Differential Immune Responses and Protective Efficacy Induced by Components of a Tuberculosis Polyprotein Vaccine, Mtb72F, Delivered as Naked DNA or Recombinant Protein

Vaccine-elicited 10-kilodalton culture filtrate protein-specific CD8+ T cells are sufficient to mediate protection against Mycobacterium tuberculosis infection

The 10-kDa culture filtrate protein (CFP-10) and 6-kDa early secretory antigen of T cells (ESAT-6) are secreted in abundance by Mycobacterium tuberculosis and are frequently recognized by T cells from infected people. The genes encoding these proteins have been deleted from the genome of the vaccine strain Mycobacterium bovis bacillus Calmette-Guérin (BCG), and it is hypothesized that these proteins are important targets of protective immunity. Indeed, vaccination with ESAT-6 elicits protective CD4+ T cells in C57BL/6 mice. We have previously shown that M. tuberculosis infection of C3H mice elicits CFP-10-specific CD8+ and CD4+ T cells. Here we demonstrate that immunization with a CFP-10 DNA vaccine stimulates a specific T-cell response only to the H-2K(k)-restricted epitope CFP-10(32-39). These CFP-10(32-39)-specific CD8+ cells undergo a rapid expansion and accumulate in the lung following challenge of immunized mice with aerosolized M. tuberculosis. Protective immunity is induced by CFP-10 DNA vaccination as measured by a CFU reduction in the lung and spleen 4 and 8 weeks after challenge with M. tuberculosis. These data demonstrate that CFP-10 is a protective antigen and that CFP-10(32-39)-specific CD8+ T cells elicited by vaccination are sufficient to mediate protection against tuberculosis.

Evaluation of the anti-tuberculosis activity generated by different multigene DNA vaccine constructs

Development of multigenic constructs expressing Mycobacterium tuberculosis (Mtb) antigens may be a strategy to obtain improved DNA vaccines against tuberculosis (TB). Several multigenic constructs expressing two or three Mtb antigens as fusion proteins were developed, both as tPA- and ubiquitin-fusion proteins. To demonstrate proper protein expression and intracellular turnover all multiantigens were tagged with the HA epitope and constructs were used to transfect rhabdomyosarcoma (RD) cells. Antigen expression was demonstrated by immunofluorescence using anti-HA antibodies. C57Bl/6 mice were immunized with selected constructs and protective activity was assessed following aerogenic challenge with Mtb. Several of these constructs induced a significant level of protection in the lung and in the spleen. Immunization with the construct expressing tPA85B-ES6 induced a level of protection that approached that provided by BCG. Immunization with a combination of these constructs induced levels of protection that were not superior to those elicited by a single combination, and immunization with a construct expressing five Mtb antigens could not provide an improved level of protection compared to tPA85B-ES6. We conclude that the activity of a DNA vaccine based on tPA85B-ES6 cannot be enhanced by broadening the antigen repertoire with other highly immunogenic secreted Mtb proteins.

The challenges of eliciting neutralizing antibodies to HIV-1 and to influenza virus

The ability to elicit broadly neutralizing antibody responses against HIV-1 is a crucial goal for a prophylactic HIV-1 vaccine. Here, we discuss the difficulties of achieving broad HIV-1 neutralization in the context of both the effective annual human influenza virus vaccine and the need to develop a pandemic influenza vaccine. Immunogen-design strategies are underway to target functionally conserved regions of the HIV-1 envelope glycoproteins, and similar strategies might be applicable to pandemic influenza virus vaccine development. Efforts to develop broadly neutralizing vaccines against either HIV-1 or influenza virus might establish a paradigm for future vaccines against highly variable pathogens.

Adjuvant modulation of the cytokine balance in Mycobacterium tuberculosis subunit vaccines; immunity, pathology and protection

It is known that protection against tuberculosis is mediated primarily by T helper type 1 (Th1) cells but the influence of the Th1/Th2 balance of a vaccination response on the subsequent protection and pathology during infection has not been studied in detail. We designed a panel of Ag85B-ESAT-6 subunit vaccines based on adjuvants with different Th1/Th2-promoting activities and studied cellular responses, bacterial replication and pathology in the lungs of mice infected with Mycobacterium tuberculosis. All vaccines induced cell-mediated and humoral responses but with markedly different interferon-gamma : interleukin-5 (IFN-gamma : IL-5) and immunoglobulin G1 (IgG1) : IgG2 ratios. The vaccines promoted different levels of control of bacterial replication with the most efficient protection being exerted by cationic liposomes containing monophosphoryl lipid A and low to completely absent immunity with conventional aluminium. The level of protection correlated with the amount of IFN-gamma produced in response to the vaccine whereas there was no inverse correlation with the level of IL-5. Characterizing a protective response was an accelerated recruitment of IL-17 and IFN-gamma-producing lymphocytes resulting in the early formation of granulomas containing clustered inducible nitric oxide synthase-activated macrophages. In comparison, non-protected mice exhibited a different inflammatory infiltrate rich in neutrophil granulocytes. This study indicates that the adjuvant component of a tuberculosis vaccine may be crucial in determining the kinetics by which effective granulomas, pivotal in controlling bacterial growth, are formed.

Immune responses in mice to Mycobacterium avium subsp. paratuberculosis following vaccination with a novel 74F recombinant polyprotein

Johne's disease (JD) is a chronic infectious disease of ruminants caused by Mycobacterium avium subsp. paratuberculosis (MAP). Here, we report the cloning and expression of a 74kDa recombinant polyprotein (Map74F) and its protective efficacy against MAP infection in mice. Map74F was generated by the sequential linkage of the ORFs of the approximately 17.6-kDa C-terminal fragment of Map3527 to the full-length ORF of Map1519, followed at the C-terminus with approximately 14.6-kDa N-terminal portion of Map3527. Mice immunized with Map74F had a significant IgG1 response but not IgG2a. In immunized animals, the IgG1/IgG2a ratio increased until 4 weeks after MAP challenge. The ratio decreased from 8 weeks indicating a shift to a Th1 response. Antigen specific IFN-gamma response, CD3+ and CD4+ T cells increased significantly in immunized mice. Following challenge, MAP burden was significantly lower in liver, spleen and mesenteric lymph nodes of immunized animals compared to control animals indicating protection against MAP infection. This was further evident by the improved liver and spleen pathology of the immunized animals, which had fewer granulomas and lower numbers of acid-fast bacilli. Results of this study indicated that immunization of mice with Map74F protected mice against MAP infection.

