Myeloid-Derived Suppressor Cells in Sepsis

Immunophenotyping of Peripheral Blood Mononuclear Cells in Septic Shock Patients With High-Dimensional Flow Cytometry Analysis Reveals Two Subgroups With Differential Responses to Immunostimulant Drugs

Sepsis is associated with a dysregulated inflammatory response to infection. Despite the activation of inflammation, an immune suppression is often observed, predisposing patients to secondary infections. Therapies directed at restoration of immunity may be considered but should be guided by the immune status of the patients. In this paper, we described the use of a high-dimensional flow cytometry (HDCyto) panel to assess the immunophenotype of patients with sepsis. We then isolated peripheral blood mononuclear cells (PBMCs) from patients with septic shock and mimicked a secondary infection by stimulating PBMCs for 4 h in vitro with lipopolysaccharide (LPS) with or without prior exposure to either IFN-γ, or LAG-3Ig. We evaluated the response by means of flow cytometry and high-resolution clustering cum differential analysis and compared the results to PBMCs from healthy donors. We observed a heterogeneous immune response in septic patients and identified two major subgroups: one characterized by hypo-responsiveness (Hypo) and another one by hyper-responsiveness (Hyper). Hypo and Hyper groups showed significant differences in the production of cytokines/chemokine and surface human leukocyte antigen-DR (HLA-DR) expression in response to LPS stimulation, which were observed across all cell types. When pre-treated with either interferon gamma (IFN-γ) or lymphocyte-activation gene 3 (LAG)-3 recombinant fusion protein (LAG-3Ig) prior to LPS stimulation, cells from the Hypo group were shown to be more responsive to both immunostimulants than cells from the Hyper group. Our results demonstrate the importance of patient stratification based on their immune status prior to any immune therapies. Once sufficiently scaled, this approach may be useful for prescribing the right immune therapy for the right patient at the right time, the key to the success of any therapy.

Myeloid-Derived Suppressor Cells: Facilitators of Cancer and Obesity-Induced Cancer

Immature myeloid cells at varied stages of differentiation, known as myeloid-derived suppressor cells (MDSC), are present in virtually all cancer patients. MDSC are profoundly immune-suppressive cells that impair adaptive and innate antitumor immunity and promote tumor progression through nonimmune mechanisms. Their widespread presence combined with their multitude of protumor activities makes MDSC a major obstacle to cancer immunotherapies. MDSC are derived from progenitor cells in the bone marrow and traffic through the blood to infiltrate solid tumors. Their accumulation and suppressive potency are driven by multiple tumor- and host-secreted proinflammatory factors and adrenergic signals that act via diverse but sometimes overlapping transcriptional pathways. MDSC also accumulate in response to the chronic inflammation and lipid deposition characteristic of obesity and contribute to the more rapid progression of cancers in obese individuals. This article summarizes the key aspects of tumor-induced MDSC with a focus on recent progress in the MDSC field.

Neutrophils Encompass a Regulatory Subset Suppressing T Cells in Apparently Healthy Cattle and Mice

Neutrophils that reside in the bone marrow are swiftly recruited from circulating blood to fight infections. For a long time, these first line defenders were considered as microbe killers. However their role is far more complex as cross talk with T cells or dendritic cells have been described for human or mouse neutrophils. In cattle, these new roles are not documented yet. We identified a new subset of regulatory neutrophils that is present in the mouse bone marrow or circulate in cattle blood under steady state conditions. These regulatory neutrophils that display MHC-II on the surface are morphologically indistinguishable from classical MHC-II^neg neutrophils. However MHC-II^pos and MHC-II^neg neutrophils display distinct transcriptomic profiles. While MHC-II^neg and MHC-II^pos neutrophils display similar bacterial phagocytosis or killing activity, MHC-II^pos only are able to suppress T cell proliferation under contact-dependent mechanisms. Regulatory neutrophils are highly enriched in lymphoid organs as compared to their MHC-II^neg counterparts and in the mouse they express PDL-1, an immune checkpoint involved in T-cell blockade. Our results emphasize neutrophils as true partners of the adaptive immune response, including in domestic species. They open the way for discovery of new biomarkers and therapeutic interventions to better control cattle diseases.

Immunopathogenesis of Craniotomy Infection and Niche-Specific Immune Responses to Biofilm

Bacterial infections in the central nervous system (CNS) can be life threatening and often impair neurological function. Biofilm infection is a complication following craniotomy, a neurosurgical procedure that involves the removal and replacement of a skull fragment (bone flap) to access the brain for surgical intervention. The incidence of infection following craniotomy ranges from 1% to 3% with approximately half caused by Staphylococcus aureus (S. aureus). These infections present a significant therapeutic challenge due to the antibiotic tolerance of biofilm and unique immune properties of the CNS. Previous studies have revealed a critical role for innate immune responses during S. aureus craniotomy infection. Experiments using knockout mouse models have highlighted the importance of the pattern recognition receptor Toll-like receptor 2 (TLR2) and its adaptor protein MyD88 for preventing S. aureus outgrowth during craniotomy biofilm infection. However, neither molecule affected bacterial burden in a mouse model of S. aureus brain abscess highlighting the distinctions between immune regulation of biofilm vs. planktonic infection in the CNS. Furthermore, the immune responses elicited during S. aureus craniotomy infection are distinct from biofilm infection in the periphery, emphasizing the critical role for niche-specific factors in dictating S. aureus biofilm-leukocyte crosstalk. In this review, we discuss the current knowledge concerning innate immunity to S. aureus craniotomy biofilm infection, compare this to S. aureus biofilm infection in the periphery, and discuss the importance of anatomical location in dictating how biofilm influences inflammatory responses and its impact on bacterial clearance.

