The study found that parasites engineered to lack the molecule known as lipophosphoglycan (LPG) were ten times more vulnerable to attack by an immune defense known as complement, which is found in the bloodstream. And although parasites that lack LPG easily enter macrophages (immune cells that the parasite normally infects), they were quickly destroyed once inside the cells.
The findings are published online and in the Aug. 5 issue of the Proceedings of the National Academy of Sciences.
"This study helps us better understand how these parasites are transmitted and how they establish infections," says principal investigator Stephen M. Beverley, Ph.D., the Marvin A. Brennecke Professor of Molecular Microbiology and head of the department. "It also could help efforts leading to the development of a vaccine to prevent this devastating disease."
About 12 million people are thought to be infected with leishmania parasites worldwide. The parasite is spread by the bite of infected sand flies and can cause leishmaniasis, a sometimes fatal and disfiguring disease.
Cutaneous leishmaniasis causes open, slow-healing and sometimes disfiguring skin sores. Visceral leishmaniasis spreads to several organs and is usually fatal if left untreated. About 1.5 million new cases of cutaneous leishmaniasis and 500,000 new cases of visceral leishmaniasis occur each year. The mainly tropical and subtropical disease takes its greatest toll in poor countries such as India and war-torn countries such as Sudan. It also arises as an opportunistic infection in people with AIDS.
Beverley and a group of his colleagues genetically engineered a strain of leishmania parasites that lacked LPG. They tested the ability of these modified parasites to cause infection in animals, survive exposure to human complement in blood serum and enter, survive and infect macrophages in mice. Macrophages normally patrol the body and engulf or phagocytize, invading microbes and other material. Inside the macrophage, microbes are destroyed or "digested" by corrosive substances known as oxidants. Beverley and the team of investigators found that modified leishmania parasites were more likely to be destroyed by these oxidants as normal parasites. They then used genetically engineered mice that lacked the oxidative defense system to prove that this played a key role in disease.
Parasites without LPG, however, retained the ability to resist other macrophage defenses including exposure to acid substances and enzymes that attack proteins. Thus, while LPG is critical to the survival of Leishmania, other molecules must collaborate with it. Beverley and his colleagues are studying several of these associated molecules, including one that is structurally related to LPG. "Learning more about the biochemistry of these proteins and the genes that encode them will give us some unique targets for the development of new drugs which are badly needed to treat this disease," Beverley says.
Spath GF, Garraway LA, Turco SJ, Beverley SM. The role(s) of lipophosphoglycan (LPG) in the establishment of Leishmania major infections in mammalian hosts. Proceedings of the National Academy of Sciences, Aug. 5, 2003.
Funding from the National Institute of Allergy and Infectious Diseases, the Deutsche Akademische Austauschdienst and the Human Frontiers Science Program supported this research.
Journal
Proceedings of the National Academy of Sciences