Effect of Cell Density on Development of the Antiviral State in Interferon-Producing Cells: A Possible Model of in Vivo Conditions

Tissue culture conditions which mimic the high cell densities n vivo were used to elucidate further the virus, cell, and interferon interactions. To help define some of the variables which govern the action of interferon in the same cells which produce the interferon, the technique of altering the effective interferon concentration by varying the extracellular volume was used. Specifically, mouse L cells were treated with a high-multiplicity Newcastle disease virus at 0° and then resuspended at 37° in two different volumes of prewarmed medium such that each had the same total number of cells but the concentration of cells in one culture was 5 × 10⁶ and that in the other was 5 × 10⁴. Under these conditions both cell suspensions produced the same total amount of interferon, but at interferon concentrations of 320 and 4 units/ ml, respectively. Antiviral resistance developed more rapidly and to higher levels in the more concentrated cells. It was calculated that at the high cell density (closer to that of solid tissue) the production of the antiviral protein occurred not later than 60-75 min after the induction by NDV and the production of interferon itself must have occurred even earlier. Also, the finding that interferon-producing cells developed different levels of antiviral activity in relation to the interferon concentration in the surrounding fluid, rather than to the same internal (total) amount of interferon produced, supports the view that interferon must be externalized to induce resistance. Finally the possibility was raised that secreted interferon might more efficiently induce resistance in the interferon-producing cell than in nonproducing cells.