Survival of Pathogenic Bacterium Inside Neutrophils

Scientists analyzed how neutrophils from healthy blood donors respond to Anaplasma phagocytophilum, a tick-borne bacterium that causes granulocytic anaplasmosis in people, dogs, horses and cows. A. phagocytophilum is carried by the same tick that transmits Lyme disease and was first identified in humans in 1996. Human granulocytic anaplasmosis (HGA) — formerly called human granulocytic ehrlichiosis — is prevalent in Minnesota and along the East Coast. HGA typically causes mild symptoms that include fever, muscle aches and nausea. Some 362 U.S. cases were reported to the Centers for Disease Control and Prevention in 2003.

HGA is considered an emerging infectious disease, and Dr. Borjesson is working to understand how it affects blood cells — and neutrophils in particular. "Few people know about this pathogen, but it is important because it is transmitted by ticks and causes disease in both animals and humans," Dr. Borjesson says.

Neutrophils, which make up about 60 percent of all white blood cells, are the largest cellular component of the human immune system — billions exist inside each human. Typically, neutrophils ingest and then kill harmful bacteria by producing molecules that are toxic to cells, including a bleach-like substance called hypochlorous acid. Once the bacteria are killed, the involved neutrophils self-destruct in a process known as apoptosis. Recent evidence suggests that this process is vital to resolving human infections.

A. phagocytophilum is unusual in that it can delay apoptosis in human neutrophils, which presumably allows some of the bacteria to replicate and cause infection.

"This particular bacterium specifically seeks out neutrophils — possibly the most lethal of all host defense cells — and remarkably, can alter their function, multiply within them and thereby cause infection," says NIAID Director Anthony S. Fauci, M.D.

Dr. DeLeo says the findings contrast with what is known about other bacterial pathogens, most notably Staphylococcus aureus, which is of great interest because of its increasing resistance to antibiotic treatment. S. aureus, often simply referred to as "staph," are bacteria commonly found on the skin and in the noses of healthy people. Occasionally, staph can cause infection; most are minor, such as pimples, boils and other skin conditions. However, staph bacteria can also cause serious and sometimes fatal infections, such as bloodstream infections, surgical wound infections and pneumonia.

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