Elongation factor 1 (EF1α) promoter in a lentiviral backbone improves expression of the CD20 suicide gene in primary T lymphocytes allowing efficient rituximab-mediated lysis

Background and Objectives. CD20 has been proposed as a novel suicide gene system for the treatment of graft-versus-host disease (GVHD), a fatal complication of allogeneic bone marrow transplantation: indeed expression of the human non-immunogenic exogenous CD20 protein allows positive immunoselection of transduced cells as well as their killing in vitro with rituximab. Lentiviral vectors are promising tools in the field of gene therapy. We therefore searched for a lentivector giving good efficiency of transduction of human T lymphocytes activated by the sole addition of interleukin (IL)-2 and high expression levels of the CD20 transgene. Design and Methods. The T-cell line CEM and peripheral T lymphocytes activated by phytohemagglutinin (PHA) and/or IL-2 were transduced with two different vectors carrying the CD20 transgene driven by either the phosphoglycerate kinase (PGK) or elongation factor 1α (EF1α) promoter, and using different multiplicities of infection (MOIs). Results. Both the PGK- and EF1α-CD20 vectors allowed efficient transduction of the CEM cell line and PHA-activated T cells, reaching 99% and 90% in the different targets, respectively. However EF1α-CD20 led to much higher expression levels of the transgene (mean fluorescence intensity 588-618 compared to 53 for PGK-CD20). Furthermore lymphocytes activated with IL-2 alone could be efficiently transduced with EF1α-CD20, reaching 10-25% positivity for CD20 (mean fluorescence intensity 409-424), allowing adequate immunoselection and strong complement-mediated lysis. Interpretation and Conclusions. EF1α-CD20 may represent a good candidate vector for gene therapy with the CD20 suicide system in the setting of allogeneic bone marrow transplants.