Type I interferons drive inflammasome-independent emergency monocytopoiesis during endotoxemia

Activation of Wnt/β-catenin signal induces DCs to differentiate into immune tolerant regDCs in septic mice

BACKGROUND

Sepsis is a common complication among patients in intensive care units, and has a high mortality rate, with no effective therapies to date. As immunosuppression has become the research focus of sepsis, the regulatory role of dendritic cells (DCs) in the immune response to sepsis has received attention.

OBJECTIVE

To investigate the role of the Wnt/β-catenin signaling pathway in inducing the differentiation of splenic DCs in mice with sepsis caused by cecal ligation and puncture (CLP).

METHODS

C57bl/6 mice were randomly divided into three groups, namely the sham, 24 h post-CLP, and 72 h post-CLP groups. Levels of regulatory T cells (Tregs) among splenic mononuclear cells, suppressor T cells (TSs), and surface markers, such as major histocompatibility complex class II (MHC-II), co-stimulatory molecules (CD80 and CD86), negative co-stimulatory molecule death-ligand 1 (PD-L1), CC chemokine receptor-5 (CCR5), and CC chemokine receptor-7 (CCR7), were analyzed via flow cytometry for each group of mice post-surgery. CD11c+ DCs were purified from the splenic mononuclear cells of each group, and the expression of β-catenin, Wnt5a, and Wnt3a was detected using RT-PCR and western blotting.Each group of DCs was incubated with LPS-containing culture solution, and the supernatant of the culture solution was collected after 24 hours to detect the level of Tumor necrosis factor-α(TNF-α), interleukin (IL)-6, IL-12, and IL-10.

RESULTS

Compared with that in the sham group, the expression of β-catenin, Wnt5a, and Wnt3a in splenic DCs of the other two groups of mice increased with prolonged CLP exposure (P<0.05). Meanwhile, the proportion of Tregs and TSs increased in the mouse spleens after CLP, and levels of DC surface molecules, such as CCR5, CCR7, CD80, CD86, and MHC-II, decreased to different degrees, whereas those of PD-L1 increased. These results suggested that DCs differentiate towards regulatory DCs (regDCs) after CLP in mice. The results of ELISA showed that the longer the exposure time after CLP, the lower the ability of DCs to secrete TNF-α and IL-12, but the higher the level of IL-10 and IL-6.

CONCLUSION

The Wnt/β-catenin signaling pathway activates and induces regDCs differentiation in the splenic DCs of mice with sepsis and participates in the regulation of immune tolerance in the organism.

Systemic innate myeloid responses to acute ischaemic and haemorrhagic stroke

Acute ischaemic and haemorrhagic stroke account for significant disability and morbidity burdens worldwide. The myeloid arm of the peripheral innate immune system is critical in the immunological response to acute ischaemic and haemorrhagic stroke. Neutrophils, monocytes, and dendritic cells (DC) contribute to the evolution of pathogenic local and systemic inflammation, whilst maintaining a critical role in ongoing immunity protecting against secondary infections. This review aims to summarise the key alterations to myeloid immunity in acute ischaemic stroke, intracerebral haemorrhage (ICH), and subarachnoid haemorrhage (SAH). By integrating clinical and preclinical research, we discover how myeloid immunity is affected across multiple organ systems including the brain, blood, bone marrow, spleen, and lung, and evaluate how these perturbations associate with real-world outcomes including infection. These findings are placed in the context of the rapidly developing field of human immunology, which offers a wealth of opportunity for further research.

Usp22 is an intracellular regulator of systemic emergency hematopoiesis

Emergency hematopoiesis is a concerted response aimed toward enhanced protection from infection, involving multiple cell types and developmental stages across the immune system. Despite its importance, the underlying molecular regulation remains poorly understood. The deubiquitinase USP22 regulates the levels of monoubiquitinated histone H2B (H2Bub1), which is associated with activation of interferon responses upon viral infection. Here, we show that in the absence of infection or inflammation, mice lacking Usp22 in all hematopoietic cells display profound systemic emergency hematopoiesis, evident by increased hematopoietic stem cell proliferation, myeloid bias, and extramedullary hematopoiesis. Functionally, loss of Usp22 results in elevated phagocytosis by neutrophilic granulocytes and enhanced innate protection against Listeria monocytogenes infection. At the molecular level, we found this state of emergency hematopoiesis associated with transcriptional signatures of myeloid priming, enhanced mitochondrial respiration, and innate and adaptive immunity and inflammation. Augmented expression of many inflammatory genes was linked to elevated locus-specific H2Bub1 levels. Collectively, these results demonstrate the existence of a tunable epigenetic state that promotes systemic emergency hematopoiesis in a cell-autonomous manner to enhance innate protection, identifying potential paths toward immune enhancement.

