Interferon Induction by Viruses. XI. Early Events in the Induction Process

Abstract

The mechanism by which NDV, Sindbis virus, and VSV enter "aged" primary chick embryo cells to initiate IFN induction was studied by using NH₄Cl, a lysosomotropic weak base that compromises low pH-dependent membrane fusion. NH₄Cl was used to perturb the early steps in virus entry into the cytosol thought to result ultimately in presentation of dsRNA to the cell's first-stage recognition system for IFN induction. Three parameters were monitored: (i) the nature of the dose (multiplicity)-response (IFN yield) curve, (ii) the activity of IFN-inducing particles (IFP), and (iii) the quantum yield of IFN. In all tests, the qualitative nature of the dose-response curve was not altered by NH₄Cl treatment. NH4Cl had no effect on infectivity of IFN induction by NDV in keeping with a mode of entry involving acidic-independent fusion through the plasma membrane. Sindbis virus infectivity and IFN-inducing particle activity were inhibited similarly in an NH₄Cl concentration-dependent manner. While the infectivity of VSV was very sensitive to the action of NH₄Cl, virtually all IFN-inducing particles were functional; however, the quantum yield of IFN they induced was reduced in an NH4Cl concentration-dependent manner. Only 1 of 6 VSV [±]DI-011 particles registered in NH₄Cl-treated cells; however, they induced more than one-half the normal quantum yield of IFN. The mechanism of entry of Sindbis virus (acidic-fusion) and VSV (acidic-endocytosis) was distinguished by the action of cytochalasin B. Infectivity and IFN induction by Sindbis virus and VSV share a common, limiting step in NH₄Cl treated cells: transfer of viral RNA from basic vacuoles into the cell cytosol. The similar sensitivity of Sindbis IFP and PFP to NH₄Cl suggests that both activities respond the same to increased intravacuolar pH; neither can be expressed. VSV-IFP, requiring only partial genomic expression (or none in the case of DI-011) can register in NH₄Cl-treated cells to varying degrees under conditions that prevent expression of the full genome.