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

The implication of targeting PD-1:PD-L1 pathway in treating sepsis through immunostimulatory and anti-inflammatory pathways

Sepsis currently remains a major contributor to mortality in the intensive care unit (ICU), with 48.9 million cases reported globally and a mortality rate of 22.5% in 2017, accounting for almost 20% of all-cause mortality worldwide. This highlights the urgent need to improve the understanding and treatment of this condition. Sepsis is now recognized as a dysregulation of the host immune response to infection, characterized by an excessive inflammatory response and immune paralysis. This dysregulation leads to secondary infections, multiple organ dysfunction syndrome (MODS), and ultimately death. PD-L1, a co-inhibitory molecule expressed in immune cells, has emerged as a critical factor in sepsis. Numerous studies have found a significant association between the expression of PD-1/PD-L1 and sepsis, with a particular focus on PD-L1 expressed on neutrophils recently. This review explores the role of PD-1/PD-L1 in immunostimulatory and anti-inflammatory pathways, illustrates the intricate link between PD-1/PD-L1 and sepsis, and summarizes current therapeutic approaches against PD-1/PD-L1 in the treatment and prognosis of sepsis in preclinical and clinical studies.

Role of regulation of PD-1 and PD-L1 expression in sepsis

Long term immunosuppression is problematic during sepsis. The PD-1 and PD-L1 immune checkpoint proteins have potent immunosuppressive functions. Recent studies have revealed several features of PD-1 and PD-L1 and their roles in sepsis. Here, we summarize the overall findings of PD-1 and PD-L1 by first reviewing the biological features of PD-1 and PD-L1 and then discussing the mechanisms that control the expression of PD-1 and PD-L1. We then review the functions of PD-1 and PD-L1 in physiological settings and further discuss PD-1 and PD-L1 in sepsis, including their involvement in several sepsis-related processes and their potential therapeutic relevance in sepsis. In general, PD-1 and PD-L1 have critical roles in sepsis, indicating that their regulation may be a potential therapeutic target for sepsis.

Characteristics, treatment, and outcome of diverticulitis after immune checkpoint inhibitor treatment in patients with malignancies

PURPOSE

Immune checkpoint inhibitors (ICIs) are efficacious for treating various malignancies. In addition to immune-related adverse events (irAEs), growing evidence suggests that ICIs might also be associated with diverticulitis. We aim to assess the clinical presentations and management of colonic diverticulitis among cancer patients after ICI treatment.

METHODS

A retrospective study was conducted on ICI-treated adult cancer patients between 01/2010 and 06/2020. Patients were grouped based on when diverticulitis developed relative to ICI treatment, either before (controls) or after (cases). Patient clinical characters, treatment, and outcomes were compared between both groups.

RESULTS

77 eligible patients were included: 63 patients developed diverticulitis after ICI exposure (46 had initial episode after ICI exposure, 17 had a history of diverticulitis prior then recurred after ICI exposure), and 14 had diverticulitis before ICI exposure. Diverticulitis occurred after a median of 129 days after ICI initiation. Clinical characteristics overlapped with traditional diverticulitis. 93% of patients had symptom resolution after treatment, while 23.8% experienced complications. These patients exhibited higher rates of hospitalization (87% vs 48%, P = 0.015) and surgery/interventional radiology procedures (27% vs 0, P = 0.002), and worse overall survival (P = 0.022). History of diverticulitis was not associated with a more severe disease course. Immunosuppressants (e.g., corticosteroids) were rarely required unless for concurrent ICI-mediated colitis.

CONCLUSION

Colonic diverticulitis can occur after ICI therapy at very low incidence (0.5%). Its clinical presentation, evaluation, and management are similar to traditional diverticulitis, but associated with higher complication rates requiring surgical intervention and has lower overall survival.

Publication trends of research on sepsis and programmed cell death during 2002–2022: A 20-year bibliometric analysis

Background

Sepsis is considered an intractable dysfunction that results from the disordered host immune response to uncontrolled infection. Even though the precise mechanism of sepsis remains unclear, scientific advances have highlighted the key role of various programmed cell death processes in the pathophysiology of sepsis. The current study aims to explore the worldwide research trend on programmed cell death in the setting of sepsis and assesses the achievements of publications from various countries, institutions, journals, and authors globally.

