Experimental approach. (a) Simian immunodeficiency virus (SIV)-based lentiviral vectors were used with either the nonexpressed neomycin transferase gene with a stop codon at the 5′ end (NoN) or the enhanced green fluorescent protein (GFP) gene. Vector backbones contain self-inactivating (SIN) deletions in the U3 regions of their long terminal repeats (Δ), as well as the SIV packaging sequence (ψ) which extends into the 5′ end of the gag gene, the rev-responsive element (R), and the central polypurine/central termination signal (cP), as well as the 5′ LTR from the murine stem cell virus (MSCV) as an internal promoter. (b) Overall experimental plan for nonmyeloablative conditioning, CD34⁺ cell transduction with lentiviral vectors, and autologous transplantation. Bone marrow was collected as described (see text), CD34⁺ cells were isolated, and for each recipient, half of the cells were transduced using the SIV-NoN lentiviral vector and half with the SIV-GFP lentiviral vector. Monkeys in both experimental arms (A and B) received nonmyeloablative conditioning with busulfan at the indicated dosages, and those in arm B also received fludarabine; blood levels of the drugs were measured. The CD34⁺ cells were combined and reinfused into their autologous donors for hematopoietic stem cells (HSC) transplants. Post-transplant follow-up included biweekly evaluations in the first month to detect acute toxicity then monthly evaluations to measure gene marking and immune responses for 6–12 months.

Busulfan pharmacokinetics after first dose. Busulfan was administered at dosages of either 80, 120, or 160 mg/m² and serum levels of busulfan were measured at sequential time-points after the first dose. The area under the curve (AUC) of busulfan serum concentrations were calculated for each monkey. Horizontal bars indicate the mean value for each dosage group.

Absolute neutrophil and platelets counts after nonmyeloablative conditioning. Complete blood counts were assessed every 2–4 days for the month following conditioning and transplant. (a) Absolute neutrophil counts (ANC) and (b) platelet counts for each of the three groups of six recipients are shown. Monkeys from arm A (busulfan only) are plotted with solid lines and those from arm B (busulfan + fludarabine) are plotted with dashed lines.

Nadir values of absolute neutrophils counts (ANC), platelet, and lymphocyte counts versus busulfan serum concentrations [area under the curve (AUC)]. The lowest value of (a) ANC (cells/µl) (R² = 0.5502), (b) platelet counts (×10⁵/µl) (R² = 0.5005), and (c) lymphocyte counts (cells/µl) (R² = 0.0036) recorded in the first month following conditioning and transplant are shown versus the busulfan AUC determined for each recipient. Recipients in arm A (busulfan only) are shown with diamond symbols and those in arm B (busulfan + fludarabine) are indicated with square symbols.

Gene marking levels in blood and bone marrow cells. Peripheral blood and bone marrow samples were obtained from recipients #3A–#3F monthly post-transplant and cell subpopulations were isolated. Gene marking levels by the SIV-NoN and SIV-GFP vectors (vector copies/cell) were determined by quantitative PCR (qPCR). Mononuclear cell fractions were isolated from peripheral blood (Bld MNC; purple X) and bone marrow (BM MNC; green triangle), and the bone marrow mononuclear cells were further separated into CD34⁺ (BM CD34⁺; blue diamond) and CD34⁻ (BM CD34⁻; brown square) cell fractions during each of the 12-month time points that were evaluated. At 1 year post-transplant, peripheral blood cells were fractionated further to produce populations of peripheral blood CD2⁺ T lymphocytes (Bld CD2⁺; blue cross), CD4⁺ T lymphocytes (Bld CD4⁺; red dash), CD8⁺ T lymphocytes (Bld CD8⁺; green dash), CD20⁺ B lymphocytes (Bld CD20⁺; purple diamond), and CD34⁺ progenitor cells (Bld CD34⁺; orange circle). Genomic DNA isolated from these samples was evaluated for vector copies per cell of both the NoN and GFP genes by qPCR. SIV, Simian immunodeficiency virus.

Gene marking levels in bone marrow CD34⁺ cells. (a) Average gene marking in bone marrow CD34⁺ cells during months 4–6. The levels (vector copy/cell) of NoN (blue bar) and GFP (red bar) genes measured in the bone marrow CD34⁺ cell samples from months 4–6 after transplant were averaged and are shown for all 18 recipients. The conditioning regimens each received prior to transplant are indicated on the x-axis (see ). (b) Geometric mean gene marking in bone marrow CD34⁺ cells during months 4–6. The 18 recipients were clustered into four categories based upon the busulfan area under the curve (AUC) measured for each and the geometric means of the average levels of marking the NoN (blue bar) and GFP (red bar) vectors in bone marrow CD34⁺ cells were calculated. (c) Average gene marking in bone marrow CD34⁺ cells during months 7–12. The levels (vector copy/cell) of NoN (blue bar) and GFP (red bar) genes measured in the bone marrow CD34⁺ cell samples from months 7–12 after transplant were averaged and are shown for the six recipients in the Group 3 (#3A–#3F).

Evaluation of immune responses to the green fluorescent protein (GFP) transgene product. Plasma and peripheral blood mononuclear cells (PBMCs) were collected from subjects monthly after autologous transplant of CD34⁺ cells transduced with SIV-GFP and SIV-Non. Recipients in Group 3 were immunized with recombinant GFP (rGFP) at 6 months post-transplant and with autologous PBMCs transduced with the SIV-GFP lentiviral vector at 9 months post-transplant (GFP-PBMC). (a) Serum antibody levels were determined by ELISA to rGFP. n.d., not detected. (b) Cellular responses to GFP (red squares), tetanus toxoid (green triangles), hepatitis B surface antigen proteins (purple X), and to anti-CD3 monoclonal antibody (blue diamonds) were assessed by measuring the frequency of PBMCs producing interferon γ (IFNγ) by enzyme-linked immunosorbent spot (ELISPOT). SIV, Simian immunodeficiency virus.