In line herewith, CD137L−/− mice show defects in inducing CD8+ T-cell responses against viral infections, such as influenza (33), MHV-68 (34), and LCMV (35), and further study revealed that secondary rather than primary CD8+ T-cell responses against influenza infection are impaired in these mice (120). This evidence concerns the gene TNFSF9 and viral infectious disease.