Rational design of vaccines against tuberculosis directed by basic immunology

Tuberculosis represents a serious problem for public health worldwide, and effective vaccines are urgently required. This represents a significant challenge as the causative bacterial agent, Mycobacterium tuberculosis, has developed strategies to persist in infected hosts despite the presence of potent T-cell-mediated immune responses. New advances in basic immunology are giving us improved understanding of what constitutes a protective immune response and ways this response is manipulated by the bacillus. Such insights should inform us how to design more effective vaccination strategies against intracellular pathogens.

An analysis of the epitope knowledge related to Mycobacteria

BACKGROUND

Tuberculosis, caused by the bacterium Mycobacterium tuberculosis, remains a leading cause of infectious disease morbidity and mortality, and is responsible for more than 2 million deaths a year. Reports about extremely drug resistant (XDR) strains have further heightened the sense of urgency for the development of novel strategies to prevent and treat TB. Detailed knowledge of the epitopes recognized by immune responses can aid in vaccine and diagnostics development, and provides important tools for basic research. The analysis of epitope data corresponding to M. tuberculosis can also identify gaps in our knowledge, and suggest potential areas for further research and discovery. The Immune Epitope Database (IEDB) is compiled mainly from literature sources, and describes a broad array of source organisms, including M. tuberculosis and other Mycobacterial species.

DESCRIPTION

A comprehensive analysis of IEDB data regarding the genus Mycobacteria was performed. The distribution of antibody/B cell and T cell epitopes was analyzed in terms of their associated recognition cell type effector function and chemical properties. The various species, strains and proteins which the epitope were derived, were also examined. Additional variables considered were the host in which the epitopes were defined, the specific TB disease state associated with epitope recognition, and the HLA associated with disease susceptibility and endemic regions were also scrutinized. Finally, based on these results, standardized reference datasets of mycobacterial epitopes were generated.

CONCLUSION

All current TB-related epitope data was cataloged for the first time from the published literature. The resulting inventory of more than a thousand different epitopes should prove a useful tool for the broad scientific community. Knowledge gaps specific to TB epitope data were also identified. In summary, few non-peptidic or post-translationally modified epitopes have been defined. Most importantly epitopes have apparently been defined from only 7% of all ORFs, and the top 30 most frequently studied protein antigens contain 65% of the epitopes, leaving the majority of M. tuberculosis genome unexplored. A lack of information related to the specific strains from which epitopes are derived is also evident. Finally, the generation of reference lists of mycobacterial epitopes should also facilitate future vaccine and diagnostic research.

GlaxoSmithKline Adjuvant Systems in vaccines: concepts, achievements and perspectives

The need for potentiating immune responses to recombinant or subunit antigens has prompted GlaxoSmithKline (GSK) Biologicals to develop various Adjuvant Systems for the design of prophylactic and therapeutic vaccines. Adjuvant Systems are formulations of classical adjuvants mixed with immunomodulators, specifically adapted to the antigen and the target population. They can activate the appropriate innate immune system and subsequently impact on adaptive immune responses. AS04 is an Adjuvant System that has demonstrated significant achievements in several vaccines against viral diseases. AS02, another Adjuvant System, is being evaluated in various contexts, where a strong T-cell response is needed to afford protection. Likewise, AS01 has been developed for vaccines where the induction of a yet stronger T-cell-mediated immune response is required. Altogether, the promising clinical results strongly support the concept of Adjuvant Systems and allow for further development of new vaccines, best adapted to the target population and the immune mechanisms of protection.

A Toll-like receptor-2-directed fusion protein vaccine against tuberculosis

A fusion protein designated CSU-F36 was constructed that consisted of acylated Rv1411, a potent Toll-like receptor-2 agonist, fused to ESAT-6, a well-characterized immunogenic protein from Mycobacterium tuberculosis. The CSU-F36 fusion protein strongly induced interleukin 12 secretion from macrophages and induced the increased accumulation of CD4 T cells capable of secreting gamma interferon in the lungs of infected mice. These mice were significantly protected from low-dose aerosol challenge with M. tuberculosis, even with CSU-F36 delivered in a simple depot material. This "natural adjuvant"-containing system could potentially bypass the need for more expensive TH1-inducing adjuvants and could be applied to many mycobacterial proteins to provide effective and cheap new vaccines against tuberculosis.

Recombinant BCG Tokyo (Ag85A) protects cynomolgus monkeys (Macaca fascicularis) infected with H37Rv Mycobacterium tuberculosis

One tuberculosis vaccine candidate that has shown a promising degree of protective efficacy in guinea pigs is recombinant BCG Tokyo (Ag85A)(rBCG-Ag85A[Tokyo]). As a next step, cynomolgus monkeys were utilized because they are susceptible to Mycobacterium tuberculosis and develop a continuous course of infection that resembles that in humans both clinically and pathologically. The recombinant BCG vaccine was administered once intradermally in the back skin to three groups of cynomolgus monkeys, and its protective efficacy was compared for 4 months with that of its parental BCG Tokyo strain. Vaccination of the monkeys with the rBCG-Ag85A[Tokyo] resulted in a reduction of tubercle bacilli CFU (p<0.01) and lung pathology in animals challenged intratracheally with 3000 CFU H37Rv M. tuberculosis. Vaccination prevented an increase in the old tuberculin test after challenge with M. tuberculosis and reaction of M. tuberculosis-derived antigen. Thus, it was shown in monkeys that rBCG-Ag85A[Tokyo] induced higher protective efficacy than BCG Tokyo. This warrants further clinical evaluation.

