Granzyme a expression reveals distinct cytolytic CTL subsets following influenza A virus infection

CTL mediate anti-viral immunity via targeted exocytosis of cytolytic granules containing perforin and members of the granzyme (grz) serine protease family. Here, we provide the first analysis of grzA protein expression by murine anti-viral CTL. During the progression of influenza A virus infection, CTL expressed two divergent cytolytic phenotypes: grzA^-B⁺ and grzA⁺B⁺. CTL lacked grzA expression during the initial rounds of antigen-driven division. High levels of grzA were expressed by influenza-specific CTL early post infection (day 6), particularly in tissues associated with the infected respiratory tract (bronchoalveolar lavage, lung). Following resolution of influenza infection, a small population of memory CTL expressed grzA. Interestingly, individual influenza A virus-derived epitope-specific CTL expressed different levels of grzA. The grzA expression hierarchy was determined to be K^bPB1₇₀₃ = D^bF2₆₂ = K^bNS2₁₁₄ > D^bNP₃₆₆ = D^bPA₂₂₄ and inversely correlated with CTL magnitude. Therefore following influenza infection, a CTL cytolytic hierarchy was established relating to the different profiles of antigen expression and relative immunodominance. Analysis of CTL grzA expression during influenza virus immunity has enabled a more detailed insight into the cytolytic mechanisms of virus elimination.