A single-radial-immunodiffusion technique for the assay of rabies glycoprotein antigen: application for potency tests of vaccines against rabies.

An assay for rabies glycoprotein antigen based on single-radial-immunodiffusion (SRD) is described. Rabies glycoprotein antigen at concentrations of 0.7 microgram/ml or greater (approx. 1 international unit, IU) produced well-defined SRD reactions zones in immunoplates containing antibody to purified glycoprotein. Plots of zone area against relative antigen concentration were linear. The method was found to be of suitable sensitivity fo in vitro potency assays of inactivated cell culture rabies vaccines. Qualitative differences were detected between rabies vaccines prepared by two different methods when these were analysed in sucrose gradients for glycoprotein antigen associated with intact virions or in 'soluble' form associated with subviral structures. In vaccines prepared by zonal ultracentrifugation the glycoprotein was totally associated with intact virus, whilst in those prepared by ultrafiltration comparable quantities of subviral antigen were also detected. The SRD test appears to have considerable potential for assays of the antigenic content of rabies vaccines and has the advantage of reducing reliance on conventional in vivo tests for immunogenicity which employ infectious virus.