Tuberculosis subunit vaccines: from basic science to clinical testing

More than 80 years after the introduction of Bacillus Calmette-GuErin, the first tuberculosis vaccine, new vaccines for tuberculosis are finally in clinical trials. The selection of antigens on which new subunit vaccines are based represent the first fulfillment of the promise of proteomics and genomics, and the delivery systems for these antigens are likewise the first fruits of the improved understanding of how the host immune system recognizes pathogens. However, clinical trials are still at Phase I and there remain formidable obstacles to the registration of the first new TB vaccines. Here the authors review the vaccines in clinical trials and discuss the different approaches they take to stimulating immunity to Mycobacterium tuberculosis infection, focusing on recombinant subunit vaccines. The challenges that confront these approaches and how they are being addressed are then discussed.

New TB vaccines: is there a requirement for CD8 T cells?

MHC class I-restricted CD8(+) T cells are necessary to mount an immune response against Mycobacterium tuberculosis. M. tuberculosis antigens can enter MHC class I cross-processing pathways through a number of different mechanisms, including via the uptake of antigen-containing apoptotic vesicles released by infected cells. A study in this issue of the JCI by Hinchey and colleagues shows that M. tuberculosis inhibits host cell apoptosis and thus may interfere with optimal cross-priming and action of CD8(+) T cells (see the related article beginning on page 2279). M. tuberculosis genetically modified to induce apoptosis is shown to be more effective in priming CD8(+) T cells in vivo and therefore may be a more effective vaccine against tuberculosis than the currently utilized M. bovis BCG vaccine.

DNA polymorphisms in the pepA and PPE18 genes among clinical strains of Mycobacterium tuberculosis: implications for vaccine efficacy

Tuberculosis continues to be a leading cause of death worldwide. Development of an effective vaccine against Mycobacterium tuberculosis is necessary to reduce the global burden of this disease. Mtb72F, consisting of the protein products of the pepA and PPE18 genes, is the first subunit tuberculosis vaccine to undergo phase I clinical trials. To obtain insight into the ability of Mtb72F to induce an immune response capable of recognizing different strains of M. tuberculosis, we investigated the genomic diversity of the pepA and PPE18 genes among 225 clinical strains of M. tuberculosis from two different geographical locations, Arkansas and Turkey, representing a broad range of genotypes of M. tuberculosis. A combination of single nucleotide polymorphisms (SNPs) and insertion/deletions resulting in amino acid changes in the PPE18 protein occurred in 47 (20.9%) of the 225 study strains, whereas SNPs resulted in amino acid changes in the PepA protein in 14 (6.2%) of the 225 study strains. Of the 122 Arkansas study strains and the 103 Turkey study strains, 32 (26.2%) and 15 (14.6%), respectively, had at least one genetic change leading to an alteration of the amino acid sequence of the PPE18 protein, and many of the changes occurred in regions previously reported to be potential T-cell epitopes. Thus, immunity induced by Mtb72F may not recognize a proportion of M. tuberculosis clinical strains.

Vaccines for bovine tuberculosis: current views and future prospects

Bovine tuberculosis, caused by Mycobacterium bovis, is rapidly increasing in cattle herds in developed countries such as the UK, New Zealand and the USA. In addition, persistence of M. bovis in other parts of the world may account for up to 10% of cases of human tuberculosis. Thus, a rise in the number of M. bovis infections poses an increased human health risk and is also a major economic problem. In the UK, the incidence of bovine tuberculosis continues to rise despite the use of a skin test and slaughter control policy, highlighting the need for an effective vaccination strategy to control the spread of disease. The only vaccine currently available for human, (and bovine), tuberculosis is Bacillus Calmette-Guérin, which is known to have variable efficacy for both species. In this article, the authors discuss potential strategies by which Bacillus Calmette-Guérin vaccination may be improved to allow highly efficacious vaccination of cattle. These strategies are also highly applicable to the fight against tuberculosis in humans.

Development of new tuberculosis vaccines: a global perspective on regulatory issues

What are the regulatory challenges for testing and introducing investigative TB vaccines into countries where the disease is endemic?

Chronic helminth infections may negatively influence immunity against tuberculosis and other diseases of public health importance

Tuberculosis (TB) has once again become a major public health threat owing to the combined effects of deteriorating socioeconomic situations and the emergence of the HIV/AIDS pandemic. The only vaccine available against TB, although effective in reducing the burden of childhood TB, shows enormous variability in its efficacy against pulmonary TB, which is the most common form of the disease in adults. Most areas of high TB incidence and poor TB vaccine efficacy have a high prevalence of intestinal helminth infections. Such infections have been shown to cause a range of immunomodulation characterized by enhanced T helper 2-type cytokine profile, high immunoglobulin E levels and upregulated regulatory T-cell activity, as well as chronic immune activation. An altered background immune profile could have adverse effects on the outcome of subsequent infections and vaccinations. In support of this hypothesis, studies conducted in animals and humans living in worm-endemic areas have shown that helminths impair resistance against a number of infections of major public health importance, including TB, malaria and HIV/AIDS. Understanding such interactions could assist in the design of vaccines against these diseases.

Childhood tuberculosis: old and new vaccines

The world is witnessing an escalation of the tuberculosis (TB) epidemic, particularly in sub-Saharan Africa and South-East Asia. The problem has been compounded by the evolution of the human immunodeficiency virus pandemic, the increase in multidrug-resistant TB and the emergence of extensively drug-resistant TB. This has led to renewed interest in vaccines aimed at preventing tuberculosis. The current Bacille Calmette-Guérin (BCG) vaccine prevents the invasive complications of childhood tuberculosis, such as meningitis and miliary disease, but provides variable protection against adult pulmonary disease. This review discusses the history of the BCG vaccine, the reasons for its variable efficacy, protective immunity and TB, and the evolution of and obstacles to development of new candidate vaccines. Several new TB vaccines have demonstrated promising results in animal models; a number have gone into phase I clinical trials in humans, and it is anticipated that phase III trials will commence by 2009. Licensing of an effective new TB vaccine by 2015 is thus a possibility.