Immunological Endotyping of Chronic Critical Illness After Severe Sepsis

Improved management of severe sepsis has been one of the major health care accomplishments of the last two decades. Due to enhanced recognition and improved management of severe sepsis, in-hospital mortality has been reduced by up to 40%. With that good news, a new syndrome has unfortunately replaced in-hospital multi-organ failure and death. This syndrome of chronic critical illness (CCI) includes sepsis patients who survive the early "cytokine or genomic storm," but fail to fully recover, and progress into a persistent state of manageable organ injury requiring prolonged intensive care. These patients are commonly discharged to long-term care facilities where sepsis recidivism is high. As many as 33% of sepsis survivors develop CCI. CCI is the result, at least in part, of a maladaptive host response to chronic pattern-recognition receptor (PRR)-mediated processes. This maladaptive response results in dysregulated myelopoiesis, chronic inflammation, T-cell atrophy, T-cell exhaustion, and the expansion of suppressor cell functions. We have defined this panoply of host responses as a persistent inflammatory, immune suppressive and protein catabolic syndrome (PICS). Why is this important? We propose that PICS in survivors of critical illness is its own common, unique immunological endotype driven by the constant release of organ injury-associated, endogenous alarmins, and microbial products from secondary infections. While this syndrome can develop as a result of a diverse set of pathologies, it represents a shared outcome with a unique underlying pathobiological mechanism. Despite being a common outcome, there are no therapeutic interventions other than supportive therapies for this common disorder. Only through an improved understanding of the immunological endotype of PICS can rational therapeutic interventions be designed.

Myeloid-derived suppressor cells (MDSC): When good intentions go awry

MDSC are a heterogeneous population of immature myeloid cells that are released by biological stress such as tissue damage and inflammation. Conventionally, MDSC are known for their detrimental role in chronic inflammation and neoplastic conditions. However, their intrinsic functions in immunoregulation, wound healing, and angiogenesis are intended to protect from over-reactive immune responses, maintenance of immunotolerance, tissue repair, and homeostasis. Paradoxically, under certain conditions, MDSC can impair protective immune responses and exacerbate the disease. The transition from protective to harmful MDSC is most likely driven by environmental and epigenetic mechanisms induced by prolonged exposure to unresolved inflammatory triggers. Here, we review several examples of the dual impact of MDSC in conditions such as maternal-fetal tolerance, self-antigens immunotolerance, obesity-associated cancer, sepsis and trauma. Moreover, we also highlighted the evidence indicating that MDSC have a role in COVID-19 pathophysiology. Finally, we have summarized the evidence indicating epigenetic mechanisms associated with MDSC function.

Effects of vitamin D on macrophages and myeloid-derived suppressor cells (MDSCs) hyperinflammatory response in the lungs of COVID-19 patients

Vitamin D regulates homeostasis, anti-microbial response, and inflammation. The vitamin D receptors are expressed in the macrophages and other immune cells, regulating the transcription of many different genes, including those coding the anti-microbial peptides. One of the most severe complications of the SARS-CoV-2 infection is the acute respiratory distress syndrome (ARDS) caused by the hyperinflammatory response (commonly called cytokine storm) of the lung macrophages. Studies showed that Vitamin D deficiency increases the severity of the ARDS in COVID-19 infection. We discuss here how the vitamin D supplementation may influence macrophage and myeloid-derived suppressor cells (MDSCs) inflammatory response, subdue the hyperinflammatory response, and lessen the ARDS in COVID-19 patients.

Bifunctional Janus Particles as Multivalent Synthetic Nanoparticle Antibodies (SNAbs) for Selective Depletion of Target Cells

Monoclonal antibodies (mAb) have had a transformative impact on treating cancers and immune disorders. However, their use is limited by high development time and monetary cost, manufacturing complexities, suboptimal pharmacokinetics, and availability of disease-specific targets. To address some of these challenges, we developed an entirely synthetic, multivalent, Janus nanotherapeutic platform, called Synthetic Nanoparticle Antibodies (SNAbs). SNAbs, with phage-display-identified cell-targeting ligands on one "face" and Fc-mimicking ligands on the opposite "face", were synthesized using a custom, multistep, solid-phase chemistry method. SNAbs efficiently targeted and depleted myeloid-derived immune-suppressor cells (MDSCs) from mouse-tumor and rat-trauma models, ex vivo. Systemic injection of MDSC-targeting SNAbs efficiently depleted circulating MDSCs in a mouse triple-negative breast cancer model, enabling enhanced T cell and Natural Killer cell infiltration into tumors. Our results demonstrate that SNAbs are a versatile and effective functional alternative to mAbs, with advantages of a plug-and-play, cell-free manufacturing process, and high-throughput screening (HTS)-enabled library of potential targeting ligands.

