Antibodies to a new linear site at the topographical or functional interface between the haemagglutinin and fusion proteins protect against measles encephalitis

The haemagglutinin protein (H) of measles virus (MV) binds to susceptible cells and collaborates with the fusion protein (F) to mediate fusion of the virus with the cell membrane. Binding and fusion activity of the virus can be monitored by haemagglutination and haemolysis, respectively, of monkey erythrocytes. Most monoclonal antibodies (MAbs) with haemolysis inhibiting activity (HLI⁺) are either MV-F specific and do not inhibit haemagglutination (HI^-), or they bind to MV-H and are HI⁺ by interfering with virus binding. We describe here a small panel of H-specific MAbs (BH47, BH59, BH103, BH129) which bind to a new linear neutralizing epitope, H244-250 (SELSQLS; NE domain), and which prevent virus-cell fusion (HLI⁺) but not virus binding (HI^-). These antibodies also protect against MV encephalitis in an animal model. They do not compete with an HLI⁺/HI⁺ antibody (BH216) which binds to the haemagglutinin noose epitope (HNE). The antibodies described here and the HNE-specific antibodies are functionally distinct and define two topographically non-overlapping interfaces, supposedly with a bias towards the host cell MV-receptor and the fusion protein respectively. The proximity of the CD46 downregulating amino acid Arg-243 may suggest a functional link between the domain described here and the CD46 binding domain. This new protective linear site is also of potential interest for the design of a subunit-based vaccine.