Effect of Recombinant Human Macrophage Colony-Stimulating Factor in Irradiated Murine Recipients of T-Cell-Depleted Allogeneic or Non-Depleted Syngeneic Bone Marrow Transplants

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Recombinant macrophage colony-stimulating factor (rM-CSF), which reacts exclusively with cells of monocyte lineage, was evaluated in the murine bone marrow (BM) transplant setting for in vivo effects on recipient survival, hematologic recovery, and engraftment. Two types of fully allogeneic donors were selected based on the expression (BALB/c), or lack of expression (DBA/1), of hybrid hematopoietic histocompatibility (Hh1) antigens. These antigens are established targets for monocyte and/or natural killer (NK) cell-mediated graft rejection. Irradiated C57BL/6 mice were used as recipients for all experiments. Recipients of T-cell-depleted (TCD) BALB/c BM and a 14-day continuous subcutaneous infusion of 16.8 micrograms/d rM-CSF (n = 30) showed a significant decrease in donor cell engraftment as compared with recipients of donor BM administered pumps delivering saline. These mice administered rM-CSF also displayed significantly reduced levels of circulating leukocytes (predominantly lymphocytes) on day 14 posttransplant (compared with saline controls). Neither engraftment effects nor leukocyte effects were observed when C57BL/6 recipients were administered Hh1 nonexpressing TCD DBA/1 BM cells (n = 30), suggesting that the monocyte/macrophage population is important in long-term alloengraftment in certain donor-recipient strain combinations in which donor Hh1 antigens can serve as target antigens for host effector cells, but are not important in strain combinations in which they are not recognized. Circulating tumor necrosis factor alpha (TNF alpha) levels measured at two time periods during rM-CSF infusion were not elevated. Thus, the reduction in alloengraftment is not likely to be directly related to TNF alpha. However, in vivo elimination of NK cells in the BALB/c into C57BL/6 model prevented the impairment of engraftment mediated by rM-CSF. Thus, rM-CSF-mediated inhibition of alloengraftment is contingent on the presence of host NK cells with antidonor reactivity. Survival was unaffected when rM-CSF was administered in either allogeneic BM transplant model, but was significantly reduced when rM-CSF was administered to C57BL/6 recipients of syngeneic BM transplants. These data are the first analyzing the effects of rM-CSF in murine allogeneic BM transplantation and extend our previous studies using the BALB/c into C57BL/6 model in which in vivo infusions of recombinant granulocyte-macrophage CSF, but not recombinant granulocyte-CSF, lead to decreases in alloengraftment. These data show that rM-CSF-induced stimulation of monocytes may increase BM graft rejection in instances in which NK cells are involved in the rejection process. These data may have future clinical implications for the use of rM-CSF in allogeneic BM transplantation.