Molecular cloning of a macrophage-derived, interferon-inducible secreted immunoglobulin-binding protein

We describe the molecular cloning of a 1803-bp cDNA coding for a product te rmed interferon-induced immunoglobulin-binding protein (IIBP) from a librar y of IFN-a-induced primary bone marrow macrophages. The open reading frame encodes a protein of 26-kDa containing two immunoglobulin-like and one Fc r eceptor-like domain. Due to the constitutive release of low amounts of IFN- beta, the IIBP mRNA is already present in macrophage-colony-stimulating fac tor-cultured macrophages. Its expression could be blocked in the presence o f either anti-IFN-beta or inteuleukin-4, which down-regulates the endogenou s IFN-beta production. Upon addition of rIFN-alpha(4) a 3-5-fold superinduc tion of IIBP mRNA was observed. Rat monoclonal antibodies detected a protei n of the predicted size exclusively in cell culture supernatants of primary bone marrow macrophages and a B-cell line. In immunoprecipitation experime nts an unknown 30-kDa protein co-precipitated. The secreted IIBP showed con siderable binding to nonspecific rat IgG2a and could be precipitated using mouse IgG2a, IgG2b and IgG3 antibodies of irrelevant specificities, indicat ing that this gene product is a novel secreted immunoglobulin-binding prote in with a new IgG isotype binding pattern that differs to that of known Fe receptors.