Material and methods

Associated publications during 2002–2022 with the topical subject of sepsis and programmed cell death were extracted from the Web of Science. VOSviewer was utilized to evaluate and map the published trend in the relevant fields.

Results

All 2,037 relevant manuscripts with a total citation of 71,575 times were screened out by the end of 1 January 2022. China accounted for the largest number of publications (45.07%) and was accompanied by corporate citations (11,037) and H-index (48), which ranked second globally. The United States has been ranked first place with the highest citations (30,775) and H-index (88), despite a low publication number (29.95%), which was subsequent to China. The journal Shock accounted for the largest number of publications in this area. R. S. Hotchkiss, affiliated with Washington University, was considered to have published the most papers in the relevant fields (57) and achieved the highest citation frequencies (9,523). The primary keywords on the topic of programmed cell death in sepsis remarkably focused on “inflammation” “immunosuppression”, and “oxidative stress”, which were recognized as the core mechanisms of sepsis, eventually attributing to programmed cell death. The involved research on programmed cell death induced by immune dysregulation of sepsis was undoubtedly the hotspot in the pertinent areas.

Conclusions

The United States has been academically outstanding in sepsis-related research. There appears to be an incompatible performance between publications and quantity with China. Frontier advances may be consulted in the journal Shock. The leading-edge research on the scope of programmed cell death in sepsis should preferably focus on immune dissonance-related studies in the future.

[Expression and Role of PD-L1 in a Mouse Model of Necrotizing Enterocolitis]

Objective

To investigate the expression and role of programmed death ligand-1 (PD-L1) in a mouse model of necrotizing enterocolitis (NEC).

Methods

A total of 20 wild-type C57 BL/6 J mice were randomly assigned to the control and the model groups. Mice in the control group were breastfed, while mice in the model group were given lipopolysaccharide, formula feeding, hypoxia, and cold stimulation for NEC induction. Then, the intestines of the mice were collected in order to assess the pathological changes through HE staining, to examine PD-L1 expression and localization with immunofluorescence co-localization, and to evaluate intestinal PD-L1 expression with Western blot. Peripheral blood was collected for flow cytometry to examine leukocyte subpopulations and their PD-L1 expression. On the other hand, 14 PD-L1 (+/+) mice and 14 PD-L1 (-/-) mice were randomly divided into their respective genotype control groups and model groups. The same induction method as was already mentioned was adopted for the model groups. The intestines of the mice were collected for HE staining to evaluate the pathological change and peripheral blood was collected to examine the expression of inflammatory factors.

Results

The NEC mouse model was successfully constructed. PD-L1 was widely expressed in enterocytes and inflammatory cells in the mouse intestines and in T cells, monocytes, and neutrophils in peripheral blood. The expression of PD-L1 in NEC mouse intestines increased in comparison with that of the control group. In the peripheral blood of NEC mice, the proportion of T cells and monocytes and their PD-L1 expression showed no significant changes compared with those of the control group, while the proportion of neutrophils and their PD-L1 expression increased by about 140% and 150%, respectively, in comparison with those of the control group ( P<0.05). According to the results of the PD-L1 gene mouse experiment, the control groups of PD-L1 (+/+) mice and PD-L1 (-/-) mice showed no significant difference in their intestinal conditions and serum inflammatory factor levels, while the PD-L1 (-/-) NEC mouse had worse intestinal pathological changes and increased mean pathological scores compared with those of PD-L1 (+/+) NEC mouse ( P<0.05). In addition, serum interleukin (IL)-10 in PD-L1 (-/-) NEC mouse decreased by about 44% compared with that of PD-L1 (+/+) NEC mice, and chemokine (C-X-C motif) ligand 1/IL-6/IL-1β all increased by more than 25% (all P<0.05).

Conclusion

PD-L1 is widely expressed in inflammatory cells and enterocytes in mice. Knocking out PD-L1 aggravates the degree of NEC inflammation and intestinal pathological changes. PD-L1 plays a protective role by reducing inflammation in the pathogenesis of NEC, the mechanism of which may be related to the regulation of neutrophils/enterocytes.

