Antibodies to CD9, a tetraspan transmembrane protein, inhibit canine distemper virus-induced cell-cell fusion but not virus-cell fusion

Canine distemper virus (CDV) causes a life-threatening disease in several c arnivores including domestic dogs, Recently, we identified a molecule, CD9, a member of the tetraspan transmembrane protein family, which facilitates, and antibodies to which inhibit, the infection of tissue culture cells wit h CDV (strain Onder-stepoort), Here we describe that an anti-CD9 monoclonal antibody (MAb K41) did not interfere with binding of CDV to cells and upta ke of virus. In addition, in single-step growth experiments, MAb K41 did no t induce differences in the levels of viral mRNA and proteins. However, the virus release of syncytium-forming strains of CDV, the virus-induced cell- cell fusion in lytically infected cultures, and the cell-cell fusion of uni nfected with persistently CDV-infected HeLa cells were strongly inhibited b y MAb K41. These data indicate that anti-CD9 antibodies selectively block v irus-induced cell-cell fusion, whereas virus-cell fusion is not affected.