Proteolytic cleavage by plasmin of the HA polypeptide of influenza virus: host cell activation of serum plasminogen

Virulence factors of influenza A viruses: WSN virus neuraminidase required for plaque production in MDBK cells

The genetic basis for the distinctive capacity of influenza A/WSN/33 (H0N1) virus (WSN virus) to produce plaques on bovine kidney (MDBK) cells was found to be related to virus neuraminidase. Recombinant viruses that derived only the neuraminidase of WSN virus were capable of producing plaques, whereas recombinant viruses identical to WSN except for neuraminidase did not produce plaques. With viruses that do not contain WSN neuraminidase, infectivity of virus yields from MDBK cells was increased approximately 1,000-fold after in vitro treatment with trypsin. In contrast, no significant increase in infectivity was observed after trypsin treatment of viruses containing WSN neuraminidase. In addition, polyacrylamide gel analysis of proteins of WSN virus obtained after infection of MDBK cells demonstrated that hemagglutinin was present in the cleaved form (HA1 + HA2), whereas only uncleaved hemagglutinin was obtained with a recombinant virus that derived all of its genes from WSN virus except its neuraminidase. These data are in accord with the hypothesis that neuraminidase may facilitate production of infectious particles by removing sialic acid residues and exposing appropriate cleavage sites on hemagglutinin.

The segments of influenza viral mRNA

Influenza viral mRNA, i.e., complementary RNA (cRNA), isolated from infected cells , was resolved into six different species by electrophoresis in 2.1% acrylamide gels containing 6 M urea. The cRNA's were grouped into three size classes: L (large), M (medium-size), and S (small). Similarly, when gels were sliced for analysis, the virion RNA (vRNA) also distributed into six peaks because the three largest vRNA segments were closely spaced and were resolved only when the gels were autoradiographed or stained. Because of their attached polyadenylic acid [poly(A)]sequences, the cRNA segments migrated more slowly than did the corresponding vRNA segments during gel electrophoresis. After removal of the poly(A) by RNase H, the cRNA and vRNA segments comigrated, indicating that they were approximately the same size. One of the cRNA segments, S2, was shown by annealing to contain the genetic information in the vRNA segment with which it comigrated, strongly suggesting that each cRNA segment was transcribed from the vRNA segment of the same size. In contrast to the vRNA segments, which when isolated from virions were present in approximately 1:1 molar ratios, the segments of the isolated cRNA were present in unequal amounts, with the segments M2 and S2 predominating, suggesting that different amounts of the cRNA segments were synthesized in the infected cell. The predominant cRNA segments, M2 and S2, and also the S1 segment, were active as mRNA's in wheat germ extracts. The M2 cRNA was the mRNA for the nucleocapsid protein; S1 for the membrane protein; and S2 for the nonstructural protein NS1.

Survival of airborne influenza virus: effects of propagating host, relative humidity, and composition of spray fluids

Influenza A virus, strain WSNH, propagated in bovine, human and chick embryo cell cultures and aerosolized from the cell culture medium, was maximally stable at low relative humidity (RH), minimally stable at mid-range RH, and moderately stable at high RH. Most lots of WSNH virus propagated in embryonated eggs and aerosolized from the allantoic fluid were also least stable at mid-range RH, but two preparations after multiple serial passage in eggs showed equal stability at mid-range and higher RH's. Airborne stability varied from preparation to preparations of virus propagated both in cell culture and embryonal eggs. There was no apparent correlation between airborne stability and protein content of spray fluid above 0.1 mg/ml, but one preparation of lesser protein concentration was extremely unstable at 50 to 80 per cent RH. Polyhydroxy compounds exerted a protective effect on airborne stability.

Interaction of concanavalin A with the surface of virus-infected cells

Infection of untransformed cells with a wide-range of non-oncogenic enveloped viruses causes a significant increase in their susceptibility to agglutination by concanavalin A (Con A). The increased Con A agglutinability of these cells is not caused by an increase in the number of Con A sites on the cell surface but involves alteration in the surface properties of infected cells to allow redistribution of Con A receptors to form "patches" following binding of Con A to the cell surface. Similarities between Con A-mediated agglutination of normal cells infected with non-oncogenic viruses and the agglutination response to cells transformed by oncogenic viruses will be reviewed. Finally, the use of Con A as an experimental tool to modify the replication and cytopathogenicity of non-oncogenic viruses grown in mammalian cells will be presented.