The biochemistry and cell biology of
Leishmania is similar to that of other
kinetoplastids. They share the same main morphological features: a single
flagellum which has an invagination - the flagellar pocket - at its base; a
kinetoplast, which is found in the single
mitochondrion; and a subpelicular array of microtubules, which make up the main part of the
cytoskeleton.
Lipophosphoglycan coat Leishmania possesses a
lipophosphoglycan coat over the outside of the cell. Lipophosphoglycan is a trigger for
toll-like receptor 2, a signalling receptor involved in triggering an
innate immune response in mammals. The precise structure of lipophosphoglycan varies depending on the species and
lifecycle stage of the parasite. The glycan component is particularly variable and different lipophosphoglycan variants can be used as a
molecular marker for different lifecycle stages.
Lectins, a group of
proteins which bind different glycans, are often used to detect these lipophosphoglycan variants. For example,
peanut agglutinin binds a particular lipophosphoglycan found on the surface of the infective form of
L. major. Lipophosphoglycan is used by the parasite to promote its survival in the host and the mechanisms by which the parasite does this center around modulating the immune response of the host. This is vital, as the
Leishmania parasites live within
macrophages and need to prevent the macrophages from killing them. Lipophosphoglycan has a role in resisting the
complement system, inhibiting the
oxidative burst response, inducing an
inflammation response and preventing
natural killer T cells recognising that the macrophage is infected with the
Leishmania parasite.
Intracellular mechanism of infection In order to avoid destruction by the
immune system and thrive, the
Leishmania 'hides' inside its host's cells. This location enables it to avoid the action of the
humoral immune response (because the pathogen is safely inside a cell and outside the open bloodstream), and furthermore it may prevent the immune system from destroying its host through nondanger surface signals which discourage
apoptosis. The primary cell types
Leishmania infiltrates are
phagocytotic cells such as
neutrophils and
macrophages. Usually, a phagocytotic immune cell like a macrophage will ingest a pathogen within an enclosed
endosome and then fill this endosome with enzymes which digest the pathogen. However, in the case of
Leishmania, these enzymes have no effect, allowing the parasite to multiply rapidly. This uninhibited growth of parasites eventually overwhelms the host macrophage or other immune cell, causing it to die. Transmitted by the
sandfly, the
protozoan parasites of
L. 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 (PMNs) 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 Upon
microbial infection, PMNs move out from the bloodstream through the vessels' endothelial layer, to the site of the infected tissue (dermal tissue after fly bite). They immediately initiate the first immune response and phagocytize the invader by recognition of foreign and activating surfaces on the parasite. Activated PMN secrete
chemokines,
IL-8 particularly, to attract further
granulocytes and stimulate phagocytosis. Further,
L. major increases the secretion of IL-8 by PMNs. This mechanism is observed during infection with other
obligate intracellular parasites, as well. For microbes like these, multiple intracellular survival mechanisms exist. Surprisingly, the coinjection of apoptotic and viable pathogens causes by far a more fulminate course of disease than injection of only viable parasites. When the anti-inflammatory signal
phosphatidylserine usually found on apoptotic cells, is exposed on the surface of dead parasites,
L. major switches off the
oxidative burst, thereby preventing killing and degradation of the viable pathogen. In the case of
Leishmania, progeny are not generated in PMNs, but in this way they can survive and persist untangled in the primary site of infection. The promastigote forms also release
Leishmania chemotactic factor (LCF) to actively recruit 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 PMNs 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 PMNs, 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 to 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,
L. 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 two or three days. The pathogens are not dronish; instead they take over the command at the primary site of infection. They induce the production by PMNs of the chemokines MIP-1α and MIP-1β (
macrophage inflammatory protein) to recruit macrophages. An important factor in prolonging infection is the inhibition of
the adaptive immune system. This occurs especially during the intercellular phases, when amastigotes search for new macrophages to infect and are more susceptible to immune responses. Nearly all types of
phagocytes are targeted. For example,
mincle has been shown to be targeted by
L. major. Interaction between mincle and a protein released by the parasite results in a weakened immune response in
dendritic cells.
Silent phagocytosis theory To save the integrity of the surrounding tissue from the
toxic cell components and
proteolytic enzymes contained in neutrophils, the apoptotic PMNs are silently cleared by macrophages. Dying PMNs 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 PMNs are taken up into macrophages, employing an absolutely
physiological and nonphlogistic process. The strategy of this "silent phagocytosis" has the following advantages for the parasite: • Taking up apoptotic cells silences macrophage killing activity leading to a survival of the pathogens. • Pathogens inside of PMNs 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. However, studies have shown this is unlikely, as the pathogens are seen to leave apoptotic cells and no evidence is known of macrophage uptake by this method. == Molecular biology ==