Dissecting and exploiting vector components of Leishmania transmission.

Leishmaniasis is a zoonotic vector-borne disease, caused by a parasitic protozoa transmitted by the bite of a bloodfeeding sandfly. Symptoms range from painful skin ulcers, through to infection of the liver and spleen, which is fatal if not treated. Wild rodents and domestic dogs are major reservoirs of infection, which can amplify human infection in endemic areas. There is an urgent need for new preventive measures to control leishmaniasis, in people and in the reservoir hosts, since we have a limited range of effective drugs and currently there is no vaccine. Immunological studies of leishmaniasis have traditionally used mice infected by needle injection of parasites into the skin. It is now very clear, from my own work and that of others, that this does not properly reflect the lesions that develop after natural infection initiated by sandfly bite. I have therefore developed, and evaluated, a protocol by which infection can be induced in the most naturalistic way possible, using infected sandfly bites. Sandflies carry Leishmania parasites in their gut. The parasite produces a gel - promastigote secretory gel (PSG) - that acts as a plug in the fly's gut, forcing it to expel the plug before it can feed on animals. I have shown that infected sand flies regurgitate both parasites and this PSG gel-plug during bloodfeeding and have found that co-injection of purified PSG gel with parasites greatly exacerbated Leishmania infections in mice. This revealed PSG to be a missing and important part of natural infection. I have shown that to promote Leishmania infection in the skin, the gel entice immune cells (macrophages) to the site of the sandfly bite. Macrophages normally kill invading parasites by ingesting and digesting them. However, within the first few days of infection, the gel modifies the function of the macrophages (via a process known as alternative activation) so that they engulf but do not kill the Leishmania; indeed these alternatively activated macrophages provide essential nutrients to the parasites so that they can grow faster. In this way Leishmania parasites are very cunning - they make PSG in order to inhibit the immune system so that they can establish a skin infection. Macrophages receive instructions from T lymphocytes to either kill or not kill the Leishmania that they ingest. How the PSG, macrophage and T cells interact in the skin following natural infection is poorly understood and characterisation of these interactions is a major focus of this project. I have gathered evidence showing that PSG operates through a newly described immune pathway involving the receptor ST2 and its corresponding cytokine, interleukin-33 (IL-33) which are involved in the body's response to allergy and wound healing (eg. a sandfly bite). Therefore, I propose to investigate the immune mechanism(s) which allow PSG to promote Leishmania infection in the skin via this novel signalling pathway, and how the wound response to the sandfly bite interacts with PSG to promote long term infection. From my studies, a striking feature of PSG in skin is its ability to facilitate wound closure, dependent on IL-33. Therefore, an intriguing possibility may arise as a spin-off from my proposed research - that PSG may be developed as a novel treatment for speeding up wound healing in humans and in animals. This may increase healing from surgery, or aid the healing of chronic wounds which are a persistent problem of the elderly, diabetic and obese in humans, and a recurrent problem in livestock. It is expected that the proposed work will not only reveal novel insights into the earliest immune events that govern natural Leishmania infection, but may also reveal potentially exploitable information regarding the response of skin to infection and wounding. I also hope that my studies will help us to design more effective Leishmania vaccines that will help to control the spread of the infection from its animal reservoir into humans