High-salt diet suppresses autoimmune demyelination by regulating the blood-brain barrier permeability.

Sodium chloride, "salt," is an essential component of daily food and vitally contributes to the body's homeostasis. However, excessive salt intake has often been held responsible for numerous health risks associated with the cardiovascular system and kidney. Recent reports linked a high-salt diet (HSD) to the exacerbation of artificially induced central nervous system (CNS) autoimmune pathology through changes in microbiota and enhanced TH17 cell differentiation [M. Kleinewietfeld et al., Nature 496, 518-522 (2013); C. Wu et al., Nature 496, 513-517 (2013); N. Wilck et al., Nature 551, 585-589 (2017)]. However, there is no evidence that dietary salt promotes or worsens a spontaneous autoimmune disease. Here we show that HSD suppresses autoimmune disease development in a mouse model of spontaneous CNS autoimmunity. We found that HSD consumption increased the circulating serum levels of the glucocorticoid hormone corticosterone. Corticosterone enhanced the expression of tight junction molecules on the brain endothelial cells and promoted the tightening of the blood-brain barrier (BBB) thereby controlling the entry of inflammatory T cells into the CNS. Our results demonstrate the multifaceted and potentially beneficial effects of moderately increased salt consumption in CNS autoimmunity.

Oligodendrocytes enforce immune tolerance of the uninfected brain by purging the peripheral repertoire of autoreactive CD8+ T cells.

Myelin-specific CD8(+) T cells are thought to contribute to the pathogenesis of multiple sclerosis. Here we modeled this contribution in mice with CD8(+) T cells recognizing ovalbumin (OVA) expressed in oligodendrocytes (ODCs). Surprisingly, even very high numbers of activated OVA-reactive CD8(+) T cells failed to induce disease and were cleared from the immune system after antigen encounter in the central nervous system (CNS). Peripheral infection with OVA-expressing Listeria (Lm-OVA) enabled CD8(+) T cells to enter the CNS, leading to the deletion of OVA-specific clones after OVA recognition on ODCs. In contrast, intracerebral infection allowed OVA-reactive CD8(+) T cells to cause demyelinating disease. Thus, in response to infection, CD8(+) T cells also patrol the CNS. If the CNS itself is infected, they destroy ODCs upon cognate antigen recognition in pursuit of pathogen eradication. In the sterile brain, however, antigen recognition on ODCs results in their deletion, thereby maintaining tolerance.

A Mouse Model of Post-Stroke Pneumonia Induced by Intra-Tracheal Inoculation with Streptococcus pneumoniae.

BACKGROUND: Stroke-induced immunodeficiency increases the risk of infectious complications, which adversely affects neurological outcome. Among those, pneumonia affects as many as one third of stroke patients and is the main contributor to mortality in the post-acute phase of stroke. Experimental findings on post-stroke susceptibility to spontaneous pneumonia in mice are contradictory. Here, we established a mouse model inducing standardized bacterial pneumonia and characterized the impaired pulmonary cellular and humoral immune responses after experimental stroke. METHODS: Bacterial pneumonia was induced by intra-tracheal inoculation with Streptococcus pneumoniae at different time points after transient middle cerebral artery occlusion (MCAO). Bacterial counts in lungs and blood, histological changes, and cytokine production in the lungs were assessed. Furthermore, we investigated the effect of pneumonia on stroke outcome. RESULTS: Intra-tracheal inoculation resulted in reproducible pneumonia and bacteraemia, and demonstrated post-stroke susceptibility to streptococcal pneumonia developing with a delay of at least 24 h after MCAO. Higher bacterial counts in mice infected 3 days after stroke induction correlated with reduced neutrophil and macrophage infiltration in the lungs and lower levels of pro-inflammatory cytokines in the broncho-alveolar lavage compared to sham-operated animals. Pneumonia increased mortality without affecting brain-infiltrating leukocytes. CONCLUSIONS: In this standardized mouse model of post-stroke pneumonia, we describe attenuated leukocyte infiltration and cytokine production in response to bacterial infection in the lungs that has a profound effect on outcome.

