Wolf in sheep’s clothing

Leprosy bacteria (red) and interferon-beta (green).
Date:6 March 2013 Tags:, ,

In a study published online in the journal Science, researchers at UCLA demonstrate that certain cunning bacteria – including the type that causes tuberculosis – can pretend to be viruses when infecting humans, allowing them to hijack the body’s immune response so that they can hide out, unhindered, inside our cells. The findings may also help explain how viral infections such as flu make us more susceptible to subsequent bacterial infections, such as pneumonia.

The study is particularly relevant to tuberculosis, which kills 1,4 million people worldwide each year. In the case of a recent Los Angeles outbreak, which may have exposed up to 4 500 individuals to the bacterium that causes the deadly disease, the findings could provide clues as to how flu, and a lack of vitamin D, may have given the tuberculosis bacterium an edge. The outbreak is occurring during winter, when homeless individuals are driven to crowded shelters, when influenza is peaking and when people’s vitamin D levels, typically boosted by sunlight exposure, are low. The UCLA study offers critical insight into how various bacteria may manipulate such factors to their advantage.

The protection our immune system provides against bacteria-based diseases and infections depends on the critical response of T cells – white blood cells that play a central role in fighting infections – and in particular, on the release of a protein called interferon-gamma. Interferon-gamma utilises the vitamin D hormone to alert and activate cells to destroy invading bacteria.

The research team found that bacteria can pretend to be viruses, triggering the immune system to launch an attack with a different protein, called interferon-beta, which is designed to fight viruses, not bacteria. Not only is interferon-beta ineffective against bacteria, but it can also block the action of interferon-gamma, to the advantage of bacteria. Further, if a real virus were to infect the body, triggering interferon-beta, it would divert the attention of the immune response, preventing an attack on the bacterial invader. The researchers say this may explain why the flu can lead to a more serious bacteria-based infection like pneumonia.

“Like a wolf in sheep’s clothing, the bacteria can fool the immune system into launching an attack against the wrong type of infection, thus weakening the response against the bacteria,” says first author Rosane Teles, a researcher in the dermatology division at the Geffen School of Medicine. – Rachel Champeau, UCLA.

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