Next generation: tuberculosis vaccines that elicit protective CD8+ T cells

Tuberculosis continues to cause considerable human morbidity and mortality worldwide, particularly in people coinfected with HIV. The emergence of multidrug resistance makes the medical treatment of tuberculosis even more difficult. Thus, the development of a tuberculosis vaccine is a global health priority. Here we review the data concerning the role of CD8+ T cells in immunity to tuberculosis and consider how CD8+ T cells can be elicited by vaccination. Many immunization strategies have the potential to elicit CD8+ T cells and we critically review the data supporting a role for vaccine-induced CD8+ T cells in protective immunity. The synergy between CD4+ and CD8+ T cells suggests that a vaccine that elicits both T-cell subsets has the best chance at preventing tuberculosis.

H2-M3-restricted CD8+ T cells induced by peptide-pulsed dendritic cells confer protection against Mycobacterium tuberculosis

One of the oligopolymorphic MHC class Ib molecules, H2-M3, presents N-formylated peptides derived from bacteria. In this study, we tested the ability of an H2-M3-binding peptide, TB2, to induce protection in C57BL/6 mice against Mycobacterium tuberculosis. Immunization with bone marrow-derived dendritic cell (BMDC) pulsed with TB2 or a MHC class Ia-binding peptide, MPT64(190-198) elicited an expansion of Ag-specific CD8+ T cells in the spleen and the lung. The number of TB2-specific CD8+ T cells reached a peak on day 6, contracted with kinetics similar to MPT64(190-198)-specific CD8+ T cells and was maintained at an appreciable level for at least 60 days. The TB2-specific CD8+ T cells produced less effector cytokines but have stronger cytotoxic activity than MPT64(190-198)-specific CD8+ T cells. Mice immunized with TB2-pulsed BMDC as well as those with MPT64(190-198)-pulsed BMDC showed significant protection against an intratracheal challenge with M. tuberculosis H37Rv. However, histopathology of the lung in mice immunized with TB2-pulsed BMDC was different from mice immunized with MPT64(190-198)-pulsed BMDC. Our results suggest that immunization with BMDC pulsed with MHC class Ib-restricted peptides would be a useful vaccination strategy against M. tuberculosis.

Translational mini-review series on vaccines: Development and evaluation of improved vaccines against tuberculosis

Tuberculosis (TB) continues to be a major global health disaster, despite the widespread use of BCG and effective drug therapies. The development of an efficacious new TB vaccine would be an important component of disease control in the future. Many approaches are being utilised to enhance understanding of the requirements of a successful vaccine. Numerous vaccines are being designed and assessed in a series of animal models, with a few progressing to clinical trials. Here, the steps involved in the development and evaluation of TB vaccines will be discussed, including description of the most frequently used animal models and the processes involved in advancing vaccines to phase III trials.

Increased levels of interferon-gamma primed by culture filtrate proteins antigen and CpG-ODN immunization do not confer significant protection against Mycobacterium tuberculosis infection

The results of various animal model studies of tuberculosis (TB) suggest that culture filtrate proteins (CFPs), which are antigens secreted by Mycobacterium tuberculosis, are largely responsible for improvements in TB vaccines. The great obstacle to developing protein subunit vaccines is that adjuvants are required in order to stimulate relevant protective immune responses. Acting as immune adjuvants, CpG-oligodeoxynucleotides (CpG-ODNs) promote the activation of Th1 cells and of pro-inflammatory cytokines. To evaluate the adjuvant role of CpG-ODNs in conferring enhanced immunogenic capacity and protection against M. tuberculosis, we immunized mice with CFP antigen combined with synthetic CpG-ODNs (CFP/CpG) or with incomplete Freund's adjuvant (IFA) (CFP/IFA). Immunization with CFP/CpG induced a T helper 1 (Th1)-biased response accompanied by a higher immunoglobulin G2a (IgG2a) antibody/IgG1 antibody ratio, elevated production of interferon-gamma (IFN-gamma) by spleen cells and in lungs. However, CFP/IFA-immunized mice presented higher levels of IgG1 antibodies, as well as increased production of IFN-gamma, interleukin (IL)-5, and IL-10 by spleen cells, together with lower levels of IFN-gamma in the lungs. Despite the stronger Th1 response seen in both groups, believed to be necessary for protection against TB, only mice immunized with CFP/IFA were protected after M. tuberculosis infection. Lung histology revealed that lung parenchyma were better preserved in CFP/IFA-immunized mice, which also presented intense lymphocyte recruitment to the lesion, whereas CFP/CpG-immunized mice presented severe pulmonary injury accompanied by necrosis. Based on the data presented, we discuss the widely accepted paradigm that high levels of IFN-gamma are directly correlated with protection against experimental TB.

A tuberculosis vaccine based on phosphoantigens and fusion proteins induces distinct gammadelta and alphabeta T cell responses in primates

Phosphoantigens are mycobacterial non-peptide antigens that might enhance the immunogenicity of current subunit candidate vaccines for tuberculosis. However, their testing requires monkeys, the only animal models suitable for gammadelta T cell responses to mycobacteria. Thus here, the immunogenicity of 6-kDa early secretory antigenic target-mycolyl transferase complex antigen 85B (ESAT-6-Ag85B) (H-1 hybrid) fusion protein associated or not to a synthetic phosphoantigen was compared by a prime-boost regimen of two groups of eight cynomolgus. Although phosphoantigen activated immediately a strong release of systemic Th1 cytokines (IL-2, IL-6, IFN-gamma, TNF-alpha), it further anergized blood gammadelta T lymphocytes selectively. By contrast, the hybrid H-1 induced only memory alphabeta T cell responses, regardless of phosphoantigen. These latter essentially comprised cytotoxic T lymphocytes specific for Ag85B (on average + 430 cells/million PBMC) and few IFN-gamma-secreting cells (+ 40 cells/million PBMC, equally specific for ESAT-6 and for Ag85B). Hence, in macaques, a prime-boost with the H-1/phosphoantigen subunit combination induces two waves of immune responses, successively by gammadelta T and alphabeta T lymphocytes.

