A ribonucleotide reductase inhibitor reverses burn-induced inflammatory defects

Persistent inflammation, immunosuppression, and catabolism syndrome (PICS): a review of definitions, potential therapies, and research priorities

Persistent Inflammation, Immunosuppression, and Catabolism Syndrome (PICS) is a clinical endotype of chronic critical illness. PICS consists of a self-perpetuating cycle of ongoing organ dysfunction, inflammation, and catabolism resulting in sarcopenia, immunosuppression leading to recurrent infections, metabolic derangements, and changes in bone marrow function. There is heterogeneity regarding the definition of PICS. Currently, there are no licensed treatments specifically for PICS. However, findings can be extrapolated from studies in other conditions with similar features to repurpose drugs, and in animal models. Drugs that can restore immune homeostasis by stimulating lymphocyte production could have potential efficacy. Another treatment could be modifying myeloid-derived suppressor cell (MDSC) activation after day 14 when they are immunosuppressive. Drugs such as interleukin (IL)-1 and IL-6 receptor antagonists might reduce persistent inflammation, although they need to be given at specific time points to avoid adverse effects. Antioxidants could treat the oxidative stress caused by mitochondrial dysfunction in PICS. Possible anti-catabolic agents include testosterone, oxandrolone, IGF-1 (insulin-like growth factor-1), bortezomib, and MURF1 (muscle RING-finger protein-1) inhibitors. Nutritional support strategies that could slow PICS progression include ketogenic feeding and probiotics. The field would benefit from a consensus definition of PICS using biologically based cut-off values. Future research should focus on expanding knowledge on underlying pathophysiological mechanisms of PICS to identify and validate other potential endotypes of chronic critical illness and subsequent treatable traits. There is unlikely to be a universal treatment for PICS, and a multimodal, timely, and personalised therapeutic strategy will be needed to improve outcomes for this growing cohort of patients.

Candida-induced granulocytic myeloid-derived suppressor cells are protective against polymicrobial sepsis

IMPORTANCE

Polymicrobial intra-abdominal infections are serious clinical infections that can lead to life-threatening sepsis, which is difficult to treat in part due to the complex and dynamic inflammatory responses involved. Our prior studies demonstrated that immunization with low-virulence Candida species can provide strong protection against lethal polymicrobial sepsis challenge in mice. This long-lived protection was found to be mediated by trained Gr-1+ polymorphonuclear leukocytes with features resembling myeloid-derived suppressor cells (MDSCs). Here we definitively characterize these cells as MDSCs and demonstrate that their mechanism of protection involves the abrogation of lethal inflammation, in part through the action of the anti-inflammatory cytokine interleukin (IL)-10. These studies highlight the role of MDSCs and IL-10 in controlling acute lethal inflammation and give support for the utility of trained tolerogenic immune responses in the clinical treatment of sepsis.

MDSCs in sepsis-induced immunosuppression and its potential therapeutic targets

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection. In sepsis, a complicated immune response is initiated, which varies over time with sustained excessive inflammation and immunosuppression. Identifying a promising way to orchestrate sepsis-induced immunosuppression is a challenge. Myeloid-derived suppressor cells (MDSCs) comprise pathologically activated neutrophils and monocytes with potent immunosuppressive activity. They play an important part in inhibiting innate and adaptive immune responses, and have emerged as part of the immune response in sepsis. MDSCs numbers are persistently high in sepsis patients, and associated with nosocomial infections and other adverse clinical outcomes. However, their characteristics and functional mechanisms during sepsis have not been addressed fully. Our review sheds light on the features and suppressive mechanism of MDSCs. We also review the potential applications of MDSCs as biomarkers and targets for clinical treatment of sepsis.

Burn-Induced Local and Systemic Immune Response: Systematic Review and Meta-Analysis of Animal Studies

As burn injuries are often followed by a derailed immune response and excessive inflammation, a thorough understanding of the occurring reactions is key to prevent secondary complications. This systematic review, that includes 247 animal studies, shows the post-burn response of 14 different immune cell types involved in immediate and long-term effects, in both wound tissue and circulation. Peripheral blood neutrophil and monocyte numbers increased directly after burns, whereas thrombocyte numbers increased near the end of the first week. Lymphocyte numbers, however, were decreased for at least two weeks. In burn wound tissue, neutrophil and macrophage numbers accumulated during the first three weeks. Burns also altered cellular functions as we found increased migratory potential of leukocytes, impaired antibacterial activity of neutrophils and enhanced inflammatory mediator production by macrophages. Neutrophil surges were positively associated with burn size and were highest in rats. Altogether, this comprehensive overview of the temporal immune cell dynamics shows that unlike normal wound healing, burn injury induces a long-lasting inflammatory response. It provides a fundamental research basis to improve experimental set-ups, burn care and outcome.

