Sepsis induces lymphocyte apoptosis and prevention of lymphocyte death may improve the chances of surviving this disorder. We compared the efficacy of a selective caspase-3 inhibitor to a polycaspase inhibitor and to caspase-3-/- mice. Both inhibitors prevented lymphocyte apoptosis and improved survival. Caspase-3-/- mice shared a decreased, but not total, block of apoptosis. The polycaspase inhibitor caused a very substantial decrease in bacteremia. Caspase inhibitors did not benefit RAG-1-/- mice, which had a > tenfold increase in bacteremia compared to controls. Adoptive transfer of T cells that overexpressed the anti-apoptotic protein Bcl-2 increased survival. T cells stimulated with anti-CD3 and anti-CD28 produced increased interleukin 2 and interferon gamma by 6 h. Thus, caspase inhibitors enhance immunity by preventing lymphocyte apoptosis and lymphocytes act rapidly, within 24 h, to control infection.

Sepsis induces extensive lymphocyte apoptosis, a process which may be beneficial to host survival by down-regulating the inflammatory response or, alternatively, harmful by impairing host defenses. To determine the beneficial vs. adverse effects of lymphocyte apoptosis in sepsis, we blocked lymphocyte apoptosis either by N-benzyloxycarbonyl-Val-Ala-Asp(O-methyl) fluoromethyl ketone (z-VAD), a broad-spectrum caspase inhibitor, or by use of Bcl-2 Ig transgenic mice that selectively overexpress the antiapoptotic protein Bcl-2 in a lymphoid pattern. Both z-VAD and Bcl-2 prevented lymphocyte apoptosis and resulted in a marked improvement in survival. z-VAD did not decrease lymphocyte tumor necrosis factor-␣ production. Considered together, these two studies employing different methods of blocking lymphocyte apoptosis provide compelling evidence that immunodepression resulting from the loss of lymphocytes is a central pathogenic event in sepsis, and they challenge the current paradigm that regards sepsis as a disorder resulting from an uncontrolled inflammatory response. Caspase inhibitors may represent a treatment strategy in this highly lethal disorder.apoptosis ͉ shock ͉ endotoxin ͉ mortality ͉ programmed cell death

Dendritic cells (DCs) are a group of APCs that have an extraordinary capacity to interact with T and B cells and modulate their responses to invading pathogens. Although a number of defects in the immune system have been identified in sepsis, few studies have examined the effect of sepsis on DCs, which is the purpose of this study. In addition, this study investigated the effect of sepsis on macrophages, which are reported to undergo apoptosis, and MHC II expression, which has been noted to be decreased in sepsis. Spleens from 26 septic patients and 20 trauma patients were evaluated by immunohistochemical staining. Although sepsis did not decrease the number of macrophages, sepsis did cause a dramatic reduction in the percentage area of spleen occupied by FDCs, i.e., 2.9 ± 0.4 vs 0.7 ± 0.2% in trauma and septic patients, respectively. The number of MHC II-expressing cells, including interdigitating DCs, was decreased in septic, compared with trauma, patients. However, sepsis did not appear to induce a loss of MHC II expression in those B cells, macrophages, or DCs that were still present. The dramatic loss of DCs in sepsis may significantly impair B and T cell function and contribute to the immune suppression that is a hallmark of the disorder.

In sepsis, both necrotic and apoptotic cell death can occur. Apoptotic cells induce anergy that could impair the host response, whereas necrotic cells cause immune activation that might result in enhanced antimicrobial defenses. We determined whether adoptive transfer of apoptotic or necrotic cells impacted survival in a clinically relevant sepsis model. We also evaluated the effects of adoptive transfer of apoptotic or necrotic cells on the prototypical TH1 and TH2 cytokines IFN-␥ and IL-4, respectively. C57BL6͞J mice had adoptive transfer of apoptotic (irradiated) or necrotic (freeze thaw) splenocytes. Controls received saline. Apoptotic cells greatly increased mortality, whereas necrotic splenocytes markedly improved survival, P < 0.05. The contrasting effects that apoptotic or necrotic cells exerted on survival were mirrored by opposite effects on splenocyte IFN-␥ production with greatly decreased and increased production, respectively. Importantly, either administration of anti-IFN-␥ antibodies or use of IFN-␥ knockout mice prevented the survival benefit occurring with necrotic cells. This study demonstrates that the type of cell death impacts survival in a clinically relevant model and identifies a mechanism for the immune suppression that is a hallmark of sepsis. Necrotic cells (and likely apoptotic cells) exert their effects via modulation of IFN-␥. Sepsis is the leading cause of death in many intensive-care units and currently ranks as the 12th most common cause of death in America (1). Septic patients are severely immune suppressed as typified by their loss of delayed type hypersensitivity, inability to eradicate their primary infection, and a predisposition to develop secondary nosocomial infections (2-6). A feature illustrative of the immune suppression in septic patients is their failure to respond to skin testing with antigens derived from microbes to which previous exposure occurred (positive controls) (2, 7). Animal studies indicate that the immune defect in sepsis may be critical to the pathogenesis and resultant mortality (8-10). Evidence to support this contention is also provided by a recent clinical trial using IFN-␥. Administration of this cytokine, which is a potent macrophage activator and an inducer of the TH1 response, improved survival in patients with sepsis (11).A number of defects in the immune system have been reported in sepsis. These abnormalities include a shift from a proinflammatory TH1 to an antiinflammatory TH2 lymphocyte profile, a loss in cellular MHC II expression, and a profound apoptosisinduced depletion of CD4 T and B cells (5,(11)(12)(13)(14)(15). The sepsis-induced apoptosis of lymphocytes may be particularly important not only because of the extensive lymphocyte loss but also because of a potential immunosuppressive effect of apoptotic cells on the immune system. Recent work has demonstrated that uptake of apoptotic cells by phagocytic cells stimulates immune tolerance by the release of antiinflammatory cytokines and suppression of release of proinflammatory cytokin...

Dendritic cells are a phenotypically diverse group of APC that have unique capabilities to regulate the activity and survival of B and T cells. Although proper function of dendritic cells is essential to host control of invading pathogens, few studies have examined the impact of sepsis on dendritic cells. The purpose of this study was to determine the effect of sepsis on splenic interdigitating dendritic cells (IDCs) and follicular dendritic cells (FDCs) using a clinically relevant animal model. Immunohistochemical staining for FDCs showed that sepsis induced an initial marked expansion in FDCs that peaked at 36 h after onset. The FDCs expanded to fill the entire lymphoid zone otherwise occupied by B cells. Between 36 and 48 h after sepsis, there was a profound caspase 3 mediated apoptosis induced depletion of FDCs such that only a small contingent of cells remained. In contrast to the initial increase in FDCs, IDC numbers were decreased to ∼50% of control by 12 h after onset of sepsis. IDC death occurred by caspase 3-mediated apoptosis. Such profound apoptosis induced loss of FDCs and IDCs may significantly compromise B and T cell function and impair the ability of the host to survive sepsis.