New vaccines against tuberculosis

In September 2000, recognizing the effect of communicable diseases as obstacles to development in poorer countries, the European Commission assembled a special round table on 'accelerated action targeted at major communicable diseases within the context of poverty reduction'. The three major communicable diseases discussed were tuberculosis (TB), malaria and HIV. One outcome of this discussion was a workshop examining issues related to the fight against TB in Africa, which took place in Gorée, Sénégal, in May 2001. The timing was propitious, as new vaccines for TB (recombinant MVA and BCG, and adjuvanated recombinant fusion proteins or peptide constructs), are just beginning to enter human clinical trials. All but the last of these have shown promise in animal models, up to and including non-human primates, and all are strongly immunogenic and apparently safe. Humans trials for safety and efficacy are thus the logical next step. This review summarizes recent advances in tuberculosis vaccine development, with a special emphasis on issues raised at the Gorée meeting about testing and deploying new generation vaccines in TB-endemic areas such as Africa.     

Development of vaccines against tuberculosis

Development of an effective vaccine against tuberculosis (TB) hinges on an improved understanding of the human immune responses to Mycobacterium tuberculosis. A successful vaccination strategy should be able to stimulate the appropriate arm of the immune system with concomitant generation of the memory cells. In the absence of a perfect strategy, while long term efforts of TB researchers continue to resolve the nature of protective immunity against TB and other related issues, the current approach, dictated by the urgency of a TB vaccine, employs available knowledge and technology to develop new TB vaccines and channel the promising ones to clinical trials. While Indian scientists have contributed in several areas towards the development of a TB vaccine, this review is an attempt to summarize their contributions mainly pertaining to the discovery of new antigens, immune responses elicited by antigens against TB and development of new vaccines and their evaluation in animal models.     

Novel vaccines against M. tuberculosis

CDC and ACET in U.S.A. reported that novel vaccines instead of BCG are required for the protection against infection of Mycobacterium tuberculosis worldwide. However, no novel vaccine for clinical use has not yet been developed in the world including U.S.A. and Europe. We have developed two novel tuberculosis (TB) vaccines; a DNA vaccine combination expressing mycobacterial heat shock protein 65 (HSP 65) and interleukin-12 (IL-12) by using the hemagglutinating virus of Japan (HVJ)-liposome (HSP 65 + IL-12/HVJ). A mouse IL-12 expression vector (mIL-12 DNA) encoding single-chain IL-12 proteins comorised of p40 and p35 subunits were constructed. In a mouse model, a single gene gun vaccination with the combination of HSP 65 DNA and mIL-12 DNA provided a remarkably high degree of protection against challenge with virulent Mycobacterium tuberculosis; bacterial numbers were 100 fold lower in the lungs compared to BCG-vaccinated mice. To explore the clinical use of the DNA vaccines, we evaluated HVJ-liposome encapsulated HAP 65 DNA and mIL-12 DNA (HSP 65 + mIL-12/ HVJ). The HVJ-liposome method improved the protective efficacy of the HSP 65 DNA vaccine compared to gene gun vaccination. This vaccine provide remarkable protective efficacy in mouse and guinea pig models, as compared to the current by available BCG vaccine. HSP 65 + IL-12/HVJ vaccine induced CD8+cytoxic T lymphocyte activity against HSP 65 antigen. Protective efficacy of this vaccine was associated with the emergence of IFN-gamma-secreting T cells and activation of proliferative T cells as well as CTL induction upon stimulation with the HSP 65 and antigens from M. tuberculosis. Furthermore, we extended our studies to a cynomolgus monkey model, which is currently the best animal model of human tuberculosis, to evaluate the HSP 65 + IL-12/HVJ vaccine. Vaccination with HSP 65 + IL-12/HVJ provided better protective efficacy as assessed by the Erythrocyte Sedimentation Rate, chest X-ray findings, and immune responses than BCG. Most importantly, HSP 65 + IL-12/HVJ resulted in an increased survival for over a year. This is the first report of successful DNA vaccination against M. tuberculosis in the monkey model. Novel TB vaccines using the monkey model will be discussed in this issue. The development of novel vaccines against tuberculosis was also studied in murine and cynomolgus monkey systems. Four distinct methods; DNA vaccination (1. plasmid, 2. adenovirus vector, 3. adenoassouated virus), 4. recombinant BCG, and 5. subunit (recombinant protein) were used for the development of novel vaccines. Genes (HSP 65 gene, IL-12 gene as well as Ag 85A-, 85B-, MPB51-gene) and IL-6 related genes (IL-6 gene + IL-6R gene +gp130 gene) were administered into the Balb/c mice infected (i.v. or intra-tracheal injection) with Mycobacterium tuberculosis (M. tuberculosis). Elimination of M. tuberculosis in lungs, liver, and spleen of these mice and survival were studied in these models. HSP 65 gene + IL-12 gene vaccination, or recombinant BCG (BA51 : Antigen 85B(-) + Antigen 85A(-) + MPB51-gene recombinant BCG) were more prophylactically efficient than parental BCG Tokyo vaccination. In contrast, IL-6 related genes vaccination using adenovirus vector showed therapeutic effect on M. tuberculosis infected mice. Cytotoxic T cells (CTL) activity against M. tuberculosis in the spleen cells from mice treated with IL-6 related genes vaccination were significantly augmented. Furthermore, NOD-SCID-PBL/hu mice treated with anti-IL-2 receptor beta-chain antibody provide an useful tool for analyzing in vivo human T cell immunity against tuberculosis. In conclusion, we demonstrate the development of a novel HVJ-liposome DNA vaccine encapsulating HSP 65 DNA plus IL-12 DNA. These results suggest that HSP 65 + IL-12/HVJ could be a promising candidate for a new tuberculosis DNA vaccine, which is superior to the currently available BCG vaccine. The goal of our study is to develop a new tuberculosis vaccine superior to BCG. To this aim, we believe that the protective efficacy and protective immune responses for vaccine candidates should be addressed in larger animals, such as nonhuman primates, before proceeding to human clinical trials. Although other DNA vaccine candidates that appear to protect against virulent M. tuberculosis in mice better than BCG have failed to provide better protection than BCG in guinea pigs against aerosol challenge of a low dose of virulent M. tuberculosis, some of them are being prepared to enter early human clinical trials. More recently, we evaluated the HSP 65 + hIL-12/HVJ vaccine in the cynomolgus monkey model, which is currently the best non-human primate animal model of human tuberculosis. Monkeys were subsequently challenged with virulent M. tuberculosis by the intra-tracheal route after the third vaccination. This challenge dose normally causes death from acute respiratory infection within 4-6 months. In this particular experiment, monkeys vaccinated with HSP 65 + hIL-12/HVJ induced HSP 65-specific T-cell proliferation and improvement of chest X-P findings, resulting in an increased survival for over a year, superior to BCG group. Thus, we are taking advantage of the availability of multiple animal models (mouse, guinea pig, and monkey) to accumulate essential data of the HVJ-liposome DNA vaccine, including the vaccine efficacy and safety, for up-coming Phase I clinical trials.     

