Deletion of the 19kDa antigen does not alter the protective efficacy of BCG

Manipulation of BCG vaccine: a double-edged sword

Mycobacterium bovis Bacillus Calmette-Guérin (BCG), an attenuated vaccine derived from M. bovis, is the only licensed vaccine against tuberculosis (TB). Despite its protection against TB in children, the protective efficacy in pulmonary TB is variable in adolescents and adults. In spite of the current knowledge of molecular biology, immunology and cell biology, infectious diseases such as TB and HIV/AIDS are still challenges for the scientific community. Genetic manipulation facilitates the construction of recombinant BCG (rBCG) vaccine that can be used as a highly immunogenic vaccine against TB with an improved safety profile, but, still, the manipulation of BCG vaccine to improve efficacy should be carefully considered, as it can bring in both favourable and unfavourable effects. The purpose of this review is not to comprehensively review the interaction between microorganisms and host cells in order to use rBCG expressing M. tuberculosis (Mtb) immunodominant antigens that are available in the public domain, but, rather, to also discuss the limitations of rBCG vaccine, expressing heterologous antigens, during manipulation that pave the way for a promising new vaccine approach.

The lpqS knockout mutant of Mycobacterium tuberculosis is attenuated in macrophages

Lipoproteins of Mycobacterium tuberculosis (M. tuberculosis) represent an important class of cell envelop proteins. On the whole, 99 putative lipoproteins have been identified in the genome of M. tuberculosis. Earlier investigations on individual mycobacterial lipoproteins demonstrate that some lipoproteins elicit a strong immune response, while others are virulence factors in different models of infection. LpqS is an uncharacterized lipoprotein encoded by the open reading frame Rv0847 of M. tuberculosis. In the present study, we have characterized this putative lipoprotein LpqS with respect to the virulence of M. tuberculosis. A mutant of the M. tuberculosis H37Rv strain not producing LpqS (ΔlpqS) was generated by specialized transduction. The deletion mutant showed reduced growth in Sauton's minimal media and was highly sensitive to SDS and copper, compared to the wild type when grown on solid media. In vitro infection studies showed that the mutant was attenuated for growth in PMA-activated THP-1 cells. Complementation of the mutant with a single copy of the gene cloned under the hsp60 promoter partially restored the phenotype of the wild type strain H37Rv. Thus lpqS plays an important role in sensing the host macrophage environment and might be required for the intracellular survival of M. tuberculosis. Cotranscription of lpqS with the genes downstream cysK2, Rv0849 and Rv0850 was also demonstrated by Reverse transcription PCR (RT-PCR) of intergenic regions.

Virulence factors of the Mycobacterium tuberculosis complex

The Mycobacterium tuberculosis complex (MTBC) consists of closely related species that cause tuberculosis in both humans and animals. This illness, still today, remains to be one of the leading causes of morbidity and mortality throughout the world. The mycobacteria enter the host by air, and, once in the lungs, are phagocytated by macrophages. This may lead to the rapid elimination of the bacillus or to the triggering of an active tuberculosis infection. A large number of different virulence factors have evolved in MTBC members as a response to the host immune reaction. The aim of this review is to describe the bacterial genes/proteins that are essential for the virulence of MTBC species, and that have been demonstrated in an in vivo model of infection. Knowledge of MTBC virulence factors is essential for the development of new vaccines and drugs to help manage the disease toward an increasingly more tuberculosis-free world.

Mycobacterial lipopeptides elicit CD4+ CTLs in Mycobacterium tuberculosis-infected humans

