T-lymphocyte function after retroviral-mediated thymidine kinase gene transfer and G418 selection.

Generation of an efficient graft-versus-leukemia (GVL) effect in patients with hematological malignancies who relapse after allogeneic bone marrow transplantation depends in part upon the number of infused T lymphocytes. Currently, a GVL reaction cannot be achieved without inducing concomitant graft-versus-host disease (GVHD); thus, one strategy is to try to modulate this GVL/GVHD ratio. We engineered human T lymphocytes with herpes simplex virus-thymidine kinase and neomycin resistance genes, with an LXSN-derived vector that confers a ganciclovir-specific sensitivity to the transduced T cells. We analyzed proliferation, interleukin-2 production, alloreactivity in a mixed lymphocyte culture, and clonogenicity during the different stages of retroviral infection and G418 selection. Our results confirm that a sufficient number of transduced T lymphocytes can be obtained after selection for clinical studies. Their proliferative activity, alloresponsiveness, and ability to produce and respond to interleukin-2 were retained. Compared with control populations, their clonogenicity, as assessed by limiting dilution assays, was reduced after retroviral infection and G418 selection by 1.6 and 2.9 logs, respectively, with both viral supernatant incubation and coculture procedures. This study shows that infection and selection with the thymidine kinase-neomycin resistance gene retroviral vector significantly reduces the number of functional T lymphocytes. This finding should be taken into account when establishing the dose of T lymphocytes necessary to trigger a modulated GVL/GVHD effect.

5-Azacytidine prevents transgene methylation in vivo.

Retroviral sequence can silence transgene expression in vitro and in vivo. We report that this effect can be efficiently prevented by in vivo administration of the demethylating agent 5-azacytidine (aza-C). We engineered the U937 human cell line with a retroviral vector consisting of the thymidine kinase suicide gene (tk), which induces sensitivity to ganciclovir (gcv) and through an IRES sequence, the bacterial beta-galactosidase gene (lacZ) as a marker gene. About 90% of the U937 cells expressed the transgene. By injecting the transduced U937 cells in severe combined immunodeficient disease (SCID) mice, we generated a tumor which, during in vivo treatment with aza-C, maintained the high expression of lacZ and tk genes at the baseline values. LacZ-positive cells in the tumour masses after death was weak (1-2%) in the control group, while in mice treated with aza-C it was maintained at 90%. The delay in tumour onset was significantly longer when animals were treated with both aza-C and gcv (P < 0.0001) compared with animals treated with gcv or with aza-C alone. The prevention of silencing phenomena has important implications for gene therapy, because an efficient transduction associated with appropriate drug therapy, might be a powerful strategy for successful application of gene therapy protocols.

Retroviral transfer of herpes simplex virus-thymidine kinase and beta-galactosidase genes into U937 cells with bicistronic vector.

In this study we describe a new retroviral vector utilizing an internal ribosome entry site (IRES) from encephalomyocarditis virus to co-express two genes. One is the herpes simplex virus type 1 thymidine kinase gene (HSV-TK) which induces sensitivity to ganciclovir, and the second is the bacterial beta-galactosidase gene (LacZ) which was revealed by an histochemical staining with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal). We engineered the U937 human cell line to co-express both genes and monitored transduced cells using X-Gal staining. Several transduced clones were selected. The clones exhibiting X-Gal positive cells were sensitive to ganciclovir treatment (1 microgram/ml) while X-Gal negative clones were not. Monoclonal cell lines showed a single copy of the provirus integrated in their genome with the TK-IRES-LacZ sequence stably inserted in all clones. The band distribution pattern of the proviral DNA differed only at the long terminal repeat (LTR) level. Northern blot analysis of an X-Gal positive/ganciclovir sensitive clone showed an mRNA band of 6 kb with both LacZ and TK probes. An X-Gal negative/ganciclovir resistant clone was negative with both probes. This report shows: (1) a therapeutic gene can be linked to a marker gene by an IRES element achieving equivalent expression of both proteins; (2) the co-expression of a marker gene makes fluorescein-di-beta-D-galactopyranoside staining possible, and consequently separation of cells expressing the LacZ gene by fluorescence activated cell sorting. Thus the cells expressing the HSV-Tk gene are enriched; (3) the use of a marker gene such as LacZ could open up interesting perspectives in gene therapy protocols because of the opportunity to monitor the transduced cells using a simple cytochemical stain.

beta-galactosidase transduced T lymphocytes: a comparison between stimulation by either PHA and IL-2 or a mixed lymphocyte reaction.

BACKGROUND: Retroviral-mediated gene transfer stably introduces exogenous genes into normal and neoplastic cells of the hematopoietic system. METHODS: We used two retroviral vectors [the first, FLac, expresses a chimeric protein (Sh-ble::LacZ) between the product of the phleomycin resistance gene (Sh-ble) and the bacterial beta-galactosidase encoded by the LacZ gene; the second, NuNL vector, contains a fusion sequence (LacZ::Neo) that expresses the LacZ and the neomycin resistance genes] to transduce T lymphocytes derived from the peripheral blood of healthy human donors. Two lymphocyte activation procedures were employed: a) phytohemagglutinin/interleukin-2 (PHA/IL-2) polyclonal stimulation; b) allogeneic stimulation in a mixed irradiated or non irradiated lymphocyte reaction, both supplemented with IL-2 (MLR/IL-2). Infection was achieved by co-cultivating activated T cells with the producing amphotropic cell line pretreated with mitomycin C for 96 hours. Infection and transduction efficiency were assayed by LacZ gene expression, which is detected as indigo blue staining with the chromogenic substrate 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X..Gal). RESULTS: The highest percentage of transduced T cells was reached on the 3rd PHA/IL-2 and on 9th MLR/IL-2 activation days. In these conditions with FLac vector we obtained up to 80% X-Gal+ cells after PHA/IL-2 activation and 66% and 44%, respectively, with non irradiated and irradiated MLR/IL-2, respectively. Up to 40% X-Gal+ cells were obtained with NuNL vector after PHA/IL-2 stimulation, 40% with irradiated and 48% with non irradiated MLR/IL-2 activated cells. In term of transduction efficiency, large variability was observed among patients. There were no immunophenotypical differences between FLac or NuNL vector-transduced cells activated by either of the two techniques and the control cells. CONCLUSIONS: Our results indicate that: a) the use of FLac or NuNL vector retroviral-mediated gene transfer into T-lymphocytes derived from peripheral blood and stimulated by either PHA/IL-2 or a MLR produces a high percentage of transduced T cells; b) MLR is a good system for generating a transduced alloreactive lymphocyte population. The combination of high transduction efficiency and the capacity to obtain alloreactive transduced lymphocytes should open up the possibility of generating new in vitro and in vivo studies with selectable genes for in vivo therapeutic use.