In vivo selection of hematopoietic progenitor cells and temozolomide dose intensification in rhesus macaques through lentiviral transduction with a drug resistance gene

Major limitations to gene therapy using HSCs are low gene transfer efficiency and the inability of most therapeutic genes to confer a selective advantage on the gene-corrected cells. One approach to enrich for gene-modified cells in vivo is to include in the retroviral vector a drug resistance gene, such as the P140K mutant of the DNA repair enzyme O⁶-methylguanine-DNA methyltransferase (MGMT*). We transplanted 5 rhesus macaques with CD34⁺ cells transduced with lentiviral vectors encoding MGMT* and a fluorescent marker, with or without homeobox B4 (HOXB4), a potent stem cell self-renewal gene. Transgene expression and common integration sites in lymphoid and myeloid lineages several months after transplantation confirmed transduction of long-term repopulating HSCs. However, all animals showed only a transient increase in gene-marked lymphoid and myeloid cells after O⁶-benzylguanine (BG) and temozolomide (TMZ) administration. In 1 animal, cells transduced with MGMT* lentiviral vectors were protected and expanded after multiple courses of BG/TMZ, providing a substantial increase in the maximum tolerated dose of TMZ. Additional cycles of chemotherapy using 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) resulted in similar increases in gene marking levels, but caused high levels of nonhematopoietic toxicity. Inclusion of HOXB4 in the MGMT* vectors resulted in no substantial increase in gene marking or HSC amplification after chemotherapy treatment. Our data therefore suggest that lentivirally mediated gene transfer in transplanted HSCs can provide in vivo chemoprotection of progenitor cells, although selection of long-term repopulating HSCs was not seen.

Retroviral vector–mediated gene transfer into HSCs has the potential to improve treatment for various genetic, malignant, and infectious diseases (1, 2). However, a critical limitation to stem cell gene therapy is the low gene transfer efficiency in large animal models and clinical trials (3, 4).

In a recent gene therapy trial for X-linked SCID, although engraftment of transduced HSCs was low, as evidenced by the low gene marking in myeloid cells, patients demonstrated near-complete repopulation with genetically modified T lymphocytes (5). In vivo selective survival advantage of lymphocytes carrying the therapeutic transgene (the IL-2 common γ–receptor chain, encoded by Il2rg) contributed substantially to clinically beneficial levels of genetically corrected lymphocytes after gene therapy. These results strongly suggest that providing an in vivo selective advantage to transduced cells may be an attractive method of enhancing the repopulation of transduced cells in vivo. However, for most blood disorders, there will be no constitutive selective advantage of the gene-corrected cells.

One potential approach to in vivo selection of transduced HSCs is to incorporate drug resistance genes to convey protection against chemotherapeutic agents (6). Successful in vivo selection was previously documented in murine HSC transplantation models using several drug resistance genes, including multidrug resistance protein–1 (MDR-1; refs. 7, 8), dihydrofolate reductase (DHFR; ref. 9), and O⁶-methylguanine-DNA methyltransferase (MGMT; refs. 10–12). However, in more clinically relevant large-animal models, in vivo selection using DHFR (13) and MDR-1 (14, 15) was variable and transient, with levels of gene-modified cells returning to baseline within a few weeks. MGMT is one of the most promising drug resistance genes that encodes for the DNA repair protein O⁶-alkylguanine-DNA-alkyl-transferase (AGT). This protein confers resistance to the cytotoxic effects of alkylating agents, including nitrosoureas [e.g. 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU)] and temozolomide (TMZ). To allow hematopoietic selection, O⁶-benzylguanine (BG) is coadministered with TMZ for inactivation of endogenous MGMT activity. Several mutant forms of MGMT that give rise to altered AGT are highly resistant to inactivation by BG, but retain their ability to repair DNA damage (16, 17). The most commonly used mutant is MGMT P140K (MGMT*), which is almost completely resistant to BG, yet has a modest reduction in the rate of DNA adduct repair compared with wild-type AGT. This mutant form of MGMT can be incorporated in the gene transfer retroviral vectors to facilitate a greater selective advantage for primitive hematopoietic cells containing the vector. In vivo selection of HSCs using MGMT* was successfully achieved in murine (10–12) and canine models (18–20) and in human NOD/SCID-repopulating cells (21, 22). The same approach has also been proposed as a means to protect patients against dose-limiting hematopoietic toxicity following administration of chemotherapy combining alkylating agents and BG for the treatment of brain tumors (23). Protection of the BM compartment would allow administration of increased doses of chemotherapy agents to effect an improved therapeutic response.

A second approach to achieving therapeutic levels of gene-corrected cells is to confer a selective growth advantage upon gene-transduced cells. Homeobox B4 (HOXB4) is a potent stem cell self-renewal gene. In murine transplantation models, overexpression of HOXB4 has previously been shown to induce ex vivo expansion and self renewal of HSCs without compromising differentiation or homeostatic regulation of HSC pool size in murine transplantation models (24–26). More recently, HOXB4 function was examined in a clinically relevant nonhuman primate transplantation model: HOXB4 overexpression in rhesus CD34⁺ cells was able to expand short-term repopulating cells, but had a less pronounced effect on long-term repopulating cells (27).

