The role of the P2X7 receptor in the intestinal inflammatory response to the parasite, Toxoplasma gondii
- Publication Type:
- Thesis
- Issue Date:
- 2011
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Toxoplasma gondii is an obligate intracellular protozoan parasite belonging to the
phylum Apicomp lexa. It is found throughout the world, infecting a variety of warmblooded
animals; overall it infects one in three people worldwide, though local
prevalence rates vary widely.
Experimental oral infection of susceptible strain s of mice w ith cysts of T. gondii is
known to provoke an acute inflammatory response in the intestine, known as
toxoplasmic ileitis. The hypothesis tested in this thesis was that the purinergic P2X7
receptor plays a critical role in intestinal inflammation. This was tested by infecting
mice with 10 cysts of the ME49 st rain of T. gondii - three strains of mouse were used:
BALB/c mice, which are known to be resistant to toxoplasmic ileitis; C57BL/6J mice,
which are classified as susceptible to toxoplasmic ileitis; and P2X7 receptor knockout
mice (which are C57BL/6J mice whose gene for the P2X7 receptor has been deleted).
Following infection, experiments were done to assess the effect of the absence of the
P2X7 receptor on: (a) weight loss and intestinal pathology; (b) parasite control; (c)
changes in pro- and anti-inflammatory mediators; and (d) activation of key
transcrip ion factors and intracellular signalling pathways.
In vivo studies showed that absence of the P2X7 receptor rendered mice acutely
susceptible to oral infection with T. gondii ME49. P2X7 receptor knockout mice lost
weight at a much faster rate and had significantly higher levels of intestinal pathology
(including swelling, angiogenesis, intestinal villous breakdown and infiltration of
inflammatory cells) than either C57BL/6J or BALB/c mice.
In vitro studies confirmed that activation of the P2X7 receptor could induce killing of T.
gondii ME49. However, intestinal parasite burdens were no different in mice with or
without the P2X7 receptor, indicating that the inflammatory pathology in the intestines
of P2X7 receptor knockout mice was not due to uncontrolled replication of T. gondii.
The pathology and weight loss experienced by P2X 7 receptor knockout mice infected
with T. gondii was associated w ith significantly elevated ileal levels of the proinflammatory
cytokines, IFN-y, MCP-1, TNF, IL-6, IL-12, IL-1~ and IL-18, compared with
C57BL/6J or BALB/c mice. There were no significant differences observed in the
production of the anti-inflammatory cytokines IL-10 and TGF-~ between the three
strains of mice, ruling out any dysregulation of anti-inflammatory pathway activation in
the development of toxoplasmic ileitis in P2X7 receptor-deficient mice.
The single most outstanding difference between the P2X7 receptor knockout mice and
mice with functional P2X7 receptors was an overproduction of nitric oxide.
Transcription of iNOS, which codes for the enzyme that generates nitric oxide, is
activated by the transcription factor, NFKB. A series of in vivo experiments
demonstrated that, without the P2X7 receptor, mice lack the ability to regulate key
elements of NF-KB activation, which may have contributed to the over-production of
nitric oxide and inflammatory cytokines seen in these mice.
The experiments presented in this thesis improve understanding of the role played by
the P2X7 receptor in the intestinal immune response and pathology induced by
infection and reveal a previously unrecognised role for this receptor in the regulation
of intestinal inflammation.
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