Role of myeloid derived suppressor cells in sepsis

Sepsis is a serious and menacing organ dysfunction that occur due to dysregulated response of the host towards the infection. This organ dysfunction may lead to sepsis with intense cellular, metabolic and circulatory dysregulation, multiple organ failure and high mortality. Lymphopenia is observed in two-third of sepsis patients and a significant depletion of lymphocytes occurs in non-survivors compared to sepsis survivors. Myeloid derived suppressor cells (MDSCs) gave new insights into sepsis-associated lymphopenia. If MDSC expansion and its tissue-infiltration persist, it can induce significant pathophysiology including lymphopenia, host immunosuppression and immune-paralysis that contributes to worsened patient outcomes. This review focuses on MDSCs and its subsets, the role of MDSCs in infection, sepsis and septic shock.

Myeloid-Derived Suppressor Cells (MDSCs) and the Immunoinflammatory Response to Injury (Mini Review)

ABSTRACT

Myeloid-derived suppressor cells (MDSCs) are a heterogenous population of immature myeloid cells hallmarked by their potent immunosuppressive function in a vast array of pathologic conditions. MDSCs have recently been shown to exhibit marked expansion in acute inflammatory states including traumatic injury, burn, and sepsis. Although MDSCs have been well characterized in cancer, there are significant gaps in our knowledge of their functionality in trauma and sepsis, and their clinical significance remains unclear. It is suggested that MDSCs serve an important role in quelling profound inflammatory responses in the acute setting; however, MDSC accumulation may also predispose patients to developing persistent immune dysregulation with increased risk for nosocomial infections, sepsis, and multiorgan failure. Whether MDSCs may serve as the target for novel therapeutics or an important biomarker in trauma and sepsis is yet to be determined. In this review, we will discuss the current understanding of MDSCs within the context of specific traumatic injury types and sepsis. To improve delineation of their functional role, we propose a systemic approach to MDSC analysis including phenotypic standardization, longitudinal analysis, and expansion of clinical research.

The Hematopoietic Stem/Progenitor Cell Response to Hemorrhage, Injury, and Sepsis: A Review of Pathophysiology

ABSTRACT

Hematopoietic stem/progenitor cells (HSPC) have both unique and common responses following hemorrhage, injury, and sepsis. HSPCs from different lineages have a distinctive response to these "stress" signals. Inflammation, via the production of inflammatory factors, including cytokines, hormones, and interferons, has been demonstrated to impact the differentiation and function of HSPCs. In response to injury, hemorrhagic shock, and sepsis, cellular phenotypic changes and altered function occur, demonstrating the rapid response and potential adaptability of bone marrow hematopoietic cells. In this review, we summarize the pathophysiology of emergency myelopoiesis and the role of myeloid-derived suppressor cells, impaired erythropoiesis, as well as the mobilization of HSPCs from the bone marrow. Finally, we discuss potential therapeutic options to optimize HSPC function after severe trauma or infection.

Dysregulated Immunity and Immunotherapy after Sepsis

Implementation of protocolized surveillance, diagnosis, and management of septic patients, and of surgical sepsis patients in particular, is shown to result in significantly increased numbers of patients surviving their initial hospitalization. Currently, most surgical sepsis patients will rapidly recover from sepsis; however, many patients will not rapidly recover, but instead will go on to develop chronic critical illness (CCI) and experience dismal long-term outcomes. The elderly and comorbid patient is highly susceptible to death or CCI after sepsis. Here, we review aspects of the Persistent Inflammation, Immunosuppression, and Catabolism Syndrome (PICS) endotype to explain the underlying pathobiology of a dysregulated immune system in sepsis survivors who develop CCI; then, we explore targets for immunomodulatory therapy.

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.

One-hit wonder: Late after burn injury, granulocytes can clear one bacterial infection but cannot control a subsequent infection

OBJECTIVE

Burn injury induces an acute hyperactive immune response followed by a chronic immune dysregulation that leaves those afflicted susceptible to multiple secondary infections. Many murine models are able to recapitulate the acute immune response to burn injury, yet few models are able to recapitulate long-term immune suppression and thus chronic susceptibility to bacterial infections seen in burn patients. This has hindered the field, making evaluation of the mechanisms responsible for these susceptibilities difficult to study. Herein we describe a novel mouse model of burn injury that promotes chronic immune suppression allowing for susceptibility to primary and secondary infections and thus allows for the evaluation of associated mechanisms.

