Toxoplasma gondii is an intracellular protozoan parasite that infects many different vertebrate species asexually and undergoes a sexual cycle after infecting cats (http://www.cdc.gov/parasites/toxoplasmosis/, 2013). Parasite oocysts are potentially introduced into the human environment in cat feces. T. gondii is of interest in clinical medicine because humans can serve as accidental intermediate hosts when they ingest oocysts in, for example, undercooked, contaminated meat or ingest parasites in contaminated drinking water. Mother-to-child transmission can also occur. In most healthy individuals the infection does not cause illness, but in individuals with immune deficiencies and in fetuses it can cause substantial morbidity. In the case of congenital infection of a fetus, morbidity, including vision loss, cognitive deficits, and seizures tends to be more severe with earlier infection and mortality can result in either miscarriage or stillborn birth. L. David Sibley (Washington University) and colleagues (Etheridge et al., 2014) have now further clarified the molecular basis for the variation in virulence among different T. gondii lineages for mice, an important prey species for cats and therefore an important intermediate host species.
There are three lineages of T. gondii in North America and Europe, and these lineages vary substantially in virulence for mice. With respect to typical laboratory mice, Type I is highly virulent, Type II exhibits an intermediate degree of virulence, and Type III is avirulent. In previous work (Saeij et al., 2006; Taylor et al., 2006), Sibley and his associates used genetic crosses among these three lineages to identify key genes that contribute to virulence. Specifically, they identified a gene (ROP18) that encodes a protein (ROP18) released from the secretory organelle known as the rhoptry (2006 a, b). In the most recent study in this line of investigation, Etheridge et al. reveal that two other genes (ROP5, ROP17) and their gene products (ROP5, ROP17) contribute critically to virulence in mice.
These parasite proteins promote virulence by inactivating multiple mouse proteins, including those known as immunity-related GTPases, or IRGs, that are expressed at increased levels by cells exposed to interferon-gamma (IFNg) and are involved in killing parasites inside mouse cells (Fentress et al., 2010). The parasites reside in vesicular structures known as parasitophorous vacuoles inside infected host cells, and the IRGs bind to the membrane of this vacuole, and to each other, and disrupt the vacuole leading to the death of the parasites within.
The T. gondii ROP proteins phosphorylate the IRG proteins and the covalent modifications functionally inactivate these host proteins. At a molecular level, the ROP proteins form two different heterodimers, ROP5/ROP17 and ROP5/ROP18. ROPs 17 and 18 both have kinase activity (i.e., they attached a phosphate group to a substrate molecule, such as a mouse IRG protein) whereas ROP 5 is a non-enzymatic protein (pseudokinase) and binds to IRG proteins in a manner that facilitates the ability of ROP17 or ROP18 to bind to its IRG substrate and phosphorylate it. In the absence of ROP5, ROPs -17 and -18 are ineffective at inactivating mouse IRG proteins.
After recently hearing Dr. Sibley present his research on T. gondii virulence factors in our departmental immunology lecture series, I asked him to comment on the likelihood that the explanation for T. gondii virulence in mice that he provided in his lecture and in Etheridge et al. and related papers may be different in other hosts. In his response to my question and subsequent discussion, it became clear that the detailed molecular basis for T. gondii virulence in humans or even other mouse species does not conform to the pattern in the typical inbred mouse strains used in the lab.
To start with humans do not appear have IRG proteins (Könen-Waisman and Howard, 2007). So, the ROP proteins that are so crucial in lab mice are not the key to virulence in humans. Furthermore, the degrees of virulence of T. gondii of Types I, II, and III that are so discernible in mice, are not important in the human context. While IFNg is important in humans as in mice for cell-autonomous immunity to T. gondii, the key effect of IFNg is increased expression of indoleamine 2,3-dioxygenase (IDO), an enzyme that degrades the amino acid tryptophan. Conversely, IDO is not important in cell-autonomous mouse immunity to T. gondii.
There are also other important human-mouse differences in cell-autonomous immunity to T. gondii. Toll-like receptor (TLR)11 and TLR12 are activated in laboratory mice by a T. gondii protein (profilin-like protein, PRF) that leads to interleukin-12 (IL-12) production that in turn elicits IFNg secretion by T cells and natural killer (NK) cells (Gazzinelli, 2014). Humans do not have functional genes for either TLR11 or TLR12.
But one need not study human-T. gondii interactions to find significant differences in host-parasite relations in comparison to those seen in laboratory mice infected with T. gondii. Jonathan Howard and colleagues (Lilue et al., (2013) recognized that the high virulence of some T. gondii strains for laboratory mice is not necessarily advantageous from an evolutionary perspective because rapid death of the host prevents the encystment that is essential for the continuation of the parasite life cycle and transmission to the definitive host. Therefore, they studied inbred strains derived from wild mice and found an enormous degree of sequence polymorphism among genes encoding IRG proteins as well as copy number variation. One wild-derived IRG haplotype (a set of loci on one chromosome) reduces the effectiveness of T. gondii ROP proteins, specifically the ROP5/ROP18 complex, in protecting the parasite from host immunity. While the ability of mice possessing this resistance haplotype to control the parasite may strike some as a ‘defeat’ for the parasite, earlier immune control by the host can permit the surviving parasites to encyst thereby promoting parasite transmission.
Lilue et al. (2013) argue that the mouse is selected by more virulent strains of T. gondii to develop more robust mechanisms of immunity and subversion of parasite molecules that subvert host immune mechanisms. However, more virulent parasites, selected by the mice with more potent immunity, might kill less resistant mice too quickly to achieve high rates of transmission to cats. Less virulent parasites will likely fail to achieve high rates of transmission to the definitive host from highly resistant mice but will probably be more successful in less resistant mice.
These dynamics might therefore favor diversification of both host genes related to immunity against the parasite and parasite genes related to countering host immunity. Howard and colleagues also caution that the patterns of variation in host genes encoding IRGs and other host defense molecules are very likely to reflect selection mediated by pathogens other than T. gondii.
Parasites – Toxoplasmosis (Toxoplasma infection). http://www.cdc.gov/parasites/toxoplasmosis/; last updated: 1/10/13; last accessed: 5/31/14.
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