Tsungda Hsu

Growth of Mycobacterium tuberculosis biofilms containing free mycolic acids and harbouring drug‐tolerant bacteria

Successful treatment of human tuberculosis requires 6–9u2003months therapy with multiple antibiotics. Incomplete clearance of tubercle bacilli frequently results in disease relapse, presumably as a result of reactivation of persistent drug‐tolerant Mycobacterium tuberculosis cells, although the nature and location of these persisters are not known. In other pathogens, antibiotic tolerance is often associated with the formation of biofilms – organized communities of surface‐attached cells – but physiologically and genetically defined M. tuberculosis biofilms have not been described. Here, we show that M. tuberculosis forms biofilms with specific environmental and genetic requirements distinct from those for planktonic growth, which contain an extracellular matrix rich in free mycolic acids, and harbour an important drug‐tolerant population that persist despite exposure to high levels of antibiotics.

Mycobacterium tuberculosis nuoG Is a Virulence Gene That Inhibits Apoptosis of Infected Host Cells

The survival and persistence of Mycobacterium tuberculosis depends on its capacity to manipulate multiple host defense pathways, including the ability to actively inhibit the death by apoptosis of infected host cells. The genetic basis for this anti-apoptotic activity and its implication for mycobacterial virulence have not been demonstrated or elucidated. Using a novel gain-of-function genetic screen, we demonstrated that inhibition of infection-induced apoptosis of macrophages is controlled by multiple genetic loci in M. tuberculosis. Characterization of one of these loci in detail revealed that the anti-apoptosis activity was attributable to the type I NADH-dehydrogenase of M. tuberculosis, and was mainly due to the subunit of this multicomponent complex encoded by the nuoG gene. Expression of M. tuberculosis nuoG in nonpathogenic mycobacteria endowed them with the ability to inhibit apoptosis of infected human or mouse macrophages, and increased their virulence in a SCID mouse model. Conversely, deletion of nuoG in M. tuberculosis ablated its ability to inhibit macrophage apoptosis and significantly reduced its virulence in mice. These results identify a key component of the genetic basis for an important virulence trait of M. tuberculosis and support a direct causal relationship between virulence of pathogenic mycobacteria and their ability to inhibit macrophage apoptosis.

Mycolic Acid Modification by the mmaA4 Gene of M. tuberculosis Modulates IL-12 Production

Mycobacterium tuberculosis has evolved many strategies to evade elimination by the host immune system, including the selective repression of macrophage IL-12p40 production. To identify the M. tuberculosis genes responsible for this aspect of immune evasion, we used a macrophage cell line expressing a reporter for IL-12p40 transcription to screen a transposon library of M. tuberculosis for mutants that lacked this function. This approach led to the identification of the mmaA4 gene, which encodes a methyl transferase required for introducing the distal oxygen-containing modifications of mycolic acids, as a key locus involved in the repression of IL-12p40. Mutants in which mmaA4 (hma) was inactivated stimulated macrophages to produce significantly more IL-12p40 and TNF-α than wild-type M. tuberculosis and were attenuated for virulence. This attenuation was not seen in IL-12p40-deficient mice, consistent with a direct linkage between enhanced stimulation of IL-12p40 by the mutant and its reduced virulence. Treatment of macrophages with trehalose dimycolate (TDM) purified from the ΔmmaA4 mutant stimulated increased IL-12p40, similar to the increase observed from ΔmmaA4 mutant-infected macrophages. In contrast, purified TDM isolated from wild-type M. tuberculosis inhibited production of IL-12p40 by macrophages. These findings strongly suggest that M. tuberculosis has evolved mmaA4-derived mycolic acids, including those incorporated into TDM to manipulate IL-12-mediated immunity and virulence.

