A Chronic Need for IL-21

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Science  19 Jun 2009:
Vol. 324, Issue 5934, pp. 1525-1526
DOI: 10.1126/science.1176487

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Chronic viral infections provoke a war of attrition between cytotoxic T cells (the CD8+ subclass) of the immune system and infected host cells. Although the CD8+ T cell response gradually whittles away at the viral load, the cells progressively become “exhausted” and lose function during a chronic infection (1). This exhaustion becomes more severe in the absence of helper CD4+ T cells, potentially leading to viral persistence and loss of virus-specific CD8+ T cells (1, 2). But how do CD4+ cells help? Three reports in this issue, by Elsaesser et al. on page 1569 (3), Yi et al. on page 1572 (4), and Fröhlich et al. on page 1576 (5), suggest an unexpected role for a specific cytokine produced by CD4+ T cells called interleukin-21 (IL-21) in controlling chronic viral infections.