Efficient Gene Transfer in Primitive CD34+/CD38loHuman Bone Marrow Cells Reselected after Long-Term Exposure to GALV-Pseudotyped Retroviral Vector

Retrovirus vectors can be made in the absence of helper virus by using retrovirus packaging cell lines. Helper-free virus is critical for a variety of gene transfer studies. The most useful packaging cell lines contain helper virus DNA from which the signal required for packaging of the viral RNA genome into virions has been deleted. However, we showed that the ability to package virus is conferred at very low frequency to cells infected with virus from these packaging cell lines, presumably by low-frequency transmission of the deleted virus genome. In addition, these packaging cell lines can interact with some retroviral vectors to yield replication-competent virus. We constructed packaging cell lines containing helper virus DNA that had several alterations in addition to deletion of the packaging signal. The new packaging cells retained the useful features of previously available lines but did not yield helper virus after introduction of any of the vectors tested, and transfer of the packaging function was not detected.

A major goal of current hematopoiesis research is to develop in vitro methods suitable for the measurement and characterization of stem cells with long-term in vivo repopulating potential. Previous studies from several centers have suggested the presence in normal human or murine marrow of a population of very primitive cells that are biologically, physically, and pharmacologically different from cells detectable by short-term colony assays and that can give rise to the latter in long-term cultures (LTCs) containing a competent stromal cell layer. In this report, we show that such cultures can be used to provide a quantitative assay for human "LTC-initiating cells" based on an assessment of the number of clonogenic cells present after 5-8 weeks. Production of derivative clonogenic cells is shown to be absolutely dependent on the presence of a stromal cell feeder. When this requirement is met, the clonogenic cell output (determined by assessment of 5-week-old cultures) is linearly related to the input cell number over a wide range of cell concentrations. Using limiting dilution analysis techniques, we have established the frequency of LTC-initiating cells in normal human marrow to be approximately 1 per 2 X 10(4) cells and in a highly purified CD34-positive subpopulation to be approximately 1 per 50-100 cells. The proliferative capacity exhibited by individual LTC-initiating cells cultured under apparently identical culture conditions was found to be highly variable. Values for the number of clonogenic cells per LTC-initiating cell in 5-week-old cultures ranged from 1 to 30 (the average being 4) with similar levels being detected in positive 8-week-old cultures. Some LTC-initiating cells are multipotent as evidenced by their generation of erythroid as well as granulopoietic progeny. The availability of a system for quantitative analysis of the proliferative and differentiative behavior of this newly defined compartment of primitive human hematopoietic cells should facilitate future studies of specific genetic or microenvironmental parameters involved in the regulation of these cells.

We have constructed hybrid retrovirus packaging cell lines that express the gibbon ape leukemia virus env and the Moloney murine leukemia virus gag-pol proteins. These cells were used to produce a retrovirus vector at over 10(6) CFU/ml, with a host range that included rat, hamster, bovine, cat, dog, monkey, and human cells. The gag-pol and env expression plasmids were separately transfected to reduce the potential for helper virus production, which was not observed. The NIH 3T3 mouse cells from which the packaging lines were made are not infectable by gibbon ape leukemia virus; thus, the generation and spread of possible recombinant viruses in the packaging cells is greatly reduced. These simian virus-based packaging cells extend the host range of currently available murine and avian packaging cells and should be useful for efficient gene transfer into higher mammals.