Immunity to Protozoan Parasites
Eric Denkers

Toxoplasma gondii is a protozoan parasite which infects humans and other animals and resides for the lifetime of the host in tissues of the brain. Toxoplasma is one of the most common parasitic infections in humans. In North America, between 30 and 50% of the population carry the parasite. T. gondii is an excellent parasite because, in most people, it causes no sign of illness. Thus, parasite and host co-exist in a biologically stable relationship that allows both to survive. However, for T. gondii to exist in this quiescent state requires active host immunity in the form of a strong T cell response. In AIDS patients who are also infected with Toxoplasma, the HIV-induced decline in T cell immunity can result in loss of an ability to control the parasite. In this clinical situation, T. gondii becomes an extremely serious infection which can become life-threatening if not treated. During pregnancy, the parasite can cross the placental barrier and infect the fetus where it can cause serious damage.

In this exploration, we will examine the ability of cells of the immune system to produce interferon-gamma in response to T. gondii. Interferon-gamma is a protein molecule which is very effective at controlling growth of the parasite. T cells are a major source of interferon-gamma. Synthesis of this biological molecule will be assessed by reverse-transcriptase polymerase chain reaction (RT-PCR) amplification. In this widely used technique, RNA from immune cells exposed to T. gondii is subjected to an enzymatic amplification process which results in an increase in interferon-gamma RNA from levels too low to detect, to levels high enough to see by chemical (fluorescence) staining. By comparison to nonstimulated cells, we will be able to determine if the gene encoding the interferon-gamma protein has been switched on in response to Toxoplasma.

Note: Students who are immunocompromised or who believe themselves to be pregnant should not participate in this exploration.