CD4+ lymphocytes control gut epithelial apoptosis and mediate survival in sepsis

Liensinine alleviates mouse intestinal injury induced by sepsis through inhibition of oxidative stress, inflammation, and cell apoptosis

Sepsis is a clinical syndrome triggered by an imbalanced host response to pathogens that can lead to multiple organ dysfunction. The immune response and barrier function of the gut play an important role in the pathogenesis and progression of sepsis. This study aimed to explore the potential role of natural alkaloid Liensinine in the treatment of intestinal injury caused by sepsis and its possible molecular mechanism. In this study, a mouse model of sepsis was established by injecting LPS to explore the protective effect of Liensinine on intestinal injury in sepsis. The results showed that Liensinine could reduce the intestinal damage caused by LPS and increase the number of goblet cells. Furthermore, it decreased the release of inflammatory cytokines by inhibiting NF-kB phosphorylation and NLRP3 inflammasome synthesis. Liensinine also reduced the oxidative stress and ROS accumulation caused by LPS, and played an anti-oxidative stress role by regulating the Nrf2/keap1 signaling pathway. In addition, Liensinine alleviated the inhibition of intestinal autophagy caused by LPS by inhibiting the PI3K/Akt/mTOR pathway. And then it reduced the excessive apoptosis of intestinal cells. This study provides valuable insights for sepsis prevention and treatment, offering a potential therapeutic candidate to protect against intestinal injury and regulate the inflammatory response in sepsis.

Overexpression of BCL-2 in the Intestinal Epithelium Prevents Sepsis-Induced Gut Barrier Dysfunction via Altering Tight Junction Protein Expression

Sepsis induces both intestinal hyperpermeability and epithelial apoptosis. While each has been implicated in mediating sepsis mortality, the relationship between these two processes is unclear. We hypothesized that preventing intestinal apoptosis would prevent gut barrier dysfunction. To test this hypothesis, transgenic mice that overexpress the anti-apoptotic protein Bcl-2 in the gut epithelium (Fabpl-Bcl-2 mice) and wild-type (WT) mice were subjected to sham laparotomy or cecal ligation and puncture and orally gavaged with fluorescein isothiocyanate conjugated-dextran (FD-4) 5 h before sacrifice. Serum FD-4 concentration was assayed to measure intestinal permeability, and jejunal tight junctions were assayed for mRNA and protein expression. Baseline FD-4 concentration was similar between WT and Fabpl-Bcl-2 mice. Intestinal permeability increased 6, 12, 24, and 48 h following sepsis in WT mice; however, FD-4 concentration was significantly lower at each timepoint in Fabpl-Bcl-2 mice. In addition, there were no statistically significant changes in permeability between septic and sham transgenic mice. Intestinal mRNA expression of claudin 3, claudin 5, and occludin was lower in septic Fabpl-Bcl-2 mice, while claudin 4 mRNA levels were higher in Fabpl-Bcl-2 mice. In contrast, no differences were detected in claudins 2, 7, 15, JAM-A, or ZO-1. Protein levels followed the same trend for all tight junction mediators different between WT and Fabpl-Bcl-2 mice except occludin was significantly higher in transgenic mice. Together these results demonstrate that decreasing intestinal epithelial apoptosis prevents hyperpermeability following sepsis via tight junction alterations which may be at least partially responsible for improved survival conferred by Bcl-2 overexpression.

Unmasking Unique Immune Altering Aspects of the Microbiome as a Tool to Correct Sepsis-Induced Immune Dysfunction

Background: Sepsis-related mortality is driven by immune dysfunction. A bidirectional micro-organism-immune cell cross talks exists. Gut Bacteroides fragilis-T-cell crosstalk maintains innate immune cell/pathogen homeostasis. Commensal gut Clostridia spp. suppress inflammation and induce gut tolerance. Probiotics are administered to restore immune microbiome homeostasis. Individual microbial components have an immunomodulatory effect. However, probiotic therapies for sepsis-induced immune disruptions are rarely tailored to specific immune responses. Thus, we ask the question as to how components of the intestinal microbiome, often found in probiotic therapies, affect lymphocyte phenotypic profile? Methods: T-lymphocytes were cultured with either monomicrobial or polymicrobial combinations. Microbes used were Bacteroides fragilis, Clostridium perfringens, or Lactobacillus acidophilus. Cytokines, measured by enzyme-linked immunosorbent assay (ELISA)-included interleukin (IL)-6, IL-10, IL-22, and IL-33. Flow cytometry was used for T-cell phenotyping for program-death receptor-1 (PD-1) and B- and T-lymphocyte attenuator (BTLA). T-cell DNA was extracted to assess global epigenetic changes. For translation, IL-33 was measured from surgical intensive care unit (ICU) patients with sepsis with either monomicrobial or polymicrobial infection. Results: Lactobacillus consistently induced IL-22 and IL-33. Bacteroides fragilis induced IL-33 only under polymicrobial (pB) conditions. Within surgical ICU patients, IL-33 levels were higher in polymicrobial versus monomicrobial patients. PD-1⁺ expression was lowest with either monomicrobial Bacteroides fragilis or Bacteroides fragilis predominant polymicrobial context. Conversely Bacteroides fragilis exposure induced a distinct PD-1^-high subpopulation. B- and T-lymphocyte attenuator-positive expression did not differ after individual microbes. Among polymicrobial conditions, Bacteroides fragilis predominant (pB) and Lactobacillus acidophilus predominant (pL) increased BTLA⁺ expression. DNA methylation was most increased in response to Clostridium perfringens in monomicrobial and in response to Bacteroides fragilis in polymicrobial conditions. Conclusion: Unique microbe/lymphocyte interactions occur. Bacteroides fragilis induced a T-cell phenotype consistent with potential long-term immune recovery. This work begins to discover how varying microbes may induce unique functional and phenotypic T-lymphocyte responses.