Mycobacterium tuberculosis-specific CD8+ T cells and their role in immunity

There are more cases of tuberculosis in the world today than at any other time in history. The global epidemic has generated intense interest into the immunological mechanisms that control infection. Although CD4+ T cells play a critical role in host immunity to Mycobacterium tuberculosis, there is considerable interest in understanding the role of other T cell subsets in preventing disease development following infection. CD8+ T cells are required for optimum host defense following M. tuberculosis infection, which has led to investigation into how this protective effect is mediated. A critical review of recent literature regarding the role of CD8+ T cells in protective immunity to M. tuberculosis infection is now required to address the strengths and weaknesses of these studies. In this article, we evaluate the evidence that CD8+ T cells are critical in immunity to M. tuberculosis infection. We discuss the specific mycobacterial proteins that are recognized by CD8+ T cells elicited during infection. Finally, we examine the effector mechanisms of CD8+ T cells generated during infection and synthesize recent studies to consider the protective roles that these T cells serve in vivo.

Current status of TB vaccines

During last 10 years, there has been extensive work for the development of potential tuberculosis vaccine candidates using the mice and guinea pig models. Though till date several promising candidates have been identified and at least eight vaccines have entered clinical evaluation. These recent advances in the clinical testing of new TB vaccines are very exciting and promising. However, there is a need to continue the search for additional vaccine candidates or vaccination strategies.

Tuberculosis subunit vaccine development: Impact of physicochemical properties of mycobacterial test antigens

Tuberculosis caused by Mycobacterium tuberculosis continues to be one of the major public health problems in the world. The eventual control of this disease will require the development of a safe and effective vaccine. One of the approaches receiving a great deal of attention recently is subunit vaccination. An efficacious antituberculous subunit vaccine requires the identification and isolation of key components of the pathogen that are capable of inducing a protective immune response. Clues to identify promising subunit vaccine candidates may be found in their physicochemical and immunobiological properties. In this article, we review the evidence that the physicochemical properties of mycobacterial components can greatly impact the induction of either protective or deleterious immune response and consequently influence the potential utility as an antituberculous subunit vaccine.

Tuberculosis subunit vaccine design: the conflict of antigenicity and immunogenicity

The attempts to find an effective antituberculous subunit vaccine are based on the assumption that it must drive a Th1 response. In the absence of effective correlates of protection, a vast array of mycobacterial components are being evaluated worldwide either on the basis of their ability to be recognized by T lymphocytes in in vitro assays during early stage of animal or human infection (antigenicity) or their capacity to induce T cell response following immunization in animal models (immunogenicity). The putative vaccine candidates selected using either of these strategies are then subjected to challenge studies in different animal models to evaluate the protective efficacy. Here we review the outcome of this current scheme of selection of vaccine candidates using an 'antigenicity' or 'immunogenicity' criterion on the actual protective efficacy observed in experimental animal models. The possible implications for the success of some of the leading vaccine candidates in clinical trials will also be discussed.

Vaccine strategies against latent tuberculosis infection

The leading tuberculosis (TB) vaccines currently in clinical trials are all designed as prophylactic vaccines. Although these vaccines are highly active, they will most probably not result in sterilizing immunity and, therefore, will not solve the global problem of latent TB. An attractive strategy is to target the remaining dormant bacteria with vaccines based upon antigens induced as the bacteria change from active multiplication to non-replicating dormancy (latency antigens) or during reactivation as dormant bacteria resume active metabolism (resuscitation antigens). These late-stage antigens might have potential as post-exposure vaccines or could form the basis for a multi-stage vaccine strategy, in which they are combined with prophylactic vaccines based on early antigens from replicating bacteria.

BCG vaccination confers poor protection against M. tuberculosis HN878-induced central nervous system disease

Using a rabbit model of tuberculous meningitis (TBM), we compared the protective efficacy of Mycobacterium bovis bacillus Calmette-Guérin (BCG) vaccination against central nervous system infection with the virulent M. tuberculosis clinical isolate HN878 and the laboratory strain H37Rv. Although BCG clearly provided protection against infection with either challenge strain, protection against disease manifestations was significantly poorer in rabbits infected with HN878. BCG was less efficient in protecting against HN878 dissemination to the liver and spleen and against HN878-induced inflammation, loss of body weight, lung and brain pathology, and signs of disease. We suggest that the efficacy of newly developed vaccines should be tested in animal models not only against challenge with M. tuberculosis H37Rv but also with different clinical isolates including the highly virulent strains of the W-Beijing family.

Chimaeric Protein Improved Immunogenicity Compared with Fusion Protein of Ag85B and ESAT-6 Antigens of Mycobacterium tuberculosis

Antigen 85B (Ag85B) and ESAT-6 are important immunodominant antigens of Mycobacterium tuberculosis, and both are very promising vaccine candidate molecules. In this study, we relied on the T-cell epitopes of Ag85B and ESAT-6 to design a chimaeric protein by inserting ESAT-6 into Ag85B from the amino acids 167-182. We found the ratio of IgG2b/IgG1 and the secretion of interferon (IFN)-gamma in the mice vaccinated with the new protein with adjuvant MPL and TDM were higher than the mice immunized with fusion protein Ag85B-ESAT-6, which have been reported and could induce levels of protective immunity similar to BCG in the mouse model of tuberculosis (TB) infection. These results suggest that the chimaeric protein Ag85B(N)-ESAT-6-Ag85B(C) is a strong candidate for further study and the T-cell epitopes of the antigens should be considered when we design the subunit vaccine.

