Caltech scientists show why anti-HIV antibodies are ineffective at blocking...( PASADENA Calif.--Some 25 years after...)

PASADENA, Calif.--Some 25 years after the AIDS epidemic spawned a worldwide search for an effective vaccine against the human immunodeficiency virus (HIV), progress in the field seems to have effectively become stalled. The reason? According to new findings from a team of researchers from the California Institute of Technology (Caltech), it's at least partly due to the fact that our body's natural HIV antibodies simply don't have a long enough reach to effectively neutralize the viruses they are meant to target.

Their findings were published last week in the online early edition of the Proceedings of the National Academy of Sciences (PNAS).

"This study helps to clarify the obstacles that antibodies face in blocking infection," says Pamela Bjorkman, the Max Delbrck Professor of Biology at Caltech and a Howard Hughes Medical Institute Investigator, "and will hopefully shed more light on why developing an effective vaccine for HIV has proven so elusive."

Y-shaped antibodies are best at neutralizing viruses--i.e., blocking their entry into cells and preventing infection--when both arms of the Y are able to reach out and bind to their target proteins at more or less the same time. In the case of HIV, antibodies that can block infection target the proteins that stud the surface of the virus, which stick out like spikes from the viral membrane. But an antibody can only bind to two spikes at the same time if those spikes fall within its span--the distance the antibody's structure allows it to stretch its two arms.

"When both arms of an antibody are able to bind to a virus at the same time," says Joshua Klein, a Caltech graduate student in biochemistry and molecular biophysics and the PNAS paper's first author, "there can be a hundred- to thousandfold increase in the strength of the interaction, which can sometimes translate into an equally dramatic increase in its ability to neutralize a virus. Having antibodie
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