Gene therapy strategies: can we eradicate HIV?

Despite the tremendous advances in antiretroviral combination therapy over the last decade, eradication of HIV from the infected organism is still an elusive goal. Lifelong therapy is associated with potential long-term toxicity, adherence problems, and development of drug resistance. Thus, gene therapy approaches targeting viral eradication are still attractive. Here a number of studies have failed to show a clear clinical benefit yet. Current approaches were mainly limited by a low number of transduced cells and genotoxicity. The use of new vector systems and the right choice of target cells and improved transduction protocols may overcome these obstacles. Recent reports on the use of newly developed transgenes either allowing for an enrichment of transduced cells by an in vivo selection advantage or restoration of a functional immune system which is resistant to HIV infection nourished the hope for continuous progress in this field. Indeed the intriguing finding that HIV seems to be eradicated in an individual case study after stem cell transplantation with a mutant coreceptor (CCR5 delta 32 deletion) underlines the proof of the concept.     

Persisting multilineage transgene expression in the clonal progeny of a hematopoietic stem cell.

Many applications of hematopoietic gene therapy require selection for clones with active transgene expression. However, it was unclear whether the clonal progeny of a retrovirally transduced hematopoietic stem cell would be capable of maintaining transgene expression through serial repopulation and multilineage differentiation. Such investigations require simultaneous analyses of clonality, multilineage activity and transgene copy numbers. Using a mouse model, the present study demonstrates that a single hematopoietic stem cell expressing a marker gene from one or two insertions of a simple retroviral vector actively maintains multilineage transgene expression in the vast majority (80-99%) of bone marrow and peripheral blood cells. Gene expression persisted through serial transplantations for at least 97 weeks post gene transfer and was observed in the lymphoid (B, T and NK cells), myeloid (CD11b(+), Gr-1(+)), erythroid (Ter119(+), mature red blood cells) and megakaryocytic (as indicated by platelets) progeny. Therefore, a single immunoselection for hematopoietic stem cells expressing the transgene in vivo was sufficient to establish a completely chimeric hematopoiesis. These observations imply that the retroviral vectors used in this study contain cis-elements that mediate expression through massive clonal expansion and multilineage differentiation, provided the insertion occurred in genetic loci permissive for expression in hematopoietic stem cells.     

Aldehyde dehydrogenase activity as a marker for the quality of hematopoietic stem cell transplants

Taking advantage of fluorescent substrates for their metabolic marker aldehyde dehydrogenase (ALDH), hematopoietic stem cells (HSC) were defined as SSC(lo)ALDH(br) - reflecting their low orthogonal light scattering and bright fluorescence intensity in flow cytometry. Based thereon, we investigated the usefulness of ALDH activity for characterizing HSC graft quality, particularly under stress conditions. We first compared the expression of ALDH vs CD34 in bone marrow and peripheral blood stem cell (PBSC) samples over 7 days. We noted that (i) only ALDH activity but not CD34 expression strongly reflected colony-forming ability over time, and that (ii) PBSC grafts stored at room temperature lost most of their progenitor cells within just 48 h. We then retrospectively related ALDH and CD34 expression as well as granulocyte-macrophage colony-forming units (CFU-GM) potential for 19 cryopreserved allogeneic PBSC grafts to engraftment data. Strikingly, in all six patients who received markedly decreased numbers of SSC(lo)ALDH(br) cells, this was associated not only with almost complete loss of CFU-GM potential but also with delayed establishment/permanent absence of full hematopoietic donor cell chimerism, whereas all other patients showed early complete donor chimerism. In conclusion, we suggest to measure ALDH activity as a surrogate marker for HSC activity, and to transport and store PBSC under controlled cooling conditions.     

Restoration of SHIP activity in a human leukemia cell line downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3beta signaling and leads to an increased transit time through the G1 phase of the cell cycle.

The inositol 5-phosphatase SHIP (SHIP-1) is a negative regulator of signal transduction in hematopoietic cells and targeted disruption of SHIP in mice leads to a myeloproliferative disorder. We analyzed the effects of SHIP on the human leukemia cell line Jurkat in which expression of endogenous SHIP protein is not detectable. Restoration of SHIP expression in Jurkat cells with an inducible expression system caused a 69% reduction of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) and a 65% reduction of Akt kinase activity, which was associated with reduced phosphorylation of glycogen synthase kinase 3beta (GSK-3beta) (Ser-9) without changing the phosphorylation of Bad (Ser-136), FKHR (Ser-256) or MAPK (Thr-202/Tyr-204). SHIP-expressing Jurkat cells showed an increased transit time through the G1 phase of the cell cycle, but SHIP did not cause a complete cell cycle arrest or apoptosis. Extension of the G1 phase was associated with an increased stability of the cell cycle inhibitor p27(Kip1) and reduced phosphorylation of the retinoblastoma protein Rb at serine residue 780. Our data indicate that restoration of SHIP activity in a human leukemia cell line, which has lost expression of endogenous SHIP, downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3beta signaling and leads to an increased transit time through the G1 phase of the cell cycle.     