METHODS

C57Bl/6 mice receiving a full-thickness contact burn were infected with Pseudomonas aeruginosa 14 days (primary infection) and/or 17 days (secondary infection) after burn or sham injury. The survival, pulmonary and systemic bacterial load as well as frequency and function of innate immune cells (neutrophils and macrophages) were evaluated.

RESULTS

Following secondary infection, burn mice were less effective in clearance of bacteria compared to sham injured or burn mice following a primary infection. Following secondary infection both neutrophils and macrophages recruited to the airways exhibited reduced production of anti-bacterial reactive oxygen and nitrogen species and the pro-inflammatory cytokineIL-12 while macrophages demonstrated increased expression of the anti-inflammatory cytokine interleukin-10 compared to those from sham burned mice and/or burn mice receiving a primary infection. In addition the BALF from these mice contained significantly higher level so of the anti-inflammatory cytokine IL-4 compared to those from sham burned mice and/or burn mice receiving a primary infection.

CONCLUSIONS

Burn-mediated protection from infection is transient, with a secondary infection inducing immune protection to collapse. Repeated infection leads to increased neutrophil and macrophage numbers in the lungs late after burn injury, with diminished innate immune cell function and an increased anti-inflammatory cytokine environment.

Burn-Related Dysregulation of Inflammation and Immunity in Experimental and Clinical Studies

The purpose of this study was to evaluate burn-related variations of inflammation and immunity. Fifty-five mice were divided randomly into sham burn and burn groups. Eighty-seven hospitalized burn patients were also reviewed. In mice, neutrophils and monocytes were elevated significantly on post burn day (PBD 1). Lymphocytes were reduced on PBDs 1 and 3. Levels of serum tumor necrosis factor-α and interleukin-6 were highest on PBD 1. Interleukin-1β levels were the highest on PBD 3. On PBD 3, CD4CD25T regulatory cells/CD4 cells in spleen were higher. On PBDs 1, 3, 7, and 14, percentage of splenic dendritic cells were significantly lower than the sham burn group. In patients, neutrophils and monocytes were significantly elevated on PBD 1. Levels declined but remained elevated at most days to PBD 7. Lymphocytes in burn groups 1 and 2 were reduced on PBDs 1 and 3, respectively. Our results exhibited that severe burn injury initiated a hyperinflammatory response and immunosuppression. PBDs 1 to 3 were important for changes in inflammation and immunosuppression.

Persistent Inflammation, Immunosuppression and Catabolism Syndrome

Following advances in critical care, in-hospital multiple organ failure-related mortality is declining. Consequently, incidence of chronic critical illness is increasing. These patients linger in the intensive care unit, have high resource utilization, and poor long-term outcomes. Within this population, the authors propose that a substantial subset of patients have a new phenotype: persistent inflammation, immunosuppression, and catabolism syndrome. There is evidence that myelodysplasia with expansion of myeloid-derived suppressor cells, innate and adaptive immune suppression, and protein catabolism with malnutrition are major contributors. Optimal care of these patients will require novel multimodality interventions.

Effect of mild hypothermia on the increase of CD11b+ Gr-1+ myeloid-derived suppressor cells induced by lipopolysaccharide in a mouse model of sepsis

PURPOSE

This study aimed to investigate the influence of mild hypothermia on the number of CD11b+ Gr-1+ myeloid-derived suppressor cells (MDSCs) induced by lipopolysaccharide (LPS) injection in mice with sepsis.

METHODS

BALB/c mice were administered LPS to establish a mouse model of sepsis. Then, these mice were randomly divided into 3 groups: the mild hypothermia plus LPS group, the normothermia plus LPS group, and the LPS group. The normal control group was injected the same amount of 0.9% sodium chloride solution. The ratio of CD11b+ Gr-1+ MDSCs in the mouse spleen and bone marrow was determined at 6, 12, 24, 48, and 72 hours after LPS injection and after injected 0.9% sodium chloride solution.

