Activation of the ppp(A2'p)nA system in interferon-treated, Herpes simplex virus-infected cells and evidence for novel inhibitors of the ppp(A2'p)nA-dependent RNase

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Abstract
High doses (100–1000 reference units/ml) of α or β interferons are required to inhibit the growth of herpes simplex virus types I and II (HSV-I and HSV-II) in human Chang cells. In contrast, much lower doses (10–100 reference units/ml) of interferon inhibit replication of encephalomyocarditis virus (EMCV) in these cells. In the HSV-infected cells these high doses did not prevent the virus-induced shut off of host protein synthesis. The interferons were more effective in reducing the virus yield of HSV-I than of HSV-II. At the above concentrations they inhibited HSV-I protein synthesis but had little apparent effect on that of HSV-II. Similar amounts of (2′-5′)oligo(adenylate)s were synthesised in response to HSV-I, HSV-II and EMCV infection of Chang cells after treatment with α or β interferons. No (i.e. < 1 nM) (2′-5′)oligo(adenylate)s were found in control cells or on virus infection alone. Only low levels of ppp(A2′p)nA-specific rRNA cleavage were observed in the interferon-treated HSV-infected cells. In contrast, high levels were found in response to EMCV, despite the fact that ppp(A2′p)nA accumulated to similar levels with each of the three viruses in these cells. High-performance liquid chromatographic analysis of material from interferon-treated Chang cells 18 h after infection with HSV-I or HSV-II, combined with radiobinding, radioimmune and rRNA cleavage assays, confirmed the presence of ppp(A2′p)nA and ppp(A2′p)nA at greater than nanomolar concentration. In addition, apparently equivalent amounts of two other putative (2′-5′)oligo(adenylate) derivatives which compete in the radiobinding and radioimmune assays, were present. These compounds were only weak activators of the ppp(A2′p)nA-dependent RNase and under appropriate conditions were capable of inhibiting the activation of this RNase by authentic ppp(A2′p)nA. The presence of these potentially inhibitory compounds provides a possible explanation for the relatively low levels of activation of the ppp(A2′p)nA-dependent RNase in interferon-treated, HSV-infected Chang cells.