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Slj/+ mice display a slight macrocytic anaemia due to a defect in their haemopoietic organ stroma. They have a deficient endogenous spleen colony (CFU-end) formation following sublethal doses of gamma-radiation compared with their normal +/+ littermates, which is likely to be due to the low pre-irradiation CFU-S content of the Slj/+ spleen. CFU-S in these congenic mice do not differ in their sensitivity to gamma-irradiation or stem cell-activating factor. While injection of +/+ mice with 10 micrograms of lipopolysaccharide-W (LPS) one day prior to irradiation led to a substantial increase in their survival, the survival of Slj/+ mice was only slightly increased. Irradiation induced a similar dose-related reduction in the numbers of CFU-S in the spleen and femora of LPS-injected Slj/+ mice compared to similarly treated +/+ mice when measured directly after irradiation. At Day 9 after irradiation, injection of LPS led to a significantly higher CFU-end formation and higher numbers of CFU-S and nucleated cells in the Slj/+ spleens compared to LPS-injected +/+ mice. No such differences in the radioprotective effect of LPS were observed in the +/+ and Slj/+ mice with respect to the splenic and femoral 59Fe-incorporation and the femoral CFU-S numbers at Day 9. These data strongly suggest a contribution by immigrating CFU-S to the CFU-S numbers and endogenous colony formation in at least the Slj/+ spleen after LPS injection and subsequent sublethal irradiation. The observations also imply that the splenic organ stroma may play a mediatory role in the radioprotective action of LPS. In addition, the data represent an extreme example of a lack of correlation between animal survival and haemopoietic parameters. Caution should be taken when applying endogenous colony counts as a means of screening potential anti-radiation drugs.

A tentative characterization of haemopoietic stem cells with respect to their organ distribution, seeding fraction and colony formation in the spleen, radiosensitivity and humoral regulation was attempted in mice heterozygous for the mutant allele Slj and in their normal littermates. Slj/+ mice were characterized by a deficient CFU-s content of the blood and spleen and had slightly lower femoral CFU-s numbers. This CFU-s distribution could not be explained by differences in seeding efficiency 'f' between CFU-s of Slj/+ and +/+ origin in lethally irradiated recipients used in the CFU-s assay. The seeding fraction of CFU-s of +/+ origin did not differ in +/+ and Slj/+ recipients. However, in irradiated Slj/+ recipient mice a 30% decrease was observed in the number of the colonies derived from splenic and femoral CFU-s of both +/+ and Slj/+ origin. The serum level of SHSF (splenic haemopoiesis stimulating factor) was decreased in Slj/+ mice, but significantly increased in Sl/Sld mice, as compared to their respective normal +/+ littermates. Endogenous colony formation in Slj/+ spleens was deficient in comparison to that observed in +/+ spleens, and distinct sex differences were observed. However, mutant and normal CFU-s from spleen and bone marrow had a similar survival following in-vitro gamma irradiation. Femurs and spleens of both Slj/+ and +/+ origin were implanted into both Slj/+ and +/+ hosts. Six weeks later the Slj/+ grafts contained less CFU-s than the +/+ grafts. These data show that the splenic stroma of Slj/+ mice is not defective in its capacity to lodge injected CFU-s but is deficient in its ability to maintain CFU-s under 'steady-state' conditions and stimulate their colony formation in a 'perturbed state'. Some of the characteristics of Slj/+ mice segregate them from Sl/Sld mice, i.e. a deficient splenic CFU-s content, normal seeding fractions 'f' of CFU-s from spleen and bone marrow in the presence of an almost compensated anemia, and decreased serum levels of SHSF. The study of the Slj trait may be a useful extension of the current Sl/Sld model for exploration of hereditary defects in haematopoietic stroma.