Drug resistance gene–mediated in vivo selection can amplify the percentage of gene-transduced HSCs; however, it is accompanied by a decrease in the overall number of HSCs. In murine transplantation studies, this limitation can be overcome by coexpressing HOXB4 with the drug resistance gene to allow selective regeneration of transduced cells after treatment with cytotoxic drugs (28). In this study, we examined the effects of an MGMT*-mediated drug-resistant system with or without an HOXB4-mediated HSC self-renewal system in transplanted rhesus macaques. Our findings indicate that in vivo chemoprotection of progenitor cells occurred, but selection of long-term repopulating HSCs was not seen.

CD34⁺ cells transduced with MGMT* lentiviral vectors show transient chemoprotection after in vivo treatment with BG and TMZ. We investigated whether in vivo chemoprotection could be achieved after transduction with MGMT*-expressing lentiviral vectors based on SIV and autologous transplantation of mobilized peripheral blood (PB) or BM CD34⁺ cells in 5 rhesus macaques (Table 1). Gene transfer efficiency in the CD34⁺ cell population before transplant in rhesus macaques varied between 18% and 53%, as determined by flow cytometry (Table 1).

Table 1

Reconstitution of irradiated rhesus macaques with transduced, autologous CD34⁺ cells

In a first set of experiments, AMD3100-mobilized PB CD34⁺ cells from animals RQ4876 and RQ4795 were transduced using SIV vectors expressing GFP-MGMT* fusion protein (Figure 1A). We administered 2 daily doses of 500 cGy total body irradiation before infusion of 5.6–6.4 × 10⁶ CD34⁺ cells/kg. Hematopoietic recovery was documented between 7 and 17 days (Table 1). At 3–6 months after transplant, each animal received the first of 4 successive BG/TMZ treatments given at 1- to 2-month intervals. Substantial myelosuppression was seen in both animals after drug administration (Figure 2A).

Figure 1

Vector constructs. (A) For the GFP-MGMT* SIV-based lentiviral vectors, expression of a GFP-MGMT* fusion gene was driven by the promoter activity of the MSCV LTR. (B and C) For MGMT*-P2A-GFP (B) and MGMT*-P2A-YFP (C) SIV vectors (referred to herein as MGMT*-GFP and MGMT*-YFP, respectively), P2A was used to generate a multicistronic expression cassette inserted in the pCL20cSLFR MSCV-GFP backbone. The primer pairs (M3, M5, Y3, Y5, G3, and G5) used for construction of the vectors are described in Methods. (D) For MGMT*-P2A-HOXB4-T2A-GFP SIV vectors (referred to herein as MGMT*-HOXB4-GFP), T2A was used to generate a multicistronic expression cassette inserted in the pCL20cSLFR MSCV-GFP backbone. MGMT*, HOXB4, and GFP were amplified and 2A-tagged from their respective cDNAs using primer pairs M5, M3, H5, H3, G5, and G3 as described in Methods.

Figure 2

In vivo selection in SIV-transduced animals RQ4876 and RQ4795 after multiple cycles of BG/BCNU and/or BG/TMZ. Both animals were transplanted with CD34⁺ cells transduced with lentiviral GFP-MGMT* vector. (A) ANC, hemoglobin (HGB), and platelet (PLT) counts in PB over time for RQ4876 and RQ4795. (B) Percent GFP⁺ granulocytes (Gran), lymphocytes (Lymph), and monocytes (Mono) from PB for RQ4876 and RQ4795. Arrows denote cycles of drug administration; “†” indicates time of animal death.

Animal RQ4876 showed a rapid increase in transduced PB cells of all lineages in vivo after drug treatment. Steady-state marking levels of 2%, 6%, and 5% in granulocytes, lymphocytes, and monocytes, respectively, increased to peak levels of 52%, 33%, and 36% in the respective lineages after the first (granulocytes), second (lymphocytes), or third (monocytes) dose of BG/TMZ. The fourth dose of BG/TMZ led to only moderate increases in GFP⁺ cells in all lineages. Treatment with BG/TMZ was well tolerated, with little to no cumulative toxicity. The proportion of GFP⁺ cells subsequently declined gradually over a period of 200 days after the last BG/TMZ administration; marking levels in PB stabilized to values similar to those observed at steady state before the first treatment with BG/TMZ, with 2% GFP⁺ granulocytes, 6% GFP⁺ lymphocytes, and 3% GFP⁺ monocytes (Figure 2B). Lentiviral vector provirus copy numbers were also determined by real-time PCR at several time points in DNA from PB (data not shown). Overall, there was a good correlation between flow cytometry and real-time PCR results, which suggests that on average, repopulating cells contained no more than 1 vector copy and that gene silencing was not responsible for the gradual decrease in gene marking levels in vivo.

Animal RQ4795 had very low steady-state gene marking levels in PB granulocytes (0.1%), lymphocytes (0.01%), and monocytes (0.2%; Figure 2B). We tested the safety and feasibility of improving very low baseline gene marking levels with in vivo infusion of BG/TMZ. No change in GFP⁺ granulocytes was detected at any time point after BG/TMZ administration. A peak increase in GFP⁺ lymphocytes (0.2%) and monocytes (1.5%) was observed after the second dose of BG/TMZ, but the increase was only transient, returning to baseline levels within 30 days. The third and fourth doses of BG/TMZ resulted in substantial myelosuppression, but had no impact on gene marking levels in any lineage.

