Polymicrobial Sepsis Diminishes Dendritic Cell Numbers and Function Directly Contributing to Impaired Primary CD8 T Cell Responses In Vivo

Defining Parameters That Modulate Susceptibility and Protection to Respiratory Murine Coronavirus MHV1 Infection

Patients infected with SARS-CoV-2 experience variable disease susceptibility, and patients with comorbidities such as sepsis are often hospitalized for COVID-19 complications. However, the extent to which initial infectious inoculum dose determines disease outcomes and whether this can be used for immunological priming in a genetically susceptible host has not been completely defined. We used an established SARS-like murine model in which responses to primary and/or secondary challenges with murine hepatitis virus type 1 (MHV-1) were analyzed. We compared the response to infection in genetically susceptible C3H/HeJ mice, genetically resistant C57BL/6J mice, and genetically diverse, variably susceptible outbred Swiss Webster mice. Although defined as genetically susceptible to MHV-1, C3H/HeJ mice displayed decreasing dose-dependent pathological changes in disease severity and lung infiltrate/edema, as well as lymphopenia. Importantly, an asymptomatic dose (500 PFU) was identified that yielded no measurable morbidity/mortality postinfection in C3H/HeJ mice. Polymicrobial sepsis induced via cecal ligation and puncture converted asymptomatic infections in C3H/HeJ and C57BL/6J mice to more pronounced disease, modeling the impact of sepsis as a comorbidity to β-coronavirus infection. We then used low-dose infection as an immunological priming event in C3H/HeJ mice, which provided neutralizing Ab-dependent, but not circulating CD4/CD8 T cell-dependent, protection against a high-dose MHV-1 early rechallenge. Together, these data define how infection dose, immunological status, and comorbidities modulate outcomes of primary and secondary β-coronavirus infections in hosts with variable susceptibility.

Sepsis leads to lasting changes in phenotype and function of naïve CD8 T cells

Sepsis, an amplified immune response to systemic infection, is characterized by a transient cytokine storm followed by chronic immune dysfunction. Consequently, sepsis survivors are highly susceptible to newly introduced infections, suggesting sepsis can influence the function and composition of the naïve CD8 T cell pool and resulting pathogen-induced primary CD8 T cell responses. Here, we explored the extent to which sepsis induces phenotypic and functional changes within the naïve CD8 T cell pool. To interrogate this, the cecal ligation and puncture (CLP) mouse model of polymicrobial sepsis was used. In normal, non-septic mice, we show type-I interferon (IFN I)-mediated signaling plays an important role in driving the phenotypic and functional heterogeneity in the naïve CD8 T cell compartment leading to increased representation of Ly6C+ naïve CD8 T cells. In response to viral infection after sepsis resolution, naïve Ly6C+ CD8 T cells generated more primary effector and memory CD8 T cells with slower conversion to a central memory CD8 T cell phenotype (Tcm) than Ly6C- naïve CD8 T cells. Importantly, as a potent inducer of cytokine storm and IFN I production, sepsis leads to increased representation of Ly6C+ naïve CD8 T cells that maintained their heightened ability to respond (i.e., effector and memory CD8 T cell accumulation and cytokine production) to primary LCMV infection. Lastly, longitudinal analyses of peripheral blood samples obtained from septic patients revealed profound changes in CD8 T cell subset composition and frequency compared to healthy controls. Thus, sepsis has the capacity to alter the composition of naïve CD8 T cells, directly influencing primary CD8 T cell responses to newly introduced infections.

Comprehensive single-cell analysis reveals novel anergic antigen-presenting cell subtypes in human sepsis

Background

Sepsis is a life-threatening condition with high mortality. A few studies have emerged utilizing single-cell RNA sequencing (scRNA-seq) to analyze gene expression at the single-cell resolution in sepsis, but a comprehensive high-resolution analysis of blood antigen-presenting cells has not been conducted.

Methods

All published human scRNA-seq data were downloaded from the single cell portal database. After manually curating the dataset, we extracted all antigen-presenting cells, including dendritic cells (DCs) and monocytes, for identification of cell subpopulations and their gene profiling and intercellular interactions between septic patients and healthy controls. Finally, we further validated the findings by performing deconvolution analysis on bulk RNA sequencing (RNA-seq) data and flow cytometry.

Results

Within the traditional DC populations, we discovered novel anergic DC subtypes characterized by low major histocompatibility complex class II expression. Notably, these anergic DC subtypes showed a significant increase in septic patients. Additionally, we found that a previously reported immunosuppressive monocyte subtype, Mono1, exhibited a similar gene expression profile to these anergic DCs. The consistency of our findings was confirmed through validation using bulk RNA-seq and flow cytometry, ensuring accurate identification of cell subtypes and gene expression patterns.

Conclusions

This study represents the first comprehensive single-cell analysis of antigen-presenting cells in human sepsis, revealing novel disease-associated anergic DC subtypes. These findings provide new insights into the cellular mechanisms of immune dysregulation in bacterial sepsis.

The Calm after the Storm: Implications of Sepsis Immunoparalysis on Host Immunity

The immunological hallmarks of sepsis include the inflammation-mediated cytokine storm, apoptosis-driven lymphopenia, and prolonged immunoparalysis. Although early clinical efforts were focused on increasing the survival of patients through the first phase, studies are now shifting attention to the long-term effects of sepsis on immune fitness in survivors. In particular, the most pertinent task is deciphering how the immune system becomes suppressed, leading to increased incidence of secondary infections. In this review, we introduce the contribution of numerical changes and functional reprogramming within innate (NK cells, dendritic cells) and adaptive (T cells, B cells) immune cells on the chronic immune dysregulation in the septic murine and human host. We briefly discuss how prior immunological experience in murine models impacts sepsis severity, immune dysfunction, and clinical relevance. Finally, we dive into how comorbidities, specifically autoimmunity and cancer, can influence host susceptibility to sepsis and the associated immune dysfunction.

Screening and identification of the core immune-related genes and immune cell infiltration in severe burns and sepsis

Severe burns often have a high mortality rate due to sepsis, but the genetic and immune crosstalk between them remains unclear. In the present study, the GSE77791 and GSE95233 datasets were analysed to identify immune-related differentially expressed genes (DEGs) involved in disease progression in both burns and sepsis. Subsequently, weighted gene coexpression network analysis (WGCNA), gene enrichment analysis, protein-protein interaction (PPI) network construction, immune cell infiltration analysis, core gene identification, coexpression network analysis and clinical correlation analysis were performed. A total of 282 common DEGs associated with burns and sepsis were identified. Kyoto Encyclopedia of Genes and Genomes pathway analysis identified the following enriched pathways in burns and sepsis: metabolic pathways; complement and coagulation cascades; legionellosis; starch and sucrose metabolism; and ferroptosis. Finally, six core DEGs were identified, namely, IL10, RETN, THBS1, FGF13, LCN2 and MMP9. Correlation analysis showed that some core DEGs were significantly associated with simultaneous dysregulation of immune cells. Of these, RETN upregulation was associated with a worse prognosis. The immune-related genes and dysregulated immune cells in severe burns and sepsis provide potential research directions for diagnosis and treatment.

