Antagonists or altruists: do viral mutants modulate T-cell responses?

The ability of altered peptide ligands (APLs) to modify or downregulate T-cell responses in vitro has generated much interest. Recently, it was proposed that viral variants may contain mutations in cytotoxic T lymphocyte (CTL) epitopes that allow mutual antagonism, and that these combine to downregulate the host immune response. Here, Miles Davenport discusses some inherent problems with this simple model and proposes a closer evaluation of the effects of viral competition on host-viral interactions.