In a second set of experiments, we tested whether alternative SIV MGMT*-expressing vectors in combination with minor modifications to transduction conditions and escalating doses of TMZ could result in improved longer-term gene marking levels. Rhesus competitive repopulation experiments were set up using equal numbers of mobilized PB or BM CD34⁺ cells transduced with SIV vectors expressing MGMT* alone (containing GFP or yellow fluorescent protein [YFP]) or MGMT*-HOXB4-GFP (see Methods and Figure 1). Multicistronic cassettes containing GFP or YFP were generated in each vector using the 2A peptide from porcine teschovirus–1 (P2A) and thosea asigna virus (T2A) (Figure 1, B–D). All 3 animals in this study — RQ4513, RQ4099, and RQ4152 — were mobilized with G-CSF plus SCF and received 2- to 3-fold more CD34⁺ cells/kg compared with the AMD3100-mobilized animals RQ4876 and RQ4795 after total body irradiation conditioning (Table 1). Hematopoietic reconstitution was rapid, with the absolute neutrophil count (ANC) reaching 500 cells/μl or more by 8–11 days after transplant. All animals showed a total of 3%–6% marked PB granulocytes and monocytes, and 1%–12% marked PB lymphocytes, at 60 days after transplant (Figures 3, 4, 5). After stable engraftment, we evaluated the drug combination BG/TMZ. The dose of BG was fixed at 120 mg/m², but, in contrast to animals RQ4876 and RQ4795, the dose of TMZ was gradually escalated from 170 to 450 mg/m²/d for a 5-day period. All animals showed significant myelosuppression after each treatment (Supplemental Figure 1; supplemental material available online with this article; doi: 10.1172/JCI37506DS1).

Figure 3

In vivo selection in SIV-transduced animal RQ4513 after 2 cycles of BG/TMZ and 2 cycles of BG/BCNU. Rhesus competitive repopulation experiments were set up using equal numbers of mobilized PB CD34⁺ cells transduced with SIV vectors expressing MGMT*-YFP or MGMT*-HOXB4-GFP. (A) Percent GFP⁺ and YFP⁺ granulocytes from PB. (B) Absolute number of YFP⁺ granulocytes. (C) Percent GFP⁺ and YFP⁺ monocytes from PB. (D) Percent GFP⁺ and YFP⁺ B cells and T cells from PB. Arrows denote cycles of drug administration; “†” indicates time of animal death.

Figure 4

In vivo selection in SIV-transduced animal RQ4099 after 1 cycle of BG/TMZ. Rhesus competitive repopulation experiments were set up using equal numbers of mobilized PB CD34⁺ cells transduced with SIV vectors expressing MGMT*-YFP or MGMT*-HOXB4-GFP. (A) Percent GFP⁺ and YFP⁺ granulocytes and monocytes from PB. (B) Percent GFP⁺ and YFP⁺ B cells and T cells from PB. Arrows denote cycles of drug administration.

Figure 5

In vivo selection in SIV-transduced animal RQ4152 after multiple cycles of BG/TMZ and 1 cycle of BG/BCNU. Rhesus competitive repopulation experiments were set up using equal numbers of mobilized PB CD34⁺ cells transduced with SIV vectors expressing MGMT*-GFP or MGMT*-HOXB4-GFP. (A) Percent GFP⁺ granulocytes, B cells, T cells, and monocytes from PB for RQ4152. (B) Absolute number of GFP⁺ granulocytes. Arrows denote cycles of drug administration.

The first animal in this group, RQ4513, was treated with 2 courses of BG/TMZ at 12 and 15 weeks after transplant. The ANC nadirs were 780 cells/μl at day 16 after the first treatment and 210 cells/μl at day 14 after the second treatment. The percentage of GFP⁺ and YFP⁺ granulocytes (Figure 3A) and monocytes (Figure 3C) increased from 4% to 40% (YFP) and 4% to 50% (GFP), but gradually declined back to 4% (GFP) and 10% (YFP) over a period of 50 days after the second dose of BG/TMZ. The absolute number of YFP-transduced granulocytes increased from 40 cells/μl before drug treatment to approximately 200 cells/μl after 2 courses of BG/TMZ, which suggests that YFP-transduced CD34⁺ cells were protected and expanded after BG/TMZ treatment (Figure 3B). Similarly, the percentage of GFP⁺ and YFP⁺ lymphocytes doubled after the second BG/TMZ administration, but GFP⁺ lymphocytes rapidly declined back to pretreatment levels (Figure 3D).

The second animal, RQ4099, was treated with a single course of BG/TMZ at week 10 after transplant. The ANC nadir was 190 cells/μl at day 14 after treatment. The percentage of GFP⁺ and YFP⁺ granulocytes and monocytes increased from a steady-state level of 3%–5% to a peak of 25%–30%, but returned to baseline gene marking levels within 50 days after BG/TMZ administration (Figure 4A). In vivo enrichment of GFP⁺ and YFP⁺ B cells was also transient (Figure 4B). In T cells, GFP and YFP marking levels were 1% at steady state and peaked at 13% (GFP) and 23% (YFP) after BG/TMZ treatment. The proportion of gene marked T cells subsequently declined and stabilized at 4% (GFP) and 5% (YFP) (Figure 4B). The animal died approximately 3 months after BG/TMZ treatment due to pulmonary fibrosis. This complication is occasionally seen in animals not treated with BG/TMZ and is likely secondary to radiation received as a conditioning regimen prior to transplant.

