In-depth studies of innate immunity require efficient genetic manipulation of macrophages, which is especially difficult in primary macrophages. We have developed a lentiviral system for inducible gene expression both in macrophage cell lines and in primary macrophages. A transgenic mouse strain C3H.TgN(SRA-rtTA) that expresses reverse tetracycline transactivator (rtTA) under the control of macrophage-specific promoter, a modified human scavenger receptor A (SR-A) promoter was generated. For gene delivery, we constructed a dual-promoter lentiviral vector, in which expression of a "gene-of-interest" is driven by a doxycycline-inducible promoter and the expression of a selectable surface marker is driven by an independent constitutive promoter UBC. This vector is used for transduction of bone marrow-derived macrophage precursors. The transduced cells can be enriched to 95-99% purity using marker-specific monoclonal antibodies, expanded and differentiated into mature macrophages or myeloid dendritic cells. We also successfully used this approach for inducible protein expression in hard to transfect macrophage cell lines.Because many proteins, which are expressed by activated or infected macrophages, possess cytotoxic, anti-proliferative or pro-apoptotic activities, generation of stable macrophage cell lines that constitutively express those proteins is impossible. Our method will be especially useful to study immunity-related macrophage proteins in their physiological context during macrophage activation or infection.

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