MicroRNA-150-regulated vectors allow lymphocyte-sparing transgene expression in hematopoietic gene therapy

Endogenous microRNA (miRNA) expression can be exploited for cell type-specific transgene expression as the addition of miRNA target sequences to transgenic cDNA allows for transgene downregulation specifically in cells expressing the respective miRNAs. Here, we have investigated the potential of miRNA-150 target sequences to specifically suppress gene expression in lymphocytes and thereby prevent transgene-induced lymphotoxicity. Abundance of miRNA-150 expression specifically in differentiated B and T cells was confirmed by quantitative reverse transcriptase PCR. Mono- and bicistronic lentiviral vectors were used to investigate the effect of miRNA-150 target sequences on transgene expression in the lymphohematopoietic system. After in vitro studies demonstrated effective downregulation of transgene expression in murine B220(+) B and CD3(+) T cells, the concept was further verified in a murine transplant model. Again, marked suppression of transgene activity was observed in B220(+) B and CD4(+) or CD8(+) T cells whereas expression in CD11b(+) myeloid cells, lin(-) and lin(-)/Sca1(+) progenitors, or lin(-)/Sca1(+)/c-kit(+) stem cells remained almost unaffected. No toxicity of miRNA-150 targeting in transduced lymphohematopoietic cells was noted. Thus, our results demonstrate the suitability of miRNA-150 targeting to specifically suppress transgene expression in lymphocytes and further support the concept of miRNA targeting for cell type-specific transgene expression in gene therapy approaches.