DAMP signaling is a key pathway inducing immune modulation after brain injury.

Acute brain lesions induce profound alterations of the peripheral immune response comprising the opposing phenomena of early immune activation and subsequent immunosuppression. The mechanisms underlying this brain-immune signaling are largely unknown. We used animal models for experimental brain ischemia as a paradigm of acute brain lesions and additionally investigated a large cohort of stroke patients. We analyzed release of HMGB1 isoforms by mass spectrometry and investigated its inflammatory potency and signaling pathways by immunological in vivo and in vitro techniques. Features of the complex behavioral sickness behavior syndrome were characterized by homecage behavior analysis. HMGB1 downstream signaling, particularly with RAGE, was studied in various transgenic animal models and by pharmacological blockade. Our results indicate that the cytokine-inducing, fully reduced isoform of HMGB1 was released from the ischemic brain in the hyperacute phase of stroke in mice and patients. Cytokines secreted in the periphery in response to brain injury induced sickness behavior, which could be abrogated by inhibition of the HMGB1-RAGE pathway or direct cytokine neutralization. Subsequently, HMGB1-release induced bone marrow egress and splenic proliferation of bone marrow-derived suppressor cells, inhibiting the adaptive immune responses in vivo and vitro. Furthermore, HMGB1-RAGE signaling resulted in functional exhaustion of mature monocytes and lymphopenia, the hallmarks of immune suppression after extensive ischemia. This study introduces the HMGB1-RAGE-mediated pathway as a key mechanism explaining the complex postischemic brain-immune interactions.

Idarucizumab Improves Outcome in Murine Brain Hemorrhage Related to Dabigatran.

Lack of specific antidotes is a major concern in intracerebral hemorrhage (ICH) related to direct anticoagulants including dabigatran (OAC-ICH). We examined the efficacy of idarucizumab, an antibody fragment binding to dabigatran, in a mouse model of OAC-ICH. Dabigatran etexilate (DE) dose-dependently prolonged diluted thrombin time and tail-vein bleeding time, which were reversed by idarucizumab. Pretreatment with DE increased intracerebral hematoma volume and cerebral hemoglobin content. Idarucizumab in equimolar dose prevented excess hematoma expansion for both DE doses. In more extensive ICH, idarucizumab significantly reduced mortality. Thus, idarucizumab prevents excess intracerebral hematoma formation in mice anticoagulated with dabigatran and reduces mortality.

Amplification of regulatory T cells using a CD28 superagonist reduces brain damage after ischemic stroke in mice.

BACKGROUND AND PURPOSE: Neuroinflammation plays an important role in ischemic brain injury. Regulatory T cells (Treg) are important endogenous immune modulators. We tested the hypothesis that Treg amplification with a CD28 superagonistic monoclonal antibody (CD28SA) reduces brain damage in murine cerebral ischemia. METHODS: Cerebral ischemia was induced by coagulation of the distal middle cerebral artery or by 60 minutes filament occlusion of the proximal middle cerebral artery in C57BL6 mice. 150 µg CD28SA was injected intraperitoneally 3 or 6 hours after ischemia onset. Outcome was determined by infarct volumetry and behavioral testing. Brain-infiltrating leukocyte subpopulations were analyzed by flow cytometry and immunohistochemistry 3 and 7 days after middle cerebral artery occlusion. RESULTS: CD28SA reduced infarct size in both models and attenuated functional deficit 7 days after stroke induction. Mice treated with CD28SA increased numbers of Treg in spleen and brain. Tregs were functionally active and migrated into the brain where they accumulated and proliferated in the peri-infarct area. More than 60% of brain infiltrating Treg produced interleukin-10 in CD28SA compared with 30% in control. CONCLUSIONS: In vivo expansion and amplification of Treg by CD28SA attenuates the inflammatory response and improves outcome after experimental stroke.

Boosting regulatory T cells limits neuroinflammation in permanent cortical stroke.

