Functional innervation of hepatic iNKT cells is immunosuppressive following stroke

Stroke-induced immunosuppression and poststroke infection

Infections occur commonly after stroke and are strongly associated with an unfavourable functional outcome of these patients. Approaches for effective management of poststroke infection remain scarce, presenting an urgent need for preventive anti-infection strategies for patients who have suffered a stroke. Emerging evidence indicates that stroke impairs systemic immune responses and increases the susceptibility to infections, suggesting that the modification of impaired immune defence could be beneficial. In this review, we summarised previous attempts to prevent poststroke infections using prophylactic antibiotics and the current understanding of stroke-induced immunosuppression. Further elucidation of the immune mechanisms of stroke will pave the way to tailored design of new treatment to combat poststroke infection via modifying the immune system.

Effector CD4+ T cells recognize intravascular antigen presented by patrolling monocytes

Although effector CD4⁺ T cells readily respond to antigen outside the vasculature, how they respond to intravascular antigens is unknown. Here we show the process of intravascular antigen recognition using intravital multiphoton microscopy of glomeruli. CD4⁺ T cells undergo intravascular migration within uninflamed glomeruli. Similarly, while MHCII is not expressed by intrinsic glomerular cells, intravascular MHCII-expressing immune cells patrol glomerular capillaries, interacting with CD4⁺ T cells. Following intravascular deposition of antigen in glomeruli, effector CD4⁺ T-cell responses, including NFAT1 nuclear translocation and decreased migration, are consistent with antigen recognition. Of the MHCII⁺ immune cells adherent in glomerular capillaries, only monocytes are retained for prolonged durations. These cells can also induce T-cell proliferation in vitro. Moreover, monocyte depletion reduces CD4⁺ T-cell-dependent glomerular inflammation. These findings indicate that MHCII⁺ monocytes patrolling the glomerular microvasculature can present intravascular antigen to CD4⁺ T cells within glomerular capillaries, leading to antigen-dependent inflammation.

Monocytes constitutively adhere and crawl along the glomerular endothelium and are thought to contribute to glomerulonephritis. Here the authors use multiphoton microscopy to show local antigen presentation by MHCII⁺ monocytes to T cells in glomerular capillaries of mice.

iNKT Cell Emigration out of the Lung Vasculature Requires Neutrophils and Monocyte-Derived Dendritic Cells in Inflammation

iNKT cells are a subset of innate T cells that recognize glycolipids presented on CD1d molecules and protect against bacterial infections, including S. pneumoniae. Using lung intravital imaging, we examined the behavior and mechanism of pulmonary iNKT cell activation in response to the specific iNKT cell ligand α-galactosylceramide or S. pneumoniae infection. In untreated mice, the major fraction of iNKT cells resided in the vasculature, but a small critical population resided in the extravascular space in proximity to monocyte-derived DCs. Administration of either α-GalCer or S. pneumoniae induced CD1d-dependent rapid recruitment of neutrophils out of the vasculature. The neutrophils guided iNKT cells from the lung vasculature via CCL17. Depletion of monocyte-derived DCs abrogated both the neutrophil and subsequent iNKT cell extravasation. Moreover, impairing iNKT cell recruitment by blocking CCL17 increased susceptibility to S. pneumoniae infection, suggesting a critical role for the influx of iNKT cells in host defense.

Immune Responses in the Liver

The liver is a key, frontline immune tissue. Ideally positioned to detect pathogens entering the body via the gut, the liver appears designed to detect, capture, and clear bacteria, viruses, and macromolecules. Containing the largest collection of phagocytic cells in the body, this organ is an important barrier between us and the outside world. Importantly, as portal blood also transports a large number of foreign but harmless molecules (e.g., food antigens), the liver's default immune status is anti-inflammatory or immunotolerant; however, under appropriate conditions, the liver is able to mount a rapid and robust immune response. This balance between immunity and tolerance is essential to liver function. Excessive inflammation in the absence of infection leads to sterile liver injury, tissue damage, and remodeling; insufficient immunity allows for chronic infection and cancer. Dynamic interactions between the numerous populations of immune cells in the liver are key to maintaining this balance and overall tissue health.

An insight into intestinal mucosal microbiota disruption after stroke

Recent work from our laboratory has provided evidence that indicates selective bacterial translocation from the host gut microbiota to peripheral tissues (i.e. lung) plays a key role in the development of post-stroke infections. Despite this, it is currently unknown whether mucosal bacteria that live on and interact closely with the host intestinal epithelium contribute in regulating bacterial translocation after stroke. Here, we found that the microbial communities within the mucosa of gastrointestinal tract (GIT) were significantly different between sham-operated and post-stroke mice at 24 h following surgery. The differences in microbiota composition were substantial in all sections of the GIT and were significant, even at the phylum level. The main characteristics of the stroke-induced shift in mucosal microbiota composition were an increased abundance of Akkermansia muciniphila and an excessive abundance of clostridial species. Furthermore, we analysed the predicted functional potential of the altered mucosal microbiota induced by stroke using PICRUSt and revealed significant increases in functions associated with infectious diseases, membrane transport and xenobiotic degradation. Our findings revealed stroke induces far-reaching and robust changes to the intestinal mucosal microbiota. A better understanding of the precise molecular events leading up to stroke-induced mucosal microbiota changes may represent novel therapy targets to improve patient outcomes.

