Recombinant human parainfluenza virus type 2 vaccine candidates containing a 3′ genomic promoter mutation and L polymerase mutations are attenuated and protective in non-human primates

Previously, we identified several attenuating mutations in the L polymerase protein of human parainfluenza virus type 2 (HPIV2) and genetically stabilized those mutations using reverse genetics (Nolan et al., 2005). Here we describe the discovery of an attenuating mutation at nucleotide 15 (15T→C) in the 3′ genomic promoter that was also present in the previously characterized mutants. We evaluated the properties of this promoter mutation alone and in various combinations with the L polymerase mutations. Amino acid substitutions at L protein positions 460 (460A or 460P) or 948 (948L), or deletion of amino acids 1724 and 1725 (Δ1724), each conferred a temperature sensitivity (ts) phenotype whereas the 15T→C mutation did not. The 460A and 948L mutations each contributed to restricted replication in the lower respiratory tract of African green monkeys, but the Δ1724 mutation increased attenuation only in certain combinations with other mutations. We constructed two highly attenuated viruses, rV94(15C)/460A/948L and rV94(15C)/948L/Δ1724, that were immunogenic and protective against challenge with wild-type HPIV2 in African green monkeys and, therefore, appear to be suitable for evaluation in humans.