Development of live recombinant vaccine candidates against tuberculosis

Exploring functional genomics for the development of novel intervention strategies against tuberculosis

Tuberculosis (TB) remains a serious threat to humankind, and humans have encountered the causative agent of TB, Mycobacterium tuberculosis (MTB), for more than 10,000 years. Despite rapid advances in technology, efforts to besiege this robust pathogen seem to fail. The availability of genome sequences of several MTB complex strains open a new era of MTB research, the functional genomics, which will provide guidelines for novel control measures. In recent years, a series of methods have been developed to explore the mechanisms employed by MTB to persist and cause disease in the host. DNA array technology enables us to perform comparative genomics of different MTB strains and to examine the gene expression profiles of MTB growing under diverse living conditions. The generated transcriptome data can be exploited for design of new drugs, especially against multidrug-resistant (MDR) strains, development of more efficient vaccines, and identification of biomarkers for better diagnosis.

Is a new tuberculosis vaccine necessary and feasible? A Cuban opinion

Mycobacterium tuberculosis kills more human beings worldwide than any other pathogen. An estimated two billion people are already infected with the bacterium. In 2006, tuberculosis (TB) will kill nearly one million more people than in 1992. In Cuba, TB is not a serious health problem any more and we are striving to eliminate it in the near future. The most widely applied human vaccine in the world is BCG. It is also a safe vaccine except when it is applied to immunocompromised persons. Its protective efficacy is a controversial topic. In spite of this, more than 80 years of experience with this vaccine has demonstrated that BCG is effective, at least in significantly preventing childhood TB, including the meningeal and disseminated forms of the disease, but does not protect against the predominant pulmonary form of the disease in adults, which means that our best TB vaccine now is inadequate; we therefore need a new vaccine. The following facts, apart from the experience with BCG, support discussion about the feasibility of a new and better TB vaccine: Less than 10% of the 2 billion TB infected persons develop active disease. It has been demonstrated that HIV + TB co-infection increases 30 times the risk of contracting active TB and it increases the risk of being killed by the bacterium. Some new vaccine candidates, now under development and evaluation, are showing promising results in preclinical studies, and a few of them have entered clinical trials. There seems to be a consensus that a new TB vaccine will be feasible, but some challenging issues must be positively solved, such as, the lack of universally accepted correlates for protection, improved diagnostics, and final vaccine efficacy evaluation conducted on large phase III clinical trials in underdeveloped countries. The ethical, economical, organizational and scientific questions involved in this global task are enormous, but feasible.

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.

Progress in the development of vaccines and diagnostic reagents to control tuberculosis in cattle

The sharp rise of bovine tuberculosis (TB) in Great Britain and the continuing problem of wild life reservoirs in countries such as New Zealand and Great Britain have resulted in increased research efforts into the disease. Two of the goals of this research are to develop (1) cattle vaccines against TB and (2) associated diagnostic reagents that can differentiate between vaccinated and infected animals (differential diagnosis). This review summarises recent progress and describes efforts to increase the protective efficacy of the only potential TB vaccine currently available, Mycobacterium bovis BCG, and to develop specific reagents for differential diagnosis. Vaccination strategies based on DNA or protein subunit vaccination, vaccination with live viral vectors as well as heterologous prime-boost scenarios are discussed. In addition, we outline results from studies aimed at developing diagnostic reagents to allow the distinction of vaccinated from infected animals, for example antigens that are not expressed by vaccines like Mycobacterium bovis Bacille-Calmette-Guérin, but recognised strongly in Mycobacterium bovis infected cattle.

Preclinical testing of new vaccines for tuberculosis: A comprehensive review

The past decade has seen an explosive increase in the development of potential new tuberculosis vaccine candidates, as well as the establishment of at least two testing centers. Various animal models, but particularly the mouse and guinea pig models, have provided a lot information about how new vaccines can reduce disease progression and how this influences the pathology of the disease, but there is still much to learn at the immunological level, particularly in terms of the nature of the T cell response that is needed to confer long lived resistance. Several categories of vaccine candidates have been tried to date, and there are at least five individual vaccines moving towards clinical evaluation. There are still areas of the field that are poorly developed however. These include the fact that we have no models of post- exposure vaccination, or any models of latent disease. In addition, no standardized models of safety/toxicology exist as yet, which will be needed before extensive clinical development of the new vaccines.

Tuberculosis Vaccines

Tuberculosis continues to be a major cause of disease and death throughout the developing world. Chemotherapy is the current method of control but with the continuing emergence of drug resistance, coupled with the reticence of major drug companies to invest in drug discovery, the identification of new vaccines to combat tuberculosis is a pressing need. Rational vaccine design requires knowledge of the protective immune response and, while this is not fully understood, it is clear that induction of a T-helper-1 type of immunity is critical to host resistance. A variety of animal models, but especially the mouse and guinea pig, can be used to determine the protective efficacy of new vaccines. These mostly consist of relatively short-term prophylactic models in which animals are vaccinated and then challenged by the aerosol infection route to determine their capacity to reduce the lung bacterial load. Several promising vaccine types have emerged, including subunit vaccines, DNA vaccines and vaccines based upon living vectors, such as recombinant bacillus Calmette-Guérin (BCG) vaccines and auxotrophic or gene disrupted mutants of Mycobacterium tuberculosis. A few of these have already entered early stage clinical trials.

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.