FLT3 ligand preserves the uncommitted CD34+CD38- progenitor cells during cytokine prestimulation for retroviral transduction

Restriction of HIV-1-based lentiviral vectors in adult primary marrow-derived and peripheral mobilized human CD34+ hematopoietic stem and progenitor cells occurs prior to viral DNA integration

Background

Gene therapy is currently being attempted using a number of delivery vehicles including lentiviral-based vectors. The delivery and insertion of a gene using lentiviral-based vectors involves multiple discrete steps, including reverse transcription of viral RNA into DNA, nuclear entry, integration of viral DNA into the host genome and expression of integrated genes. Transduction of murine stem cells by the murine leukemia viruses is inefficient because the expression of the integrated DNA is profoundly blocked. Transduction of human stem cells by lentivirus vectors is also inefficient, but the cause and specific part of the retroviral lifecycle where this block occurs is unknown.

Results

Here we demonstrate that the dominant point of restriction of an HIV-1-based lentiviral vector in adult human hematopoietic stem and progenitor cells (HSPCs) from bone marrow and also those obtained following peripheral mobilization is prior to viral DNA integration. We specifically show that restriction of HSPCs to an HIV-1-based lentiviral vector is prior to formation of nuclear DNA forms.

Conclusions

Murine restriction of MLV and human cellular restriction of HIV-1 are fundamentally different. While murine restriction of MLV occurs post integration, human restriction of HIV-1 occurs before integration.

Serum Flt3 ligand variation as a predictive indicator of hematopoietic stem cell mobilization

Flt3 ligand (FL) is a good indicator of bone marrow (BM) cellularity, having a reciprocal relationship with white blood cell (WBC) count in aplastic anemia and chemotherapy-induced neutropenia. In this report, serum FL level was measured by enzyme-linked immunosorbent assay (ELISA), in 16 consecutive patients undergoing autologous peripheral stem cell transplantation, with an average of 12 selected levels for each patient based on major changes of WBC counts at different procedure stations. We found a significant increase of serum FL level at the WBC nadir after mobilization chemotherapy and a more dramatic increase at the WBC nadir post transplantation, consistent with a more profound BM aplasia after myeloablative chemotherapy as compared to high-dose cyclophosphamide used for mobilization. Hence, we reproduced the reciprocal relationship between serum FL and BM cellularity. A direct correlation between the increase of FL level after mobilization chemotherapy and the length of mobilization was also established, which may help physicians, at the individual patient level, to predict the time of stem cell collection. Finally, we showed a direct correlation between the peripheral CD34+ count at the time of stem cell collection and the peak FL level after transplantation, which can reflect BM stromal cell function. Our results suggest that variation of serum FL level may be used as predictive indicator of hematopoietic stem cell (HSC) mobilization.

Impaired progenitor cell function in HIV-negative infants of HIV-positive mothers results in decreased thymic output and low CD4 counts

Hematologic and immunologic functions were examined in 19 HIV-negative infants of HIV-positive mothers and 19 control infants of HIV-negative mothers. Control infants were selected to match for gestational age, weight, and mode of delivery. Cord blood was obtained from all infants and used for flow cytometric determination of lymphocyte subsets, including the naive CD4 count. Furthermore, to determine thymic output, cord blood mononuclear cells were used for determination of T-cell receptor excision circles (TRECs). Evaluation of progenitor cell function was done by means of colony-forming cell assay and fetal thymic organ cultures (FTOCs). Lower naive CD4 counts (459.3 +/- 68.9 vs 1128.9 +/- 146.8 cells/microL, P <.001) and reduced thymic output in infants of HIV-positive mothers were found (frequency of CD4(+) cells with TRECs was 3.6% +/- 0.7% compared with 14.3% +/- 2.2% in controls, P <.001). In combination with lower red blood cell counts in infants of HIV-positive mothers, this finding suggested impairment of progenitor cell function. Indeed, progenitors from infants of HIV-positive mothers had decreased cloning efficiency (15.7% +/- 2.6% vs 55.8% +/- 15.9%, P =.009) and seemed to generate fewer T cells in FTOCs. In conclusion, lower numbers of naive CD4(+) cells and reduced thymic output in HIV-negative infants of HIV-positive mothers may be due to impaired progenitor cell function.