Apoptosis of lymphocytes and monocytes infected with influenza virus might be the mechanism of combating virus and causing secondary infection by influenza.

Influenza affects most of the world's population annually, often causing a secondary infection, but pathological mechanisms of influenza virus infection remain unclear. We have found that influenza viruses have a selective preference for infecting monocytes and mature immune effector cells. This paper provides evidence that influenza virus infection increases the expression of granzyme B (GrB) in monocytes, activated T and B cells. All GrB(+) cells had cytolytic function. GrB(+)CD62L(high) central memory (T(CM)) cells were fast response population to virus infection when compared with GrB(+)CD62L(low) population. The influenza virus-infected PBMC could be killed by GrB(+) cells. We propose the following mechanism for influenza: (i) influenza virus within the respiratory tract overcomes humoral defenses; (ii) free virus is directly engulfed by the immune system effector cells and free virus also infects epithelial cells; (iii) virus-infected epithelial cells and the immune system cells are killed by cytotoxic cells. These indicated that an immune system that was combating a virus infection needs to sacrifice some of its immune system cells. Therefore, influenza viruses might temporally destroy the human immune system's line of defense, resulting in susceptibility to a secondary infection. This might be a prevalent mechanism existing in cell-mediated immune responses.