Rare infectious bacteria thrive by hijacking human immune response

Tuesday, December 04, 2012 by: David Gutierrez, staff writer
Tags: infectious disease, bacteria, immune response

(NaturalNews) The bacteria responsible for a rare but dangerous tick-borne illness owe their success to a never-before-seen adaptation, according to a study conducted by researchers from Ohio State University and Japan's Osaka University and Nagahama Institute of Bioscience and Technology, and published in the Proceedings of the National Academy of Sciences.

The bacteria, known as Anaplasma phagocytophilum (Ap), hijacks an immune response designed to kill it and actually creates a food factory within the human body's white blood cells.

"This study shows how bacteria subvert natural processes," lead author Yasuko Rikihisa said. "They are creating their own food supply through a cellular mechanism that hurts other infectious bacteria. And because this process doesn't cause inflammation, they do it very gently, becoming an insider that eventually kills the host cell."

Ap causes a hard-to-treat disease known as human granulocytic anaplasmosis, which can be lethal in the elderly or those with compromised immune systems. Approximately 1,000 people every year become ill from the disease, which is primarily transmitted by black-legged and western black-legged ticks. This represents an increase from 348 cases in 2003, according to the Centers for Disease Control and Prevention.

Subverting the body's defenses

Many cells possess an adaptation known as autophagy, an emergency response that allows them to digest some of their own substance to survive starvation conditions. But the immune systems of many animals, including humans, have developed a way to exploit this process, hijacking the autophagy systems of bacteria and inducing invaders to essentially kill themselves. This technique is regularly used against bacteria that actually invade the body's cells, such as those that cause salmonellosis or shigellosis.

Unlike most bacteria, which seek to avoid autophagy, Ap enters the body's white blood cells and then secretes a chemical known as Ats-1 to initiate the process. In response, the white blood cell produces a chemical called Beclin 1. But Ats-1 binds to Beclin 1, preventing it from launching autophagy.

"We believe this is the first bacterial protein that has been found to do this," Rikihisa said.

Instead, the two attached proteins form a little "bubble" that fills with broken down pieces of the host cell. This bubble then attaches to the compartment that the bacteria form for their own growth. By this mechanism, the bacteria leech a steady stream of nutrients from the host cell to fuel their own reproduction.

To the rest of the cell (and the body); however, this entire process just looks like the normal process of autophagy, so no immune response is marshaled against it. Eventually, the bacteria grow to such numbers that they rupture the walls of the white blood cells and pour into the bloodstream en masse - and only then does the body realize that an invasion is underway.

In a group of related studies, the researchers confirmed that Ap's rate of reproduction is related to both its own production of Ats-1 and its host's production of Beclin 1. The researchers hope that this new understanding could help develop new drugs to target Ap more effectively.

Sources:

http://researchnews.osu.edu/archive/bactbind.htm