Over-expression of Programmed Death Ligand 1 (PD-L1) in Refractory Inflammatory Bowel Disease (IBD)

Programmed death ligand 1 (PD-L1) dysregulation has been implicated in chronic inflammatory diseases, but its role in regulating intestinal mucosa inflammation is still unclear. The aim of this study was to assess PD-L1 expression in the intestinal mucosa of patients with refractory inflammatory bowel disease (IBD) compared to controls. We evaluated PD-L1 expression by immunohistochemistry in colectomy specimens of patients with ulcerative colitis (UC) and Crohn disease (CD) compared to controls. PD-L1 expression was assessed in colonic epithelium and inflammatory cells, along with the location of the inflammatory cells expressing PD-L1. All cases were stained with CD3, CD4, CD8, FOXP3, CD20, CD68, and CD90 immunostains to determine the types of cells expressing PD-L1. The UC group showed significantly higher PD-L1 expression in the colonic epithelium compared to both CD and control groups (both P<0.001), and CD was also significantly higher than the control group (P=0.004). Both UC and CD groups showed similar PD-L1 expression in the inflammatory infiltrate, but significantly higher than the control group (both P<0.001). Among both IBD groups, higher IBD activity was associated with higher levels of PD-L1 expression in the colonic epithelium (P<0.05) and inflammatory infiltrate (P<0.001). When comparing PD-L1 expression to lineage specific markers, CD3+, CD4+ T cells, CD68+ macrophages, and CD90+ colonic stromal cells appeared to be expressing PD-L1. These findings implicate a role for PD-L1 in the dysregulation of the immune response in refractory IBD. Further studies are warranted to better understand the role of the immune regulatory pathways in intestinal mucosa.

Negative Immune Checkpoint Protein, VISTA, Regulates the CD4+ Treg Population During Sepsis Progression to Promote Acute Sepsis Recovery and Survival

Sepsis is a systemic immune response to infection that is responsible for ~35% of in-hospital deaths and over 24 billion dollars in annual treatment costs. Strategic targeting of non-redundant negative immune checkpoint protein pathways can cater therapeutics to the individual septic patient and improve prognosis. B7-CD28 superfamily member V-domain Immunoglobulin Suppressor of T cell Activation (VISTA) is an ideal candidate for strategic targeting in sepsis. We hypothesized that immune checkpoint regulator, VISTA, controls T-regulatory cells (Treg), in response to septic challenge, thus playing a protective role/reducing septic morbidity/mortality. Further, we investigated if changes in morbidity/mortality are due to a Treg-mediated effect during the acute response to septic challenge. To test this, we used the cecal ligation and puncture model as a proxy for polymicrobial sepsis and assessed the phenotype of CD4+ Tregs in VISTA-gene deficient (VISTA-/-) and wild-type mice. We also measured changes in survival, soluble indices of tissue injury, and circulating cytokines in the VISTA-/- and wild-type mice. We found that in wild-type mice, CD4+ Tregs exhibit a significant upregulation of VISTA which correlates with higher Treg abundance in the spleen and small intestine following septic insult. However, VISTA-/- mice have reduced Treg abundance in these compartments met with a higher expression of Foxp3, CTLA4, and CD25 compared to wild-type mice. VISTA-/- mice also have a significant survival deficit, higher levels of soluble indicators of liver injury (i.e., ALT, AST, bilirubin), and increased circulating proinflammatory cytokines (i.e., IL-6, IL-10, TNFα, IL-17F, IL-23, and MCP-1) following septic challenge. To elucidate the role of Tregs in VISTA-/- sepsis mortality, we adoptively transferred VISTA-expressing Tregs into VISTA-/- mice. This adoptive transfer rescued VISTA-/- survival to wild-type levels. Taken together, we propose a protective Treg-mediated role for VISTA by which inflammation-induced tissue injury is suppressed and improves survival in early-stage murine sepsis. Thus, enhancing VISTA expression or adoptively transferring VISTA+ Tregs in early-stage sepsis may provide a novel therapeutic approach to ameliorate inflammation-induced death.

PD-1 signaling pathway in sepsis: Does it have a future?