Development of systemic immune dysregulation in a rat trauma model of biomaterial-associated infection

Orthopedic biomaterial-associated infections remain a major clinical challenge, with Staphylococcus aureus being the most common pathogen. S. aureus biofilm formation enhances immune evasion and antibiotic resistance, resulting in a local, indolent infection that can persist long-term without symptoms before eventual hardware failure, bone non-union, or sepsis. Immune modulation is an emerging strategy to combat host immune evasion by S. aureus. However, most immune modulation strategies are focused on local immune responses at the site of infection, with little emphasis on understanding the infection-induced and orthopedic-related systemic immune responses of the host, and their role in local infection clearance and tissue regeneration. This study utilized a rat bone defect model to investigate how implant-associated infection affects the systemic immune response. Long-term systemic immune dysregulation was observed with a significant systemic decrease in T cells and a concomitant increase in immunosuppressive myeloid-derived suppressor cells (MDSCs) compared to non-infected controls. Further, the control group exhibited a regulated and coordinated systemic cytokine response, which was absent in the infection group. Multivariate analysis revealed high levels of MDSCs to be most correlated with the infection group, while high levels of T cells were most correlated with the control group. Locally, the infection group had attenuated macrophage infiltration and increased levels of MDSCs in the local soft tissue compared to non-infected controls. These data reveal the widespread impacts of an orthopedic infection on both the local and the systemic immune responses, uncovering promising targets for diagnostics and immunotherapies that could optimize treatment strategies and ultimately improve patient outcomes.

Monitoring immunomodulation in patients with sepsis

Introduction: This review aims to summarize current progress of the last ten years in the development of biomarkers used for classifying the immune response of the septic host and for monitoring the efficacy of the applied adjunctive immunotherapy.Areas covered: An extensive search of the literature was performed. In this review the authors discuss available biomarkers of host immune response in sepsis toward two directions; immunosuppression and hyperinflammation. Ferritin, sCD163, sIL-2 ra, and IL-18 may help in the diagnosis of macrophage activation syndrome (MAS) complicating sepsis whereas lymphopenia, decreased HLA-DR expression on monocytes, overexpression of Programmed cell death protein-1 (PD-1)/Programmed death-ligand 1 (PD-L1) and IL-10 are indicators of sepsis-induced immunosuppression. Novel approaches in the classification of immune state in sepsis include Myeloid-Derived Suppressor Cells (MDSC) and specific endotypes, defined by gene expression and molecular techniques.Expert opinion: HLA-DR and ferritin are the most commonly used biomarkers to monitor immunomodulation in clinical practice whereas developing specific sepsis endotypes is the future target. New immunotherapy trials in sepsis need to incorporate biomarkers for a personalized treatment.

Frontline Science: Extracellular CIRP generates a proinflammatory Ly6G+ CD11bhi subset of low-density neutrophils in sepsis

Extracellular cold-inducible RNA-binding protein (eCIRP) is a damage-associated molecular pattern. Neutrophils present in the mononuclear cell fraction of Ficoll gradient separation are called low-density neutrophils (LDNs). Here we report the novel role of eCIRP on LDNs' heterogeneity in sepsis. Sepsis was induced in male C57BL/6 wild-type (WT) and CIRP^-/- mice by cecal ligation and puncture (CLP). At 20 h after CLP, LDNs in the blood were isolated by Ficoll gradient separation, followed by staining the cells with anti-Ly6G and anti-CD11b Abs and detection by flow cytometry. Sepsis or recombinant murine CIRP (rmCIRP) injection in mice resulted in significant increase in the frequency (%) and number of Ly6G⁺ CD11b^hi and Ly6G⁺ CD11b^lo LDNs in the blood compared to sham- or vehicle-treated mice. At 20 h of CLP, CIRP^-/- mice had significantly lower frequency and number of Ly6G⁺ CD11b^hi and Ly6G⁺ CD11b^lo LDNs in the blood compared to WT mice. In sepsis mice or rmCIRP-injected mice, compared to Ly6G⁺ CD11b^lo LDNs, the expression of CXCR4, ICAM-1, and iNOS and formation of reactive oxygen species, and neutrophil extracellular traps in Ly6G⁺ CD11b^hi LDNs in the blood were significantly increased. Treatment of WT bone marrow-derived neutrophils (BMDNs) with rmCIRP increased Ly6G⁺ CD11b^hi LDN frequency, whereas treatment of TLR4^-/- BMDNs with rmCIRP significantly decreased the frequency of Ly6G⁺ CD11b^hi LDNs. BMDNs' stimulation with rmCIRP increased the expression of transcription factors in LDNs. eCIRP induces the formation of a proinflammatory phenotype Ly6G⁺ CD11b^hi of LDNs through TLR4. Targeting eCIRP may provide beneficial outcomes in sepsis by decreasing proinflammatory Ly6G⁺ CD11b^hi LDNs.

Myeloid-Derived Suppressor Cells in Pregnancy and the Neonatal Period

During pregnancy, the immune systems of mother and offspring are challenged by their close adjacency to balance tolerance and rejection. After birth the neonate has to continue this balance towards its new environment by tolerating commensals while rejecting pathogens and towards its developing tissues to avoid inflammatory damage while overcoming immunosuppression. Our group was the first to link immunosuppressive features of myeloid derived suppressor cells (MDSC) to materno-fetal tolerance, neonatal susceptibility to infection and inflammation control. Here we summarize recent advances in this dynamic field.

