Analysis of antigen-antibody reactions of two plant viruses by density-gradient centrifugation and electron microscopy

Abstract

Electron microscopy and density-gradient centrifugation were used for a sensitive and specific detection of serological reactions of plant viruses. Addition of a limited amount of homologous antiserum to tobacco mosaic and southern bean mosaic viruses aggregated only part of the virus. The unaggregated virus sedimented at a normal rate and could be measured by density-gradient centrifugation. The amount of unaggregated virus was nearly a linear function of the logarithm of the antiserum concentrations in the 20% to 80% reaction range. The amount of antiserum necessary to aggregate 50% of the virus could be determined by interpolation. This 50% point depended upon the initial concentration of virus. Removal of a zone by a specific antiserum could be used to identify zones, to determine relations between antigenic materials of different zones, and to provide a specific measure of one antigen-antibody combination in the presence of other nonrelated antigens. Centrifugation of mixtures with antibody excess gave no zones that could be attributed to “soluble antigen” or to antibody-coated virus. Density-gradient centrifugation gave an accurate determination of the precipition reaction and indicated that virus was aggregated with dilutions of antiserum beyond the end point of visible precipitation. Electron microscopy confirmed that low concentrations of virus were aggregated by low concentrations of antiserum.