Sepsis is characterized by high mortality and poor prognosis and is one of the leading causes of death among patients in the intensive care unit (ICU). In the past, drugs that block early inflammatory responses have done little to reverse the progression of sepsis. Programmed cell death receptor 1 (PD-1) and its two ligands, programmed cell death receptor ligand 1(PD-L1) and programmed cell death receptor ligand 2 (PD-L2), are negative regulatory factors of the immune response of the body. Recently, the role of the PD-1 signaling pathway in sepsis has been widely studied. Studies showed that the PD-1 signaling pathways are closely related to the mortality and prognosis of sepsis patients. In the immunotherapy of sepsis, whether in animal experiments or clinical trials, anti-PD-1/PD-L1 antibodies have shown good promise. In this review, firstly, we focus on the immunosuppressive mechanism of sepsis and the structure and function of the PD-1 signaling pathway. The variety of the PD-1 signaling pathways in sepsis is introduced. Then, the relationship between the PD-1 signaling pathway and immune cells and organ dysfunction and the regulatory factors of the PD-1 signaling pathway in sepsis is discussed. Finally, the application of the PD-1 signaling pathway in sepsis is specifically emphasized.

Ghrelin Alleviates Intestinal Dysfunction in Sepsis Through the KLF4/MMP2 Regulatory Axis by Activating SIRT1

Intestinal barrier dysfunction is an important contributor to morbidity caused by sepsis. This study investigates the molecular mechanism by which Ghrelin affects intestinal dysfunction in rat model of sepsis. A rat model of sepsis was established by cecal ligation and puncture (CLP), revealing that Ghrelin was downregulated when sepsis occurs. Increases in the levels of inflammatory factors tumor necrosis factor α (TNF-α), interleukin-1 (IL-1β), IL-6, gastrin, γ-H2AX and 8-OHdG was also detected in this model system, as was an overall increase in oxidative stress. Introduction of exogenous Ghrelin inhibited these increases in inflammatory response and oxidative stress, leading to a reduction of overall sepsis-induced intestinal dysfunction. Ghrelin was then shown to activate SIRT1 expression in vitro, while SIRT1 was found to co-express with KLF4, which in turn was predicted to bind to matrix metalloproteinase 2 (MMP2) promoter. Finally, gain- and loss-of-function experiment demonstrated that SIRT1 upregulated the expression of KLF4 to downregulate MMP2. Collectively, Ghrelin inhibits the oxidative stress and intestinal dysfunction to attenuate sepsis by activating SIRT1 and regulating a KLF4/MMP2 regulatory axis.

Survival and Pulmonary Injury After Neonatal Sepsis: PD1/PDL1's Contributions to Mouse and Human Immunopathology

Morbidity and mortality associated with neonatal sepsis remains a healthcare crisis. PD1-/- neonatal mice endured experimental sepsis, in the form of cecal slurry (CS), and showed improved rates of survival compared to wildtype (WT) counterparts. End-organ injury, particularly of the lung, contributes to the devastation set forth by neonatal sepsis. PDL1-/- neonatal mice, in contrast to PD1-/- neonatal mice did not have a significant improvement in survival after CS. Because of this, we focused subsequent studies on the impact of PD1 gene deficiency on lung injury. Here, we observed that at 24 h post-CS (but not at 4 or 12 h) there was a marked increase in pulmonary edema (PE), neutrophil influx, myeloperoxidase (MPO) levels, and cytokine expression sham (Sh) WT mice. Regarding pulmonary endothelial cell (EC) adhesion molecule expression, we observed that Zona occludens-1 (ZO-1) within the cell shifted from a membranous location to a peri-nuclear location after CS in WT murine cultured ECs at 24hrs, but remained membranous among PD1-/- lungs. To expand the scope of this inquiry, we investigated human neonatal lung tissue. We observed that the lungs of human newborns exposed to intrauterine infection had significantly higher numbers of PD1+ cells compared to specimens who died from non-infectious causes. Together, these data suggest that PD1/PDL1, a pathway typically thought to govern adaptive immune processes in adult animals, can modulate the largely innate neonatal pulmonary immune response to experimental septic insult. The potential future significance of this area of study includes that PD1/PDL1 checkpoint proteins may be viable therapeutic targets in the septic neonate.

