CD9 and infection: Mice infected with influenza virus via the intranasal route were euthanized at various times post-infection and BALf was collected, filtered (0.2 μm) and exosomes were purified by ultra-centrifugation (14) followed by further enrichment by immune-absorption onto magnetic beads coated with antibodies against the tetraspanin CD9, a common exosomal marker (15) (Supplementary Figure 1).