Lipopeptides rather than lipopolysaccharide favor the development of dendritic cell dysfunction similar to polymicrobial sepsis in mice

CD40L-Activated DC Promotes Th17 Differentiation and Inhibits Th2 Differentiation in Sepsis-Induced Lung Injury via cGAS-STING Signaling

Immune hemostasis due to an infection plays a vital role in sepsis-induced multiple organ dysfunction. Dendritic cells (DC) and T helper (Th) cells are the key members of the immune system maintaining immune homeostasis. This study aimed to explore the effect and mechanism of CD40L on the activation of DC and activated DC-induced Th2/Th17 differentiation. A CD40L knockout and cecal ligation and puncture (CLP) mouse model was established via cecal ligation. HE staining was used to evaluate the pathological changes. The gene expressions were studied using quantitative real-time polymerase chain reaction (qRT-PCR), while a transwell system was used to perform the co-culture of DC and T-cells. Flow cytometry was performed to detect the subtype of T and DC cells. ELISA was used to assess the amount of inflammatory factors. CD40L was highly expressed in the plasma of CLP mice. Knock out of CD40L inhibited the activation of DC cell and Th17 differentiation while promoting the Th2 differentiation. The mechanistic investigations revealed that CD40L promoted the activation of cGAS-STING pathway. Rescue experiments indicated that CD40L mediated DC activation via cGAS-STING signaling. Moreover, co-culturing of CD and CD+4 T-cells demonstrated that silencing of CD40L in DC suppressed the DC activation and inhibited Th17 differentiation while promoting Th2 differentiation. These findings revealed a relationship between CD40L, DC activation, and Th2/Th17 differentiation balance in sepsis-induced acute lung injury for the first time. These findings are envisaged to provide novel molecular targets for sepsis-induced lung injury treatment.

Helicobacter pylori induces a novel form of innate immune memory via accumulation of NF-кB proteins

Helicobacter pylori is a widespread Gram-negative pathogen involved in a variety of gastrointestinal diseases, including gastritis, ulceration, mucosa-associated lymphoid tissue (MALT) lymphoma and gastric cancer. Immune responses aimed at eradication of H. pylori often prove futile, and paradoxically play a crucial role in the degeneration of epithelial integrity and disease progression. We have previously shown that H. pylori infection of primary human monocytes increases their potential to respond to subsequent bacterial stimuli - a process that may be involved in the generation of exaggerated, yet ineffective immune responses directed against the pathogen. In this study, we show that H. pylori-induced monocyte priming is not a common feature of Gram-negative bacteria, as Acinetobacter lwoffii induces tolerance to subsequent Escherichia coli lipopolysaccharide (LPS) challenge. Although the increased reactivity of H. pylori-infected monocytes seems to be specific to H. pylori, it appears to be independent of its virulence factors Cag pathogenicity island (CagPAI), cytotoxin associated gene A (CagA), vacuolating toxin A (VacA) and γ-glutamyl transferase (γ-GT). Utilizing whole-cell proteomics complemented with biochemical signaling studies, we show that H. pylori infection of monocytes induces a unique proteomic signature compared to other pro-inflammatory priming stimuli, namely LPS and the pathobiont A. lwoffii. Contrary to these tolerance-inducing stimuli, H. pylori priming leads to accumulation of NF-кB proteins, including p65/RelA, and thus to the acquisition of a monocyte phenotype more responsive to subsequent LPS challenge. The plasticity of pro-inflammatory responses based on abundance and availability of intracellular signaling molecules may be a heretofore underappreciated form of regulating innate immune memory as well as a novel facet of the pathobiology induced by H. pylori.

Dendritic cell development in infection

The immune system protects from infections primarily by detecting and eliminating invading pathogens. This is predominantly mediated by innate immune cells like neutrophils, monocytes and dendritic cells (DCs) expressing specific receptors recognizing pathogen-associated molecular patterns. DC activation by pathogens leads to the initiation of antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. However, various pathogens have evolved immune evasion strategies to ensure their survival. In this review, we highlight recent findings on how various microorganisms or their structural features affect or modulate DC development and whether this has any consequences for a protective immune response.