Inflammatory mechanisms contribute substantially to secondary tissue injury after brain ischemia. Regulatory T cells (Tregs) are key endogenous modulators of postischemic neuroinflammation. We investigated the potential of histone deacetylase inhibition (HDACi) to enhance Treg potency for experimental stroke in mice. HDACi using trichostatin A increased the number of Tregs and boosted their immunosuppressive capacity and interleukin (IL)-10 expression. In vivo treatment reduced infarct volumes and behavioral deficits after cortical brain ischemia, attenuated cerebral proinflammatory cytokine expression, and increased numbers of brain-invading Tregs. A similar effect was obtained using tubastatin, a specific inhibitor of HDAC6 and a key HDAC in Foxp3 regulation. The neuroprotective effect of HDACi depended on the presence of Foxp3(+) Tregs, and in vivo and in vitro studies showed that the anti-inflammatory cytokine IL-10 was their main mediator. In summary, modulation of Treg function by HDACi is a novel and potent target to intervene at the center of neuroinflammation. Furthermore, this novel concept of modulating endogenous immune mechanisms might be translated to a broad spectrum of diseases, including primary neuroinflammatory and neurodegenerative disorders.

Leukocyte invasion of the brain after experimental intracerebral hemorrhage in mice.

BACKGROUND AND PURPOSE: Neuroinflammatory processes contribute to secondary neuronal damage after intracerebral hemorrhage. We aimed to characterize the time course of brain immigration of different leukocyte subsets after striatal injection of either autologous blood or collagenase in mice. METHODS: Intracerebral hemorrhage was induced by injection of either autologous blood (20 µL) or collagenase (0.03 U) in C57Bl/6J mice. Hematoma volumetry was performed on cryosections. Blood volume was measured by hemoglobin spectrophotometry. Leukocytes were isolated from hemorrhagic hemisphere 1, 3, 5, and 14 days after intracerebral hemorrhage, stained for leukocyte markers, and measured by flow cytometry. Heterologous blood injection from CD45.1 mice was used to investigate the origin of brain-invading leukocytes. RESULTS: Collagenase injection induced a larger hematoma volume but a similar blood content compared with blood injection. Cerebral leukocyte infiltration in the hemorrhagic hemisphere was similar in both models. The majority of leukocytes isolated from the brain originated from the circulation. CD4+ T lymphocytes were the predominant brain leukocyte population in both models. However, cerebral granulocyte counts were higher after collagenase compared with blood injection. CONCLUSIONS: Brain infiltration of systemic immune cells is similar in both murine intracerebral hemorrhage models. The pathophysiological impact of invading leukocytes and, in particular, of T cells requires further investigation.

Constitutive Akt1 signals attenuate B-cell receptor signaling and proliferation, but enhance B-cell migration and effector function.

Ligation of the BCR induces a complex signaling network that involves activation of Akt, a family of serine/threonine protein kinases associated with B-cell development, proliferation, and tumor formation. Here, we analyzed the effect of enhanced Akt1 signals on B-cell maturation and function. Unexpectedly, we found that peripheral B cells overexpressing Akt1 were less responsive to BCR stimuli. This correlated with a decrease in Ca(2+) -mobilization and proliferation, in an impaired activation of Erk1/2 and mammalian target of rapamycin (mTOR) kinases and poor mobilization of NFATc1 and NF-κB/p65 factors. In contrast, activation of STAT5 and migration of B cells toward the chemokine SDF1α was found to be enhanced. Akt1 Tg mice showed an altered maturation of peritoneal and splenic B1 B cells and an enhanced production of IgG1 and IgG3 upon immunization with the T-cell independent Ag TNP-Ficoll. Furthermore, mice homo-zygous for Tg Akt1 showed a severe block in the maturation of B-cell precursors in BM and a strong enrichment of plasma cells in spleen. Altogether, our data reveal that enhanced Akt1 signals modify BCR signaling strength and, thereby, B-cell development and effector function.

Preculture of PBMCs at high cell density increases sensitivity of T-cell responses, revealing cytokine release by CD28 superagonist TGN1412.

