"Once a foothold in a new host species is made, the virus HA can optimize its specificity to the new host,' say authors of a study focusing on one of the most pathogenic H5N1 virus strains isolated to date.

Bird flu is a highly contagious and deadly disease in poultry. Its spread to humans has been limited, however. As of March 14, there have been 177 documented severe infections, and nearly 100 deaths in Indonesia, Vietnam, Thailand, Cambodia, China, Iraq and Turkey, according to the World Health Organization.

"With continued outbreaks of the H5N1 virus in poultry and wild birds, further human cases are likely," says Ian Wilson, a Scripps Research professor of molecular biology and head of the laboratory that conducted the recent study. "The potential for the emergence of a human-adapted H5 virus, either by reassortment or mutation, is a clear threat to public health worldwide."

Gaining a Foothold

The study focused on A/Vietnam/1203/2004 (Viet04), one of the most pathogenic H5N1 virus strains discovered to date. The virus was originally isolated from a 10-year-old Vietnamese boy who died from the infection in 2004. The hemagglutinin (HA) structure of the Viet04 virus was found to be closely related to the HA structure of the 1918 virus that caused some 50 million deaths worldwide.

Using a recently developed microarray technology -- hundreds of microscopic assay sites on a single small surface -- researchers showed that relatively small mutations can result in switching the binding site preference of the avian virus from receptors in the intestinal tract of birds to the respiratory tract of humans. These mutations, the authors note, were already "known in [some human influenza] viruses to increase binding for these receptors."

Receptor specificity for the influenza virus is controlled by the glycoprotein HA on the virus surface. These viral HAs bind to host cell receptors containing complex glycans -- carbohydrates -- that in turn contain terminal sialic acids.

Avian viruses prefer binding to ?2-3-linked sialic acids on receptors of intestinal epithelial cells, while human viruses are usually specific for the ?2-6 linkage on epithelial cells of the lungs and upper respiratory tract. Such interactions allow the virus membrane to fuse with the membrane of the host cell so that viral genetic material can be transferred to the cell.

The switch from ?2-3 to ?2-6 receptor specificity is a critical step in the adaptation of avian viruses to a human host and appears to be one of the reasons why most avian influenza viruses, including current avian H5 strains, are not easily transmitted from human-to-human following avian-to-human infection.

However, the report does suggest that "once a foothold in a new host species is made, the virus HA can optimize its specificity to the new host."

Other Adaptation Paths Possible

"Our recombinant approach to the structural analysis of the Viet04 virus showed that when we inserted HA mutations that had already been shown to shift receptor preference in H3 HAs to the human respiratory tract, the mutations increased receptor preference of the Viet04 HA towards specific human glycans that could serve as receptors on lung epithelial cells," Wilson explains. "The effect of these mutations on the Viet04 HA increases the likelihood of binding to and infection of susceptible epithelial cells."

These results reveal only one possible route for virus adaptation, the researchers are careful to note. Other, as yet "unidentified mutations" could emerge, they say, allowing the avian virus to switch receptor specificity and make the jump to human-to-human transmission.

The glycan microarray technology, which was used to identify the mutations which could enable adaptation of H5N1 into the human population in the laboratory, could also be used to help identify new active virus strains in the field by monitoring changes in the receptor binding preference profile where infection is active, according to Jeremy M. Berg, the director of the National Institute of General Medical Sciences (NIGMS), part of the National Institutes of Health (NIH).

"This technology allows researchers to assay hundreds of carbohydrate varieties in a single experiment," Berg says. "The glycan microarray offers a detailed picture of viral receptor specificity that can be used to map the evolution of new human pathogenic strains, such as the H5N1 avian influenza, and could prove invaluable in the early identification of emerging viruses that could cause new epidemics."