The Leishmania parasite causes death and disfigurement throughout the world, but scientists have been unable to develop an effective vaccine to prevent infection. Part of the problem is that the body's immune cells seem to "forget" the parasite. But a new study suggests that some immune cells do remember the parasite, and that vaccines that rev up these cells may be the best hope for preventing leishmaniasis.
T cells help protect the body from parasites by traveling to the site of infection and recruiting other immune cells to mount an attack. Once T cells have weathered a battle with a pathogen, they usually fight it better in the future. But that's not the case for Leishmania: Immunity against the parasite seems to disappear once the organism does.
Even so, Phillip Scott and colleagues at the University of Pennsylvania suspected that T cells might retain some memory of previous encounters with Leishmania. To test this idea, the researchers injected T cells from mice that had recovered from leishmaniasis into mice that had never encountered the parasite. When the recipient mice were infected with Leishmania, the veteran T cells rapidly proliferated in the lymph nodes, a sign that the donated T cells had remembered the first infection. Moreover, these mice fought the parasite better than did control mice.
Further experiments show that this memory has lasted at least 5 months. "This suggests that future vaccines should seek to expand the population of these cells," says Scott. He adds that the findings, which are published in the online 26 September issue of Nature Medicine, may be applicable to other infectious diseases such as tuberculosis, for which no highly effective vaccine exists.
"This study will be held up as a way that other people should be analyzing protective immunity," says David Sacks, who heads the Intracellular Parasite Biology Section of the National Institutes of Allergy and Infectious Diseases (NIAID). Robert Seder, an immunologist also at NIAID, says it remains to be seen how long the protection will last.