TL1A Aggravates Cytokine-Induced Acute Gut Inflammation and Potentiates Infiltration of Intraepithelial Natural Killer Cells in Mice

BACKGROUND

The tumor necrosis factor alpha (TNFα)-homologous cytokine TL1A is emerging as a major player in intestinal inflammation. From in vitro experiments on human lymphocytes, TNF-like molecule 1A (TL1A) is known to activate a highly inflammatory lymphoid response in synergy with interleukin (IL)-12 and IL-18. Carriers of specific genetic polymorphisms associated with IL-12, IL-18, or TL1A signaling have increased Crohn's disease risk, and all 3 cytokines are upregulated during active disease. The study aim was to investigate whether the type 1-polarizing cytokines IL-12 and IL-18 could directly initiate intestinal pathology in mice and how TL1A would influence the resulting inflammatory response.

METHODS

Conventional barrier-bred and germ-free mice were randomly allocated to different groups and injected twice with different combinations of IL-12, IL-18, and TL1A, and killed 3 days after the first injection. All treatment groups were co-housed and fed a piroxicam-supplemented chow diet.

RESULTS

Intestinal pathology was evident in IL-12- and IL-18-treated mice and highly exacerbated by TL1A in both the colon and ileum. The cytokine-induced intestinal inflammation was characterized by epithelial damage, increased colonic levels of TNFα, IL-1β, IFN-γ, and IL-6, and various chemokines along with gut microbiota alterations exhibiting high abundance of Enterobacteriaceae. Furthermore, the inflamed ileum and colon exhibited a TL1A-specific increased infiltration of intraepithelial natural killer cells co-expressing NKG2D and IL-18Ra and a higher frequency of unconventional T cells in the colonic epithelium. Upon cytokine injection, germ-free mice exhibited similar intraepithelial lymphoid infiltration and increased colonic levels of IFNγ and TNFα.

CONCLUSIONS

This study demonstrates that TL1A aggravates IL-12- and IL-18-induced intestinal inflammation in the presence and absence of microbiota.

Mechanisms of Organ Dysfunction in Sepsis

Sepsis-associated organ dysfunction involves multiple responses to inflammation, including endothelial and microvascular dysfunction, immune and autonomic dysregulation, and cellular metabolic reprogramming. The effect of targeting these mechanistic pathways on short- and long-term outcomes depends highly on the timing of therapeutic intervention. Furthermore, there is a need to understand the adaptive or maladaptive character of these mechanisms, to discover phase-specific biomarkers to guide therapy, and to conceptualize these mechanisms in terms of resistance and tolerance.

Lymphocyte integrin expression differences between SIRS and sepsis patients

BACKGROUND

Systemic Inflammatory Response Syndrome (SIRS) and sepsis remain leading causes of death. Despite many similarities, the two entities are very distinct clinically and immunologically. T-Lymphocytes play a key pivotal role in the pathogenesis and ultimately outcome following both SIRS and sepsis. Integrins are essential in the trafficking and migration of lymphocytes. They also serve vital roles in efficient wound healing and clearance of infections. Here, we investigate whether integrin expression, specifically β1 (CD29) and β2 (CD18), are disrupted in SIRS and sepsis, and assess differences in integrin expression between these two critically ill clinical categories.

METHODS

T-Lymphocytes were isolated from whole blood collected from ICU patients exhibiting SIRS or sepsis. Samples were analyzed for CD18 (β2) and CD29 (β1) on CD3⁺ T cells through flow cytometry. Septic patients were stratified into either exclusively abdominal or non-abdominal sources of sepsis.

RESULTS

CD18 was almost ubiquitously expressed on CD3⁺ T cells irrespective of clinical condition. However, CD29 (β1 integrin) was lowest in SIRS patients (20.4% of CD3⁺ T cells) when compared with either septic patients (35.5%) or healthy volunteers (54.1%). Furthermore, there was evidence of compartmentalization in septic patients, where abdominal sources had a greater percentage of CD3⁺CD29⁺ T cells (41.7%) when compared with those with non-abdominal sources (29.5%).

CONCLUSION

Distinct differences in T-cell integrin expression exists between patients in SIRS versus sepsis, as well as relative to the source of sepsis. Further work is needed to understand cause and effect relative to the progression from SIRS into sepsis.

Adenosine promotes Foxp3 expression in Treg cells in sepsis model by activating JNK/AP-1 pathway

OBJECTIVES

Forkhead/winged helix transcription factor p3 (Foxp3) increases in CD4(+)CD25(+)Treg cells during sepsis; however, related mechanisms are unclear. Our study aimed to explore the possible molecular mechanisms of high expression of Foxp3 in Treg cells during sepsis.

METHODS

Sepsis was induced by cecal ligation and puncture (CLP) method. CD4(+)CD25(+)Treg cells were isolated from peripheral blood and identified by flow cytometry (FCM). Treg cells were cultured with or without adenosine, adenosine agonist, adenosine antagonist, SMAD family member 3 (Smad3) agonist (transforming growth factor (TGF)-β1), or C-Jun N-Terminal Kinase (JNK) inhibitor. Expression levels of Foxp3 and activator protein 1 (AP-1) were determined. The binding of c-Fos or c-Jun to the Foxp3 promoter was then evaluated by the chromatin immunoprecipitation (ChIP) assay and quantified by quantitative real-time PCR (qRT-PCR). The mRNA and protein levels of Foxp3 were determined after transfection with siRNA against c-Fos, Fra-2, c-Jun or JunD.

RESULTS

Pharmacological inhibition of both adenosine and JNK reduced Foxp3 protein levels. JNK/AP-1 activation was involved in increased levels of Foxp3 protein in CD4(+)CD25(+)Treg cells. AP-1 regulated activity of Foxp3 promoter in Treg cells, and the induction of c-Fos or c-Jun activity leads to elevated transcription of Foxp3 gene. Knockdown of c-Fos, Fra-2, c-Jun, or JunD levels also reduced Foxp3 expression.

CONCLUSION

We confirm that adenosine plays significant roles in the high expression of Foxp3. Adenosine promotes Foxp3 expression in Treg cells during sepsis via JNK/AP-1 pathway.

