LONDON ¿ Manipulating the tuberculosis bacterium to produce one of its own proteins in excess can make it susceptible to the mammalian immune response, researchers have found. The finding could lead to a completely novel generation of drugs to treat tuberculosis, and may help to improve the current vaccine, which is not totally effective.
Tuberculosis kills more people than any other infectious disease. Just as importantly, 90 percent of people infected by Mycobacterium tuberculosis initially control the infection ¿ but remain infected for the rest of their lives. Although they may have no symptoms, the disease may reactivate when they become immunosuppressed, or as they age.
The discovery, made by a team of researchers in the UK, means that it may one day be possible to tackle this enormous reservoir of infection, which is present in an estimated one in three people worldwide. The researchers report their study in a paper in Nature Medicine titled ¿Overexpression of heat-shock proteins reduces survival of Mycobacterium tuberculosis in the chronic phase of infection.¿
Graham Stewart, research fellow at the Centre for Molecular Microbiology and Infection at Imperial College of Science, Technology and Medicine in London, who is first author of the paper, told BioWorld International: ¿Our study points to a very different way of thinking about the problem of tuberculosis, which makes it a potentially significant finding.¿
Stewart, who carried out the work in the laboratory of Douglas Young, Fleming professor of Molecular Microbiology at Imperial College, together with colleagues at the University of Hertfordshire in Hatfield, UK, and at GlaxoSmithKline, of Stevenage, UK, decided to examine what happened when M. tuberculosis was genetically manipulated to produce excessive amounts of heat-shock proteins.
Heat-shock proteins, also known as molecular chaperones, are molecules found in all types of cells, and normally play a role in folding the polypeptides that form proteins, in refolding those that have folded incorrectly, and in transporting proteins around the cells. They also have functions within the immune system, promoting the presentation of antigens and acting as a danger signal that stimulates the production of molecules that modulate the immune response.
The team of researchers decided to delete a gene, whose product normally represses the production of heat-shock proteins, from the genome of M. tuberculosis. They found that these bacteria overexpressed a heat-shock protein called HSP70, and carried out tests to evaluate the pathogenicity of the mutant bacteria using a mouse model of chronic infection with M. tuberculosis. As in humans, the immune systems of these mice normally control the infection except when the animals are given immunosuppressants.
Some mice were infected with the mutant M. tuberculosis and some with wild-type bacteria. When the researchers looked at how the infections progressed, they found that during the persistent phase of infection (which follows the acute primary phase), the mutants survived less well than the wild-type bacteria.
As both mutant and wild-type bacteria had grown equally well in laboratory culture, the researchers decided to examine the murine immune response to both groups. They found that the mice infected with the mutant bacteria had an enhanced immune response, specifically to HSP70, but also a more general increase in the number of interferon-gamma-producing CD8 T cells.
Stewart said: ¿Our study shows that overexpression of HSP70 appears to allow the host¿s natural immune response to largely clear the infection during the persistent phase.¿
It may be possible, he added, to develop new drugs that make the bacteria susceptible to the immune system by controlling expression of heat-shock proteins, either by disrupting the repression system, or by inducing production of heat-shock proteins by some other route. ¿The heat-shock protein repressor itself makes a good target because it is only found in a handful of bacteria, and definitely not in eukaryotes,¿ Stewart said.
The group is currently trying to solve the structure of the heat-shock protein repressor in order to further their drug discovery efforts. They have also developed an assay to allow them to screen for drugs that induce expression of HSP70.
¿What we would like to do with the assay eventually is to run a big library of compounds through it, but first we have to prove it works in principle,¿ Stewart said.
Improvements in vaccines may also be possible. The current vaccine is a live attenuated version of M. bovis, which is most effective when given to children. Stewart explained that the group had also disrupted the heat-shock protein repressor in M. bovis, to give rise to a vaccine strain that overexpresses heat-shock proteins. This is currently being tested by Ian Orme of Colorado State University as part of a vaccine screening program funded by the National Institutes of Health.