Persistent viral infections such as HIV and hepatitis C virus pose a severe health risk to the human population,which so far cannot be addressed by efficient prophylactic vaccination.Antiviral treatment is expensive and often associated with significant adverse effects.Therapeutic vaccination could therefore represent an important addition to current treatment modalities.However,vaccination of chronically infected hosts has mostly fallen short of expectations.One important reason for this is that during the development of a chronic infection,normal memory T cells do not develop.1 Instead,some specificities of virusspecific T cells are eliminated while others develop into a dysfunctional cell type termed exhausted cells.
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