RESULTS

Compared with the control group, the number of MDSCs in the spleen in the sepsis group increased gradually, and the difference was significant at 12 hours after injection (P<.01). Moreover, the number of MDSCs was the lowest in the mild hypothermia group, and there was a significant difference than the other 2 groups at 48 hours (P<.01). The number of MDSCs in the bone marrow increased gradually, and the difference between the sepsis and control groups was significant at 24 hours (P<.01). The number of MDSCs in the mild hypothermia group was the lowest, and there was a statistically significant difference than the other 2 groups (P<.05).

CONCLUSION

Mild hypothermia inhibited the production and accumulation of MDSCs induced by LPS administration in septic mice.

Myeloid-derived suppressor cells in sepsis

Sepsis is a systemic, deleterious host response to widespread infection. Patients with sepsis will have documented or suspected infection which can progress to a state of septic shock or acute organ dysfunction. Since sepsis is responsible for nearly 3 million cases per year in China and severe sepsis is a common, expensive fatal condition in America, developing new therapies becomes a significant and worthwhile challenge. Clinical research has shown that sepsis-associated immunosuppression plays a central role in patient mortality, and targeted immune-enhancing therapy may be an effective treatment approach in these patients. As part of the inflammatory response during sepsis, there are elevations in the number of myeloid-derived suppressor cells (MDSCs). MDSCs are a heterogeneous population of immature myeloid cells that possess immunosuppressive activities via suppressing T-cell proliferation and activation. The role of MDSCs in sepsis remains uncertain. Some believe activated MDSCs are beneficial to the sepsis host by increasing innate immune responses and antimicrobial activities, while others think expansion of MDSCs leads to adaptive immune suppression and secondary infection. Herein, we discuss the complex role of MDSCs in immune regulation during sepsis, as well as the potential to target these cells for therapeutic benefit.

G-CSF drives a posttraumatic immune program that protects the host from infection

Traumatic injury is generally considered to have a suppressive effect on the immune system, resulting in increased susceptibility to infection. Paradoxically, we found that thermal injury to the skin induced a robust time-dependent protection of mice from a lethal Klebsiella pneumoniae pulmonary challenge. The protective response was neutrophil dependent and temporally associated with a systemic increase in neutrophils resulting from a reprioritization of hematopoiesis toward myeloid lineages. A prominent and specific activation of STAT3 in the bone marrow preceded the myeloid shift in that compartment, in association with durable increases in STAT3 activating serum cytokines G-CSF and IL-6. Neutralization of the postburn increase in serum G-CSF largely blocked STAT3 activation in marrow cells, reversing the hematopoietic changes and systemic neutrophilia. Daily administration of rG-CSF was sufficient to recapitulate the changes induced by injury including hematopoietic reprioritization and protection from pulmonary challenge with K. pneumoniae. Analysis of posttraumatic gene expression patterns in humans reveals that they are also consistent with a role for G-CSF as a switch that activates innate immune responses and suppresses adaptive immune responses. Our findings suggest that the G-CSF STAT3 axis constitutes a key protective mechanism induced by injury to reduce the risk for posttraumatic infection.

Myeloid-derived suppressor cells control microbial sepsis

PURPOSE

To investigate the role of myeloid-derived suppressor cells (MDSCs) during sepsis in mice. MDSCs are a heterogeneous population of cells that expand during cancer, inflammation and infection. These cells, by their ability to suppress T lymphocyte proliferation, regulate immune responses during various diseases. Their role during microbial infections is scarcely known.

METHODS

Septic shock was induced by caecal ligation and puncture in adult male BALB/c mice; sham-operated animals served as controls. Animals were killed under anaesthesia to harvest blood and organs.

RESULTS

Polymicrobial sepsis induced a progressive accumulation of MDSCs in spleens that were found to be enlarged in surviving mice. MDSCs harvested at day 10 after the onset of infection were highly responsive to LPS in terms of cytokines secretion, NF-kB activation, ROS production and arginase I activity, whereas early-appearing (day 3) MDSCs poorly responded to this stimulus. By contrast, both day 3 and day 10 MDSCs were able to inhibit T cell proliferation. Adoptive transfer of day 10 MDSCs to septic mice attenuated peritoneal cytokine production, increased bacterial clearance and dramatically improved survival rate.

CONCLUSION

These results provide new information on the role of MDSCs, suggesting a protective effect during sepsis. Pharmacologic agents known to promote the expansion of MDSCs should thus be further studied for sepsis treatment.

