Expression analysis of the soluble and membrane-associated forms of the interleukin-1 receptor-related T1 protein in primary mast cells and fibroblasts.

The murine T1 gene encodes a membrane-bound glycoprotein (T1M) and a soluble variant (T1S) which represents the ectodomain of the receptor-type form. T1 is an orphan receptor belonging to the interleukin-1 receptor family. Its biological function is currently unknown. We analyze the expression of the two T1 proteins in mast cells and fibroblasts by using a set of monoclonal antibodies (MAb) that specifically recognize the extracellular portion of the T1 receptor. To generate anti-T1 MAbs, we immunized Lewis rats with a eukaryotically expressed chimeric protein consisting of the T1-receptor ectodomain fused to a human immunoglobulin domain. The two MAbs DJ4 and DJ8 were shown to specifically detect the murine T1M protein on the surface of primary IL-3-dependent bone marrow-derived mast cells as shown by flow cytometry and immunohistochemistry. Both antibodies were also capable of immunoprecipitating the membrane-associated 110-120 kDa T1M protein from mast cell lysates. In serum-stimulated but not in quiescent NIH3T3 fibroblasts, DJ4 and DJ8 MAbs detected both the soluble T1S protein as a 45-65 kDa band on SDS polyacrylamide gels as well as the membrane-bound 95 kDa T1M protein. The T1M protein in fibroblasts was less abundantly expressed and exhibited a lower molecular weight than the mast cell-produced T1M, probably as a consequence of different protein glycosylation. The MAbs described here represent highly specific reagents and valuable tools that should facilitate the establishment of the murine T1 protein expression pattern thus contributing to the solution of the question of its function.