AM5, A PROTEIN-ASSOCIATED POLYSACCHARIDE, STIMULATES HEMATOPOIESIS AND MODULATES THE EXPRESSION OF ENDOGENOUS HEMATOPOIETIC GROWTH-FACTORS IN MURINE LONG-TERM BONE-MARROW CULTURES

We have investigated the mechanisms behind the hematopoietic stimulati on mediated by AM5, a protein-associated polysaccharide that stimulate s in vivo the murine hematopoietic system. A dose-dependent increase i n hematopoietic progenitors was observed in long-term bone marrow cult ures (LTBMCs) treated in vitro with AM5. The stimulatory effect was mo re marked in colony-forming units-granulocyte-macrophage (CFU-GM) than in CFU-spleen (CFU-S) progenitors and also more significant in the su pernatant than in the adherent layer. This stimulatory effect was reve rsible, and continuous stimulation with high doses of AM5 was conduciv e to a progressive exhaustion of the culture. The analysis of the CFU- GM stimulating activity present in the supernatant of AM5-treated cult ures revealed a dose-dependent induction of granulocyte-macrophage col ony-stimulating activity (GM-CSA), in contrast with control cultures i n which no CSA was detected. Northern blots of LTBMC-adherent layers o btained after in vitro treatment with AM5 revealed a significant mRNA expression for interleukin 1 alpha (IL-1 alpha), granulocyte-macrophag e colony-stimulating factor (GM-CSF), macrophage CSF (M-CSF) and granu locyte CSF (G-CSF), in contrast with adherent layers from untreated cu ltures which only expressed, in detectable levels, M-CSF and stem cell factor (SCF). The SCF expression was down modulated in AM5-treated cu ltures. Our results strongly suggest that the hematopoietic stimulatio n induced by AM5 is mediated by the modulated expression of endogenous hematopoietic growth factors.