Recent advances in myeloid-derived suppressor cell biology

In recent years, studying the role of myeloid-derived suppressor cells (MDSCs) in many pathological inflammatory conditions has become a very active research area. Although the role of MDSCs in cancer is relatively well established, their role in non-cancerous pathological conditions remains in its infancy resulting in much confusion. Our objectives in this review are to address some recent advances in MDSC research in order to minimize such confusion and to provide an insight into their function in the context of other diseases. The following topics will be specifically focused upon: (1) definition and characterization of MDSCs; (2) whether all MDSC populations consist of immature cells; (3) technical issues in MDSC isolation, estimation and characterization; (4) the origin of MDSCs and their anatomical distribution in health and disease; (5) mediators of MDSC expansion and accumulation; (6) factors that determine the expansion of one MDSC population over the other; (7) the Yin and Yang roles of MDSCs. Moreover, the functions of MDSCs will be addressed throughout the text.

The Critical Roles and Mechanisms of Immune Cell Death in Sepsis

Sepsis was first described by the ancient Greek physicians over 2000 years ago. The pathophysiology of the disease, however, is still not fully understood and hence the mortality rate is still unacceptably high due to lack of specific therapies. In the last decade, great progress has been made by shifting the focus of research from systemic inflammatory response syndrome (SIRS) to multiple organ dysfunction syndrome (MODS). Sepsis has been re-defined as infection-induced MODS in 2016. How infection leads to MODS is not clear, but what mediates MODS becomes the major topic in understanding the molecular mechanisms and developing specific therapies. Recently, the mechanism of infection-induced extensive immune cell death which releases a large quantity of damage-associated molecular patterns (DAMPs) and their roles in the development of MODS as well as immunosuppression during sepsis have attracted much attention. Growing evidence supports the hypothesis that DAMPs, including high-mobility group box 1 protein (HMGB1), cell-free DNA (cfDNA) and histones as well as neutrophil extracellular traps (NETs), may directly or indirectly contribute significantly to the development of MODS. Here, we provide an overview of the mechanisms and consequences of infection-induced extensive immune cell death during the development of sepsis. We also propose a pivotal pathway from a local infection to eventual sepsis and a potential combined therapeutic strategy for targeting sepsis.

Neuroendocrine Modulation of the Immune Response after Trauma and Sepsis: Does It Influence Outcome?

Although the treatment of multiple-injured patients has been improved during the last decades, sepsis and multiple organ failure (MOF) still remain the major cause of death. Following trauma, profound alterations of a large number of physiological systems can be observed that may potentially contribute to the development of sepsis and MOF. This includes alterations of the neuroendocrine and the immune system. A large number of studies focused on posttraumatic changes of the immune system, but the cause of posttraumatic immune disturbance remains to be established. However, an increasing number of data indicate that the bidirectional interaction between the neuroendocrine and the immune system may be an important mechanism involved in the development of sepsis and MOF. The aim of this article is to highlight the current knowledge of the neuroendocrine modulation of the immune system during trauma and sepsis.

JAK-STAT Targeting Offers Novel Therapeutic Opportunities in Sepsis

Sepsis is a life-threatening condition caused by exaggerated host responses to infections taking place in two phases: (i) a systemic (hyper)inflammatory response syndrome (SIRS), participating in multiple organ failure (MOF), a major complication of septic shock, followed by (ii) a compensatory anti-inflammatory response syndrome (CARS), leading to sepsis-induced immunosuppression and resulting in late infections and long-term mortality. The Janus kinase-signal transducer and activator of transcription (JAK-STAT)-dependent signaling pathway is involved in both manifestations, hence playing a key role during sepsis. It is also involved in emergency myelopoiesis, which participates in host defense. The aim of this review is to highlight and refine the recent implications of this signaling pathway in sepsis and illustrate why its central position makes it a potential biomarker and therapeutic target.

Activation of STING by cGAMP Regulates MDSCs to Suppress Tumor Metastasis via Reversing Epithelial-Mesenchymal Transition

The role of cGAMP stimulating cGAS-cGAMP-STING-IRF3 pathway to inhibit tumor growth was well-established. Herein, the efficiency and pharmacological mechanism of cGAMP on regulating tumor metastasis was investigated. The effects of cGAMP regulating CD8⁺ T cells and myeloid-derived suppressor cells (MDSCs) in tumor microenvironment was explored. In this study, we found that cGAMP boosted STING signaling pathway to activate the production of IFN-γ from CD8⁺ T cells, and decreased the population of MDSCs in vivo. The metastasis in CT26 tumor bearing mice was inhibited by cGAMP via regulating EMT process. cGAMP played an important role in suppressing the production of reactive oxygen species (ROS) and nitric oxide (NO) from MDSCs, abolished the suppressive function of MDSCs to the T cells. All in all, the results indicated that the STING agonist cGAMP activated the production of IFN-γ from CD8⁺ T cells to suppress MDSCs in vivo.