Human volunteers receiving TGN1412, a humanized CD28-specific monoclonal antibody, experienced a life-threatening cytokine release syndrome during a recent trial. Preclinical tests using human PBMCs had failed to announce the rapid release of TNF, IFN-γ, and other toxic cytokines in response to this CD28 "superagonist" (CD28SA). CD28SA activate T-lymphocytes by ligating CD28 without overt engagement of the TCR. They do, however, depend on "tonic" TCR signals, which they amplify. Here we show that short-term preculture of PBMCs at high, but not at low, cell density results in massive cytokine release during subsequent stimulation with soluble TGN1412. Restoration of reactivity was cell-contact dependent, involved functional maturation of both monocytes and T cells, was sensitive to blockade by HLA-specific mAb, and was associated with TCR polarization and tyrosine phosphorylation. CD4 effector memory T cells were identified as the main source of proinflammatory cytokines. Importantly, responses to other T-cell activating agents, including microbial antigens, were also enhanced if PBMCs were first allowed to interact under tissue-like conditions. We provide a protocol, which strongly improves reactivity of circulating T cells to soluble stimulants, thereby allowing for more reliable preclinical testing of both activating and inhibitory immunomodulatory drugs.

A novel CD4 knockout mouse strain with a spontaneous frameshift mutation in the CD4 locus.

T cells express co-receptors CD4 and CD8, which are involved in the recognition of antigen presented to T cell receptors. The expression of CD4 in thymic hematopoietic cells is crucial for the thymic development and selection of T cells. In this study, we identified a novel CD4 mutant allele that emerged spontaneously in our mouse colony. The frameshift mutation led to a truncated CD4 protein which failed to reach the plasma membrane resulting in impaired development of CD4+ helper T cells. The CRISPR mediated correction of mutant allele restored the membrane CD4 expression. Further, using an adoptive transfer of T cells, we show that this model is an ideal recipient mouse for the study of CD4+ T cells.

Antigen-specific blockade of lethal CD8 T-cell mediated autoimmunity in a mouse model of multiple sclerosis.

Increasing evidence implies CD8 T cells in tissue-specific autoimmune diseases including multiple sclerosis. mAbs specific for MHC class I molecules presenting a dominant autoantigenic peptide may allow selective immunotherapy in such settings. We demonstrate the prophylactic and therapeutic efficacy of such a mAb in a transgenic mouse model of lethal demyelinating disease in which a neo-self Ag expressed by oligodendrocytes is targeted by CD8 T cells with transgenic Ag receptors. Mechanistic studies performed in vitro and in vivo indicate that it is the low expression of MHC class I on oligodendrocytes, which makes this form of Ag-specific intervention possible.

Targeted Expression of Myelin Autoantigen in the Periphery Induces Antigen-Specific T and B Cell Tolerance and Ameliorates Autoimmune Disease.

There is a great interest in developing antigen-specific therapeutic approaches for the treatment of autoimmune diseases without compromising normal immune function. The key challenges are to control all antigen-specific lymphocyte populations that contribute to pathogenic inflammatory processes and to provide long-term protection from disease relapses. Here, we show that myelin oligodendrocyte glycoprotein (MOG)-specific tolerance can be established by ectopic expression of MOG in the immune organs. Using transgenic mice expressing MOG-specific CD4, CD8, and B cell receptors, we show that MOG expression in the bone marrow cells results in impaired development of MOG-specific lymphocytes. Ectopic MOG expression has also resulted in long-lasting protection from MOG-induced autoimmunity. This finding raises hope that transplantation of autoantigen-expressing bone marrow cells as a therapeutic strategy for specific autoantigen-driven autoimmune diseases.

Naive CD8 T-cells initiate spontaneous autoimmunity to a sequestered model antigen of the central nervous system.

