Author: Amber Dance / Source: Science News
Frances Mercer runs a fight club.
In one corner, the parasite Trichomonas vaginalis, which causes a widespread sexually transmitted infection that many people have never heard of.
In the other corner are neutrophils, the immune system cells best equipped to take down the aggressor.Watching the two battle it out, Mercer, an immunoparasitologist at California State Polytechnic University in Pomona, has learned a lot about the parasite. And she’s shown exactly how neutrophils manage to take down T. vaginalis — using a maneuver that scientists didn’t even know the immune cells possessed.
A focus on T. vaginalis has been a long time coming. In 2016, the parasite was responsible for about 156 million worldwide cases of the sexually transmitted infection called trichomoniasis in men and women. In the United States, trichomoniasis is the third most common sexually transmitted infection after HPV, or human papillomavirus, and herpes.
Today, scientists are just beginning to get a handle on how the parasite causes trouble — including increasing risk for HIV infection, infertility and preterm delivery — and how the human immune system fights back. Complicating the picture is the fact that T. vaginalis doesn’t work alone. Other microbes living inside the vagina, and some inside the parasite itself, get into the fray.
The infection “is not taken very seriously; it’s thought of as making women a bit itchy down there,” says Jane Carlton, a parasitologist at the New York University Center for Genomics and Systems Biology. As its name suggests, T. vaginalis colonizes the vagina, but it can also find a home in the urethra, which carries urine, in both men and women.
Estimate of worldwide cases of trichomoniasis in men and women in 2016
“One person’s nuisance infection is another person’s raving, itchy, hot, burning infection,” Carlton adds. About one-third or more of women who have an infection will have itch, discomfort or discharge — probably due to the action of the immune cells fighting the parasite, Mercer says.
About 5 to 10 percent of infected men suffer similar symptoms. The infection can be treated with metronidazole, a parasite-killing antibiotic, but about 5 percent of parasites are resistant to the drug.
Left untreated, trichomoniasis can have long-term consequences. The infection about doubles a woman’s chances of acquiring HIV. The risk might go up, Mercer says, because the parasite damages the barrier between the vaginal wall and the rest of the body, or because it causes an influx of the immune system’s T cells, which are the target of HIV infections. The parasite also interferes with female fertility and pregnancy. In men, T. vaginalis has been tentatively linked to infertility and prostate cancer.
Plus, the parasite can infect the same person more than once. Unlike with many other microbes, the immune system seems to be lousy at remembering how to fight T. vaginalis. The invader may even manipulate the immune response to block that memory, speculates Daniele Dessì, a microbiologist at the University of Sassari in Italy, with a touch of awe: “This is quite amazing for such a primitive organism.”
T. vaginalis is one of three species of trichomonad that infect humans. Scientists think trichomonads, single-celled protozoans, may have been among the first organisms to branch off the evolutionary tree after the rise of the original eukaryote, the ancestor of all animals, plants, fungi and protozoans.
T. vaginalis spreads by sexual contact. The infection, known as trichomoniasis, is usually treated with the antibiotic metronidazole. About 5 percent of infections are resistant to the drug.
Infects the vulva, vagina, cervix or urethra
Risks
- Nearly doubles the risk of HIV infection
- May increase the risk of transmitting HIV to sexual partners
- Raises a pregnant woman’s risk for preterm delivery and low birth weight in her newborn by 30 percent or more; treatment doesn’t always seem to reduce this risk
- Raises the risk of cervical cancer spreading in women with HPV
Infects the urethra, and sometimes the head of the penis, prostate or epididymis, where sperm mature and are stored
Risks
- May boost prostate cancer risk, though unconfirmed
- May damage sperm and interfere with fertility
“Learning about Trichomonas vaginalis is like learning about our own origins — who we were at the beginning,” says parasitologist Augusto Simoes-Barbosa of the University of Auckland in New Zealand. “Probably our ancestor was a very complex organism.”
At some point in its history — scientists don’t know when — T. vaginalis found the vagina a suitable place to live. It’s a good choice, points out Carlton: warm, moist, with plentiful sources of nutrition. Plus, men provide a convenient shuttle service to the next vagina.
Transmitted by sexual contact, the pear-shaped parasite swims into the vagina, paddling with tail-like flagella. T. vaginalis then sprawls out, amoeba-like, among the cells lining the vagina and cervix, rupturing them and eating the pieces.
Mercer first began investigating the parasite in 2013, as a postdoc in the lab of Patricia Johnson, a parasitologist at UCLA. At the time, not much was known about how T. vaginalis interacted with the immune system.
So Mercer set up her fight club, pitting the parasite against various immune cells in laboratory dishes. The parasite killed T cells and B cells, she reported with Johnson and colleagues in PLOS Neglected Tropical Diseases in 2016. The parasite seems to poison the cells, Mercer says, and may also gobble them up. B cells help the body remember past infections. So their loss may help explain why a person can get trichomoniasis multiple times. T. vaginalis was less effective at killing another type of immune cell, monocytes, which have a variety of roles, including swallowing invaders and helping T cells learn about pathogens.
One type of immune cell stood out: Neutrophils slaughtered the parasite in just 10 minutes. Neutrophils are the body’s first line of defense, so their action wasn’t a big surprise. These cells, “the foot soldiers of the immune system,” Mercer calls them, are drawn to…
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