Activation of JAK/STAT3 restores NK-cell function and improves immune defense after brain ischemia

Stroke-induced immune suppression predisposes the host to infections and can contribute to high morbidity and mortality in stroke patients. Because ischemic stroke has a profound effect on the systemic immune response, which may explain the increased susceptibility of stroke patients to infection, an urgent need persists for a better understanding of mechanisms associated with immune suppression; new and effective treatments for stroke can then be identified. NK cells play a key role in early host defense against pathogens by killing infected cells and/or producing cytokines such as IFN-γ. Because the phenotype and function of peripheral NK cells have been widely investigated in ischemic stroke, nCounter Inflammation Gene Array Analysis was used to build immune-related gene profiles of NK cells to comprehensively analyze the molecular signature of NK cells after ischemic brain injury. We observed distinct gene expression profiles reflecting different splenic NK-cell phenotypes and functional properties across the time course of transient middle cerebral artery occlusion (MCAO). Based on gene expression and pathway-network analysis, lower expression levels of signal transducer and activator of transcription-3 (STAT3) were observed in animals with MCAO compared with sham control animals. Genetic activation of STAT3 through the introduction of STAT3 clustered regularly interspaced short palindromic repeats (CRISPR) plasmid prevented the loss of NK-cell-derived IFN-γ production after MCAO, together with reduced bacterial burden and mortality. Our data suggest that brain ischemia impairs NK-cell-mediated immune defense in the periphery, at least in part through the JAK-STAT3 pathway, which can be readdressed by modulating STAT3 activation status.-Jin, W.-N., Ducruet, A. F., Liu, Q., Shi, S. X.-Y., Waters, M., Zou, M., Sheth, K. N., Gonzales, R., Shi, F.-D. Activation of JAK/STAT3 restores NK-cell function and improves immune defense after brain ischemia.

Activation of the sympathetic nervous system modulates neutrophil function

Emerging evidence has revealed that noradrenaline (NA), the main neurotransmitter of the sympathetic nervous system (SNS), regulates a variety of immune functions via binding to adrenergic receptors present on immune cells. In this study, we examined the role of NA in the regulation of neutrophil functions. Neutrophils were isolated from the bone marrow of naïve mice and treated with NA at various concentrations to assess the effect on various neutrophil functions. Additionally, we performed cremaster intravital microscopy to examine neutrophil‐endothelial cell interactions following NA superfusion in vivo. In a separate group of animals, mice were subjected to an experimental model of stroke and at 4 and 24 h neutrophils were isolated for assessment on their ability to migrate toward various chemokines. Treatment of neutrophils with NA for 4 h significantly impaired neutrophil chemotaxis and induced an N2 neutrophil phenotype with reduced expression of the genes critical for cytoskeleton remodeling and inflammation. Prolonged NA administration promoted neutrophils to release myeloperoxidase and IL‐6, but suppressed the production of interferon‐γ and IL‐10, reduced neutrophil activation and phagocytosis. Superfusion of NA over the cremaster muscle almost completely inhibited fMLP‐induced neutrophil adhesion/arrest and transmigration. Furthermore, using a mouse model of stroke, a pathological condition in which SNS activation is evident, neutrophils isolated from poststroke mice showed markedly reduced chemotaxis toward all of the chemokines tested. The findings from our study indicate that neutrophil chemotaxis, activation, and phagocytosis can all be negatively regulated in an NA‐dependent manner. A better understanding of the relationship between sympathetic activation and neutrophil function will be important for the development of effective antibacterial interventions.

Advances in the understanding and treatment of sepsis-induced immunosuppression

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

Disarming the Killers: Brain Strikes on NK Cells

Brain ischemia induces profound systemic immunosuppression, leading to infectious complications. In this issue of Immunity, Liu et al. (2017) demonstrate that distinct neuroendocrine pathways differentially inhibit natural killer (NK) cell responses in the central nervous system and the periphery after cerebral infarction.

Low Free Triiodothyronine at Admission Predicts Poststroke Infection

BACKGROUND

Poststroke infection (PSI) is common and is usually associated with a severe prognosis. We investigated the association between PSI and thyroid hormones, which are critical to immune regulation, in patients with acute stroke.

METHODS

We retrospectively enrolled 520 consecutive patients with acute ischemic stroke (326 men; age, 71.9 ± 13.2 years) admitted to our department between September 2014 and June 2016. The impact of serum thyroid hormone levels measured at admission (thyroid-stimulating hormone [TSH], free triiodothyronine [FT3], and free thyroxine [FT4]) on the PSI was evaluated using multivariate logistic regression analysis.

RESULTS

We diagnosed 107 patients (20.6%; pneumonia, 65; urinary tract infection, 19; others, 23) with PSIs. While age (P <.001), body mass index (P = .0012), preadmission modified Rankin scale score (P = .0001), National Institutes of Health Stroke Scale score on admission (P <.001), admission FT3 level (P <.001), atrial fibrillation (P <.001), and ischemic heart disease (P = .0451) were significantly associated with PSI, we found no relationship among TSH levels, FT4 levels, and PSI occurrence. After multivariate adjustment, patients with PSIs were more frequently in the Q1 quartile (≤2.25 pg/mL) than in the Q2 (2.26-2.55 pg/mL; P = .0251), Q3 (2.56-2.89 pg/mL; P = .0007), or Q4 (≥2.90 pg/mL; P = .0010) quartiles of FT3 levels. Moreover, low FT3 levels (<2.29 pg/mL) were independently associated with PSI occurrence (P = .0013).

CONCLUSIONS

Low FT3 levels at admission are independently associated with PSI occurrence.