The Gut as the Motor of Multiple Organ Dysfunction in Critical Illness

All elements of the gut - the epithelium, the immune system, and the microbiome - are impacted by critical illness and can, in turn, propagate a pathologic host response leading to multiple organ dysfunction syndrome. Preclinical studies have demonstrated that this can occur by release of toxic gut-derived substances into the mesenteric lymph where they can cause distant damage. Further, intestinal integrity is compromised in critical illness with increases in apoptosis and permeability. There is also increasing recognition that microbes alter their behavior and can become virulent based upon host environmental cues. Gut failure is common in critically ill patients; however, therapeutics targeting the gut have proven to be challenging to implement at the bedside. Numerous strategies to manipulate the microbiome have recently been used with varying success in the ICU.

Rough-Form Lipopolysaccharide Increases Apoptosis in Human CD4⁺ and CD8⁺ T Lymphocytes

Immunosuppression induced by lymphocyte apoptosis is considered an important factor in the pathogenesis of sepsis and has been demonstrated in both animal models of lipopolysaccharide (LPS)-induced endotoxemia and septic patients. As rough-form LPS (R-LPS) has recently been shown to elicit a stronger immunological response than regular smooth-form LPS (S-LPS), we aimed to assess the apoptosis-inducing capabilities of R-LPS in different subsets of lymphocytes (CD4(+) T cells, CD8(+) T cell, B cells and NK cells). Using multicolour flow cytometry on human peripheral blood mononuclear cells, we found that R-LPS increased apoptosis in CD4(+) and CD8(+) T cells assessed by annexin V and propidium iodide (AV(+) PI(-)), compared with both S-LPS-stimulated and unstimulated cells. 7-Amino-actinomycin D and staining for intracellular active caspase-3, which are considered later signs of apoptosis, did not reveal the same results. Both forms appeared to inhibit apoptosis in B cells, but no LPS-form-specific effect was seen on B or NK cells. Our results indicate that R-LPS induces a stronger AV(+) PI(-)-assessed apoptotic response in T cells than S-LPS. Our findings emphasize the importance of T cell apoptosis in endotoxemia and advocates for control of LPS form in both endotoxemia research and clinical trials with Gram-negative infections.

Immunosuppression after sepsis: systemic inflammation and sepsis induce a loss of naïve T-cells but no enduring cell-autonomous defects in T-cell function

Sepsis describes the life-threatening systemic inflammatory response (SIRS) of an organism to an infection and is the leading cause of mortality on intensive care units (ICU) worldwide. An acute episode of sepsis is characterized by the extensive release of cytokines and other mediators resulting in a dysregulated immune response leading to organ damage and/or death. This initial pro-inflammatory burst often transits into a state of immune suppression characterised by loss of immune cells and T-cell dysfunction at later disease stages in sepsis survivors. However, despite these appreciations, the precise nature of the evoked defect in T-cell immunity in post-acute phases of SIRS remains unknown. Here we present an in-depth functional analysis of T-cell function in post-acute SIRS/sepsis. We document that T-cell function is not compromised on a per cell basis in experimental rodent models of infection-free SIRS (LPS or CpG) or septic peritonitis. Transgenic antigen-specific T-cells feature an unaltered cytokine response if challenged in vivo and ex vivo with cognate antigens. Isolated CD4(+)/CD8(+) T-cells from post-acute septic animals do not exhibit defects in T-cell receptor-mediated activation at the the level of receptor-proximal signalling, activation marker upregulation or expansion. However, SIRS/sepsis induced transient lymphopenia and gave rise to an environment of immune attenuation at post acute disease stages. Thus, systemic inflammation has an acute impact on T-cell numbers and adaptive immunity, but does not cause major cell-autonomous enduring functional defects in T-cells.

Intestine-specific deletion of microsomal triglyceride transfer protein increases mortality in aged mice

Background

Mice with conditional, intestine-specific deletion of microsomal triglyceride transfer protein (Mttp-IKO) exhibit a complete block in chylomicron assembly together with lipid malabsorption. Young (8–10 week) Mttp-IKO mice have improved survival when subjected to a murine model of Pseudomonas aeruginosa-induced sepsis. However, 80% of deaths in sepsis occur in patients over age 65. The purpose of this study was to determine whether age impacts outcome in Mttp-IKO mice subjected to sepsis.

Methods

Aged (20–24 months) Mttp-IKO mice and WT mice underwent intratracheal injection with P. aeruginosa. Mice were either sacrificed 24 hours post-operatively for mechanistic studies or followed seven days for survival.

Results

In contrast to young septic Mttp-IKO mice, aged septic Mttp-IKO mice had a significantly higher mortality than aged septic WT mice (80% vs. 39%, p = 0.005). Aged septic Mttp-IKO mice exhibited increased gut epithelial apoptosis, increased jejunal Bax/Bcl-2 and Bax/Bcl-X_L ratios yet simultaneously demonstrated increased crypt proliferation and villus length. Aged septic Mttp-IKO mice also manifested increased pulmonary myeloperoxidase levels, suggesting increased neutrophil infiltration, as well as decreased systemic TNFα compared to aged septic WT mice.

Conclusions

Blocking intestinal chylomicron secretion alters mortality following sepsis in an age-dependent manner. Increases in gut apoptosis and pulmonary neutrophil infiltration, and decreased systemic TNFα represent potential mechanisms for why intestine-specific Mttp deletion is beneficial in young septic mice but harmful in aged mice as each of these parameters are altered differently in young and aged septic WT and Mttp-IKO mice.

Receptor-interacting protein kinase 3 deficiency inhibits immune cell infiltration and attenuates organ injury in sepsis

INTRODUCTION

Sepsis is defined as a systemic hyper-inflammatory immune response, with a subsequent immune-suppressive phase, which leads to multiple organ dysfunction and late lethality. Receptor-interacting protein kinase 3 (RIPK3)-dependent necrosis is implicated in driving tumor necrosis factor alpha (TNF-α)- and sepsis-induced mortality in mice. However, it is unknown if RIPK3 deficiency has any impact on immune cell trafficking, which contributes to organ damage in sepsis.

