PDCD1 and chronic hepatitis C virus infection: In this paper we demonstrate that (i) 2B4 is expressed on virus-specific CD8+ Tcells during acute and chronic hepatitis C, (ii) that 2B4 cross-linking can lead toboth inhibition and activation of HCV-specific CD8+ T cell function, dependingon expression levels of 2B4 and the intracellular adaptor molecule SAP and (iii)that 2B4 stimulation may counteract enhanced proliferation of HCV-specific CD8+T cells induced by PD1 blockade.