences in the Contribution of ubiA Mutations to High-Level Ethambutol Resistance in Mycobacterium tuberculosis.

Ethambutol (EMB) resistance can evolve through a multistep process, and mutations in the ubiA (Rv3806c) gene appear to be responsible for high-level EMB resistance in Mycobacterium tuberculosis We evaluated the prevalence of ubiA and embB (Rv3795) mutations in EMB-resistant strains originating from Africa and South Korea. No differences in embB mutation frequencies were observed between strains from both origins. However, ubiA mutations were present in 45.5% ± 6.5% of the African EMB-resistant isolates but in only 9.5% ± 1.5% of the South Korean EMB-resistant isolates. The ubiA mutations associated with EMB resistance were localized to regions encoding the transmembrane domains of the protein, whereas the embB mutations were localized to regions encoding the extramembrane domains. Larger studies are needed to investigate the causes of increased ubiA mutations as a pathway to high-level EMB resistance in African countries, such as extended EMB usage during tuberculosis treatment.

Rapid, high-throughput detection of rifampin resistance and heteroresistance in Mycobacterium tuberculosis by use of sloppy molecular beacon melting temperature coding.

Rifampin resistance in Mycobacterium tuberculosis is largely determined by mutations in an 80-bp rifampin resistance determining region (RRDR) of the rpoB gene. We developed a rapid single-well PCR assay to identify RRDR mutations. The assay uses sloppy molecular beacons to probe an asymmetric PCR of the M. tuberculosis RRDR by melting temperature (T(m)) analysis. A three-point T(m) code is generated which distinguishes wild-type from mutant RRDR DNA sequences in approximately 2 h. The assay was validated on synthetic oligonucleotide targets containing the 44 most common RRDR mutations. It was then tested on a panel of DNA extracted from 589 geographically diverse clinical M. tuberculosis cultures, including isolates with wild-type RRDR sequences and 25 different RRDR mutations. The assay detected 236/236 RRDR mutant sequences as mutant (sensitivity, 100%; 95% confidence interval [CI], 98 to 100%) and 353/353 RRDR wild-type sequences as wild type (specificity, 100%; 95% CI, 98.7 to 100%). The assay identified 222/225 rifampin-resistant isolates as rifampin resistant (sensitivity, 98.7%; 95% CI, 95.8 to 99.6%) and 335/336 rifampin-susceptible isolates as rifampin susceptible (specificity, 99.7%; 95% CI, 95.8 to 99.6%). All mutations were either individually identified or clustered into small mutation groups using the triple T(m) code. The assay accurately identified mixed (heteroresistant) samples and was shown analytically to detect RRDR mutations when present in at least 40% of the total M. tuberculosis DNA. This was at least as accurate as Sanger DNA sequencing. The assay was easy to use and well suited for high-throughput applications. This new sloppy molecular beacon assay should greatly simplify rifampin resistance testing in clinical laboratories.

Origin and Dynamics of Mycobacterium tuberculosis Subpopulations That Predictably Generate Drug Tolerance and Resistance.

