The effects of interferon-alpha and acyclovir on herpes simplex virus type-1 ribonucleotide reductase

Activation and inhibition of virus and interferon: the herpesvirus story

Viral infection of a host leads to induction of an immune system attack against the virus along with a counterattack by the virus against the host. Interferons are highly upregulated upon viral infection and function as key regulators of the host innate defense system. Whereas many RNA viruses are sensitive to the effects of interferon, several of the large DNA viruses are relatively resistant, given their capacity to encode proteins that disarms the host response. The herpesvirus family is a large and diverse family of viruses that infects vertebrates and invertebrates and possesses the ability to remain latent in their host for life. This paper summarizes the relationship between herpesviruses and interferon, including the ability of herpesviruses to induce interferon production, mechanisms by which interferon decreases herpesvirus replication, and strategies of herpesviruses to counteract these mechanisms.

Enhancement of the anti-herpetic effect of trichosanthin by acyclovir and interferon

Trichosanthin (TCS) is a type I ribosome-inactivating protein that has a wide range of pharmacological activities. The present study investigated the effectiveness of TCS on herpes simplex virus (HSV-1). The anti-viral activity and toxicity of TCS on Vero cells were measured. Results showed that the ED(50), TD(50) and the therapeutic indices were 38.5, 416.5 and 10.9 microg/ml, respectively. Anti-viral activity of TCS was substantially potentiated when it was used in conjunction with other anti-viral agents. The ED(50) of TCS was reduced 125-fold by acyclovir at a concentration of 0.001 microg/ml, which was practically devoid of significant anti-viral activity. Similarly, the ED(50) of TCS was reduced 100-fold by interferon-alpha2a at a concentration of 100 IU/ml. In conclusion, TCS is effective against HSV-1 and other anti-viral agents such as acyclovir or interferon can potentiate its action substantially.

Interferon coordinately inhibits the disruption of PML-positive ND10 and immediate-early gene expression by herpes simplex virus

Interferons (IFNs) are important components of the innate immune response, limiting herpes simplex virus (HSV) infection. In recombinant HSV-infected cells, IFN inhibited expression of beta-galactosidase from the immediate-early gene, ICP4, promoter. The extent of inhibition was dependent on IFN dose, IFN type, cell type, and multiplicity of infection (moi). IFN inhibited gene transcription, leading to a complete block in ICP4 promoter-driven gene expression in 90% of cells. The same IFN treatments resulted in an increase in the size and number of nuclear domain 10 (ND10) structures that stained positive by immunofluorescence for the promyelocytic leukemia (PML) protein. In cultures infected at low moi with a recombinant HSV producing ICP4 as a fusion protein with green fluorescence protein, the appearance of green fluorescence in the nucleus coincided with loss of PML-positive ND10 in the same nucleus, even in the rare ICP4-expressing IFN-treated cells. IFN-dependent inhibition was nearly complete when the immediate-early promoter was in the viral genome but was minimal when the promoter was stably integrated into the cellular genome. These data reveal that IFN can completely block viral gene expression in infected cells and that enhancement of the ND10 structure, which is the site of initiation of HSV replication, correlates with the block in viral gene expression.

Combined effects of interferon-alpha and acyclovir on herpes simplex virus type 1 DNA polymerase and alkaline DNase

Treatment of cells with combinations of human interferon-alpha (IFN-alpha) and the nucleoside analog, acyclovir (ACV), leads to the synergistic inhibition of herpes simplex virus type 1 (HSV-1) replication. We have examined the effect of these agents on the replication of HSV-1 DNA and the synthesis of early viral enzymes to understand the mechanism(s) responsible for this synergistic activity. Combination treatment with 100 IU/ml IFN-alpha and 5 microM ACV led to HSV-1 DNA levels more than 8-fold lower than in cells treated with ACV alone, while IFN-alpha treatment alone had no detectable effect on viral DNA synthesis. Steady state levels of DNA polymerase were reduced approximately 50% by IFN-alpha and 25% by ACV, but combination treatment did not decrease enzyme levels to an extent greater than the sum of these effects. In contrast, the activity of another early viral enzyme, alkaline DNase, was reduced less than 20% by IFN-alpha alone or combination treatment and was unaffected by ACV treatment. No decrease in the level of mRNA encoding either enzyme was detected in IFN-alpha-treated cells although ACV treatment reduced polymerase mRNA levels. These studies suggest that the synergistic anti-HSV activities of IFN-alpha with ACV could be mediated, in part, through some post-transcriptional mechanism induced by IFN-alpha treatment, leading to the reduction in production of viral early enzymes, especially DNA polymerase.