Vaccine Approaches to Prevent Tuberculosis

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), is one of the main killers among infectious pathogens in the world and represents an important factor that sustain poverty in developing countries. Failure of the BCG vaccine to protect in endemic regions, and increasing problems with multi-drug-resistant TB calls for development of better vaccines to prevent reactivation of tuberculosis. It has been estimated that an effective post-exposure vaccine will prevent 30-40% of the TB cases. New vaccines should also prevent development of TB in HIV-infected individuals. Recent characterization of M. tuberculosis H37Rv by proteomic methods has revealed a large number of novel secreted proteins that should be investigated in mouse models for latent and slowly progressive TB. There is an important balance between control of infection and tissue destruction in TB, and M. tuberculosis has developed strategies to prevent immune-mediated sterilization. Central to this strategy is inhibition of apoptosis of macrophages. Development of novel vaccines should therefore take into consideration the effects on central markers to obtain a better picture of regulation of immunity, including FasL and Bcl-2 which are essential in regulation of apoptosis.

Enhanced immunogenicity and protective efficacy with the use of interleukin-12-encapsulated microspheres plus AS01B in tuberculosis subunit vaccination

Tuberculosis subunit vaccines codelivered with interleukin-12 (IL-12)-encapsulated microspheres (IL-12EM) are designed for a sustained release of IL-12 and could induce strong Th1 immune responses specific to Ag85A and ESAT-6. The adjuvant combination of IL-12EM plus AS01B was a more efficient way to induce a sustained Th1 immunity and protection against Mycobacterium tuberculosis.

Immunogenicity of Mycobacterium tuberculosis antigens in Mycobacterium bovis BCG-vaccinated and M. bovis-infected cattle

The development of novel vaccine strategies supplementing Mycobacterium bovis BCG (BCG) constitutes an urgent research challenge. To identify potential subunit vaccine candidates, we have tested a series of eight recently identified Mycobacterium tuberculosis antigens in M. bovis-infected and BCG-vaccinated cattle. These antigens were characterized on the basis of their ability to induce in vitro gamma interferon responses in infected or BCG-vaccinated calves. We were able to establish a hierarchy of these antigens based on how frequently they were recognized in both groups of animals. In particular, we were able to prioritize frequently recognized proteins like Rv0287, Rv1174, and Rv1196 for future evaluation as subunit vaccines to be used in BCG-protein heterologous prime-boost vaccination scenarios. In addition, the antigen most dominantly recognized in M. bovis-infected cattle in this study, Rv3616c, was significantly less frequently recognized by BCG vaccinees and could be a target to improve BCG, for example, by increasing its secretion, in a recombinant BCG vaccine.

Early clinical trials with a new tuberculosis vaccine, MVA85A, in tuberculosis-endemic countries: issues in study design

Tuberculosis remains a substantial global health problem despite effective drug treatments. The efficacy of BCG, the only available vaccine, is variable, especially in tuberculosis-endemic regions. Recent advances in the development of new vaccines against tuberculosis mean that the first of these are now entering into early clinical trials. A recombinant modified vaccinia virus Ankara expressing a major secreted antigen from Mycobacterium tuberculosis, antigen 85A, was the first new tuberculosis vaccine to enter into clinical trials in September 2002. This vaccine is known as MVA85A. In a series of phase I clinical trials in the UK, MVA85A had an excellent safety profile and was highly immunogenic. MVA85A was subsequently evaluated in a series of phase I trials in The Gambia, a tuberculosis-endemic area in west Africa. This vaccine is the only new subunit tuberculosis vaccine to enter into clinical trials in Africa to date. Here, we discuss some of the issues that were considered in the protocol design of these studies including recruitment, inclusion and exclusion criteria, reimbursement of study participants, and HIV testing. These issues are highly relevant to early clinical trials with all new tuberculosis vaccines in the developing world.

Envisioning future strategies for vaccination against tuberculosis

The design of tuberculosis vaccines has entered a new era. Although several new vaccine candidates will pass Phase I clinical trials within the next year, I believe that the most effective vaccination strategy will be to combine different vaccine candidates and to use a prime-boost approach. This strategy, however, would require several years of iterative vaccine trials, unless the process is expedited by the identification of reliable biomarkers for assessing vaccine efficacy. In this Essay, I briefly summarize past and present attempts to develop a vaccine against tuberculosis, and I describe, using imagined scenarios, the tuberculosis vaccination schemes that might become available from a large repertoire of candidate schemes in the near and distant future.

Immunogenicity and protective efficacy of tuberculosis DNA vaccines combining mycolyl-transferase Ag85A and phosphate transport receptor PstS-3

DNA vaccines encoding the 32,000 MW mycolyl-transferase Ag85A and the 40,000 MW phosphate-binding protein PstS-3 can elicit protective immune responses against experimental infection with Mycobacterium tuberculosis in C57BL/6 mice. Here we have analysed the vaccine potential of a combination of both antigens using plasmid vectors expressing either a fusion protein of both antigens or the separate proteins driven by two independent promoters (in pBudCE4.1 vector). Comparable levels of Ag85A specific T helper 1 (Th1) type immune responses could be induced by the two combination vaccines and the single vaccine encoding the mycolyl-transferase, whereas induction of PstS-3 specific Th1-mediated responses was impaired in both combination vaccines. In contrast, magnitude of CD8+ mediated responses against the PstS-3 protein was comparable following combination or single DNA vaccination. Antigenic competition was also observed at the antibody level; PstS-3 specific levels being lower in mice vaccinated with the fusion vector and Ag85A specific levels being lower in mice vaccinated with the combination pBudCE4.1 vector (as compared to levels obtained following single plasmid immunization). Protection against M. tuberculosis was only modestly improved in mice vaccinated with the DNA combinations. It is possible that prior activation of Ag85A specific CD4+ T cells directed against this common mycobacterial antigen leads to cross-competition for major histocompatibility complex class II-restricted peptide complexes of the Pst-3 antigen. This may have implications for future combination vaccines using Ag85.

