BY MITCH LESLIE
Nearly 80 years old, the tuberculosis vaccine is showing its age. This stalwart of the global campaign against TB may no longer work, and Stanford researchers have discovered a possible reason why: the bacteria used to make the vaccine have jettisoned some of their genes. The discovery could guide the development of an up-to-date vaccine as well as a more accurate diagnostic test for the disease.
The TB vaccine, known as BCG, is one of the most widely used vaccines in the world. More than 1 billion people have received it, and an additional 100 million are vaccinated each year. "The vaccine is given to virtually every child in the developing world," said Marcel Behr, MD, first author of the study published in the May 28 issue of Science. "The good news is that it's safe. The bad news is that we don't know if it does anything."
Behr, an associate professor of infectious diseases and medical microbiology at McGill University in Montreal, was a member of the research team while a postdoctoral fellow at Stanford.
BCG contains live bacteria that must grow inside the body to stimulate a protective reaction from the immune system. Scientists at the Pasteur Institute in Paris created the vaccine in the early part of this century by "domesticating" a cattle bacterium that is very similar to the TB bacterium. The French scientists grew the cattle germ for more than 200 generations in a nutritious broth, a procedure that sapped the bacterium's disease-causing tendency without reducing its similarity to TB.
As far as the immune system is concerned, this cattle bacterium is a dead ringer for the TB bacterium. The early versions of the vaccine were probably quite good at stimulating the immune system to make defenses against TB, said Peter Small, MD, assistant professor of medicine and leader of the research effort. However, the vaccine has lost this ability in the years since, he said.
The investigators suspected that genetic changes in the bacteria used in BCG might be behind this deterioration. To test their idea, they used a newly developed technology called a DNA microarray, which allows the quick analysis of an organism's full complement of genes.
The researchers found that the bacteria used in BCG were missing 38 genes that occur in normal strains of the cattle bacterium. Presumably, these genes have disappeared through natural mutations since the cattle bacterium was first brought into the lab to make the vaccine.
Bacteria that had lost so many genes probably would not survive outside the lab, Small said. "But it's a pretty cush life for a bacterium to grow in a laboratory," he said. With every need met, the bacteria can survive without some formerly indispensable genes.
Once easy living robbed the bacteria of their ability to survive within the human body, they could no longer trigger the immune system. "We take the mutants that have been pampered in the lab and inject them into people," Behr said. "We think the bacteria may be dying so quickly that they aren't eliciting an immune response."
The scientists propose that the progressive weakening of the vaccine can be traced to its handling after it was introduced in the early 1920s. Until techniques for preserving living bacteria were invented in the early 1960s, the only way to keep producing the vaccine was to keep the bacteria growing in the lab. The vaccine went through more than 1,000 generations in that period, growing weaker and weaker. "Essentially, BCG is a vaccine that has been attenuated to impotence," Small said.
Knowing what's missing from BCG may help scientists craft a better TB vaccine. But the first application from these results will likely be a more accurate test for TB infection, Small said. One thing that BCG does is stimulate a positive reaction on the most commonly used test for TB infection, the PPD test. This makes it difficult to distinguish people who have TB from those who have merely been vaccinated against it.
But when the researchers compared the genes of the TB bacterium with those from the cattle bacterium, they found that the TB bacterium has 91 genes that are not present in its close relative. The products of these genes could be synthesized and included in a diagnostic test that would enable doctors to pinpoint TB infection.
Why are millions of people receiving a vaccine that may be worthless? "The reason it is given is that in certain studies it has shown benefit, and it's hard to take away something that's perceived to be beneficial," Behr said. He added that the vaccine does seem to protect children against a form of tuberculosis that often strikes the young. It just doesn't seem to work against the adult form of the disease, which primarily attacks the lungs.
BCG is not used in the United States and is given only to high-risk populations in Canada, Behr said.
Behr and Small's Stanford co-authors
include graduate student Michael Wilson; medical student Wendy
Gill; post-doctoral fellow Hugh Salamon, PhD; professor of medicine
Gary Schoolnik, MD; and technician Sangeeta Rane. SR