Leishmania

Leishmania is a genus of trypanosome protozoa, and is the parasite responsible for the disease leishmaniasis.^[1] It is spread through sandflies of the genus Phlebotomus in the Old World, and of the genus Lutzomyia in the New World. Their primary hosts are vertebrates; Leishmania commonly infects hyraxes, canids, rodents, and humans. Leishmania currently affects 12 million people in 88 countries.

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Origin

The origins of Leishmania are unclear.^[2][3] One possible theory proposes an African origin, with migration to the Americas. Another migration from the Americas to the Old World about 15 million years ago, across the Bering Strait land bridge. Another proposes a palearctic origin.^[4] Such migrations would entail migration of vector and reservoir or successive adaptations along the way. A more recent migration is that of L. infantum from Mediterranean countries to Latin America (there named L. chagasi), since European colonization of the New World, where the parasites picked up its current New World vectors in their respective ecologies. This is the cause of the epidemics now evident. One recent New World epidemic concerns foxhounds in the USA.

Pathophysiology

Leishmania cells have two morphological forms: promastigote (with an anterior flagellum)^[5] in the insect host, and amastigote (without flagella) in the vertebrate host. Infections are regarded as cutaneous, mucocutaneous, or visceral.

Cutaneous (localized and diffuse) infections appear as obvious skin reactions. The most common is the Oriental Sore (caused by Old World species L. major, L. tropica, and L. aethiopica). In the New World, the most common culprits are L. mexicana and L. (Viannia) braziliensis. Cutaneous infections are most common in Afghanistan, Brazil, Iran, Peru, Saudi Arabia and Syria.

Mucocutaneous (espundia) infections will start off as a reaction at the bite, and can go via metastasis into the mucous membrane and become fatal. Mucocutaneous infections are most common in Bolivia, Brazil and Peru.

Visceral infections are often recognized by fever, swelling of the liver and spleen, and anemia. They are known by many local names, of which the most common is probably Kala azar,^[6][7] and are caused exclusively by species of the L. donovani complex (L. donovani, L. infantum syn. L. chagasi).^[1] Found in tropical and subtropical areas of all continents except Australia, visceral infections are most common in Bangladesh, Brazil, India, Nepal and Sudan.^[1]

Treatment

Main article: Leishmaniasis

Antimonial compounds are the traditional treatments for leishmaniasis (sodium stibogluconate, meglumine antimoniate).^[8]

Resistance to the antimonials is prevalent in some parts of the world, and the most common alternative is amphotericin B^[9] (see leishmaniasis for other treatment options). Paramomycin is an inexpensive alternative with fewer side effects than amphotericin that The Institute for OneWorld Health has funded for production as an orphan drug for use in treatment of leishmaniasis, starting in India.

Molecular biology

An important aspect of the Leishmania protozoan is its glycoconjugate layer of lipophosphoglycan (LPG). This is held together with a phosphoinositide membrane anchor, and has a tripartite structure consisting of a lipid domain, a neutral hexasaccharide, and a phosphrorylated galactose-mannose, with a termination in a neutral cap. Not only do these parasites develop post-phlebotomus digestion but, it is thought to be essential to oxidative bursts, thus allowing passage for infection. Characteristics of intracellular digestion include an endosome fusing with a lysosome, releasing acid hydrolases which degrade DNA, RNA, proteins and carbohydrates.

Leishmania as component of CVBD

Canine Vector-borne Diseases (CVBD) covers diseases caused by pathogens transmitted by ectoparasites as ticks, fleas, sand flies or mosquitoes.

Other microorganism-based diseases caused by ectoparasites include Bartonella, Borrelia, Babesia, Dirofilaria, Ehrlichia, and Anaplasma.

Neutrophil granulocytes - the Trojan horses for Leishmania parasites

The strategy of the "Trojan horse" as a mechanism of pathogenicity of intracellular microorganisms is, to avoid the immune system and its memory function cleverly, with phagocytosis of infected and apoptotic neutrophils by macrophages, employing the non-danger surface signals of apoptotic cells.

Transmitted by the sandfly, the protozoan parasites of the genus Leishmania major may switch the strategy of the first immune defense from eating/inflammation/killing to eating/no inflammation/no killing of their host phagocyte' and corrupt it for their own benefit. They use the willingly phagocytosing polymorphonuclear neutrophil granulocytes (PMN) rigorously as a tricky hideout, where they proliferate unrecognized from the immune system and enter the long-lived macrophages to establish a “hidden” infection.

