Th2 responses to inactivated influenza virus can be converted to Th1 responses and facilitate recovery from heterosubtypic virus infection

Immunization with live influenza virus expands Th1 memory cells and facilit ates more rapid recovery after heterosubtypic virus challenge. Immunization with inactivated virus generates a Th2 response and does not lead to heter osubtypic immunity. Creation of a Th1 priming environment by the inclusion of interleukin (IL)-12 with antibodies to IL-4 converted the response again st inactivated virus to a Th1 response that was able to facilitate virus cl earance upon heterosubtypic virus challenge. Evaluation of memory responses of mice immunized by the various protocols demonstrated that the type of i mmunization imprints T cell memory, dictating the nature of the response to subsequent infection. After live virus challenge, expansion of Th1 cells s eems to facilitate the generation of cytotoxic T lymphocytes from naive pre cursors. This latter finding may be the mechanism by which inactivated viru s immunization in a Th1 cytokine context mediates heterosubtypic immunity.