METHODS

To study this, male wild-type (WT) and RIPK3-deficient (Ripk3-/-) mice on C57BL/6 background were subjected to sham operation or cecal ligation and puncture (CLP)-induced sepsis. Blood and tissue samples were collected 20 hours post-CLP for various measurements.

RESULTS

In our severe sepsis model, the mean survival time of Ripk3-/- mice was significantly extended to 68 hours compared to 41 hours for WT mice. Ripk3-/- mice had significantly decreased plasma levels of TNF-α and IL-6 and organ injury markers compared to WT mice post-CLP. In the lungs, Ripk3-/- mice preserved better integrity of microscopic structure with reduced apoptosis, and decreased levels of IL-6, macrophage inflammatory protein (MIP)-2 and keratinocyte-derived chemokine (KC), compared to WT. In the liver, the levels of MIP-1, MIP-2 and KC were also decreased in septic Ripk3-/- mice. Particularly, the total number of neutrophils in the lungs and liver of Ripk3-/- mice decreased by 59.9% and 66.7%, respectively, compared to WT mice post-CLP. In addition, the number of natural killer (NK) and CD8T cells in the liver decreased by 64.8% and 53.4%, respectively, in Ripk3-/- mice compared to WT mice post-sepsis.

CONCLUSIONS

Our data suggest that RIPK3 deficiency modestly protected from CLP-induced severe sepsis and altered the immune cell trafficking in an organ-specific manner attenuating organ injury. Thus, RIPK3 acts as a detrimental factor in contributing to the organ deterioration in sepsis.

Inhibition of IKKβ in enterocytes exacerbates sepsis-induced intestinal injury and worsens mortality

OBJECTIVES

Nuclear factor-κB is a critical regulator of cell-survival genes and the host inflammatory response. The purpose of this study was to investigate the role of enterocyte-specific NF-kB in sepsis through selective ablation of IkB kinase.

DESIGN

Prospective, randomized controlled study.

SETTING

Animal laboratories in university medical centers.

SUBJECTS AND INTERVENTIONS

Mice lacking functional NF-kB in their intestinal epithelium (Vil-Cre/Ikkβ) and wild-type mice were subjected to sham laparotomy or cecal ligation and puncture. Animals were killed at 24 hours or followed 7 days for survival.

MEASUREMENTS AND MAIN RESULTS

Septic wild-type mice had decreased villus length compared with sham mice, whereas villus atrophy was further exacerbated in septic Vil-Cre/Ikkβ mice. Sepsis induced an increase in intestinal epithelial apoptosis compared with sham mice, which was further exacerbated in Vil-Cre/Ikkβ mice. Sepsis induced intestinal hyperpermeability in wild-type mice compared with sham mice, which was further exacerbated in septic Vil-Cre/Ikkβ mice. This was associated with increased intestinal expression of claudin-2 in septic wild-type mice, which was further increased in septic Vil-Cre/Ikkβ mice. Both, pro-inflammatory and anti-inflammatory cytokines were increased in serum following cecal ligation and puncture, and interleukin 10 and monocyte chemoattractant protein-1 levels were higher in septic Vil-Cre/Ikkβ mice than in septic wild-type mice. All septic mice were bacteremic, but no differences in bacterial load were identified between wild-type and Vil-Cre/Ikkβ mice. To determine the functional significance of these results, animals were followed for survival. Septic wild-type mice had lower mortality than septic Vil-Cre/Ikkβ mice (47% vs 80%, p<0.05). Antitumor necrosis factor administration decreased intestinal apoptosis, permeability, and mortality in wild-type septic mice, and a similar improvement in intestinal integrity and survival were seen when antitumor necrosis factor was given to Vil-Cre/Ikkβ mice.

CONCLUSIONS

Enterocyte-specific NF-kB has a beneficial role in sepsis by partially preventing sepsis-induced increases in apoptosis and permeability, which are associated with worsening mortality.

Inhibition of intestinal epithelial apoptosis improves survival in a murine model of radiation combined injury

World conditions place large populations at risk from ionizing radiation (IR) from detonation of dirty bombs or nuclear devices. In a subgroup of patients, ionizing radiation exposure would be followed by a secondary infection. The effects of radiation combined injury are potentially more lethal than either insult in isolation. The purpose of this study was to determine mechanisms of mortality and possible therapeutic targets in radiation combined injury. Mice were exposed to IR with 2.5 Gray (Gy) followed four days later by intratracheal methicillin-resistant Staphylococcus aureus (MRSA). While either IR or MRSA alone yielded 100% survival, animals with radiation combined injury had 53% survival (p = 0.01). Compared to IR or MRSA alone, mice with radiation combined injury had increased gut apoptosis, local and systemic bacterial burden, decreased splenic CD4 T cells, CD8 T cells, B cells, NK cells, and dendritic cells, and increased BAL and systemic IL-6 and G-CSF. In contrast, radiation combined injury did not alter lymphocyte apoptosis, pulmonary injury, or intestinal proliferation compared to IR or MRSA alone. In light of the synergistic increase in gut apoptosis following radiation combined injury, transgenic mice that overexpress Bcl-2 in their intestine and wild type mice were subjected to IR followed by MRSA. Bcl-2 mice had decreased gut apoptosis and improved survival compared to WT mice (92% vs. 42%; p<0.01). These data demonstrate that radiation combined injury results in significantly higher mortality than could be predicted based upon either IR or MRSA infection alone, and that preventing gut apoptosis may be a potential therapeutic target.

Pathways mediating resolution of inflammation: when enough is too much

Patients with critical illness, and in particular sepsis, are now recognized to undergo unifying, pathogenic disturbances of immune function. Whilst scientific and therapeutic focus has traditionally been on understanding and modulating the initial pro-inflammatory limb, recent years have witnessed a refocusing on the development and importance of immunosuppressive 'anti-inflammatory' pathways. Several mechanisms are known to drive this phenomenon; however, no overriding conceptual framework justifies them. In this article we review the contribution of pro-resolution pathways to this phenotype, describing the observed immune alterations in terms of either a failure of resolution of inflammation or the persistence of pro-resolution processes causing inappropriate 'injurious resolution'-a novel hypothesis. The dysregulation of key processes in critical illness, including apoptosis of infiltrating neutrophils and their efferocytosis by macrophages, are discussed, along with the emerging role of specialized cell subtypes Gr1(+) CD11b(+) myeloid-derived suppressor cells and CD4(+) CD25(+) FoxP3(+) T-regulatory cells.

