Chimerism and gene therapy - Lessons learned from non-conditioned murine bone marrow transplantation models.

OBJECTIVE: Hematopoietic stem and progenitor cells (HSPCs) can be used as a vector for gene therapies. In order to predict the number of HSPCs cells necessary to achieve the target level of chimerism in an autologous setting, syngeneic male bone marrow (BM) cells were transplanted into 35 non-conditioned female BALB/c mice. METHOD: The resulting chimerism was determined at 6-53 weeks using qPCR, cell subpopulation sorting, and colony-forming units (CFU) analysis. RESULTS: After the transplantation of 125.8 ± 2.5 million nucleated BM cells, the BM of recipients contained 20.0 ± 2.8% donor cells, representing a chimerism of 0.16 ± 0.02% per one million transplanted nucleated BM cells. Chimerism levels in the BM, neutrophils, and B cells were comparable, whereas in T cells it was lower, and in CFU was approximately twice greater than in BM. CONCLUSION: By extrapolating our murine data, and data from some previous studies to a human non-conditioned autologous CD34+ HSPC transplantation setting, we conclude that approximately 44 million CD34+ HSPCs would be needed to achieve 20% donor chimerism in a 70-kg human, which could serve as a starting point for the future use of HSCPs in gene and cell therapy.

Assessment of stability of CD34+ cell products enriched by immunoselection from peripheral blood mononuclear cells during refrigerated storage.

Durable engraftment of transplanted CD34+ cells largely depends on the quality of the cell product. Limited data are currently available about extended storage of immunoselected CD34+ cells. The aim of our study was to assess the stability of CD34+ cell product with the cells stored in high concentration (80×106 in 6mL) in small bags intended for cell implantation. Cell products were prepared by leukapheresis and immunoselection (Clinimacsplus procedure) from 13 patients with chronic dilated cardiomyopathy. CD34+ cell products were stored at 2-8°C and analyzed at time 0 (fresh products), 24, 48h, 4 and 6 days. Product viability, absolute number of viable CD34+ cells and apoptosis were determined by flow cytometry. Microbiological contamination of the cell products was tested by BACTEC system. The mean viability of CD34+ cells decreased by 2.7% within 24h, by 13.4% within 48h and by 37.5% within 6 days. The mean recovery of viable CD34+ cells was 91.1%, 74.8%, 66.3% and 56.2% at 24, 48h, 4 and 6 days, respectively. The mean fraction of early apoptotic cells in fresh and stored products was 4.9±3.5% at 0h, 5.9±3.8% at 24h, 4.2±3.1% at 48h, 6.3±2.6% at 4 days and 9.3±4.6% at 6 days. All products were negative for microbial contamination.

DNA Methylation and Hydroxymethylation Profile of CD34+-Enriched Cell Products Intended for Autologous CD34+ Cell Transplantation.

Epigenetic dysregulation has been shown to limit functional capacity of aging hematopoietic stem cells, which may contribute to impaired outcome of hematopoietic stem cell-based therapies. The aim of our study was to gain better insight into the epigenetic profile of CD34+-enriched cell products intended for autologous CD34+ cell transplantation in patients with cardiomyopathy. We found global DNA methylation content significantly higher in immunoselected CD34+ cells compared to leukocytes in leukapheresis products (2.33 ± 1.03% vs. 1.84 ± 0.86%, p = 0.04). Global DNA hydroxymethylation content did not differ between CD34+ cells and leukocytes (p = 0.30). By measuring methylation levels of 94 stem cell transcription factors on a ready-to-use array, we identified 15 factors in which average promoter methylation was significantly different between leukocytes and CD34+ cells. The difference was highest for HOXC12 (58.18 ± 6.47% vs. 13.34 ± 24.18%, p = 0.0009) and NR2F2 (51.65 ± 25.89% vs. 7.66 ± 21.43%, p = 0.0045) genes. Our findings suggest that global DNA methylation and hydroxymethylation patterns as well as target methylation profile of selected genes in CD34+-enriched cell products do not differ significantly compared to leukapheresis products and, thus, can tell us little about the functional capacity and regenerative properties of CD34+ cells. Future studies should examine other CD34+ cell graft characteristics, which may serve as prognostic tools for autologous CD34+ cell transplantation.

The effect of CD34+ cell telomere length and hTERT expression on the outcome of autologous CD34+ cell transplantation in patients with chronic heart failure.

Age-related telomere attrition in stem/progenitor cells may diminish their functional capacity and thereby impair the outcome of cell-based therapies. The aim of the present study was to investigate the effect of CD34+ cell telomere length and hTERT expression on the clinical outcome of autologous CD34+ cell transplantation. We studied 43 patients with cardiomyopathy. Their peripheral blood CD34+ cells were mobilized with granulocyte colony-stimulating factor, enriched by immunoselection and delivered transendocardially. Relative telomere length and expression levels of hTERT were measured using a real-time PCR assay. Immunoselected CD34+ cells had longer telomere length compared to leukocytes in leukapheresis products (p=0.001). In multivariate analysis, CD34+ cell telomere length was not associated with the clinical outcome (b=3.306, p=0.540). While hTERT expression was undetectable in all leukapheresis products, 94.4% of the CD34+ enriched cell products expressed hTERT. Higher CD34+hTERT expression was associated with a better clinical outcome on univariate analysis (b=87.911, p=0.047). Our findings demonstrate that CD34+ cell telomere length may not influence the clinical outcome in cardiomyopathy patients treated with autologous CD34+ cell transplantation. Larger studies are needed to validate the impact of the CD34+hTERT expression on the clinical outcome of autologous CD34+ cell transplantation.