Flu causes immune system paradox
US researchers believe they have found a way that influenza makes people more prone to potentially deadly pneumonia.
The finding could have important implications as 'flu deaths' generally don't result from the influenza virus itself.
Instead many so-called flu deaths result after the patient develops pneumonia due to secondary infection with bacteria such as Streptococcus pneumoniae, Staphylococcus aureus, known as golden staph, or Klebsiella pneumoniae.
They hope the discovery will provide a new avenue of research for those developing ways to combat pneumonia after flu infection, and a potential target for reversing viral-induced suppression of the immune system.
How flu makes people more sensitive to pneumonia-causing secondary bacterial infections is not well understood.
Their study, using mice, looked at signalling molecules called interferons (type 1 IFNs). These interferons are normally produced by the immune system to limit viral replication and control infection.
But the researchers found they make individuals with flu more sensitive to bacterial pneumonia, by impairing the body's ability to mount an adequate immune response to the bacteria.
They do this by decreasing production of factors that attract neutrophils to bacterial infection sites. Neutrophils are immune cells central to the initial antibacterial immune response.
University of Newcastle immunologist and microbiologist, Professor Richard Barry, says it's a paradox that interferon 'downgrades' some genes which stimulate bacterial growth and adhesion, yet at the same time switches off genes that enable stimulation of neutrophils.
"Once you get flu and it's severe enough for a secondary bacterial infection, genes necessary to stimulate neutrophil production are inhibited," he says. "The virus and the interferon override the body's ability to produce enough neutrophils."
Barry explains how a bacterial infection secondary to flu such as 'strep throat' can be lethal.
"Bacteria creep down the throat and into the depths of the lungs where they normally aren't present," he says. "Macrophages (white blood cells) and neutrophils in the lungs normally gobble up bacteria, but bacterial pneumonia can overwhelm these cells because there's so many bacteria."
Recent research indicates most of the 50 to 100 million deaths in the 1918 'Spanish' flu pandemic may have been due to secondary 'super infection' caused by Streptococcus pneumoniae.
Barry says today's antibiotic range, which was not available early last century, provides some protection. But, he says, antibiotic resistance may make the task of controlling a flu pandemic particularly difficult.