Nadia Naffakh

Human influenza A viruses replicate in the upper respiratory tract at a temperature of about 33 degrees C, whereas avian viruses replicate in the intestinal tract at a temperature close to 41 degrees C. In the present study, we analyzed the influence of low temperature (33 degrees C) on RNA replication of avian and human viruses in cultured cells. The kinetics of replication of the NP segment were similar at 33 and 37 degrees C for the human A/Puerto-Rico/8/34 and A/Sydney/5/97 viruses, whereas replication was delayed at 33 degrees C compared to 37 degrees C for the avian A/FPV/Rostock/34 and A/Mallard/NY/6750/78 viruses. Making use of a genetic system for the in vivo reconstitution of functional ribonucleoproteins, we observed that the polymerase complexes derived from avian viruses but not human viruses exhibited cold sensitivity in mammalian cells, which was determined mostly by residue 627 of PB2. Our results suggest that a reduced ability of the polymerase complex of avian viruses to ensure replication of the viral genome at 33 degrees C could contribute to their inability to grow efficiently in humans.

Following circulation of avian influenza H5 and H7 viruses in poultry, the hemagglutinin (HA) can acquire additional glycosylation sites, and the neuraminidase (NA) stalk becomes shorter. We investigated whether these features play a role in the pathogenesis of infection in mammalian hosts. From 1996 to 2007, H5N1 viruses with a short NA stalk have become widespread in several avian species. Compared to viruses with a long-stalk NA, viruses with a short-stalk NA showed a decreased capacity to elute from red blood cells and an increased virulence in mice, but not in chickens. The presence of additional HA glycosylation sites had less of an effect on virulence than did NA stalk length. The short-stalk NA of H5N1 viruses circulating in Asia may contribute to virulence in humans.

Although transmission of avian influenza viruses to mammals, particularly humans, has been repeatedly documented, adaptation and sustained transmission in the new host is a rare event that in the case of humans may result in pandemics. Host restriction involves multiple genetic determinants. Among the known determinants of host range, key determinants have been identified on the genes coding for the nucleoprotein and polymerase proteins that, together with the viral RNA segments, form the ribonucleoproteins (RNPs). The RNP genes form host-specific lineages and harbor host-associated genetic signatures. The functional significance of these determinants has been studied by reassortment and reverse genetics experiments, underlining the influence of the global genetic context. In some instances the molecular mechanisms have been approached, pointing to the importance of the polymerase activity and interaction with cellular host factors. Better knowledge of determinants of host restriction will allow monitoring of the pandemic potential of avian influenza viruses.