A second group was injected 1 day later and so on through day 5 P.I. day 8 postinfection. Anti-CD137Cinjected 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- 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 activationCinduced 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 (4C8), 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 cellCdependent humoral immunity in Triamcinolone hexacetonide vivo (11). Both attributes have made this reagent attractive for therapeutic use in treating cancer and autoimmune disease, and it is currently FGF17 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 virus-specific 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 cellCmediated antiviral immune responses. Anti-CD137Cinjected lymphocytic choriomeningitis virusCinfected (LCMV-infected) mice rapidly produced high levels of IL-10 and developed sustained TNF- production during the late stages of infection, leading to Fas expression on CD8+ T cells. Blockade of IL-10 signaling fully rescued virus-specific CD4+ T cells but minimally affected CD8+ T cell function and survival. In T cells from virus-infected anti-CD137Cinjected IL-10C/C mice treated with antiCTNF- mAbs, Fas was downregulated, CD8+ T cells were not deleted, and antiviral immunity remained intact. When anti-CD137 treatment was withheld until day 4 P.I., the magnitude of antiviral immunity was significantly enhanced, demonstrating that anti-CD137 mAb treatment could induce in a time-dependent manner 2 diametrically opposed outcomes. Results Anti-CD137 mAb treatment during LCMV infection suppressed antiviral T cell immunity. CD137 ligands deliver costimulatory signals to CD8+ T cells in vivo and paradoxically suppress CD4+ T helper cells despite being costimulatory for both in vitro (11, 13, 14). This has raised the question of whether CD137 signaling differentially regulates CD4+ and CD8+ T cell effector function. We addressed this question in 2 viral infection models because its resolution has important ramifications for the therapeutic use of CD137 ligands. LCMV Armstrong induces an acute infection in mice that is rapidly cleared while eliciting long-lasting immunity. Antiviral immunity was measured in LCMV-infected mice given 200 g of Triamcinolone hexacetonide agonist anti-CD137 mAbs or rat IgG on day 1 P.I. Consistent with earlier reports (11, 13C15), we found that this treatment markedly suppressed CD4+ T cells as measured by T cellCdependent LCMV-specific IgG (Figure ?(Figure1A),1A), enumerating LCMV-specific antibody-secreting cells in the BM (Figure ?(Figure1B)1B) and measuring IFN-+ CD4+ T cells (Figure ?(Figure1C).1C). In dose response studies we found that 30 g of mAb was sufficient to induce suppression when given on day 1 P.I. (data not shown). That anti-CD137 costimulates CD8+ T cells has been widely reported in the literature, and at no time has it been shown to be inhibitory. Remarkably, in this study we found complete suppression of antiviral CD8+ Triamcinolone hexacetonide T cell responses. We found a marked reduction in tetramer-positive CD8+ T cells (Figure ?(Figure1D),1D), reduction in numbers of virus-specific CD8+ T cells (Figure ?(Figure1E),1E), absence of IFN-+ CD8+ T cells (Figure ?(Figure1F),1F), and failure to induce antiviral CTL responses (Figure ?(Figure1G).1G)..