The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus (NDV) plays a crucial role in the process of infection. However, the exact contribution of the HN gene to NDV pathogenesis is not known. In this study, the role of the HN gene in NDV virulence was examined. By use of reverse genetics procedures, the HN genes of a virulent recombinant NDV strain, rBeaudette C (rBC), and an avirulent recombinant NDV strain, rLaSota, were exchanged. The hemadsorption and neuraminidase activities of the chimeric viruses showed significant differences from those of their parental strains, but heterotypic F and HN pairs were equally effective in fusion promotion. The tissue tropism of the viruses was shown to be dependent on the origin of the HN protein. The chimeric virus with the HN protein derived from the virulent virus exhibited a tissue predilection similar to that of the virulent virus, and vice versa. The chimeric viruses with reciprocal HN proteins either gained or lost virulence, as determined by a standard intracerebral pathogenicity index test of chickens and by the mean death time in chicken embryos (a measure devised to classify these viruses), indicating that virulence is a function of the amino acid differences in the HN protein. These results are consistent with the hypothesis that the virulence of NDV is multigenic and that the cleavability of F protein alone does not determine the virulence of a strain.Newcastle disease virus (NDV), the only member of the genus Avulavirus, belongs to the family Paramyxoviridae (25). NDV is an important pathogen of many species of birds; it invokes trade barriers and causes significant economic losses in the commercial poultry industry worldwide. NDV isolates display a spectrum of virulence in chickens, from a fatal to an inapparent infection (1). Strains of NDV are classified into three major pathotypes, depending on the severity of disease produced in chickens. Avirulent strains are termed lentogenic, intermediately virulent strains are termed mesogenic, and highly virulent strains are termed velogenic.The surfaces of NDV particles contain two important functional glycoproteins: the fusion (F) and hemagglutinin-neuraminidase (HN) proteins. In general, membrane glycoproteins drive the assembly and budding of enveloped RNA viruses (41) and are the key players in determining host range and tissue tropism. The F protein mediates both virus-cell and cell-cell fusion (14). The F protein is synthesized as a nonfusogenic precursor, F0, and becomes fusogenic only after cleavage by host cell proteases into disulfide-linked F1 and F2 polypeptides (36). The cleavability of F protein is directly related to the virulence of viruses in vivo. A high content of basic amino acid residues at the F0 cleavage site is correlated with virulence (3, 47). Recent studies with recombinant NDV generated by reverse-genetics techniques showed that modification of a lentogenic F cleavage site to a velogenic cleavage site increased the virulence of the strain (32, 33) but did not reach the vi...

The alphavirus Sindbis virus (SINV) causes encephalomyelitis in mice byAlphaviruses in the family Togaviridae are enveloped, plusstrand, mosquito-borne RNA viruses that can cause encephalomyelitis. Sindbis virus (SINV), the prototype alphavirus, causes arthritis and rash in humans (39, 48) and encephalomyelitis in mice, a small-animal model for study of the pathogenesis of acute encephalitis (32). Age is an important determinant of outcome, and neonatal mice die within the first few days after infection, while older mice clear SINV from the central nervous system (CNS) within 6 to 8 days without signs of paralysis or neurological damage (33,40). Maturity of the infected neuron determines the level of virus replication and the susceptibility to SINV-induced cell death independent of the immune response (28,43,46,56). Immature neurons replicate SINV to higher titers and are susceptible to virus-induced apoptosis, while mature neurons are intrinsically more resistant to SINV replication and survive virus infection (3, 4, 43). Recovery from infection requires immune-mediated clearance of virus from these surviving infected neurons.Because mature neurons are terminally differentiated cells with limited capacity for regeneration, recovery that does not result in neuronal damage requires noncytolytic, rather than the more traditional cytolytic, immune mechanisms for virus clearance. Antibody is produced, T cells begin to infiltrate the CNS 3 to 4 days after infection, and virus clearance begins shortly thereafter (19,50,52). Type I interferon (IFN) is essential for initial control of virus replication (5, 6, 17, 69), and both humoral (6, 43, 77) and cellular (3, 37) arms of the adaptive immune response play important roles in clearance. Mice deficient in all components of adaptive immunity (SCID or Rag Ϫ/Ϫ ) develop persistent nonfatal infection, and passive transfer of SINV antibody results in clearance of infectious virus from the CNS and decreased viral RNA without neurologic damage, indicating an important role for antibody in noncytolytic clearance (34). However, mice deficient in antibody (MT) are able to reduce levels of SINV in the cortex and hippocampus of the brain compared to SCID mice and to clear infectious virus from the brain stem and spinal cord through local production of IFN-␥, indicating a role for T cells in clearance from some, but not all, types of neurons (3). Studies with mice deficient in production of both IFN-␥ and antibody indicate a synergistic role for these mediators in clearing SINV from the CNS, but the mechanisms are not known (5).To identify mechanisms of immune-mediated clearance, various in vitro systems have been developed. CSM14.1 neuronal cells that have been differentiated in vitro become persistently infected with SINV, and treatment with IFN-␥ results in virus clearance and improved cell survival (4, 79). Characterization of the response of infected differentiated neurons to treatment with IFN-␥ has shown a transient increase in synthesis of viral RNA and protein 6 h after tre...

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Respiratory syncytial virus (RSV) infection is the leading cause of hospitalization and infant mortality under six months of age worldwide; therefore, the prevention of RSV infection in all infants represents a significant unmet medical need. Here we report the isolation of a potent and broadly neutralizing RSV monoclonal antibody derived from a human memory B-cell. This antibody, RB1, is equipotent on RSV A and B subtypes, potently neutralizes a diverse panel of clinical isolates in vitro and demonstrates in vivo protection. It binds to a highly conserved epitope in antigenic site IV of the RSV fusion glycoprotein. RB1 is the parental antibody to MK-1654 which is currently in clinical development for the prevention of RSV infection in infants.