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Background: Respiratory viral infections are known to be a risk factor for the exacerbation of asthma. In a model of asthma with influenza A virus infection, dendritic cells play an important role during antigen sensitization and antigen challenge. However, the role of T cells in the asthma-influenza A model was unclear. The aim of the present study was to assess the roles of T cells during antigen sensitization and antigen challenge in the asthmatic model. Methods: C57BL/6 mice were infected with influenza A virus and were sensitized with inhaled antigen during the acute phase of the virus infection. Anti-CD4 or anti-CD8 monoclonal antibodies were administered during antigen sensitization or antigen challenge to evaluate the role of CD4+ or CD8+ T cells in this murine model of asthma. The onset of asthma was determined by eosinophil recruitment into the lung. Results: During antigen sensitization, depletion of either CD4+ or CD8+ T cells resulted in an absence of eosinophil infiltration into the lung after antigen challenge. However, during antigen challenge, depletion of CD4+ T cells did cause such an absence of eosinophil infiltration. Conclusions: During antigen sensitization, both CD4+ and CD8+ T cells were required in C57BL/6 mice for exacerbation of asthma. During antigen challenge, CD4+ T cells were important for the onset of asthma, whereas CD8+ T cells do not affect eosinophil recruitment into the lung.