A new type-II membrane protein in erythropoietic organs enhances erythropoiesis

Erythropoiesis is regulated by erythropoietin and microenvironment of erythropoietic organs such as fetal liver and spleen in mice. We developed in vitro erythropoietic microenvironment by established stromal cell lines from erythropoietic organs, in which proliferation and differentiation of erythroid progenitor cells are supported by direct cell-to-cell contact between erythroid progenitor cells and the stroma cells. To identify the functional molecules of the stromal cells, we established monoclonal antibodies which detected specific molecules of the erythropoietic organs. Among those monoclonal antibodies, 11 D exhibited specific immunohistochemical staining of the red pulp of spleen where erythropoiesis dominates. Using this monoclonal antibody, we cloned a new gene (SMAP-1) which codes a type-II membrane protein. Over expression of its antisense cDNA in a stroma cell line caused a decrease in the erythropoietic supporting activity. Thus, SMAP-1 may play an important role in the erythropoietic stimulatory activity of the stromal cells.