Spinal cord injury-induced immunodeficiency is mediated by a sympathetic-neuroendocrine adrenal reflex

Acute spinal cord injury (SCI) causes systemic immunosuppression and life-threatening infections, thought to result from noradrenergic overactivation and excess glucocorticoid release via hypothalamus-pituitary-adrenal axis stimulation. Instead of consecutive hypothalamus-pituitary-adrenal axis activation, we report that acute SCI in mice induced suppression of serum norepinephrine and concomitant increase in cortisol, despite suppressed adrenocorticotropic hormone, indicating primary (adrenal) hypercortisolism. This neurogenic effect was more pronounced after high-thoracic level (Th1) SCI disconnecting adrenal gland innervation, compared with low-thoracic level (Th9) SCI. Prophylactic adrenalectomy completely prevented SCI-induced glucocorticoid excess and lymphocyte depletion but did not prevent pneumonia. When adrenalectomized mice were transplanted with denervated adrenal glands to restore physiologic glucocorticoid levels, the animals were completely protected from pneumonia. These findings identify a maladaptive sympathetic-neuroendocrine adrenal reflex mediating immunosuppression after SCI, implying that therapeutic normalization of the glucocorticoid and catecholamine imbalance in SCI patients could be a strategy to prevent detrimental infections.

CD1d-Restricted pathways in hepatocytes control local natural killer T cell homeostasis and hepatic inflammation

Invariant natural killer T (iNKT) cells recognize lipid antigens presented by CD1d and play a central role in regulating immunity and inflammation in peripheral tissues. However, the mechanisms which govern iNKT cell homeostasis after thymic emigration are incompletely understood. Here we demonstrate that microsomal triglyceride transfer protein (MTP), a protein involved in the transfer of lipids onto CD1d, regulates liver iNKT cell homeostasis in a manner dependent on hepatocyte CD1d. Mice with hepatocyte-specific loss of MTP exhibit defects in the function of CD1d and show increased hepatic iNKT cell numbers as a consequence of altered iNKT cell apoptosis. Similar findings were made in mice with hepatocyte-specific loss of CD1d, confirming a critical role of CD1d in this process. Moreover, increased hepatic iNKT cell abundance in the absence of MTP is associated with susceptibility to severe iNKT cell-mediated hepatitis, thus demonstrating the importance of CD1d-dependent control of liver iNKT cells in maintaining immunological homeostasis in the liver. Together, these data demonstrate an unanticipated role of parenchymal cells, as shown here for hepatocytes, in tissue-specific regulation of CD1d-restricted immunity and further suggest that alterations in lipid metabolism may affect iNKT cell homeostasis through effects on CD1d-associated lipid antigens.

Lymphatic drainage system of the brain: A novel target for intervention of neurological diseases

The belief that the vertebrate brain functions normally without classical lymphatic drainage vessels has been held for many decades. On the contrary, new findings show that functional lymphatic drainage does exist in the brain. The brain lymphatic drainage system is composed of basement membrane-based perivascular pathway, a brain-wide glymphatic pathway, and cerebrospinal fluid (CSF) drainage routes including sinus-associated meningeal lymphatic vessels and olfactory/cervical lymphatic routes. The brain lymphatic systems function physiological as a route of drainage for interstitial fluid (ISF) from brain parenchyma to nearby lymph nodes. Brain lymphatic drainage helps maintain water and ion balance of the ISF, waste clearance, and reabsorption of macromolecular solutes. A second physiological function includes communication with the immune system modulating immune surveillance and responses of the brain. These physiological functions are influenced by aging, genetic phenotypes, sleep-wake cycle, and body posture. The impairment and dysfunction of the brain lymphatic system has crucial roles in age-related changes of brain function and the pathogenesis of neurovascular, neurodegenerative, and neuroinflammatory diseases, as well as brain injury and tumors. In this review, we summarize the key component elements (regions, cells, and water transporters) of the brain lymphatic system and their regulators as potential therapeutic targets in the treatment of neurologic diseases and their resulting complications. Finally, we highlight the clinical importance of ependymal route-based targeted gene therapy and intranasal drug administration in the brain by taking advantage of the unique role played by brain lymphatic pathways in the regulation of CSF flow and ISF/CSF exchange.

Antibody-dependent fragmentation is a newly identified mechanism of cell killing in vivo

The prevailing view is that therapeutic antibodies deplete cells through opsonization and subsequent phagocytosis, complement-dependent lysis or antibody-dependent cellular-cytotoxicity. We used high resolution in vivo imaging to identify a new antibody-dependent cell death pathway where Kupffer cells ripped large fragments off crawling antibody-coated iNKT cells. This antibody-dependent fragmentation process resulted in lethality and depletion of crawling iNKT cells in the liver sinusoids and lung capillaries. iNKT cell depletion was Fcy-receptor dependent and required iNKT cell crawling. Blood, spleen or joint iNKT cells that did not crawl were not depleted. The antibody required high glycosylation for sufficiently strong binding of the iNKT cells to the Fc Receptors on Kupffer cells. Using an acetaminophen overdose model, this approach functionally depleted hepatic iNKT cells and affected the severity of liver injury. This study reveals a new mechanism of antibody-dependent killing in vivo and raises implications for the design of new antibodies for cancer and auto-reactive immune cells.

Sepsis biomarkers reprofiling to predict stroke-associated infections

We aimed to evaluate the usefulness of sepsis biomarkers to predict stroke-associated infections. Soluble triggering receptor expressed on myeloid cells-1 (sTREM-1), mid-regional pro-adrenomedullin (MR-proADM), presepsin (sCD14), and soluble urokinase-type plasminogen activator receptor (suPAR), were explored in 125 blood samples collected at different time-points. At baseline, MR-proADM was an independent predictor of infection [>0.94pg/mL, OR=3.63 (1.16-11.33), p=0.026], as well as suPAR at 24h [>2185.8pg/mL, OR=5.81 (1.05-32.26), p=0.044]. Both MR-proADM and suPAR were raised in patients with infections throughout the first week after stroke. These results are especially relevant for MR-proADM given its early elevation, which would allow early preventive interventions.

