C. S. Chung

Inhibition of Fas/Fas ligand signaling improves septic survival: differential effects on macrophage apoptotic and functional capacity.

Signaling through the Fas/Fas ligand (FasL) pathway plays a central role in immune‐cell response and function; however, under certain pathological conditions such as sepsis, it may contribute to the animals or patients morbidity and mortality. To determine the contribution of FasL to mortality, we conducted survival studies by blocking Fas/FasL with Fas receptor fusion protein (FasFP) in vivo. C3H/HeN mice received FasFP or the saline vehicle (veh) immediately (0 h) or delayed (12 h), after sepsis induced by cecal ligation and puncture (CLP). Subsequently, we examined the effect of FasFP treatment (12 h post‐CLP) on macrophage apoptosis and functional capacities. Peritoneal and splenic macrophages and Kupffer cells from sham‐veh‐, CLP‐veh‐, sham‐FasFP‐, or CLP‐FasFP‐treated mice were harvested 24 h after CLP and stimulated with lipopolysaccharide (LPS) for 24 h. The results indicate that only delayed (12 h) but not 0 h administration of FasFP demonstrated a significant increase in survival. The ability of all macrophage populations to release interleukin (IL)‐6 was significantly depressed, and IL‐10 release was augmented after CLP. FasFP treatment attenuated the increased IL‐10 release in Kupffer cells. However, althogh enhanced susceptibility to LPS‐induced apoptosis could be suppressed in CLP mouse Kupffer cells by FasFP, FasFP did not change the peritoneal or splenic macrophage response. Furthermore, FasFP attenuated the elevated plasma levels of liver enzymes after sepsis. These data indicate that in vivo inhibition of Fas/FasL signaling has tissue‐specific effects on the induction of macrophage apoptosis, functional changes, and liver damage, which may contribute to the hosts ability to ward off a septic challenge.

Increased inducible apoptosis in CD4+ T lymphocytes during polymicrobial sepsis is mediated by Fas ligand and not endotoxin

Recent studies suggest that increased lymphocyte apoptosis (Ao) detected in peripheral blood T cells from burn patients appears to contribute to decreased lymphocyte immunoresponsiveness. However, while it is known that sepsis induces a marked depression in the splenocyte immune response (i.e. decreased interleukin‐2, interferon‐γ production and proliferation) in response to the T‐cell mitogen concanavalin A (Con A), it is unknown whether this depression is associated with an increase in inducible Ao and if so, which mediators control this process. To assess this, splenocytes were harvested from mice at 24 hr (a period associated with decreased Con A response) after the onset of polymicrobial sepsis [caecal ligation and puncture (CLP)] or sham‐CLP (Sham) and then stimulated with 2·5 μg Con A/ml (24 hr). Septic mouse splenocytes stimulated with Con A, while not showing a change in their phenotypic make‐up, did exhibit a marked increase in the percentage of splenocyte that were Ao+ which was associated with altered cytokine release. This appears to be due to an increase in the percentage of Ao+ cells in the CD4+ CD8− population and was associated with enhanced Fas antigen expression as well as an increase in mRNA for the Fas–FasL gene family. To determine if the changes in Ao are due to either endotoxin (a product of Gram‐negative bacteria seen in CLP mice) or the expression of Fas ligand (FasL; a mediator of activation‐induced lymphocyte Ao), a second set of studies examining Con A‐inducible Ao was performed with splenocytes harvested from septic endotoxin‐tolerant C3H/HeJ and the FasL‐deficient C3H/HeJ‐Fasl gld mice. The results show that increased splenocyte Ao detected following CLP is due to a FasL‐mediated process and not to endotoxin. Thus the inadvertent up‐regulation of FasL‐mediated splenocyte Ao may contribute to the depression of splenocyte immune responses seen during polymicrobial sepsis.

Epithelial cell apoptosis and neutrophil recruitment in acute lung injury-a unifying hypothesis? What we have learned from small interfering RNAs.

In spite of protective ventilatory strategies, Acute Lung Injury (ALI) remains associated with high morbidity and mortality. One reason for the lack of therapeutic options might be that ALI is a co-morbid event associated with a diverse family of diseases and, thus, may be the result of distinct pathological processes. Among them, activated neutrophil- (PMN-) induced tissue injury and epithelial cell apoptosis mediated lung damage represent two potentially important candidate pathomechanisms that have been put forward. Several approaches have been undertaken to test these hypotheses, with substantial success in the treatment of experimental forms of ALI. With this in mind, we will summarize these two current hypotheses of ALI briefly, emphasizing the role of apoptosis in regulating PMN and/or lung epithelial cell responses. In addition, the contribution that Fas-mediated inflammation may play as a potential biological link between lung cell apoptosis and PMN recruitment will be considered, as well as the in vivo application of small interfering RNA (siRNA) as a novel approach to the inhibition of ALI and its therapeutic implications.

