Win32 Virus Removal

The elderly are normally the most susceptible to flu viruses, so it was something of a shock to find that they were largely spared in the recent waves of pandemic H1N1 influenza.

Experts have speculated that their apparent resistance to the virus may have arisen because they were exposed to a similar virus in the past and developed some antibodies that protected them.

Two new studies released last month demonstrate that this is the case, and that the virus they had been exposed to was the one that caused the 1918 “Spanish flu” pandemic that killed millions worldwide.

Both groups studied hemagglutinin (the “H” in H1N1), the spike-shaped protein that sits on the surface of the flu virus. Hemagglutinins are the portion of the virus that binds to host cells, allowing the virus to enter.

It is also the portion of the virus that is recognized by antibodies, allowing the immune system to destroy the virus. Hemagglutinin proteins are highly malleable, undergoing frequent mutation of amino acids.

As few as three or four mutations are normally enough to prevent an old antibody from recognizing a newly mutated virus — which is why seasonal flu vaccines have to be produced from scratch each year.

The hemagglutinin sequence of pandemic H1N1 differs from that of the Spanish flu virus hemagglutinin by about 20 percent, which should be enough to allow it to avoid detection by the immune system.

Virologist Ian A. Wilson of the Scripps Research Institute in La Jolla, Calif., and his colleagues studied the three-dimensional structure of the two hemagglutinins.

They reported in the online version of the journal Science that, despite 80 years of mutations, the two proteins had virtually identical amino acid sequences at a crucial binding site known as antigenic site Sa. That allows the two viruses to be recognized by the same antibodies.

With colleagues at Vanderbilt, they confirmed this by demonstrating that antibodies against the Spanish flu virus were capable of binding to and neutralizing the swine flu virus.

In a separate study, virologist Dr. Gary J. Nabel of the National Institute of Allergy and Infectious Diseases and his colleagues reported in the journal Science Translational Medicine that antibodies produced against the Spanish flu virus protected mice against infection with the swine flu virus and that antibodies against the swine flu virus protected mice from infection by the Spanish flu virus.

“This is a surprising result,” Nabel said in a statement. “We wouldn’t have expected that cross-reactive antibodies would be generated against viruses separated by so many years.”