Mice infected by the wild type 627E and the PB2 mutants all induced increased levels of pro-inflammatory cytokines (TNF-α, IP-10, MCP-1, MIP-1α, RANTES, MCP-3, IFN-α, MIP-1β and GM-CSF) in the lung compared to the uninfected control at both 3 and 6 days post-infection although the wild type 627E and PB2 mutants with K at 591 and N at 701 showed no significant weight loss (Figs 6 and 7). Here, CSF2 is linked to infection.