In vitro culture during retroviral transduction improves thymic repopulation and output after total body irradiation and autologous peripheral blood progenitor cell transplantation in rhesus macaques

Barcoding of Macaque Hematopoietic Stem and Progenitor Cells: A Robust Platform to Assess Vector Genotoxicity

Gene therapies using integrating retrovirus vectors to modify hematopoietic stem and progenitor cells have shown great promise for the treatment of immune system and hematologic diseases. However, activation of proto-oncogenes via insertional mutagenesis has resulted in the development of leukemia. We have utilized cellular bar coding to investigate the impact of different vector designs on the clonal behavior of hematopoietic stem and progenitor cells (HSPCs) during in vivo expansion, as a quantitative surrogate assay for genotoxicity in a non-human primate model with high relevance for human biology. We transplanted two rhesus macaques with autologous CD34+ HSPCs transduced with three lentiviral vectors containing different promoters and/or enhancers of a predicted range of genotoxicities, each containing a high-diversity barcode library that uniquely tags each individual transduced HSPC. Analysis of clonal output from thousands of individual HSPCs transduced with these barcoded vectors revealed sustained clonal diversity, with no progressive dominance of clones containing any of the three vectors for up to almost 3 years post-transplantation. Our data support a low genotoxic risk for lentivirus vectors in HSPCs, even those containing strong promoters and/or enhancers. Additionally, this flexible system can be used for the testing of future vector designs.

The multi-differentiation potential of peripheral blood mononuclear cells

Peripheral blood is a large accessible source of adult stem cells for both basic research and clinical applications. Peripheral blood mononuclear cells (PBMCs) have been reported to contain a multitude of distinct multipotent progenitor cell populations and possess the potential to differentiate into blood cells, endothelial cells, hepatocytes, cardiomyogenic cells, smooth muscle cells, osteoblasts, osteoclasts, epithelial cells, neural cells, or myofibroblasts under appropriate conditions. Furthermore, transplantation of these PBMC-derived cells can regenerate tissues and restore function after injury. This mini-review summarizes the multi-differentiation potential of PBMCs reported in the past years, discusses the possible mechanisms for this multi-differentiation potential, and describes recent techniques for efficient PBMC isolation and purification.

Human and rhesus plasmacytoid dendritic cell and B-cell responses to Toll-like receptor stimulation

Interferon-α (IFN-α) produced at high levels by human plasmacytoid dendritic cells (pDCs) can specifically regulate B-cell activation to Toll-like receptor (TLR) 7/8 stimulation. To explore the influence of IFN-α and pDCs on B-cell functions in vivo, studies in non-human primates that closely resemble humans in terms of TLR expression on different subsets of immune cells are valuable. Here, we performed a side-by side comparison of the response pattern between human and rhesus macaque B cells and pDCs in vitro to well-defined TLR ligands and tested whether IFN-α enhanced B-cell function comparably. We found that both human and rhesus B cells proliferated while pDCs from both species produced high levels of IFN-α in response to ligands targeting TLR7/8 and TLR9. Both human and rhesus B-cell proliferation to TLR7/8 ligand and CpG class C was significantly increased in the presence of IFN-α. Although both human and rhesus B cells produced IgM upon stimulation, only human B cells acquired high expression of CD27 associated with plasmablast formation. Instead, rhesus B-cell differentiation and IgM levels correlated to down-regulation of CD20. These data suggest that the response pattern of human and rhesus B cells and pDCs to TLR7/8 and TLR9 is similar, although some differences in the cell surface phenotype of the differentiating cells exist. A more thorough understanding of potential similarities and differences between human and rhesus cells and their response to potential vaccine components will provide important information for translating non-human primate studies into human trials.

Generation and identification of thymic epithelial progenitor cells pTEC by in-vitro processing of human thymic fragments for allotransplantation

The procedure of generation and identification of stromal progenitor cells derived from human thymic fragments (PL patent 378431) has been described in this article. Our aim was to prepare material for transplantation in elderly people. The method is based on in-vitro processing of thymic fragments to get rid of all immunogenic elements of lymphocytes, endothelial cells, macrophages, and fibroblasts. In the thymic culture process, this organ dies out in the incubation medium and epithelial cells emerge out of the organ. After about 4 weeks from the start of the culture, the population of various developmental forms of epithelial cells was generated, namely CK AE1/AE3+, SDF-1 alpha+ and a weak expression of FGF+ S-100+. Finally, we obtained approximately 3 million cells as a monolayer. The progenitor cells were experimentally transplanted into a 72-year-old volunteer in order to prove that they do not induce neither a local nor a systemic rejection response.