Impact of Infection on Stroke Morbidity and Outcomes

Each year, millions of persons worldwide are disabled by stroke. The burden of stroke is expected to increase as a consequence of growth in our elderly population. Outcome is dependent upon limitation of secondary medical processes in the acute setting that lead to deterioration and increased long-term disability. The prevalence of infection after stroke is greater that seen in other medical conditions with similar acuity and its impact upon morbidity and mortality is substantial. Physical impairment and immune modulation are chief determinants in rate of infection after stroke. Each of these factors has been a target for therapeutic intervention. Current best practices for acute stroke management implement strategies for prevention, prompt identification, and treatment of infection. Novel therapies are currently being explored which have the opportunity to greatly minimize infectious complications following stroke. Fever commonly accompanies infection and independently influences stroke outcome. Targeted temperature management provides an additional chance to improve stroke recovery.

Impact of aging immune system on neurodegeneration and potential immunotherapies

The interaction between the nervous and immune systems during aging is an area of avid interest, but many aspects remain unclear. This is due, not only to the complexity of the aging process, but also to a mutual dependency and reciprocal causation of alterations and diseases between both the nervous and immune systems. Aging of the brain drives whole body systemic aging, including aging-related changes of the immune system. In turn, the immune system aging, particularly immunosenescence and T cell aging initiated by thymic involution that are sources of chronic inflammation in the elderly (termed inflammaging), potentially induces brain aging and memory loss in a reciprocal manner. Therefore, immunotherapeutics including modulation of inflammation, vaccination, cellular immune therapies and "protective autoimmunity" provide promising approaches to rejuvenate neuroinflammatory disorders and repair brain injury. In this review, we summarize recent discoveries linking the aging immune system with the development of neurodegeneration. Additionally, we discuss potential rejuvenation strategies, focusing aimed at targeting the aging immune system in an effort to prevent acute brain injury and chronic neurodegeneration during aging.

An unexplored brain-gut microbiota axis in stroke

Microbiota research, in particular that of the gut, has recently gained much attention in medical research owing to technological advances in metagenomics and metabolomics. Despite this, much of the research direction has focused on long-term or chronic effects of microbiota manipulation on health and disease. In this addendum, we reflect on our recent publication that reported findings addressing a rather unconventional hypothesis. Bacterial pneumonia is highly prevalent and is one of the leading contributors to stroke morbidity and mortality worldwide. However, microbiological cultures of samples taken from stroke patient with a suspected case of pneumonia often return with a negative result. Therefore, we proposed that post-stroke infection may be due to the presence of anaerobic bacteria, possibly those originated from the host gut microbiota. Supporting this, we showed that stroke promotes intestinal barrier breakdown and robust microbiota changes, and the subsequent translocation of selective bacterial strain from the host gut microbiota to peripheral tissues (i.e. lung) induces post-stroke infections. Our findings were further supported by various elegant studies published in the past 12 months. Here, we discuss and provide an overview of our key findings, supporting studies, and the implications for future advances in stroke research.

Intravital Imaging of Myeloid Cells: Inflammatory Migration and Resident Patrolling

Myeloid cell recruitment to sites of infection and injury started out as a simple model that has been referred to as the universal concept of leukocyte recruitment. However, as we gain more insight into the different mechanisms, it is becoming clear that each organ and perhaps even each cell has its own unique mechanism of recruitment. Moreover, as the ability to visualize specific cell types in specific organs becomes more accessible, it is also becoming clear that there are resident populations of leukocytes, some within the tissues and others attached to the vasculature of tissues, the latter poised to affect the local environment. In this review, we will first highlight the imaging approaches that have allowed us to gain spectacular insight into locale and function of specific cell types, and then we will discuss what we have learned from this approach as far as myeloid cells are concerned. We will also highlight some of the gaps in our knowledge, which exist almost certainly because of the challenges of being able to visualize certain compartments of the body.

Immune responses in perinatal brain injury

The perinatal period has often been described as immune deficient. However, it has become clear that immune responses in the neonate following exposure to microbes or as a result of tissue injury may be substantial and play a role in perinatal brain injury. In this article we will review the immune cell composition under normal physiological conditions in the perinatal period, both in the human and rodent. We will summarize evidence of the inflammatory responses to stimuli and discuss how neonatal immune activation, both in the central nervous system and in the periphery, may contribute to perinatal hypoxic-ischemic brain injury.

Mechanisms and Therapeutic Relevance of Neuro-immune Communication

Active research at the frontiers of immunology and neuroscience has identified multiple points of interaction and communication between the immune system and the nervous system. Immune cell activation stimulates neuronal circuits that regulate innate and adaptive immunity. Molecular mechanistic insights into the inflammatory reflex and other neuro-immune interactions have greatly advanced our understanding of immunity and identified new therapeutic possibilities in inflammatory and autoimmune diseases. Recent successful clinical trials using bioelectronic devices that modulate the inflammatory reflex to significantly ameliorate rheumatoid arthritis and inflammatory bowel disease provide a path for using electrons as a therapeutic modality for targeting molecular mechanisms of immunity. Here, we review mechanisms of peripheral sensory neuronal function in response to immune challenges, the neural regulation of immunity and inflammation, and the therapeutic implications of those mechanistic insights.