Immune Checkpoints: Novel Therapeutic Targets to Attenuate Sepsis-Induced Immunosuppression

Sepsis is a leading cause of death in intensive care units and survivors develop prolonged immunosuppression and a high incidence of recurrent infections. No definitive therapy exists to treat sepsis and physicians rely on supportive care including antibiotics, intravenous fluids, and vasopressors. With the rising incidence of antibiotic resistant microbes, it is becoming increasingly critical to discover novel therapeutics. Sepsis-induced leukocyte dysfunction and immunosuppression is recognized as an important contributor towards increased morbidity and mortality. Pre-clinical and clinical studies show that specific cell surface inhibitory immune checkpoint receptors and ligands including PD-1, PD-L1, CTLA4, BTLA, TIM3, OX40, and 2B4 play important roles in the pathophysiology of sepsis by mediating a fine balance between host immune competency and immunosuppression. Pre-clinical studies targeting the inhibitory effects of these immune checkpoints have demonstrated reversal of leukocyte dysfunction and improved host resistance of infection. Measurement of immune checkpoint expression on peripheral blood leukocytes may serve as a means of stratifying patients to direct individualized therapy. This review focuses on advances in our understanding of the role of immune checkpoints in the host response to infections, and the potential clinical application of therapeutics targeting the inhibitory immune checkpoint pathways for the management of septic patients.

Sepsis target validation for repurposing and combining complement and immune checkpoint inhibition therapeutics

Introduction: Sepsis is a disease that occurs due to an adverse immune response to infection by bacteria, viruses and fungi and is the leading pathway to death by infection. The hallmarks for maladapted immune reactions in severe sepsis, which contribute to multiple organ failure and death, are bookended by the exacerbated activation of the complement system to protracted T-cell dysfunction states orchestrated by immune checkpoint control. Despite major advances in our understanding of the condition, there remains to be either a definitive test or an effective therapeutic intervention.Areas covered: The authors consider a combinational drug therapy approach using new biologics, and mathematical modeling for predicting patient responses, in targeting innate and adaptive immune mediators underlying sepsis. Special consideration is given for emerging complement and immune checkpoint inhibitors that may be repurposed for sepsis treatment.Expert opinion: In order to overcome the challenges inherent to finding new therapies for the complex dysregulated host response to infection that drives sepsis, it is necessary to move away from monotherapy and promote precision for personalized combinatory therapies. Notably, combinatory therapy should be guided by predictive systems models of the immune-metabolic characteristics of an individual's disease progression.

Intestinal Epithelium-Derived Luminally Released Extracellular Vesicles in Sepsis Exhibit the Ability to Suppress TNF-a and IL-17A Expression in Mucosal Inflammation

Sepsis is a systemic inflammatory disorder induced by a dysregulated immune response to infection resulting in dysfunction of multiple critical organs, including the intestines. Previous studies have reported contrasting results regarding the abilities of exosomes circulating in the blood of sepsis mice and patients to either promote or suppress inflammation. Little is known about how the gut epithelial cell-derived exosomes released in the intestinal luminal space during sepsis affect mucosal inflammation. To study this question, we isolated extracellular vesicles (EVs) from intestinal lavage of septic mice. The EVs expressed typical exosomal (CD63 and CD9) and epithelial (EpCAM) markers, which were further increased by sepsis. Moreover, septic-EV injection into inflamed gut induced a significant reduction in the messaging of pro-inflammatory cytokines TNF-α and IL-17A. MicroRNA (miRNA) profiling and reverse transcription and quantitative polymerase chain reaction (RT-qPCR) revealed a sepsis-induced exosomal increase in multiple miRNAs, which putatively target TNF-α and IL-17A. These results imply that intestinal epithelial cell (IEC)-derived luminal EVs carry miRNAs that mitigate pro-inflammatory responses. Taken together, our study proposes a novel mechanism by which IEC EVs released during sepsis transfer regulatory miRNAs to cells, possibly contributing to the amelioration of gut inflammation.

Herpes Virus Entry Mediator (HVEM) Expression Promotes Inflammation/ Organ Injury in Response to Experimental Indirect-Acute Lung Injury

Therapeutic interventions to treat acute lung injury (ALI) remain largely limited to lung-protective strategies, as a real molecular pathophysiologically driven therapeutic intervention has yet to become available. While we have previously documented the expression of herpes virus entry mediator (HVEM) on leukocytes of septic mice and critically ill patients, its functional role in shock/sepsis-induced ALI has not yet been studied. Inasmuch, a murine model of indirect ALI (iALI) was induced by hemorrhagic shock (HEM) followed by cecal ligation and puncture (CLP), septic challenge and HVEM-siRNA or phosphate buffered saline was administrated by intratracheal instillation 2 h after hemorrhage to determine the role of HVEM in the development of experimental iALI. Indices of lung injury were measured. HVEM expression was significantly elevated in iALI mice. Compared with phosphate buffered saline treated iALI mice, HVEM knock-down by siRNA caused a reduction of cytokine/chemokine levels, myeloperoxidase activity, broncho-alveolar lavage fluid (BALF) cell count and protein concentration. HVEM-siRNA treatment reduced inflammation and attenuated pulmonary architecture destruction as well as provided an early (60 h post HEM-CLP) survival benefit in iALI mice. This ability of anti-HVEM treatment to prevent the development of iALI and provide a transient survival benefit implies that mitigating signaling through HVEM may be a novel target worth further investigation.

