Transcriptional tissue specificity was engineered directly into Moloney Murine Leukaemia Virus (Mo-MLV)-derived retroviral vectors by replacing the viral enhancer in the 3′ long terminal repeat (LTR) with two different lengths (2.5 kbp or 769 bp) of the murine tyrosinase promoter/enhancer. The hybrid tyrosinase-LTR was transferred to the proviral 5′ LTR following viral packaging and infection of target cell lines. Hybrid tyrosinase-LTR-driven IL-2 production was barely above background levels in infected nonmelanoma cell lines containing intact provirus, whereas infected melanoma cell lines expressed high levels of IL-2 and the larger tyrosinase promoter/enhancer fragment directed higher levels of transgene expression. By replacing the viral enhancer with the tyrosinase promoter/enhancer sequences, promoter interference effects which we have previously observed when the tyrosinase promoter was included as an internal promoter within a similar retroviral vector were effectively abolished. Our data show that the hybrid tyrosinase-LTR behaves as a tightly regulated melanocytic-specific regulatory element when embedded in an enhancer-deleted Mo-MLV LTR. The use of other heterologous cellular promoter/enhancer elements in similar vectors should allow the development of simpler, targeted retroviral vectors for the expression of genes in selected cell types and may eventually provide for the development of safer, more efficient vectors for use in human gene therapy.
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