Initial responses to tuberculosis treatment are poor predictors of final therapeutic outcomes in drug-susceptible disease, suggesting that treatment success depends on features that are hidden within a small minority of the overall infecting Mycobacterium tuberculosis population. We developed a multitranswell robotic system to perform numerous parallel cultures of genetically barcoded M. tuberculosis exposed to steady-state concentrations of rifampicin to uncover these difficult-to-eliminate minority populations. We found that tolerance emerged repeatedly from at least two subpopulations of barcoded cells, namely, one that could not grow on solid agar media and a second that could form colonies, but whose kill curves diverged from the general bacterial population within 4 and 16 days of drug exposure, respectively. These tolerant subpopulations reproducibly passed through a phase characterized by multiple unfixed resistance mutations followed by emergent drug resistance in some cultures. Barcodes associated with drug resistance identified an especially privileged subpopulation that was rarely eliminated despite 20 days of drug treatment even in cultures that did not contain any drug-resistant mutants. The association of this evolutionary scenario with a defined subset of barcodes across multiple independent cultures suggested a transiently heritable phenotype, and indeed, glpK phase variation mutants were associated with up to 16% of the resistant cultures. Drug tolerance and resistance were eliminated in a ΔruvA mutant, consistent with the importance of bacterial stress responses. This work provides a window into the origin and dynamics of bacterial drug-tolerant subpopulations whose elimination may be critical for developing rapid and resistance-free cures. IMPORTANCE Tuberculosis is unusual among bacterial diseases in that treatments which can rapidly resolve symptoms do not predictably lead to a durable cure unless treatment is continued for months after all clinical and microbiological signs of disease have been eradicated. Using a novel steady-state antibiotic exposure system combined with chromosomal barcoding, we identified small hidden Mycobacterium tuberculosis subpopulations that repeatedly enter a state of drug tolerance with a predisposition to develop fixed drug resistance after first developing a cloud of unfixed resistance mutations. The existence of these difficult-to-eradicate subpopulations may explain the need for extended treatment regimen for tuberculosis. Their identification provides opportunities to test genetic and therapeutic approaches that may result in shorter and more effective TB treatments.

Allelic exchange and mutant selection demonstrate that common clinical embCAB gene mutations only modestly increase resistance to ethambutol in Mycobacterium tuberculosis.

Mutations within codon 306 of the Mycobacterium tuberculosis embB gene modestly increase ethambutol (EMB) MICs. To identify other causes of EMB resistance and to identify causes of high-level resistance, we generated EMB-resistant M. tuberculosis isolates in vitro and performed allelic exchange studies of embB codon 406 (embB406) and embB497 mutations. In vitro selection produced mutations already identified clinically in embB306, embB397, embB497, embB1024, and embC13, which result in EMB MICs of 8 or 14 microg/ml, 5 microg/ml, 12 microg/ml, 3 microg/ml, and 4 microg/ml, respectively, and mutations at embB320, embB324, and embB445, which have not been identified in clinical M. tuberculosis isolates and which result in EMB MICs of 8 microg/ml, 8 microg/ml, and 2 to 8 microg/ml, respectively. To definitively identify the effect of the common clinical embB497 and embB406 mutations on EMB susceptibility, we created a series of isogenic mutants, exchanging the wild-type embB497 CAG codon in EMB-susceptible M. tuberculosis strain 210 for the embB497 CGG codon and the wild-type embB406 GGC codon for either the embB406 GCC, embB406 TGC, embB406 TCC, or embB406 GAC codon. These new mutants showed 6-fold and 3- to 3.5-fold increases in the EMB MICs, respectively. In contrast to the embB306 mutants, the isogenic embB497 and embB406 mutants did not have preferential growth in the presence of isoniazid or rifampin (rifampicin) at their MICs. These results demonstrate that individual embCAB mutations confer low to moderate increases in EMB MICs. Discrepancies between the EMB MICs of laboratory mutants and clinical M. tuberculosis strains with identical mutations suggest that clinical EMB resistance is multigenic and that high-level EMB resistance requires mutations in currently unknown loci.

Rapid detection of Mycobacterium tuberculosis and rifampin resistance by use of on-demand, near-patient technology.

