An anti-fusion regulatory protein-1 monoclonal antibody suppresses human parainfluenza virus type 2-induced cell fusion

Fusion regulatory protein-1 (FRP-1) regulates virus-mediated cell fusion and induces polykaryocyte formation of monocytes without any fusogen. We have recently reported that FRP-1 and the 4F2/CD98 heavy chain are identical molecules. Cell fusion in Newcastle disease virus (NDV)-infected HeLa cells was enhanced when cells were incubated with anti-FRP-1 MAb. Anti-FRP-1 MAbs also induced human immunodeficiency virus gp160-mediated cell fusion. However, HBJ127, an anti-FRP-1/4F2/CD98 MAb that enhanced cell fusion in NDV-infected cells, delayed human parainfluenza virus type 2 (HPIV-2)-induced cell fusion in HeLa cells, although these viruses belong to the same genus Rubulavirus. No anti-FRP-1 MAbs enhanced cell fusion in HPIV-2-infected HeLa cells. Anti-FRP-1 MAbs including HBJ127 showed no effect on virus growth and expression levels of virus-specific polypeptides in HPIV-2-infected HeLa cells, indicating that the delay in cell fusion by an anti-FRP-1 MAb is not due to suppression of virus replication. When HeLa cells were transfected with an expression vector harbouring HPIV-2 HN and F genes, cell fusion was also suppressed by HBJ127, but the effect was weak in comparison with virus-infected cells. These data indicate anti-FRP-1 antibodies not only induce/enhance, but also inhibit/delay virus-induced cell fusion and therefore FRP-1 molecules are multifunctional.