A Parasite that Dupes to Conquer

[Tormod]

If microbes seem cruel in their attempt to survive by establishing infections, they lose no love from their hosts.

Humans have numerous "innate" immune defense mechanisms that can be downright hostile to pathogens, automatically responding to a foreign invader and often wiping them out. In the intricate opera of the natural world, however, pathogens can adapt and reemerge to enjoy a strong second act-sometimes even evolving the ability to use the human immune system to their own advantage.

A recent study by researchers at the National Institute of Allergy and Infectious Diseases (NIAID) published in the journal Science last month demonstrates one such adaptation in Leishmania parasites. Common in parts of North Africa, Asia, and the Americas, these parasites invade when someone is bitten by a sand fly, which have these parasites growing in their digestive tract. While the sand fly is feeding, these parasites get expelled into the skin and cause a painful ulcerous disease that strikes more than a million people a year.

Taking advantage of major advances in microscopes in the last decade that allow them to directly visualize the parasite in living tissue, the NIAID researchers took time-lapse 3D video images of the parasite as they established infections in mice. Cells within the mice were prepared with chemicals designed to absorb light and glow with a certain characteristic color. What the videos reveal is that one of the main immune cells that acts as a first line of defense and is supposed to protect against invading parasites is actually critical for the establishment of Leishmania infection.

Called neutrophils, the human body makes billions of these cells every day. Their lives, to borrow a phrase from Thomas Hobbes, are nasty, brutish, and short. Usually living for less than a day, they troll the bloodstream and tissues in search of foreign pathogens and toxic matter, which they normally engulf and destroy with an onslaught of toxic chemicals. Neutrophils cannot destroy Leishmania, however, and the parasites use them as safe havens until they are eventually released and taken up by the definitive host cell for the parasite, a second type of immune cell in the skin called the macrophage. Inside macrophage cells, the parasites find a long-term home for the infection—but only if the neutrophils are there first.

In the end, what this means is that the bite of a sand fly stings twice. First, the creature expels parasites where it bites. And second, in biting, the sand fly attracts the very cells the parasites need to initiate an infection. The researchers say that this insight may influence how they design vaccines in the future. "A vaccine that works against parasites inoculated by a needle may not work against those transmitted by a sand fly," says Nathan Peters, who led the research. A jab, after all, is not the same as a bite.

Source: Inside Science News/American Institute of Physics