INVITRO AND INVIVO EFFECTS OF MDP-LYS(L18) ON MOUSE MEGAKARYOCYTE PROGENITOR CELLS (CFU-MEG)

We investigated the in vitro and in vivo effects of MDP-Lys(L18), a derivative of muramyl dipeptide (MDP), on megakaryocyte progenitor cells (megakaryocyte colony-forming units, CFU-Meg) in the mouse bone marrow and spleen. When CFU-Meg culture was performed with a suboptimum concentration (2%) of pokeweed mitogen-stimulated mouse spleen-conditioned medium (PWM-SCM), addition of 0.1-20-mu-g/ml of MDP-Lys(L18) increased the number of megakaryocyte colonies. The size of the megakaryocyte colonies (the number of megakaryocytes per colony) was also significantly increased by the addition of MDP-Lys(L18) under the same culture conditions in comparison with cultures without MDP-Lys(L18). MDP-Lys(L18) itself did not stimulate megakaryocyte colony formation without PWM-SCM, and it failed to enhance megakaryocyte colony formation in cultures with an optimum PWM-SCM concentration (10%). Furthermore, no effect of MDP-Lys(L18) was observed in cultures of phagocytic cell-depleted bone marrow cells. However, MDP-Lys(L18) enhanced megakaryocyte colony formation in cultures of T-lymphocyte-depleted bone marrow cells. The culture supernatant from a macrophage cell line, J774.1, plus MDP-Lys(L18) enhanced in vitro megakaryocyte colony formation in cultures with a suboptimum PWM-SCM concentration. Although interleukin 1 (IL-1)beta in the culture supernatant of J774.1 plus MDP-Lys(L18) was increased in a dose-dependent manner in response to MDP-Lys(L18), the effect of the culture supernatant was not blocked by an anti-IL-1 antibody, and IL-1-beta failed to enhance megakaryocyte colony formation in the presence of suboptimum PWM-SCM levels. The enhancement of megakaryocyte colony formation by MDP-Lys(L18) could be neutralized, however, by an anti-interleukin 6 (IL-6) antibody. Intraperitoneal administration of MDP-Lys(L18) (100-mu-g daily for 3 days) significantly increased the number of bone marrow and spleen megakaryocyte colonies at 24 to 72 h after the final injection. These in vitro and in vivo observations strongly suggest that MDP-Lys(L18) indirectly enhances the proliferation and differentiation of mouse CFU-Meg via colony-stimulating factor(s) other than IL-1, probably as a result of the stimulation of macrophages to produce IL-6.