Flow Cytometric Characterization of T Cell Subsets and Microglia After Repetitive Mild Traumatic Brain Injury in Rats

Although, there is growing awareness in the progressive neurodegeneration of chronic traumatic encephalopathy, changes of immune reactions remain equivocal at best. Thus, in a clinically relevant rat repetitive mild traumatic brain injury (rmTBI) model, some immunologic cells (T cell subsets, microglia) in the injured brain and peripheral blood were analyzed by flow cytometry and immunofluorescence. In the injured brain, CD3⁺ T cells showed a bimodal increase during 42 days post-injury (dpi). CD3⁺CD4⁺ T cells firstly increased and then decreased, while CD3⁺CD8⁺ T cells had reversed tendency. CD86⁺/CD11b⁺ M1-like microglia increased at 42 dpi and CD206⁺/CD11b⁺ M2-like microglia peaked at 7 dpi. In addition, peripheral immune suppression was implicated in the chronic phase after rmTBI. Taken together, the study provided useful information on long-term dynamic changes of some immune cells after rmTBI in rats.

C-reactive protein in the detection of post-stroke infections: systematic review and individual participant data analysis

We conducted a systematic review and individual participant data meta-analysis to explore the role of C-reactive protein (CRP) in early detection or prediction of post-stroke infections. CRP, an acute-phase reactant binds to the phosphocholine expressed on the surface of dead or dying cells and some bacteria, thereby activating complement and promoting phagocytosis by macrophages. We searched PubMed up to May-2015 for studies measuring CRP in stroke and evaluating post-stroke infections. Individual participants' data were merged into a single database. CRP levels were standardized and divided into quartiles. Factors independently associated with post-stroke infections were determined by logistic regression analysis and the additional predictive value of CRP was assessed by comparing areas under receiver operating characteristic curves and integrated discrimination improvement index. Data from seven studies including 699 patients were obtained. Standardized CRP levels were higher in patients with post-stroke infections beyond 24 h. Standardized CRP levels in the fourth quartile were independently associated with infection in two different logistic regression models, model 1 [stroke severity and dysphagia, odds ratio = 9.70 (3.10-30.41)] and model 2 [age, sex, and stroke severity, odds ratio = 3.21 (1.93-5.32)]. Addition of CRP improved discrimination in both models [integrated discrimination improvement = 9.83% (0.89-18.77) and 5.31% (2.83-7.79), respectively], but accuracy was only improved for model 1 (area under the curve 0.806-0.874, p = 0.036). In this study, CRP was independently associated with development of post-stroke infections, with the optimal time-window for measurement at 24-48 h. However, its additional predictive value is moderate over clinical information. Combination with other biomarkers in a panel seems a promising strategy for future studies.

Effect of PD-1: PD-L1 in Invariant Natural Killer T-Cell Emigration and Chemotaxis Following Sepsis

Invariant natural killer T-cells (iNKT) are a subset of T-cells that play a regulatory role in sepsis. Following cecal ligation and puncture (CLP), iNKT cells emigrate from the liver and into the circulation and peritoneum in a manner dependent upon coinhibitory molecule Programmed Cell Death Receptor 1 (PD-1). We hypothesized that the effect of PD-1 on iNKT-cell emigration was dependent upon the direct PD-1:PD-L1 interaction, and that PD-1 and PD-L1 would play a role in chemotaxis and chemokine receptor expression. Adoptive transfer of Vybrant-labeled wild-type (WT) cells showed the donor iNKT cells migrated from the liver to the peritoneum following CLP, but PD-L1 deficient donor iNKT cells did not. In a chemotaxis assay, WT-iNKT cells chemotaxed to CXCL12, but PD-1 and PD-L1 deficient iNKT cells did not. Using flow cytometry to evaluate chemokine receptor expression, peritoneal iNKT expression of CXCR4 increased following CLP in the WT, PD-1, and PD-L1 deficient animals, and CXCR6 increased in the WT and PD-1 deficient animals. In conclusion here we document that the hepatic emigration of iNKT cells following CLP to the peritoneum appears dependent upon the direct PD-1:PD-L1 interaction; however, although PD-1 and PD-L1 appear to play a role in chemotaxis, this is unlikely a reflection of iNKT-cell chemokine receptor expression changes.

Prolonged Activation of Invariant Natural Killer T Cells and TH2-Skewed Immunity in Stroke Patients

Background

Infection is highly prevalent and contribute significantly to mortality of stroke patients. In addition to the well described robust systemic lymphocytopenia and skewed T helper 2 (T_H2)-immunity after stroke, emerging experimental evidence demonstrate that the development of infection poststroke is attributed by the activation of invariant natural killer T (iNKT) cells. In this prospective study, we examined the levels of a broad spectrum of inflammatory mediators, the activation status of iNKT cell in the blood of patients with various degree of stroke severity, and investigate whether these parameters differ in patients who later develop poststroke infections.

Methods and results

We obtained blood from stroke patients and matching controls to perform flow cytometry and multiplex measurement of inflammatory mediators. Our data suggest a pronounced activation of iNKT cells in stroke patients as compared with matched Healthy and Hospital control patients. The magnitude of iNKT activation is positively correlated with the severity of stroke, supporting the hypothesis that iNKT cells may contribute in the modulation of the host immune response after stroke-associated brain injury. In addition, stroke severity is closely correlated with decreased T_H1/T_H2 ratio, increased production of interleukin (IL)-10, with infected stroke patients showing exacerbated production of IL-10.

Conclusion

Stroke triggers a robust and sustained shift in systemic immunity in patients, including specific lymphopenia, robust activation of iNKT cells, systemic production of IL-10, and a prolonged T_H2-skewed immunity, all are potential contributors to severe immune suppression seen in patients after stroke. Future studies with large sample size will provide potential causality relationship insights.