Current nucleic acid amplification methods to detect Mycobacterium tuberculosis are complex, labor-intensive, and technically challenging. We developed and performed the first analysis of the Cepheid Gene Xpert System's MTB/RIF assay, an integrated hands-free sputum-processing and real-time PCR system with rapid on-demand, near-patient technology, to simultaneously detect M. tuberculosis and rifampin resistance. Analytic tests of M. tuberculosis DNA demonstrated a limit of detection (LOD) of 4.5 genomes per reaction. Studies using sputum spiked with known numbers of M. tuberculosis CFU predicted a clinical LOD of 131 CFU/ml. Killing studies showed that the assay's buffer decreased M. tuberculosis viability by at least 8 logs, substantially reducing biohazards. Tests of 23 different commonly occurring rifampin resistance mutations demonstrated that all 23 (100%) would be identified as rifampin resistant. An analysis of 20 nontuberculosis mycobacteria species confirmed high assay specificity. A small clinical validation study of 107 clinical sputum samples from suspected tuberculosis cases in Vietnam detected 29/29 (100%) smear-positive culture-positive cases and 33/39 (84.6%) or 38/53 (71.7%) smear-negative culture-positive cases, as determined by growth on solid medium or on both solid and liquid media, respectively. M. tuberculosis was not detected in 25/25 (100%) of the culture-negative samples. A study of 64 smear-positive culture-positive sputa from retreatment tuberculosis cases in Uganda detected 63/64 (98.4%) culture-positive cases and 9/9 (100%) cases of rifampin resistance. Rifampin resistance was excluded in 54/55 (98.2%) susceptible cases. Specificity rose to 100% after correcting for a conventional susceptibility test error. In conclusion, this highly sensitive and simple-to-use system can detect M. tuberculosis directly from sputum in less than 2 h.

Transcriptional regulation of multi-drug tolerance and antibiotic-induced responses by the histone-like protein Lsr2 in M. tuberculosis.

Multi-drug tolerance is a key phenotypic property that complicates the sterilization of mammals infected with Mycobacterium tuberculosis. Previous studies have established that iniBAC, an operon that confers multi-drug tolerance to M. bovis BCG through an associated pump-like activity, is induced by the antibiotics isoniazid (INH) and ethambutol (EMB). An improved understanding of the functional role of antibiotic-induced genes and the regulation of drug tolerance may be gained by studying the factors that regulate antibiotic-mediated gene expression. An M. smegmatis strain containing a lacZ gene fused to the promoter of M. tuberculosis iniBAC (PiniBAC) was subjected to transposon mutagenesis. Mutants with constitutive expression and increased EMB-mediated induction of PiniBAC::lacZ mapped to the lsr2 gene (MSMEG6065), a small basic protein of unknown function that is highly conserved among mycobacteria. These mutants had a marked change in colony morphology and generated a new polar lipid. Complementation with multi-copy M. tuberculosis lsr2 (Rv3597c) returned PiniBAC expression to baseline, reversed the observed morphological and lipid changes, and repressed PiniBAC induction by EMB to below that of the control M. smegmatis strain. Microarray analysis of an lsr2 knockout confirmed upregulation of M. smegmatis iniA and demonstrated upregulation of genes involved in cell wall and metabolic functions. Fully 121 of 584 genes induced by EMB treatment in wild-type M. smegmatis were upregulated ("hyperinduced") to even higher levels by EMB in the M. smegmatis lsr2 knockout. The most highly upregulated genes and gene clusters had adenine-thymine (AT)-rich 5-prime untranslated regions. In M. tuberculosis, overexpression of lsr2 repressed INH-mediated induction of all three iniBAC genes, as well as another annotated pump, efpA. The low molecular weight and basic properties of Lsr2 (pI 10.69) suggested that it was a histone-like protein, although it did not exhibit sequence homology with other proteins in this class. Consistent with other histone-like proteins, Lsr2 bound DNA with a preference for circular DNA, forming large oligomers, inhibited DNase I activity, and introduced a modest degree of supercoiling into relaxed plasmids. Lsr2 also inhibited in vitro transcription and topoisomerase I activity. Lsr2 represents a novel class of histone-like proteins that inhibit a wide variety of DNA-interacting enzymes. Lsr2 appears to regulate several important pathways in mycobacteria by preferentially binding to AT-rich sequences, including genes induced by antibiotics and those associated with inducible multi-drug tolerance. An improved understanding of the role of lsr2 may provide important insights into the mechanisms of action of antibiotics and the way that mycobacteria adapt to stresses such as antibiotic treatment.

