Megakaryocytes develop in densely seeded normal mouse bone marrow (BM) cells cultured in agar or in liquid medium. This formation of megakaryocytes is enhanced by the myeloid differentiation-inducing protein MGI-2, which we have shown to be interleukin-6 (IL-6). Monoclonal antibody (MoAb) that specifically neutralizes mouse IL-6 but not human IL-6 inhibited megakaryocyte development in cells cultured either with or without the addition of mouse IL-6 but did not inhibit megakaryocyte development induced by human IL-6. This MoAb to mouse IL- 6 that does not neutralize mouse IL-3 also inhibited mouse IL-3-induced megakaryocyte development. Antibody to mouse GM-CSF did not inhibit the formation of megakaryocytes. The results show that the induction of megakaryocyte development by IL-3 is due to the production of IL-6 in the BM cultures. The present experiments demonstrate a new property of IL-6 and indicate that IL-6 is a regulatory protein of normal megakaryocyte development.

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