Role of T cells for cytokine production and outcome in a model of acute septic peritonitis

CD47-amyloid-β-CD74 signaling triggers adaptive immunosuppression in sepsis

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. However, how this dysregulation occurs remains to be elucidated. In this study, we use single-cell RNA sequencing (scRNA-seq) and conventional RNA-seq to analyze the immune landscape of sepsis and observe that adaptive immunity is acutely and strongly suppressed. This systemic immunosuppression occurs not only in the peripheral blood but also in all other immune compartments, including the spleen, lymph nodes, and bone marrow. Clinical data show that these adaptive immunity-related genes may have the potential to be used to distinguish patients with sepsis from those with common infections. CD47 is found to play a pivotal role in this immunosuppression by inducing the production of amyloid-β (Aβ), which interacts with CD74 on B cells, leading to B-cell suppression and subsequent adaptive immunosuppression. Blocking CD47-Aβ signaling significantly reduces organ injury and improves the survival rate of septic mice by restoring phagocytic cell functions and alleviating B-cell suppression and adaptive immunosuppression.

Pparγ Expression in T Cells as a Prognostic Marker of Sepsis

Translating murine data to the human situation, we proposed that the level of peroxisome proliferator-activated receptor γ (PPARγ) expression in T cells from septic patients correlates with clinical outcome. In this preliminary report, we analyzed PPARγ mRNA expression in CD3 T cells derived from blood of a very small number of septic patients (n = 18) on various days up to 2 weeks after the initial diagnosis. CD3 T cell count was determined by flow cytometry. T cells from n = 11 healthy donors were included as controls. Maximal PPARγ mRNA expression was observed on the day of sepsis diagnosis (day 0; 5,896 ± 1,523 copies PPARγ mRNA/25 ng mRNA, P < 0.05 vs. controls). In contrast, the number of CD3 T cells was significantly decreased in septic patients compared with healthy controls (296 ± 31 vs. 1,803 ± 134 T cells/μL blood, P < 0.001). Setting two arbitrary limits: patients with a PPARγ expression in T cells higher than 7,000 copies/25 ng mRNA, of whom five of six patients died during the ICU stay, and patients with a T cell count below 100 T cells/μL blood, of whom five of eight patients died, we identified a correlation between sepsis survival and low T cell number, paired with high T cell-specific PPARγ expression. Among all 18 sepsis patients, four fulfilled the criteria for both arbitrary settings and all four of these patients subsequently died. We suggest that both high PPARγ expression in T cells and low absolute T cell number in blood of septic patients may have the potential as a new prognostic marker for a poor sepsis outcome.

FTY720-Induced Lymphopenia Does Not Aggravate Mortality in a Murine Model of Polymicrobial Abdominal Sepsis

BACKGROUND

FTY720 is an immunosuppressive molecule licensed for the treatment of chronic relapsing multiple sclerosis (MS). It attenuates the adaptive immune response by sequestering T cells within secondary lymphoid organs via its action as functional antagonist of sphingosine-1-phasphate. To date, it is unknown whether FTY-induced lymphopenia puts MS patients at increased risk for severe forms of postoperative infectious complications such as abdominal sepsis.

OBJECTIVES

To determine the effect of FTY720-induced lymphopenia on survival to sepsis secondary to postoperative intraabdominal infections in a murine model of polymicrobial sepsis.

METHODS

Detailed analysis of cellular dynamics in secondary lymphoid organs and of cytokine profiles was performed in FTY720-treated or placebo-treated C57BL/6 mice after induction of colon ascendens stent peritonitis (CASP). Furthermore, survival analysis was performed in FTY720-treated and placebo-treated animals in severe CASP. Fifty percent of each group were treated with broad spectrum antibiotics.

RESULTS

FTY720 treatment resulted in remodeling of cell populations present in the peripheral blood, the peritoneal cavity, and the spleen after CASP induction. Both lymphoid and myeloid cell lines were affected. However, survival in lymphopenic FTY720-treated animals was similar to placebo-treated mice following CASP. Antibiotic treatment increases survival in untreated as well as FTY720-treated animals to a similar extent.

DISCUSSION

Our data demonstrate that inhibition of T-cell migration and induction of peripheral lymphopenia did not affect survival in a model of severe murine sepsis. The presence of reduced T- and B-cell numbers in the peripheral blood during a septic challenge did not negatively affect sepsis mortality in our model of severe abdominal sepsis. The absence of increased mortality under FTY720 treatment in the CASP model suggests that FTY720 treatment will probably not result in increased mortality in MS patients suffering from sepsis.

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.

The role of CXCL10 in the pathogenesis of experimental septic shock

Introduction

The chemokine CXCL10 is produced during infection and inflammation to activate the chemokine receptor CXCR3, an important regulator of lymphocyte trafficking and activation. The goal of this study was to assess the contributions of CXCL10 to the pathogenesis of experimental septic shock in mice.

Methods

Septic shock was induced by cecal ligation and puncture (CLP) in mice resuscitated with lactated Ringer’s solution and, in some cases, the broad spectrum antibiotic Primaxin. Studies were performed in CXCL10 knockout mice and mice treated with anti-CXCL10 immunoglobulin G (IgG). Endpoints included leukocyte trafficking and activation, core body temperature, plasma cytokine concentrations, bacterial clearance and survival.

