Host F1 Mice Pretreated With Granulocyte Colony-Stimulating Factor Accept Parental Bone Marrow Grafts in Hybrid Resistance System

In the setting of hybrid resistance, parental C57BL/6 bone marrow (BM) grafts are vigorously rejected by lethally irradiated (C57BL/6xDBA/2) F1 mice. However, F1 mice pretreated by continuous administration of granulocyte colony-stimulating factor (G-CSF ) with a miniosmotic pump before BM grafting developed day-8 splenic colonies of donor origin. This inhibitory effect on rejection was reversible because F1 mice regained the capacity to reject parental BM when the pump ceased functioning. The appearance of only a small number of colonies with the administration of G-CSF soon after BM grafting suggested the importance in producing this inhibitory effect of pre-exposure of host mice to G-CSF. Because G-CSF administration with a syngeneic combination did not influence the number of colonies, an altered distribution of grafted precursors was unlikely. The absence of a reduction in the number of NK1.1-positive cells in G-CSF–treated mice suggested functional impairment of natural killer cells, major effectors in hybrid resistance, but further study is necessary to elucidate the mechanism underlying this phenomenon. However, our results indicate the importance of G-CSF as a regulator in a certain type of immune response and raise the possibility of clinical application in transplantation medicine.

T- and B-lymphocyte differentiation potentials of spleen colony-forming cells

Cells that generate splenic colonies within 8 days (day-8 colony- forming units-spleen [CFU-s]) are generally thought to differentiate only into erythroid/myeloid cells. The T and B lymphocyte differentiation potentials of day-8 CFU-s were evaluated and compared with those of day-12 and 5-fluorouracil (5-FU) CFU-s. This was achieved by analyzing, after intravenous and intrathymic injection, the lymphocyte progeny of cells contained within individual splenic colonies collected at day 8 and day 12 post-bone marrow cell transfer into irradiated congenic recipients. A large majority of day-8 spleen colonies generated T cells when transferred intrathymically. After intravenous (IV) injection of day-8 colonies, donor-type thymocytes emerged in 33% of the animals reconstituted with only 1 day-8 colony, but in 83% of those inoculated with a pool of 5 colonies. All post-5-FU and 75% of day-12 colonies gave rise to thymocytes after IV injection. B cells were generated by a high proportion of day-8 colonies, and by all day-12 and post 5-FU colonies. These results demonstrate that progenitors of T and B lymphocytes are generated within spleen colonies produced by at least some day-8 CFU-s and virtually all day-12 CFU-s. Whether these progenitors are CFU-s themselves or committed precursors remains an open question.

T- and B-lymphocyte differentiation potentials of spleen colony-forming cells

Cells that generate splenic colonies within 8 days (day-8 colony- forming units-spleen [CFU-s]) are generally thought to differentiate only into erythroid/myeloid cells. The T and B lymphocyte differentiation potentials of day-8 CFU-s were evaluated and compared with those of day-12 and 5-fluorouracil (5-FU) CFU-s. This was achieved by analyzing, after intravenous and intrathymic injection, the lymphocyte progeny of cells contained within individual splenic colonies collected at day 8 and day 12 post-bone marrow cell transfer into irradiated congenic recipients. A large majority of day-8 spleen colonies generated T cells when transferred intrathymically. After intravenous (IV) injection of day-8 colonies, donor-type thymocytes emerged in 33% of the animals reconstituted with only 1 day-8 colony, but in 83% of those inoculated with a pool of 5 colonies. All post-5-FU and 75% of day-12 colonies gave rise to thymocytes after IV injection. B cells were generated by a high proportion of day-8 colonies, and by all day-12 and post 5-FU colonies. These results demonstrate that progenitors of T and B lymphocytes are generated within spleen colonies produced by at least some day-8 CFU-s and virtually all day-12 CFU-s. Whether these progenitors are CFU-s themselves or committed precursors remains an open question.

The Role of the Lymphocyte in Haemopoiesis

According to current dogma, circulating blood cells are all derived from the same progenitor, which therefore must be both pluripotent and capable of prolific self-replication. In the irradiated mouse, such haemopoietic stem cells (HSC) give rise to splenic colonies, and thus are designated as CFU-S (colony forming units-spleen). Definitive identification of a similar entity in man so far has proved elusive. However, primitive unipotent cells, committed to development along a single pathway, can be detected in human blood-forming tissues under appropriate culture conditions. Operationally defined as CFU-E (erythrocyte), CFU-GM (granulocyte/macrophage) etc., the ontogenetic relationships of these cells to each other and to the HSC have been the objects of exhaustive study. A population of lymphocytes, classed as ‘null’ cells, do not exhibit the surface membrane markers which characterize commitment to differentiation in the thymusdependent (T-cell) or bursa- equivalent (B cell) lineages. Accumulating evidence points to the null lymphocyte as a potential precursor of haemopoietic tissue. In bone marrow, the activity of terminal deoxynucleotidyl transferase (TdT) is concentrated in null cells, but the relevance of this unique enzyme to lymphocytic differentiation remains uncertain. Nevertheless, it appears that the functional heterogeneity, expressed within the family of lymphocytes, extends to haemopoiesis. The roles of various lymphocyte populations, in the generation and functional control of blood-forming tissue, are examined in this review.