The microepidemiology of vaccinial infection as studied in HeLa cell stationary cultures

The dissemination of vaccinia virus in HeLa cell cultures grown on glass was studied by two different experimental approaches. The addition of antiserum immediately after the adsorption of 30–200 PFU of virus per culture failed to prevent the formation of foci of infection. Antibody, however, restricted the rate of progression of the infection. Infected cells were found to be capable of initiating foci of infection in the continued presence of antibody, even in cell sheets previously treated with antiserum. Virus release into the medium was very slow and scanty, since plaque counts in cultures receiving antiserum as late as 30 hours after the termination of the virus adsorption period were only slightly higher than those in cultures receiving antiserum immediately after virus adsorption. Infectivity assays of the fluid and cellular phases of monolayer cultures which had adsorbed 40–200 PFU of virus before the addition of an antibody-free maintenance medium showed a rise in the number of infected cells as early as 7 hours postadsorption, and confirmed the slow and limited release of newly formed virus into the medium. The rate at which the number of infected cells increased following inoculation of cultures with virus particles or infected cells was found to be dependent on the density of the cell population. It is concluded that passage of virus from an infected cell to immediately adjoining cells is predominantly responsible for the progress of vaccinial infection in HeLa cell monolayer cultures. The possible routes involved in this passage are discussed.