TLR2-induced CD8+ T-cell deactivation shapes dendritic cell differentiation in the bone marrow during sepsis

Sepsis is associated with profound immune dysregulation that increases the risk for life-threatening secondary infections: Dendritic cells (DCs) undergo functional reprogramming due to yet unknown changes during differentiation in the bone marrow (BM). In parallel, lymphopenia and exhaustion of T lymphocytes interfere with antigen-specific adaptive immunity. We hypothesized that there exists a link between T cells and the modulation of DC differentiation in the BM during murine polymicrobial sepsis. Sepsis was induced by cecal ligation and puncture (CLP), a model for human bacterial sepsis. At different time points after CLP, the BM and spleen were analyzed in terms of T-cell subpopulations, activation, and Interferon (IFN)-γ synthesis as well as the number of pre-DCs. BM-derived DCs were generated in vitro. We observed that naïve and virtual memory CD8⁺ T cells, but not CD4⁺ T cells, were activated in an antigen-independent manner and accumulated in the BM early after CLP, whereas lymphopenia was evident in the spleen. The number of pre-DCs strongly declined during acute sepsis in the BM and almost recovered by day 4 after CLP, which required the presence of CD8⁺ T cells. Adoptive transfer experiments and in vitro studies with purified T cells revealed that Toll-like receptor 2 (TLR2) signaling in CD8⁺ T cells suppressed their capacity to secrete IFN-γ and was sufficient to change the transcriptome of the BM during sepsis. Moreover, the diminished IFN-γ production of CD8⁺ T cells favored the differentiation of DCs with increased production of the immune-activating cytokine Interleukin (IL)-12. These data identify a novel role of CD8⁺ T cells in the BM during sepsis as they sense TLR2 ligands and control the number and function of de novo differentiating DCs.

Redox regulation of the immune response

The immune-inflammatory response is associated with increased nitro-oxidative stress. The aim of this mechanistic review is to examine: (a) the role of redox-sensitive transcription factors and enzymes, ROS/RNS production, and the activity of cellular antioxidants in the activation and performance of macrophages, dendritic cells, neutrophils, T-cells, B-cells, and natural killer cells; (b) the involvement of high-density lipoprotein (HDL), apolipoprotein A1 (ApoA1), paraoxonase-1 (PON1), and oxidized phospholipids in regulating the immune response; and (c) the detrimental effects of hypernitrosylation and chronic nitro-oxidative stress on the immune response. The redox changes during immune-inflammatory responses are orchestrated by the actions of nuclear factor-κB, HIF1α, the mechanistic target of rapamycin, the phosphatidylinositol 3-kinase/protein kinase B signaling pathway, mitogen-activated protein kinases, 5' AMP-activated protein kinase, and peroxisome proliferator-activated receptor. The performance and survival of individual immune cells is under redox control and depends on intracellular and extracellular levels of ROS/RNS. They are heavily influenced by cellular antioxidants including the glutathione and thioredoxin systems, nuclear factor erythroid 2-related factor 2, and the HDL/ApoA1/PON1 complex. Chronic nitro-oxidative stress and hypernitrosylation inhibit the activity of those antioxidant systems, the tricarboxylic acid cycle, mitochondrial functions, and the metabolism of immune cells. In conclusion, redox-associated mechanisms modulate metabolic reprogramming of immune cells, macrophage and T helper cell polarization, phagocytosis, production of pro- versus anti-inflammatory cytokines, immune training and tolerance, chemotaxis, pathogen sensing, antiviral and antibacterial effects, Toll-like receptor activity, and endotoxin tolerance.

Kynurenine Pathway-An Underestimated Factor Modulating Innate Immunity in Sepsis-Induced Acute Kidney Injury?

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection, and it accounts for about half of the cases of acute kidney injury (AKI). Although sepsis is the most frequent cause of AKI in critically ill patients, its pathophysiological mechanisms are not well understood. Sepsis has the ability to modulate the function of cells belonging to the innate immune system. Increased activity of indoleamine 2,3-dioxygenase 1 (IDO1) and production of kynurenines are the major metabolic pathways utilized by innate immunity cells to maintain immunological tolerance. The activation of the kynurenine pathway (KP) plays a dual role in sepsis—in the early stage, the induction of IDO1 elicits strong proinflammatory effects that may lead to tissue damage and septic shock. Afterwards, depletion of tryptophan and production of kynurenines contribute to the development of immunosuppression that may cause the inability to overpower opportunistic infections. The presented review provides available data on the various interdependencies between elements of innate immunity and sepsis-induced AKI (SAKI) with particular emphasis on the immunomodulatory significance of KP in the above processes. We believe that KP activation may be one of the crucial, though underestimated, components of a deregulated host response to infection during SAKI.

Sepsis leads to lasting changes in phenotype and function of memory CD8 T cells

The global health burden due to sepsis and the associated cytokine storm is substantial. While early intervention has improved survival during the cytokine storm, those that survive can enter a state of chronic immunoparalysis defined by transient lymphopenia and functional deficits of surviving cells. Memory CD8 T cells provide rapid cytolysis and cytokine production following re-encounter with their cognate antigen to promote long-term immunity, and CD8 T cell impairment due to sepsis can pre-dispose individuals to re-infection. While the acute influence of sepsis on memory CD8 T cells has been characterized, if and to what extent pre-existing memory CD8 T cells recover remains unknown. Here, we observed that central memory CD8 T cells (TCM) from septic patients proliferate more than those from healthy individuals. Utilizing LCMV immune mice and a CLP model to induce sepsis, we demonstrated that TCM proliferation is associated with numerical recovery of pathogen-specific memory CD8 T cells following sepsis-induced lymphopenia. This increased proliferation leads to changes in composition of memory CD8 T cell compartment and altered tissue localization. Further, memory CD8 T cells from sepsis survivors have an altered transcriptional profile and chromatin accessibility indicating long-lasting T cell intrinsic changes. The sepsis-induced changes in the composition of the memory CD8 T cell pool and transcriptional landscape culminated in altered T cell function and reduced capacity to control L. monocytogenes infection. Thus, sepsis leads to long-term alterations in memory CD8 T cell phenotype, protective function and localization potentially changing host capacity to respond to re-infection.