Regulation of dendritic cell function improves survival in experimental sepsis through immune chaperone

Dendritic cells (DCs) are professional Ag-presenting cells that play a critical role in both innate and adaptive immune responses. DCs recognize and respond to bacteria through multiple PRRs, including TLRs. Heat shock protein gp96/grp94 is a master essential chaperone for TLRs in the endoplasmic reticulum. We generated DC-specific gp96-knockout (KO) mice and showed that gp96 KO DCs were unable to respond to multiple TLR ligands. TLR-mediated hyperinflammatory response can lead to sepsis. However, the roles of neither DCs nor the DC-intrinsic gp96 in the process are completely understood. In a LPS-induced sepsis model, we hereby found that deletion of gp96 in DCs significantly reduced serum TNF-α levels and improved survival. Furthermore, using the well-defined polymicrobial sepsis model of cecal ligation and puncture, we found that DC-specific ablation of gp96 improved survival with significantly attenuated liver and renal injuries, decreased circulating inflammatory cytokines, altered DC maturation and activation, and increased serum Ig. Collectively, we demonstrate that deletion of gp96 in DCs is beneficial in protecting mice against sepsis induced by both endotoxemia and polymicrobial infections. We conclude that targeting gp96 in DCs may provide a potential novel approach for reducing the morbidity and mortality of sepsis.

Recent advances in endotoxin tolerance

Endotoxin tolerance is defined as a reduced capacity of a cell to respond endotoxin (lipopolysaccharide, LPS) challenge after an initial encounter with endotoxin in advance. The body becomes tolerant to subsequent challenge with a lethal dose of endotoxin and cytokines release and cell/tissue damage induced by inflammatory reaction are significantly reduced in the state of endotoxin tolerance. The main characteristics of endotoxin tolerance are downregulation of inflammatory mediators such as tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), and C-X-C motif chemokine 10 (CXCL10) and upregulation of anti-inflammatory cytokines such as IL-10 and transforming growth factor β (TGF-β). Therefore, endotoxin tolerance is often regarded as the regulatory mechanism of the host against excessive inflammation. Endotoxin tolerance is a complex pathophysiological process and involved in multiple cellular signal pathways, receptor alterations, and biological molecules. However, the exact mechanism remains elusive up to date. To better understand the underlying cellular and molecular mechanisms of endotoxin tolerance, it is crucial to investigate the comprehensive cellular signal pathways, signaling proteins, cell surface molecules, proinflammatory and anti-inflammatory cytokines, and other mediators. Endotoxin tolerance plays an important role in reducing the mortality of sepsis, endotoxin shock, and other endotoxin-related diseases. Recent reports indicated that endotoxin tolerance is also related to other diseases such as cystic fibrosis, acute coronary syndrome, liver ischemia-reperfusion injury, and cancer. The aim of this review is to discuss the recent advances in endotoxin tolerance mainly based on the cellular and molecular mechanisms by outline the current state of the knowledge of the involvement of the toll-like receptor 4 (TLR4) signaling pathways, negative regulate factor, microRNAs, apoptosis, chromatin modification, and gene reprogramming of immune cells in endotoxin tolerance. We hope to provide a new idea and scientific basis for the rational treatment of endotoxin-related diseases such as endotoxemia, sepsis, and endotoxin shock clinically.

Understanding the Cellular Origin of the Mononuclear Phagocyte System Sheds Light on the Myeloid Postulate of Immune Paralysis in Sepsis

Sepsis, in essence, is a serious clinical condition that can subsequently result in death as a consequence of a systemic inflammatory response syndrome including febrile leukopenia, hypotension, and multiple organ failures. To date, such life-threatening organ dysfunction remains one of the leading causes of death in intensive care units, with an increasing incidence rate worldwide and particularly within the rapidly growing senior population. While most of the clinical trials are aimed at dampening the overwhelming immune response to infection that spreads through the bloodstream, based on several human immunological investigations, it is now widely accepted that susceptibility to nosocomial infections and long-term sepsis mortality involves an immunosuppressive phase that is characterized by a decrease in some subsets of dendritic cells (DCs). Only recently substantial advances have been made in terms of the origin of the mononuclear phagocyte system that is now likely to allow for a better understanding of how the paralysis of DCs leads to sepsis-related death. Indeed, the unifying view of each subset of DCs has already improved our understanding of the pivotal pathways that contribute to the shift in commitment of their progenitors that originate from the bone marrow. It is quite plausible that this anomaly in sepsis may occur at the single level of DC-committed precursors, and elucidating the immunological basis for such a derangement during the ontogeny of each subset of DCs is now of particular importance for restoring an adequate cell fate decision to their vulnerable progenitors. Last but not least, it provides a direct perspective on the development of sophisticated myelopoiesis-based strategies that are currently being considered for the treatment of immunosenescence within different tissue microenvironments, such as the kidney and the spleen.

