We investigated the in vivo function of the B7 family member B7-H3 (also known as B7RP-2) by gene targeting. B7-H3 inhibited T cell proliferation mediated by antibody to T cell receptor or allogeneic antigen-presenting cells. B7-H3-deficient mice developed more severe airway inflammation than did wild-type mice in conditions in which T helper cells differentiated toward type 1 (T(H)1) rather than type 2 (T(H)2). B7-H3 expression was consistently enhanced by interferon-gamma but suppressed by interleukin 4 in dendritic cells. B7-H3-deficient mice developed experimental autoimmune encephalomyelitis several days earlier than their wild-type littermates, and accumulated higher concentrations of autoantibodies to DNA. Thus, B7-H3 is a negative regulator that preferentially affects T(H)1 responses.

4-1BB (CD137) is a costimulatory member of the TNFR family expressed on activated T cells. Its ligand, 4-1BBL, is expressed on activated APC. In the mouse, CD8 T cells are preferentially activated by agonistic anti-murine 4-1BB Abs. However, murine 4-1BBL can stimulate both CD4 and CD8 T cells. To date, there are only limited data on the effects of 4-1BBL on human T cell responses. To further understand the role of 4-1BBL in human T cell responses, we compared human CD4 and CD8 T cell responses to transfected human 4-1BBL plus TCR-mediated stimulation. Both human CD4 and CD8 T cells responded to 4-1BBL. The presence of 4-1BBL on the APC led to increased expansion, cytokine production, and the development of cytolytic effector function by human T cells. In unfractionated T cell cultures, CD4 and CD8 T cells could expand to a similar extent in response to signals through the TCR and 4-1BB, as measured by CFSE labeling and by quantitating T cell numbers in the cultures. In contrast to the results with total T cells, isolated CD8 T cells produced less IL-2 and expanded to a lesser extent than isolated CD4 T cells responding to 4-1BBL. Thus, 4-1BBL is most effective when both CD4 and CD8 T cells are included in the cultures. CD28 and 4-1BB were found to synergize in the induction of IL-2 by human T cells, and CTLA-Ig partially blocked 4-1BBL-dependent IL-2 production. However, a portion of the 4-1BBL-mediated effects were independent of CD28-B7 interaction.

Effective adjuvants capable of inducing strong cytotoxic T cell responses in humans are lacking. In this study, we tested 4-1BBL as an adjuvant for activation of human memory antiviral CD8 T cell responses ex vivo. A recombinant replication-defective 4-1BBL adenovirus was used to convert autologous monocytes into efficient antigen-presenting cells after overnight incubation, bypassing the need to generate dendritic cells. Together with viral peptides, 4-1BBL led to robust memory responses of human EpsteinBarr virus-and influenza virus-specific cytotoxic T cells, with expansion of peptide-specific CD8 effector cells; up-regulation of Bcl-x L, granzyme A, and perforin; enhanced cytotoxic activity; and increased cytokine production. The response was significant even at a 100-fold lower peptide dose, compared with responses obtained with control adenovirus. Adenovirus-delivered B7.1 also expanded and activated virus-specific CD8 T cells, but 4-1BBL was more effective in driving the T cells toward a more fully differentiated CD27 ؊ effector state. Thus, 4-1BBL is a promising adjuvant for human memory CD8 T cells and will likely be most effective in the boost phase of a prime-boost strategy.E fficient activation of the cellular arm of the immune system requires a specific T cell antigen receptor signal delivered upon recognition of peptide͞MHC together with costimulatory signals. It has now been well established that dendritic cells are potent antigen-presenting cells (APC) for initiation of immune responses (1). Upon maturation, these cells up-regulate costimulatory molecules required for T cell activation. As a result, there is now considerable interest in the use of dendritic cells loaded with antigens as adjuvants for therapeutic vaccines (2, 3). A limitation of this approach is the need to derive the syngeneic dendritic cells in culture, a process that takes 7 days. In this report we describe the conversion of monocytes into efficient APC for activation of T cell memory responses by overnight incubation with recombinant adenoviruses expressing costimulatory molecules.Although the best characterized costimulatory molecule is CD28, recently other costimulatory molecules have been characterized (4-6). The emerging picture is that CD28 is important for the initial activation of an immune response and that other costimulatory ligand-receptor pairs act later to help sustain and diversify the response (4-6).4-1BB is an inducible costimulatory member of the tumor necrosis factor receptor family expressed on activated CD4 and CD8 T cells. Its ligand, 4-1BBL, is expressed on activated APC (6, 7). 4-1BB can enhance both the proliferation and the survival of murine CD4 and CD8 T cells (8)(9)(10)(11)(12)(13)(14). Recent evidence in mouse models suggests that 4-1BB͞4-1BBL interaction plays an important role in the memory CD8 T cell response to viruses (15-18).In humans, several studies have looked at the role of 4-1BBL or anti-4-1BB in polyclonal activation of T cells, but the specific role of 4-1BBL in activation of antigen-s...