Redefining the gut as the motor of critical illness

The gut is hypothesized to play a central role in the progression of sepsis and multiple organ dysfunction syndrome. Critical illness alters gut integrity by increasing epithelial apoptosis and permeability and by decreasing epithelial proliferation and mucus integrity. Additionally, toxic gut-derived lymph induces distant organ injury. Although the endogenous microflora ordinarily exist in a symbiotic relationship with the gut epithelium, severe physiological insults alter this relationship, leading to induction of virulence factors in the microbiome, which, in turn, can perpetuate or worsen critical illness. This review highlights newly discovered ways in which the gut acts as the motor that perpetuates the systemic inflammatory response in critical illness.

Is suppression of apoptosis a new therapeutic target in sepsis?

Sepsis remains as a leading cause of death in critically ill patients. Unfortunately, there have been very few successful specific therapeutic agents that can significantly reduce the attributable mortality and morbidity of sepsis. Developing novel therapeutic strategies to improve outcomes of sepsis remains an important focus of ongoing research in the field of critical care medicine. Apoptosis has recently been identified as an important mechanism of cell death and evidence suggests that prevention of cell apoptosis can improve survival in animal models of sepsis and endotoxaemia. In this review article, we summarise the critical role of apoptosis of the immune cells in the pathophysiology of sepsis and propose that blocking cell-signaling pathways leading to apoptosis may present a promising specific therapy for sepsis. Various methods to inhibit apoptosis including the cell surface Fas receptor pathway inhibitors, caspase inhibitors, over-expression of anti-apoptotic genes and small interfering ribonucleic acid therapy are discussed.

Foxp3+ regulatory T cells are required for recovery from severe sepsis

The role of regulatory T cells (Tregs) in bacterial sepsis remains controversial because antibody-mediated depletion experiments gave conflicting results. We employed DEREG mice (DEpletion of REGulatory T cells) and a caecal ligation and puncture model to elucidate the role of CD4⁺Foxp3⁺ Tregs in sepsis. In DEREG mice natural Tregs can be visualized easily and selectively depleted by diphtheria toxin because the animals express the diphtheria toxin receptor and enhanced green fluorescent protein as a fusion protein under the control of the foxp3 locus. We confirmed rapid Treg-activation and an increased ratio of Tregs to Teffs in sepsis. Nevertheless, 24 h after sepsis induction, Treg-depleted and control mice showed equally strong inflammation, immune cell immigration into the peritoneum and bacterial dissemination. During the first 36 h of disease survival was not influenced by Treg-depletion. Later, however, only Treg-competent animals recovered from the insult. We conclude that the suppressive capacity of Tregs is not sufficient to control overwhelming inflammation and early mortality, but is a prerequisite for the recovery from severe sepsis.

The endogenous bacteria alter gut epithelial apoptosis and decrease mortality following Pseudomonas aeruginosa pneumonia

The endogenous bacteria have been hypothesized to play a significant role in the pathophysiology of critical illness, although their role in sepsis is poorly understood. The purpose of this study was to determine how commensal bacteria alter the host response to sepsis. Conventional and germ-free (GF) C57Bl/6 mice were subjected to Pseudomonas aeruginosa pneumonia. All GF mice died within 2 days, whereas 44% of conventional mice survived for 7 days (P = 0.001). Diluting the dose of bacteria 10-fold in GF mice led to similar survival in GF and conventional mice. When animals with similar mortality were assayed for intestinal integrity, GF mice had lower levels of intestinal epithelial apoptosis but similar levels of proliferation and intestinal permeability. Germ-free mice had significantly lower levels of tumor necrosis factor and interleukin 1β in bronchoalveolar lavage fluid compared with conventional mice without changes in systemic cytokine production. Under conventional conditions, sepsis unmasks lymphocyte control of intestinal epithelial apoptosis, because sepsis induces a greater increase in gut apoptosis in Rag-1 mice than in wild-type mice. However, in a separate set of experiments, gut apoptosis was similar between septic GF Rag-1 mice and septic GF wild-type mice. These data demonstrate that the endogenous bacteria play a protective role in mediating mortality from pneumonia-induced sepsis, potentially mediated through altered intestinal apoptosis and the local proinflammatory response. In addition, sepsis-induced lymphocyte-dependent increases in gut epithelial apoptosis appear to be mediated by the endogenous bacteria.

Intestine-specific Mttp deletion decreases mortality and prevents sepsis-induced intestinal injury in a murine model of Pseudomonas aeruginosa pneumonia

BACKGROUND

The small intestine plays a crucial role in the pathophysiology of sepsis and has been referred to as the "motor" of the systemic inflammatory response. One proposed mechanism is that toxic gut-derived lipid factors, transported in mesenteric lymph, induce systemic injury and distant organ failure. However, the pathways involved are yet to be defined and the role of intestinal chylomicron assembly and secretion in transporting these lipid factors is unknown. Here we studied the outcome of sepsis in mice with conditional, intestine-specific deletion of microsomal triglyceride transfer protein (Mttp-IKO), which exhibit a block in chylomicron assembly together with lipid malabsorption.

METHODOLOGY/PRINCIPAL FINDINGS

Mttp-IKO mice and controls underwent intratracheal injection with either Pseudomonas aeruginosa or sterile saline. Mttp-IKO mice exhibited decreased seven-day mortality, with 0/20 (0%) dying compared to 5/17 (29%) control mice (p<0.05). This survival advantage in Mttp-IKO mice, however, was not associated with improvements in pulmonary bacterial clearance or neutrophil infiltration. Rather, Mttp-IKO mice exhibited protection against sepsis-associated decreases in villus length and intestinal proliferation and were also protected against increased intestinal apoptosis, both central features in control septic mice. Serum IL-6 levels, a major predictor of mortality in human and mouse models of sepsis, were elevated 8-fold in septic control mice but remained unaltered in septic Mttp-IKO mice. Serum high density lipoprotein (HDL) levels were reduced in septic control mice but were increased in septic Mttp-IKO mice. The decreased levels of HDL were associated with decreased hepatic expression of apolipoprotein A1 in septic control mice.