Neural regulation of immunity: molecular mechanisms and clinical translation

Studies bridging neuroscience and immunology have identified neural pathways that regulate immunity and inflammation. Recent research using methodological advances in molecular genetics has improved our understanding of the neural control of immunity. Here we outline mechanistic insights, focusing on translational relevance and conceptual developments. We also summarize findings from recent clinical studies of bioelectronic neuromodulation in inflammatory and autoimmune diseases.

Neuro-Immune Interactions at Barrier Surfaces

Multidirectional interactions between the nervous and immune systems have been documented in homeostasis and pathologies ranging from multiple sclerosis to autism, and from leukemia to acute and chronic inflammation. Recent studies have addressed this crosstalk using cell-specific targeting, novel sequencing, imaging, and analytical tools, shedding light on unappreciated mechanisms of neuro-immune regulation. This Review focuses on neuro-immune interactions at barrier surfaces-mostly the gut, but also including the skin and the airways, areas densely populated by neurons and immune cells that constantly sense and adapt to tissue-specific environmental challenges.

Immunomodulation after ischemic stroke: potential mechanisms and implications for therapy

Brain injuries are often associated with intensive care admissions, and carry high morbidity and mortality rates. Ischemic stroke is one of the most frequent causes of injury to the central nervous system. It is now increasingly clear that human stroke causes multi-organ systemic disease. Brain inflammation may lead to opposing local and systemic effects. Suppression of systemic immunity by the nervous system could protect the brain from additional inflammatory damage; however, it may increase the susceptibility to infection. Pneumonia and urinary tract infection are the most common complications occurring in patients after stroke. The mechanisms involved in lung-brain interactions are still unknown, but some studies have suggested that inhibition of the cholinergic anti-inflammatory pathway and release of glucocorticoids, catecholamines, and damage-associated molecular patterns (DAMPs) are among the pathophysiological mechanisms involved in communication from the ischemic brain to the lungs after stroke. This review describes the modifications in local and systemic immunity that occur after stroke, outlines mechanisms of stroke-induced immunosuppression and their role in pneumonia, and highlights potential therapeutic targets to reduce post-stroke complications. Despite significant advances towards a better understanding of the pathophysiology of ischemic stroke-induced immunosuppression and stroke-associated pneumonia (SAP) in recent years, many unanswered questions remain. The true incidence and outcomes of SAP, especially in intensive care unit settings, have yet to be determined, as has the full extent of stroke-induced immunosuppression and its clinical implications.

Immunity and stroke, the hurdles of stroke research translation

Immunomodulatory therapies after stroke have the potential to provide clinical benefit to a subset of patients, but risk subverting the protective, healing aspects of the innate immune response. Neutrophils clear necrotic cerebral tissue and are important in immunomodulation, but can also contribute to tissue injury. Human trials for immunomodulatory stroke treatments in the sub-acute time frame have attempted to prevent peripheral neutrophil infiltration, but none have been successful and one trial demonstrated harm. These unselected trials had broad inclusion criteria and appear to not have had a specific treatment target. Unfortunately, due to the heterogeneous nature of brain ischemia in humans resulting in variation in clinical severity, the negative effect of thrombolytic drugs on the blood-brain barrier, and the heterogeneity of immune response, it may only be a subset of stroke patients who can realistically benefit from immunomodulation therapies. Translational research strategies require both an understanding of lab practices which create highly controlled environments in contrast to clinical practice where the diagnosis of stroke does not require the identification of a vessel occlusion. These differences between lab and clinical practices can be resolved through the integration of appropriate patient selection criteria and use of advanced imaging and ridged patient selection practices in clinical trials which will be an important part to the success of any future trials of translational research such as immunomodulation.

Inflammation after Ischemic Stroke: The Role of Leukocytes and Glial Cells

The immune response after stroke is known to play a major role in ischemic brain pathobiology. The inflammatory signals released by immune mediators activated by brain injury sets off a complex series of biochemical and molecular events which have been increasingly recognized as a key contributor to neuronal cell death. The primary immune mediators involved are glial cells and infiltrating leukocytes, including neutrophils, monocytes and lymphocyte. After ischemic stroke, activation of glial cells and subsequent release of pro- and anti-inflammatory signals are important for modulating both neuronal cell damage and wound healing. Infiltrated leukocytes release inflammatory mediators into the site of the lesion, thereby exacerbating brain injury. This review describes how the roles of glial cells and circulating leukocytes are a double-edged sword for neuroinflammation by focusing on their detrimental and protective effects in ischemic stroke. Here, we will focus on underlying characterize of glial cells and leukocytes under inflammation after ischemic stroke.

Pre-Stroke Use of Beta-Blockers Does Not Lower Post-Stroke Infection Rate: An Exploratory Analysis of the Preventive Antibiotics in Stroke Study

BACKGROUND

Stroke-associated infections occur frequently and are associated with unfavorable outcome. Previous cohort studies suggest a protective effect of beta-blockers (BBs) against infections. A sympathetic drive may increase immune suppression and infections.

AIM

This study is aimed at investigating the association between BB treatment at baseline and post-stroke infection in the Preventive Antibiotics in Stroke Study (PASS), a prospective clinical trial.

METHODS

We performed an exploratory analysis in PASS, 2,538 patients with acute phase of stroke (24 h after onset) were randomized to ceftriaxone (intravenous, 2 g per day for 4 days) in addition to stroke unit care, or standard stroke unit care without preventive antibiotic treatment. All clinical data, including use of BBs, was prospectively collected. Infection was diagnosed by the treating physician, and independently by an expert panel blinded for all other data. Multivariable analysis was performed to investigate the relation between BB treatment and infection rate.