ACTIVATION OF COMMON ANTIVIRAL PATHWAYS CAN POTENTIATE INFLAMMATORY RESPONSES TO SEPTIC SHOCK

ABSTRACT Induction of the antiviral cytokine interferon &agr;/&bgr; (IFN-&agr;/&bgr;) is common in many viral infections. The impact of ongoing antiviral responses on subsequent bacterial infection is not well understood. In human disease, bacterial superinfection complicating a viral infection can result in significant morbidity and mortality. We injected mice with polyinosinic-polycytidylic (PIC) acid, a TLR3 ligand and known IFN-&agr;/&bgr; inducer as well as nuclear factor &kgr;B (NF-&kgr;B) activator to simulate very early antiviral pathways. We then challenged mice with an in vivo septic shock model characterized by slowly evolving bacterial infection to simulate bacterial superinfection early during a viral infection. Our data demonstrated robust induction of IFN-&agr; in serum within 24 h of PIC injection with IFN-&agr;/&bgr;-dependent major histocompatibility antigen class II up-regulation on peritoneal macrophages. PIC pretreatment before septic shock resulted in augmented tumor necrosis factor alpha and interleukins 6 and 10 and heightened lethality compared with septic shock alone. Intact IFN-&agr;/&bgr; signaling was necessary for augmentation of the inflammatory response to in vivo septic shock and to both TLR2 and TLR4 agonists in vitro. To assess the NF-&kgr;B contribution to PIC-modulated inflammatory responses to septic shock, we treated with parthenolide, an NF-&kgr;B inhibitor before PIC and septic shock. Parthenolide did not inhibit IFN-&agr; induction by PIC. Inhibition of NF-&kgr;B by parthenolide did reduce IFN-&agr;-mediated potentiation of the cytokine response and lethality from septic shock. Our data demonstrate that pathways activated early during many viral infections can have a detrimental impact on the outcome of subsequent bacterial infection. These pathways may be critical to understanding the heightened morbidity and mortality from bacterial superinfection after viral infection in human disease.

Lymphocyte Activation, Anergy, and Apoptosis in Polymicrobial Sepsis

Sepsis and multiple organ failure (MOF) are reported to be responsible for up-wards of 60% of the deaths which occur in the surgical intensive care unit [1], de- spite the use of specific antibiotics, aggressive operative intervention and nutritional support, and also the use of new therapeutic agents in clinical trails, such as antibodies to endotoxin and the pro-inflammatory cytokines [1, 2]. A number of studies have suggested that the link between cell and organ dysfunction associated with MOF lies in the initial presentation of sepsis (Gram-negative, Grampositive, and/or fungal in nature) [3]. In this respect, it has been reported that patients exposed to various forms of trauma and soft tissue injury as well as those with sepsis exhibit a broad spectrum of alterations in both innate and acquired immunity which are eventually deleterious to host survival.

The Apoptotic Response of the Lymphoid Immune System to Trauma, Shock, and Sepsis

Despite the rapid development and many significant advances in preventive methods and clinical care, trauma remains the most common cause of mortality during the first thirty years of life. It has been reported that approximately 50% of the deaths occur due to exsanguination or central nervous system complications within one hour following trauma and another 30% of victims die during the following 1–2 hours due to major internal injury [1]. Of those patients who survive the initial insult, more than 50% die over the next few days to week(s) due to secondary infections (i.e., sepsis) and/or multiple organ dysfunction [2]. Studies have indicated that there are marked alterations in both innate and acquired immune functions in patients exposed to major trauma such as thermal injury, soft tissue trauma or prolonged surgical procedures. These alterations include suppressed neutrophil bacterial killing function, defective opsonization, altered macrophage function, decreased humoral immunity, suppression of T- and B-lymphocyte responsiveness and altered pro- and anti-inflammatory mediator/cytokine release [3]. A number of studies indicate that traumatic injury and/or shock result in profound immunosuppression which predisposes the patients to septic infection and/or multiple organ failure (MOF).

A Novel Role for Programmed Cell Death Receptor Ligand-1 (PD-L1) in Sepsis-Induced Intestinal Dysfunction.

Studies imply that intestinal barrier dysfunction is a key contributor to morbid events associated with sepsis. Recently, the co-inhibitory molecule programmed death-ligand1 (PD-L1) has been shown to be involved in the regulation of intestinal immune tolerance and/or inflammation. Our previous studies showed that PD-L1 gene deficiency reduced sepsis-induced intestinal injury morphologically. However, it is not known how PD-L1 expression impacts intestinal barrier dysfunction during sepsis. Here we tested the hypothesis that PD-L1 expressed on intestinal epithelial cells (IECs) has a role in sepsis-induced intestinal barrier dysfunction. To address this, C57BL/6 or PD-L1 gene knockout mice were subjected to experimental sepsis and PD-L1 expression, intestinal permeability and tissue cytokine levels were assessed. Subsequently, septic or nonseptic colonic samples (assigned by pathology report) were immunohistochemically stained for PD-L1 in a blinded fashion. Finally, human Caco2 cells were used for in vitro studies. The results demonstrated that PD-L1 was constitutively expressed and sepsis significantly upregulates PD-L1 in IECs from C57BL/6 mice. Concurrently, we observed increased PD-L1 expression in colon tissue samples from septic patients. PD-L1 gene deficiency reduced ileal permeability and tissue levels of IL-6, TNF-α and MCP-1, and prevented ileal tight junction protein loss compared with WT after sepsis. Comparatively, while Caco2 cell monolayers also responded to inflammatory cytokine stimulation with elevated PD-L1 expression, increased monolayer permeability and altered/decreased monolayer tight junction protein morphology/expression, these changes were reversed by PD-L1 blocking antibody. Together these data indicate that ligation of PD-L1 plays a novel role in mediating the pathophysiology of sepsis-induced intestinal barrier dysfunction.