Respiratory Syncytial Virus is a major cause of childhood lower respiratory tract infections. Seventy percent of all children are infected in their first year of life, and nearly all children by age three. Although the majority of patients suffer a relatively mild upper respiratory tract infection, 3% of infected children are hospitalized and require oxygen support. Risk factors for a severe RSV infection are pre-maturity, congenital heart disease and bronchopulmonary dysplasia, although genetic predisposition also seems to play a role. Unfortunately however, the precise mechanism(s) of severe lower respiratory tract disease remain unknown. T cells play an important role in clearing the virus from the infected lung. However, T cells may also be involved in immune pathology that can be associated with respiratory disease. The scope of this thesis was to investigate the mechanism of the initiation of the cellular immune response after RSV infection. New insights of the interaction of the virus with the immune system can provide new directions for the rational design of a vaccine. In the first part of this thesis we focused on characterization of T cell epitopes. One CD4 T cell epitope was found to be HLA-DP4 restricted. The HLA-DP4 restricted epitope is an interesting finding since HLA-DP4 is the most prevalent HLA class II molecule, and epitopes presented by this HLA allele can therefore be used as a tool to measure RSV specific T cell responses in a large population. Furthermore five CD8 T cell epitopes were identified. All epitopes were proven to be processed and presented by RSV infected antigen presenting cells. The information obtained from these studies allowed us to monitor virus specific CD4+ and CD8+ T cell responses and extend the knowledge on the interplay of RSV with the adaptive and innate immune systems. Furthermore, we addressed the question whether mutations in the NS proteins of natural RSV isolates would lead to altered cytotoxic T cell responses, because the NS proteins interfere with the host capacity to induce type I interferon production and thus interfere with the capacity of the cell to suppress viral replication. We found three novel amino acid changes in NS proteins in virus isolates. The mutations lead to altered CD8 T cell responses.\ud Dendritic cells are considered professional antigen presenting cells crucial for the initiation of immune responses. They have the ability to direct and polarize T cells. Since DC are abundantly present in the airways, the effect of RSV infection on DC and how RSV infection impacted on the ability of DC to initiate T cell activation was studied in the second part of this thesis. We have found that RSV-infected myeloid DC secreted an a as yet un-identified soluble suppressive factor that was responsible for the inefficient expansion of naive T cells and an impaired cytokine production by the cells that did expand. In contrast to the inhibitory effect on proliferation by both naive and memory T cells RSV-DC were not impaired in the ability to induce IFN- in memory CD4 and CD8 T cells. Moreover, antigen processing and presentation via the HLA class I and II route were intact in RSV infected DCs. We observed that exposure of DC to inactivated RSV did not lead to the inhibitory phenotype of DC. These DC were able to present viral peptides via both HLA class I and II, which demonstrated that DC could internalize inactivated virus and stimulate CD8 T cells by cross-presentation
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