Cloning and overexpression of Th1 stimulatory proteins for their prophylactic potential against experimental Visceral Leishmaniasis

Abstract

Leishmaniasis is a zoonotic protozoan disease caused by Leishmania. The disease is widely distributed, affecting approximately 12 million people worldwide; causing a wide spectrum of diseases. Leishmaniasis affects the poorest of the poor and is associated with malnutrition, displacement, poor housing, illiteracy, gender discrimination, weakness of the immune system and lack of resources. Leishmaniasis has been considered a tropical affliction that constitutes one of the six entities on the list of most important diseases of World Health Organization/Tropical Disease Research (WHO/TDR) viz. Malaria, Schistosomiasis, Filariasis, Chagas disease, African Trypanosomiasis, Leishmaniasis, Leprosy, Tuberculosis (Desjeux et al., 2001). Leishmania are the protozoan parasites that shuttle between sand fly vector where they multiply as the free promastigotes in the gut lumen and mammalian host where they propagate as intracellular amastigotes in the mononuclear phagocytes (Kedzierski et al., 2006). These parasites are characterized by diverse complexity (Desjeux et al., 2001). A total of about 21 Leishmania species have been identified to be pathogenic to human. Leishmania is one of the several genera within the family Trypanosomatidae, and is characterized by the possession of a kinetoplast. Humans are infected via bite of sand flies (subfamily phlebotominae). Wild and domesticated animals as well as humans can act as a reservoir of infection. Most forms of leishmaniasis are originally infections of small mammals (‘reservoir hosts’), which play a major role in the epidemiology of the disease. Old World (Europe, Asia, and Africa) forms of Leishmania are transmitted by sand flies of the genus Phlebotomus, while New World (America) forms mainly by flies of the genus Lutzomyia. Sand flies become infected by ingesting blood from infected reservoir hosts or from infected people. Transmission of parasite may be anthroponotic (from one human to another) or zoonotic (from animal to human). IIndia, the disease is completely anthroponotic where as in certain parts of the world there are one or more reservoirs (zoonotic host) e.g. dogs in the Mediterranean region and rodents in South Africa. Leishmaniasis has a long history dating back as far as the first century AD. As early as this period, pre-Incan pottery from Ecuador and Peru displayed depictions of skin lesions and facial deformities that are typical of cutaneous and mucocutaneous leishmaniasis. Incan text from the 15th and 16th century and accounts from Spanish conquistadors noted the presence of skin lesions on agricultural workers returning from the Andes. These ulcers resembled leprosy lesions and were labelled, “white leprosy,” “Andean sickness,” or “valley sickness.” In Africa and India, reports in the mid-18 th century describe the disease now known as visceral leishmaniasis, as “kal-azar” or “black fever.” In 1756, Alexander Russell made an important advance in the discovery of Leishmaniasis after examining a Turkish patient. According to Russell, "After it is cicatrised, it leaves an ugly scar, which remains through life, and for many months has a livid colour. When they are not irritated, they seldom give much pain." Russell called this disease, "Aleppo boil." The disease became known as Leishmaniasis after William Leishman, a Glasgwegian doctor serving with the British Army in India, developed one of the earliest stains of Leishmania in 1901. In Dum Dum, a town near Calcutta, Leishman discovered ovoid bodies in the spleen of a British soldier who was experiencing bouts of fever, anaemia, muscular atrophy and swelling of the spleen. Leishman described this illness as “dum dum fever” and published his findings in 1903. Charles Donovan also recognized these symptoms in other kal-azar patients and published his discovery a few weeks after Leishman. After examining the parasite using Leishman's stain, these amastigotes were known as Leishman-Donovan bodies and officially, this species became known as, L. donovani. By linking this protozoan with kal-azar, Leishman and Donovan discovered the genus,1.2 Risk factors and definition of the problem Leishmaniasis is classified as one of the "most neglected diseases," based on the limited resources invested in diagnosis, treatment and control, and its strong association with poverty. The burden of leishmaniasis falls disproportionately on the poorest segments of the global population. Within endemic areas, increased infection risk is mediated through poor housing conditions and environmental sanitation, lack of personal protective measures and economically driven migration and employment that bring nonimmune hosts into contact with infected sand flies Leishmaniasis is endemic in 88 countries, with more than 350 million people at risk. The estimated incidence is 2 million new cases per year, 0.5 million VL and l.5 million CL (Desjeux, 2004a). India, Nepal and Bangladesh harbour an estimated 67% of the global VL disease burden (Hotez et al., 2004), the commitment of the governments of these countries to launch a regional VL elimination programme is welcome. The target of this programme is to eliminate VL as a public health problem in these countries by 2015, by using a local approach to reduce the annual incidence of VL to less than 1 case per 10,000 individuals. Patients with leishmaniasis belong disproportionately to the segments of the global population with neither voice nor power to influence decision-makers, and least able to afford the high cost of treatment. 1.3 Types of Leishmaniasis There are a number of types of protozoa that can cause leishmaniasis. Each type exists in specific locations, and there are different patterns to the kind of disease each causes. The six species of Leishmania recognized to cause disease in humans are very similar morphologically but produce strikingly different pathological responses. The only feature common to all is the chronicity of disease manifestations. Leishmaniasis is spreading in several areas of the world as a result of epidemiological changes which sharply increase the overlapping of AIDS and VL. A patient with leishmaniasis may present with one of three quite distinct clinical syndromes - visceral, cutaneous, or mucosal. 1.3.1 Cutaneous Leishmaniasis (CL) This is the most common form of Leishmaniasis, also known as ‘Oriental sore’ which first appears as a persistent insect bite. Simple skin lesions appear at the site of sand fly bite (Fig.1.3.1) which self-heal within few months but leaves scars. The incubation period can last from few days to months. Gradually, the lesion enlarges, remaining red, but without noticeable heat or pain. Resolution of the lesion involves immigration of leucocytes, which isolate the infected area leading to necrosis of the infected tissues, and formation of a healing granuloma in the floor of the lesion. The disease is mostly prevalent in Mediterranean region, Central Asia and many places of Central Africa (Chatterjee and Ghosh, 1957) . Man is the definitive host whereas gerbils, cats, dogs, and rodent act as the natural reservoir of CL. Sand flies of genus Phlebotomus serve as transmitter for this disease. CL is usually caused by L. major, L. tropica, L. aethiopica, in old world and by L. mexicana, L. venezuelensis, L. amazonensis, L. braziliensis, L. panamensis, L. guyanensis and L. peruviana in new world. This is a chronic, progressive, polyparasitic variant that develops in context of leishmanial specific anergy and is manifested by disseminated non-ulcerative skin lesions, which can resemble lesions of lepromatous leprosy (Fig.1.3.2). DCL is restricted to Venezuela and Dominican Republic in the western hemisphere, and to Ethiopia and Kenya in Africa. Its main causative organisms are L. aethiopica (old world) and L mexicana species complex (new world).