The third animal, RQ4152, had pretreatment gene marking levels of 5% in granulocytes, 3% in monocytes, 10% in B cells, and 1% in T cells. The animal was treated with 7 courses of BG/TMZ, with TMZ doses ranging from 170 to 450 mg/m²/d. The ANC nadir was 850 cells/μl at day 6 after the first treatment (170 mg/m² TMZ) and 7 cells/μl at day 9 after the last treatment (450 mg/m² TMZ). After each course of drug treatment, there was a rapid increase in GFP marking in all lineages. The highest marking achieved after the last course of BG/TMZ treatment was 94% for myeloid cells and 57% for lymphoid lineages (Figure 5A). The proportion of GFP⁺ cells subsequently declined and leveled off at 10% for granulocytes, 5% for monocytes, 17% for B cells, and 30% for T cells at 150 days after the last treatment (Figure 5A). Correspondingly, the absolute number of transduced granulocytes increased after each drug treatment with, for example, levels of 80 cells/μl before the fourth course peaking to 698 cells/μl after the seventh course and stabilizing at approximately 250 cells/μl (Figure 5B). These results indicate that MGMT*-transduced cells were protected and expanded following multiple courses of BG/TMZ treatments with TMZ dose escalation up to 450 mg/m². In pediatric patients, the maximum tolerated dose of TMZ without BG is 185 mg/m² (29); therefore, gene therapy resulted in substantial dose escalation due to protection from hematopoietic toxicity. As shown by pharmacokinetic data (Table 2), the absorption and clearance of TMZ in animal RQ4152 at the time of the seventh dose of TMZ was comparable to that seen in treated pediatric patients. Moreover, the AUC for TMZ was about 2-fold greater than that seen in pediatric patients (Table 2), confirming that BM protection was achieved against BG/TMZ by MGMT* overexpression in this animal. This is particularly interesting given that BG is known to significantly enhance TMZ toxicity, necessitating TMZ dose reductions (30). Given the relative lack of nonhematopoietic toxicity seen in our series with BG/TMZ, we conclude that hematopoietic protection via transfer of MGMT* allows for substantial dose escalation and administration of repeated drug courses.

Table 2

Pharmacokinetic study of RQ4152 after the seventh course of TMZ/BG treatment

Polyclonal hematopoiesis is sustained after BG/TMZ treatment. The clonal composition before and after BG/TMZ treatment was analyzed by linear amplification–mediated PCR (LAM-PCR) in animals RQ4152 and RQ4513, which showed the highest in vivo gene marking levels. DNA isolated from PBLs before and after 1, 2, 6, and 7 courses of BG/TMZ was used for LAM-PCR. Gel electrophoresis showed contribution of multiple clones at all time points analyzed (Supplemental Figure 2). Individual bands from LAM-PCR products were picked from the gel and sequenced directly after PCR amplification. Using previously described selection criteria (31), we retrieved and analyzed 254 independent unequivocal retroviral integration sites (ISs; 152 for RQ4152, Supplemental Table 1; and 102 for RQ4513, Supplemental Table 2). The majority of these ISs occurred within transcription units, mainly within introns (Table 3), consistent with data from previous reports (31–33). Also, in agreement with previously published data (31–33), SIV integration events did not favor locations upstream or downstream of transcription units (Table 3), indicating no preference for integration near transcription start sites. In animals RQ4152 and RQ4513, 7.9% and 3.9% of ISs identified, respectively, were in or near cancer-associated genes (CAGs; Table 3 and Supplemental Tables 1 and 2). We identified 8 clones that appeared repetitively before and after drug treatment, which suggests that the same clones contributed to hematopoiesis after in vivo selection with BG/TMZ.

Table 3

IS distribution and association with CAGs for RQ4152 and RQ4513

In vivo treatment with BG/BCNU does not result in long-term chemoprotection and causes high levels of nonhematopoietic toxicity. In animals RQ4876, RQ4513, and RQ4152, which showed evidence of increased gene marking after multiple BG/TMZ treatments, we tested whether chemoprotection could be achieved using BCNU as an alternative alkylating agent. Several months after the last dose of BG/TMZ, we administered 1–3 successive infusions of 120 mg/m² BG and 20 mg/m² BCNU (hereafter referred to as BG/BCNU). In animal RQ4876, a moderate but steady increase in marking levels was seen after each BG/BCNU dose. Pretreatment levels of 2% in granulocytes, 6% in lymphocytes, and 5% in monocytes peaked at 9%, 17%, and 10%, respectively, after the third dose (Figure 2B). BG/BCNU treatment also resulted in a rapid increase in GFP/YFP marking in all hematopoietic lineages in animal RQ4513, with maximal levels of 90% in granulocytes and monocytes, 60% in B cells, and 40% in T cells (Figure 3). A more modest effect was seen in animal RQ4152 (Figure 5).

