Abstract
To identify the active component of Sendai virus that induces interferon in mouse spleen cells, infectious and noninfectious viruses, envelope particles derived from them, and isolated hemagglutinin-neuraminidase (HN) glycoproteins were examined for interferon induction. The interaction between membranous structures containing Sendai virus HN glycoprotein and the receptors on the cell surface was shown to be sufficient for interferon induction in mouse spleen cells, suggesting that the actual inducer of interferon in mouse spleen cells is the HN glycoprotein of Sendai virus. When mouse spleen cells were stimulated in vitro with Sendai virus grown in eggs or LLC-MK2 cells or with membranous structures containing glycoproteins obtained from these viruses, interferon could be detected in the culture fluid. Furthermore, isolated HN glycoprotein per se could induce interferon in the cells. A linear correlation was found between the titer of interferon induced and the hemagglutinating activity of the membranous structure containing the HN glycoprotein. It was concluded from these findings that HN glycoprotein was the active component of Sendai virus responsible for interferon induction in mouse spleen cells and that viral RNA and F glycoprotein were not required. The results also showed that the interaction between HN glycoprotein and receptors on the cell surface triggered production of type I interferon (IFN-alpha and IFN-beta). Although when Sendai virus was incubated at 56 degrees C for 5 min it lost its hemolytic and hemagglutinating activities, it induced a considerable amount of interferon in the culture fluid of mouse spleen cells. The interferon-inducing ability of heat-inactivated virus could be absorbed with mouse spleen cells but not with sheep erythrocytes or mouse erythrocytes, indicating that the inactivated virus retained ability to bind to mouse lymphoid cells.