Enhancement of Endotoxin‐Induced Virus‐Inhibiting Factor or Interferon Production by Pretreatment of the Rabbit with Newcastle Disease Virus

ABSTRACT Rabbits pretreated by intravenous injection with Newcastle disease virus (NDV) produced high‐titered virus‐inhibiting factor (IF) or interferon in the serum upon intravenous injection of Escherichia coli endotoxin 24 hr later as compared with the IF production induced by endotoxin in rabbits without the NDV pretreatment. IF assays of different organs revealed that the NDV pretreatment rendered the spleen rather hyporeactive but other organs such as the liver, lungs and kidneys hyperreactive to the subsequent IF induction by endotoxin. The enhanced IF titer observed in the serum seems to be the summation of IF released from various organs rendered hyporeactive or hyperreactive by the NDV pretreatment. It is postulated that the late‐appearing, heat‐ and acid‐stable IF and the early‐appearing, heat‐ and acid‐labile IF are produced by different processes in different types of cells. The production of early IF consists of two steps, formation of “preinterferon” and conversion of preinterferon to early IF. Preinterferon formation is induced by virus but not by endotoxin, while the conversion of preinterferon to early IF is effected by both endotoxin and virus. The formation of late IF is induced only by virus, taking a one‐step process. This hypothesis seems to explain the findings in the present study.