All animals that received BG/BCNU eventually developed substantial nonhematopoietic toxicity, in contrast to what was seen using BG/TMZ. RQ4876 developed severe tachypnea, fevers, and failure to thrive with weight loss 1 month after the third dose of BG/BCNU. X-rays revealed diffuse pulmonary infiltrates consistent with pneumonitis (Supplemental Figure 3A). Euthanasia was performed 35 days after the last administration of BG/BCNU. Animal RQ4513 developed severe pulmonary and gastrointestinal complications, and euthanasia was performed 2 weeks after the second cycle of BG/BCNU. Specimens collected at necropsy revealed extensive pulmonary edema and widespread colonic necrosis with pseudomembrane formation (Supplemental Figure 3B). Animal RQ4152 was clinically well after receiving 7 courses of BG/TMZ, but developed severe gastrointestinal complications and died 7 days after 1 course of BG/BCNU. At necropsy, specimens were consistent with a diagnosis of severe necrohemorrhagic colitis (Supplemental Figure 3C). While lower doses of BCNU may have been less toxic, we observed no significant decrease in complete blood count and no change in vector marking levels when a single test dose of 5 mg/m² was administered in animal RQ4876 (data not shown).

Because all animals were euthanized shortly after BG/BCNU administration, long-term persistence of improved marking levels could not be assessed. In the BCNU-treated animal RQ4876, follow-up was possible for a slightly longer period of time after BCNU administration (about 80 days); in this animal, a clear trend down in marking levels was evident at the time of euthanasia (Figure 3B), which suggests a lack of persistent chemoprotection with BCNU, as was seen after TMZ administration.

Absence of long-term chemoprotection in vivo is not caused by lack of transduction of long-term repopulating HSCs. We investigated whether the lack of stable selection in vivo could be the result of poor transduction of long-term repopulating HSCs. Several lines of evidence indicated transduction of HSCs. First, in all animals, persistence of substantial expression of the GFP transgene in T cells, B cells, granulocytes, and monocytes for several months after transplant was an indication of HSC transduction (Figure 2B and Figures 3–5). Second, in animal RQ4876, 10% of CD34⁺ cells isolated from the BM 630 days after transplant were GFP⁺, as assessed by flow cytometry (data not shown). Third, we also obtained molecular evidence of transduction of HSCs in animal RQ4876. Using DNA isolated from the BM of this animal more than 1 year after transplant, we performed LAM-PCR and isolated 10 ISs. Demonstration of common lentiviral ISs in sorted subpopulations of the PB (T cells, B cells, and granulocytes) collected before BG/TMZ treatment (day 60 after transplant), after the last dose of BG/TMZ (day 374), and after the last dose of BG/BCNU (day 630) was performed by PCR using 1 vector long-terminal repeat (LTR) primer and 1 genomic primer spanning a region of each IS. We were able to detect 2 common ISs (Figure 6), providing a clear demonstration of the transduction of HSCs contributing in the long term to the different hematopoietic lineages. Interestingly, of the 2 common ISs, the one designated IS 1 (Figure 6) was undetectable prior to drug treatment, but its contribution to all lineages increased over time after BG/TMZ and BG/BCNU treatment, a characteristic of HSC dynamics. Overall, these observations rule out the lack of transduction of HSCs as a possible mechanism for the absence of long-term chemoprotection. Given the consistent data among the 5 animals described in the present study, we believe that these conclusions can be generalized to all animals.

Figure 6

Transduction of long-term repopulating rhesus macaque HSCs with lentiviral vectors. Using DNA isolated from the BM of rhesus macaque RQ4876 more than 1 year after transplant, we performed LAM-PCR and isolated 10 ISs. Demonstration of common lentiviral ISs in sorted subpopulations of the PB — T cells, B cells, and granulocytes — collected before and after BG/TMZ or BG/BCNU selection was subsequently performed by PCR using 1 vector LTR primer and 1 genomic primer spanning a region of each IS. We detected 2 common ISs, providing a clear demonstration of the transduction of HSCs contributing in the long term to the different hematopoietic lineages. DNA extracted from wbcs of a nontransplanted animal was used as a negative control. Each DNA sample was amplified using β-actin–specific primers as a positive control.