Use of the bovine model of tuberculosis for the development of improved vaccines and diagnostics

Over the past few years there has been a resurgence in research into bovine tuberculosis due to the sharp rise of the disease in countries such as Great Britain and to the continuing problem of wild-life reservoirs in countries such as New Zealand. One of the goals of this research is to develop cattle vaccines against TB. The initial testing of candidate vaccines is carried out in laboratory animals, initially mice and subsequently guinea pigs. A unique feature of the cattle vaccination programme is that candidate vaccines which show promise in laboratory models can then be tested in the natural host species, cattle, before progressing to clinical trials. This is a major advantage over the strategy for developing a vaccine for human tuberculosis where, of course, it is impossible to test a candidate vaccine by experimentally challenging the host species with the pathogen. The most commonly used model for testing vaccine candidates in cattle consists of an intra-tracheal challenge of between 10(3) and 10(4) colony forming units of Mycobacterium bovis. The pathology observed following challenge is similar to human tuberculosis giving rise to a marked granulomatous reaction and a predominantly cellular immune response. Using this model we have been able to make a number of significant advances towards a bovine TB vaccine. First we have developed antigen cocktails that, when used in a whole blood gamma interferon assay, can differentiate between M. bovis infected and BCG vaccinated animals. Next we have developed immune correlates of pathology, which allow us to assess whether the vaccine is protecting animals against challenge before post mortem examination. Finally we have been able to use the model to develop a vaccine that improves the efficacy of BCG against M. bovis challenge.     

Plasmodium simium and Saimiri boliviensis as a model system for testing candidate vaccines against Plasmodium vivax

Observations on Plasmodium simium infections in Saimiri boliviensis boliviensis monkeys suggest that this host-parasite combination would be a suitable model for the testing of candidate vaccines against Plasmodium vivax. To evaluate the normal course of infections, parasitemia in 52 splenectomized S. boliviensis boliviensis monkeys infected with P. simium were analyzed. The mean maximum parasite count for 31 monkeys after injection with trophozoite-infected erythrocytes was 77,580/microL. Twenty-one monkeys were infected via sporozoites, and prepatent periods ranged from 14 to 24 days with a median of 15 days. The mean maximum parasite count was 29,234/microL. The mean maximum parasite count for monkeys previously infected with Old World P. vivax was 26,337/microL versus 56,362/microL for those previously infected with New World P. vivax, possibly suggesting a closer antigenic relationship between P. simium and the Old World parasites.     

