Identification of a novel bovine serum protein which is involved in human T-cell leukemia virus type I (HTLV-I)-induced syncytium formation

Abstract

By immunizing rats with cocultured HTLV-I-positive ILT8M2 and HTLV-I-negative MOLT-4-cells, we isolated a monoclonal antibody (mAb), designated as mAb R21, which enhances the syncytium formation induced by coculturing ILT8M2 cells with MOLT-4 cells. The antigen recognized by mAb R21 was found on the surface of all T-cell, fibroblastoid, and epithelial cells lines, and a part of B-cell and myelomonocytoid cell lines. MAb R21 reacted with an approximately 17-kDa protein from ILT8M2 and MOLT-4 cell lysates in both nonreducing and reducing conditions by immunoblotting. Immunoprecipitation experiments using surface-labeled cells revealed that a 17-kDa protein is present on the surface of both ILT8M2 and MOLT-4 cells. Since the enhancing activity by mAb R21 of syncytium formation was observed only in the presence of a factor contained in fetal calf serum (FCS) which seems to bind to mAb R21, we purified this serum factor from FCS using a mAb R21-coupled Sepharose 4B column. The purified protein, designated as R21 protein, was revealed to be O-glycosylated but not N-glycosylated protein of approximately 17 kDa. The partial amino acid sequence of this protein indicates that R21 protein is a novel bovine serum protein which has approximately 90% amino acid homology with bovine platelet factor 4, a member of CXC chemokine family. These results indicate that the R21 protein on the surface of cells and/or in FCS may play an important role in the process of HTLV-I-induced syncytium formation by as yet unknown mechanism.