Monocyte Chemotactic S19 Ribosomal Protein Homodimer Sharing Receptor Ligand Characteristics with Complement-derived Leukocyte Chemotactic Factor, C5a

Abstract

A monocyte specific chemotactic factor, S19 ribosomal protein homodimer, was originally isolated from rheumatoid arthritis-synovial tissue. The homodimer but not the monomer shared an immunologic epitope with the complement-derived leukocyte chemotactic factor, C5a. The monocyte chemotactic effect of the S19 protein homodimer was inhibited by an anti-C5a receptor monoclonal antibody or by a synthetic C5a receptor antagonist. The S19 protein homodimer competitively inhibited the binding of radiolabeled C5a to polymorphonuclear leukocytes. Furthermore, the S19 protein homodimer inhibited the chemotactic response of polymorphonuclear leukocytes to C5a in vitro and in vivo. These data indicate that the S19 protein homodimer possess a three-dimensional structure similar to C5a in terms of the receptor ligand, although homology between their primary structures is only 4%. The S19 protein homodimer with these opposite effects in the leukocyte chemotaxis, agonistic to monocytes and antagonistic to polymorphonuclear leukocytes, seems to be a factor to induce the monocyte/macrophage predominant infiltration in chronic inflammation.