RESULTS

Infection, as defined by the physician, occurred in 348 of 2,538 patients (14%). Multivariable analysis showed that the use of BBs at baseline was associated with the development of infection during clinical course (adjusted OR (aOR) 1.61, 95% CI 1.19-2.18; p < 0.01). BB use at baseline was also associated with the development of pneumonia (aOR 1.56, 95% CI 1.05-2.30; p = 0.03). Baseline BB use was not associated with mortality (aOR 1.14, 95% CI 0.84-1.53; p = 0.41) or unfavorable outcome at 3 months (aOR 1.10, 95% CI 0.89-1.35; p = 0.39).

CONCLUSIONS

Patients treated with BBs prior to stroke have a higher rate of infection and pneumonia.

Antigen Presentation After Stroke

Stroke induces a local inflammatory reaction and a plethora of innate immune responses in the brain where antigen-presenting cells become prominent. However, to date, it is still unclear whether antigen presentation is relevant to the neuropathological and functional outcome of stroke. Stroke does not trigger overt autoimmune reactions, but neural antigens have been found in lymphoid tissues of patient with stroke and it is unknown whether they promote tolerance or immune reactions that under certain conditions might contribute to the functional worsening observed in some patients. Autoantibodies to neural molecules have also been reported in patients with stroke, but the subclass of antibodies is important for their function, and the contribution of such findings to stroke outcome is not yet clear. Notably, stroke induces immunodepression highlighted by a transient lymphopenia, lymphoid organ atrophy, and monocyte deactivation. While these effects might reduce the chances of autoreactivity, they increase the risk of infection in patients with stroke and most frequently in those with severe stroke. Therefore any potential brain protective effect of stroke-induced immunodepression by attenuating or preventing lymphocyte-mediated brain damage is confounded by stroke severity and an increased incidence of infections. Systemic inflammation due to a number of comorbidities that are frequent in patients with stroke is also associated to a poor outcome. Herein, we review some relevant findings regarding the identification of neural antigens in stroke and discuss their potential contribution to the functional outcome of stroke.

Infiltration of invariant natural killer T cells occur and accelerate brain infarction in permanent ischemic stroke in mice

Invariant natural killer T (iNKT) cells are a unique subset of T cells that have been implicated in inflammation, atopy, autoimmunity, infections, and cancer. Although iNKT cells have been extensively studied over the past decade, its role in the pathogenesis of ischemic brain injury is still largely unknown. In our study, we determined whether iNKT cells infiltration occur in a mouse model of permanent cerebral ischemia. C57BL6/J male mice were treated with either alpha-galactosylceramide (α-GalCer) or vehicle control before undergoing permanent middle cerebral artery occlusion (pMCAO). α-GalCer, a glycolipid antigen, specifically activates iNKT cells by a CD1d-restricted mechanism. Using flow cytometry, 10,000 leukocytes (CD45 high cells) from the ischemic hemisphere and peripheral blood respectively were analyzed to determine the number of NK1.1⁺CD3⁺ cells at 3, 12, 24 and 48h post-pMCAO. Cerebral infarct size, brain edema and morphological characteristics were measured at the stipulated time points by 2,3,5-triphenyltetrazolium chloride (TTC) staining, weighing, and H&E staining. The levels of IFN-γ and TNF-α in brain tissue and serum were assessed by immunohistochemistry and ELISA respectively. We found that the number of iNKT cells started increasing from 12h (PB sample) and 24h (ischemic hemisphere sample) respectively in the vehicle treated group. iNKT cells infiltration occurred at an earlier time-point compared in the α-GalCer treated group (T=3H vs T=12H in PB sample; T=12H vs T=24H in ischemic hemisphere sample). Brain water content at 12h and 24h was significantly higher in pMCAO+α-GalCer mice compared to pMCAO+vehicle mice which was in turn higher than mice that underwent sham surgery. Aggravated morphological abnormalities in HE-stained neurons and significantly increased neurons with pyknotic nuclei and cavitation in the ischemic region were observed at 24h in the pMCAO+α-GalCer and pMCAO+vehicle groups. Cerebral infarct volume, neurological deficit Scores and brain edema were significantly increased at 24h in the pMCAO+α-GalCer group compared to pMCAO+vehicle group. In the pMCAO+vehicle group, the serum concentrations of TNF-α and IFN-γ were increased at 12h and 24h post-pMCAO, and remained elevated up to 48h. In mice treated with pMCAO+α-GalCer, TNF-α and IFN-γ were both increased at 12h post-pMCAO, and remained elevated up to 48h. Immunohistochemistry showed that protein expression of TNF-α and IFN-γ in brain tissues was higher in α-GalCer-treated mice. Our results demonstrate that within 48h of focal permanent cerebral ischemia, iNKT cells infiltrate into the brain and contribute to brain infarction.

Neural circuitry and immunity

Research during the last decade has significantly advanced our understanding of the molecular mechanisms at the interface between the nervous system and the immune system. Insight into bidirectional neuro-immune communication has characterized the nervous system as an important partner of the immune system in the regulation of inflammation. Neuronal pathways, including the vagus nerve-based inflammatory reflex, are physiological regulators of immune function and inflammation. In parallel, neuronal function is altered in conditions characterized by immune dysregulation and inflammation. Here, we review these regulatory mechanisms and describe the neural circuitry modulating immunity. Understanding these mechanisms reveals possibilities to use targeted neuromodulation as a therapeutic approach for inflammatory and autoimmune disorders. These findings and current clinical exploration of neuromodulation in the treatment of inflammatory diseases define the emerging field of Bioelectronic Medicine.

