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Serial observations of blast cell colony development from spleen cells of mice treated with 5-fluorouracil (5-FU) four days earlier revealed that either form of human interleukin-1 (IL-1 alpha or IL-1 beta) hastens the emergence of interleukin-3 (IL-3)-dependent blast cell colonies. This activity was essentially indistinguishable from the effect of interleukin-6 (IL-6) or granulocyte colony-stimulating factor (G-CSF) in the same system, an effect that we have ascribed previously to a shortening of the G0 period of the dormant stem cells. We also analyzed the time courses of colony formation from cultures of day-2 post-5-FU marrow cells supported by IL-1 alpha, IL-6, or G-CSF alone or in combination with IL-3. In the presence of IL-3, G-CSF and IL-6 but not IL-1 alpha hastened the development of colonies and increased the numbers of multilineage colonies relative to cultures of IL-3 alone. This observation, together with our previous data from the human system, suggests that the synergistic effect of IL-1 is likely due to induction of secondary growth factors, including IL-6 and G-CSF, by accessory cells in culture. The effect of IL-6 on G0 was confirmed by analysis of the cycling status of progenitor cells in short-term culture. While neither IL-3 nor IL-6 alone had any effect on the cycling status, the combination of factors resulted in a rapid recruitment of quiescent cells into cell cycle (within 48 hours) as represented by a twofold increase in the numbers of multipotential progenitors and a significant increase in the sensitivity of these cells to 3H-thymidine with high specific activity. Combinational testing of all of these synergistic factors revealed that the target cell populations for the IL-1, IL-6, and G-CSF overlap considerably, suggesting that they all may act through a common mechanism. This is further supported by our finding that cells from blast cell colonies grown in the presence of a combination of any one of the synergistic factors with IL-3 replate with higher efficiency and yield more multilineage secondary colonies than those from colonies grown in IL-3 alone. These findings provide further evidence that IL-1, IL-6, and G- CSF serve to integrate the immediate host responses to infection through augmentation of effector cells and antibody production as well as the longer term host responses by recruitment of dormant hemopoietic stem cells into active cell cycling.

In cultures of spleen cells from normal mice, recombinant human granulocyte colony-stimulating factor (G-CSF) supported the formation of multipotential blast cell colonies. Serial replating of the blast cell colonies in the presence of G-CSF, however, failed to demonstrate any direct effect of G-CSF on murine multipotential progenitors. We therefore examined the effects of G-CSF in combination with murine interleukin 3 on proliferation of murine blast cell colony-forming cells. The time course of total colony formation and multilineage colony formation by spleen cells harvested from mice 4 days after injection of 5-fluorouracil at 150 mg/kg was significantly shortened in cultures containing both factors in contrast with cultures supported by either factor alone. Serial observations of individual multipotential blast cell colonies (mapping) revealed that blast cell colonies emerged at random time intervals in the presence of interleukin 3 or G-CSF. The appearance of blast cell colonies, however, was significantly hastened in cultures containing both factors relative to cultures grown with either factor. In cultures of day-2 post-5-fluorouracil bone marrow cells, G-CSF in concentrations as low as 1 unit/ml revealed synergism with interleukin 3 in supporting the proliferation of multipotential progenitors. This synergistic activity may explain the previous in vivo studies suggesting the effects of G-CSF on apparent multipotential stem cells.

Currently available evidence suggests that in the steady state, the majority of hematopoietic stem cells are dormant in cell cycle and reside in the so-called G0 period. Studies in our laboratory indicated that once a stem cell leaves G0, its subsequent proliferation requires the presence of interleukin-3 (IL-3). Recently it was reported that interleukin-1 (IL-1) may stimulate stem cells to become sensitive to IL- 3. In a separate study, we observed that interleukin-6 (IL-6, also known as B cell stimulatory factor-2/interferon beta 2) possesses synergism with IL-3, shortening the G0 period of murine hematopoietic stem cells. We report here that human IL-6 and IL-3 act synergistically in support of the proliferation of progenitors for human blast cell colonies and that IL-1 alpha reveals no synergism with IL-3 when tested against purified human marrow progenitors. Panned My-10+ human marrow cells were plated in culture and on day 14 of incubation, either IL-3, IL-6, IL-1 alpha or a combination of these factors was added to the cultures. Blast cell colony formation was analyzed daily between days 18 and 32 of culture. IL-6 or IL-1 alpha alone failed to support blast cell colony formation. In the presence of IL-3 alone, blast cell colonies continued to emerge between days 21 and 27. When a combination of IL-3 and IL-6 was added, blast cell colonies developed earlier than in cultures with IL-3 alone and twice as many blast cell colonies were identified. IL-1 alpha failed to augment IL-3-dependent blast cell colony formation. Replating studies of the individual blast cell colonies revealed various types of single as well as multilineage colonies. These observations suggest that IL-6 shortens the G0 period of human hematopoietic stem cells and that the reported synergistic activities of IL-1 on primitive hematopoietic cells may be indirect.

The human megakaryocytic cell line CMK was positive for the expression of platelet glycoprotein(GP) Ilb/IIIa but few cells were positive for GPIb. To establish a GPIb-positive CMK subclone, GPIb-positive CMK cells were purified, micromanipulated and cloned, but resulting sublines were all negative for GPIb expression. The combined presence of interleukin-3 (IL-3) and the anti-virus agent ribavirin strongly induced expression of GPIb on the cells. However, the megakaryocyte growth and development factor (MGDF)/thrombopoietin (TPO) failed to enhance GPIb expression. CMK cells can produce platelet-like particles during culture, but both cells themselves and particles are negative for GPIb expression. To utilize these particles as platelet substitutes, further appropriate culture systems are necessary.

Interleukin-6 (IL-6, also known as B-cell stimulatory factor 2/interferon beta 2) was previously shown to support the proliferation of granulocyte/macrophage progenitors and indirectly support the formation of multilineage and blast cell colonies in cultures of spleen cells from normal mice. We report here that IL-3 and IL-6 act synergistically in support of the proliferation of murine multipotential progenitors in culture. The time course of total colony formation by spleen cells isolated from mice 4 days after injection of 5-fluorouracil (150 mg/kg) was significantly shortened in cultures containing both lymphokines relative to cultures supported by either of the two factors. Serial observations (mapping) of individual blast cell colonies in culture revealed that blast cell colonies emerged after random time intervals in the presence of IL-3. The average time of appearance in IL-6 alone was somewhat delayed, and in cultures containing both factors the appearance of multilineage blast cell colonies was significantly hastened relative to cultures grown in the presence of the individual lymphokines. In cultures of day-2 post-5-fluorouracil bone marrow cells, IL-6 failed to support colony formation; IL-3 alone supported the formation of a few granulocyte/macrophage colonies, but the combination of factors acted synergistically to yield multilineage and a variety of other types of colonies. In this system, IL-1 alpha also acted synergistically with IL-3, but the effect was smaller, and no multilineage colonies were seen. Together these results indicate that IL-3 and IL-6 act synergistically to support the proliferation of hemopoietic progenitors and that at least part of the effect results from a decrease in the G0 period of the individual stem cells.