Platelet MHC class I mediates CD8+ T-cell suppression during sepsis

Circulating platelets interact with leukocytes to modulate host immune and thrombotic responses. In sepsis, platelet-leukocyte interactions are increased and have been associated with adverse clinical events, including increased platelet-T-cell interactions. Sepsis is associated with reduced CD8+ T-cell numbers and functional responses, but whether platelets regulate CD8+ T-cell responses during sepsis remains unknown. In our current study, we systemically evaluated platelet antigen internalization and presentation through major histocompatibility complex class I (MHC-I) and their effects on antigen-specific CD8+ T cells in sepsis in vivo and ex vivo. We discovered that both human and murine platelets internalize and proteolyze exogenous antigens, generating peptides that are loaded onto MHC-I. The expression of platelet MHC-I, but not platelet MHC-II, is significantly increased in human and murine platelets during sepsis and in human megakaryocytes stimulated with agonists generated systemically during sepsis (eg, interferon-γ and lipopolysaccharide). Upregulation of platelet MHC-I during sepsis increases antigen cross-presentation and interactions with CD8+ T cells in an antigen-specific manner. Using a platelet lineage-specific MHC-I-deficient mouse strain (B2Mf/f-Pf4Cre), we demonstrate that platelet MHC-I regulates antigen-specific CD8+ T-cell proliferation in vitro, as well as the number and functional responses of CD8+ T cells in vivo, during sepsis. Loss of platelet MHC-I reduces sepsis-associated mortality in mice in an antigen-specific setting. These data identify a new mechanism by which platelets, through MHC-I, process and cross-present antigens, engage antigen-specific CD8+ T cells, and regulate CD8+ T-cell numbers, functional responses, and outcomes during sepsis.

Sepsis and multiple sclerosis: Causative links and outcomes

Sepsis is a life-threatening condition characterized by an acute cytokine storm followed by prolonged dysfunction of the immune system in the survivors. Post-septic lymphopenia and functional deficits of the remaining immune cells lead to increased susceptibility to secondary infections and other morbid conditions causing late death in the patients. This state of post-septic immunoparalysis may also influence disorders stemming from inappropriate or overactive immune responses, such as autoimmune and immunoinflammatory diseases, including multiple sclerosis. In addition, ongoing autoimmunity likely influences the susceptibility to and outcome of sepsis. This review article addresses the bidirectional relationship between sepsis and multiple sclerosis, with a focus on the immunologic mechanisms of the interaction and potential directions for future studies.

Immune Intervention in Sepsis

Sepsis is a host immune disorder induced by infection. It can lead to multiple organ dysfunction syndrome (MODS), which has high morbidity and mortality. There has been great progress in the clinical diagnosis and treatment of sepsis, such as improvements in pathogen detection technology, innovations regarding anti-infection drugs, and the development of organ function support. Abnormal immune responses triggered by pathogens, ranging from excessive inflammation to immunosuppression, are recognized to be an important cause of the high mortality rate. However, no drugs have been approved specifically for treating sepsis. Here, we review the recent research progress on immune responses in sepsis to provide a theoretical basis for the treatment of sepsis. Constructing and optimizing a dynamic immune system treatment regimen based on anti-infection treatment, fluid replacement, organ function support, and timely use of immunomodulatory interventions may improve the prognosis of sepsis patients.

Inflammation and Cell Death of the Innate and Adaptive Immune System during Sepsis

Sepsis is a life-threatening medical condition that occurs when the host has an uncontrolled or abnormal immune response to overwhelming infection. It is now widely accepted that sepsis occurs in two concurrent phases, which consist of an initial immune activation phase followed by a chronic immunosuppressive phase, leading to immune cell death. Depending on the severity of the disease and the pathogen involved, the hosts immune system may not fully recover, leading to ongoing complications proceeding the initial infection. As such, sepsis remains one of the leading causes of morbidity and mortality world-wide, with treatment options limited to general treatment in intensive care units (ICU). Lack of specific treatments available for sepsis is mostly due to our limited knowledge of the immuno-physiology associated with the disease. This review will provide a comprehensive overview of the mechanisms and cell types involved in eliciting infection-induced immune activation from both the innate and adaptive immune system during sepsis. In addition, the mechanisms leading to immune cell death following hyperactivation of immune cells will be explored. The evaluation and better understanding of the cellular and systemic responses leading to disease onset could eventuate into the development of much needed therapies to combat this unrelenting disease.

NK Cell-Derived IL-10 Supports Host Survival during Sepsis

The dysregulated sepsis-induced cytokine storm evoked during systemic infection consists of biphasic and interconnected pro- and anti-inflammatory responses. The contrasting inflammatory cytokine responses determine the severity of the septic event, lymphopenia, host survival, and the ensuing long-lasting immunoparalysis state. NK cells, because of their capacity to elaborate pro- (i.e., IFN-γ) and anti-inflammatory (i.e., IL-10) responses, exist at the inflection of sepsis-induced inflammatory responses. Thus, NK cell activity could be beneficial or detrimental during sepsis. In this study, we demonstrate that murine NK cells promote host survival during sepsis by limiting the scope and duration of the cytokine storm. Specifically, NK cell-derived IL-10, produced in response to IL-15, is relevant to clinical manifestations in septic patients and critical for survival during sepsis. This role of NK cells demonstrates that regulatory mechanisms of classical inflammatory cells are beneficial and critical for controlling systemic inflammation, a notion relevant for therapeutic interventions during dysregulated infection-induced inflammatory responses.

New Insights into the Immune System Using Dirty Mice

The mouse (Mus musculus) is the dominant organism used to investigate the mechanisms behind complex immunological responses because of their genetic similarity to humans and our ability to manipulate those genetics to understand downstream function. Indeed, our knowledge of immune system development, response to infection, and ways to therapeutically manipulate the immune response to combat disease were, in large part, delineated in the mouse. Despite the power of mouse-based immunology research, the translational efficacy of many new therapies from mouse to human is far from ideal. Recent data have highlighted how the naive, neonate-like immune system of specific pathogen-free mice differs dramatically in composition and function to mice living under barrier-free conditions (i.e., "dirty" mice). In this review, we discuss major findings to date and challenges faced when using dirty mice and specific areas of immunology research that may benefit from using animals with robust and varied microbial exposure.

Sepsis impedes EAE disease development and diminishes autoantigen-specific naive CD4 T cells

Evaluation of sepsis-induced immunoparalysis has highlighted how decreased lymphocyte number/function contribute to worsened infection/cancer. Yet, an interesting contrast exists with autoimmune disease development, wherein diminishing pathogenic effectors may benefit the post-septic host. Within this framework, the impact of cecal ligation and puncture (CLP)-induced sepsis on the development of experimental autoimmune encephalomyelitis (EAE) was explored. Notably, CLP mice have delayed onset and reduced disease severity, relative to sham mice. Reduction in disease severity was associated with reduced number, but not function, of autoantigen (MOG)-specific pathogenic CD4 T cells in the CNS during disease and draining lymph node during priming. Numerical deficits of CD4 T cell effectors are associated with the loss of MOG-specific naive precursors. Critically, transfer of MOG-TCR transgenic (2D2) CD4 T cells after, but not before, CLP led to EAE disease equivalent to sham mice. Thus, broad impairment of antigenic responses, including autoantigens, is a hallmark of sepsis-induced immunoparalysis.

Sepsis is a life-threatening condition that can happen when the immune system overreacts to an infection and begins to damage tissues and organs in the body. It causes an extreme immune reaction called a cytokine storm, where the body releases uncontrolled levels of cytokines, proteins that are involved in coordinating the body’s response to infections. This in turn activates more immune cells, resulting in hyperinflammation.