Results

CXCL10 was present at high concentrations in plasma and peritoneal cavity during CLP-induced septic shock. Survival was significantly improved in CXCL10 knockout (CXCL10KO) mice and mice treated with anti-CXCL10 IgG compared to controls. CXCL10KO mice and mice treated with anti-CXCL10 IgG showed attenuated hypothermia, lower concentrations of interleukin-6 (IL-6) and macrophage inhibitory protein-2 (MIP-2) in plasma and lessened natural killer (NK) cell activation compared to control mice. Compared to control mice, bacterial burden in blood and lungs was lower in CXCL10-deficient mice but not in mice treated with anti-CXCL10 IgG. Treatment of mice with anti-CXCL10 IgG plus fluids and Primaxin at 2 or 6 hours after CLP significantly improved survival compared to mice treated with non-specific IgG under the same conditions.

Conclusions

CXCL10 plays a role in the pathogenesis of CLP-induced septic shock and could serve as a therapeutic target during the acute phase of septic shock.

Is boosting the immune system in sepsis appropriate?

A relative immunosuppression is observed in patients after sepsis, trauma, burns, or any severe insults. It is currently proposed that selected patients will benefit from treatment aimed at boosting their immune systems. However, the host immune response needs to be considered in context with pathogen-type, timing, and mainly tissue specificity. Indeed, the immune status of leukocytes is not universally decreased and their activated status in tissues contributes to organ failure. Accordingly, any new immune-stimulatory therapeutic intervention should take into consideration potentially deleterious effects in some situations.

WAVE1 mediates suppression of phagocytosis by phospholipid-derived DAMPs

Clearance of invading pathogens is essential to preventing overwhelming inflammation and sepsis that are symptomatic of bacterial peritonitis. Macrophages participate in this innate immune response by engulfing and digesting pathogens, a process called phagocytosis. Oxidized phospholipids (OxPL) are danger-associated molecular patterns (DAMPs) generated in response to infection that can prevent the phagocytic clearance of bacteria. We investigated the mechanism underlying OxPL action in macrophages. Exposure to OxPL induced alterations in actin polymerization, resulting in spreading of peritoneal macrophages and diminished uptake of E. coli. Pharmacological and cell-based studies showed that an anchored pool of PKA mediates the effects of OxPL. Gene silencing approaches identified the A-kinase anchoring protein (AKAP) WAVE1 as an effector of OxPL action in vitro. Chimeric Wave1(-/-) mice survived significantly longer after infection with E. coli and OxPL treatment in vivo. Moreover, we found that endogenously generated OxPL in human peritoneal dialysis fluid from end-stage renal failure patients inhibited phagocytosis via WAVE1. Collectively, these data uncover an unanticipated role for WAVE1 as a critical modulator of the innate immune response to severe bacterial infections.

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.

Genetic deletion of the HIF-1α isoform I.1 in T cells enhances antibacterial immunity and improves survival in a murine peritonitis model

Hypoxia-adenosinergic suppression and redirection of the immune response has been implicated in the regulation of antipathogen and antitumor immunity, with hypoxia-inducible factor 1α (HIF-1α) playing a major role. In this study, we investigated the role of isoform I.1, a quantitatively minor alternative isoform of HIF-1α, in antibacterial immunity and sepsis survival. By using the cecal ligation and puncture model of bacterial peritonitis, we studied the function of I.1 isoform in T cells using mice with total I.1 isoform deficiency and mice with T-cell-targeted I.1 knockdown. We found that genetic deletion of the I.1 isoform resulted in enhanced resistance to septic lethality, significantly reduced bacterial load in peripheral blood, increased M1 macrophage polarization, augmented levels of proinflammatory cytokines in serum, and significantly decreased levels of the anti-inflammatory cytokine IL-10. Our data suggest a previously unrecognized immunosuppressive role for the I.1 isoform in T cells during bacterial sepsis. We interpret these data as indicative that the activation-inducible isoform I.1 hinders the contribution of T cells to the antibacterial response by affecting M1/M2 macrophage polarization and microbicidal function.

Different peripheral neuroendocrine responses to Trypanosoma cruzi infection in mice lacking adaptive immunity

Trypanosoma cruzi infection in mice triggers neuroendocrine responses that affect the course of the disease. To analyze the contribution of adaptive immunity to these responses, comparative studies between normal C57Bl/6J and recombinase activator gene 1 (RAG-1)-deficient mice, which lack mature B and T lymphocytes, were performed. There was no difference between both types of mice in basal body weight. Following infection, higher parasitemia, increased IL-1β and IL-6 blood levels, less marked changes in lymphoid organs weight, no cardiomegaly, and earlier mortality were observed in RAG-1-deficient, compared with normal mice. The response of the hypothalamus-pituitary-adrenal axis after infection occurred earlier and was more intense in RAG-1-deficient mice than in normal mice. Noradrenaline concentration and serotonergic metabolism in the spleen, lymph nodes, and heart differed between RAG-1-deficient and normal mice. Our studies indicate that the absence of adaptive immunity to T. cruzi influences the neuroendocrine response to the infection with this parasite.

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.