Lentiviral vectors (LVs) are promising tools for in vivo gene delivery, either to correct genetic defects or for vaccination. Intravenous administration of LVs results in stable transduction and expression of the transgene in antigen presenting cells (APCs) from the spleen. Therefore, it was decided to investigate the reasons for and the consequences of sustained antigen expression in these cells after systemic in vivo administration of LVs. Intravenous injection of a LV encoding green fluorescent protein (GFP) resulted in transduction of lymphocytes, macrophages and all subsets of dendritic cells (DCs) in the spleen, detected 5 days later. In the case of macrophages and DCs, the percentage of transduced cells increased between 5 and 30 days after injection. The transduction of dividing precursors contributes to the persistence of the transgene-expressing DCs, as shown by BrdU incorporation. Expression of ovalbumin (OVA) resulted in a reduced number of transgeneexpressing cells after 30 days. However, the remaining transduced cells stimulated proliferation and activation of OVA-specific CD8+ T cells up to 3 months after LV administration, in spite of a reduction in the activation status of transduced DCs over time. Mice also maintained cytolytic activity against OVA-pulsed targets following a single immunisation. In conclusion, this thesis shows that LVs can transduce DCs and macrophages, leading to persistent antigen expression. These modified APCs are functional and capable of activating T cells. Therefore, LVs can be used as tools for persistent genetic modification of APCs, opening the opportunity for their use in long-term immunomodulation
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