Specific for Influenza A Viruses

We characterized the human CD8+ T cell response against influenza A viruses by a flow cytometry–based assay. Peripheral blood mononuclear cells (PBMCs) were incubated with inactivated influenza virus prepa-ration, for 17 h, and were stained for intracellular interferon-g. Major histocompatibility complex class I–restricted memory CD8+ T cells specific for influenza antigens were detected in PBMCs from all 19 adult donors, at an average frequency of 0.39%. On average, 83 % of influenza virus–specific CD8+ T cells expressed the differentiation-associated marker CD27, a percentage that is significantly higher than that of CD8+ T cells specific for pp65 of human cytomegalovirus (53%). These observations indicate that class I–restricted immunity against influenza A viruses is characterized by the persistence, after clearance of infection, of circulating antigen-specific CD8+ T cells. The different patterns of CD27 expression in influenza virus – and cytomega-lovirus-specific CD8+ T cells suggest that influenza virus–specific memory and effector CD8+ T cells can be differentiated by phenotypic analysis. Influenza A viruses are single-stranded, segmented neg-ative-sense RNA viruses that are characterized by their broad host range. The influenza A viruses, including the H1N1, H3N2, and H2N1 serotypes, cause annual epidemics of acute respiratory infection that often result in significant morbidity and mortality, especially in chil-dren and in the elderly [1]. Virus-specific CD8 cy-totoxic T lymphocytes (CTLs) have been implicated as necessary for the clearance of influenza viruses durin