Positive selection results in frequent reversible amino acid replacements in the G protein gene of human respiratory syncytial virus

Human respiratory syncytial virus (HRSV) is the major cause of lower respiratory tract infections in children under 5 years of age and the elderly, causing annual disease outbreaks during the fall and winter. Multiple lineages of the HRSVA and HRSVB serotypes co-circulate within a single outbreak and display a strongly temporal pattern of genetic variation, with a replacement of dominant genotypes occurring during consecutive years. In the present study we utilized phylogenetic methods to detect and map sites subject to adaptive evolution in the G protein of HRSVA and HRSVB. A total of 29 and 23 amino acid sites were found to be putatively positively selected in HRSVA and HRSVB, respectively. Several of these sites defined genotypes and lineages within genotypes in both groups, and correlated well with epitopes previously described in group A. Remarkably, 18 of these positively selected tended to revert in time to a previous codon state, producing a "flipflop" phylogenetic pattern. Such frequent evolutionary reversals in HRSV are indicative of a combination of frequent positive selection, reflecting the changing immune status of the human population, and a limited repertoire of functionally viable amino acids at specific amino acid sites.

Genetic stability of the attachment glycoprotein of human respiratory syncytial viruses during serial passages in cell cultures.

Thirteen isolates of human respiratory syncytial viruses (HRSV) of groups A and B were isolated in HEp-2 cells from nasopharyngeal aspirates (NPA) from the children with acute respiratory infections. Three isolates of HRSV of group A were propagated in HEp-2 cells in 20 serial passages. Nucleotide sequences of the products obtained by RT-PCR from the glycoprotein (G) hypervariable region of the original virus isolates in NPA and those after one or several passages were compared. All the isolates analyzed showed no changes during passaging in HEp-2 cells.