BioWorld International Correspondent
LONDON - A more effective vaccine to protect against tuberculosis may one day be available to help fight the current pandemic of this disease. A new study suggests that the efficacy of the attenuated strain of Mycobacterium bovis, from which the BCG (bacilli Calmette-Guerin) vaccine is made, can be boosted by adding back in some of the genes that it lacks, but which are present in virulent forms of Mycobacterium tuberculosis.
The study, by Stewart Cole of the Institut Pasteur in Paris, and colleagues, together with collaborators at the Liverpool School of Tropical Medicine in the UK and the Centre for Applied Microbiology and Research in Salisbury, UK, is published in Nature Medicine (April 14 advance online publication) in a paper titled "Recombinant BCG exporting ESAT-6 confers enhanced protection against tuberculosis."
Douglas Young, Fleming professor of medical microbiology at Imperial College, London, told BioWorld International, "This is an exciting time in terms of the development of new vaccines to protect against tuberculosis, with several candidates either entering or about to enter human clinical trials. This study is important because it gives us a basic understanding of what strategies might be successful for making effective vaccines, which we can apply to design improved vaccines, rather than the shot-in-the-dark approach which has sometimes been used in the past."
Young, who wrote a News & Views article for the same issue of Nature Medicine titled "Building a better tuberculosis vaccine," added, "The strain developed by Cole and his colleagues can be considered as a vaccine candidate in its own right. At the same time, information from the study will be useful in designing further candidates based on modified BCG or attenuated Mycobacterium tuberculosis."
The starting point for Cole and his colleagues was information from studies involving comparative genomics that found that more than 100 coding sequences present in M. tuberculosis were missing from the BCG strain. Some of those were lost when M. bovis and M. tuberculosis diverged from each other during evolution and others during the passage of M. bovis during the development of the BCG strain.
Among the missing sequences are those encoding two proteins called CFP-10 and ESAT-6, both of which induce a Th1 immune response - exactly the type of immune response that is required to stimulate T cells to activate macrophages to kill intracellular pathogens such as M. tuberculosis. Studies using those proteins as subunit vaccines in animal models have shown that they can protect against tuberculosis but less effectively than BCG.
Scientists working on developing new vaccines have theorized that proteins actively secreted by the bacteria are particularly important for protection against tuberculosis, and Cole's team set out to test which of the "lost genes" needed to be put back into BCG for secretion of ESAT-6 to occur.
They examined which of the nine genes within the cluster known as the "region of deletion-1," or RD1, were necessary for that process. They found the entire cluster was needed for maximum secretion of ESAT-6 by the cells of the live vaccine.
Next, they carried out experiments to test the protective ability of BCG to which they had added back the genes encoding the complete RD1 region. The tests showed that mice and guinea pigs that had received the candidate vaccine were better protected against challenge with M. tuberculosis than those who received the traditional form of BCG. Animals given the modified BCG vaccine had lower numbers of bacteria in their spleens than those who received unaltered BCG, as well as less tissue damage to the spleen normally associated with tuberculosis.
Concluding their paper, Cole and his colleagues wrote: "Ultimately, the robust enhancement in protection observed on re-incorporation of the RD1 locus is a compelling reason to include this genetic modification in any recombinant BCG vaccine, even if it may require a balancing attenuating mutation."
Young, in his article, suggested that if RD1 is effective in stimulating an immune response simply because it allows greater expression of antigen, then it may be possible to boost the immune response to BCG by delivering the proteins present in RD1 as a subunit vaccine. Alternatively, he speculated, RD1-mediated secretion may allow the live cells of the vaccine to persist in the body for longer, and so generate greater numbers of activated T cells. "In planning how to build rationally and progressively on improvements to BCG, it would be useful to disentangle these different mechanisms," Young wrote.