Hypoxia shapes the immune landscape in lung injury and promotes the persistence of inflammation

Hypoxemia is a defining feature of acute respiratory distress syndrome (ARDS), an often-fatal complication of pulmonary or systemic inflammation, yet the resulting tissue hypoxia, and its impact on immune responses, is often neglected. In the present study, we have shown that ARDS patients were hypoxemic and monocytopenic within the first 48 h of ventilation. Monocytopenia was also observed in mouse models of hypoxic acute lung injury, in which hypoxemia drove the suppression of type I interferon signaling in the bone marrow. This impaired monopoiesis resulted in reduced accumulation of monocyte-derived macrophages and enhanced neutrophil-mediated inflammation in the lung. Administration of colony-stimulating factor 1 in mice with hypoxic lung injury rescued the monocytopenia, altered the phenotype of circulating monocytes, increased monocyte-derived macrophages in the lung and limited injury. Thus, tissue hypoxia altered the dynamics of the immune response to the detriment of the host and interventions to address the aberrant response offer new therapeutic strategies for ARDS.

Spatiotemporal Adaptations of Macrophage and Dendritic Cell Development and Function

Macrophages and conventional dendritic cells (cDCs) are distributed throughout the body, maintaining tissue homeostasis and tolerance to self and orchestrating innate and adaptive immunity against infection and cancer. As they complement each other, it is important to understand how they cooperate and the mechanisms that integrate their functions. Both are exposed to commensal microbes, pathogens, and other environmental challenges that differ widely among anatomical locations and over time. To adjust to these varying conditions, macrophages and cDCs acquire spatiotemporal adaptations (STAs) at different stages of their life cycle that determine how they respond to infection. The STAs acquired in response to previous infections can result in increased responsiveness to infection, termed training, or in reduced responses, termed paralysis, which in extreme cases can cause immunosuppression. Understanding the developmental stage and location where macrophages and cDCs acquire their STAs, and the molecular and cellular players involved in their induction, may afford opportunities to harness their beneficial outcomes and avoid or reverse their deleterious effects. Here we review our current understanding of macrophage and cDC development, life cycle, function, and STA acquisition before, during, and after infection. We propose a unified framework to explain how these two cell types adjust their activities to changing conditions over space and time to coordinate their immunosurveillance functions. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Systemic bacterial infections affect dendritic cell development and function

Dendritic cells (DCs) are critical in host defense against infection. DC depletion is an early event in the course of sepsis that may impair the host defense mechanisms. Here, we addressed whether DC depletion and dysfunction are pathogen-independent, mediated via pattern recognition receptors, and are due to impaired DC development upon systemic infection with the Gram-negative bacterium Escherichia coli and the Gram-positive pathogen Staphylococcus aureus. Infection with E. coli and S. aureus led to reduced numbers of splenic DC subsets and of DC progenitors in the bone marrow (BM) with this effect persisting significantly longer in mice infected with S. aureus than with E. coli. The reduction of DC subsets and their progenitors was mainly TLR-independent as was the infection-induced monopoiesis. Moreover, de novo DC development was impaired in mice infected with S. aureus, and BM cells from E. coli or S. aureus infected mice favored macrophage differentiation in vitro. As a consequence of reduced DC numbers and their reduced expression of MHC II less CD4⁺ and CD8⁺ T cells, especially Th1 and IFN-γ producing CD8⁺ T cells, could be detected in S. aureus compared to E. coli infected mice. These differences are reflected in the rapid killing of E. coli as opposed to an increase in bacterial load in S. aureus. In summary, our study supports the idea that systemic bacterial infections generally affect the number and development of DCs and thereby the T cell responses, but the magnitude is pathogen-dependent.

Alcohol Consumption Accumulation of Monocyte Derived Macrophages in Female Mice Liver Is Interferon Alpha Receptor Dependent

Monocytes develop in the bone marrow from the hematopoietic stem cells and represent heterogeneous phagocyte cells in the circulation. In homeostatic and inflammatory conditions, after recruitment into tissues, monocytes differentiate into macrophages and dendritic cells. Alcohol use causes about 3.3 million worldwide deaths per year, which is about 5.9% of all deaths. In the United States and Europe, alcohol use disorders represent the fifth leading cause of death. Females are more susceptible to alcoholic liver injury in both humans and mice. Strikingly, we still do not know how much of this difference in tissue injury is due to the differential effect of alcohol and its toxic metabolites on a) parenchymal or resident cells and/or b) immune response to alcohol. Therefore, we used a model of chronic alcohol exposure in mice to investigate the dynamics of monocytes, an innate immune cell type showed to be critical in alcoholic liver injury, by using immunophenotypic characterization. Our data reveal a sex-dimorphism of alcohol response of hepatic monocytes in female mice that is interferon receptor alpha dependent. This dimorphism could shed light on potential cellular mechanism(s) to explain the susceptibility of females to alcoholic immunopathogenesis and suggests an additional targetable pathway for alcoholic liver injury in females.