Immunologic timeline of Ebola virus disease and recovery in humans

A complete understanding of human immune responses to Ebola virus infection is limited by the availability of specimens and the requirement for biosafety level 4 (BSL-4) containment. In an effort to bridge this gap, we evaluated cryopreserved PBMCs from 4 patients who survived Ebola virus disease (EVD) using an established mass cytometry antibody panel to characterize various cell populations during both the acute and convalescent phases. Acute loss of nonclassical monocytes and myeloid DCs, especially CD1c+ DCs, was noted. Classical monocyte proliferation and CD38 upregulation on plasmacytoid DCs coincided with declining viral load. Unsupervised analysis of cell abundance demonstrated acute declines in monocytic, NK, and T cell populations, but some populations, many of myeloid origin, increased in abundance during the acute phase, suggesting emergency hematopoiesis. Despite cell losses during the acute phase, upregulation of Ki-67 correlated with recovery of cell populations over time. These data provide insights into the human immune response during EVD.

Selective targeting of different populations of myeloid-derived suppressor cells by histone deacetylase inhibitors

Myeloid-derived suppressor cells (MDSCs) are widely implicated in negative regulation of immune responses in cancer. Inhibition of class I histone deacetylases (HDAC) with entinostat has anti-MDSC activity. However, as single agent, it did not delay tumor growth in EL4 and LLC tumor models. Here, we found that entinostat reduced immune suppressive activity of only one type of MDSC-polymorphonuclear, PMN-MDSC, whereas it had no effect on monocytic M-MDSC or macrophages. M-MDSC had high amount of class II HDAC-HDAC6, which was further increased after the treatment of mice with entinostat. Inhibition of HDAC6 with ricolinostat reduced suppressive activity of M-MDSC, but did not affect PMN-MDSC or delayed tumor growth. However, combination of entinostat and ricolinostat abrogated suppressive activity of both populations of MDSC and substantially delayed tumor progression. Thus, inactivation of MDSC required targeting of both major subsets of these cells via inhibitors of class I and class II HDAC.

Distinct Neutrophil Populations in the Spleen During PICS

While mortality after acute sepsis has decreased, the long-term recovery for survivors is still poor, particularly those developing persistent inflammation, immunosuppression, and catabolism syndrome (PICS). While previously thought that activated neutrophils responding to the acute phase of sepsis migrate to the spleen to undergo cell death and contribute to immunosuppression, our data show a significant accumulation of distinct, yet functional, neutrophil populations in the spleen in a murine model of PICS. The exact role and function of neutrophils in this response is still unclear. The objective of our study was to better define the immune function of splenic neutrophils to determine if this could give insight into the pathogenesis of PICS. Using a murine model of cecal ligation and puncture (CLP), which demonstrates all characteristics of PICS by 8 days, spleens were harvested, and neutrophils were identified by Ly6G and CD11b expression via flow cytometry. Nearly all splenic neutrophils expressed CD54, but there were distinct CD54^hi and CD54^lo cells, with the majority being CD54^lo cells during PICS. The CD54^hi population showed traditional, proinflammatory properties, but a relatively decreased chemotactic response, while CD54^lo cells had significantly higher chemotaxis, yet significantly decreased proinflammatory functions. Using 5-ethynyl-2′-deoxyuridine (EdU) incorporation, we found that the CD54^hi population on day 2 after CLP may be participating in emergency myelopoiesis. However, the vast majority of the CD54^lo population were paused in the G₁ phase at this time point and not proliferating. By day 8 after CLP, most of the CD54^hi cells in the spleen were no longer proliferating, while the CD54^lo cells were, indicating that CD54^lo dominate in extramedullary myelopoiesis at later time points. Almost none of the neutrophils produced arginase or inducible nitric oxide synthase (iNOS), indicating that these are not suppressor cells. Overall, our data demonstrate that neutrophil accumulation in the spleen during PICS is related to extramedullary myelopoiesis, leading to the production of immature neutrophils. While not suppressor cells, the majority have greater chemotactic function but less inflammatory responsiveness, which may contribute to the immunosuppression seen in PICS. Attention to these distinct neutrophil populations after septic or other systemic inflammatory responses is therefore critical to understanding the mechanisms of PICS.

Scald Injury-Induced T Cell Dysfunction Can Be Mitigated by Gr1+ Cell Depletion and Blockage of CD47/CD172a Signaling

Infection is a common and severe complication of burn injury: Sepsis accounts for 47% of postburn mortality. Burn-induced T cell suppression likely contributes to the increased infection susceptibility in burn patients. However, little is known about the kinetics of T cell dysfunction after burn and its underlying mechanisms. In this study, we show in a murine scald injury model that T cell activation of both CD4⁺ and CD8⁺ T cells as well as T cell cytokine production is suppressed acutely and persistently for at least 11 days after burn injury. Purified T cells from scald-injured mice exhibit normal T cell functions, indicating an extrinsically mediated defect. We further show that T cell dysfunction after burn appears to be cell-to-cell contact dependent and can be ameliorated by depletion of myeloid-derived suppressor cells. These cells expand after burn injury, particularly a subset expressing the checkpoint inhibitor CD172a, and infiltrate germinal centers. Expression of CD172a appears to be driven by ingestion of immature reticulocytes. Immature reticulocytes are drastically increased in the spleen of scald mice and may contribute to immunosuppression through more direct mechanisms as well. Overall, our study newly identifies two cell populations, myeloid-derived suppressor cells and immature reticulocytes, as well as the CD47/CD172a-signaling pathways as mediators of T cell suppressors after burn and thus opens up new research opportunities in the search for new therapies to combat increased infection susceptibility and the associated morbidity and mortality in burn victims.