In searching for immunogenic molecules with the potential to induce protective immune responses against tuberculosis, we developed an ex vivo model to study frequency, phenotype, and effector functions of human T lymphocytes recognizing hydrophobic Ags of Mycobacterium tuberculosis (M.Tb). To obtain unbiased results, we characterized T lymphocytes responding to a crude cell wall extract (chloroform methanol extract of M.Tb (M.Tb-CME)) containing a broad spectrum of mycobacterial glycolipids and lipopeptides. A significant proportion of T lymphocytes recognized M.Tb-CME (290 IFN-gamma+ T cells/10(5) PBMCs) and developed to effector memory cells as determined by the expression of CD45RO and the chemokine receptors CXCR3 and CCR5. Expanded lymphocytes fulfilled all criteria required for an efficient immune response against tuberculosis: 1) release of macrophage-activating Th1 cytokines and chemokines required for the spatial organization of local immune responses, 2) cytolytic activity against Ag-pulsed macrophages, and 3) recognition of infected macrophages and killing of the intracellular bacteria. Phenotypically, M.Tb-CME-expanded cells were CD4+ and MHC class II restricted, challenging current concepts that cytotoxic and antimicrobial effector cells are restricted to the CD8+ T cell subset. Pretreatment of M.Tb-CME with protease or chemical delipidation abrogated the biological activity, suggesting that responses were directed toward mycobacterial lipopeptides. These findings suggest that lipidated peptides are presented by M.Tb-infected macrophages and elicit CD4+ cytolytic and antimicrobial T lymphocytes. Our data support an emerging concept to include hydrophobic microbial Ags in vaccines against tuberculosis.

Progress in serodiagnosis of Mycobacterium tuberculosis infection

One-third of the world population is estimated to have Mycobacterium tuberculosis infection. Accurate and timely identification of infected individuals is critical for treatment and control. The current diagnostic methods lack the desired sensitivity and specificity, require sophisticated equipment and skilled workforce or take weeks to yield results. Diagnosis of extrapulmonary TB, TB-HIV co-infection, childhood TB and sputum smear-negative pulmonary TB pose serious challenges. Interest in developing serodiagnostic methods is increasing because detection of antibody is rapid, simple and relatively inexpensive, and does not require a living cell for detection. Three types of tests, namely screening tests to overcome diagnostic delay, specific tests for diagnosis of extrapulmonary TB and other bacteriologically negative cases, and tests for vaccine-induced immunity need critical consideration. Several factors must be considered to develop serodiagnostic methods for TB. Antigen recognition by infected individuals is highly heterogeneous due to stage of disease, differences in HLA types, strain of the bacilli, health of the patient and bacillary load. With advances in molecular biological techniques, a number of novel antigens have been identified. Some of these antigens have proven valuable in detecting specific antibodies in some of the most challenging TB patients. The best example is a fusion protein containing several M. tuberculosis proteins (e.g. CFP-10, MTB8, MTB48, MTB81 and the 38-kDa protein) which showed encouraging results in detecting antibodies in sera of patients, including TB-HIV co-infection. This review presents progress made in the serodiagnosis of TB during the last decade.

A mutant of Mycobacterium tuberculosis lacking the 19-kDa lipoprotein Rv3763 is highly attenuated in vivo but retains potent vaccinogenic properties

A mutant of Mycobacterium tuberculosis (Delta 19) lacking the 19-kDa lipoprotein grows well in culture in vitro but was shown here to be essentially incapable of any significant replication in mice, even in mice lacking the gamma interferon gene. Despite this, mice inoculated with Delta 19 were equally protected against an aerosol delivered challenge with M. tuberculosis compared to the conventional BCG vaccine. Cellular responses, including the generation of activated CD4 and CD8 cells secreting gamma interferon, were produced in similar numbers, and lung cells, particularly dendritic macrophages, exhibited high levels of Class-II MHC expression. These data show that despite being highly attenuated, the Delta 19 mutant strongly retained vaccinogenic properties.

Immunological properties of recombinant Mycobacterium bovis bacillus Calmette-Guérin strain expressing fusion protein IL-2-ESAT-6

