Bird flu and blood cells

One nasty (and usually fatal) consequence of infection with bird flu (influenza A/H5N1) in humans is that the virus doesn't just infect the lungs but becomes disseminated to many different organs. We know that a bird-like receptor that the virus can use to get into cells is found in several other organs, including the lining of blood vessels and neural tissues. Central nervous system involvement is frequently a hallmark of fatal bird flu cases. The virus probably gets to a lot of other organs, as well. But how? An examination of the blood of a fatal case in a pregnant woman suggests answer -- white blood cells:

In the present study, we investigated organs obtained at autopsy from an H5N1 virus-infected pregnant woman and from her fetus in an attempt to study the mechanism of systemic dissemination of H5N1 virus. Neutrophils were abundant in the placenta, and therefore, we evaluated blood cells in the placental villi obtained at autopsy to determine whether neutrophils were infected by H5N1 virus.

[snip]

The unequivocal evidence of H5N1 viral proteins and nucleotide sequences in the nuclei and cytoplasm of neutrophils of patients with avian influenza indicates novel mechanisms of pathogenesis. These cells may serve as a viral carrier in systemic circulation and may cause multiple organ infection, as was reported elsewhere. (Zhao et al., Clinical Infectious Diseases 2008;47)

What this means is that viral proteins and viral genetic material was found inside the kind of white blood cells (neutrophils) that make up the majority of white cells in our blood and that constitute the vanguard of our innate (non-specific) immune response. The neutrophils are like wandering policemen, engulfing foreign particles, bacteria, viruses and whatnot, and digesting them so they are no longer harmful. In this case it seems they have somehow internalized H5N1 virus, either by their normal "gobbling" process or because they have receptors on their surfaces that allow the virus to enter the cell and infect it. Once inside the neutrophil we don't know if the virus becomes disabled or remains infective. If the former, it may still cause the neutrophil to die, thus imparing the immune response, and if the latter, it may be the way the virus gets around the body, inside a cell and protected by antibodies in the serum.

Is this another piece of the puzzle? Perhaps. Maybe it's a piece of another picture or a peripheral piece of the bird flu one. Or maybe a central piece. We'll have to see.