Enhancement of T Cell Development in Murine Bone Marrow Transplant Recipients by Transfer of the IL-7 or SCF Gene into Marrow-Derived Mesenchymal Stem Cells

The ability of the thymus to generate T cells diminishes with increasing age, the use of chemotherapy, bone marrow transplantation (BMT), anti-retroviral therapy for HIV, and graft-versus-host disease (GVHD) which can lead to a major clinical problem. Therefore, developing a clinically relevant strategy for the rapid development of T lymphocytes is crucial for treating immune deficiency. Stem cell factor (SCF: also known as kit ligand) and interleukin-7 (IL-7) are stroma–derived cytokines that induce proliferation, differentiation, and survival of developing immature T cells in the thymus. Studies have shown that administration of recombinant human IL-7 following murine BMT resulted in improved thymopoiesis and immune function. However, our previous studies have shown that that IL-7 treatment post-HSCT to enhance immune reconstitution in the allogeneic setting may have adverse effects because of the dual role of IL-7 in supporting both thymopoiesis and mature T lymphocyte expansion. Therefore it raises the question of whether IL-7 treatment after allogeneic BMT will increase the frequency or severity of GVHD. The purpose of this study was to examine whether:

1. administration of IL-7 and SCF with infusion of mature T cell depleted (TCD) BM cells can induce enhancement of donor-derived immune reconstitution more rapidly than treatment with either cytokine alone and

2. whether IL-7 and SCF are synergistic and partially complementary signals for the proliferation, survival, and differentiation of immature T cells.

To evaluate the combinatory effect of IL-7 and SCF in T cell development following BMT, we developed a gene therapy approach using retrovirally-mediated transduction of BM-derived mesenchymal stem cells (MSC) with the human IL-7 or murine SCF gene (soluble isoform). C57BL/6J (CD45.2) recipient mice were irradiated (1300 cGy) and co-transplanted with 1 × 10 6 T cell depleted (TCD) bone marrow cells from congenic donor B6.SJL mice (CD45.1) and different doses (0.1 × 10 6 or 0.3 × 10 6) of eGFP (control), IL-7, SCF, or combination of IL-7 and SCF MSC. At day 30 following BMT, we observed that transplantation of both IL-7 and SCF MSC resulted in significantly higher numbers of donor-derived thymocytes and peripheral lymphocytes than either IL-7 or SCF MSC transplantation alone. Most noticeably, the number of donor-derived immature and mature T cells recovered from the animals receiving transplantation of 0.1 × 10 6 IL-7 MSC and 0.3 × 10 6 SCF MSC was similar to that of animals receiving 0.3 × 10 6 IL-7 MSC alone, demonstrating that the reduced proliferative signals produced by 0.1 × 10 6 IL-7 MSC can be compensated by co-transplantation of 0.3 × 10 6 SCF MSC. Moreover, transplantation of IL-7 and SCF MSC significantly increased the number of donor-derived common lymphoid progenitors (CLP [Lin-, Sca-1 low, Thy1-, c-Kit low, IL-7R+]) in the BM, suggesting that transplanted CLPs are induced to differentiate or expand more rapidly in response to IL-7 and SCF and may have contributed to increased immune reconstitution. Collectively, our findings demonstrate that IL-7 and SCF gene therapy may be a therapeutically useful method to promote enhancement of T cell development in de novo. Furthermore, the experiments resulted in important knowledge about complementary signals provided between IL-7 and SCF, and suggest various doses of IL-7 and SCF therapy may enhance development of T cells with limited expansion of mature T cells responsible for causing GVHD in allogeneic BMT setting.