CONCLUSIONS/SIGNIFICANCE

These studies suggest that strategies directed at blocking intestinal chylomicron secretion may attenuate the progression and improve the outcome of sepsis through effects mediated by metabolic and physiological adaptations in both intestinal and hepatic lipid flux.

Mechanisms of methicillin-resistant Staphylococcus aureus pneumonia-induced intestinal epithelial apoptosis

Methicillin-resistant Staphylococcus aureus (MRSA) pneumonia-induced sepsis is a common cause of morbidity in the intensive care unit. Although pneumonia is initiated in the lungs, extrapulmonary manifestations occur commonly. In light of the key role the intestine plays in the pathophysiology of sepsis, we sought to determine whether MRSA pneumonia induces intestinal injury. FVB/N mice were subjected to MRSA or sham pneumonia and killed 24 h later. Septic animals had a marked increase in intestinal epithelial apoptosis by both hematoxylin-eosin and active caspase 3 staining. Methicillin-resistant S. aureus-induced intestinal apoptosis was associated with an increase in the expression of the proapoptotic proteins Bid and Bax and the antiapoptotic protein Bcl-xL in the mitochondrial pathway. In the receptor-mediated pathway, MRSA pneumonia induced an increase in Fas ligand but decreased protein levels of Fas, FADD, pFADD, TNF-R1, and TRADD. To assess the functional significance of these changes, MRSA pneumonia was induced in mice with genetic manipulations in proteins in either the mitochondrial or receptor-mediated pathways. Both Bid-/- mice and animals with intestine-specific overexpression of Bcl-2 had decreased intestinal apoptosis compared with wild-type animals. In contrast, Fas ligand-/- mice had no alterations in apoptosis. To determine if these findings were organism-specific, similar experiments were performed in mice subjected to Pseudomonas aeruginosa pneumonia. Pseudomonas aeruginosa induced gut apoptosis, but unlike MRSA, this was associated with increased Bcl-2 and TNF-R1 and decreased Fas. Methicillin-resistant S. aureus pneumonia thus induces organism-specific changes in intestinal apoptosis via changes in both the mitochondrial and receptor-mediated pathways, although the former may be more functionally significant.

The outcome of polymicrobial sepsis is independent of T and B cells

The contribution of the adaptive and innate immune systems to the pathogenesis and outcome of sepsis remains a fundamental yet controversial question. Here, we use mice lacking the recombination activating gene 1 (Rag-1) to study the role of T and B cells in sepsis after cecal ligation and puncture (CLP). Spleens of Rag-1 mice were atrophic and completely devoid of CD3 T cells and CD19 B cells. Wild-type mice and Rag-1 mice (both on a C57BL/6J background) underwent CLP or sham surgery. Both wild-type and Rag-1 mice developed clinical signs of sepsis within the first day after CLP. This included severe hypothermia as measured by a decrease in body surface temperature and organ dysfunction as detected by plasma increases in blood urea nitrogen and lactate dehydrogenase levels. Survival curves of wild-type and Rag-1 mice after CLP were superimposable, with 35% survival in the wild-type group and 27% survival in the Rag-1 group, respectively (not significant, P = 0.875). Using multiplex bead-based assays, the mediator concentrations for 23 cytokines and chemokines were measured in plasma of wild-type and Rag-1 mice 8 h after CLP or sham surgery. Compared with sham surgery mice, the highest mediator levels were observed for granulocyte colony-stimulating factor, keratinocyte chemoattractant, IL-6, monocyte chemotactic protein 1, and IL-10. Levels for most mediators were unaffected by the absence of T and B lymphocytes. Only the concentrations of IL-6 and IL-17 were found to be significantly lower in Rag-1 mice compared with wild-type mice. In conclusion, the absence of T and B cells in the CLP model used does not appear to affect the acute outcome of severe sepsis.

Cecal ligation and puncture followed by methicillin-resistant Staphylococcus aureus pneumonia increases mortality in mice and blunts production of local and systemic cytokines

Mortality in the intensive care unit frequently results from the synergistic effect of two temporally distinct infections. This study examined the pathophysiology of a new model of intra-abdominal sepsis followed by methicillin-resistant Staphylococcus aureus (MRSA) pneumonia. Mice underwent cecal ligation and puncture (CLP) or sham laparotomy followed 3 days later by an intratracheal injection of MRSA or saline. Both CLP/saline and sham/MRSA mice had 100% survival, whereas animals with CLP followed by MRSA pneumonia had 67% 7-day survival. Animals subjected to CLP/MRSA had increased bronchoalveolar lavage concentrations of MRSA compared with sham/MRSA animals. Animals subjected to sham/MRSA pneumonia had increased bronchoalveolar lavage levels of interleukin 6 (IL-6), tumor necrosis factor α, and granulocyte colony-stimulating factor compared with those given intratracheal saline, whereas CLP/MRSA mice had a blunted local inflammatory response with markedly decreased cytokine levels. Similarly, animals subjected to CLP/saline had increased peritoneal lavage levels of IL-6 and IL-1β compared with those subjected to sham laparotomy, whereas this response was blunted in CLP/MRSA mice. Systemic cytokines were upregulated in both CLP/saline and sham/MRSA mice, and this was blunted by the combination of CLP/MRSA. In contrast, no synergistic effect on pneumonia severity, white blood cell count, or lymphocyte apoptosis was identified in CLP/MRSA mice compared with animals with either insult in isolation. These results indicate that a clinically relevant model of CLP followed by MRSA pneumonia causes higher mortality than could have been predicted from studying either infection in isolation, and this was associated with a blunted local (pulmonary and peritoneal) and systemic inflammatory response and decreased ability to clear infection.

