Enumeration of antigenic sites of influenza virus hemagglutinin

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Abstract
The antigenic sites on the hemagglutinin of X-31 (H3) influenza virus were defined by using a competitive radioimmunoassay with a panel of mouse monoclonal antibodies which includes those known to select variants with substitutions of particular amino acids. The capacity of each monoclonal antibody to block the binding of other radioiodinated monoclones to purified hemagglutinin permitted classification of the panel into 4 separate groups, each of which defined a particular antigenic site on the hemagglutinin molecule. Three of these are located on the polypeptide backbone and correspond to the hinge, the loop, and the tip interface of the X-ray crystallographic model. Nonreciprocal blocking of certain anti-interface antibodies by anti-loop antibody suggests that much of the exposed surface of the head of the hemagglutinin molecule extending from the loop to the interface may be a continuum of epitopes. A 4th antigenic site is carbohydrate in nature, presumably situated on the antigenic oligosaccharide side chains. These 4 domains are in addition to 2 antigenic sties defined by monoclonal antibodies that inhibit neither hemagglutination nor infectivity.