Thrombopoietin: its role from early hematopoiesis to platelet production

Thrombopoietin (TPO), also referred to as MpI ligand, is the most potent cytokine that physiologically regulates platelet production. With the availability of sufficient amounts of recombinant forms of the protein, the biological in vitro and in vivo activities of this cytokine have been extensively studied. The objective of this review is to summarize the published data focusing on TPO production and regulation and to discuss the pleiotropic biological action of this hormone. The review also highlights the results so far obtained in preclinical and clinical trials.

The material examined in this review includes data published by the author and articles or abstracts published in Journals covered by Medline. The author has contributed to the isolation of TPO, has been working in the field for several years and has contributed original papers on the TPO/MpI system in normal and pathologic situations.

TPO is a hormone constitutively produced by the liver and kidneys. Plasma levels of TPO are regulated through receptor-mediated uptake, internalization and catabolism. First thought to be a lineage dominant factor promoting megakaryocytopoiesis, several lines of evidence indicate that TPO has pleiotropic effects on hematopoiesis. In vitro studies show that TPO alone, or in combination with early acting cytokines, stimulates the proliferation and enhances the expansion of primitive CD34+ CD38- hematopoietic progenitor cells. In vivo studies with c-mpl- and TPO-null mice reveal that the molecule sustains the survival and proliferation of early committed progenitor cells of various type. Preclinical and clinical trials indicate that recombinant TPO molecules increase platelet counts and megakaryocyte numbers in normal or mildly thrombocytopenic states. However, no significant effects of TPO administration on platelet recovery have so far been reported in patients subjected to intensive chemotherapy regimens. Recombinant molecules appear to be safe to administer and very little toxicity is reported. TPO augments the number of erythroid and myeloid committed progenitor cells in marrow, and mobilized stem cells in peripheral blood.

The potential clinical use of TPO is still unclear. With the increased knowledge of the multiple effects of TPO on hematopoiesis, it is expected that future carefully monitored clinical trials will provide more information regarding the eventual benefits of this cytokine in the treatment of thrombocytopenia. At present, one successful application of TPO appears to be its addition in cytokine cocktails used to expand hematopoietic stem cells ex vivo.