Potential role for ESAT6 in dissemination of M. tuberculosis via human lung epithelial cells

ESAT6 has recently been demonstrated to cause haemolysis and macrophage lysis. Our studies demonstrate that ESAT6 causes cytolysis of type 1 and type 2 pneumocytes. Both types of pneumocytes express membrane laminin, and ESAT6 exhibits dose‐dependent binding to both cell types and to purified human laminin. While minimal ESAT6 was detected on the surface of Mycobacterium tuberculosis grown in vitro, exogenously provided ESAT6 specifically associated with the bacterial cell surface, and the bacterium‐associated ESAT6 retained its cytolytic ability. esat6 transcripts were upregulated ∼4‐ to ∼13‐fold in bacteria replicating in type 1 cells, and ∼3‐ to ∼5 fold in type 2 cells. In vivo, laminin is primarily concentrated at the basolateral surface of pneumocytes where they rest on the basement membrane, which is composed primarily of laminin and collagen. The upregulation of esat6 transcripts in bacteria replicating in pneumocytes, the specific association of ESAT6 with the bacterial surface, the binding of ESAT6 to laminin and the lysis of pneumocytes by free and bacterium‐associated ESAT6 together suggest a scenario wherein Mycobacterium tuberculosis replicating in pneumocytes may utilize surface ESAT6 to anchor onto the basolateral laminin‐expressing surface of the pneumocytes, and damage the cells and the basement membrane to directly disseminate through the alveolar wall.

Efficacy and immunogenicity of Mycobacterium bovis ΔRD1 against aerosol M. bovis infection in neonatal calves

An attenuated Mycobacterium bovisRD1 deletion (DeltaRD1) mutant of the Ravenel strain was constructed, characterized, and sequenced. This M. bovis DeltaRD1 vaccine strain administered to calves at 2 weeks of age provided similar efficacy as M. bovis bacillus Calmette Guerin (BCG) against low dose, aerosol challenge with virulent M. bovis at 3.5 months of age. Approximately 4.5 months after challenge, both DeltaRD1- and BCG-vaccinates had reduced tuberculosis (TB)-associated pathology in lungs and lung-associated lymph nodes and M. bovis colonization of tracheobronchial lymph nodes as compared to non-vaccinates. Mean central memory responses elicited by either DeltaRD1 or BCG prior to challenge correlated with reduced pathology and bacterial colonization. Neither DeltaRD1 or BCG elicited IFN-gamma responses to rESAT-6:CFP-10 prior to challenge, an emerging tool for modern TB surveillance programs. The DeltaRD1 strain may prove useful for bovine TB vaccine programs, particularly if additional mutations are included to improve safety and immunogenicity.

Specialized Transduction Designed for Precise High-Throughput Unmarked Deletions in Mycobacterium tuberculosis

ABSTRACT Specialized transduction has proven to be useful for generating deletion mutants in most mycobacteria, including virulent Mycobacterium tuberculosis. We have improved this system by developing (i) a single-step strategy for the construction of allelic exchange substrates (AES), (ii) a temperature-sensitive shuttle phasmid with a greater cloning capacity than phAE87, and (iii) bacteriophage-mediated transient expression of site-specific recombinase to precisely excise antibiotic markers. The methods ameliorate rate-limiting steps in strain construction in these difficult-to-manipulate bacteria. The new methods for strain construction were demonstrated to generalize to all classes of genes and chromosomal loci by generating more than 100 targeted single- or multiple-deletion substitutions. These improved methods pave the way for the generation of a complete ordered library of M. tuberculosis null strains, where each strain is deleted for a single defined open reading frame in M. tuberculosis. IMPORTANCE This work reports major advances in the methods of genetics applicable to all mycobacteria, including but not limited to virulent M. tuberculosis, which would facilitate comparative genomics to identify drug targets, genetic validation of proposed pathways, and development of an effective vaccine. This study presents all the new methods developed and the improvements to existing methods in an integrated way. The work presented in this study could increase the pace of mycobacterial genetics significantly and will immediately be of wide use. These new methods are transformative and allow for the undertaking of construction of what has been one of the most fruitful resources in model systems: a comprehensive, ordered library set of the strains, each of which is deleted for a single defined open reading frame. This work reports major advances in the methods of genetics applicable to all mycobacteria, including but not limited to virulent M. tuberculosis, which would facilitate comparative genomics to identify drug targets, genetic validation of proposed pathways, and development of an effective vaccine. This study presents all the new methods developed and the improvements to existing methods in an integrated way. The work presented in this study could increase the pace of mycobacterial genetics significantly and will immediately be of wide use. These new methods are transformative and allow for the undertaking of construction of what has been one of the most fruitful resources in model systems: a comprehensive, ordered library set of the strains, each of which is deleted for a single defined open reading frame.