Lack of association of novel mutation Asp397Gly in aftB gene with ethambutol resistance in clinical isolates of Mycobacterium tuberculosis.

To discover additional genotypic indicators for ethambutol (EMB) resistant M. tuberculosis, we studied polymorphisms in arabinofuranosyl transferase encoding genes aftA (Rv3792), aftB (Rv3805) and aftC (Rv2673) in 38 EMB resistant and 34 EMB susceptible isolates from India and a repository established by the World Health Organization (WHO) Special Programme for Research and Training in Tropical Disease (TDR) by DNA sequencing. The results were correlated with the minimum inhibitory concentration (MIC) of EMB and mutations in embB (Rv3795). The most common non-synonymous polymorphism identified in aftB was Asp397Gly in 12/38 (31.6%) EMB resistant and 3/34 (8.8%) EMB susceptible isolates. Interestingly, 10/12 (83.3%) EMB resistant isolates with aftB Asp397Gly mutation also carried embB306, embB402 or embB497 mutations. Association of Asp397Gly polymorphism with EMB resistance was statistically significant (p 0.0216). However, overexpression of the mutant aftB in M. tuberculosis H37Rv did not exhibit any change in the MIC. Whole genome sequencing of a panel of Indian isolates and SNP cluster grouping (SCG) of TDR strains revealed an association between aftB mutation Asp397Gly and Beijing genotype or SCG2, a cluster group representing the Beijing genotype. To conclude, though aftBAsp397Gly mutation is not associated with EMB resistance, this mutation may be a phylogenetic marker for the Beijing clade.

Transfer of embB codon 306 mutations into clinical Mycobacterium tuberculosis strains alters susceptibility to ethambutol, isoniazid, and rifampin.

Implicated as a major mechanism of ethambutol (EMB) resistance in clinical studies of Mycobacterium tuberculosis, mutations in codon 306 of the embB gene (embB306) have also been detected in EMB-susceptible clinical isolates. Other studies have found strong associations between embB306 mutations and multidrug resistance, but not EMB resistance. We performed allelic exchange studies in EMB-susceptible and EMB-resistant clinical M. tuberculosis isolates to identify the role of embB306 mutations in any type of drug resistance. Replacing wild-type embB306 ATG from EMB-susceptible clinical M. tuberculosis strain 210 with embB306 ATA, ATC, CTG, or GTG increased the EMB MIC from 2 microg/ml to 7, 7, 8.5, and 14 microg/ml, respectively. Replacing embB306 ATC or GTG from two high-level EMB-resistant clinical strains with wild-type ATG lowered EMB MICs from 20 microg/ml or 28 microg/ml, respectively, to 3 microg/ml. All parental and isogenic mutant strains had identical isoniazid (INH) and rifampin (RIF) MICs. However, embB306 CTG mutants had growth advantages compared to strain 210 at sub-MICs of INH or RIF in monocultures and at sub-MICs of INH in competition assays. CTG mutants were also more resistant to the additive or synergistic activities of INH, RIF, or EMB used in different combinations. These results demonstrate that embB306 mutations cause an increase in the EMB MIC, a variable degree of EMB resistance, and are necessary but not sufficient for high-level EMB resistance. The unusual growth property of embB306 mutants in other antibiotics suggests that they may be amplified during treatment in humans and that a single mutation may affect antibiotic susceptibility against multiple first-line antibiotics.

Rifampin resistance, Beijing-W clade-single nucleotide polymorphism cluster group 2 phylogeny, and the Rv2629 191-C allele in Mycobacterium tuberculosis strains.

