Known as cystatin-9, the substance is one of a group of naturally occurring human proteins that function to moderate immune responses. In cell culture and mouse experiments, the researchers found that it also directly affected the bacteria responsible for tularemia, acting against them both in cultured macrophages (the immune cells that make up the first line of defense against invading bacteria) and mice.
“When we looked at cystatin-9 in the lungs with Francisella tularensis, we saw that it increased killing of the bacteria,” said associate professor Tonyia Eaves-Pyles, lead author of a paper on the work now online in Molecular Medicine. “Moreover, it did this without inducing the out-of-control immune response that generally accompanies tularemia 24 hours after the bacteria has been inhaled.”
That runaway immune response, Eaves-Pyles said, is more dangerous to the infected person than the infection itself, because it causes extensive tissue damage. But treatment with cystatin-9 moderates immune responses without completely shutting them down, allowing the immune system to continue fighting the bacterial invasion.
At the same time, cystatin-9 also has an effect on Francisella tularensis, disrupting the bacterial cell wall and making the bacteria less virulent. “So it’s two-fold,” Eaves-Pyles said. “Cystatin-9 is having effects on the host and it’s also having direct effects on the bacteria.”
Best known in connection with its natural hosts, rabbits, Francisella tularensis was weaponized by both the United States and the Soviet Union. Inhaling as few as 10 of the bacteria is enough to kill an untreated victim.
Prompt treatment with antibiotics can be effective, but the bacteria can go undetected by the host for 24 to 72 hours or longer after infection because the infection initially presents as cold or flu symptoms. This extended time allows the bacteria to replicate and spread out of the lungs, making it difficult to know in time whether a person has been exposed.
Eaves-Pyles envisions cystatin-9 being used as a prophylactic by U.S. military or other personnel in situations where they are likely to encounter aerosolized Francisella tularensis. The protein is small, easily produced and stable, making it particularly suitable for field applications.
In the lab, she expects it to lead to a better understanding of the interactions that govern immune responses to infection.
“Once we really start defining the mechanism of cystatin-9, we’re going to see how it affects protein changes., and those proteins may be able to be targeted specifically and used to better understand what is a protective immune response versus an unrestrained, damaging inflammatory response,” Eaves-Pyles said. “So I think it’s going to give us a lot of information not just about Francisella but other human pathogens as well.”
Other authors of the paper include research associates Jignesh Patel and Aaron Miller, associate professor Yingzi Cong, graduate students Anthony Cao and Eric Carlsen, professors Nisha Garg, Richard Pyles, Vsevolod Popov, Lynn Soong and Csaba Szabo, and postdoctoral fellow Ciro Coletta, all from UTMB; Emma Arigi and Igor Almeida of the University of Texas at El Paso; Bernard Arulanandam of the University of Texas at San Antonio; and Monisha Dhiman of the Central University of Punjab, India. This research was supported by the National Institutes of Health.
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