CD137-mediated signals costimulate T cells and protect them from activation-induced apoptosis; they induce curative antitumor immunity and enhance antiviral immune responses in mice. In contrast, anti-CD137 agonistic mAbs can suppress T-dependent humoral immunity and reverse the course of established autoimmune disease. These results have provided a rationale for assessing the therapeutic potential of CD137 ligands in human clinical trials. In this study, we report that a single 200-μg injection of anti-CD137 given to otherwise naive BALB/c or C57BL/6 mice led to the development of a series of immunological anomalies. These included splenomegaly, lymphadenopathy, hepatomegaly, multifocal hepatitis, anemia, altered trafficking of B cells and CD8 T cells, loss of NK cells, and a 10-fold increase in bone marrow (BM) cells bearing the phenotype of hemopoietic stem cells. These events were dependent on CD8 T cells, TNF-α, IFN-γ, and type I IFNs. BM cells up-regulated Fas, and there was a significant increase in the number of CD8+ T cells that correlated with a loss of CD19+ and Ab-secreting cells in the BM. TCR Vαβ usage was random and polyclonal among liver-infiltrating CD8 T cells, and multifocal CD8+ T cell infiltrates were resolved upon termination of anti-CD137 treatment. Anti-CD137-treated mice developed lymphopenia, thrombocytopenia, and anemia, and had lowered levels of hemoglobin and increased numbers of reticulocytes.

CD137 is expressed on activated T cells and ligands to this costimulatory molecule have clinical potential for amplifying CD8 T cell immunity to tumors and viruses, while suppressing CD4 autoimmune T cell responses.To understand the basis for this dichotomy in T cell function, CD4 and CD8 antiviral immunity was measured in lymphocytic choriomeningitis virus (LCMV) Armstrong-or A/PR8/34 influenza-infected mice injected with anti-CD137 mAbs. We found that the timing of administration of anti-CD137 mAbs profoundly altered the nature of the antiviral immune response during acute infection. Antiviral immunity progressed normally for the first 72 hours when the mAb was administered early in infection before undergoing complete collapse by day 8 postinfection. Anti-CD137-injected LCMV-infected mice became tolerant to, and persistently infected with, LCMV Armstrong. Elevated levels of IL-10 early in the response was key to the loss of CD4 + T cells, whereas CD8 + T cell deletion was dependent on a prolonged TNF-α response, IL-10, and upregulation of Fas. Blocking IL-10 function rescued CD4 antiviral immunity but not CD8 + T cell deletion. Anti-CD137 treatment given beyond 72 hours after infection significantly enhanced antiviral immunity. Mice treated with anti-CD137 mAb 1 day before infection with A/PR8/34 virus experienced 80% mortality compared with 40% mortality of controls. When treatment was delayed until day 1 postinfection, 100% of the infected mice survived. These data show that anti-CD137 mAbs can induce T cell activation-induced cell death or enhance antiviral immunity depending on the timing of treatment, which may be important for vaccine development. Introduction CD137, a TNF receptor family member, is an activation-inducible T cell costimulatory receptor (1, 2) that is expressed on cells of the innate and adaptive immune system (reviewed in ref.3). CD137 ligands induce T cell proliferation and cytokine production in vitro (4-8), whereas CD137 ligands confer potent costimulatory effects upon CD8 + T cells in vivo (9, 10). Although CD137 crosslinking costimulates CD4 + T cells to proliferate in vitro, remarkably, it suppresses the development of T cell-dependent humoral immunity in vivo (11). Both attributes have made this reagent attractive for therapeutic use in treating cancer and autoimmune disease, and it is currently being evaluated for use as an adjuvant in vaccine design (12). Herein we assess the effect of altering the timing of anti-CD137 treatment during viral infection and in so doing reconcile the dichotomy between CD137-induced CD8 + T cell activation and CD4 + T cell suppression. We found that virusspecific CD4 + and CD8 + T cell proliferation proceeds normally over the first 3 days of infection. By day 8 postinfection (P.I.), large numbers of virus-specific CD4 + and CD8 + T cells were deleted, thus leading to the loss of humoral and T cell-mediated antiviral immune responses. Anti-CD137-injected lymphocytic choriomeningitis virus-infected (LCMV-infected) mice rapidly produced high

CD137 (4-1BB), is an inducible T-cell costimulatory receptor and a member of the tumor necrosis factor receptor (TNFR) superfamily. It is expressed on activated T cells and activated natural killer (NK) cells, but is constitutively expressed on a population of splenic dendritic cells (DCs). The natural counter receptor for CD137 is 4-1BB ligand, a member of the TNF superfamily that is weakly expressed on naïve or resting B cells, macrophages, and DCs. Upon activation, the level of 4-1BBL expression increases on these cells. In T cells CD137-induced signals lead to the recruitment of TRAF family members and activation of several kinases, including ASK-1, MKK, MAPK3/ MAPK4, p38, and JNK/SAPK. Kinase activation is then followed by the activation and nuclear translocation of several transcription factors, including ATF-2, Jun, and NF-kappaB. CD137-mediated T-cell costimulation as measured by enhanced proliferation and cytokine production can be induced by anti-CD137 monoclonal antibodies (MAbs) or by employing immobilized 4-1BB ligand. In addition to augmenting suboptimal TCR-induced proliferation, CD137-mediated signaling protects T cells, and in particular, CD8+ T cells from activation-induced cell death (AICD). Although studies with CD137-deficient or 4-1BBL-deficient mice failed to demonstrate any loss of essential immunological function, or other noteworthy deficits, we have found that 4-1BBL-deficient mice failed to generate a strong antiviral immune response following lymphocytic choriomeningitis virus (LCMV) peptide vaccination. We further found that although compromised, the immune response to LCMV vaccination in these mice could be fully restored by injecting them with anti-CD137 MAbs at the time of vaccination. Finally, we have found that injecting normal mice with anti-CD137 MAbs had profound effects on their ability to develop immune responses to allo- and autoantigens. The results of these studies discussed in this article provide a rationale for assessing the potential use of anti-CD137 MAbs for therapeutic purposes.

Background-Dendritic cell (DC) therapy is a promising technology for the treatment of HIV infected individuals. HIV-1 Gag-and Nef RNA-loaded DC have previously been shown to induce immune responses ex vivo following coculture with autologous lymphocytes. However, polyfunctionality and memory responses following coculture have not been evaluated. In addition, little is known regarding whether specific HIV-1 proteome components, such as highly conserved regions of the HIV-1, could enhance clinical responses following DC therapy.

Material