Rifampin resistance is a key prognostic marker for treatment success in tuberculosis patients. Recently, Wang et al. demonstrated that Rv2629 A191C mutations were present in 99.1% of rifampin-resistant and 0% of rifampin-susceptible clinical Mycobacterium tuberculosis isolates and that overexpression of the Rv2629 191C allele in Mycobacterium smegmatis produced an eightfold increase in rifampin resistance. These results suggested that Rv2629 could be a cause of rifampin resistance and a valuable target for rifampin resistance detection assays. We developed a molecular-beacon assay to study the association between Rv2629 191 alleles and rifampin resistance in 246 geographically and phylogenetically diverse clinical M. tuberculosis isolates. The 191C allele was present in 30/98 (30.6%) rifampin-resistant isolates and 25/148 (16.9%) rifampin-susceptible isolates and was more common in isolates from Asia. Phylogenetic analysis demonstrated complete overlap between the 191C allele and single nucleotide polymorphism cluster group 2 (SCG-2), a phylogenetic lineage that corresponds to the Beijing-W clade of M. tuberculosis. All 55 (100%) 191C isolates were SCG-2, while none of the 191 191A isolates were SCG-2 (P < 0.001). No association was found between the 191C allele and rifampin resistance in an analysis that included the SCG type (P = 1.0). Also, in contrast to the findings of Wang et al., we found that overexpression of either Rv2629 191 allele in M. smegmatis did not produce an increase in rifampin resistance. We conclude that the Rv2629 191C allele is not associated with rifampin resistance and that the allele cannot be used as a molecular target to detect rifampin resistance. The allele appears to be an excellent marker for the Beijing-W clade/SCG-2 phylogenetic group.

Polymorphisms in Rv3806c (ubiA) and the upstream region of embA in relation to ethambutol resistance in clinical isolates of Mycobacterium tuberculosis from North India.

Mutations at embB306 are the most prevalent polymorphisms associated with ethambutol (EMB) resistance, responsible for 40-60% of EMB resistant clinical cases of tuberculosis (TB). The present study analyzed additional mutations associated with EMB resistance in the embB, embC, embA and Rv3806c (ubiA) genes in 29 EMB resistant and 29 EMB susceptible clinical isolates of M. tuberculosis selected from 360 patients with TB. The entire ubiA gene, mutational hotspot regions of embB, embC, and upstream region of embA were screened for polymorphisms by DNA sequencing and the results correlated with minimum inhibitory concentrations (MIC) of EMB. The most common polymorphism identified in ubiA was at codon 149 (GAA to GAC), occurring in 5/29 (17.2%) resistant isolates and 7/29 (24%) susceptible isolates. Mutations in embB were most common at codon 306 (ATG to ATC/GTG), occurring only in EMB resistant isolates (20/29; 69%). Mutations in the upstream region of embA at -8, -11, -12 and -60 codons also occurred in EMB resistant strains (8/29; 27.5%) of which 6/8 (75%) were observed in isolates with EMB MIC ≥16 µg/ml. Though no polymorphisms associated with EMB resistance were identified in ubiA, polymorphisms upstream to embA may contribute to high level EMB resistance.

Spectrum of manifestations of Mycobacterium tuberculosis infection in primates infected with SIV.

To characterize the manifestations of coinfection with M. tuberculosis and SIV infection, we studied 12 SIV-infected rhesus monkeys, six of which were infected intrabronchially with a low dose of Mycobacterium tuberculosis H37Rv. In the six coinfected animals, M. tuberculosis antigen-stimulated lung and blood cells produced high concentrations of IFN-gamma but not IL-4 8-16 weeks after infection. Of the three coinfected animals with high levels of plasma viremia, two developed disseminated tuberculosis and the other died of bacterial peritonitis. Of three coinfected animals with moderate levels of plasma viremia, two had no clinical or radiographic evidence of tuberculosis or progressive SIV infection for 6 months after infection. At neuropsy, pulmonary granulomata were observed and acid-fast organisms or M. tuberculosis were present. These clinical, immunologic and pathologic findings are consistent with those in humans with latent tuberculosis infection (LTBI), and suggest that a model of LTBI in SIV-infected primates can be developed. Such a model will permit delineation of the immunologic and microbial factors that characterize LTBI in HIV-infected persons.

