Abstract Antigen-experienced T cells express a wide variety of costimulatory molecules, some of which can contribute to activating, inhibitory, or exhaustion pathways depending on context. In particular 2B4 (CD244/ SLAMf4) has been shown to be capable of either inhibitory or enhancing effects upon engagement of its ligand CD48 (SLAMf2). We examined the phenotypes of CD8 T cell lines and clones derived from HIV-infected or -uninfected subjects, and specific for HIV or for respiratory syncytial virus. Antiviral CTL expressed varying levels of PD-1, 2B4, and TIM-3. Cognate peptide induced rapid downregulation of both 2B4 and TIM-3, but not PD-1 on cultured CD8 T cells. 2B4 downregulation also occurred directly ex vivo and required simultaneous signaling via both TCR and 2B4, as downregulation was not observed in the absence of antigen or in the presence of CD48-blocking antibody. 2B4 downregulation was not induced by PMA or prevented by PI3-kinase inhibition, implicating a TCR-proximal signaling mechanism. Downregulation occurred rapidly upon stimulation with a low dose of peptide, concomitant with internalization of CD3 and CD8. The degree of 2B4 downregulation varied among CD8 T cells of different specificities and HLA restrictions, and the degree of downregulation correlated with the respective enhancement or suppression of antiviral IFNγ production in response to CD48 blockade. Our results indicate a role for 2B4 downregulation in modulation of antiviral CD8 T cell responses.