Open Access

Drug toxicity and viral resistance limit long-term efficacy of antiviral drug treatment for HIV infection. Thus, alternative therapies need to be explored. Previously, group of “Prof. von Laer” tested the infusion of T lymphocytes transduced with a retroviral vector (M87o) that expresses an HIV entry inhibitory peptide (maC46). Gene-modified autologous T cells were infused into 10 HIV-infected patients with advanced disease and multidrug resistant virus during antiretroviral combination therapy. T cell infusions were tolerated well with no severe side effects. A significant increase of CD4 counts was observed post infusion. At the end of the one-year follow-up, the CD4 counts of all patients were still around or above baseline. Gene-modified cells could be detected in peripheral blood, lymph nodes and bone marrow throughout the oneyear follow-up, whereby marking levels correlated with the cell dose. No significant changes of viral load were observed during the first four months. Four of the seven patients that changed their antiviral drug regimen thereafter responded with a significant decline in plasma viral load. In conclusion, the transfer of gene-modified cells was safe, led to sustained levels of gene marking and may improve immune competence in HIV-infected patients with advanced disease and multidrug resistant virus. However, the low level of gene marking and the lack of substantial long-term in vivo accumulation of gene-protected cells observed in this trial clearly demonstrate the requirement for new vectors with new strategy. In this thesis self‐inactivating lentiviral vectors harboring internal promoters and RNA elements were therefore evaluated for their potential use in a clinical gene‐therapy trial. The results from this work provide the basis for the selection of a suitable candidate vector for extensive preclinical testing. Apart from being capable of transducing non‐dividing cells, lentiviral vectors incorporate a number of additional features that are of potential value for gene therapeutic applications. These include a larger packaging capacity, higher titers than γ‐retroviral vectors and, most importantly, a reduced risk of deregulating cellular genes due to its natural integration profile. The use of internal promoters to drive expression of the therapeutic transgene maC46 should further improve the safety profile of these new‐generation vectors, while an additional artificial splice acceptor (SA) into the 5‟UTR of the transgene over all elevate transgene expression. The rationale for this is that hematopoietic stem and progenitor cells will be Summary 98 protected from enhancer‐mediated transactivation effects and also from potential side effects due to the aberrant expression of maC46 while at the same time the full clinical benefit for the patients is maintained. In order to find a suitable candidate for preclinical studies, two candidate therapeutic vectors harboring different regulatory elements were selected based on results from pilot experiments. The internal promoters used to drive expression of codon optimized maC46 were the PGK promoter and MPSV promoter. This work focuses on the transgene expression levels in lymphoid cells and antiviral activity. The issues of long term expression, propensity to methylation mediated silencing of the promoters, and genotoxicity were also touched. In a first step the performance of different vectors was evaluated in the human T cell lines. Based on promising data from ex vivo human peripheral blood mononuclear cells, the vector carrying the MPSV promoter along with intron were selected for in vivo transplantation experiments. In summary, the ex vivo data suggested the long term survival of lentiviral gene modified cells, along with maintained expression of introduced genes. It was observed that the expression of these constructs depends strongly on the activation and differentiation status of the targeted T cells. This regulation was not linked to any specific promotor. In vivo study shows that maC46 can be introduced into murine multiple hematopoietic lineages via lentiviral vector and expressed at high levels in their mulilineage progeny, without altering the hematopoiesis. There was no sign of any kind of hematopoietic or lymphoid malignancies. Although gene-modified lymphocytes persisted in-vivo, the downregulation of transgene expression was consistent with the ex-vivo observation. In contrast to that the T cells transplanted group showed delayed engraftment of donor cells and there was no expression of C46 in blood and lymphatic organs. . In conclusion, when considering HIV gene therapy focusing CD4+ T cells, potential problems of T cell activation status as related to the desired clinical effect must be addressed. These results might open the way for a gene therapy targeting mainly or exclusively activated T cells and could be exploited for immunostimulatory as well as suppressive approaches.