Tuberculosis vaccines: current status and future prospects

There is an urgent need to develop more effective tuberculosis vaccines as chemotherapy and Bacille Calmette-Guérin (BCG) have failed to control the current epidemic. BCG does have some protective effect in childhood, so using a second vaccine to boost BCG would be the most ethical and logistically feasible strategy. The cost of tuberculosis efficacy trials will be high and return on investment into the development of a tuberculosis vaccine will be low. Incentives such as orphan drug status could encourage industrial interest. As more vaccines enter into early clinical trials, there is an urgent need for the identification of correlates of protection to aid decisions about which vaccines should go forward into efficacy testing. Research efforts that focus on reducing the cost and risk of conducting clinical trials will be of direct benefit to tuberculosis vaccine development.

Expression and purification of the Mycobacterium tuberculosis complex-restricted antigen CFP21 to study its immunoprophylactic potential in mouse model

Secreted proteins encoded by different regions of difference (RDs) from the genome of Mycobacterium tuberculosis have been considered as attractive candidates for vaccination against tuberculosis owing to their absence in most BCG strains. In this study, the structural gene for the RD2 locus encoding protein CFP21 was PCR amplified and expressed as a fusion protein with hexahistidine residues in Escherichia coli. Expression of CFP21 in E. coli under transcriptional regulation of the T7 promoter yielded a protein located within inclusion bodies. The inclusion bodies were solubilized in the presence of 8M urea and the protein was purified to homogeneity under denaturing conditions at low pH using nitrilotriacetic acid (Ni-NTA) affinity chromatography. The denatured protein was renatured by gradient dialysis against a decreasing concentration of urea. The purified protein was shown to have esterase activity. CFP21 protein was evaluated for immunogenicity in C57BL/6J mice. We observed an elevated T cell proliferative response and production of IFN-gamma and IL-12 (p40). CFP21 also induced an optimum level of cytotoxic T cell activity and induced a strong humoral response as indicated by higher levels of specific IgG1 and IgG2a antibody isotypes. In addition, a moderate level of protection was observed against experimental tuberculosis. This is the first report describing esterase activity of the M. tuberculosis complex-restricted protein CFP21 and its protective potential against experimental tuberculosis.

Protective efficacy of recombinant (Ag85A) BCG Tokyo with Ag85A peptide boosting against Mycobacterium tuberculosis-infected guinea pigs in comparison with that of DNA vaccine encoding Ag85A

A recombinant form of BCG Tokyo with an Ag85A gene insert was administered once subcutaneously to guinea pigs and its protective efficacy was compared with that of a DNA vaccine encoding Ag85A from Mycobacterium tuberculosis administered twice to guinea pigs by epidermal gene gun bombardment. Vaccination with either the recombinant BCG Tokyo or Ag85A DNA significantly reduced the severity of pulmonary pathology and the number of pulmonary and splenic colony-forming units (cfu) (p<0.001). The recombinant BCG Tokyo was better than Ag85A DNA in terms of protective efficacy against M. tuberculosis. When immunogenic synthetic Ag85A peptide was further used as a booster together with recombinant BCG Tokyo (Ag85A) or Ag85A DNA, lung pathology was improved significantly and the number of pulmonary CFU was reduced significantly. Neither recombinant BCG Tokyo, Ag85A DNA, nor the parental BCG Tokyo protected the guinea pigs from hematogenous spread of tubercle bacilli to the spleen because splenic granulomas without central necrosis were recognized. The spleen tissues from guinea pigs vaccinated with recombinant BCG Tokyo or Ag85A DNA expressed IFN-gamma and IL-2 mRNA at significantly high levels (p<0.001) as evaluated by reverse transcription polymerase chain reaction. It is concluded that peptide boosting is important for the induction of higher protective efficacy by recombinant BCG Tokyo or a tuberculosis DNA vaccine and both recombinant BCG Tokyo (Ag85A) and Ag85A DNA vaccine induce Th2 cytokine mRNA expression significantly.

Advances in tuberculosis vaccine strategies

Tuberculosis (TB), an ancient human scourge, is a growing health problem in the developing world. Approximately two million deaths each year are caused by TB, which is the leading cause of death in HIV-infected individuals. Clearly, an improved TB vaccine is desperately needed. Heterologous prime-boost regimens probably represent the best hope for an improved vaccine regimen to prevent TB. This first generation of new vaccines might also complement drug treatment regimens and be effective against reactivation of TB from the latent state, which would significantly enhance their usefulness.

Progress in tuberculosis vaccine development

The first tuberculosis vaccine candidates have reached clinical testing. Novel subunit vaccine candidates aimed at boosting previous BCG-prime vaccination and novel viable attenuated vaccine candidates aimed at substituting BCG have both completed the preclinical stage. Despite these achievements, rational vaccine design against tuberculosis has not come to an end. Novel findings in basic immunology and microbiology will advance further improvements in vaccine development. These include the potential role of crosspriming to induce more potent T-cell responses, the role of memory T cells and regulatory T cells in sustaining or curtailing optimal immune responses, respectively, as well as the involvement of cytokines in T-cell migration to nonimmunologic tissue sites and in the generation of memory. Knowledge about basic mechanisms underlying optimum protection will not only have a direct impact on future vaccine design against tuberculosis but also help in the formulation of a set of biomarkers with predictive value for vaccine efficacy assessment.

Selection of novel TB vaccine candidates and their evaluation as DNA vaccines against aerosol challenge

Putative TB vaccine candidates were selected from lists of genes induced in response to in vivo-like stimuli, such as low oxygen and carbon starvation or growth in macrophages, and tested as plasmid DNA vaccines for their ability to protect against Mycobacterium tuberculosis challenge in a guinea pig aerosol infection model. This vaccination method was chosen as it induces the Th1 cell-mediated immune response required against intracellular pathogens such as M. tuberculosis. Protection was assessed in the guinea pig model in terms of mycobacteria present in the lungs at 30 days post-challenge. Protection achieved by the novel candidates was compared to BCG (positive control) and saline (negative control). Four vaccines encoding for proteins such as PE and PPE proteins, a zinc metalloprotease and an acyltransferase, gave a level of protection that was statistically better than saline in the lungs. These findings have enabled us to focus on a sub-set of vaccine candidates for further evaluation using additional vaccination strategies.