HOXB4 does not confer HSC amplification in the rhesus macaque transplant model. HOXB4 is a potent HSC amplifier in murine transplantation models. In our competitive rhesus transplantation model, we investigated whether HOXB4 could improve engraftment of transduced HSC and progenitor cells in animals RQ4513, RQ4099, and RQ4152 (Table 1). Gene transfer efficiencies in CD34⁺ cells were similar between MGMT* and MGMT*-HOXB4-GFP vectors before cell infusion based on flow and CFU analysis. For animals RQ4513 and RQ4099, the GFP/YFP ratio allowed direct measurement of competitive engraftment of MGMT*-HOXB4-GFP– versus MGMT*-transduced cells. In both animals, we observed a similar engraftment pattern. The initial engraftment levels for both MGMT*- and MGMT*-HOXB4-GFP–transduced granulocytes at 2 weeks after transplant were similar in RQ4513 (17% versus 13%; Supplemental Figure 4A) and RQ4099 (23% versus 25%; Supplemental Figure 4B). Marking levels gradually declined for both compartments before they stabilized at a similar level. In RQ4513, MGMT*-HOXB4-GFP marking stabilized at 2.6%, and MGMT* marking stabilized at 2.9%, in granulocytes 78 days after transplant (Supplemental Figure 4A). Marking levels in granulocytes in RQ4099 stabilized at 2.5% for MGMT*-HOXB4-GFP and 3.3% for MGMT* at 66 days after transplant (Supplemental Figure 4B). For animal RQ4152, because both MGMT* and MGMT*-HOXB4-GFP–transduced cells used GFP as an expression marker, Southern blot was used to distinguish the 2 compartments. Again, there was no difference in engraftment levels between MGMT*- and MGMT*-HOXB4-GFP–transduced cells (Supplemental Figure 4C, lane 1). Moreover, we did not observe any advantage for in vivo selection in the cells containing the MGMT*-HOXB4-GFP vector. For instance, in RQ4513 (Figure 3) and RQ4099 (Figure 4), marking levels in all lineages increased in parallel between MGMT*- and MGMT*-HOXB4-GFP–transduced cells after drug treatment. For RQ4152, there was also no difference in marking levels between MGMT*- and MGMT*-HOXB4-GFP–transduced cells after BG/TMZ treatment, as determined by Southern blot (Supplemental Figure 4C, lanes 2–5). At 8 months after transplant, HOXB4 expression was detected in MGMT*-HOXB4-GFP–marked PB cells sorted by flow cytometry (Supplemental Figure 5), which confirmed that the lack of amplification was not caused by absent expression of HOXB4.

MGMT*-mediated in vivo selection with lentiviral vectors occurs mainly in progenitor cells, not HSCs. In the current study, we evaluated BG/TMZ and BG/BCNU for in vivo selection of CD34⁺ cells modified with MGMT* lentiviral vectors to increase transduced blood cells in the clinically relevant rhesus macaque model. We observed that 2 animals with pretreatment steady-state in vivo marking levels of 2% and 0.2% showed transient increase in GFP expression in both myeloid and lymphoid lineages after 4 cycles of BG/TMZ treatment, indicating in vivo MGMT* gene expression and chemoprotection of progenitor cells with transient repopulating capacity. Additionally, 3 other animals with higher baseline marking levels (5% in PB granulocytes) showed a more persistent, yet still transient, increase in gene-marked cells in vivo in lymphoid and/or myeloid lineages after BG/TMZ administration. Additional cycles of chemotherapy using BCNU as an alternative alkylating agent resulted in similar increases in gene marking levels.

Overall, these results indicate that the relatively high transduction efficiencies achieved in CD34⁺ cells using lentiviral vectors support chemoprotection of progenitor cells, but not long-term repopulating stem cells. Four mechanisms can be considered in accounting for the lack of stable selection at the stem cell level. First, we demonstrated that the absence of persistent chemoprotection did not relate to the lack of transduction of long-term repopulating HSCs. Detection of transgene expression in the long term (close to 2 years in 1 animal) after transplant in both lymphoid and myeloid lineages was an indication of transduction of long-term repopulating HSCs. In a prior study, 3 rhesus macaques — transplanted with CD34⁺ cells transduced using SIV vectors and conditions similar to those used in the present study — demonstrated significant vector marking and expression of the transgene for up to 4 years after transplant (34). In animal RQ4876 in the present study, 10% of CD34⁺ cells isolated from the BM at day 630 after transplant expressed the GFP transgene. More importantly, we demonstrated 2 common lentiviral ISs in sorted subpopulations of the PB (T cells, B cells, and granulocytes) of RQ4876 collected up to 630 days after transplant. Of the 2 common ISs (Figure 6), 1 had characteristics of HSC, not progenitor cell, dynamics with amplification over time after BG/TMZ and BG/BCNU treatment. Overall, these observations indicate transduction of HSCs contributing in the long term to the different hematopoietic lineages.

Second, in spite of the observed lack of selection with TMZ, it is possible that selection of HSCs could have been achieved with BCNU at doses lower than 20 mg/m², as toxicity prevented long-term follow-up. In animal RQ4876, we administered a lower dose of BCNU (5 mg/m²), but no significant decrease in complete blood count and no change in vector marking levels was observed. While titrating the dose of BCNU may be possible in rhesus macaques for selection of transduced HSCs with decreased toxicity, it is unlikely that this drug could be used safely in patients for which individualized dose titration would not be possible.

Third, we considered the possibility that the lack of HSC selection could be related to the deleterious effect of high murine stem cell virus–driven (MSCV-driven) expression of MGMT* on cellular proliferation and engraftment, or, alternatively, to inadequate MGMT expression from this promoter in HSCs. As recently reported by Milsom and colleagues, while MGMT* expression by weaker cellular promoters (e.g., phosphoglycerate kinase) was sufficient for in vivo selection, viral promoter–driven high expression of MGMT* led to increased localization to the nucleus/chromatin, resulting in competitive repopulation defects in HSCs (35). However, the toxicity was observed with the SFFV viral promoter, causing a 10-fold higher level of MGMT* than with the MSCV promoter used in the present study. In addition, in our present study, MGMT* activity was sufficient to allow transient chemoprotection, but the low BG-noninhibitable activity detected in our transplanted animals (e.g., 7 fmol [³H]-methylated protein/mg protein detected in RQ4876) suggested that the potential toxicity of high MGMT* expression from the lentiviral constructs we used is an unlikely explanation for the lack of stable enrichment of gene modified cells in rhesus macaques.

