Researchers at the Oswaldo Cruz Foundation (Fiocruz) have identified a potential mechanism in the body of rodents that reacts to the attacks of Toxoplasma gondii protozoa. The protozoa live as parasites and cause the infectious disease known as toxoplasmosis. Under the coordination of Dr. Ricardo Gazzinelli, an immunologist, they found that the efficient defense response system of mice to a toxoplasma infection depends on the concerted action of four proteins produced by dendritic cells, the first cells of the immune system to come into contact with the parasite.
These four proteins belong to the toll-like receptors (TLRs) family, molecules expressed by immune cells that identify pieces of invading microorganisms. From an evolutionary point of view, they are part of a well preserved primary protective mechanism and are found in fish, birds and mammals. “These receptors are highly specific in recognizing molecules associated with infectious agents that threaten the survival of host organisms,” says Dr. Gazzinelli, a researcher at the René Rachou Research Center of the Oswaldo Cruz Foundation (Fiocruz), based in the state of Minas Gerais.
Two of these proteins, TLR-7 and TLR-9, were already well known to immunologists. They detect different microorganisms by recognizing stretches of their genetic material. In 2013, Dr. Gazzinelli and Warrison Andrade, then his PhD student at the Federal University of Minas Gerais (UFMG), found that these proteins act in conjunction with two other more selective ones of the same group: TLR-11 and TLR-12. Until now found only in rodents, the main intermediate hosts of toxoplasma, these receptors detect profilin, a protein that is essential to the protozoa’s motility and their ability to invade host cells, where they multiply. Whenever the toll-like receptors 11 and 12 identify profilin, they initiate a chain reaction that culminates in the production of immunity-related GTPase proteins, or IRGs, which destroy the vesicles in which the protozoa lodge.
Researchers believe that the existence of specialized proteins to identify toxoplasma is evidence of how exposure to the protozoa for millennia may have helped to shape the host’s defense system, so that both can live in relative harmony. Arising from mutations in the host’s genetic material, the genes encoding TLR-11 and TLR-12 allow rodents to survive toxoplasma infections and eliminate most of the parasites. And it’s not all bad for the parasite, which is not completely eliminated.
“This balance prevents toxoplasma from killing its intermediate host, increasing its chances of reaching the bodies of cats and other felines, its definitive hosts,” says Dr. Gazzinelli. The evolution of TLR proteins occurred over millions of years. Today each protein of this family has a similar function, but with a distinctive specificity. “Each protein of the TLR family recognizes a specific and well preserved molecule of the pathogenic microorganisms,” he says. “By performing an important role in combating invading microorganisms, they became highly conserved.” In the case of toxoplasma, the ability of the rodent’s TLR-11 and TLR-12 to recognize profilin has generated a stable balance between the parasite and its host.
Manipulate and survive
When rodents are infected, toxoplasma is surrounded by immune cells, which eliminate most of the parasites. The survivors form into cysts, usually in the host’s muscles and brain. Recent studies suggest that once the parasites are lodged inside the brain, they alter rodent behavior, causing them to lose their fear of cats. According to the researchers, this is a type of manipulation by which toxoplasma increases its chances of perpetuation, since it can only complete its reproductive cycle in the intestines of cats.
Thus IRG protein evolution follows a distinct model, based on an unstable equilibrium, in which the host develops more effective defense mechanisms against the parasite at the same time as the protozoa enhance their capacity to evade the immune response. Experiments done by German and Portuguese researchers have shown an example of this competition, which leads to a faster evolutionary process. More aggressive strains of toxoplasma neutralize the action of IRG proteins, which should be able to destroy them. But, for reasons still unknown, this neutralization occurred only in mice reared in the laboratory. The most aggressive strains of the protozoa did not disrupt the action of these proteins in wild rodents, which have a greater diversity of IRGs.
It is not known why many species do not have the IRG genes. One hypothesis is that maintaining a system of efficient IRGs would be costly for the host, which is why they have disappeared from several vertebrates.
Interestingly, according to Dr. Gazzinelli, it is this evolution of combined strategies of attack and defense that are fundamental to both organisms and their success in nature. This phenomenon, co-evolution, is due to the selection pressure to which parasite and host are subjected. “In practical terms,” he says, “toxoplasma evolves to infect the host, while the host evolves to neutralize the adaptations of the protozoa.” This process is known as Red Queen co-evolution, a reference to the book by Lewis Carroll, Alice Through the Looking Glass. In it, the Queen says to Alice: “Here, you see, it takes all the running you can do to keep in the same place.”
The protozoa can reach the human body through drinking water and the consumption of food (usually raw or rare meat) contaminated by eggs of the parasite. It is estimated that half of the Brazilian population, and one third of the world’s, may be infected. Dispersed through the feces of domestic animals, the protozoa assume their active form in the bodies of people with weakened immune systems or in pregnant women; a fetus may become infected and even die. Nevertheless, humans are considered an accidental host.
As far as we know, the immune cells of humans and other mammals do not produce the two receptors that reinforce the rodent barriers against toxoplasma. “Human beings,” says Dr. Gazzinelli, “may have developed other receptors to control the protozoa.”
GAZZINELLI, R. T. et al. Innate sensing of Toxoplasma gondii: an evolutionary tale of mice, cats and men. Cell Host & Microbe. v. 15, n. 2, p. 132-38. fev. 2014.
ANDRADE, W. A. et al. Combined action of nucleic acid-sensing Toll-like Receptors and TLR11/TLR12 heterodimers Imparts resistance to Toxoplasma gondii in mice. Cell Host & Microbe, v. 13, n. 1, p. 42-53. jan. 2013.