The live vaccine Mycobacterium bovis bacillus Calmette-Guérin (BCG) provides variable efficacy against adult pulmonary tuberculosis (TB). Recombinant BCG, expressing either immunodominant antigens or Th1 cytokines, is a promising strategy for developing a new TB vaccine. However, not much is known about whether the introduction of cytokine and specific antigen genes concurrently into the BCG strain could improve the immunogenicity of BCG. In this study, a recombinant BCG strain (rBCG) expressing the fusion protein human interleukin (IL)-2 and ESAT-6 (early secreted antigenic target-6 kDa) antigen of Mycobacterium tuberculosis was constructed. Six weeks after BALB/c mice (H-2d) were immunized with 106 colony forming units (CFUs) BCG or rBCG, splenocyte proliferation was determined with MTT [3-(4,5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide] assay, IL-4 and interferon (IFN)-gamma produced by splenocytes were tested by enzyme linked immunosorbent assay (ELISA,) and the cytotoxicity of splenocytes from immunized mice to P815 cells (H-2d) expressing ESAT-6 protein was measured using CytoTox 96 Non-Radioactive Cytotoxicity Assay. Compared with native BCG-vaccinated mice, rBCG induced stronger Th1 responses that were confirmed by high lymphoproliferative responses and IFN-gamma production to culture filtrate protein (CFP) or ESAT-6 protein. Moreover, rBCG induced significant enhanced CTL responses against P815-ESAT-6 cells. Results from rBCG-immunized mice demonstrated that introducing the il-2 and esat-6 genes into BCG could enhance Th1 type immune responses to ESAT-6. Further investigation is needed by introducing other Th1 cytokines and antigens into BCG to optimize the protective efficacy against TB.

Deciphering the molecular bases of Mycobacterium tuberculosis binding to the lectin DC-SIGN reveals an underestimated complexity

Interactions between dendritic cells and Mycobacterium tuberculosis, the aetiological agent of tuberculosis in humans, are thought to be central to anti-mycobacterial immunity. We have previously shown that M. tuberculosis binds to human monocyte-derived dendritic cells mostly through the C-type lectin DC-SIGN (dendritic-cell-specific intercellular molecule-3-grabbing non-integrin)/CD209, and we have suggested that DC-SIGN may discriminate between mycobacterial species through recognition of the mannose-capping residues on the lipoglycan lipoarabinomannan of the bacterial envelope. Here, using a variety of fast- and slow-growing Mycobacterium species, we provide further evidence that mycobacteria recognition by DC-SIGN may be restricted to species of the M. tuberculosis complex. Fine analyses of the lipoarabinomannan molecules purified from these species show that the structure and amount of these molecules alone cannot account for such a preferential recognition. We propose that M. tuberculosis recognition by DC-SIGN relies on both a potential difference of accessibility of lipoarabinomannan in its envelope and, more probably, on the binding of additional ligands, possibly including lipomannan, mannose-capped arabinomannan, as well as the mannosylated 19 kDa and 45 kDa [Apa (alanine/proline-rich antigen)] glycoproteins. Altogether, our results reveal that the molecular basis of M. tuberculosis binding to DC-SIGN is more complicated than previously thought and provides further insight into the mechanisms of M. tuberculosis recognition by the immune system.

Effect of deletion or overexpression of the 19-kilodalton lipoprotein Rv3763 on the innate response to Mycobacterium tuberculosis

The 19-kDa lipoprotein of Mycobacterium tuberculosis is an important target of the innate immune response. To investigate the immune biology of this antigen in the context of the whole bacillus, we derived a recombinant M. tuberculosis H37Rv that lacked the 19-kDa-lipoprotein gene (Delta19) and complemented this strain by reintroduction of the 19-kDa-lipoprotein gene on a multicopy vector to produce Delta19::pSMT181. The Delta19 strain multiplied less well than Delta19::pSMT181 in human monocyte-derived macrophages (MDM) (P = 0.039). Surface expression of major histocompatibility complex class II molecules was reduced in phagocytes infected with M. tuberculosis; this effect was not seen in cells infected with Delta19. Delta19 induced lower interleukin 1beta (IL-1beta) secretion from monocytes and MDM. Overexpression of the 19-kDa protein increased IL-1beta, IL-12p40, and tumor necrosis factor alpha secretion irrespective of phagocyte maturity. These data support reports that the 19-kDa lipoprotein has pleiotropic effects on the interaction of M. tuberculosis with phagocytes. However, this analysis indicates that in the context of the whole bacillus, the 19-kDa lipoprotein is only one of a number of molecules that mediate the innate response to M. tuberculosis.

