Effect of interleukin 11 on normal and pathological thrombopoiesis.

Interleukin 11 (IL-11) is a stromal cell-derived cytokine that has multiple effects on hematopoietic and nonhematopoietic systems. In vitro, it enhances the growth of early progenitors and promotes megakaryocytopoiesis and erythropoiesis. In healthy animals, IL-11 administration stimulates megakaryocyte maturation and increases peripheral platelet counts. IL-11 accelerates the recovery of peripheral neutrophil, erythrocyte, and platelet counts in mice that have undergone cytoablative treatment. Therefore, IL-11 may be useful clinically as an agent promoting recovery from hematopoiesis. However, its clinical use in patients with hematological malignancies may be restricted because IL-11 has been reported to stimulate some leukemia and myeloma cells. In the United States, phase I trials have shown that IL-11 accelerates recovery from chemotherapy-induced or bone-marrow transplantation (BMT)-induced thrombocytopenia. In Japan, phase II trials studying the thrombopoietic effect of IL-11 in patients with solid tumors postchemotherapy, in patients undergoing BMT, and in patients with aplastic or refractory anemia are now under way. Recently, thrombopoietin (TPO) has been cloned, and its thrombopoietic effect and accelerating effect on platelet count recovery in thrombopoietic states have been demonstrated in animal models. The physiological effect of TPO is restricted to hematopoiesis; therefore, it may have fewer side effects than IL-11. However, in addition to its hematopoietic effect, IL-11 administration to mice that have undergone cytoablative therapy significantly decreases morbidity and mortality due to chemotherapy-related endogenous infections caused by gut microorganisms. Therefore, IL-11 can be used in patients postchemotherapy and post-BMT not only to promote platelet recovery but also to prevent life-threatening infections. The use of in-vitro-expanded hematopoietic stem cells for BMT or as target cells for gene therapy is one of the most exciting areas in the field of medicine. Since IL-11 can expand hematopoietic progenitor-cell populations when used in combination with other cytokines, it may be useful as an ex vivo hematopoietic progenitor-cell-amplifying agent.