Cloning, expression, and purification of recombinant protein from a single synthetic multivalent construct of Mycobacterium tuberculosis

Tuberculosis remains a major infectious disease with over 8 million new cases and 2 million deaths annually. Therefore, a vaccine more potent than BCG is desperately needed. In this regard, an approximately 800 bp DNA encoding a mycobacterial synthetic gene designated as VacIII (containing ubiquitin gene UbGR and four immunogenic mycobacterial epitopes or genes of ESAT-6, Phos1, Hsp 16.3, and Mtb8.4) was sub-cloned into a bacterial expression vector of pRSET-B resulting in a 6 x His-VacIII fusion gene construction. This recombinant clone was over expressed in Escherichia coli BL-21 (DE-3). The expressed fusion protein was found almost entirely in the insoluble form (inclusion bodies) in cell lysate. The inclusion bodies were solubilized with 8M urea and the recombinant protein was purified by Ni-NTA column and dialyzed by urea gradient dialysis. This method produced a relatively high yield of recombinant VacIII protein and the cloned VacIII gene offers the potential development of other vaccine formats such as DNA vaccine and recombinant vaccine.

DNA vaccines against mycobacterial diseases

Plasmid DNA vaccination is a very powerful and easy method for the induction of strong humoral and cell-mediated immune responses in mice. The technique has also been successfully applied for the definition of immunodominant, human T-cell epitopes using HLA-transgenic mice. By virtue of its strong capacity to induce CD4+-mediated Th1 and CD8+-mediated cytotoxic T-lymphocyte responses, this vaccine approach is particularly attractive for the prophylaxis of intracellular pathogens, such as Mycobacterium tuberculosis (TB) and other pathogenic mycobacteria. In small rodents, the potential of mycobacterial DNA vaccines is well established. In humans, DNA vaccines are clearly less immunogenic and, so far, TB-specific DNA vaccines have not been assessed in humans. However, a number of studies in cattle and sheep have demonstrated the potential of mycobacterial DNA vaccines in larger animals. Also, immunization protocols combining the potent priming capacity of plasmid DNA with subsequent boosting with recombinant protein, recombinant pox-viruses or with Mycobacterium bovis bacille Calmette-Guerin (BCG) vaccine are particularly promising for future applications. The potential of mycobacterial DNA vaccines for immunotherapy and post-exposure prophylaxis is still not clear.

Tuberculosis: back on the immunologists' agenda

Although tuberculosis research was instrumental in the birth of immunology, the two disciplines soon separated. The time is ripe for a reunion. Immunology has much to offer for rational intervention measures toward tuberculosis control.

Evaluation of the Mtb72F polyprotein vaccine in a rabbit model of tuberculous meningitis

Using a rabbit model of tuberculous meningitis, we evaluated the protective efficacy of vaccination with the recombinant polyprotein Mtb72F, which is formulated in two alternative adjuvants, AS02A and AS01B, and compared this to vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) alone or as a BCG prime/Mtb72F-boost regimen. Vaccination with Mtb72F formulated in AS02A (Mtb72F+AS02A) or Mtb72F formulated in AS01B (Mtb72F+AS01B) was protective against central nervous system (CNS) challenge with Mycobacterium tuberculosis H37Rv to an extent comparable to that of vaccination with BCG. Similar accelerated clearances of bacilli from the cerebrospinal fluid, reduced leukocytosis, and less pathology of the brain and lungs were noted. Weight loss of infected rabbits was less extensive for Mtb72F+AS02A-vaccinated rabbits. In addition, protection against M. tuberculosis H37Rv CNS infection afforded by BCG/Mtb72F in a prime-boost strategy was similar to that by BCG alone. Interestingly, Mtb72F+AS01B induced better protection against leukocytosis and weight loss, suggesting that the polyprotein in this adjuvant may boost immunity without exacerbating inflammation in previously BCG-vaccinated individuals.

Development of cattle TB vaccines in the UK

In 1996, an independent scientific committee chaired by Professor John Krebs, tasked to review the problem of bovine tuberculosis (TB) in GB, concluded that vaccination of cattle offered the best long-term solution for controlling the disease in the National Herd. This view has been re-affirmed recently in the House of Commons Environment, Food and Rural Affairs Committee's report on Bovine TB (2004) and by the findings of the Independent Scientific Group Vaccine Scoping Sub-committee. Significant progress in developing TB vaccines for cattle has been made over the last 5 years. Specifically: (i) DNA or protein subunit vaccines used in combination with BCG have been shown to give superior protection against experimental challenge in cattle than BCG (heterologous prime-boost); (ii) prototype reagents that allow discrimination between vaccinated and infected animals have been developed; and (iii) and correlates of disease severity have been identified that can predict the success or failure of vaccination. These significant advances are detailed in this review with a summary of future directions that TB vaccine development for cattle is likely to take.

Protective immunity to tuberculosis with Ag85B-ESAT-6 in a synthetic cationic adjuvant system IC31

In this study, we evaluated the potential of a novel synthetic adjuvant designated IC31 for the ability to augment the immune response and protective efficacy of the well-known mycobacterial vaccine antigen, Ag85B-ESAT-6. The IC31 adjuvant, consisting of a vehicle based on the cationic peptide KLKL(5)KLK and the immunostimulatory oligodeoxynucleotide ODN1a signalling through the TLR9 receptor, was found to promote highly efficient Th1 responses. The combination of Ag85B-ESAT-6 and IC31 exhibited significant levels of protection in the mouse aerosol challenge model of tuberculosis and a detailed analysis of the immune response generated revealed the induction of CD4 T cells giving rise to high levels of IFN-gamma secretion. Furthermore, the combination of Ag85B-ESAT-6/IC31 was found to confer efficient protection in the guinea pig aerosol model of tuberculosis infection and is at present moving towards clinical testing.