We describe here a continuous perfusion bioreactor system that enables a population of unselected human mononuclear bone marrow cells obtained from adult donors to expand up to 20 to 25-fold over a two-week period. Colony-forming units of granulocyte-macrophage (CFU-GM) progenitor cells expand 10 to 30-fold. These expansions depend on the gas phase oxygen concentration, the seeding density and time of cell harvest. Under operating conditions that allow for good cell proliferation, 3 to 4 million mononuclear cells can be obtained per square centimeter, with 0.5 to 0.8% being progenitor cells. Autologous human sera supported cell expansion as efficiently as animal sera. Increasing the size of the perfusion system to produce a clinically meaningful number of CFU-GMs could have important applications in bone marrow transplantation therapies.

Despite considerable effort, the expansion of long-term culture- initiating cells (LTC-ICs) in cultures of purified hematopoietic cells has not yet been achieved. In contrast, LTC-IC expansion has been attained in cultures of bone marrow mononuclear cells (MNC) using frequent medium exchange. The use of frequent medium exchange was, therefore, examined in cultures of CD34-enriched cells. In stromal- free, CD34-enriched cell cultures, medium exchange intervals ranging from 2 days to no feeding for 14 days gave similar results. Six different growth factor combinations, reported by other groups to give optimal expansion of CD34-enriched cells, were tested in comparison with the control combination of IL-3/GM-CSF/Epo/SCF. None of the combinations resulted in improved colony-forming unit-granulocyte macrophage (CFU-GM) expansion or LTC-IC maintenance, although two were equivalent. All stromal-free cultures resulted in loss of LTC-IC to half of input. Because of the limited effect of medium exchange and growth factor variations on CD34-enriched cell cultures, the effect of preformed stroma was next examined. Preformed stroma increased cell (3- fold), CFU-GM (5-fold), and LTC-IC (3-fold) output, but only when the medium was exchanged every other day. Under these conditions, the number of LTC-IC was maintained near input level. The lack of LTC-IC expansion in CD34-enriched cell cultures prompted experiments to examine the effect of cell purification. Parallel cultures were performed at CD34+lin- cell purities of 20%, 40%, 70%, and 95%, with each well containing exactly 4,000 CD34+lin- cells in addition to the CD34- accessory cells required to give the desired percentage. Also, MNC from the same source (approximately 2% CD34+lin-) were cultured at a concentration to give 4,000 CD34+lin- cells per well. As CD34+lin- cell purity was decreased from 95% to 2%, the output of cells, CFU-GM, and LTC-IC increased by threefold to fivefold. The loss of culture performance with purification was likely due to the removal of important accessory cells, because the levels of endogenously produced leukemia inhibitory factor and IL-6 were found to decline significantly with increasing CD34+lin- cell purity. In summary, preformed stroma abrogated the decrease in cell and CFU-GM output from cultured CD34- enriched cells and led to LTC-IC maintenance. In contrast, MNC inocula resulting in a growing stromal layer during the culture led to LTC-IC expansion (3.2-fold).(ABSTRACT TRUNCATED AT 400 WORDS)

Scale-up of human hematopoietic cultures was previously described in continuously perfused systems with bone marrow mononuclear cells (BMMNC), yielding expansion of both progenitors and long-term culture-initiating cells (LTC-IC). We report here on the use of these systems for expansion of unprocessed whole BM cells (WBMC) and CD34-enriched cells. Density separation recovered 84% of CFU-GM and 65% of LTC-IC from WBMC. Subsequent CD34 selection recovered 17% of CFU-GM and 48% of LTC-IC from the MNC fraction. The unabsorbed (CD34-depleted) fraction contained 37% of CFU-GM and 38% of LTC-IC, accounting for most of the lost cells. WBMC, BMMNC, and CD34-depleted cells were each placed directly in bioreactors, whereas CD34-enriched cells were placed in bioreactors containing preformed irradiated stroma. After 14 days, an average of 3.82 x 10(7) (12.7-fold expansion), 3.54 x 10(7) (11.8-fold), 2.85 x 10(7) (9.5-fold), and 3.65 x 10(7) (1298-fold) total cells were obtained from bioreactors inoculated with WBMC, BMMNC, CD34-depleted, and CD34-enriched cells on stroma, respectively. These cultures yielded 1.64 x 10(5) (27.9-fold expansion), 1.69 x 10(5) (14.3-fold), 8.36 x 10(4) (13.0-fold), and 1.91 x 10(5) (41.4-fold) CFU-GM each, respectively. Cell recovery and expansion data were combined to determine the number of expanded CFU-GM obtained per ml of BM aspirate, allowing direct comparison of performance between the four culture inocula. WBMC generated 3.76 x 10(6) CFU-GM per ml BM aspirate, whereas MNC resulted in 1.42 x 10(6) CFU-GM. CD34-enriched cells (on irradiated stroma) gave 7.00 x 10(5) CFU-GM per ml BM aspirate, whereas CD34-depleted cells generated 4.97 x 10(5) CFU-GM. The high productivity from WBMC cultures was studied further and was found to be reproducible at different inoculum densities. WBMC cultures had elevated levels of endogenous EGF and PDGF production, which may have been responsible for the more extensive stromal development observed. Flow cytometric analysis showed that the final culture composition, with respect to T and B lymphocytes, monocytes, granulocytes, and erythrocytes, was not significantly affected by the inoculum composition and in all cases was comprised of multiple lineages. Therefore, each step in cell purification resulted in the loss of primitive and accessory cells, which in turn resulted in a net decrease in the number of expanded cells obtained per ml BM aspirate.

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Hematopoiesis, the formation of mature blood cells from stem (LTC‐IC) and progenitor (CFU‐GM) cells in the bone marrow, is a complex tissue‐forming process that leads to many important physiological functionalities. Consequently, a functioning ex vivo hematopoietic system has a variety of basic scientific and clinical uses. The design and operation of such a system presents the tissue engineer with challenges and choices. In this study, three culture variables were used to control ex vivo human hematopoiesis. Systematic variation of inoculum density (ID), medium exchange interval (MEI), and the use of preformed stroma (PFS) showed that (1) all three variables significantly influenced culture performance, (2) the three variables interacted strongly, and (3) the variables could be manipulated to achieve the optimization of different performance criteria. Donor‐to‐donor variability in culture performance was great at low ID but was minimized at higher ID. PFS had a large positive effect on cell and CFU‐GM output at low ID, but had minimal effect at higher ID. In fact, PFS caused a decrease in LTC‐IC output at high ID. The effects of PFS indicated that stromal cell elements became more limiting than proliferative cell elements as ID was reduced. In cultures without PFS, maximum cell output was obtained with high ID using a short MEI, whereas the greatest cell expansion ratio was obtained at low ID with an intermediate MEI. Maximum CFU‐GM output was obtained from cultures with high ID using a short to intermediate MEI, whereas the greatest CFU‐GM expansion ratio was obtained at intermediate ID with an intermediate MEI. The addition of PFS altered the locations of these maxima. In general, PFS moved the maxima to lower ID, and culture output became more sensitive to MEI. Therefore, the optimization of one performance criterion always resulted in a decline of the others. This study demonstrates that ex vivo tissue function is sensitive to many culture variables in an interactive fashion and that systematic multivariable studies are required to characterize tissue function. Once the effects of individual variables and their interactions are known, this knowledge can be used to optimize tissue performance with respect to desired criteria. © 1996 John Wiley & Sons, Inc.