Myeloid cells in sepsis-acquired immunodeficiency

On May 2017, the World Health Organization recognized sepsis as a global health priority. Sepsis profoundly perturbs immune homeostasis by initiating a complex response that varies over time, with the concomitant occurrence of pro- and anti-inflammatory mechanisms. Sepsis deeply impacts myeloid cell response. Different mechanisms are at play, such as apoptosis, endotoxin tolerance, metabolic failure, epigenetic reprogramming, and central regulation. This induces systemic effects on circulating immune cells and impacts progenitors locally in lymphoid organs. In the bone marrow, a progressive shift toward the release of immature myeloid cells (including myeloid-derived suppressor cells), at the expense of mature neutrophils, takes place. Circulating dendritic cell number remains dramatically low and monocytes/macrophages display an anti-inflammatory phenotype and reduced antigen presentation capacity. Intensity and persistence of these alterations are associated with increased risk of deleterious outcomes in patients. Thus, myeloid cells dysfunctions play a prominent role in the occurrence of sepsis-acquired immunodeficiency. For the most immunosuppressed patients, this paves the way for clinical trials evaluating immunoadjuvant molecules (granulocyte-macrophage colony-stimulating factor and interferon gamma) aimed at restoring homeostatic myeloid cell response. Our review offers a summary of sepsis-induced myeloid cell dysfunctions and current therapeutic strategies proposed to target these defects in patients.

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.

Role of Neutrophils and Myeloid-Derived Suppressor Cells in Glioma Progression and Treatment Resistance

Recent efforts in brain tumor research have been directed towards the modulation of the immune system for therapeutic interventions. Several human cancers, including gliomas, are infiltrated with immune cell types-including neutrophils and myeloid-derived suppressor cells-that contribute to tumor progression, invasiveness, and treatment resistance. The role of tumor-associated neutrophils and myeloid-derived suppressor cells in cancer biology remains elusive, as these cells can exert a multitude of pro-tumor and antitumor effects. In this review, we provide the current understanding and novel insights on the role of neutrophils and myeloid-derived suppressor cells in glioma progression and treatment resistance, as well as the mechanisms of pleiotropic behaviors in these cells during disease progression, with an emphasis on possible strategies to reprogram these cells towards their antitumor actions.

Immune correlates of postexposure vaccine protection against Marburg virus

Postexposure immunization can prevent disease and reduce transmission following pathogen exposure. The rapid immunostimulatory properties of recombinant vesicular stomatitis virus (rVSV)-based vaccines make them suitable postexposure treatments against the filoviruses Ebola virus and Marburg virus (MARV); however, the mechanisms that drive this protection are undefined. Previously, we reported 60–75% survival of rhesus macaques treated with rVSV vectors expressing MARV glycoprotein (GP) 20–30 minutes after a low dose exposure to the most pathogenic variant of MARV, Angola. Survival in this model was linked to production of GP-specific antibodies and lower viral load. To confirm these results and potentially identify novel correlates of postexposure protection, we performed a similar experiment, but analyzed plasma cytokine levels, frequencies of immune cell subsets, and the transcriptional response to infection in peripheral blood. In surviving macaques (80–89%), we observed induction of genes mapping to antiviral and interferon-related pathways early after treatment and a higher percentage of T helper 1 (Th1) and NK cells. In contrast, the response of non-surviving macaques was characterized by hypercytokinemia; a T helper 2 signature; recruitment of low HLA-DR expressing monocytes and regulatory T-cells; and transcription of immune checkpoint (e.g., PD-1, LAG3) genes. These results suggest dysregulated immunoregulation is associated with poor prognosis, whereas early innate signaling and Th1-skewed immunity are important for survival.

An immune-cell signature of bacterial sepsis

Dysregulation of the immune response to bacterial infection can lead to sepsis, a condition with high mortality. Multiple whole-blood gene-expression studies have defined sepsis-associated molecular signatures, but have not resolved changes in transcriptional states of specific cell types. Here, we used single-cell RNA-sequencing to profile the blood of people with sepsis (n = 29) across three clinical cohorts with corresponding controls (n = 36). We profiled total peripheral blood mononuclear cells (PBMCs, 106,545 cells) and dendritic cells (19,806 cells) across all subjects and, on the basis of clustering of their gene-expression profiles, defined 16 immune-cell states. We identified a unique CD14⁺ monocyte state that is expanded in people with sepsis and validated its power in distinguishing these individuals from controls using public transcriptomic data from subjects with different disease etiologies and from multiple geographic locations (18 cohorts, n = 1,467 subjects). We identified a panel of surface markers for isolation and quantification of the monocyte state and characterized its epigenomic and functional phenotypes, and propose a model for its induction from human bone marrow. This study demonstrates the utility of single-cell genomics in discovering disease-associated cytologic signatures and provides insight into the cellular basis of immune dysregulation in bacterial sepsis.

