Effects of interleukin-6 on hematopoiesis in allogeneic and syngeneic bone marrow chimeras

Effects of interleukin-6 (IL-6) on hematopoietic progenitor cells were analyzed in murine bone marrow chimeras. When IL-6 was injected into syngeneic [C3H/He-->C3H/He] bone marrow chimeras from day 1 to day 12, the numbers of highly proliferative potential colony-forming units (CFU-HPP) or colony-forming units mix (CFU-Mix) in spleen cells and bone marrow cells increased on day 14 although there was a marked increase in spleen cells but not in bone marrow cells on day 21. The numbers of CFU-HPP increased in spleen cells from allogeneic [BALB/c-->C3H/He] bone marrow chimeras injected with IL-6 on days 14 and 21. In syngeneic bone marrow chimeras, the numbers of colony-forming units granulocyte/macrophage (CFU-GM) and burst colony-forming units (BFU-E) increased similarly to those of CFU-HPP and CFU-Mix on day 14. On day 21, these were mainly increased in spleen cells. In allogeneic bone marrow chimeras, IL-6 decreased the numbers of CFU-GM and BFU-Mix dose-dependently on day 14. Only 10 micrograms of IL-6 increased the numbers of CFU-GM and BFU-E on day 21. In our previous work, we showed that platelet counts increased on day 14 in syngeneic bone marrow chimeras injected with IL-6, whereas platelet and leukocyte counts increased on days 14 and 24 in allogeneic bone marrow chimeras injected with IL-6, correlating inversely with the numbers of hematopoietic progenitor cells. Overall, primitive hematopoietic progenitors (i.e., CFU-HPP and CFU-Mix) existed primarily in spleen cells of allogeneic bone marrow chimeras on day 14, whereas those in spleen cells of syngeneic bone marrow chimeras were found on day 21. These findings indicate that the effect of IL-6 on hematopoiesis in allogeneic bone marrow chimeras is completely different from that in syngeneic bone marrow chimeras, probably via graft-versus-host reaction (GVHR) but not GVH disease (GVHD).