Immunopathology of RSV infection: prospects for developing vaccines without this complication.
Respiratory syncytial virus is the most important cause of lower respiratory tract infection in infants and young children. RSV clinical disease varies from rhinitis and otitis media to bronchiolitis and pneumonia. An increased incidence of asthma later in life has been associated with the more severe lower respiratory tract infections. Despite its importance as a pathogen, there is no licensed vaccine against RSV. This is due to a number of factors complicating the development of an effective and safe vaccine. The immunity to natural RSV infection is incomplete as re-infections occur in all age groups, which makes it challenging to design a protective vaccine. Second, the primary target population is the newborn infant, which has a relatively immature immune system and maternal antibodies that can interfere with vaccination. Finally, some vaccines have resulted in a predisposition for exacerbated pulmonary disease in infants, which was attributed to an imbalanced Th2-biased immune response, although the exact cause has not been elucidated. This makes it difficult to proceed with vaccine testing in infants. It is likely that an effective and safe vaccine needs to elicit a balanced immune response, including RSV-specific neutralising antibodies, CD8 T-cells, Th1/Th2 CD4 T-cells and preferably secretory IgA. Subunit vaccines formulated with appropriate adjuvants may be adequate for previously exposed individuals. However, intranasally delivered genetically engineered attenuated or vectored vaccines are currently most promising for newborns, as they are expected to induce a balanced immune response similar to that elicited to natural infection and not be subject to interference from maternal antibodies. Maternal vaccination may be the optimal strategy to protect the very young infants.