In this murine model, inhibiting gut epithelial apoptosis by overexpression of Bcl-2 was associated with a survival advantage in P aeruginosa pneumonia-induced sepsis. These results suggest that intestinal epithelial apoptosis may play a role in sepsis-related mortality.

IL-15 is a pluripotent antiapoptotic cytokine that signals to cells of both the innate and adaptive immune system and is regarded as a highly promising immunomodulatory agent in cancer therapy. Sepsis is a lethal condition in which apoptosis-induced depletion of immune cells and subsequent immunosuppression are thought to contribute to morbidity and mortality. This study tested the ability of IL-15 to block apoptosis, prevent immunosuppression, and improve survival in sepsis. Mice were made septic using cecal ligation and puncture or Pseudomonas aeruginosa pneumonia. The experiments comprised a 2 × 2 full factorial design with surgical sepsis versus sham and IL-15 versus vehicle. In addition to survival studies, splenic cellularity, canonical markers of activation and proliferation, intracellular pro- and antiapoptotic Bcl-2 family protein expression, and markers of immune cell apoptosis were evaluated by flow cytometry. Cytokine production was examined both in plasma of treated mice and splenocytes that were stimulated ex vivo. IL-15 blocked sepsis-induced apoptosis of NK cells, dendritic cells, and CD8 T cells. IL-15 also decreased sepsis-induced gut epithelial apoptosis. IL-15 therapy increased the abundance of antiapoptotic Bcl-2 while decreasing proapoptotic Bim and PUMA. IL-15 increased both circulating IFN-γ, as well as the percentage of NK cells that produced IFN-γ. Finally, IL-15 increased survival in both cecal ligation and puncture and P. aeruginosa pneumonia. In conclusion, IL-15 prevents two immunopathologic hallmarks of sepsis, namely, apoptosis and immunosuppression, and improves survival in two different models of sepsis. IL-15 represents a potentially novel therapy of this highly lethal disorder.

Sepsis continues to cause significant morbidity and mortality in critically ill patients. Studies of patients and animal models have revealed that changes in the immune response during sepsis play a decisive role in the outcome. Using a clinically relevant two-hit model of sepsis, i.e., cecal ligation and puncture (CLP) followed by the induction of Pseudomonas aeruginosa pneumonia, we characterized the host immune response. Second, AS101 [ammonium trichloro(dioxoethylene-o,o)tellurate], a compound that blocks interleukin 10 (IL-10), a key mediator of immunosuppression in sepsis, was tested for its ability to reverse immunoparalysis and improve survival. Mice subjected to pneumonia following CLP had different survival rates depending upon the timing of the secondary injury. Animals challenged with P. aeruginosa at 4 days post-CLP had ϳ40% survival, whereas animals challenged at 7 days had 85% survival. This improvement in survival was associated with decreased lymphocyte apoptosis, restoration of innate cell populations, increased proinflammatory cytokines, and restoration of gamma interferon (IFN-␥) production by stimulated splenocytes. These animals also showed significantly less P. aeruginosa growth from blood and bronchoalveolar lavage fluid. Importantly, AS101 improved survival after secondary injury 4 days following CLP. This increased survival was associated with many of the same findings observed in the 7-day group, i.e., restoration of IFN-␥ production, increased proinflammatory cytokines, and decreased bacterial growth. Collectively, these studies demonstrate that immunosuppression following initial septic insult increases susceptibility to secondary infection. However, by 7 days post-CLP, the host's immune system has recovered sufficiently to mount an effective immune response. Modulation of the immunosuppressive phase of sepsis may aid in the development of new therapeutic strategies.

Lymphocyte apoptosis plays a central role in the pathophysiology of sepsis. Lymphocyte apoptosis was examined in mice with defective death receptor pathways due to transgenic expression of a dominant negative mutant of Fas-associated death domain (FADD-DN) or Bid-/- and in mice with defective mitochondrial-mediated pathways due to loss of Bim-/-, Puma-/-, or Noxa-/-. FADD-DN transgenic and Bid-/- mice had significant albeit incomplete protection, and this protection was associated with increased survival. Surprisingly, splenic B cells were also protected in FADD-DN mice although transgene expression was confined to T cells, providing evidence for an indirect protective mechanism. Bim-/- provided virtually complete protection against lymphocyte apoptosis whereas Puma-/- and Noxa-/- mice had modest or no protection, respectively. Bim-/- mice had improved survival, and adoptive transfer of splenocytes from Bim-/- mice into Rag 1-/- mice demonstrated that this was a lymphocyte intrinsic effect. The improved survival was associated with decreased interleukin (IL) -10 and IL-6 cytokines. Collectively, these data indicate that numerous death stimuli are generated during sepsis, and it therefore appears unlikely that blocking a single "trigger" can inhibit apoptosis. If siRNA becomes practical therapeutically, proapoptotic proteins would be potential targets.

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...