Increased expression of Mycobacterium tuberculosis 19 kDa lipoprotein obliterates the protective efficacy of BCG by polarizing host immune responses to the Th2 subtype

Mycobacterium tuberculosis can not only neutralize immune effector functions, but also has the ability to modulate host-signalling cascades involved in the development of these responses. The 19 kDa antigen (Rv3763), a lipoprotein of M. tuberculosis, elicits high levels of interleukin (IL)-12 from macrophages in addition to its powerful immunomodulatory properties, leading to suppression of antigen-presentation signalling cascades. The present study was aimed at analysing the effect of overexpression of this antigen on the immunostimulatory properties of M. bovis Bacille Calmette-Guerin (BCG). We have constructed a recombinant BCG strain (rBCG19N) producing higher levels of the 19 kDa antigen in both the cytoplasmic (approximately eightfold) and extracellular (approximately fivefold) fractions as compared to the wildtype BCG. Immunization of mice with rBCG19N elicited high levels of interferon-gamma (IFN-gamma) and relatively low levels of IL-10 against the purified 19 kDa antigen. However, in response to total BCG sonicate, mice immunized with rBCG19N produced significantly high levels of IL-10 with relatively very low levels of IFN-gamma. This polarization of the host immune responses towards T-helper 2 subtype resulted in complete abrogation of the protective efficacy of BCG, when rBCG19N was used as a live vaccine against M. tuberculosis challenge in guinea pigs.

Improved protection by recombinant BCG

Mycobacterium bovis bacille Calmette Guérin (BCG) is one of the most widely used live vaccines. Technologic advancement in genome manipulation enables the construction of recombinant BCG (rBCG) strains, which can be employed as highly immunogenic vaccines against tuberculosis with improved safety profile.

Lipoproteins ofMycobacterium tuberculosis: an abundant and functionally diverse class of cell envelope components

Mycobacterium tuberculosis remains the predominant bacterial scourge of mankind. Understanding of its biology and pathogenicity has been greatly advanced by the determination of whole genome sequences for this organism. Bacterial lipoproteins are a functionally diverse class of membrane-anchored proteins. The signal peptides of these proteins direct their export and post-translational lipid modification. These signal peptides are amenable to bioinformatic analysis, allowing the lipoproteins encoded in whole genomes to be catalogued. This review applies bioinformatic methods to the identification and functional characterisation of the lipoproteins encoded in the M. tuberculosis genomes. Ninety nine putative lipoproteins were identified and so this family of proteins represents ca. 2.5% of the M. tuberculosis predicted proteome. Thus, lipoproteins represent an important class of cell envelope proteins that may contribute to the virulence of this major pathogen.

Toll-like receptor pathways in the immune responses to mycobacteria

The control of Mycobacterium tuberculosis infection depends on recognition of the pathogen and the activation of both the innate and adaptive immune responses. Toll-like receptors (TLR) were shown to play a critical role in the recognition of several pathogens. Mycobacterial antigens recognise distinct TLR resulting in rapid activation of cells of the innate immune system. Recent evidence from in vitro and in vivo investigations, summarised in this review demonstrates TLR-dependent activation of innate immune response, while the induction of adaptive immunity to mycobacteria may be TLR independent.

Mycobacterium tuberculosis LprG (Rv1411c): a novel TLR-2 ligand that inhibits human macrophage class II MHC antigen processing

MHC class II (MHC-II)-restricted CD4(+) T cells are essential for control of Mycobacterium tuberculosis infection. This report describes the identification and purification of LprG (Rv1411c) as an inhibitor of primary human macrophage MHC-II Ag processing. LprG is a 24-kDa lipoprotein found in the M. tuberculosis cell wall. Prolonged exposure (>16 h) of human macrophages to LprG resulted in marked inhibition of MHC-II Ag processing. Inhibition of MHC-II Ag processing was dependent on TLR-2. Short-term exposure (<6 h) to LprG stimulated TLR-2-dependent TNF-alpha production. Thus, LprG can exploit TLR-2 signaling to inhibit MHC-II Ag processing in human macrophages. Inhibition of MHC-II Ag processing by mycobacterial lipoproteins may allow M. tuberculosis, within infected macrophages, to avoid recognition by CD4(+) T cells.