People who survive sepsis may have long-lasing impairments in their immune system that may leave them more vulnerable to infections or cancer. But scientists do not know exactly what causes these lasting immune problems or how to treat them.

The fact that people are susceptible to cancer and infection after sepsis may offer a clue. It may suggest that the immune system is not able to attack bacteria or cancer cells. One way to explore this clue would be to test the effects of sepsis on autoimmune diseases, which cause the immune system to attack the body’s own cells. For example, in the autoimmune disease multiple sclerosis, the immune system attacks and destroys cells in the nervous system. If autoimmune disease is reduced after sepsis, it would suggest the cell-destroying abilities of the immune system are lessened.

Using this approach, Jensen, Jensen et al. show that sepsis reduces the number of certain immune cells, called CD4 T cells, which are are responsible for an autoimmune attack of the central nervous system. In the experiments, mice that survived sepsis were evaluated for their ability to develop a multiple sclerosis-like disease. Mice that survived sepsis developed less severe or no autoimmune disease. After sepsis, these animals also had fewer CD4 T cells. However, when these immune cells were reinstated, the autoimmune disease emerged.

The experiments help explain some of the immune system changes that occur after sepsis. Jensen, Jensen et al. suggest that rather than being completely detrimental, these changes may help to block harmful autoimmune responses. The experiments may also hint at new ways to combat autoimmune diseases by trying to replicate some of the immune-suppressing effects of sepsis. Studying the effect of sepsis on other autoimmune diseases in mice might provide more clues.

Microbial Exposure Enhances Immunity to Pathogens Recognized by TLR2 but Increases Susceptibility to Cytokine Storm through TLR4 Sensitization

Microbial exposures can define an individual's basal immune state. Cohousing specific pathogen-free (SPF) mice with pet store mice, which harbor numerous infectious microbes, results in global changes to the immune system, including increased circulating phagocytes and elevated inflammatory cytokines. How these differences in the basal immune state influence the acute response to systemic infection is unclear. Cohoused mice exhibit enhanced protection from virulent Listeria monocytogenes (LM) infection, but increased morbidity and mortality to polymicrobial sepsis. Cohoused mice have more TLR2⁺ and TLR4⁺ phagocytes, enhancing recognition of microbes through pattern-recognition receptors. However, the response to a TLR2 ligand is muted in cohoused mice, whereas the response to a TLR4 ligand is greatly amplified, suggesting a basis for the distinct response to Listeria monocytogenes and sepsis. Our data illustrate how microbial exposure can enhance the immune response to unrelated challenges but also increase the risk of immunopathology from a severe cytokine storm.

Polymicrobial Sepsis Impairs Antigen-Specific Memory CD4 T Cell-Mediated Immunity

Patients who survive sepsis display prolonged immune dysfunction and heightened risk of secondary infection. CD4 T cells support a variety of cells required for protective immunity, and perturbations to the CD4 T cell compartment can decrease overall immune system fitness. Using the cecal ligation and puncture (CLP) mouse model of sepsis, we investigated the impact of sepsis on endogenous Ag-specific memory CD4 T cells generated in C57BL/6 (B6) mice infected with attenuated Listeria monocytogenes (Lm) expressing the I-A^b-restricted 2W1S epitope (Lm-2W). The number of 2W1S-specific memory CD4 T cells was significantly reduced on day 2 after sepsis induction, but recovered by day 14. In contrast to the transient numerical change, the 2W1S-specific memory CD4 T cells displayed prolonged functional impairment after sepsis, evidenced by a reduced recall response (proliferation and effector cytokine production) after restimulation with cognate Ag. To define the extent to which the observed functional impairments in the memory CD4 T cells impacts protection to secondary infection, B6 mice were infected with attenuated Salmonella enterica-2W (Se-2W) 30 days before sham or CLP surgery, and then challenged with virulent Se-2W after surgery. Pathogen burden was significantly higher in the CLP-treated mice compared to shams. Similar reductions in functional capacity and protection were noted for the endogenous OVA₃₂₃-specific memory CD4 T cell population in sepsis survivors upon Lm-OVA challenge. Our data collectively show CLP-induced sepsis alters the number and function of Ag-specific memory CD4 T cells, which contributes (in part) to the characteristic long-lasting immunoparalysis seen after sepsis.

CD4 T Cell Responses and the Sepsis-Induced Immunoparalysis State

Sepsis remains a major cause of death in the United States and worldwide, and costs associated with treating septic patients place a large burden on the healthcare industry. Patients who survive the acute phase of sepsis display long-term impairments in immune function due to reductions in numbers and function of many immune cell populations. This state of chronic immunoparalysis renders sepsis survivors increasingly susceptible to infection with newly or previously encountered infections. CD4 T cells play important roles in the development of cellular and humoral immune responses following infection. Understanding how sepsis impacts the CD4 T cell compartment is critical for informing efforts to develop treatments intended to restore immune system homeostasis following sepsis. This review will focus on the current understanding of how sepsis impacts the CD4 T cell responses, including numerical representation, repertoire diversity, phenotype and effector functionality, subset representation (e.g., Th1 and Treg frequency), and therapeutic efforts to restore CD4 T cell numbers and function following sepsis. Additionally, we will discuss recent efforts to model the acute sepsis phase and resulting immune dysfunction using mice that have previously encountered infection, which more accurately reflects the immune system of humans with a history of repeated infection throughout life. A thorough understanding of how sepsis impacts CD4 T cells based on previous studies and new models that accurately reflect the human immune system may improve translational value of research aimed at restoring CD4 T cell-mediated immunity, and overall immune fitness following sepsis.

Crosstalk between Dendritic Cells and Immune Modulatory Agents against Sepsis

Dendritic cells (DCs) play a critical role in the immune system which sense pathogens and present their antigens to prime the adaptive immune responses. As the progression of sepsis occurs, DCs are capable of orchestrating the aberrant innate immune response by sustaining the Th1/Th2 responses that are essential for host survival. Hence, an in-depth understanding of the characteristics of DCs would have a beneficial effect in overcoming the obstacle occurring in sepsis. This paper focuses on the role of DCs in the progression of sepsis and we also discuss the reverse sepsis-induced immunosuppression through manipulating the DC function. In addition, we highlight some potent immunotherapies that could be used as a novel strategy in the early treatment of sepsis.

Sepsis-associated encephalopathy: a vicious cycle of immunosuppression

Sepsis-associated encephalopathy (SAE) is commonly complicated by septic conditions, and is responsible for increased mortality and poor outcomes in septic patients. Uncontrolled neuroinflammation and ischemic injury are major contributors to brain dysfunction, which arises from intractable immune malfunction and the collapse of neuroendocrine immune networks, such as the cholinergic anti-inflammatory pathway, hypothalamic-pituitary-adrenal axis, and sympathetic nervous system. Dysfunction in these neuromodulatory mechanisms compromised by SAE jeopardizes systemic immune responses, including those of neutrophils, macrophages/monocytes, dendritic cells, and T lymphocytes, which ultimately results in a vicious cycle between brain injury and a progressively aberrant immune response. Deep insight into the crosstalk between SAE and peripheral immunity is of great importance in extending the knowledge of the pathogenesis and development of sepsis-induced immunosuppression, as well as in exploring its effective remedies.