Evolution of high-level ethambutol-resistant tuberculosis through interacting mutations in decaprenylphosphoryl-ß-D-arabinose biosynthetic and utilization pathway genes.

To study the evolution of drug resistance, we genetically and biochemically characterized Mycobacterium tuberculosis strains selected in vitro for ethambutol resistance. Mutations in decaprenylphosphoryl-ß-D-arabinose (DPA) biosynthetic and utilization pathway genes Rv3806c, Rv3792, embB and embC accumulated to produce a wide range of ethambutol minimal inhibitory concentrations (MICs) that depended on mutation type and number. Rv3806c mutations increased DPA synthesis, causing MICs to double from 2 to 4 µg/ml in a wild-type background and to increase from 16 to 32 µg/ml in an embB codon 306 mutant background. Synonymous mutations in Rv3792 increased the expression of downstream embC, an ethambutol target, resulting in MICs of 8 µg/ml. Multistep selection was required for high-level resistance. Mutations in embC or very high embC expression were observed at the highest resistance level. In clinical isolates, Rv3806c mutations were associated with high-level resistance and had multiplicative effects with embB mutations on MICs. Ethambutol resistance is acquired through the acquisition of mutations that interact in complex ways to produce a range of MICs, from those falling below breakpoint values to ones representing high-level resistance.

Role of NK cell-activating receptors and their ligands in the lysis of mononuclear phagocytes infected with an intracellular bacterium.

We studied the role of NK cell-activating receptors and their ligands in the lysis of mononuclear phagocytes infected with the intracellular pathogen Mycobacterium tuberculosis. Expression of the activating receptors NKp30, NKp46, and NKG2D were enhanced on NK cells by exposure to M. tuberculosis-infected monocytes, whereas expression of DNAX accessory molecule-1 and 2B4 was not. Anti-NKG2D and anti-NKp46 inhibited NK cell lysis of M. tuberculosis-infected monocytes, but Abs to NKp30, DNAX accessory molecule-1, and 2B4 had no effect. Infection of monocytes up-regulated expression of the NKG2D ligand, UL-16 binding protein (ULBP)1, but not expression of ULBP2, ULBP3, or MHC class I-related chain A or chain B. Up-regulation of ULBP1 on infected monocytes was dependent on TLR2, and anti-ULBP1 abrogated NK cell lysis of infected monocytes. The dominant roles of NKp46, NKG2D, and ULBP1 were confirmed for NK cell lysis of M. tuberculosis-infected alveolar macrophages. We conclude that NKp46 and NKG2D are the principal receptors involved in lysis of M. tuberculosis-infected mononuclear phagocytes, and that ULBP1 on infected cells is the major ligand for NKG2D. Furthermore, TLR2 contributes to up-regulation of ULBP1 expression.

Cyclic AMP response element-binding protein positively regulates production of IFN-gamma by T cells in response to a microbial pathogen.

IFN-gamma is essential for resistance to many intracellular pathogens, including Mycobacterium tuberculosis. Transcription of the IFN-gamma gene in activated T cells is controlled by the proximal promoter element (-73 to -48 bp). CREB binds to the IFN-gamma proximal promoter, and binding is enhanced by phosphorylation of CREB. Studies in human T cell lines and in transgenic mice have yielded conflicting results about whether CREB is a positive or a negative regulator of IFN-gamma transcription. To determine the role of CREB in mediating IFN-gamma production in response to a microbial pathogen, we evaluated the peripheral blood T cell response to M. tuberculosis in healthy tuberculin reactors. EMSAs, chromatin immunoprecipitation, and Western blotting demonstrated that stimulation of PBMC with M. tuberculosis induced phosphorylation and enhanced binding of CREB to the IFN-gamma proximal promoter. Neutralization of CREB with intracellular Abs or down-regulation of CREB levels with small interfering RNA decreased M. tuberculosis-induced production of IFN-gamma and IFN-gamma mRNA expression. In addition, M. tuberculosis-stimulated T cells from tuberculosis patients, who have ineffective immunity, showed diminished IFN-gamma production, reduced amounts of CREB binding to the IFN-gamma proximal promoter, and absence of phosphorylated CREB. These findings demonstrate that CREB positively regulates IFN-gamma production by human T cells that respond to M. tuberculosis.

