Complement split product C5a mediates the lipopolysaccharide-induced mobilization of CFU-s and haemopoietic progenitor cells, but not the mobilization induced by proteolytic enzymes.

Intravenous (i.v.) injection of mice with lipopolysaccharide (LPS), and the proteolytic enzymes trypsin and proteinase, mobilizes pluripotent haemopoietic stem cells (CFU-s) as well as granulocyte-macrophage progenitor cells (GM-CFU) and the early progenitors of the erythroid lineage (E-BFU) from the haemopoietic tissues into the peripheral blood. We investigated the involvement of the complement (C) system in this process. It appeared that the early mobilization induced by LPS and other activators of the alternative complement pathway, such as Listeria monocytogenes (Lm) and zymosan, but not that induced by the proteolytic enzymes, was absent in C5-deficient mice. The mobilization by C activators in these mice could be restored by injection of C5-sufficient serum, suggesting a critical role for C5. The manner in which C5 was involved in the C activation-mediated stem cell mobilization was studied using a serum transfer system. C5-sufficient serum, activated in vitro by incubation with Lm and subsequently liberated from the bacteria, caused mobilization in both C5-sufficient and C5-deficient mice. C5-deficient serum was not able to do so. The resistance of the mobilizing principle to heat treatment (56 degrees C, 30 min) strongly suggests that it is identical with the C5 split product C5a, or an in vivo derivative of C5a. This conclusion was reinforced by the observation that a single injection of purified rat C5a into C5-deficient mice also induced mobilization of CFU-s.

Defective support of S1/S1d splenic stroma for humoral regulation of stem cell proliferation.

Humoral regulation of CFUs proliferation was investigated in S1/S1d mice characterized by a stromal defect, which severely limits in situ proliferation of in vivo colony-forming cells (CFUs). Injection of LPS-W evoked a large enhancement of CFUs numbers in the spleen of normal +/+ mice. S1/S1d mice were found to be refractory to low doses of LPS-W (up to 15 micrograms/mouse) and to have a diminished response to high doses (up to 150 micrograms). Serum transfer experiments showed that S1/S1d mice are not defective in the early elaboration (6 h) of a humoral factor (SHSF), which mediates the LPS-induced splenic stem-cell accumulation. In a serum-free in vitro system post-LPS S1/S1d and +/+ sera induced a similar degree of CFUs proliferation, indicating the ability of S1/S1d mice to produce normal levels of stem-cell-activating factor (SAF). Transfer of potent post-LPS serum from normal mice evoked a poorer splenic CFUs accumulation in S1/S1d mice as compared to normal +/+ littermates. The population size of splenic stem cells in S1/S1d mice parabiosed with normal +/+ mice also showed a limited increase in response to LPS-W injection. This diminished in vivo response of S1/S1d mice was not due to a decreased sensitivity of their CFUs for SAF, since S1/S1d and +/+ CFUs showed similar survival rates in vitro in the presence of SAF. We propose that the defective response of S1/S1d mice to LPS-induced humoral regulators is due to a nonmigratory component of the S1/S1d splenic stroma, which limits splenic CFUs proliferation either by a short-range inhibitory activity or by a deficiency of a local stimulatory activity or nutrient unlike SAF or SHSF, which might act in synergy with SAF.

Effect of serum from mice treated with lipopolysaccharide on cycling of CFUs in vitro.

Serum of lipopolysaccharide(LPS)-treated LPS-high-responder C3H/He mice was shown to increase survival of low-responder C3H/HeJ CFUs in an otherwise serum-free suspension culture by initiating cell cycling. Post-LPS serum of low-responder mice and serum of phosphate-buffered-saline-injected high-responder mice was significantly less effective in this respect. Since prolonged maintenance of CFUs was also found when cell suspensions highly enriched for stem cells were used, it seems unlikely that assessory cells mediated the effect of the post-LPS serum activity on CFUs maintenance. The serum activity did not enhance the stimulatory effect of saturating levels of highly purified stem-cell-activating factor (SAF) on CFUs maintenance in vitro. Upon injection of post-LPS serum from C3H/He mice a relatively small splenic CFUs accumulation in C3H/HeJ mice was observed.

Regulation of haemopoietic stem-cell proliferation in mice carrying the Slj allele.

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.

Mediatory role of stem cell derived cells in LPS-induced splenic CFUs accumulation.

Injection of bacterial lipopolysaccharides (LPS) in LPS-responsive mice produces a transient increase of CFUs in spleen and blood but not in bone marrow. The cellular aspects of the mechanism underlying this response of the hemopoietic system to LPS were investigated. Bone marrow cells from LPS-high responder mice (BMC-H) of the C3Heb/FeJ and C57BL/ScSn strains were transferred to lethally irradiated histocompatible LPS-low responder C3H/HeJ and C57BL/10/ScCr mice and vice versa. Six to ten weeks after reconstitution recipient mice were tested with LPS. Six days after injection of LPS, CFUs numbers in blood and spleen of low-responders reconstituted with BMC-H showed a 10-17 fold increase compared with PBS-injected controls. Lethally irradiated LPS high-responders reconstituted with low-responder bone marrow cells (BMC-L) still produced a small but significant increase of splenic and blood CFUs numbers. These results suggest that relatively radioresistant stem cell-derived cells play an important role in the generation of a stimulus inducing the splenic CFUs accumulation following LPS injection. The decrease of femoral CFUs numbers was less prominent in mice reconstituted with BMC-L than in those reconstituted with BMC-H. Thus expression of the LPS locus is evident in both medullary and extra-medullary sites.