In multiple sclerosis, CD8 T-cells are thought play a key pathogenetic role, but mechanistic evidence from rodent models is limited. Here, we have tested the encephalitogenic potential of CD8 T-cells specific for the model antigen ovalbumin (OVA) sequestered in oligodendrocytes as a cytosolic molecule. We show that in these 'ODC-OVA' mice, the neo-self antigen remains invisible to CD4 cells expressing the OVA-specific OT-II receptor. In contrast, OVA is accessible to naïve CD8 T-cells expressing the OT-I T-cell receptor, during the first 10 days of life, resulting in antigen release into the periphery. Introduction of OT-I as a second transgene leads to fulminant demyelinating experimental autoimmune encephalomyelitis with multiple sclerosis-like lesions, affecting cerebellum, brainstem, optic nerve and spinal cord. OVA-transgenic oligodendrocytes activate naïve OT-I cells in vitro, and both major histocompatibility complex class I expression and the OT-I response are further up-regulated by interferon-gamma (IFN-gamma). Release of IFN-gamma into the circulation of ODC-OVA/OT-I double transgenic mice precedes disease manifestation, and pathogenicity of OT-I cells transferred into ODC-OVA mice is largely IFN-gamma dependent. In conclusion, naïve CD8 T-cells gaining access to an 'immune-privileged' organ can initiate autoimmunity via an IFN-gamma-assisted amplification loop even if the self-antigen in question is not spontaneously released for presentation by professional antigen presenting cells.

Clusterin mRNA expression in apoptotic and activated rat thymocytes.

Clusterin is a 75-80 kDa heterodimeric glycoprotein, that is produced in most tissues but which exact biological role is still not clear. Particularly, its role in protection or promotion of apoptosis is heavily disputed, since data supporting both views have been reported in several independent studies. To clarify this issue, and also to determine whether clusterin expression itself might be affected by apoptosis, in the present study, rat thymocytes were treated with dexamethasone, -a synthetic glucocorticoid that elicits apoptosis in thymocytes-, and clusterin mRNA expression was analyzed by semi-quantitative RT-PCR before and after induction of apoptosis. Interestingly, neither the treatment with dexamethasone in vitro nor triggering of apoptosis in vivo up- regulated clusterin expression, opposing the view that clusterin is involved in apoptotic processes. On the other hand, a new clusterin mRNA isoform was detected and isolated, whose expression was restricted to freshly isolated thymocytes. This novel isoform lacks the post-translational proteolytic cleavage site and is therefore predicted to encode a monomeric protein. The biological function under normal circumstances, however, will need further investigations for clarification. While apoptosis could not modulate clusterin expression, activation of thymocytes with concanavalin A and interleukin-2 resulted in up-regulation of clusterin mRNA level, indicating that clusterin expression is rather under the control of cell activation-mediated rather than apoptosis-induced signals.

Mitogenic CD28 signals require the exchange factor Vav1 to enhance TCR signaling at the SLP-76-Vav-Itk signalosome.

Almost all physiological T cell responses require costimulation-engagement of the clonotypic TCR with MHC/Ag and CD28 by its ligands CD80/86. Whether CD28 provides signals that are qualitatively unique or quantitatively amplify TCR signaling is poorly understood. In this study, we use superagonistic CD28 Abs, which induce T cell proliferation without TCR coligation, to determine how CD28 contributes to mitogenic responses. We show that mitogenic CD28 signals require but do not activate the proximal TCR components TCRzeta and Zap-70 kinase. In cell lines lacking proximal TCR signaling, an early defect in the CD28 pathway is in phosphorylation of the adaptor molecule SLP-76, which we show is essential for recruitment of the exchange factor Vav leading to Ca(2+) flux and IL-2 production. Point mutations in CD28 that result in diminished Vav phosphorylation also result in defective Ca(2+) flux, IL-2 production, and Tec-kinase phosphorylation. Using Vav1-deficient mice, we further demonstrate the importance of Vav1 for efficient proliferation, IL-2 production, and Ca(2+) flux. Our results indicate that CD28 signals feed into the TCR signaling pathway at the level of the SLP-76 signalosome.

Induction of experimental autoimmune encephalomyelitis in transgenic mice expressing ovalbumin in oligodendrocytes.