Blockade of endothelial, but not epithelial, cell expression of PD-L1 following severe shock attenuates the development of indirect acute lung injury in mice

This study sets out to establish the comparative contribution of PD-L1 expression by pulmonary endothelial cells (ECs) and/or epithelial cells (EpiCs) to the development of indirect acute lung injury (iALI) by taking advantage of the observation that treatment with naked siRNA by intratracheal delivery in mice primarily affects lung EpiCs, but not lung ECs, while intravenous delivery of liposomal-encapsulated siRNA largely targets vascular ECs including the lung, but not pulmonary EpiCs. We showed that using a mouse model of iALI [induced by hemorrhagic shock followed by septic challenge (Hem-CLP)], PD-L1 expression on pulmonary ECs or EpiCs was significantly upregulated in the iALI mice at 24 h post-septic insult. After documenting the selective ability of intratracheal versus intravenous delivery of PD-L1 siRNA to inhibit PD-L1 expression on EpiCs versus ECs, respectively, we observed that the iALI-induced elevation of cytokine/chemokine levels (in the bronchoalveolar lavage fluid, lung lysates, or plasma), lung myeloperoxidase and caspase-3 activities could largely only be inhibited by intravenous, but not intratracheal, delivery of PD-L1 siRNA. Moreover, intravenous, but not intratracheal, delivery led to a preservation of normal tissue architecture, lessened pulmonary edema, and reduced neutrophils influx induced by iALI. In addition, in vitro mouse endothelial cell line studies showed that PD-L1 gene knockdown by siRNA or knockout by CRISPR/Cas9-mediated gene manipulation, reduced monolayer permeability, and maintained tight junction protein levels upon recombinant IFN-γ stimulation. Together, these data imply a critical role for pulmonary vascular ECs in mediating PD-1:PD-L1-driven pathological changes resulting from systemic stimuli such as Hem-CLP.

Protective Effects of Live Combined B. subtilis and E. faecium in Polymicrobial Sepsis Through Modulating Activation and Transformation of Macrophages and Mast Cells

Aims: Clinical studies showed that the use of probiotics during critical illness reduced nosocomial infection and improved clinical outcome. However, the functional mechanisms of probiotics is remains unclear. Therefore the aim of current study is to explore the protective effects and understand the underlying mechanisms for the beneficial effects of live combined Bacillus subtilis and Enterococcus faecium (LCBE) in cecal ligation puncture (CLP)-induced sepsis.

Methods and Results: Seven-week-old C57BL/6J mice were divided into three groups: sham group (6 mice), CLP-control group (20 mice, pretreatment with saline for 7 days before CLP surgery) and CLP-probiotics group (14 mice, pretreatment with LCBE enteric-coated capsules for 7 days before CLP surgery). In survival experiment, mice were monitored for 7 days after CLP. After the procedure, mice were sacrificed, and, serum, and peritoneal lavage fluid were collected and intestinal ileal samples were harvested.

Results: Our results showed that the mortality was significantly reduced in mice CLP-probiotics group vs. CLP-control group (P < 0.05). Also, treatment CLP-probiotics group decreased the injury scores CLP-probiotics group when compared to CLP-control group. Additionally, levels of pro-inflammatory cytokines IL-6 and TNF-α levels in the serum and intestinal ileal tissues of CLP-probiotics group were reduced when compared to CLP-control group (P < 0.05). However, no significant differences in anti-inflammatory levels of IL-10 and TGF-β1 were observed between CLP-control and CLP-probiotic groups. Furthermore, our experiments showed that that probiotic treatment suppressed the macrophage activation and transformation from M-type to M1-type, inhibited the mast cells (MCs) degranulation, and activation of AKT (kinase B) pathway.

Conclusion: In conclusion, our data shows that probiotics have a protective role in CLP septic mice through reducing intestinal inflammation, altering macrophage polarization and MCs degranulation, and regulating AKT signaling.