Role of embB codon 306 mutations in Mycobacterium tuberculosis revisited: a novel association with broad drug resistance and IS6110 clustering rather than ethambutol resistance.

Mutations at position 306 of embB (embB306) have been proposed as a marker for ethambutol resistance in Mycobacterium tuberculosis; however, recent reports of embB306 mutations in ethambutol-susceptible isolates caused us to question the biological role of this mutation. We tested 1,020 clinical M. tuberculosis isolates with different drug susceptibility patterns and of different geographical origins for associations between embB306 mutations, drug resistance patterns, and major genetic group. One hundred isolates (10%) contained a mutation in embB306; however, only 55 of these mutants were ethambutol resistant. Mutations in embB306 could not be uniquely associated with any particular type of drug resistance and were found in all three major genetic groups. A striking association was observed between these mutations and resistance to any drug (P < 0.001), and the association between embB306 mutations and resistance to increasing numbers of drugs was highly significant (P < 0.001 for trend). We examined the association between embB306 mutations and IS6110 clustering (as a proxy for transmission) among all drug-resistant isolates. Mutations in embB306 were significantly associated with clustering by univariate analysis (odds ratio, 2.44; P = 0.004). In a multivariate model that also included mutations in katG315, katG463, gyrA95, and kasA269, only mutations in embB306 (odds ratio, 2.14; P = 0.008) and katG315 (odds ratio, 1.99; P = 0.015) were found to be independently associated with clustering. In conclusion, embB306 mutations do not cause classical ethambutol resistance but may predispose M. tuberculosis isolates to the development of resistance to increasing numbers of antibiotics and may increase the ability of drug-resistant isolates to be transmitted between subjects.

NK cells regulate CD8+ T cell effector function in response to an intracellular pathogen.

We studied the role of NK cells in regulating human CD8+ T cell effector function against mononuclear phagocytes infected with the intracellular pathogen Mycobacterium tuberculosis. Depletion of NK cells from PBMC of healthy tuberculin reactors reduced the frequency of M. tuberculosis-responsive CD8+IFN-gamma+ cells and decreased their capacity to lyse M. tuberculosis-infected monocytes. The frequency of CD8+ IFN-gamma+ cells was restored by soluble factors produced by activated NK cells and was dependent on IFN-gamma, IL-15, and IL-18. M. tuberculosis-activated NK cells produced IFN-gamma, activated NK cells stimulated infected monocytes to produce IL-15 and IL-18, and production of IL-15 and IL-18 were inhibited by anti-IFN-gamma. These findings suggest that NK cells maintain the frequency of M. tuberculosis-responsive CD8+IFN-gamma+ T cells by producing IFN-gamma, which elicits secretion of IL-15 and IL-18 by monocytes. These monokines in turn favor expansion of Tc1 CD8+ T cells. The capacity of NK cells to prime CD8+ T cells to lyse M. tuberculosis-infected target cells required cell-cell contact between NK cells and infected monocytes and depended on interactions between the CD40 ligand on NK cells and CD40 on infected monocytes. NK cells link the innate and the adaptive immune responses by optimizing the capacity of CD8+ T cells to produce IFN-gamma and to lyse infected cells, functions that are critical for protective immunity against M. tuberculosis and other intracellular pathogens.