Resistin expression in human monocytes is controlled by two linked promoter SNPs mediating NFKB p50/p50 binding and C-methylation

Resistin is a key cytokine associated with metabolic and inflammatory diseases. Especially in East Asian populations, the expression levels are strongly influenced by genetic polymorphisms. Mechanisms and functional implications of this genetic control are still unknown. By employing reporter assays, EMSA, inhibition studies, bisulphite sequencing, ChIP-Seq and gene-editing we show that the p50/p50 homodimer known to act as repressor for a number of pro-inflammatory genes plays a central role in the genetic regulation of resistin in monocytes along with promoter methylation. In the common RETN haplotype p50/p50 constitutively dampens the expression by binding to the promoter. In an Asian haplotype variant however this interaction is disrupted by the A allele of rs3219175. The SNP is in very close linkage to rs34861192, a CpG SNP, located 280 bp upstream which provides an allele-specific C-methylation site. rs34861192 is located in a 100 bp region found to be methylated in the common but not in the Asian haplotype, resulting in the latter having a higher basal expression, which also associates with elevated histone acetylation (H3K27ac). Genotype associations within cohort data of 200 East Asian individuals revealed significant associations between this haplotype and the plasma levels of factors such as TGF-b, S100B, sRAGE and IL-8 as well as with myeloid DC counts. Thus, the common RETN haplotype is tightly regulated by the epigenetic mechanism linked to p50/p50-binding. This control is lost in the Asian haplotype, which may have evolved to balance the antagonistic RETN effects on pathogen protection vs. metabolic and inflammatory disease induction.

Pathological alteration and therapeutic implications of sepsis-induced immune cell apoptosis

Sepsis is a life-threatening organ dysfunction syndrome caused by dysregulated host response to infection that leads to uncontrolled inflammatory response followed by immunosuppression. However, despite the high mortality rate, no specific treatment modality or drugs with high efficacy is available for sepsis to date. Although improved treatment strategies have increased the survival rate during the initial state of excessive inflammatory response, recent trends in sepsis show that mortality occurs at a period of continuous immunosuppressive state in which patients succumb to secondary infections within a few weeks or months due to post-sepsis “immune paralysis.” Immune cell alteration induced by uncontrolled apoptosis has been considered a major cause of significant immunosuppression. Particularly, apoptosis of lymphocytes, including innate immune cells and adaptive immune cells, is associated with a higher risk of secondary infections and poor outcomes. Multiple postmortem studies have confirmed that sepsis-induced immune cell apoptosis occurs in all age groups, including neonates, pediatric, and adult patients, and it is considered to be a primary contributing factor to the immunosuppressive pathophysiology of sepsis. Therapeutic perspectives targeting apoptosis through various strategies could improve survival in sepsis. In this review article, we will focus on describing the major apoptosis process of immune cells with respect to physiologic and molecular mechanisms. Further, advances in apoptosis-targeted treatment modalities for sepsis will also be discussed.

Notch signaling pathway regulates T cell dysfunction in septic patients

Sepsis disrupts innate and adaptive immune response, and immune disorders may also impact clinical course of sepsis. Notch signaling pathway plays a vital role in T cell modulation and differentiation. The aim of current study was to investigate the immunoregulatory function of Notch signaling pathway to T cells in patients with sepsis and septic shock. Twenty-seven sepsis patients, twenty-five septic shock patients, and twenty-one normal controls (NCs) were enrolled. Notch receptors mRNA levels were semi-quantified by real-time PCR. The absolute numbers of CD3⁺, CD4⁺, and CD8⁺ T cells were measured by flow cytometry. Key transcriptional factors of CD4⁺ T cells, cytotoxic molecules in CD8⁺ T cells, and cytotoxicity of CD8⁺ T cells were investigated. The regulatory activities of Notch signaling inhibition by γ-secretase inhibitor (GSI) on purified CD4⁺ and CD8⁺ T cells from sepsis and septic shock patients were also assessed. Notch1 mRNA relative level was significantly elevated in sepsis and septic shock patients when compared with NCs. CD4⁺ and CD8⁺ T cells were dysfunctional in sepsis and septic shock, which presented as decreased cell accounts, down-regulation of Th1/Th17 transcriptional factors and cytotoxic molecules (perforin, granzyme B, and FasL), and reduced cytotoxicity of CD8⁺ T cells. Notch signaling inhibition by GSI increased Th1 and Th17 differentiation of CD4⁺ T cells. Moreover, GSI stimulation not only promoted perforin, granzyme B, and FasL mRNA expression in CD8⁺ T cells, but also elevated CD8⁺ T cell-induced target cell death and IFN-γ/TNF-α production in sepsis and septic shock. The current data suggest that Notch signaling pathway contributes to T cell dysfunction and limited immune response in sepsis.

Sepsis-Induced State of Immunoparalysis Is Defined by Diminished CD8 T Cell-Mediated Antitumor Immunity

Patients who survive sepsis experience long-term immunoparalysis characterized by numerical and/or functional lesions in innate and adaptive immunity that increase the host's susceptibility to secondary complications. The extent to which tumor development/growth is affected in sepsis survivors remains unknown. In this study, we show cecal ligation and puncture (CLP) surgery renders mice permissive to increased B16 melanoma growth weeks/months after sepsis induction. CD8 T cells provide partial protection in this model, and tumors from sepsis survivors had a reduced frequency of CD8 tumor-infiltrating lymphocytes (TILs) concomitant with an increased tumor burden. Interestingly, the postseptic environment reduced the number of CD8 TILs with high expression of activating/inhibitory receptors PD-1 and LAG-3 (denoted PD-1^hi) that define a tumor-specific CD8 T cell subset that retain some functional capacity. Direct ex vivo analysis of CD8 TILs from CLP hosts showed decreased proliferation, IFN-γ production, and survival compared with sham counterparts. To increase the frequency and/or functional capacity of PD-1^hi CD8 TILs in tumor-bearing sepsis survivors, checkpoint blockade therapy using anti-PD-L1/anti-LAG-3 mAb was administered before or after the development of sepsis-induced lesions in CD8 TILs. Checkpoint blockade did not reduce tumor growth in CLP hosts when therapy was administered after PD-1^hi CD8 TILs had become reduced in frequency and/or function. However, early therapeutic intervention before lesions were observed significantly reduced tumor growth to levels seen in nonseptic hosts receiving therapy. Thus, sepsis-induced immunoparalysis is defined by diminished CD8 T cell-mediated antitumor immunity that can respond to timely checkpoint blockade, further emphasizing the importance of early cancer detection in hosts that survive sepsis.

