Infection-triggered regulatory mechanisms override the role of STAT 4 in control of the immune response to influenza virus antigens.

Accurate control of the balance of the T1 and T2 cells during antiviral immunity is essential for optimizing immune effector functions and for avoiding potentially severe immunopathology. We examined the in vivo role of the signal transducer and activator of transcription (STAT) 4 in regulating the T1/T2 balance during the response to live influenza virus and isolated viral proteins. We found that the differentiation of gamma interferon (IFN-gamma)-producing Th1 and Tc1 cells after inoculation of live virus occurred independently of STAT 4 expression. Influenza virus-specific T2 and Tc2 responses were well controlled in such STAT 4-deficient mice unless IFN-gamma was eliminated as well. In contrast, the STAT 4-dependent signaling pathway played a more essential role in regulating the T1/T2 balance after immunization with viral proteins and, in particular, inactivated nonreplicating virus. Pulmonary infection was cleared even in the absence of both functional STAT 4 genes and functional IFN-gamma genes because virus-neutralizing antibodies were still generated, consistent with a substantial redundancy in different antiviral effector pathways. Thus, replicating agents such as live influenza virus can elicit IFN-gamma and control T2 immunity independently of STAT 4, whereas the profile of immunity to isolated proteins is more reliant on an intact STAT 4 signaling pathway.