Proteolytic cleavage of the viral glycoproteins and its significance for the virulence of Newcastle disease virus

In search of a molecular basis underlying the variations in virulence observed with different strains of Newcastle disease virus, a comparative study has been carried out on the biosynthesis and function of the viral glycoproteins. Five virulent (Italien, Herts, Field Pheasant, Texas, Warwick) and five avirulent strains (La Sota, B1, F, Queensland, Ulster) have been analyzed. They were grown in five different host systems (embryonated eggs, cultures of BHK21-F, MDBK, chick embryo, and chick chorioallantoic membrane cells). Glycoprotein F (MW 56,000) which is responsible for hemolysis and cell fusion has been found with all strains to be derived by proteolytic cleavage from the precursor glycoprotein Fo (MW 68,000). With strains Queensland and Ulster, in addition, a precursor glycoprotein HNo (MW 82,000) has been identified which is converted, again by proteolytic cleavage, into the hemagglutinin-neuraminidase glycoprotein HN (MW 74,000). Cleavage of Fo is necessary for the expression of cell fusing and hemolytic activity, and the available evidence suggests that cleavage of HNo is paralleled by an enhancement of hemagglutinating and neuraminidase activity. However, activation of the glycoproteins is not required for virus assembly. Thus, virus particles containing the precursor Fo may be formed which have a reduced infectivity. Infectivity is even lower, if both glycoproteins are present in the uncleaved form. After in vitro treatment with trypsin, such particles display full biological activity. Whether the glycoproteins are cleaved in vivo depends on the virus strain and on the host cell. With virulent strains, cleavage occurs in all host systems analyzed, and the virions formed contain HN and F. With avirulent strains, however, this is the case only in the embryonated egg and in cultures of chorioallantoic membrane cells. All other cells produce particles containing uncleaved glycoproteins. From these observations the following conclusions can be drawn: only a few host systems are permissive for avirulent strains, i.e., they produce highly infectious virus; other systems are nonpermissive for these strains, i.e., they produce defective virus; in contrast, all host systems studied are permissive for virulent strains. This concept is supported by the finding that multiple replication cycles and plaque formation occur only in permissive cells or in a nonpermissive culture after substitution of trypsin. Thus, plaque assays are now available for avirulent strains, either by the use of MDBK and chick embryo cells in the presence of trypsin or by the use of chorioallantoic membrane cells. These observations demonstrate striking differences in host range between virulent and avirulent strains which are determined by the susceptibility of the envelope glycoproteins to proteolytic cleavage. It is suggested that these differences account at least in part for the variations in the virulence of Newcastle disease virus.