SECRETED AND MEMBRANE-BOUND ISOFORMS OF T1, AN ORPHAN RECEPTOR-RELATED TO IL-1-BINDING PROTEINS, ARE DIFFERENTLY EXPRESSED IN-VIVO

The murine T1 gene encodes a membrane-bound glycoprotein (T1-M), highl y similar to interleukin-1 (IL-1) receptor type I, and a soluble varia nt (T1-S) representing its isolated extracellular domain. In vivo, the expression pattern of both T1 isoforms differs drastically. The T1-M receptor is abundantly expressed in single cells of the major hemopoie tic organs (embryonic liver, spleen, bone marrow). It is restricted to few hemopoietic cell types throughout ontogenesis. By contrast, the s oluble T1-S protein is predominantly expressed in selected nonhemopoie tic embryonic tissues (developing skin, bone, and retina) and deposite d in extracellular matrix. Despite the similarity of the T1 ligand-bin ding domain to all IL-1-binding proteins, it does not exhibit affinity to either IL-1 alpha or -beta. Thus, T1-M likely represents a novel o rphan receptor of selected hemopoietic cells. The matrix-associated T1 -S variant might act to create a reservoir of the putative T1 ligand i n some differentiating tissues. (C) 1995 Academic Press, Inc.