In contrast, the internal MSCV promoter used in the present study, although active in the rhesus CD34⁺ progenitor cells and blood progeny, may have not provided adequate expression of the MGMT* transgene in HSCs. The MSCV promoter was chosen at the outset of these experiments based on our extensive experience with it in the murine system, where we have previously shown it to be active in HSCs (36–38). Interestingly, a recent study showed that while the MSCV LTR is expressed in undifferentiated mouse ES cells, it has much lower transcriptional activity than that of the human elongation factor–1α (EF1α) promoter or the hybrid CAG promoter (CMV enhancer, chicken β-actin promoter and intron, and globin polyadenylation signal; ref. 39). In the context of self-inactivating (SIN) HIV-based lentiviral vectors, the EF1α promoter gave about a 6-fold increase in expression relative to that of the MSCV LTR, as judged by GFP fluorescence, while the CAG vector gave 2- to 3-fold better expression (39). The promoter MND may also offer advantages for expression in HSCs. It includes deletion of the negative control region in the 5′ region of the U3 portion of the LTR, and it contains 2 copies of the 72-bp enhancer repeat, compared with 1 copy in MSCV (40–43). Another promoter with potential activity in HSCs is the ubiquitin-C promoter. SIN lentiviral vectors using this promoter to drive GFP expression were previously found to express well in the primitive embryonic blastula cells of mice after injection of vector into single-cell embryos (44). Together, these data raise the possibility that either of the latter regulatory elements may provide better expression than the MSCV promoter in HSCs, thus allowing better long-term in vivo selection.

Fourth, the lack of selection at the stem cell level may be attributed to the ability of untransduced stem cells to resist the cytotoxic effects of TMZ. The cytotoxicity of TMZ is largely attributed to the induction of single-strand methyl adducts at the O⁶ position of guanines, leading to mispairing with thymidine residues. When TMZ-treated cells divide, DNA replication is inhibited, triggering apoptosis and cell death. In contrast to rapidly dividing progenitor cells, HSCs cycle slowly in vivo in large animals (once per 23–36 weeks at steady state; ref. 45); therefore, the TMZ-induced O⁶-methylguanine DNA adducts may persist longer in these cells with no deleterious effects. During this period of quiescence, in the absence of BG, endogenous MGMT levels would be expected to recover, leading to repair of O⁶-methylguanine DNA adducts in both transduced and untransduced HSCs. When the untransduced HSCs eventually divide, the TMZ-induced DNA lesions may have been repaired, preventing the cells from undergoing apoptosis. Given the marked differences in estimates of the average HSC replication rate between mice (once per 2.5 weeks; refs. 45, 46) and large animals (once per 23–36 weeks; ref. 45), the same reasoning could also explain why in vivo selection of HSCs using MGMT* was successfully achieved in mice (10–12). In our rhesus macaque study, BG was given once daily for 5 days; perhaps continuous administration of BG for a longer period of time could have prevented recovery of endogenous MGMT in slowly dividing HSCs and allowed for a stable increase in gene marking levels in vivo. Another option would be to design a treatment protocol that forces HSC cycling using appropriate hematopoietic cytokines during the period of cytotoxic drug treatment.

An alternative explanation for the increased resistance of untransduced HSCs to BG inactivation may be related to higher levels of endogenous MGMT compared with more mature progenitor cells. Indeed, high levels of expression of molecules such as ABCG2, MDR-1, and ALDH in HSCs (47–50) are postulated to be of importance in protecting HSCs from genotoxic or metabolic damage. Moreover, in highly purified murine long-term repopulating HSCs (defined as CD34^–Lin^–c-Kit⁺Sca1⁺), MGMT expression was 2–3 times higher than that present in more mature progenitor cells (defined as Lin^–c-Kit⁺Sca1^–), as determined previously by query of a GEO DataSet Browser for microarray data of MGMT expression in hematopoietic cells (51). Optimization of inhibition of endogenous HSC MGMT through a pharmacokinetic and pharmacodynamic approach could improve in vivo selection in HSCs.

BG/BCNU has more nonhematopoietic toxicity than does BG/TMZ for MGMT*-mediated in vivo selection. An important consideration regarding the ultimate use of the MGMT* selection system in humans is whether it can be used with acceptable toxicity. In our rhesus macaque study, euthanasia was performed in 3 of 3 (100%) animals that received BG/BCNU. In contrast, animal RQ4795 survived treatment with BG/TMZ alone without complications, and animal RQ4099, also treated with BG/TMZ alone, was euthanized following complications attributed to radiation-induced pulmonary fibrosis. These observations indicate a higher toxicity of the alkylating agent BCNU compared with TMZ. Gastrointestinal complications were observed with BCNU, including necrohemorrhagic colitis in animal RQ4152 and colonic necrosis with pseudomembrane formation in animal RQ4513. Pulmonary complications were also seen in 2 animals, including pulmonary congestion/edema (RQ4513) and infections (RQ4876). These complications have never or only rarely been detected in more than 200 transplanted rhesus macaques at our institutions, which suggests that they are causally related to the administration of BG/BCNU. In contrast, TMZ toxicity was highly specific for hematopoietic cells, and multiple courses were well tolerated without cumulative or nonhematopoietic toxicity.