Isoniazid treatment of Mycobacterium bovis in cattle as a model for human tuberculosis

Cattle infected with Mycobacterium bovis spoligotype 9 were treated with Isoniazid (INH) from three to 14 weeks post infection, rested for four weeks to allow INH depletion and then challenged with M. bovis spoligotype 35. Post mortem examination (PME) 35 weeks after the initial infection showed partial protection against infectious challenge following INH-attenuated infection compared with the spoligotype 35 challenge controls. Antigen-specific IFN-γ responses decreased over time with INH therapy, following a similar pattern to that observed in the treatment of M. tuberculosis infection in humans. Following cessation of therapy, specific IFN-γ responses increased more strongly in those calves that were visibly lesioned at PME. IFN-γ responses were also used to identify two antigens, TB10.4 and Acr2, that induced anamnestic responses in INH-treated, re-challenged calves, suggesting a role for both antigens in protective immunity. Specific IL-10 responses were observed in all calves following treatment with INH suggesting a role for IL-10 in the resolution of infection.     

Aotus nancymaae as a potential model for the testing of anti-sporozoite and liver stage vaccines against Plasmodium falciparum

The Santa Lucia strain of Plasmodium falciparum was transmitted to Aotus lemurinus griseimembra, A. azarae boliviensis, A. vociferans, and A. nancymaae monkeys by bite and by intravenous inoculation of sporozoites dissected from Anopheles freeborni, An. stephensi, An. gambiae, An. albimanus, and An. maculatus mosquitoes. The data obtained from these infections indicate that A. nancymaae can be considered a suitable host model when combined with the Santa Lucia strain of P. falciparum for the testing of candidate anti-sporozoite and liver stage vaccines.     

Human immune recognition-based multicomponent subunit vaccines against tuberculosis.

The cell-mediated immune response, with its shift in favour of type-1 over type-2 T-helper cell immune response, is generally regarded as essential to protection against mycobacterial infections. The aim of this study was to evaluate the protective potential of two multicomponent subunit vaccines (MSV-1 and MSV-2) against tuberculosis (TB) based on human immune recognition. MSV-1 consisted of five immunodominant antigens (TB10.4, early secretory antigenic target (ESAT)-6, culture filtrate protein (CFP)-8, CFP-10 and CFP-15) selected from a group of polypeptides, which induced a predominant T-cell response in immune human subjects, whereas MSV-2 consisted of antigens (CFP-11, CFP-21, CFP-22.5, Mycobacterium tuberculosis protein (MPT)-64 and CFP-31) selected from a group of polypeptides which induced a subdominant T-cell response along with the antibody response. Both of these sets of polypeptides were extensively recognised in healthy individuals with significant interferon gamma release compared to the diseased population. In C57BL/6J mice, at the level of the lungs, the order of protective efficacy for the test vaccines was: bacille Calmette-Guerin (BCG)>MSV-2>MSV-1. The protective efficacy of MSV-1 was found to be significantly less than that of MSV-2 and BCG at the level of spleen, whereas that of MSV-2 was comparable to that of BCG. The results of this study indicate that high T-helper cell type 1 response-inducing polypeptides selected on the basis of human immune recognition do not necessarily impart protection during vaccination experiments.     

New vaccines against tuberculosis. The status of current research

The increasing realization that the current vaccine for tuberculosis, bacille Calmette-Guérin (BCG), is of varying effectiveness, and is less protective in adults than in children, has prompted new research for a replacement. New research has resulted in innovative approaches, including the use of sub-unit vaccines, auxotropic vaccines, DNA vaccines, and recombinant vaccines, among others. This article reviews these approaches and test results in animal models, and discusses their potential for use in humans.     

Prospects for better tuberculosis vaccines

Tuberculosis remains one of the top three infectious disease killers. Treatment is long and expensive and drug resistant strains of Mycobacterium tuberculosis are already on the rise. The current vaccine, BCG, is ineffective in parts of the world where the disease is most widespread and therefore the search for a novel, more effective vaccine is paramount. In this review we discuss the current state of vaccine research, including the identification of candidate antigens and the current methods used for their evaluation.