We have used the 5' flanking sequence of the myelin basic protein gene known to include the core promoter and a strong oligodendrocyte (ODC)-specific enhancer to target expression of the well-studied model antigen ovalbumin (OVA) to ODC in transgenic mice. OVA protein was detected in a tissue- and cell-specific manner in these "ODC-OVA" mice. Without immunization, CD4 T cells and B cells remained ignorant of the neo-self antigen expressed in the central nervous system (CNS), as indicated by unimpaired development and lack of activation of OVA/IA(b)-specific TCR transgenic T cells in these mice, and the ability to mount normal OVA-specific recall and antibody responses. Upon immunization with OVA in complete Freund's adjuvant, about half of the transgenic mice developed neurological symptoms characteristic of experimental autoimmune encephalomyelitis (EAE). Mononuclear infiltrates in the brain and spinal cord contained both macrophages and T cells, similar to classical models of EAE induced by immunization with CNS antigens in adjuvant. The wealth of immunological reagents available to study and manipulate the OVA-specific response should make this new model useful for the investigation of components and mechanisms involved in CNS-specific autoimmunity.

Active protein kinase B regulates TCR responsiveness by modulating cytoplasmic-nuclear localization of NFAT and NF-kappa B proteins.

T cell activation leads to the induction of the transcription factors of the NFAT and NF-kappa B families, important regulators of T cell activation and function. In this study we demonstrate that TCR/CD3-stimulated T cells from mice expressing a constitutively active form of protein kinase B (myr PKB alpha) lack significant nuclear accumulation/shuttling of NFATc1 and NFATp as well as NF-kappa Bp65 and RelB proteins. Notably, despite this deficit in nuclear NFAT and NF-kappa B proteins, myr PKB T cells show lower activation threshold for proliferation, enhanced cell cycle progression and increased production of Th1 and Th2 cytokines similar to signals provided by CD28 costimulation. The enhanced T cell response correlates with increased expression of cyclins D3 and B1 and cytokine-induced Src homology 2 protein, and inactivation of the forkhead transcription factor FKHR. In addition, coimmunoprecipitation studies indicate a direct regulation of NFATc1 by active PKB. Together, our results demonstrate that the positive regulatory role of myr PKB on TCR responsiveness, subsequent cell division, and effector function is linked to a negative regulatory mechanism on the nuclear accumulation/shuttling of NFAT and NF-kappa B proteins.

Mitogenic signals through CD28 activate the protein kinase Ctheta-NF-kappaB pathway in primary peripheral T cells.

Mitogenic anti-CD28 antibody stimulates all peripheral T cells to proliferate in the absence of TCR ligation, providing an exception to the two-signal requirement of T cell responses. This antibody preferentially recognizes a mobilized signaling-competent form of CD28, normally induced following TCR ligation, thus providing a unique non-physiological tool to dissect CD28-specific signals leading to T cell proliferation. The protein kinase C (PKC)theta-NF-kappaB pathway has recently been shown to integrate TCR- and CD28-derived signals in co-stimulation. We now demonstrate that this pathway is activated by mitogenic anti-CD28 antibody stimulation. In contrast to conventional anti-CD28 antibody, mitogenic anti-CD28 antibody induced activation of phospholipase Cgamma and Ca(2+) flux in peripheral rat T cells despite no or low levels of inducible tyrosine phosphorylation of TCRzeta chain, TCRzeta-associated protein of 70 kDa (ZAP-70) or linker for activation of T cells (LAT)-critical components of the TCR signaling machinery. Nevertheless, PKCtheta kinase activity in vitro was increased following mitogenic anti-CD28 antibody stimulation, as was membrane association of both PKCtheta and Bcl10. As downstream targets of PKCtheta activation, NF-kappaB components translocated to the nucleus at levels comparable to those after TCR-CD28 co-stimulation. NF-kappaB translocation was diminished by PKCtheta inhibition, as was induction of the NF-kappaB/AP-1 responsive activation marker CD69. We propose that co-stimulation is a sequential process in which appropriate TCR engagement is required to mobilize CD28 into a signaling-competent form which then activates the PKCtheta-NF-kappaB pathway necessary for IL-2 production and proliferation.