The parasites that cause leishmaniasis use a quirky trick to convince the immune system to effectively invite them into cells for dinner. The researchers, from Imperial College London, say their findings improve understanding of the way Leishmania parasites establish an infection and could aid the search for a vaccine against this neglected tropical disease. Leishmania parasites are transmitted by sand flies. After the parasites infect a sand fly, they make a sticky gel so that when the fly bites a human, it regurgitates this gel into the body. Today's research, which was funded by the Wellcome Trust, shows that the gel persuades immune cells known as macrophages to feed the parasites, rather than killing them.
Leishmaniasis is an infection caused by Leishmania parasites that affects around 12 million people per year, mainly in tropical and sub-tropical countries. Symptoms include disfiguring and painful skin ulcers and in severe cases the infection can also spread to the internal organs. Patients with the infection often suffer from social exclusion because of their disfigurement. There is currently no vaccine to protect against infection and although treatments are available, they are not always effective and access to drugs is limited in many areas.
Leishmania-infected sand flies carry the parasites in their midgut. The parasites produce a gel that turns into a plug, stopping anything from passing in or out of the fly's gut. The fly must regurgitate the gel plug before it can feed on human blood. When the fly bites a person, its barbed mouth parts tear the skin so when it regurgitates parasites along with the gel plug, the skin becomes infected.
Today's study shows that the gel's work doesn't stop there - it also helps the parasites to establish an infection by enticing macrophages to the bite site. Macrophages usually kill invading pathogens by eating and digesting them. However, according to the new research, the gel persuades macrophages to engulf the parasites and feed them rather than digest them. This happens within the first few days following infection, enabling the parasites to establish themselves and infect the skin.
The new findings may help scientists develop a vaccine against Leishmania infection
Previous research suggested that the sand fly's saliva could be involved in manipulating the immune system. Today's study suggests that the gel has an even bigger effect than the saliva on establishing infection.
Imperial College London