Uptake and survival

By a microbial infection PMN move out from the bloodstream and through the vessels’ endothelial layer, to the site of the infected tissue (dermal tissue after fly bite). They immediately start their business there as the first immune response and phagocyte the invader because of the foreign and activating surfaces. In that processes an inflammation emerges. Activated PMN secrete chemokines, IL-8 particularly, to attract further granulocytes and stimulate them to phagocytosis. Furthermore Leishmania major increases the secretion of IL-8 by PMN. In the parasites case, that may not sound reasonable at first. We can observe this mechanism on other obligate intracellular parasites, too. For microbes like these, there are several ways to survive inside cells. Surprisingly, the co-injection of apoptotic and viable pathogens causes by far a more fulminate course of disease than injection of only viable parasites. Exposing on the surface of dead parasites the anti-inflammatory signal phosphatidylserine, usually found on apoptotic cells, Leishmania major switches off the oxidative burst, so killing and degradation of the co-injected viable pathogen is not achieved. In case of Leishmania progeny is not generated in PMN, but in this way they can survive and persist untangled on the primary site of infection. The promastigote forms also release LCF (Leishmania chemotactic factor) to recruit actively neutrophils but not other leukocytes , for instance monocytes or NK cells. In addition to that, the production of interferon gamma (IFNγ)-inducible protein 10 (IP10) by PMN is blocked in attendance of Leishmania, what involves the shut down of inflammatory and protective immune response by NK and Th1 cell recruitment. The pathogens stay viable during phagocytosis since their primary hosts, the PMN, expose apoptotic cell associated molecular pattern (ACAMP) signaling “no pathogen”.

Persistency and attraction

The lifespan of neutrophil granulocytes is quite short. They circulate in bloodstream for about 6 or 10 hours after leaving bone marrow, whereupon they undergo spontaneous apoptosis. Microbial pathogens have been reported to influence cellular apoptosis by different strategies. Obviously because of the inhibition of caspase3-activation Leishmania major can induce the delay of neutrophils apoptosis and extend their lifespan for at least 2–3 days. The fact of extended lifespan is very beneficial for the development of infection because the final host cells for these parasites are macrophages, which normally migrate to the sites of infection within 2 or 3 days. The pathogens are not dronish; instead they take over the command at the primary site of infection. They induce the production by PMN of the chemokines MIP-1α and MIP-1β (macrophage inflammatory protein) to recruit macrophages.

Silent phagocytosis

To save the integrity of the surrounding tissue from the toxic cell components and proteolytic enzymes contained in neutrophils, the apoptotic PMN are silently cleared by macrophages. Dying PMN expose the "eat me"-signal phosphatidylserine which is transferred to the outer leaflet of the plasma membrane during apoptosis. By reason of delayed apoptosis the parasites that persist in PMN are taken up into macrophages, employing an absolutely physiological and non-phlogistic process. The strategy of this "silent phagocytosis" has following advantage for the parasite:

• Taking up apoptotic cells silences macrophage killing activity leading to a survival of the pathogens.

• Pathogens inside of PMN have no direct contact to the macrophage surface receptors, because they can not see the parasite inside the apoptotic cell. So the activation of the phagocyte for immune activation does not occur.

Literature:

• Zandbergen et al. "Leishmania disease development depends on the presence of apoptotic promastigotes in the virulent inoculum", PNAS, Sept. 2006 (PDF)

• Laskay et al. "Neutrophil granulocytes - Trojan horses for "Leishmania major" and other intracellular microbes?", TRENDS in microbiology, May 2003

• Shaw, J. J. (1969). The haemoflagellates of sloths. London, H. K. Lewis & Co. Ltd. (PDF)

References

• A discussion list (Leish-L) is also available with over 600 subscribers to the list, ranging from molecular biologists to public health workers, from many countries both inside and outside endemic regions. Comments and questions are welcomed.

• KBD: Kinetoplastid Biology and Disease, is a website devoted to leishmaniasis, sleeping sickness and Chagas (American leishmaniasis). It contains free access to full length peer review articles on these subjects. The site contains many articles relating to the unique kinetoplastid organelle and genetic material therein.