Anthrax Targets Immune Cells to Kill

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) - Anthrax targets immune system cells, grappling them, sneaking inside and then multiplying until the cell bursts.

The newborn swarms of anthrax bacteria spill out and look for other cells to infest.

``We don't entirely understand how anthrax kills,'' Darrell Galloway, a molecular biologist at Ohio State University who is trying to make a new anthrax vaccines, said in a telephone interview.

The Bacillus anthracis bacteria starts out as a spore, which is a microbe's way of shutting down and surviving hard times. It has been shown to persist for as long as 100 years, and can survive being freeze-dried, buried, and shot through a nozzle as an aerosol.

The spores wait until they find a congenial environment, which for this bacterium is a warm, wet place -- like deep inside a lung.

There, they seek out a place to replicate. For as-yet unknown reasons, they look for immune system cells known as phagocytes, a type of cells that include macrophages. These are the cells that sweep the body for invaders, usually engulfing and destroying the enemy.

But when they encounter anthrax bacteria, the anthrax uses a molecular doorway to get into the cells and do what bacteria do best -- multiply.

CELLS STRETCH AND BURST OPEN

So many of the rod-shaped bacteria are produced that eventually the phagocyte stretches to its limits and bursts. Scores of rod-shaped anthrax bacilli spill out into the blood or lymph, and seek other cells to infect.

As they grow, the bacteria produce a waste byproduct of poisons. One of these is known as lethal factor.

``Like a killer who cuts the telephone line before entering the house, the poisons will move throughout the body and slice up a protein called MAPKK,'' bioterrorism and public health expert Michael Osterholm of the University of Minnesota writes in his book ``Living Terrors''.

This particular protein is vital to cell function.

``These macrophages that get targeted by toxin will, in a fairly short period of time, die,'' Galloway said.

But as they die, they send out a distress call -- a release of signaling chemicals called cytokines. These overstimulate the immune system, causing the victim -- whether a sheep or a human -- to go into shock.

``From what we understand, the individual basically dies from shock-like symptoms,'' Galloway said.

``This is in part why the effect of anthrax seems so sudden, why an individual may seem to be recovering and then suddenly die -- because of the peculiar role of the macrophages.''

In contagious diseases -- those that spread from person to person or animal to animal -- it does not benefit the microbe to kill its host, at least not right away. That is why particularly deadly strains of flu, like the one that caused a worldwide pandemic in 1918, die out.

But anthrax benefits from killing its host, which is usually a grazing animal.

The animal dies and rots into the soil. There, the bacteria revert to their spore state, waiting, sometimes for years, until another animal comes by and ingests the spore as it grazes or perhaps gets some infected dirt into a cut.

Luckily, naturally occurring Bacillus anthracis is easy to kill, succumbing to a wide range of antibiotics from penicillin to ciprofloxacin. Although health experts fear that someone, somewhere may have genetically engineered anthrax that can resist antibiotics, none has been found.