Understanding the interactions of hosts and parasites
Analyzing the infection
of a host organism by parasites has remained a challenge because previous studies
— conducted postmortem or in vitro — could not accurately demonstrate
immunological reactions or the dynamics of cell invasion. Recently, scientists have
been able to image the infection routes of parasites in a host in vivo with transgenic
parasites expressing green or red fluorescent proteins.
Volker Heussler of the Bernhard Nocht
Institute for Tropical Medicine in Hamburg, Germany, and Christian Doerig of the
University of Glasgow in the UK report on imaging techniques that elucidate host-parasite
interactions. Wide-field and spinning-disk fluorescence microscopies have been
employed to follow sporozoites in the skin and blood vessels of mice. Intravital
microscopy has been used to track the infection routes of Plasmodium parasites.
Observations at the cellular level
have shown parasites to attach to blood vessels in the liver, glide along the endothelial
cell layer and migrate through several cells before settling down in a final hepatocyte.
The authors report the use of intravital microscopy to demonstrate the protrusion
of infected cells toward blood vessels of the liver, thus confirming a hypothesis
on the infection of red blood cells.
Although intravital microscopy holds
many advantages, the authors note several drawbacks to the technique. Excitation
of the fluorescent proteins could damage the parasites or result in photobleaching.
Laser-scanning multiphoton microscopy has lower energy and greater penetration;
however, it has poor resolution and a low sampling speed not optimal for three-dimensional
Heussler and Doerig expect intravital
microscopy to further improve the study of immune responses against pathogens, particularly
the interaction of infected cells with major organs such as the lungs and brain.
They believe that this will open doors to drug discovery and vaccine development.
(Trends in Parasitology, May 2006, pp. 192-195.)
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