Cutting Edge: Polymicrobial Sepsis Has the Capacity to Reinvigorate Tumor-Infiltrating CD8 T Cells and Prolong Host Survival

Malignancy increases sepsis incidence 10-fold and elevates sepsis-associated mortality. Advances in treatment have improved survival of cancer patients shortly after sepsis, but there is a paucity of information on how sepsis impacts cancer growth, development, and prognosis. To test this, cecal ligation and puncture surgery was performed on B16 melanoma-bearing mice to show that sepsis has detrimental effects in hosts with advanced tumors, leading to increased mortality. Surprisingly, mice experiencing cecal ligation and puncture-induced sepsis earlier during tumor development exhibited CD8 T cell-dependent attenuation of tumor growth. Sepsis-resistant CD8 tumor-infiltrating T cells showed increased in vivo activation, effector IFN-γ cytokine production, proliferation, and expression of activation/inhibitory PD-1/LAG-3 receptors because of a sepsis-induced liberation of tumor Ags. Sepsis-reinvigorated CD8 tumor-infiltrating T cells were also amenable to (anti-PD-L1/LAG-3) checkpoint blockade therapy, further prolonging cancer-associated survival in sepsis survivors. Thus, sepsis has the capacity to improve tumor-specific CD8 T cell responses, leading to better cancer prognosis and increased survival.

Myalgic encephalomyelitis or chronic fatigue syndrome: how could the illness develop?

A model of the development and progression of chronic fatigue syndrome (myalgic encephalomyelitis), the aetiology of which is currently unknown, is put forward, starting with a consideration of the post-infection role of damage-associated molecular patterns and the development of chronic inflammatory, oxidative and nitrosative stress in genetically predisposed individuals. The consequences are detailed, including the role of increased intestinal permeability and the translocation of commensal antigens into the circulation, and the development of dysautonomia, neuroinflammation, and neurocognitive and neuroimaging abnormalities. Increasing levels of such stress and the switch to immune and metabolic downregulation are detailed next in relation to the advent of hypernitrosylation, impaired mitochondrial performance, immune suppression, cellular hibernation, endotoxin tolerance and sirtuin 1 activation. The role of chronic stress and the development of endotoxin tolerance via indoleamine 2,3-dioxygenase upregulation and the characteristics of neutrophils, monocytes, macrophages and T cells, including regulatory T cells, in endotoxin tolerance are detailed next. Finally, it is shown how the immune and metabolic abnormalities of chronic fatigue syndrome can be explained by endotoxin tolerance, thus completing the model.

Sepsis-Induced T Cell Immunoparalysis: The Ins and Outs of Impaired T Cell Immunity

Sepsis results in a deluge of pro- and anti-inflammatory cytokines, leading to lymphopenia and chronic immunoparalysis. Sepsis-induced long-lasting immunoparalysis is defined, in part, by impaired CD4 and CD8 αβ T cell responses in the postseptic environment. The dysfunction in T cell immunity affects naive, effector, and memory T cells and is not restricted to classical αβ T cells. Although sepsis-induced severe and transient lymphopenia is a contributory factor to diminished T cell immunity, T cell-intrinsic and -extrinsic factors/mechanisms also contribute to impaired T cell function. In this review, we summarize the current knowledge of how sepsis quantitatively and qualitatively impairs CD4 and CD8 T cell immunity of classical and nonclassical T cell subsets and discuss current therapeutic approaches being developed to boost the recovery of T cell immunity postsepsis induction.

Polymicrobial Sepsis Chronic Immunoparalysis Is Defined by Diminished Ag-Specific T Cell-Dependent B Cell Responses

Immunosuppression is one hallmark of sepsis, decreasing the host response to the primary septic pathogens and/or secondary nosocomial infections. CD4 T cells and B cells are among the array of immune cells that experience reductions in number and function during sepsis. “Help” from follicular helper (Tfh) CD4 T cells to B cells is needed for productive and protective humoral immunity, but there is a paucity of data defining the effect of sepsis on a primary CD4 T cell-dependent B cell response. Using the cecal ligation and puncture (CLP) mouse model of sepsis induction, we observed reduced antibody production in mice challenged with influenza A virus or TNP-KLH in alum early (2 days) and late (30 days) after CLP surgery compared to mice subjected to sham surgery. To better understand how these CD4 T cell-dependent B cell responses were altered by a septic event, we immunized mice with a Complete Freund's Adjuvant emulsion containing the MHC II-restricted peptide 2W1S₅₆₋₆₈ coupled to the fluorochrome phycoerythrin (PE). Immunization with 2W1S-PE/CFA results in T cell-dependent B cell activation, giving us the ability to track defined populations of antigen-specific CD4 T cells and B cells responding to the same immunogen in the same mouse. Compared to sham mice, differentiation and class switching in PE-specific B cells were blunted in mice subjected to CLP surgery. Similarly, mice subjected to CLP had reduced expansion of 2W1S-specific T cells and Tfh differentiation after immunization. Our data suggest CLP-induced sepsis impacts humoral immunity by affecting the number and function of both antigen-specific B cells and CD4 Tfh cells, further defining the period of chronic immunoparalysis after sepsis induction.

Polymicrobial sepsis influences NK-cell-mediated immunity by diminishing NK-cell-intrinsic receptor-mediated effector responses to viral ligands or infections

The sepsis-induced cytokine storm leads to severe lymphopenia and reduced effector capacity of remaining/surviving cells. This results in a prolonged state of immunoparalysis, that contributes to enhanced morbidity/mortality of sepsis survivors upon secondary infection. The impact of sepsis on several lymphoid subsets has been characterized, yet its impact on NK-cells remains underappreciated–despite their critical role in controlling infection(s). Here, we observed numerical loss of NK-cells in multiple tissues after cecal-ligation-and-puncture (CLP)-induced sepsis. To elucidate the sepsis-induced lesions in surviving NK-cells, transcriptional profiles were evaluated and indicated changes consistent with impaired effector functionality. A corresponding deficit in NK-cell capacity to produce effector molecules following secondary infection and/or cytokine stimulation (IL-12,IL-18) further suggested a sepsis-induced NK-cell intrinsic impairment. To specifically probe NK-cell receptor-mediated function, the activating Ly49H receptor, that recognizes the murine cytomegalovirus (MCMV) m157 protein, served as a model receptor. Although relative expression of Ly49H receptor did not change, the number of Ly49H⁺ NK-cells in CLP hosts was reduced leading to impaired in vivo cytotoxicity and the capacity of NK-cells (on per-cell basis) to perform Ly49H-mediated degranulation, killing, and effector molecule production in vitro was also severely reduced. Mechanistically, Ly49H adaptor protein (DAP12) activation and clustering, assessed by TIRF microscopy, was compromised. This was further associated with diminished AKT phosphorylation and capacity to flux calcium following receptor stimulation. Importantly, DAP12 overexpression in NK-cells restored Ly49H/D receptors-mediated effector functions in CLP hosts. Finally, as a consequence of sepsis-dependent numerical and functional lesions in Ly49H⁺ NK-cells, host capacity to control MCMV infection was significantly impaired. Importantly, IL-2 complex (IL-2c) therapy after CLP improved numbers but not a function of NK-cells leading to enhanced immunity to MCMV challenge. Thus, the sepsis-induced immunoparalysis state includes numerical and NK-cell-intrinsic functional impairments, an instructive notion for future studies aimed in restoring NK-cell immunity in sepsis survivors.