The Generation and Identity of Human Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) are cells of myeloid lineage with a potent immunosuppressive capacity. They are present in cancer patients as well as in patients with severe inflammatory conditions and infections. MDSCs exist as two main subtypes, the granulocytic (G-MDSCs) and the monocytic (Mo-MDSCs) type, as defined by their surface phenotype and functions. While the functions of MDSCs have been investigated in depth, the origin of human MDSCs is less characterized and even controversial. In this review, we recapitulate theories on how MDSCs are generated in mice, and whether this knowledge is translatable into human MDSC biology, as well as on problems of defining MDSCs by their immature cell surface phenotype in relation to the plasticity of myeloid cells. Finally, the challenge of pharmacological targeting of MDSCs in the future is envisioned.

Immunotherapy in sepsis - brake or accelerate?

Sepsis, a life threating syndrome characterized by organ failure after infection, is the most common cause of death in hospitalized patients. The treatment of sepsis is generally supportive in nature, involving the administration of intravenous fluids, vasoactive substances and oxygen plus antibiotics to eliminate the pathogen. No drugs have been approved specifically for the treatment of sepsis, and clinical trials of potential therapies have failed to reduce mortality - suggesting that new approaches are needed. Abnormalities in the immune response elicited by the pathogen, ranging from excessive inflammation to immunosuppression, contribute to disease pathogenesis. Although hundreds of immunomodulatory agents are potentially available, it remains unclear which patient benefits from which immune therapy at a given time point. Results indicate the importance of personalized therapy, specifically the need to identify the type of intervention required by each individual patient at a given point in the disease process. To address this issue will require using biomarkers to stratify patients based on their individual immune status. This article reviews recent and ongoing clinical investigations using immunostimulatory or immunosuppressive therapies against sepsis including non-pharmacological and novel preclinical approaches.

Myeloid-derived suppressor cell function and epigenetic expression evolves over time after surgical sepsis

Background

Sepsis is an increasingly significant challenge throughout the world as one of the major causes of patient morbidity and mortality. Central to the host immunologic response to sepsis is the increase in circulating myeloid-derived suppressor cells (MDSCs), which have been demonstrated to be present and independently associated with poor long-term clinical outcomes. MDSCs are plastic cells and potentially modifiable, particularly through epigenetic interventions. The objective of this study was to determine how the suppressive phenotype of MDSCs evolves after sepsis in surgical ICU patients, as well as to identify epigenetic differences in MDSCs that may explain these changes.

Methods

Circulating MDSCs from 267 survivors of surgical sepsis were phenotyped at various intervals over 6 weeks, and highly enriched MDSCs from 23 of these samples were co-cultured with CD3/CD28-stimulated autologous T cells. microRNA expression from enriched MDSCs was also identified.

Results

We observed that MDSC numbers remain significantly elevated in hospitalized sepsis survivors for at least 6 weeks after their infection. However, only MDSCs obtained at and beyond 14 days post-sepsis significantly suppressed T lymphocyte proliferation and IL-2 production. These same MDSCs displayed unique epigenetic (miRNA) expression patterns compared to earlier time points.

Conclusions

We conclude that in sepsis survivors, immature myeloid cell numbers are increased but the immune suppressive function specific to MDSCs develops over time, and this is associated with a specific epigenome. These findings may explain the chronic and persistent immune suppression seen in these subjects.

Sepsis Immunometabolism: From Defining Sepsis to Understanding How Energy Production Affects Immune Response

OBJECTIVES

This review will examine current definitions and trends in sepsis management as well pathophysiologic mechanisms in animal and ex vivo studies that correlate decreased energy production with deranged inflammatory response during the septic process.

DATA SOURCES

The latest articles in the literature that focus on the role of immunometabolism and associated mechanisms in sepsis were selected.

STUDY SELECTION

The most relevant, original articles were included in the review.

DATA EXTRACTION

All pertinent data for sepsis definitions as well as changes in immunometabolic pathways during the septic process was reviewed and assessed for inclusion in this article.

DATA SYNTHESIS

Sepsis is a major cause of multiple organ dysfunction. It is the principal cause of death resulting from infection and one of the most expensive conditions treated in the United States. Despite current efforts to accurately define sepsis, novel treatments and highly trained providers, mortality rates for sepsis remain high, prompting a need for further investigation of underlying immunometabolic mechanisms to identify potential treatment targets. The definition of sepsis has shifted and changed in the past few decades due to poorly defined criteria, as well as unclear guidelines for providers with regards to management of severe sepsis and septic shock. The early identification of patients with a systemic inflammatory response that will progress to septic shock is critical since recent traditional therapeutic approaches, such as early goal-directed therapy, IV immunoglobulin, and anti-tumor necrosis factor-α antibodies have failed.

CONCLUSIONS

There are no effective anti-sepsis drug therapies due to complex inflammatory and metabolic interactions. Further studies regarding the interface between innate immunity and metabolism should be investigated to effectively address septic patient mortality rates.

C-Reactive Protein Promotes the Expansion of Myeloid Derived Cells With Suppressor Functions

Previously we established that human C-reactive protein (CRP) exacerbates mouse acute kidney injury and that the effect was associated with heightened renal accumulation of myeloid derived cells with suppressor functions (MDSC). Herein we provide direct evidence that CRP modulates the development and suppressive actions of MDSCs in vitro. We demonstrate that CRP dose-dependently increases the generation of MDSC from wild type mouse bone marrow progenitors and enhances MDSC production of intracellular reactive oxygen species (iROS). When added to co-cultures, CRP significantly enhanced the ability of MDSCs to suppress CD3/CD28-stimulated T cell proliferation. Experiments using MDSCs from FcγRIIB deficient mice (FcγRIIB^-/-) showed that CRP's ability to expand MDSCs and trigger their increased production of iROS was FcγRIIB-independent, whereas its ability to enhance the MDSC T cell suppressive action was FcγRIIB-dependent. Importantly, CRP also enabled freshly isolated primary human neutrophils to suppress proliferation of autologous T cells. These findings suggest that CRP might be an endogenous regulator of MDSC numbers and actions in vivo.

