Diminished effector and memory CD8+ circulating T lymphocytes in patients with severe influenza caused by the AH1N1 pdm09 virus

The T cell immune response to viral infection includes the expansion of naïve T cells, effector cell differentiation and the induction of long-lived memory cells. We compared the differentiation of CD8+ T cells in patients with severe or mild pneumonia induced by influenza infection occurring during the 2009 influenza outbreak and compared their T cell subsets with those in blood samples obtained from healthy volunteers before the AH1N1 influenza outbreak in Mexico. Patients with severe influenza exhibited significantly lower numbers of effector memory CD8+CD26 high CD45RO+CCR7+ phenotype and lower numbers of central memory CD8+CD26high CD62L+CCR7+, CD26 high CD62L+CD127+ or CD26 high CD45RO+CD57 low phenotypes than patients with mild influenza or unexposed healthy subjects. Effector T cells with CD8+CD26CD62L low CD57+ phenotype were significantly diminished in severe influenza patients compared to those in patients with mild influenza or unexposed healthy subjects. These results suggest that low levels of circulating CD8+ T effector and central memory cells are associated with influenza severity.