Significance and Impact of Study: This study demonstrated the protective effects and mechanisms involved in the protective role of live combined Bacillus subtilis and Enterococcus faecium (LCBE) in CLP-induced septic mice model.

A novel role for programmed cell death receptor ligand 2 in sepsis-induced hepatic dysfunction

The liver is an organ that, when dysfunctional in a septic patient, is strongly associated with morbidity and mortality. Understanding the pathophysiology of liver failure during sepsis may lead to improved diagnostics and potential therapeutic targets. Historically, programmed cell death receptor (PD) ligand 1 (PD-L1) has been considered the primary ligand for its checkpoint molecule counterpart, PD-1, with PD-L2 rarely in the immunopathological spotlight. PD-1 and PD-L1 contribute to liver dysfunction in a murine cecal ligation and puncture (CLP) model of sepsis, but virtually nothing is known about PD-L2's role in sepsis. Therefore, our central hypothesis was that sepsis-induced changes in hepatic PD-L2 expression contributed to worsened liver function and, subsequently, more pronounced morbidity and mortality. We found that although PD-L1 gene deficiency attenuated the hepatic dysfunction seen in wild-type mice after CLP, the loss of PD-L2 appeared to actually worsen indices of liver function along with a trend toward higher liver tissue vascular permeability. Conversely, some protective effects of PD-L2 gene deletion were noted, such as reduced liver/peritoneal bacterial load and reduced IL-6, IL-10, and macrophage inflammatory protein 2 levels following CLP. These diverse actions, as well as the unique expression pattern of PD-L2, may explain why no overt survival advantage could be witnessed in the septic PD-L2^-/- mice. Taken together, these data suggest that although PD-L2 has some selective effects on the hepatic response seen in the septic mouse, these factors are not sufficient to alter septic mortality in this adult murine model. NEW & NOTEWORTHY Our study shows not only that ligands of the checkpoint protein PD-1 respond inversely to a stressor such as septic challenge (PD-L2 declines, whereas PD-L1 rises) but also that aspects of liver dysfunction increase in septic mice lacking the PD-L2 gene. Furthermore, these differences in PD-L2 gene-deficient animals culminated in the abrogation of the survival advantage seen in the septic PD-L1-knockout mice, suggesting that PD-L2 may have roles beyond a simple immune tolerogen.

Transcriptional profiling of primary endometrial epithelial cells following acute HIV-1 exposure reveals gene signatures related to innate immunity

PROBLEM

Genital epithelial cells (GECs) line the mucosal surface of the female genital tract (FGT) and are the first cells that interface with both commensal microbiota and sexually transmitted pathogens. Despite the protective barrier formed by GECs, the FGT is a major site of HIV-1 infection. This highlights the importance of studying the interaction of HIV-1 and GECs.

METHOD OF STUDY

Using microarray analysis, we characterized the transcriptional profile of primary endometrial GECs grown in the presence or absence of physiological levels of E2 (10^-9  mol/L) or P4 (10^-7  mol/L) following acute exposure to HIV-1 for 6 hours.

RESULTS

Acute exposure of primary endometrial GECs to HIV-1 resulted in the expression of genes related to inflammation, plasminogen activation, adhesion and diapedesis and interferon response. Interestingly, exposure to HIV-1 in the presence of E2 and P4 resulted in differential transcriptional profiles, suggesting that the response of primary endometrial GECs to HIV-1 exposure is modulated by female sex hormones.

CONCLUSION

The gene expression signature of endometrial GECs indicates that the response of these cells may be key to determining host susceptibility to HIV-1 and that sex hormones modulate these interactions. This study allows us to explore possible mechanisms that explain the hormone-mediated fluctuation of HIV-1 susceptibility in women.

A novel role for coinhibitory receptors/checkpoint proteins in the immunopathology of sepsis

Coinhibitory molecules, such as PD-1, CTLA-4, 2B4, and BTLA, are an important new family of mediators in the pathophysiology of severe bacterial and/or fungal infection, as well as the combined insults of shock and sepsis. Further, the expression of these molecules may serve as indicators of the immune status of the septic individual. Using PD-1:PD-L as an example, we discuss in this review how such checkpoint molecules may affect the host response to infection by mediating the balance between effective immune defense and immune-mediated tissue injury. Additionally, we explore how the up-regulation of PD-1 and/or PD-L1 expression on not only adaptive immune cells (e.g., T cells), but also on innate immune cells (e.g., macrophages, monocytes, and neutrophils), as well as nonimmune cells during sepsis and/or shock contributes to functional alterations often with detrimental sequelae.