Spectrum of Trained Innate Immunity Induced by Low-Virulence Candida Species against Lethal Polymicrobial Intra-abdominal Infection

Polymicrobial intra-abdominal infections (IAI) are clinically prevalent and cause significant morbidity and mortality, especially those involving fungi. Our laboratory developed a mouse model of polymicrobial IAI and demonstrated that coinfection with Candida albicans and Staphylococcus aureus (C. albicans/S. aureus) results in 80 to 90% mortality in 48 to 72 h due to robust local and systemic inflammation.

Polymicrobial intra-abdominal infections (IAI) are clinically prevalent and cause significant morbidity and mortality, especially those involving fungi. Our laboratory developed a mouse model of polymicrobial IAI and demonstrated that coinfection with Candida albicans and Staphylococcus aureus (C. albicans/S. aureus) results in 80 to 90% mortality in 48 to 72 h due to robust local and systemic inflammation. Surprisingly, inoculation with Candida dubliniensis and S. aureus resulted in minimal mortality, and rechallenge of mice with lethal C. albicans/S. aureus conferred >90% protection up to 60 days postinoculation. Protection was mediated by Gr-1⁺ polymorphonuclear leukocytes, indicating a novel form of trained innate immunity (TII). The purpose of this study was to determine the microbial requirements and spectrum of innate-mediated protection. In addition to Candida dubliniensis, several other low-virulence Candida species (C. glabrata, C. auris, and C. albicansefg1Δ/Δ cph1Δ/Δ) and Saccharomyces cerevisiae conferred significant protection with or without S. aureus. For C. dubliniensis-mediated protection, hyphal formation was not required, with protection conferred as early as 7 days after primary challenge but not at 120 days, and also following multiple lethal C. albicans/S. aureus rechallenges. This protection also extended to a lethal intravenous (i.v.) C. albicans challenge but had no effect in the C. albicans vaginitis model. Finally, studies revealed the ability of the low-virulence Candida species that conferred protection to invade the bone marrow by 24 h post-primary challenge, with a positive correlation between femoral bone marrow fungal infiltration at 48 h and protection upon rechallenge. These results support and further extend the characterization of this novel TII in protection against lethal fungal-bacterial IAI and sepsis.

Epigenetics in Sepsis: Understanding Its Role in Endothelial Dysfunction, Immunosuppression, and Potential Therapeutics

Sepsis has a complex pathophysiology in which both excessive and refractory inflammatory responses are hallmark features. Pro-inflammatory cytokine responses during the early stages are responsible for significant endothelial dysfunction, loss of endothelial integrity, and organ failure. In addition, it is now well-established that a substantial number of sepsis survivors experience ongoing immunological derangement and immunosuppression following a septic episode. The underpinning mechanisms of these phenomena are incompletely understood yet they contribute to a significant proportion of sepsis-associated mortality. Epigenetic mechanisms including DNA methylation, histone modifications, and non-coding RNAs, have an increasingly clear role in modulating inflammatory and other immunological processes. Recent evidence suggests epigenetic mechanisms are extensively perturbed as sepsis progresses, and particularly play a role in endothelial dysfunction and immunosuppression. Whilst therapeutic modulation of the epigenome is still in its infancy, there is substantial evidence from animal models that this approach could reap benefits. In this review, we summarize research elucidating the role of these mechanisms in several aspects of sepsis pathophysiology including tissue injury and immunosuppression. We also evaluate pre-clinical evidence for the use of "epi-therapies" in the treatment of poly-microbial sepsis.

Candida/Staphylococcal Polymicrobial Intra-Abdominal Infection: Pathogenesis and Perspectives for a Novel Form of Trained Innate Immunity

Polymicrobial sepsis is difficult to diagnose and treat and causes significant morbidity and mortality, especially when fungi are involved. In vitro, synergism between Candida albicans and various bacterial species has been described for many years. Our laboratory has developed a murine model of polymicrobial intra-abdominal infection with Candida albicans and Staphylococcus aureus, demonstrating that polymicrobial infections cause high levels of mortality, while monoinfections do not. By contrast, closely related Candida dubliniensis does not cause synergistic lethality and rather provides protection against lethal polymicrobial infection. This protection is thought to be driven by a novel form of trained innate immunity mediated by myeloid-derived suppressor cells (MDSCs), which we are proposing to call "trained tolerogenic immunity". MDSC accumulation has been described in patients with sepsis, as well as in in vivo sepsis models. However, clinically, MDSCs are considered detrimental in sepsis, while their role in in vivo models differs depending on the sepsis model and timing. In this review, we will discuss the role of MDSCs in sepsis and infection and summarize our perspectives on their development and function in the spectrum of trained innate immune protection against fungal-bacterial sepsis.