CTL play an important role in the control of influenza virus infections. They eliminate virus-infected cells, on which surface they recognize foreign antigens derived from endogenously expressed viral antigens presented by MHC class I molecules. Thus, they contribute to the clearance of the virus from the infected tissue and prevent the spread of viral infection. Although CTL do not prevent influenza infection, their beneficial effect on the course of infection was observed after the adoptive transfer of virus-specific CTL clones to mice, resulting in direct lysis of infected cells [72] [73] [74] . In addition, depletion of CTL in infected mice led to higher titers of the virus in lungs, increased mortality and more severe disease [75] . Depending on their antigen specificity, CTLs may be subtype-specific or, in case they recognize the internal antigens, they are broadly cross-reactive with various influenza A viral strains. Early studies in mice showed that the majority of influenza-specific CTLs were reactive across subtypes [76, 77] , what underlines their important role in heterosubtypic immunity. This high crossreactivity is explained by the antigenically conserved targets of CTL represented mostly by inner influenza antigens (e.g. NP, M1 and PB1, PB2) [78] [79] [80] [81] . However, some conserved T-cell epitopes were identified also on variable surface influenza antigens [82] [83] [84] .