CD9-dependent regulation of Canine distemper virus-induced cell-cell fusion segregates with the extracellular domain of the haemagglutinin

Antibodies to CD9, a member of the tetraspan transmembrane-protein family, selectively inhibit Canine distemper virus (CDV)-induced cell–cell fusion. Neither CDV-induced virus–cell fusion nor cell–cell fusion induced by the closely related morbillivirus Measles virus (MV) is affected by anti-CD9 antibodies. As CDV does not bind CD9, an unknown, indirect mechanism is responsible for the observed inhibition of cell–cell fusion. It was investigated whether this effect was restricted to only one viral glycoprotein, either the haemagglutinin (H) or the fusion (F) protein, which form a fusion complex on the surface of virions and infected cells, or whether it is dependent on both in transient co-transfection assays. The susceptibility to CD9 antibodies segregates with the H protein of CDV. By exchanging portions of the H proteins of CDV and MV, it was determined that the complete extracellular domain, including the predicted stem structure (stem 1, barrel strand 1 and stem 2) and globular head domain, of the CDV-H protein mediates the effect. This suggests that interaction of the CDV-H protein with an unknown cellular receptor(s) is regulated by CD9, rather than F protein-mediated membrane fusion.