Novel anti-c-Mpl monoclonal antibodies identified multiple differentially glycosylated human c-Mpl proteins in megakaryocytic cells but not in human solid tumors.

Thrombopoietin and its cognate receptor, c-Mpl, are the primary molecular regulators of megakaryocytopoiesis and platelet production. To date the pattern of c-Mpl expression in human solid tumors and the distribution and biochemical properties of c-Mpl proteins in hematopoietic tissues are largely unknown. We have recently developed highly specific mouse monoclonal antibodies (MAb) against human c-Mpl. In this study we used these antibodies to demonstrate the presence of full-length and truncated human c-Mpl proteins in various megakaryocytic cell types, and their absence in over 100 solid tumor cell lines and in the 12 most common primary human tumor types. Quantitative assays showed a cell context-dependent distribution of full-length and truncated c-Mpl proteins. All forms of human c-Mpl protein were found to be modified with extensive N-linked glycosylation but different degrees of sialylation and O-linked glycosylation. Of note, different variants of full-length c-Mpl protein exhibiting differential glycosylation were expressed in erythromegakaryocytic leukemic cell lines and in platelets from healthy human donors. This work provides a comprehensive analysis of human c-Mpl mRNA and protein expression on normal and malignant hematopoietic and non-hematopoietic cells and demonstrates the multiple applications of several novel anti-c-Mpl antibodies.