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| Credit: Image by Ke Hu and John Murray. Image is licensed under the Creative Commons Attribution 2.5 Generic license |
The life cycle, briefly, of Toxoplasma gondii is as follows (in case you missed the myriads of posts that talk about Toxoplasma). Infective cysts when ingested, rupture in the new host, releasing the parasite. These invade the host cells and become the quickly replicating tachyzoite stage that cause the acute phase of the infection. This attracts the immune system, so that much of the infection is cleared. Some of the tachyzoites, however, persist, hitch rides to immunologically safe locations and become the slow replicating bradyzoites. These then form the bradycysts in important tissues like the CNS, the retina, the heart, causing a persistent chronic infection. When there is immunosuppression, the bradyzoites revert back and wreck havoc.
In an older article in Nature Reviews, the mechanisms of immune evasion and the recruitment of host anti-inflammatory pathways by the parasite are reviewed.
The author approaches the parasite from an evolutionary angle. Although the approach seems to give too much credit to a single celled protozoa, the anthropomorphic notion works well for the crafty Toxoplasma, the sole goal of which is to transmit itself to new hosts. Hence, it must do all that it can to multiply in its hosts, without killing them. To promote host survival, a powerful immune response is induced, which ensures that the host is not killed no matter what. But, Toxoplasma being clever, subverts the immune response. Thus, the very mechanism which ensures host survival is used to increase parasite persistence and transmission.
The immune responses to Toxoplasma are summarized breifly below.
In the initial acute phase of the infection, NK cells are important for the initial immune response. IL12, which is secreted by the macrophages, neutrophils and DCs, triggers the NK cells, and also triggers the interferon gamma dependent responses. DCs secrete IL12 when they harbor replicating Toxoplasma, or if they detect parasite derived molecules, even when other costimulatory signals are absent. Parasite derived molecules include cyclophilin 18 (a prolysylisomerase), which binds to the chemokine recptor CCR5 on DCs and trigger IL12 production. Incidentally, CCR5 also happens to be a coreceptor for HIV invasion, suggesting the hypothesis that C18 could inhibit monocyte infection by certain HIV strains.
TLRs take up some responsibility for IL12 induction. Evidence for this is provided by the fact that in MyD88 deficient mice, there is a marked reduction in the amount of IL12 secreted upon infection.TLR2 seems to play a role in the overall immunity against Toxoplasma, with TLR2 deficient mice showing inefficient nitric oxide production by macrophages.MAP kinase p38 is involved in IL12 signalling. A Interferon inducible transcription factor (IRF8) promotes IL12 gene transcription. Absence of IRF8 also causes an absence of the DC subset CD8alpha +, which is mainly involved in IL12 secretion.
So, why this intense fascination with IL12? What exactly does IL12 do ?
IL12 activates NK cell and causes them to produce INF gamma, which in turn causes the proliferation of Type I CD4+ and CD8+ T cells, both of which also produce IFN gamma.
Macrophages infected with T. gondii produce nitric oxide in response to IFN gamma. Again, this does not completely eliminate the parasite, as some still escape exposure to NO.
Two strategies for immune evasion are mentioned, followed by an elaborate discussion of the second. The first is that the parasite becomes less susceptible to microbicidal activity by producing immunosuppressive molecules such as peroxiredoxins, that act against the effector molecules of the host. The second is that T. gondii uses mechanisms that exploit host anti-inflamatory responses.
Every effector mechanism in the body has a counteracting mechanism that keeps the former in check, ensuring homeostasis. This is true of the inflammatory response as well. A check to the wildly raging IFN gamma-dependent response is offered by IL10, which normally keeps in check the stimulated leukocytes in the acute phase of the infection. Now, IL10 is an antiinflammatory molecule that inactivates the microbicidal pathways triggered by IFN gamma, inhibits antigen processing and presentation by APCs, and inhibits cytotoxic cytokines producd by the T cells.
There is an upregulation of IL10 production that occurs concurrent to a T. gondii infection. If IL10 is neutralized in chronic Toxoplasmosis, severe CNS pathology results. Also, mice that produce little or no IL10 have uncontrolled INF gamma and TNF production in response to T. gondii, that results in severe leukocyte infiltration and hence severe inflammation. These mice also die in the acute phase of the infection. However, experts in the field disagree about the actual role played by IL10.
The second mechanism that is utilized is that of the potent antiinflammatory molecule lipoxin A4. LXA4 is generated by the action of 5-lipooxygenase on arachidonic acid via the intermediate leukotriene A4, which is acted upon by 15-lipoxygenase. Macrophages, but not DCs, participate in the production of lipoxins, even though the DCs are the main targets of LXA4. This mechanism operates independently of IL10. When mice that were IL10-deficient and those that were 5-lipoxygenase deficient, were challenged with T. gondii, severe infiltration with lymphocytes and severe necrosis of the liver without any CNS pathology was observed in the former, while there was infiltration of both the CNS and the liver in the latter. In mice that lacked 5-lipooxgenase, administration of IL 10 caused parasite reactivation and proliferation, because macrophage activity was inhibited.
The cellular source of 15-lipoxygenase, the enzyme that acts on leukotriene A4 to convert it to LX4, has been a mystery, suggesting that perhaps Toxoplasma itself produces the molecule. Proteomic analysis has showed that Toxoplasma tachyzoites carry a lipoxgenase that is similar to a plant derived type I lipoxygenase. Whether this is 15 lipoxygenase is yet to be confirmed. Also, the genes that code for the enzyme are yet to be identified in Toxoplasma. It is speculated that host cell invasion or immune attack of infected cells might trigger an upregulation of the lipoxygenase in intracellular forms.
5-lipoxygenase, the enzyme that converts arachidonic acid to leukotriene A4, is present in the host cells, and can be stimulated by parasite extracts. Since lipoxins dampen the immune response, host survival is ensured.
Two other examples of lipoxygenase modulation, that the author mentions, are: a pathogen coded lipoxygenases (a 15-lipoxygenase-like molecule) in Pseudomonas aeruginosa and a putative one in Mycobacterium tuberculosis.
In P. aeruginosa infections in cystic fibrosis patients, the authors speculate that the bacteria might use the lipooxygenase to produce lipoxins that might suppress inflammation, and so persist by not activating the immune system in immunocompetent patients. The actual relevance of this in clinical situations is yet to be elucidated because CF patients do not produce lipoxins in the lungs. The authors further speculate that mutations in the CFTR and other genes might also affect lipooxgenase generation directly in CF patients.
Mice that did not produce LXA4 endogenously seemed more resistant to infection with Mycobacterium tuberculosis. They survived longer, had lower bacterial counts and a more pronounced type I CMI to the bacteria.
The author suggests that a contradiction emerges. Lipoxins seem to be protective in Toxoplama infections and yet detrimental in M. tuberculosis infections, suggesting that biological reasons are responsible. For Toxoplasma gondii, host survival is necessary to enable predation of the host. So, lipoxins dampen a fierce immune response, disallowing complete elimination of the parasite. In tuberculosis however, high bacterial numbers need to be generated to ensure transmission . Here, lipoxins dampen the immune response to allow proliferation. The author concludes that lipoxin mediated immunomodulation is a new field with many unanswered questions and potential therapeutic applications.
Reference :
Aliberti, J., 2005. Host persistence: exploitation of anti-inflammatory pathways by Toxoplasma gondii. Nat Rev Immunol 5, 162-170.
