Quantitative Determination of Residual Virus on Red Cells by Direct Infectivity Titrations

Abstract

Summary A method is described for determination of infective virus particles attached to chicken or human red cells. The technique involves photoelectric standardization of virus-red cell suspensions after thorough washing and subsequent titration of infectivity in embryonated eggs. Application of this method to red cells following maximal elution of Newcastle disease and influenza viruses indicated the persistence of infective virus. Results of experiments with receptor destroying enzyme (RDE) or inelutable virus suggested the possibility that infective virus may attach to red cells at points other than those susceptible to enzymatic destruction. Determination of the maximum number of ID50/chicken red cell yielded values of 83 and 166 ID50 for influenza and NDV respectively. Electron microscope counts by others indicates that the number of observable particles/red cell is at least 10-fold higher. Comparison of the ID50/HA ratio by this technique with the classical method of simultaneous HA and ID50 titrations was made for influenza and NDV using both chicken and human red cells. Ratios by the classical method gave slightly higher values (3 to 5 times) with chicken red cells but much higher ratios (100 times) with human red cells than obtained by the technique reported.