The dual functions of receptor interacting protein 1 in fas-induced hepatocyte death during sepsis

In examining the liver's response to sepsis, our laboratory has found that septic hepatocytes exhibit a higher degree of necrosis when compared with septic thymocytes, which typically die through the canonical apoptotic pathway. Recently, an adaptor protein associated with the Fas/TNF death receptor pathway, receptor interacting protein 1 (RIP1), has been shown to be critical for determining whether a cell's death is apoptotic or necrotic. We propose to test the central hypothesis that RIP1 activation by death receptor (Fas) during sepsis determines whether the hepatocytes' fate is apoptotic versus necrotic. We approached this problem by delivering RIP1 siRNA in vivo to C57BL/6 mice and observing changes in mortality after septic challenge. Contrary to our hypothesis, RIP1-silenced mice did not survive as long as scrambled sequence injected controls (22.2% vs. 50.0% 14 days after cecal ligation and puncture, respectively). When we used a pharmacological/synthetic antagonist of RIP1 kinase, necrostatin 1 (Nec1), and examined the mortality of Nec1-treated mice, there was no difference from the RIP1 siRNA-treated mice (20.0% vs. 22.2%, respectively). Furthermore, we carried out a series of comparative histological studies, which indicated that septic mice pretreated with Nec1 exhibited a preservation of liver glycogen stores (represented by periodic acid Schiff stain) versus siRNA-treated mice, which exhibit lower glycogen stores as well as altered morphology. Furthermore, the histological studies also revealed that Nec1 treatment in septic mice increases caspase 3 activity. We speculate that these contradictatory findings are due to the dual-signaling responsibilities of RIP1, where the RIP1 kinase domain can induce death through Fas ligation while also initiating prosurvival signaling through nuclear factor κB (NF-κB).

B cells enhance early innate immune responses during bacterial sepsis

Type I interferon–responsive B cells provide early protection against bacterial sepsis.

Microbes activate pattern recognition receptors to initiate adaptive immunity. T cells affect early innate inflammatory responses to viral infection, but both activation and suppression have been demonstrated. We identify a novel role for B cells in the early innate immune response during bacterial sepsis. We demonstrate that Rag1^−/− mice display deficient early inflammatory responses and reduced survival during sepsis. Interestingly, B cell–deficient or anti-CD20 B cell–depleted mice, but not α/β T cell–deficient mice, display decreased inflammatory cytokine and chemokine production and reduced survival after sepsis. Both treatment of B cell–deficient mice with serum from wild-type (WT) mice and repletion of Rag1^−/− mice with B cells improves sepsis survival, suggesting antibody-independent and antibody-dependent roles for B cells in the outcome to sepsis. During sepsis, marginal zone and follicular B cells are activated through type I interferon (IFN-I) receptor (IFN-α/β receptor [IFNAR]), and repleting Rag1^−/− mice with WT, but not IFNAR^−/−, B cells improves IFN-I–dependent and –independent early cytokine responses. Repleting B cell–deficient mice with the IFN-I–dependent chemokine, CXCL10 was also sufficient to improve sepsis survival. This study identifies a novel role for IFN-I–activated B cells in protective early innate immune responses during bacterial sepsis.

Prevention of lymphocyte apoptosis in septic mice with cancer increases mortality

Lymphocyte apoptosis is thought to have a major role in the pathophysiology of sepsis. However, there is a disconnect between animal models of sepsis and patients with the disease, because the former use subjects that were healthy prior to the onset of infection while most patients have underlying comorbidities. The purpose of this study was to determine whether lymphocyte apoptosis prevention is effective in preventing mortality in septic mice with preexisting cancer. Mice with lymphocyte Bcl-2 overexpression (Bcl-2-Ig) and wild type (WT) mice were injected with a transplantable pancreatic adenocarcinoma cell line. Three weeks later, after development of palpable tumors, all animals received an intratracheal injection of Pseudomonas aeruginosa. Despite having decreased sepsis-induced T and B lymphocyte apoptosis, Bcl-2-Ig mice had markedly increased mortality compared with WT mice following P. aeruginosa pneumonia (85 versus 44% 7-d mortality; p = 0.004). The worsened survival in Bcl-2-Ig mice was associated with increases in Th1 cytokines TNF-α and IFN-γ in bronchoalveolar lavage fluid and decreased production of the Th2 cytokine IL-10 in stimulated splenocytes. There were no differences in tumor size or pulmonary pathology between Bcl-2-Ig and WT mice. To verify that the mortality difference was not specific to Bcl-2 overexpression, similar experiments were performed in Bim(-/-) mice. Septic Bim(-/-) mice with cancer also had increased mortality compared with septic WT mice with cancer. These data demonstrate that, despite overwhelming evidence that prevention of lymphocyte apoptosis is beneficial in septic hosts without comorbidities, the same strategy worsens survival in mice with cancer that are given pneumonia.

The role of heat shock protein 70 in mediating age-dependent mortality in sepsis

Sepsis is primarily a disease of the aged, with increased incidence and mortality occurring in aged hosts. Heat shock protein (HSP) 70 plays an important role in both healthy aging and the stress response to injury. The purpose of this study was to determine the role of HSP70 in mediating mortality and the host inflammatory response in aged septic hosts. Sepsis was induced in both young (6- to 12-wk-old) and aged (16- to 17-mo-old) HSP70(-/-) and wild-type (WT) mice to determine whether HSP70 modulated outcome in an age-dependent fashion. Young HSP70(-/-) and WT mice subjected to cecal ligation and puncture, Pseudomonas aeruginosa pneumonia, or Streptococcus pneumoniae pneumonia had no differences in mortality, suggesting HSP70 does not mediate survival in young septic hosts. In contrast, mortality was higher in aged HSP70(-/-) mice than aged WT mice subjected to cecal ligation and puncture (p = 0.01), suggesting HSP70 mediates mortality in sepsis in an age-dependent fashion. Compared with WT mice, aged septic HSP70(-/-) mice had increased gut epithelial apoptosis and pulmonary inflammation. In addition, HSP70(-/-) mice had increased systemic levels of TNF-α, IL-6, IL-10, and IL-1β compared with WT mice. These data demonstrate that HSP70 is a key determinant of mortality in aged, but not young hosts in sepsis. HSP70 may play a protective role in an age-dependent response to sepsis by preventing excessive gut apoptosis and both pulmonary and systemic inflammation.