Dendritic cell development in infection

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

Emerging Roles of Autophagy and Inflammasome in Ehrlichiosis

Human monocytic ehrlichiosis (HME) is a potentially life-threatening tick-borne rickettsial disease (TBRD) caused by the obligate intracellular Gram-negative bacteria, Ehrlichia. Fatal HME presents with acute ailments of sepsis and toxic shock-like symptoms that can evolve to multi-organ failure and death. Early clinical and laboratory diagnosis of HME are problematic due to non-specific flu-like symptoms and limitations in the current diagnostic testing. Several studies in murine models showed that cell-mediated immunity acts as a “double-edged sword” in fatal ehrlichiosis. Protective components are mainly formed by CD4 Th1 and NKT cells, in contrast to deleterious effects originated from neutrophils and TNF-α-producing CD8 T cells. Recent research has highlighted the central role of the inflammasome and autophagy as part of innate immune responses also leading to protective or pathogenic scenarios. Recognition of pathogen-associated molecular patterns (PAMPS) or damage-associated molecular patterns (DAMPS) triggers the assembly of the inflammasome complex that leads to multiple outcomes. Recognition of PAMPs or DAMPs by such complexes can result in activation of caspase-1 and -11, secretion of the pro-inflammatory cytokines IL-1β and IL-18 culminating into dysregulated inflammation, and inflammatory cell death known as pyroptosis. The precise functions of inflammasomes and autophagy remain unexplored in infections with obligate intracellular rickettsial pathogens, such as Ehrlichia. In this review, we discuss the intracellular innate immune surveillance in ehrlichiosis involving the regulation of inflammasome and autophagy, and how this response influences the innate and adaptive immune responses against Ehrlichia. Understanding such mechanisms would pave the way in research for novel diagnostic, preventative and therapeutic approaches against Ehrlichia and other rickettsial diseases.

Type I interferons differentially modulate maternal host immunity to infection by Listeria monocytogenes and Salmonella enterica serovar Typhimurium during pregnancy

PROBLEM

IFN-alpha receptor deficiency (IFNAR^-/- ) enhances immunity to Listeria monocytogenes (LM) and Salmonella enterica serovar Typhimurium (ST) in the non-pregnant state by inhibiting pathogen-induced immune cell death. However, the roles of IFNAR signaling in modulating immunity to infection during pregnancy are not well understood.

METHOD OF STUDY

C57BL/6J wild-type (WT) and IFNAR^-/- mice were infected systemically with LM or ST. Bacterial burden in spleen and individual placentas was enumerated at day 3 post-infection. Immune cell numbers and percentages were quantified in spleen and individual placentas, respectively, through flow cytometry. Cytokine expression in serum, spleen, and individual placentas was measured through cytometric bead array.

RESULTS

IFNAR^-/- mice exhibited decreased splenic monocyte numbers in non-pregnant and pregnant state, and an altered distribution of placental immune cell types in the non-infected state. IFNAR^-/- mice controlled LM infection more effectively than WT mice even during pregnancy. This correlated with enhanced serum IL-12 expression, despite reduced splenic monocyte numbers relative to WT controls. In contrast, pregnant IFNAR^-/- mice unlike their non-pregnant counterparts exhibited increased susceptibility to ST infection, which was associated with decreased serum IL-12 expression.

CONCLUSION

Type I IFN responses differentially impact host resistance to LM and ST infection during pregnancy through modulation of immune cell distribution and cytokine responses.

Alteration of Flt3-Ligand-dependent de novo generation of conventional dendritic cells during influenza infection contributes to respiratory bacterial superinfection

Secondary bacterial infections contribute to the excess morbidity and mortality of influenza A virus (IAV) infection. Disruption of lung integrity and impaired antibacterial immunity during IAV infection participate in colonization and dissemination of the bacteria out of the lungs. One key feature of IAV infection is the profound alteration of lung myeloid cells, characterized by the recruitment of deleterious inflammatory monocytes. We herein report that IAV infection causes a transient decrease of lung conventional dendritic cells (cDCs) (both cDC1 and cDC2) peaking at day 7 post-infection. While triggering emergency monopoiesis, IAV transiently altered the differentiation of cDCs in the bone marrow, the cDC1-biaised pre-DCs being particularly affected. The impaired cDC differentiation during IAV infection was independent of type I interferons (IFNs), IFN-γ, TNFα and IL-6 and was not due to an intrinsic dysfunction of cDC precursors. The alteration of cDC differentiation was associated with a drop of local and systemic production of Fms-like tyrosine kinase 3 ligand (Flt3-L), a critical cDC differentiation factor. Overexpression of Flt3-L during IAV infection boosted the cDC progenitors' production in the BM, replenished cDCs in the lungs, decreased inflammatory monocytes' infiltration and lowered lung damages. This was associated with partial protection against secondary pneumococcal infection, as reflected by reduced bacterial dissemination and prolonged survival. These findings highlight the impact of distal viral infection on cDC genesis in the BM and suggest that Flt3-L may have potential applications in the control of secondary infections.

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.