Neural Control of Inflammation: Implications for Perioperative and Critical Care

Inflammation and immunity are regulated by neural reflexes. Recent basic science research has demonstrated that a neural reflex, termed the inflammatory reflex, modulates systemic and regional inflammation in a multiplicity of clinical conditions encountered in perioperative medicine and critical care. In this review, the authors describe the anatomic and physiologic basis of the inflammatory reflex and review the evidence implicating this pathway in the modulation of sepsis, ventilator-induced lung injury, postoperative cognitive dysfunction, myocardial ischemia-reperfusion injury, and traumatic hemorrhage. The authors conclude with a discussion of how these new insights might spawn novel therapeutic strategies for the treatment of inflammatory diseases in the context of perioperative and critical care medicine.

Strain-Related Differences in the Immune Response: Relevance to Human Stroke

There are significant differences in the immune response and in the susceptibility to autoimmune diseases among rodent strains. It would thus be expected that the contribution of the immune response to cerebral ischemic injury would also differ among rodent strains. More importantly, there are significant differences between the immune responses of rodents and humans. All of these factors are likely to impact the successful translation of immunomodulatory therapies from experimental rodent models to patients with stroke.

Sex-Related Differences in the Risk of Hospital-Acquired Sepsis and Pneumonia Post Acute Ischemic Stroke

BACKGROUND AND OBJECTIVE

Infectious complications after ischemic stroke are frequent and lead to neurological deterioration, poor functional outcomes, and higher mortality. Local and systemic inflammatory responses to brain ischemia differ between males and females, but little is known about differences in poststroke susceptibility to infection by sex. The purpose of this study was to compare sex-related differences in the risk of hospital-acquired sepsis and pneumonia after acute ischemic stroke (AIS).

MATERIALS AND METHODS

This is a retrospective, secondary analysis of the 2010-2011 California State Inpatient Database. Previously validated International Classification of Disease, Ninth Revision (ICD-9) codes were used to identify adult hospitalizations for AIS. The primary outcome was hospital-acquired sepsis or pneumonia, also identified using ICD-9 codes. Associations between sex and hospital-acquired sepsis or pneumonia were adjusted for baseline characteristics and comorbidities using multivariable logistic regression.

RESULTS

There were 91,643 hospitalizations for AIS included in this analysis, of which 1027 had hospital-acquired sepsis and 1225 had hospital-acquired pneumonia. The in-hospital mortality without infection was 4.6%; the presence of hospital-acquired infections was associated with higher mortality for sepsis (32.7%) and pneumonia (21.9%). Female (versus male) sex was associated with lower adjusted odds of hospital-acquired sepsis (odds ratio [OR] .74, 95% confidence interval [CI] .65-.84) and pneumonia (OR .69, 95% CI .62-.78). This difference was similar across age strata. Among hospitalizations with either hospital-acquired sepsis or pneumonia, sex did not influence mortality.

CONCLUSIONS

Female sex was associated with a lower risk of hospital-acquired sepsis and pneumonia after AIS. Further investigation is needed to determine the mechanisms underlying this clinical observation.

Identification and treatment of the Staphylococcus aureus reservoir in vivo

Kubes et al. show that methicillin-resistant Staphylococcus aureus (MRSA) survive and proliferate inside Kupffer cells. Intracellular MRSA is resistant to neutrophil-killing and antibiotics treatment and, when released into the circulation, can infect other organs.

Methicillin-resistant Staphylococcus aureus (MRSA) bacteremia is reaching epidemic proportions causing morbidity, mortality, and chronic disease due to relapses, suggesting an intracellular reservoir. Using spinning-disk confocal intravital microscopy to track MRSA-GFP in vivo, we identified that within minutes after intravenous infection MRSA is primarily sequestered and killed by intravascular Kupffer cells (KCs) in the liver. However, a minority of the Staphylococci overcome the KC’s antimicrobial defenses. These bacteria survive and proliferate for many days within this intracellular niche, where they remain undetected by recruited neutrophils. Over time, the KCs lyse, releasing bacteria into the circulation, enabling dissemination to other organs such as the kidneys. Vancomycin, the antibiotic of choice to treat MRSA bacteremia, could not penetrate the KCs to eradicate intracellular MRSA. However, based on the intravascular location of these specific macrophages, we designed a liposomal formulation of vancomycin that is efficiently taken up by KCs and diminished the intracellular MRSA. Targeting the source of the reservoir dramatically protected the liver but also dissemination to other organs, and prevented mortality. This vancomycin formulation strategy could help treat patients with Staphylococcal bacteremia without a need for novel antibiotics by targeting the previously inaccessible intracellular reservoir in KCs.

Inflammatory responses in brain ischemia

Brain infarction causes tissue death by ischemia due to occlusion of the cerebral vessels and recent work has shown that post stroke inflammation contributes significantly to the development of ischemic pathology. Because secondary damage by brain inflammation may have a longer therapeutic time window compared to the rescue of primary damage following arterial occlusion, controlling inflammation would be an obvious therapeutic target. A substantial amount of experimentall progress in this area has been made in recent years. However, it is difficult to elucidate the precise mechanisms of the inflammatory responses following ischemic stroke because inflammation is a complex series of interactions between inflammatory cells and molecules, all of which could be either detrimental or beneficial. We review recent advances in neuroinflammation and the modulation of inflammatory signaling pathways in brain ischemia. Potential targets for treatment of ischemic stroke will also be covered. The roles of the immune system and brain damage versus repair will help to clarify how immune modulation may treat stroke.