Identification of the adult human hemangioblast

Recent studies show that human CD133(+) (previously known as AC133(+)) cells from mobilized peripheral blood consist of stem cells with either hematopoietic or endothelial potential. To test whether this population also contains individual precursors with both capacities, the defining characteristics of the elusive adult hemangioblast, we developed a culture system that allows single-cell analyses of differentiation. In the presence of vascular endothelial growth factor (VEGF), stem cell growth factor (SCGF), and FLT-3 ligand, CD133(+)-enriched cells were first expanded and the amplified cells were transduced with a vector encoding an enhanced green fluorescent protein (EGFP) marker gene. Single EGFP(+) cells were then cocultured with corresponding non-transduced cells from the same experiment, yielding 50-100 marked cells in 8% of the wells after 2 weeks. The resultant cells were divided and differentiated with either granulocyte colony-stimulating factor (G-CSF) or with SCGF and VEGF. These culture conditions resulted in the formation of neutrophil or endothelial cells, respectively, as identified morphologically and by phenotypic staining. Dual differentiation of EGFP(+) cells could be observed in one-quarter of clones from single-seeded cells, suggesting that 2% of EGFP(+) cells were in fact human hemangioblasts. These cells could be expanded for at least 28 days without losing this dual capacity. Hence, this culture system may be of clinical relevance in the development of cellular therapies for disorders involving hematopoiesis and the vascular system. In addition, our results provide important information related to the development of the vasculature and the potential role of hemangioblasts in vasculogenesis in adult humans.     

Differences in proportion and dynamics of recipient hematopoiesis following hematopoietic cell transplantation in CML and IMF

Since decades myeloablation followed by allogeneic stem cell transplantation offered the only opportunity to cure leukemia patients and only recently the development of STI571 created a further alternative in chronic myeloid leukemia (CML). While among all leukemias this transplantation regimen had the best outcome in CML, trials with reduced intensity conditioning regimens (RIC) were rather humbling and recurrence of the neoplastic clone occurred frequently. However, the same therapy in patients with idiopathic myelofibrosis (IMF) resulted in a more favorable outcome. Therefore, long-term mixed chimerism (mCh) was determined on bone marrow (BM) biopsies derived from five IMF patients and from eight CML patients of the pre STI era following sex-mismatched transplantation. All patients presented lasting hematologic remission and were matched concerning age, sex and appearance of GvHD. Analysis of late transplant period (day +100) revealed a concentration of host cells within the CD34+ precursor cell compartment in both diseases. However, in IMF BM biopsies only up to 8% recipient CD34+ precursors but in CML biopsies up to 26% recipient CD34+ precursors were detected. Taken into account that in CML up to 10% of the host BM CD34+ precursors bear the BCR-ABL translocation our data suggest that the neoplastic CD34+ progenitor cell population might dispose of better strategies to escape immune surveillance in CML than in IMF.     

Depletion of alloreactive donor T cells using immunomagnetic cell selection

Donor T cells support both engraftment and immune reconstitution after allogeneic BMT. Moreover, they may exert potent anti-tumor effects (graft-versus-leukemia, GVL), which are used for adoptive immunotherapy. On the other hand, infusion of allogeneic T cells is frequently associated with the manifestation of immune reactions against healthy tissue, which may lead to life-threatening graft-versus-host disease (GVHD). To overcome this problem, we developed a new strategy for the exclusive depletion of alloreactive cells from donor leukocytes. We activated donor T cells by co-cultivation with a stroma layer of recipient cells and analyzed activation kinetics of CD3+, CD4+ and CD8+ T cells. Based on these data, activated cells were then depleted based on expression of activation-induced antigens using magnetic cell sorting (MACS). Alloreactivity of donor T cells was remarkably decreased after depletion of cells expressing either CD25 or CD69, as was shown in suitable in vitro assays. The lowest level of alloreactivity was found when both CD25- and CD69-positive cells were depleted. Importantly, depleted cell fractions preserved reactivity against third-party cells. In summary, we found that MACS-based ex vivo depletion of alloreactive cells may be a suitable way to prevent GVHD and therefore improve allogeneic BMT and adoptive immunotherapy.     

Antithymocyte globulin induces complement-dependent cell lysis and caspase-dependent apoptosis in myeloma cells

OBJECTIVE: Allogeneic stem cell transplantation is a potentially curative therapy for patients with multiple myeloma. Polyclonal antithymocyte globulins (ATG) or monoclonal anti-CD52 (Alemtuzumab) are included in conditioning regimens to enhance engraftment and reduce risk of severe graft-vs-host disease. Because both agents have been reported to induce depletion of B cells, we sought to investigate their cytotoxic activity on myeloma cells. MATERIALS AND METHODS: Complement-mediated and complement-independent activity of ATG-Fresenius and Alemtuzumab was investigated on four myeloma cell lines (RPMI-8226, U266, KMS-12-BM, and EJM) and bone marrow samples from six myeloma patients. Cytotoxicity was determined by staining with annexin V-fluorescein isothiocyanate and 7-amino-actinomycin D followed by flow cytometry. RESULTS: ATG at a concentration of 500 microg mL(-1) induced up to 100% and 85% complement-dependent killing of myeloma cell lines and primary myeloma samples respectively. In the absence of complement ATG still could induce up to 50% and 80% apoptosis in myeloma cell lines and primary myeloma samples, respectively. Preincubation of myeloma cells with a general caspase inhibitor abrogated ATG-induced complement-independent cell death. Alemtuzumab-mediated myeloma cytotoxicity was only observed in KMS-12-BM cells, and in none of the patient samples. CONCLUSION: ATG induces marked cytotoxic activity both in myeloma cell lines and in primary myeloma samples. Further elucidation of antibodies and antigens involved may pave the way for antibody-based myeloma therapy.