Sepsis is an exaggerated host response to infection that can initially lead to significant morbidity/mortality and a long-lasting state of immunoparalysis in sepsis survivors. Sepsis-induced immunoparalysis functionally impairs numerous lymphocyte populations, including NK-cells. However, the scope and underlying mechanisms of NK-cell impairment and the consequences for NK-cell-mediated pathogen control remain underappreciated. NK-cells contribute to early host control of pathogens through a balance of activating and inhibitory receptors, and alterations in the number and capacity of NK-cells to exert receptor-mediated immunity can lead to dramatic impairment in host control of infection. The present study defines sepsis-induced numerical and cell-intrinsic functional impairments in NK-cell response to cytokine stimulation and receptor signaling that contribute to impaired host capacity to mount NK-cell-mediated effector responses and provide protection to bacterial and/or viral pathogens. Impairments in receptor signaling were due to reduced expression of adaptor protein DAP12. Importantly, the diminished ability of NK-cells from CLP hosts to provide anti-viral (MCMV) immunity is partially restored by IL-2 complex (IL-2c) therapy, which increased the number, but not function, of protective Ly49H⁺ NK-cells. Thus, these findings define sepsis-induced changes of the NK-cell compartment and provide insight into potential therapeutic interventions aimed at resolving sepsis-induced immunoparalysis in sepsis survivors.

Bacterial coinfection restrains antiviral CD8 T-cell response via LPS-induced inhibitory NK cells

Infection of specific pathogen-free mice with lymphocytic choriomeningitis virus (LCMV) is a widely used model to study antiviral T-cell immunity. Infections in the real world, however, are often accompanied by coinfections with unrelated pathogens. Here we show that in mice, systemic coinfection with E. coli suppresses the LCMV-specific cytotoxic T-lymphocyte (CTL) response and virus elimination in a NK cell- and TLR2/4-dependent manner. Soluble TLR4 ligand LPS also induces NK cell-mediated negative CTL regulation during LCMV infection. NK cells in LPS-treated mice suppress clonal expansion of LCMV-specific CTLs by a NKG2D- or NCR1-independent but perforin-dependent mechanism. These results suggest a TLR4-mediated immunoregulatory role of NK cells during viral-bacterial coinfections.

Alterations of the Innate Immune System in Susceptibility and Resilience After Social Defeat Stress

Dysregulation of innate immune responses has frequently been reported in stress-associated psychiatric disorders such as major depression. In mice, enhanced circulating cytokine levels as well as altered innate immune cell numbers have been found after stress exposure. In addition, stress-induced recruitment of peripheral monocytes to the brain has been shown to promote anxiety-like behavior. However, it is yet unclear whether specific differences in the innate immune system are associated with stress susceptibility or resilience in mice. Utilizing chronic social defeat, a model of depression and stress vulnerability, we characterized peripheral and brain-invading myeloid cells in stress-susceptible and resilient animals. In all defeated animals, we found reduced percentages of CD11c⁺ dendritic cells (DCs) by flow cytometry in the spleen when compared to non-defeated controls. Exclusively in susceptible mice conventional DCs of the spleen showed up-regulated expression of MHC class II and co-stimulatory CD80 molecules pointing toward an enhanced maturation phenotype of these cells. Susceptible, but not resilient animals further exhibited an increase in inflammatory Ly6C^hi monocytes and higher numbers of spleen-derived CD11b⁺ cells that produced the proinflammatory cytokine tumor necrosis factor (TNF) upon lipopolysaccharide (LPS) stimulation. Increased percentages of peripheral CD45^hi CD11b⁺ cells immigrated into the brain of defeated mice, regardless of resilience or susceptibility. However, cellular infiltrates in the brain of susceptible mice contained higher percentages of CC chemokine receptor 2 (CCR2⁺) Ly6C^hi monocytes representing an inflammatory phenotype. Thus, we defined specific stress-related immune signatures involving conventional DCs and inflammatory Ly6C^hi monocytes in susceptible and resilient mice. Together, our findings suggest an impact of the innate immune system in vulnerability to stress-related disorders such as major depression.

Endotoxin Tolerant Dendritic Cells Suppress Inflammatory Responses in Splenocytes via Interleukin-1 Receptor Associated Kinase (IRAK)-M and Programmed Death-Ligand 1 (PDL-1)

BACKGROUND Sepsis causes the highest mortality in non-cardiovascular intensive care units worldwide. Recent research has demonstrated that the late phase of sepsis, characterized as septic immunosuppression, is the central pathophysiological mechanism of immune dysfunction. Investigating the suppressive mechanism of immune cells may identify possible targets for therapy. MATERIAL AND METHODS We used LPS 2-hit model for dendritic cells (DCs) to establish endotoxin tolerance, and co-cultured with splenocytes. Co-culture responses and gene expressions were evaluated. RESULTS Endotoxin tolerant DCs showed irresponsiveness in pro-inflammatory cytokine production and expressed negative regulator genes of inflammation. When co-cultured with splenocytes, suppression of inflammatory responses and T cells apoptosis were observed with elevated expression of IRAK-M and PDL-1, and interference and neutralization of these 2 molecules led to partly reversed suppression of inflammation. CONCLUSIONS Our research found direct regulation of endotoxin tolerant DCs to other immune cells and suggested a possible mechanism via IRAK-M and PDL-1. This may inform research on septic immunosuppression and suggests possible therapeutic targets for sepsis.

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.

Autophagy: A Potential Therapeutic Target for Reversing Sepsis-Induced Immunosuppression

Sepsis remains the leading cause of mortality in intensive care units and an intractable condition due to uncontrolled inflammation together with immune suppression. Dysfunction of immune cells is considered as a major cause for poor outcome of septic patients but with little specific treatments. Currently, autophagy that is recognized as an important self-protective mechanism for cellular survival exhibits great potential for maintaining immune homeostasis and alleviating multiple organ failure, which further improves survival of septic animals. The protective effect of autophagy on immune cells covers both innate and adaptive immune responses and refers to various cellular receptors and intracellular signaling. Multiple drugs and measures are reportedly beneficial for septic challenge by inducing autophagy process. Therefore, autophagy might be an effective target for reversing immunosuppression compromised by sepsis.