Sphingosine 1-Phosphate- and C-C Chemokine Receptor 2-Dependent Activation of CD4+ Plasmacytoid Dendritic Cells in the Bone Marrow Contributes to Signs of Sepsis-Induced Immunosuppression

Sepsis is the dysregulated response of the host to systemic, mostly bacterial infection, and is associated with an enhanced susceptibility to life-threatening opportunistic infections. During polymicrobial sepsis, dendritic cells (DCs) secrete enhanced levels of interleukin (IL) 10 due to an altered differentiation in the bone marrow and contribute to the development of immunosuppression. We investigated the origin of the altered DC differentiation using murine cecal ligation and puncture (CLP), a model for human polymicrobial sepsis. Bone marrow cells (BMC) were isolated after sham or CLP operation, the cellular composition was analyzed, and bone marrow-derived DCs (BMDCs) were generated in vitro. From 24 h on after CLP, BMC gave rise to BMDC that released enhanced levels of IL-10. In parallel, a population of CD11c^hiMHCII⁺CD4⁺ DCs expanded in the bone marrow in a MyD88-dependent manner. Prior depletion of the CD11c^hiMHCII⁺CD4⁺ DCs from BMC in vitro reversed the increased IL-10 secretion of subsequently differentiating BMDC. The expansion of the CD11c^hiMHCII⁺CD4⁺ DC population in the bone marrow after CLP required the function of sphingosine 1-phosphate receptors and C-C chemokine receptor (CCR) 2, the receptor for C-C chemokine ligand (CCL) 2, but was not associated with monocyte mobilization. CD11c^hiMHCII⁺CD4⁺ DCs were identified as plasmacytoid DCs (pDCs) that had acquired an activated phenotype according to their increased expression of MHC class II and CD86. A redistribution of CD4⁺ pDCs from MHC class II⁻ to MHC class II⁺ cells concomitant with enhanced expression of CD11c finally led to the rise in the number of CD11c^hiMHCII⁺CD4⁺ DCs. Enhanced levels of CCL2 were found in the bone marrow of septic mice and the inhibition of CCR2 dampened the expression of CD86 on CD4⁺ pDCs after CLP in vitro. Depletion of pDCs reversed the bias of splenic DCs toward increased IL-10 synthesis after CLP in vivo. Thus, during polymicrobial sepsis, CD4⁺ pDCs are activated in the bone marrow and induce functional reprogramming of differentiating BMDC toward an immunosuppressive phenotype.

Sepsis lethality via exacerbated tissue infiltration and TLR-induced cytokine production by neutrophils is integrin α3β1-dependent

Integrin-mediated migration of neutrophils to infected tissue sites is vital for pathogen clearance and therefore host survival. Although β2 integrins have been shown to mediate neutrophil transendothelial migration during systemic and local inflammation, relatively little information is available regarding neutrophil migration in sepsis beyond the endothelial cell layer. In this study, we report that integrin α3β1 (VLA-3; CD49c/CD29) is dramatically upregulated on neutrophils isolated from both human septic patients and in mouse models of sepsis. Compared with the α3β1 (low) granulocytes, α3β1 (high) cells from septic animals displayed hyperinflammatory phenotypes. Administration of a α3β1 blocking peptide and conditional deletion of α3 in granulocytes significantly reduced the number of extravasating neutrophils and improved survival in septic mice. In addition, expression of α3β1 on neutrophils was associated with Toll-like receptor-induced inflammatory responses and cytokine productions. Thus, our results show that α3β1 is a novel marker of tissue homing and hyperresponsive neutrophil subtypes in sepsis, and blocking of α3β1 may represent a new therapeutic approach in sepsis treatment.