High-efficiency retroviral gene transfer into murine high-proliferative-potential cells cycle-activated by cytosine arabinoside

Bone marrow stromal dysfunction in mice administered cytosine arabinoside

OBJECTIVE

The aim of the study was to investigate ex-vivo the bone marrow (BM) stroma of mice under conditions of low- and high-dose cytosine arabinoside (Ara-C), a cycle-specific drug (S-phase) and to assess possible stromal damage, apart from the killing of hematopoietic cells. Stroma consists of mesenchymal elements generally not in the cell cycle; therefore it could not be a target for the killing effect of Ara- C.

MATERIALS AND METHODS

The stromal function was studied by the following: the incidence of stromal stem cells, i.e. CFU-F; formation of stromal layers under growth conditions of long-term culture (LTC) followed by irradiation and overlayering of test cells in contact and non-contact co-cultures; subsequent culture of the test cells in a semi-solid medium to assay the incidence of hyperproliferative potential cells (HPPC); production of GM-CSF, IL-3, IL-4, IL-6 and IFNgamma in the conditioned medium (CM) of confluent stromal layers. All tests and assays were carried out on BM specimens, 1-4 d after Ara-C administration and on controls.

RESULTS

Low-dose Ara-C induces a marked decrease of CFU-F, compensated by cycle induction of pre-CFU-F, young-type stromal stem cells. High-dose Ara-C causes a CFU-F decrease to almost zero level. The time length to layer confluency is normal after low-dose Ara-C ( approximately 10 d) and prolonged after a high dose ( approximately 30 d). The confluent layers from mice receiving low- or high-dose Ara-C support hematopoiesis adequately. Among the growth factors and cytokines assayed, only IL-6 is detected in CM layers. IL-6 decreases after a low dose of Ara-C and increases after a high dose. The cause of IL-6 fluctuations is yet to be investigated. It is, however, evident that IL-6 is not an essential factor in support of hematopoiesis.

CONCLUSIONS

Taken together, the current study in mice indicates that Ara-C administration, in particular a high dose, induces bone marrow stromal damage and/or disfunction. The long period of time to reach layer confluency after a high Ara-C dose might reflect the in-vivo situation of slow stromal regeneration.

Gene transfer into hematopoietic stem cells of nonhuman primates

Nonhuman primates provide an appropriate preclinical large-animal model to test the efficacy of bone marrow gene therapy procedures. Successful retroviral vector-mediated gene transfer into monkey pluripotent hematopoietic stem cells (PHSC) has closed the gap between gene transfer experiments in mouse models and clinical application of bone marrow gene therapy. After initial bone marrow transplant failures, ex vivo bone marrow culture conditions were found that sufficiently supported maintenance of the long-term repopulating ability of genetically modified autologous monkey grafts. The efficiency of gene transfer into primate PHSC has, however, remained at least one order of magnitude lower than has been achieved in mice. Similar gene transfer efficiencies have been obtained with total bone marrow grafts, CD34+ bone marrow grafts, and mobilized peripheral blood progenitor cell grafts; however, various attempts to increase the transduction efficiency have been without significant success. Primate PHSC seem to require quite different culture conditions for their maintenance and transduction than mouse PHSC, in particular regarding hematopoietic growth factor addition. In contrast to observations in other species, some form of conditioning appeared essential for engraftment of transduced PHSC in monkeys. Although it has been shown that mouse retroviruses can replicate in monkeys and are capable of inducing neoplasms, experiments in monkeys have sufficiently confirmed the safety of current gene transfer procedures to allow their clinical application.