Advances in the understanding and treatment of sepsis-induced immunosuppression

Sepsis is defined as a life-threatening organ dysfunction that is caused by a dysregulated host response to infection. Sepsis can induce acute kidney injury and multiple organ failures and represents the most common cause of death in the intensive care unit. Sepsis initiates a complex immune response that varies over time, with the concomitant occurrence of both pro-inflammatory and anti-inflammatory mechanisms. As a result, most patients with sepsis rapidly display signs of profound immunosuppression, which is associated with deleterious consequences. Scientific advances have highlighted the role of metabolic failure, epigenetic reprogramming, myeloid-derived suppressor cells, immature suppressive neutrophils and immune alterations in primary lymphoid organs (the thymus and bone marrow) in sepsis. An improved understanding of the mechanisms underlying this immunosuppression as well as of the similarities between sepsis-induced immunosuppression and immune defects in cancer or immunosenescence has led to novel therapeutic strategies aimed at stimulating immune function in patients with sepsis. Trials assessing the therapeutic benefit of IL-7, granulocyte-macrophage colony-stimulating factor (GM-CSF) and antibodies against programmed cell death protein 1 (PD1) and programmed cell death 1 ligand 1 (PDL1) for the treatment of sepsis are in progress. The reappraisal of sepsis pathophysiology has also resulted in a novel approach to the design of clinical trials evaluating sepsis treatments, based on an evaluation of the immune status and biomarker-based stratification of patients.

The potential mechanism of extracellular high mobility group box-1 protein mediated p53 expression in immune dysfunction of T lymphocytes

In the present study, we examined the activity of p53 protein in Jurkat cells treated with high mobility group box-1 protein (HMGB1), thereafter we investigated the mechanism of extracellular HMGB1 mediated p53 expression in immune dysfunction of T lymphocytes. mRNA expression of p53, mdm2, and p21 was determined by Real-time reverse transcription-polymerase chain reaction(RT-PCR). The apoptotic rate of Jurkat cells was analyzed by flow cytometry. Expressions of bcl-2, bax, caspase-3, phosphorylated (p) extracellular signal-regulated kinase (ERK)1/2, ERK1/2, p-p38 mitogen-activated protein kinase (MAPK), p38 MAPK, and p-c-jun amino-terminal kinase (JNK)1/2 and JNK1/2 were simultaneously determined by Western blotting. After treatment with HMGB1 (100 ng/ml or 1000 ng/ml), the proliferative activity of Jurkat cells was significantly decreased, and a low and medium concentration of HMGB1 induced an up-regulation of p53 mRNA, p-p53 and p53 protein expression. Meanwhile, levels of mdm2 and p21 were elevated by incubated with HMGB1 (100 ng/ml) for 24 or 48 hours. Moreover, the proliferation of Jurkat cells in response to HMGB1 (100 ng/ml) in the vector group was significantly depressed. The bax and caspase-3 levels in p53 shRNA-expressed cells treated with HMGB1 (100 ng/ml) was markedly decreased, whereas expression of bcl-2 was obviously enhanced. Among ERK1/2, p38 MAPK and JNK1/2 signaling, only p38 MAPK pathway could be significantly activated by treatment with HMGB1, and the specific inhibitor of p38 MAPK was used, p53 and p-p53 expression induced by HMGB1 were significantly down-regulated. Taken together, our data strongly indicated that HMGB1 might enhance p53 expression, which was associated with both the proliferative activity as well as apoptosis of T cells.

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.

Polymicrobial sepsis impairs bystander recruitment of effector cells to infected skin despite optimal sensing and alarming function of skin resident memory CD8 T cells

Sepsis is a systemic infection that enhances host vulnerability to secondary infections normally controlled by T cells. Using CLP sepsis model, we observed that sepsis induces apoptosis of circulating memory CD8 T-cells (TCIRCM) and diminishes their effector functions, leading to impaired CD8 T-cell mediated protection to systemic pathogen re-infection. In the context of localized re-infections, tissue resident memory CD8 T-cells (TRM) provide robust protection in a variety of infectious models. TRM rapidly 'sense' infection in non-lymphoid tissues and 'alarm' the host by enhancing immune cell recruitment to the site of the infection to accelerate pathogen clearance. Here, we show that compared to pathogen-specific TCIRCM, sepsis does not invoke significant numerical decline of Vaccinia virus induced skin-TRM keeping their effector functions (e.g., Ag-dependent IFN-γ production) intact. IFN-γ-mediated recruitment of immune cells to the site of localized infection was, however, reduced in CLP hosts despite TRM maintaining their 'sensing and alarming' functions. The capacity of memory CD8 T-cells in the septic environment to respond to inflammatory cues and arrive to the site of secondary infection/antigen exposure remained normal suggesting T-cell-extrinsic factors contributed to the observed lesion. Mechanistically, we showed that IFN-γ produced rapidly during sepsis-induced cytokine storm leads to reduced IFN-γR1 expression on vascular endothelium. As a consequence, decreased expression of adhesion molecules and/or chemokines (VCAM1 and CXCL9) on skin endothelial cells in response to TRM-derived IFN-γ was observed, leading to sub-optimal bystander-recruitment of effector cells and increased susceptibility to pathogen re-encounter. Importantly, as visualized by intravital 2-photon microscopy, exogenous administration of CXCL9/10 was sufficient to correct sepsis-induced impairments in recruitment of effector cells at the localized site of TRM antigen recognition. Thus, sepsis has the capacity to alter skin TRM anamnestic responses without directly impacting TRM number and/or function, an observation that helps to further define the immunoparalysis phase in sepsis survivors.

Murine Cytomegalovirus Disrupts Splenic Dendritic Cell Subsets via Type I Interferon-Dependent and -Independent Mechanisms

Dendritic cells (DC) are well-known modulators of immunity. This heterogeneous population is composed of defined subsets that exhibit functional specialization and are critical in initiating responses to pathogens. As such, many infectious agents employ strategies to disrupt DC functioning in attempts to evade the immune system. In some instances, this manifests as an outright loss of these cells. Previous work has suggested that, in the absence of an efficient natural killer (NK) cell response, murine cytomegalovirus (MCMV) induces large amounts of interferon (IFN)-I. This heightened IFN-I response is thought to contribute to conventional DC (cDC) loss and delayed development of T cell immunity. However, the precise role of IFN-I in such cDC loss remains unclear. We investigated the effects of licensed NK cells and IFN-I signaling on splenic cDC subsets during MCMV infection and found that a licensed NK cell response partially protects cDC numbers, but does not prevent increases in serum IFN-I. This suggested that high residual IFN-I could contribute to cDC loss. Therefore, we used multiple strategies to modulate IFN-I signaling during MCMV infection including plasmacytoid DC depletion, IFN-I receptor (IFNAR) blockade, and genetic ablation of IFNAR expression. Interestingly, restriction of IFN-I signals did not substantially preserve either CD8⁺ or CD4⁺ DC total numbers, but resulted in significant retention and/or accumulation of the splenic CD8⁻ CD4⁻ [double negative (DN)] subset. However, the DN DC effect manifested in a DC-extrinsic manner since IFNAR-deficient cells were not preferentially retained over their IFNAR wild-type counterparts in a mixed-chimera setting. Our results show that IFN-I signaling is not responsible for overt cDC toxicity in the setting of acute MCMV infection and emphasize that additional mechanisms contribute to DC loss and require exploration.