A second-generation anti TB vaccine is long overdue

Mycobacterium bovis BCG vaccine significantly reduces the risk of tuberculosis by 50% and continues to be used to prevent tuberculosis around the world. However, it has been shown to be ineffective in some geographical regions. The existence of different BCG strains was described more than 60 years ago, these vary in their antigenic content but the genetic mutations in BCG strains have yet been shown to affect their protection. After the declaration of tuberculosis as a global emergency in 1993, current research attempts to develop a novel more-effective vaccine. Using new technologies, recombinant, auxotroph, DNA, subunit and phylogenetically closely related mycobacteria, naturally or genetically attenuated, have been used as vaccines in animal models, but their protective efficacy, is less than that offered by the current BCG vaccine. Today it is mandatory that a major effort be made to understand how different BCG vaccine strains influence immune response and why in some cases vaccines have failed, so we can rationally develop the next generation of tuberculosis vaccines to reduce the prevalence from 10% to less than 2 % for developed countries.

Alternate class I MHC antigen processing is inhibited by Toll-like receptor signaling pathogen-associated molecular patterns: Mycobacterium tuberculosis 19-kDa lipoprotein, CpG DNA, and lipopolysaccharide

Pathogen-associated molecular patterns (PAMPs) signal through Toll-like receptors (TLRs) to activate immune responses, but prolonged exposure to PAMPs from Mycobacterium tuberculosis (MTB) and other pathogens inhibits class II MHC (MHC-II) expression and Ag processing, which may allow MTB to evade CD4(+) T cell immunity. Alternate class I MHC (MHC-I) processing allows macrophages to present Ags from MTB and other bacteria to CD8(+) T cells, but the effect of PAMPs on this processing pathway is unknown. In our studies, MTB and TLR-signaling PAMPs, MTB 19-kDa lipoprotein, CpG DNA, and LPS, inhibited alternate MHC-I processing of latex-conjugated Ag by IFN-gamma-activated macrophages. Inhibition was dependent on TLR-2 for MTB 19-kDa lipoprotein (but not whole MTB or the other PAMPs); inhibition was dependent on myeloid differentiation factor 88 for MTB and all of the individual PAMPs. Inhibition of MHC-II and alternate MHC-I processing was delayed, appearing after 16 h of PAMP exposure, as would occur in chronically infected macrophages. Despite inhibition of alternate MHC-I Ag processing, there was no inhibition of MHC-I expression, MHC-I-restricted presentation of exogenous peptide or conventional MHC-I processing of cytosolic Ag. MTB 19-kDa lipoprotein and other PAMPs inhibited phagosome maturation and phagosome Ag degradation in a myeloid differentiation factor 88-dependent manner; this may limit availability of peptides to bind MHC-I. By inhibiting both MHC-II and alternate MHC-I Ag processing, pathogens that establish prolonged infection of macrophages (>16 h), e.g., MTB, may immunologically silence macrophages and evade surveillance by both CD4(+) and CD8(+) T cells, promoting chronic infection.

Immune evasion by Mycobacterium tuberculosis: living with the enemy

Mycobacterium tuberculosis is successful as a pathogen because of its ability to persist in an immunocompetent host. This bacterium lives within the macrophage, a cell whose function is the elimination of microbes. Recent advances have improved our understanding of how M. tuberculosis evades two major antimicrobial mechanisms of macrophages: phagolysosome fusion and the production of toxic reactive nitrogen intermediates. M. tuberculosis also modulates antigen presentation to prevent the detection of infected macrophages by CD4(+) T cells.