Ex vivo expansion of retrovirally transduced primate CD34+ cells results in overrepresentation of clones with MDS1/EVI1 insertion sites in the myeloid lineage after transplantation

Activation of proto-oncogenes by retroviral insertion is an important issue delaying clinical development of gene therapy. We have reported the nonrandom persistence of hematopoietic clones with vector insertions within the MDS1/EVI1 locus following transplantation of rhesus macaques. We now ask whether prolonged culture of transduced CD34(+) cells before transplantation selects for clones with insertions in the MDS1/EVI11 or other proto-oncogene loci. CD34(+) cells were transduced with standard retroviral vectors for 4 days and then continued in culture for an additional 6 days before transplantation. A 15% of insertions identified in granulocytes 6 months post-transplant were in MDS1/EVI11, significantly increased compared to the frequency in animals transplanted with cells immediately following transduction. MDS1/EVI1 clones became more dominant over time post-transplantation in one animal that was followed long term, accompanied by an increased overall copy number of vector-containing granulocytes, with one MDS1/EVI1 clone eventually accounting for 100% of transduced granulocytes and marrow colony-forming unit (CFU). This vector insertion increased the expression of Evi1 mRNA. There was no overrepresentation of MDS1/EVI1 insertions contributing to lymphoid lineages. Strategies involving prolonged ex vivo expansion of transduced cells may increase the risk of genotoxicity.

Hematopoietic stem-cell behavior in nonhuman primates

Little is known about the behavior of hematopoietic stem cells (HSCs) in primates because direct observations and competitive-repopulation assays are not feasible. Therefore, we used 2 different and independent experimental strategies, the tracking of transgene expression after retroviral-mediated gene transfer (N = 11 baboons; N = 7 rhesus macaques) and quantitation of the average telomere length of granulocytes (N = 132 baboons; N = 14 macaques), together with stochastic methods, to study HSC kinetics in vivo. The average replication rate for baboon HSCs is once per 36 weeks according to gene-marking analyses and once per 23 weeks according to telomere-shortening analyses. Comparable results were derived from the macaque data. These rates are substantially slower than the average replication rates previously reported for HSCs in mice (once per 2.5 weeks) and cats (once per 8.3 weeks). Because baboons and macaques live for 25 to 45 years, much longer than mice ( approximately 2 years) and cats (12-18 years), we can compute that HSCs undergo a relatively constant number ( approximately 80-200) of lifetime replications. Thus, our data suggest that the self-renewal capacity of mammalian stem cells in vivo is defined and evolutionarily conserved.

Keratinocyte growth factor augments immune reconstitution after autologous hematopoietic progenitor cell transplantation in rhesus macaques

Opportunistic infections contribute to morbidity and mortality after peripheral blood progenitor cell (PBPC) transplantation and are related to a deficient T-cell compartment. Accelerated T-cell reconstitution may therefore be clinically beneficent. Keratinocyte growth factor (KGF) has been shown to protect thymic epithelial cells in mice. Here, we evaluated immune reconstitution after autologous CD34(+) PBPC transplantation in rhesus macaques conditioned with myeloablative total body irradiation in the absence or presence of single pretotal body irradiation or repeated peritransplant KGF administration. All KGF-treated animals exhibited a well-preserved thymic architecture 12 months after graft. In contrast, thymic atrophy was observed in the majority of animals in the control group. The KGF-treated animals showed higher frequencies of naive T cells in lymph nodes after transplantation compared with the control animals. The animals given repeated doses of KGF showed the highest levels of T-cell receptor excision circles (TRECs) and the lowest frequencies of Ki67(+) T cells, which suggest increased thymic-dependent reconstitution in these animals. Of note, the humoral response to a T-cell-dependent neo-antigen was significantly higher in the KGF-treated animals compared with the control animals. Thus, our findings suggest that KGF may be a useful adjuvant therapy to augment T-cell reconstitution after human PBPC transplantation.