Repression of retrovirus-mediated transgene expression by interferons: implications for gene therapy

Retrovirus-mediated gene transfer is commonly used in gene therapy protocols and has the potential to provide long-term expression of the transgene. Although expression of a retrovirus-delivered transgene is satisfactory in cultured cells, it has been difficult to achieve consistent and high-level expression in vivo. In this investigation, we explored the possibility of modulating transgene expression by host-derived cytokines. Normal human keratinocytes and dermal fibroblasts were transduced with recombinant retroviruses expressing a reporter gene (lacZ). Treatment of transduced cells with a proinflammatory cytokine, gamma interferon (IFN-gamma), significantly reduced lacZ expression to less than 25% of that of nontreated cells. The inhibition was concentration dependent (peak at 5 ng/ml) and time dependent (maximal at 16 h for transcript and 24 h for protein); expression remained repressed in the continued presence of IFN-gamma but returned to normal levels 24 h after IFN-gamma withdrawal. The decrease in beta-galactosidase activity appeared to result from decrease in steady-state lacZ mRNA levels. Inhibitors of transcription and translation blocked IFN-gamma-induced repression, suggesting involvement of newly synthesized protein intermediates. Similar results were obtained by treatment of transduced cells with IFN-alpha but not with other proinflammatory cytokines, including tumor necrosis factor alpha, interleukin-2 (IL-1), IL-4, and granulocyte colony-stimulating factor. Although the level of lacZ mRNA was reduced by >70% following IFN treatment, the rate of lacZ transcription was not significantly different from that for nontreated cells. These results suggest that IFN-mediated regulation of transgene expression is at a posttranscriptional level. Interestingly, IFN-gamma also suppressed transgene expression driven by a cellular promoter (involucrin) inserted in an internal position in the retroviral vector. The presence of the overlapping 3' untranslated regions in transcripts initiated from the internal promoter and the long terminal repeat is suggestive of a posttranscriptional regulation, likely at the level of RNA stabilization. These results provide direct evidence for modulatory effects of IFNs on retrovirus-mediated transgene expression and suggest that gene therapy results may be altered by host inflammatory responses.