Innate Lymphoid Cell Are Sequentially Recruited to the Lungs Following Influenza A Virus Infection in Mouse

Influenza A virus (IAV) induces an inflammatory response in the airways that triggers the accumulation of immune cells. Innate lymphoid cells (ILC) of group 2 (ILC2) accumulate in the lungs following IAV infection. However, whether other ILC subtypes are present and the factors contributing to their accumulation in the airways like recruitment and/or proliferation remain unknown. Infection of mice with the PR8 virus (H1N1) induced the sequential accumulation of every ILC subset in the lungs. NK cells began to accumulate at day 3, followed by ILC2 and NCR22 at days 4-6, ILC1 and ILC3 at day 6, and LTi at day 8. These ILCs were activated as they expressed the activation markers CD69 and Sca1 and produced cytokines. Using the X31 virus (H3N2) and the more virulent MAp2009 (H1N1), we demonstrated that viral subtype and virulence has little influence on ILC accumulation. However, viral dose was directly correlated to the magnitude of the accumulation. Analysis of Ki-67 expression revealed that 25% of the different ILC subsets proliferate in the lungs, correlating with high levels of IL-2 and IL-33 transcripts. The number rose to 75% for ILC2, suggesting that cellular expansion is a major factor for their accumulation, while others ILCs are probably recruited. Finally, we detected high transcript levels of CCR2 and CXCR4 in the ILC1 and ILC2 population. Studies are performed to confirm the importance of the corresponding chemokines for ILC migration.