CD8 ؉ T lymphocytes play a major role in cellular-mediated immune responses to foreign antigen. We have previously demonstrated that costimulation of purified human CD8 ؉ T cells induces de novo expression of the CD4 molecule and that ligation of CD4 on this cell type modulates CD8 ؉ T cell activity in vitro. Herein, we investigate how the CD4 molecule expressed on murine CD8 ؉ T cells contributes to CD8 ؉ cell responses in vivo by employing adoptive transfer of CD8 cells from CD4 knockout mice into severe combined immunodeficient (SCID) recipients. Transfer of these cells into syngeneic SCID mice resulted in a decreased immune response to infection by lymphocytic choriomeningitis virus. These decreased responses occurred even in the presence of CD4 ؉ T cells, indicating that this was truly a CD8-cell defect. Similarly, transfer of CD8 ؉ T cells incapable of expressing CD4 into allogeneic SCID mice resulted in a decreased response to alloantigens compared with that of normal CD8 ؉ T cells. Therefore, CD4 expression on CD8 T lymphocytes modulates cytotoxic T lymphocyte function and is critical in vivo for optimal cell-mediated immunity to viral and alloantigens.antiviral response ͉ CD8 ϩ T cells ͉ alloantigen response ͉ cytotoxic T lymphocytes T cell development in the thymus is characterized by the appearance and disappearance of cell-surface CD4 and CD8, which are classic markers of ontogeny. It was previously thought that after synchronous expression of CD4 and CD8, the developing thymocyte becomes CD4 or CD8 single positive, permanently turning off expression of one of these molecules. These single-positive cells are then exported to the periphery to perform their respective functions: CD4 ϩ cells serving primarily as T helper (Th) cells, and CD8 ϩ cells serving primarily as cytotoxic T lymphocytes (CTLs). Our current investigation focuses on the CD8 ϩ T cell population. CD8 ϩ T cells become activated to perform their function when they encounter a specific antigen presented in the context of MHC class I and are costimulated by one of many molecules present on antigen-presenting cells (APCs) (1). A well described series of events is triggered after activation of CD8 ϩ T cells, primarily resulting in CTLs that kill antigen-expressing target cells.In contrast to the notion that commitment to the CD4 or CD8 lineage is complete or permanent after thymopoiesis, extrathymic CD8 ϩ CD4 ϩ cells have been identified in many organisms, including humans, monkeys, mice, rats, swine, and chickens (reviewed in ref.2). Various disease conditions appear to affect the levels of CD8 ϩ CD4 ϩ T cells. In humans, infection with HIV, human T cell leukemia virus, Epstein-Barr virus, human herpesvirus 6, and increasing age have been linked with increased percentages of CD8 ϩ CD4 ϩ T cells in the peripheral blood (3-6). In mice, increases in the CD8 ϩ CD4 ϩ population have been observed after inoculation with reovirus (7), or recombinant adenovirus (8). In most cases, the rise in the percentage of CD8 ϩ CD4 ϩ cells appears to be a dir...