Lambda interferon is the predominant interferon induced by influenza A virus infection in vivo.

The type I alpha/beta interferons (IFN-α/β) are known to play an important role in host defense against influenza A virus infection, but we have now discovered that the recently identified type III IFNs (IFN-λ) constitute the major response to intranasal infection with this virus. Type III IFNs were present at much higher levels than type I IFNs in the lungs of infected mice, and the enhanced susceptibility of STAT2-/- animals demonstrated that only signaling through the IFN-α/β or IFN-λ pathways was sufficient to mediate protection. This finding offers a possible explanation for the similar levels of antiviral protection found in wild-type (WT) mice and in animals lacking a functional type I IFN receptor (IFNAR-/-) but also argues that our current understanding of type III IFN induction is incomplete. While murine IFN-λ production is thought to depend on signaling through the type I IFN receptor, we demonstrate that intranasal influenza A virus infection leads to the robust type III IFN induction in the lungs of both WT and IFNAR-/- mice. This is consistent with previous studies showing that IFNAR-mediated protection is redundant for mucosal influenza virus infection and with data showing that the type III IFN receptor is expressed primarily by epithelial cells. However, the overlapping effects of these two cytokine families are limited by their differential receptor expression, with a requirement for IFN-α/β signaling in combating systemic disease.