Granulocyte-macrophage colony-stimulating factor expressed by recombinant respiratory syncytial virus attenuates viral replication and increases the level of pulmonary antigen-presenting cells

An obstacle to developing a vaccine against human respiratory syncytial vir us (RSV) is that natural infection typically does not confer solid immunity to reinfection. To investigate methods to augment the immune response, rec ombinant RSV (rRSV) was constructed that expresses murine granulocyte-macro phage colony-stimulating factor (mGM-CSF) from a transcription cassette ins erted into the G-F intergenic region. Replication of rRSV/mGM-CSF in the up per and lower respiratory tracts of BALB/c mice was reduced 23- to 74- and 5- to 588-fold, respectively, compared to that of the parental rRSV. Despit e this strong attenuation of replication, the level of RSV-specific serum a ntibodies induced by rRSV/mGM-CSF was comparable to, or marginally higher t han, that of the parental rRSV. The induction of RSV-specific CD8(+) cytoto xic T cells was moderately reduced during the initial infection, which migh t be a consequence of reduced antigen expression. Mice infected with rRSV/m GM-CSF had elevated levels of pulmonary mRNA for gamma interferon (IFN-gamm a) and interleukin 12 (IL-12) p40 compared to animals infected by wild-type rRSV. Elevated synthesis of IFN-gamma could account for the restriction of RSV replication, as was observed previously with an IFN-gamma -expressing rRSV. The accumulation of total pulmonary mononuclear cells and total CD4() T lymphocytes was accelerated in animals infected with rRSV/mGM-CSF compa red to that in animals infected with the control virus, and the level of IF N-gamma -positive or IL-4-positive pulmonary CD4(+) cells was elevated appr oximately twofold. The number of pulmonary lymphoid and myeloid dendritic c ells and macrophages was increased up to fourfold in mice infected with rRS V/mGM-CSF compared to those infected with the parental rRSV, and the mean e xpression of major histocompatibility complex class II molecules, a marker of activation, was significantly increased in the two subsets of dendritic cells. Enhanced antigen presentation likely accounts for the maintenance of a strong antibody response despite reduced viral replication and would be a desirable property for a live attenuated rRSV vaccine.