This paper describes the kinetics of fibroblastic colony forming units (CFU-f) in murine blood after phenylhydrazine-induced haemolytic anaemia and their subsequent migration into haemopoietic organs. Murine blood contained 5.3 +/- 0.8 CFU-f per 10(6) nucleated cells. Absence of particle ingestion and factor VIII-related antigen in addition to the enzyme pattern in CFU-f-derived cells confirmed that these cells did not have a macrophage-like or endothelial nature. Phenylhydrazine treatment of mice resulted in a 3-fold increase in blood CFU-f numbers which was accompanied by increases in blood cellularity and granulocyte-macrophage progenitor numbers. When both partners of CBA/N and CBA/T6T6 mice in parabiosis had been treated with phenylhydrazine, spleens and femoral bone marrow of both mice were shown to contain partner-derived CFU-f. These data suggest that circulating CFU-f represent a stromal cell population which can migrate into haemopoietic organs.

A possible regulatory action of phagocytic cells on erythropoiesis was investigated by infusion of inert polystyrene latex particles (LAT). LAT appeared to induce changes in the femoral content of erythroid progenitor cells. These changes were most pronounced in primitive erythroid progenitor cells (BFUe) and appeared to be gradually damped in more differentiated populations (CFUe and erythroblasts). LAT did not influence granulocyte/macrophage progenitor cells (CFUc). The effects of LAT could not be attributed to changes in the systemic erythropoietin (EP) concentration. Administration of dexamethason nullified the effect of low doses of LAT, suggesting that phagocytosis of the particles is essential to the observed effects. Erythroid burst formation was previously found to be dependent on a bone marrow associated activity, termed BFA (burst feeder activity). BFA acts as an in vitro inducer of EP-responsiveness in BFUe. In this study it was found that LAT-induced changes in femoral erythroid progenitor cell content were characteristically preceded by corresponding changes in BFA. It was concluded that BFA-associated cells probably play a role in vivo in the early differentiation of erythroid progenitor cells. The present data are interpreted as direct in vivo evidence supporting a two-step regulatory model operating in erythropoiesis and provide evidence that phagocytic cells are a component of the erythroid haemopoietic inductive micro-environment.

A variety of substances can mobilize haemopoietic stem cells (CFUs) into the peripheral blood. In this study the involvement of the complement system in the mobilization process was investigated. Pretreatment of mice with the complement-activating factor of cobra venom (CoF), which lowered the serum C3 levels to 10-25% of the normal value, could completely prevent CFUs mobilization induced by high doses of CoF, endotoxin (ET) from Salmonella typhosa, inulin, zymosan and the proteolytic enzymes proteinase and trypsin. On the other hand, mobilization induced by the polyanions dextran sulphate and the copolymer of polymethacrylic acid and styrene could not be prevented, or at least affected only slightly. There appears to be a relationship between the extent of decomplementation by CoF and the extent of CFUs mobilization induced by ET. The results indicate that certain agents mobilize CFUs via the complement system, whereas other agents induce CFUs mobilization independent of the availability of complement components.

We investigated a haemopoietic stromal defect, in mice heterozygous for the Slj allele, during haemopoietic stress induced by treatment with bacterial lipopolysaccharides (LPS) or lethal total body irradiation (TBI) and bone-marrow cell (BMC) reconstitution. Both treatments resulted in a comparable haemopoietic stem cell (CFU-s) proliferation in Slj/+ and +/+ haemopoietic organs. There was no difference in committed haemopoietic progenitor cell (BFU-e and CFU-G/M) kinetics after TBI and +/+ bone-marrow transplantation in Slj/+ and +/+ mice. The Slj/+ mice were deficient in their ability to support macroscopic spleen colony formation (65% of +/+ controls) as measured at 7 and 10 days after BMC transplantation. However, the Slj/+ spleen colonies contained the same number of BFU-E and CFU-G/M as colonies from +/+ spleens, while their CFU-s content was increased. On day 10 post-transplantation, the macroscopic 'missing' colonies could be detected at the microscopic level. These small colonies contained far fewer CFU-s than the macroscopic detectable colonies. Analysis of CFU-s proliferation-inducing activities in control and post-LPS sera revealed that Slj/+ mice are normal in their ability to produce and to respond to humoral stem-cell regulators. We postulate that Slj/+ mice have a normal number of splenic stromal 'niches' for colony formation. However, 35% of these niches is defective in its proliferative support.