Injury-induced GR-1+ macrophage expansion and activation occurs independently of CD4 T-cell influence

Burn injury initiates an enhanced inflammatory condition referred to as the systemic inflammatory response syndrome or the two-hit response phenotype. Prior reports indicated that macrophages respond to injury and demonstrate a heightened reactivity to Toll-like receptor stimulation. Since we and others observed a significant increase in splenic GR-1 F4/80 CD11b macrophages in burn-injured mice, we wished to test if these macrophages might be the primary macrophage subset that shows heightened LPS reactivity. We report here that burn injury promoted higher level TNF-α expression in GR-1, but not GR-1 macrophages, after LPS activation both in vivo and ex vivo. We next tested whether CD4 T cells, which are known to suppress injury-induced inflammatory responses, might control the activation and expansion of GR-1 macrophages. Interestingly, we found that GR-1 macrophage expansion and LPS-induced TNF-α expression were not significantly different between wild-type and CD4 T cell-deficient CD4(-/-) mice. However, further investigations showed that LPS-induced TNF-α production was significantly influenced by CD4 T cells. Taken together, these data indicate that GR-1 F4/80 CD11b macrophages represent the primary macrophage subset that expands in response to burn injury and that CD4 T cells do not influence the GR-1 macrophage expansion process, but do suppress LPS-induced TNF-α production. These data suggest that modulating GR-1 macrophage activation as well as CD4 T cell responses after severe injury may help control the development of systemic inflammatory response syndrome and the two-hit response phenotype.

Neutrophils, not monocyte/macrophages, are the major splenic source of postburn IL-10

Burn induces myeloid-derived suppressor cells (MDSCs), a heterogeneous population of immature polymorphonuclear neutrophils (PMNs) and monocytes, which protect against infection. Previous work from our laboratory demonstrated that inflammatory monocytes (iMos) were the major MDSC source of TNF-α in the postburn spleen, and we hypothesized that they were also the major source of postburn IL-10. To test this hypothesis, we examined cytokine production by postburn CCR2 knockout (KO) mice, which have fewer iMos than burn wild-type (WT) splenocytes, but equal numbers of PMNs and F4/80 macrophages. Using cell sorting and/or intracellular cytokine techniques, we examined IL-10 production by postburn PMNs and iMos. Finally, we compared IL-10 production by postburn PMNs and iMos with culture-derived MDSCs. Splenocytes from postburn CCR2 KO mice produced less IL-6 and TNF-α than WT burn splenocytes in response to LPS, but KO and WT burn splenocytes produced equal amounts of IL-10 in response to peptidoglycan. Depletion of PMNs from postburn splenocytes led to reductions in IL-10 and increases in IL-6 and TNF-α in response to peptidoglycan, but not in response to LPS. Sorting or intracellular cytokine techniques gave consistent results: Burn PMNs made more IL-10 than sham PMNs and also more IL-10 than burn or sham iMos. Polymorphonuclear neutrophil and iMos subpopulations from culture-derived MDSCs produced the same cytokine profiles in response to LPS and peptidoglycan as did the PMNs and iMos from postburn spleens: PMNs made IL-10, whereas iMos made IL-6. Finally, LPS-induced mortality of burn mice was made worse by anti-Gr-1 depletion of all PMNs and 66% of iMos from burn mice. This suggests that PMNs play a primarily anti-inflammatory role in vitro and in vivo.

A paradoxical role for myeloid-derived suppressor cells in sepsis and trauma

Myeloid-derived suppressor cells (MDSCs) are a heterogenous population of immature myeloid cells whose numbers dramatically increase in chronic and acute inflammatory diseases, including cancer, autoimmune disease, trauma, burns and sepsis. Studied originally in cancer, these cells are potently immunosuppressive, particularly in their ability to suppress antigen-specific CD8(+) and CD4(+) T-cell activation through multiple mechanisms, including depletion of extracellular arginine, nitrosylation of regulatory proteins, and secretion of interleukin 10, prostaglandins and other immunosuppressive mediators. However, additional properties of these cells, including increased reactive oxygen species and inflammatory cytokine production, as well as their universal expansion in nearly all inflammatory conditions, suggest that MDSCs may be more of a normal component of the inflammatory response ("emergency myelopoiesis") than simply a pathological response to a growing tumor. Recent evocative data even suggest that the expansion of MDSCs in acute inflammatory processes, such as burns and sepsis, plays a beneficial role in the host by increasing immune surveillance and innate immune responses. Although clinical efforts are currently underway to suppress MDSC numbers and function in cancer to improve antineoplastic responses, such approaches may not be desirable or beneficial in other clinical conditions in which immune surveillance and antimicrobial activities are required.