HOXB4 does not provide a proliferative advantage to transduced long-term repopulating cells. In addition to drug resistance genes for selection of transduced cells after transplantation, retroviral vectors armed with genes that give a proliferative advantage to the transduced cells have also been proposed to improve in vivo gene marking levels (28). Genes that favor self renewal of HSCs have been prime candidates to promote selective growth advantage of transduced HSCs for improved gene therapy. One particular transcription factor, HOXB4, has been extensively studied as a way of increasing the self renewal of HSCs and hematopoietic progenitor cells in mice (24, 25, 52), humans, and large animal models (27) both in vivo and in vitro.

In our primate competitive repopulation assay, we transduced monkey CD34⁺ cells with either a MGMT*-HOXB4-GFP vector or a vector expressing only the MGMT* gene and analyzed the competitive repopulating ability of these cells in vivo. We found no substantial difference in engraftment levels between MGMT*- and MGMT*-HOXB4-GFP–transduced cells both before and after chemotherapy treatment up to 400 days after transplant. Our data are consistent with those of Zhang, Kiem, and colleagues, who found that, in contrast to mouse studies, no significant advantage for the HOXB4-transduced cells was observed in the long term after transplant in a similar nonhuman primate competitive study (27). The same group also recently reported what we believe to be the first instances of leukemia linked to HOXB4 expression, both in the original group of monkeys now followed longer term and in dogs that received cells transduced with a HOXB4-expressing retroviral vector (53). Some 2 years after transplant, 3 of 4 animals (2 of 2 dogs and 1 of 2 nonhuman primates) developed acute myeloid leukemias, and several lines of evidence have strongly implicated HOXB4 in leukemogenesis (53). In animal RQ4152, euthanized as a result of complications of BG/BCNU treatment, we demonstrated by direct sequencing no occurrence of monoclonality, at least over the period of observation of approximately 12 months.

MGMT* lentiviral vectors can be used for TMZ dose intensification. TMZ is a chemotherapeutic alkylating agent with demonstrated efficacy for the treatment of malignant gliomas. A recent study by Stupp et al. revealed that TMZ in combination with radiation therapy yielded a statistically significantly greater prolongation of the survival time compared with radiation alone in patients with newly diagnosed gliomas (23). Increased doses of TMZ may lead to further increases in survival, but dose intensification is precluded because of increased toxicity. In the present study, we showed that BM protection was achieved by MGMT* overexpression after multiple courses of BG/TMZ treatments with TMZ dose escalation up to 450 mg/m², twice the maximum tolerated dose of TMZ in pediatric patients. This is potentially significant because the use of BG in patients to diminish MGMT activity and drug resistance in tumor cells has been associated with increased hematopoietic toxicity and has necessitated TMZ dose reductions (30). Our findings indicate that drug resistance in 5%–10% of myeloid progenitors allows for increased hematopoietic recovery and increased tolerance to TMZ. The transient chemoprotection observed in this study may be useful for patients with malignant brain tumors treated with both BG and TMZ dose intensification in order to overcome tumor cell drug resistance. While cumulative genotoxicity resulting from the combined use of alkylating agents (e.g., TMZ) and lentiviral vectors needs to considered, our study revealed no evidence of clonal dominance or leukemic transformation using this approach, at least over the period of observation covered.

View Supplemental data

We thank Arthur Nienhuis for scientific discussion. This work was supported in part by the intramural research program of NHLBI and NIAID, NIH; by NHLBI grant P01 HL 53749; by Cancer Center Support Grant P30 CA 21765; by the Assisi Foundation of Memphis; and by the American Lebanese Syrian Associated Charities.

Address correspondence to: Andre Larochelle, National Heart, Lung and Blood Institute, National Institutes of Health, 9000 Rockville Pike, Building 10CRC, Room 3E-5256, Bethesda, Maryland 20892, USA. Phone: (301) 451-7139; Fax: (301) 496-8396; E-mail: larochea@nhlbi.nih.gov.

Authorship note: Andre Larochelle, Uimook Choi, and Yan Shou contributed equally to this work.

Conflict of interest: The authors have declared that no conflict of interest exists.

Nonstandard abbreviations used: ANC, absolute neutrophil count; BCNU, 1,3-bis-(2-chloroethyl)-1-nitrosourea; BG, O⁶-benzylguanine; CAG, cancer-associated gene; HOXB4, homeobox B4; IS, integration site; LAM-PCR, linear amplification–mediated PCR; LTR, long-terminal repeat; MGMT, O⁶-methylguanine-DNA methyltransferase; MGMT*, MGMT P140K; MSCV, murine stem cell virus; P2A, 2A peptide from porcine teschovirus–1; PB, peripheral blood; T2A, 2A peptide from thosea asigna virus; TMZ, temozolomide; YFP, yellow fluorescent protein.

Reference information: J. Clin. Invest.119:1952–1963 (2009). doi:10.1172/JCI37506

See the related article at Efficient and stable MGMT-mediated selection of long-term repopulating stem cells in nonhuman primates.

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