Impaired CD4+ T-cell proliferation and effector function correlates with repressive histone methylation events in a mouse model of severe sepsis

Immunosuppression following severe sepsis remains a significant human health concern, as long-term morbidity and mortality rates of patients who have recovered from life-threatening septic shock remain poor. Mouse models of severe sepsis indicate this immunosuppression may be partly due to alterations in myeloid cell function; however, the effect of severe sepsis on subsequent CD4(+) T-cell responses remains unclear. In the present study, CD4(+) T cells from mice subjected to an experimental model of severe sepsis (cecal ligation and puncture (CLP)) were analyzed in vitro. CD4(+)CD62L(+) T cells from CLP mice exhibited reduced proliferative capacity and altered gene expression. Additionally, CD4(+)CD62L(+) T cells from CLP mice exhibit dysregulated cytokine production after in vitro skewing with exogenous cytokines, indicating a decreased capability of these cells to commit to either the T(H)1 or T(H)2 lineage. Repressive histone methylation marks were also evident at promoter regions for the T(H)1 cytokine IFN-gamma and the T(H)2 transcription factor GATA-3 in naïve CD4(+) T cells from CLP mice. These results provide evidence that CD4(+) T-cell subsets from post-septic mice exhibit defects in activation and effector function, possibly due to chromatin remodeling proximal to genes involved in cytokine production or gene transcription.

Safety of probiotic Escherichia coli strain Nissle 1917 depends on intestinal microbiota and adaptive immunity of the host

Probiotics are viable microorganisms that are increasingly used for treatment of a variety of diseases. Occasionally, however, probiotics may have adverse clinical effects, including septicemia. Here we examined the role of the intestinal microbiota and the adaptive immune system in preventing translocation of probiotics (e.g., Escherichia coli Nissle). We challenged C57BL/6J mice raised under germfree conditions (GF-raised C57BL/6J mice) and Rag1(-/-) mice raised under germfree conditions (GF-raised Rag1(-/-) mice) and under specific-pathogen-free conditions (SPF-raised Rag1(-/-) mice) with probiotic E. coli strain Nissle 1917, strain Nissle 1917 mutants, the commensal strain E. coli mpk, or Bacteroides vulgatus mpk. Additionally, we reconstituted Rag1(-/-) mice with CD4(+) T cells. E. coli translocation and dissemination and the mortality of mice were assessed. In GF-raised Rag1(-/-) mice, but not in SPF-raised Rag1(-/-) mice or GF-raised C57BL/6J mice, oral challenge with E. coli strain Nissle 1917, but not oral challenge with E. coli mpk, resulted in translocation and dissemination. The mortality rate was significantly higher for E. coli strain Nissle 1917-challenged GF-raised Rag1(-/-) mice (100%; P < 0.001) than for E. coli strain Nissle 1917-challenged SPF-raised Rag1(-/-) mice (0%) and GF-raised C57BL/6J mice (0%). Translocation of and mortality due to strain E. coli Nissle 1917 in GF-raised Rag1(-/-) mice were prevented when mice were reconstituted with T cells prior to strain E. coli Nissle 1917 challenge, but not when mice were reconstituted with T cells after E. coli strain Nissle 1917 challenge. Cocolonization experiments revealed that E. coli mpk could not prevent translocation of strain E. coli Nissle 1917. Moreover, we demonstrated that neither lipopolysaccharide structure nor flagella play a role in E. coli strain Nissle 1917 translocation and dissemination. Our results suggest that if both the microbiota and adaptive immunity are defective, translocation across the intestinal epithelium and dissemination of the probiotic E. coli strain Nissle 1917 may occur and have potentially severe adverse effects. Future work should define the possibly related molecular factors that promote probiotic functions, fitness, and facultative pathogenicity.

Protective effect of resin adsorption on septic plasma-induced tubular injury

Introduction

A pro-apoptotic effect of circulating mediators on renal tubular epithelial cells has been involved in the pathogenesis of sepsis-associated acute kidney injury (AKI). Adsorption techniques have been showed to efficiently remove inflammatory cytokines from plasma. The aim of this study was to evaluate the efficiency of the hydrophobic resin Amberchrom CG161 M to adsorb from septic plasma soluble mediators involved in tubular injury.

Methods

We enrolled in the study 10 critically ill patients with sepsis-associated AKI and we evaluated the effects of their plasma on granulocyte adhesion, apoptosis and functional alterations of cultured human kidney tubular epithelial cells. We established an in vitro model of plasma adsorption and we studied the protective effect of unselective removal of soluble mediators by the Amberchrom CG161 M resin on septic plasma-induced tubular cell injury.

Results

Plasma from septic patients induced granulocyte adhesion, apoptosis and altered polarity in tubular cells. Plasma adsorption significantly decreased these effects and abated the concentrations of several soluble mediators. The inhibition of granulocyte adhesion to tubular cells was associated with the down-regulation of ICAM-1 and CD40. Resin adsorption inhibited tubular cell apoptosis induced by septic plasma by down-regulating the activation of caspase-3, 8, 9 and of Fas/death receptor-mediated signalling pathways. The alteration of cell polarity, morphogenesis, protein reabsorption and the down-regulation of the tight junction molecule ZO-1, of the sodium transporter NHE3, of the glucose transporter GLUT-2 and of the endocytic receptor megalin all induced by septic plasma were significantly reduced by resin adsorption.

Conclusions

Septic plasma induced a direct injury of tubular cells by favouring granulocyte adhesion, by inducing cell apoptosis and by altering cell polarity and function. All these biological effects are related to the presence of circulating inflammatory mediators that can be efficiently removed by resin adsorption with a consequent limitation of tubular cell injury.