An optimized whole blood assay measuring expression and activity of NLRP3, NLRC4 and AIM2 inflammasomes

The proinflammatory protease caspase-1 plays pivotal roles in central pathways of innate immunity, thereby contributing to pathogen clearance. Beside its physiological role, dysregulated activity of caspase-1 is known to contribute to an increasing number of diseases. In this study, we optimized and validated a low-volume human whole blood assay facilitating the measurement of caspase-1 activation and inflammasome-related gene expression upon stimulation of the NLRP3, NLRC4 or AIM2 inflammasome. Using the NLRP3 inflammasome specific inhibitor MCC950, we were able to measure the activity of canonical or alternative NLRP3 pathways, AIM2 and NLRC4 inflammasomes in whole blood. Based on our data we assume a superposition of NLRP3 and NLRC4 inflammasome activities in human whole blood following stimulation with S. typhimurium. The optimized whole blood assay may be suitable for diagnostic and research purposes for pediatric patients who can only donate small amounts of blood.

Targeting Immune Cell Checkpoints during Sepsis

Immunosuppression is increasingly being recognized as one of the causes of increased morbidity and mortality during sepsis. Both innate and adaptive immune system dysfunction have been shown to cause an impaired ability to eradicate the primary infection and also lead to frequent occurrence of secondary opportunistic infections. Pre-clinical and clinical studies have shown that inhibitory immune checkpoint molecules, including programmed death-1 (PD-1), programmed death ligand-1 (PD-L1), cytotoxic T lymphocyte antigen-4 (CTLA-4), T cell membrane protein-3 (TIM-3), Lymphocyte activation-gene-3 (LAG-3) and 2B4, are upregulated during the course of sepsis. Engagement of these inhibitory molecules on various immune cells has been consistently shown to inhibit innate immune cell functions (e.g., phagocytosis, cytokine production and pathogen clearance) and also lead to impaired T cell competence. In numerous pre-clinical models of sepsis, therapeutic agents aimed at blocking engagement of inhibitory immune checkpoints on immune cells have been shown to improve innate and adaptive immune cell functions, increase host resistance to infection and significantly improve survival. Therefore, immunotherapy with immune cell checkpoint inhibitors holds significant potential for the future of sepsis therapy and merits further investigation.

The intestinal microenvironment in sepsis

The gastrointestinal tract has long been hypothesized to function as "the motor" of multiple organ dysfunction syndrome. The gastrointestinal microenvironment is comprised of a single cell layer epithelia, a local immune system, and the microbiome. These three components of the intestine together play a crucial role in maintaining homeostasis during times of health. However, the gastrointestinal microenvironment is perturbed during sepsis, resulting in pathologic changes that drive both local and distant injury. In this review, we seek to characterize the relationship between the epithelium, gastrointestinal lymphocytes, and commensal bacteria during basal and pathologic conditions and how the intestinal microenvironment may be targeted for therapeutic gain in septic patients.

New insights into the gut as the driver of critical illness and organ failure

PURPOSE OF REVIEW

The gut has long been hypothesized to be the 'motor' of multiple organ dysfunction syndrome. This review serves as an update on new data elucidating the role of the gut as the propagator of organ failure in critical illness.

RECENT FINDINGS

Under basal conditions, the gut absorbs nutrients and serves as a barrier that prevents approximately 40 trillion intraluminal microbes and their products from causing host injury. However, in critical illness, gut integrity is disrupted with hyperpermeability and increased epithelial apoptosis, allowing contamination of extraluminal sites that are ordinarily sterile. These alterations in gut integrity are further exacerbated in the setting of preexisting comorbidities. The normally commensal microflora is also altered in critical illness, with increases in microbial virulence and decreases in diversity, which leads to further pathologic responses within the host.

SUMMARY

All components of the gut are adversely impacted by critical illness. Gut injury can not only propagate local damage, but can also cause distant injury and organ failure. Understanding how the multifaceted components of the gut interact and how these are perturbed in critical illness may play an important role in turning off the 'motor' of multiple organ dysfunction syndrome in the future.