Mycobacteria lacking the RD1 region do not induce necrosis in the lungs of mice lacking interferon‐γ

The genetic region of difference 1 (RD1) in Mycobacterium tuberculosis has recently been hypothesized to encode for proteins that are cytotoxic to the host cell in nature. We demonstrate here that while M.u2003tuberculosis grew progressively in the lungs of gene disrupted mice (GKO) unable to produce interferon‐γ (IFN‐γ), similar mice infected instead with M.u2003bovis bacillus Calmette–Guérin (BCG) reproducibly exhibited an obvious slowing of the disease after about 20u2003days. Closer examination of BCG‐infected GKO mice showed a florid granulomatous inflammation in the lungs, whereas similar mice infected with M.u2003tuberculosis exhibited wholesale progressive necrosis. In the BCG‐infected GKO mice large numbers of activated effector T cells, some strongly positive for the cytokine tumour necrosis factor, as well as activated natural killer cells accumulated in the lungs. To further test the hypothesis that the differences observed were directly associated with the loss of the RD1 region, it was then shown that a mutant of M.u2003tuberculosis lacking RD1 grew progressively in both normal and GKO mice but failed to induce any degree of necrosis in either animal despite reaching similar levels in the lungs. However, when mice were infected with this mutant, in which the RD1 region had been restored by complementation, wholesale necrosis of the lungs again occurred. These data support the hypothesis that proteins encoded in the RD1 region are a major cause of necrosis and contribute significantly to the pathogenesis of the disease.

Characterization of the protective T-cell response generated in CD4-deficient mice by a live attenuated Mycobacterium tuberculosis vaccine

The global epidemic of tuberculosis, fuelled by acquired immune‐deficiency syndrome, necessitates the development of a safe and effective vaccine. We have constructed a ΔRD1ΔpanCD mutant of Mycobacterium tuberculosis (mc26030) that undergoes limited replication and is severely attenuated in immunocompromised mice, yet induces significant protection against tuberculosis in wild‐type mice and even in mice that completely lack CD4+ T cells as a result of targeted disruption of their CD4 genes (CD4–/– mice). Ex vivo studies of T cells from mc26030‐immunized mice showed that these immune cells responded to protein antigens of M.u2003tuberculosis in a major histocompatibility complex (MHC) class II‐restricted manner. Antibody depletion experiments showed that antituberculosis protective responses in the lung were not diminished by removal of CD8+, T‐cell receptor γδ (TCR‐γδ+) and NK1.1+ T cells from vaccinated CD4–/– mice before challenge, implying that the observed recall and immune effector functions resulting from vaccination of CD4–/– mice with mc26030 were attributable to a population of CD4–u2003CD8– (double‐negative) TCR‐αβ+, TCR‐γδ–, NK1.1– T cells. Transfer of highly enriched double‐negative TCR‐αβ+ T cells from mc26030‐immunized CD4–/– mice into naive CD4–/– mice resulted in significant protection against an aerosol tuberculosis challenge. Enriched pulmonary double‐negative T cells transcribed significantly more interferon‐γ and interleukin‐2 mRNA than double‐negative T cells from naive mice after a tuberculous challenge. These results confirmed previous findings on the potential for a subset of MHC class II‐restricted T cells to develop and function without expression of CD4 and suggest novel vaccination strategies to assist in the control of tuberculosis in human immunodeficiency virus‐infected humans who have chronic depletion of their CD4+ T cells.

ESX1-dependent fractalkine mediates chemotaxis and Mycobacterium tuberculosis infection in humans

Summary Mycobacterium tuberculosis-induced cellular aggregation is essential for granuloma formation and may assist establishment and early spread of M. tuberculosis infection. The M. tuberculosis ESX1 mutant, which has a non-functional type VII secretion system, induced significantly less production of the host macrophage-derived chemokine fractalkine (CX3CL1). Upon infection of human macrophages ESX1-dependent fractalkine production mediated selective recruitment of CD11b+ monocytic cells and increased infection of neighbouring cells consistent with early local spread of infection. Fractalkine levels were raised in vivo at tuberculous disease sites in humans and were significantly associated with increased CD11b+ monocytic cellular recruitment and extent of granulomatous disease. These findings suggest a novel fractalkine-dependent ESX1-mediated mechanism in early tuberculous disease pathogenesis in humans. Modulation of M. tuberculosis-mediated fractalkine induction may represent a potential treatment option in the future, perhaps allowing us to switch off a key mechanism required by the pathogen to spread between cells.