Proteins of the Rpf family: immune cell reactivity and vaccination efficacy against tuberculosis in mice

It was shown recently that Mycobacterium tuberculosis expresses five proteins that are homologous to Rpf (resuscitation promoting factor), which is secreted by growing cells of Micrococcus luteus. Rpf is required to resuscitate the growth of dormant Micrococcus luteus organisms, and its homologues may be involved in mycobacterial reactivation. Mycobacterial Rpf-like products are secreted proteins, which makes them candidates for recognition by the host immune system and anti-Rpf immune responses potentially protective against reactivated tuberculosis. Here we report that the Rpf protein itself and four out of five of its mycobacterial homologues, which were administered as subunit vaccines to C57BL/6 mice, are highly immunogenic. Rpf-like proteins elicit immunoglobulin G1 (IgG1) and IgG2a responses and T-cell proliferation and stimulate production of gamma interferon, interleukin-10 (IL-10), and IL-12 but not IL-4 or IL-5. Both humoral and T-cell responses against these antigens show a high degree of cross-reactivity. Vaccination of mice with Rpf-like proteins results in a significant level of protection against a subsequent high-dose challenge with virulent M. tuberculosis H37Rv, both in terms of survival times and mycobacterial multiplication in lungs and spleens.

Mycobacterium tuberculosis pathogenesis and molecular determinants of virulence

Tuberculosis (TB), one of the oldest known human diseases. is still is one of the major causes of mortality, since two million people die each year from this malady. TB has many manifestations, affecting bone, the central nervous system, and many other organ systems, but it is primarily a pulmonary disease that is initiated by the deposition of Mycobacterium tuberculosis, contained in aerosol droplets, onto lung alveolar surfaces. From this point, the progression of the disease can have several outcomes, determined largely by the response of the host immune system. The efficacy of this response is affected by intrinsic factors such as the genetics of the immune system as well as extrinsic factors, e.g., insults to the immune system and the nutritional and physiological state of the host. In addition, the pathogen may play a role in disease progression since some M. tuberculosis strains are reportedly more virulent than others, as defined by increased transmissibility as well as being associated with higher morbidity and mortality in infected individuals. Despite the widespread use of an attenuated live vaccine and several antibiotics, there is more TB than ever before, requiring new vaccines and drugs and more specific and rapid diagnostics. Researchers are utilizing information obtained from the complete sequence of the M. tuberculosis genome and from new genetic and physiological methods to identify targets in M. tuberculosis that will aid in the development of these sorely needed antitubercular agents.

Activation and regulation of Toll-like receptors 2 and 1 in human leprosy

The expression and activation of Toll-like receptors (TLRs) was investigated in leprosy, a spectral disease in which clinical manifestations correlate with the type of immune response mounted toward Mycobacterium leprae. TLR2-TLR1 heterodimers mediated cell activation by killed M. leprae, indicating the presence of triacylated lipoproteins. A genome-wide scan of M. leprae detected 31 putative lipoproteins. Synthetic lipopeptides representing the 19-kD and 33-kD lipoproteins activated both monocytes and dendritic cells. Activation was enhanced by type-1 cytokines and inhibited by type-2 cytokines. In addition, interferon (IFN)-gamma and granulocyte-macrophage colony-stimulating factor (GM-CSF) enhanced TLR1 expression in monocytes and dendritic cells, respectively, whereas IL-4 downregulated TLR2 expression. TLR2 and TLR1 were more strongly expressed in lesions from the localized tuberculoid form (T-lep) as compared with the disseminated lepromatous form (L-lep) of the disease. These data provide evidence that regulated expression and activation of TLRs at the site of disease contribute to the host defense against microbial pathogens.

Novel vaccination strategies based on recombinant Mycobacterium bovis BCG

In this manuscript, we will review the utilization of Mycobacterium bovis Bacille Calmette-Guerin (BCG) as a vaccine against tuberculosis (TB) and as a carrier system for heterologous antigens. BCG is one of the most widely used vaccines. Novel techniques in genome manipulation allow the construction of virulence-attenuated recombinant (r)-BCG strains that can be employed as homologous vaccines, or as heterologous antigen delivery systems, for priming pathogen-specific immunity against infectious diseases, including TB. Several approaches are available for heterologous antigen expression and compartmentalization in BCG and recent findings show the potential to modulate and direct the immune responses induced by r-BCG strains as desired. Recent achievements in complete genome analysis of various target pathogens, combined with a better understanding of protective pathogen-specific immune responses, form the basis for the rational design of a new generation of recombinant mycobacterial vaccines against a multitude of infectious diseases.