The Hemagglutinin of St. Louis Encephalitis Virus. III. Properties of Normal Inhibitors and Specific Antibody; Use of Hemagglutination-Inhibition for Diagnosis of Infection

Abstract

Summary To overcome the marked instability of the SLE hemagglutinin at the pH that is optimum for hemagglutination, inhibition tests were performed by mixing hemagglutinin diluted at pH 7.7 and 1 C with serum or tissue suspension adjusted to pH 6.5 at room temperature, and immediately adding the erythrocyte suspension. The normal inhibitor in mouse brain was extracted in larger amounts by hypotonic than by isotonic solutions, and was largely associated with particles sedimentable at 13,000 rpm for 1 hour. The normal inhibitor in brain is not destroyed during the course of infection. Human and rabbit sera contain more normal inhibitor than mouse brain extracts. Normal inhibitor in solution can remove hemagglutinin from erythrocytes and reverse hemagglutination without affecting the receptors on the red cells. The SLE normal inhibitor is only partially removed from serum by extracting with chloroform in a manner that removes practically all of the inhibitor for the Japanese B hemagglutinin. Precipitation and extraction with an appropriate amount of acetone yields a fraction which is practically devoid of normal inhibitor, but contains all the specific hemagglutination-inhibition antibody. The SLE hemagglutinin combines with specific antibody in different proportions from normal inhibitor. For each fourfold increase in concentration of hemagglutinin, the 50% inhibitory titer dropped twofold for specific antibody and fourfold for normal inhibitor. Antisera for the West Nile and Japanese B virus contained specific antibody for the SLE hemagglutinin but in much lower titer than antisera for the SLE virus. Specific hemagglutination-inhibition antibody in high titer was found in 8 patients with St. Louis encephalitis during the acute phase of the illness. Tests on human beings, without history of encephalitis, revealed that the hemagglutination-inhibition antibody was absent among residents in a nonendemic area, and present in low titer only among those residents of an endemic area who also possessed neutralizing antibody for the SLE virus.