Traumatic injury and the presence of antigen differentially contribute to T-cell recruitment in the CNS

The Immune Response in Two Models of Traumatic Injury of the Immature Brain

Traumatic brain injury (TBI) can cause major disability and increases the risk of neurodegeneration. Post-TBI, there is infiltration of peripheral myeloid and lymphoid cells; there is limited information on the peripheral immune response post-TBI in the immature brain-where injury may interfere with neurodevelopment. We carried out two injury types in juvenile mice: invasive TBI with a controlled cortical impact (CCI) and repetitive mild TBI (rmTBI) using weight drop injury and analysed the response at 5- and 35-days post-injury. In the two models, we detected the brain infiltration of immune cells (e.g., neutrophils, monocytes, dendritic cells, CD4+ T cells, and NK cells). There were increases in macrophages, neutrophils, and dendritic cells in the spleen, increases in dendritic cells in blood, and increases in CD8+ T cells and B cells in lymph nodes. These results indicate a complex peripheral immune response post-TBI in the immature brain, with differences between an invasive injury and a repetitive mild injury.

CD8+ T cells in brain injury and neurodegeneration

The interaction between the peripheral immune system and the brain is increasingly being recognized as an important layer of neuroimmune regulation and plays vital roles in brain homeostasis as well as neurological disorders. As an important population of T-cell lymphocytes, the roles of CD8+ T cells in infectious diseases and tumor immunity have been well established. Recently, increasing number of complex functions of CD8+ T cells in brain disorders have been revealed. However, an advanced summary and discussion of the functions and mechanisms of CD8+ T cells in brain injury and neurodegeneration are still lacking. Here, we described the differentiation and function of CD8+ T cells, reviewed the involvement of CD8+ T cells in the regulation of brain injury including stroke and traumatic brain injury and neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), and discussed therapeutic prospects and future study goals. Understanding these processes will promote the investigation of T-cell immunity in brain disorders and provide new intervention strategies for the treatment of brain injury and neurodegeneration.

Single Versus Repetitive Traumatic Brain Injury: Current Knowledge on the Chronic Outcomes, Neuropathology and the Role of TDP-43 Proteinopathy

Traumatic brain injury (TBI) is one of the most important causes of death and disability in adults and thus an important public health problem. Following TBI, secondary pathophysiological processes develop over time and condition the development of different neurodegenerative entities. Previous studies suggest that neurobehavioral changes occurring after a single TBI are the basis for the development of Alzheimer's disease, while repetitive TBI is considered to be a contributing factor for chronic traumatic encephalopathy development. However, pathophysiological processes that determine the evolvement of a particular chronic entity are still unclear. Human post-mortem studies have found combinations of amyloid, tau, Lewi bodies, and TAR DNA-binding protein 43 (TDP-43) pathologies after both single and repetitive TBI. This review focuses on the pathological changes of TDP-43 after single and repetitive brain traumas. Numerous studies have shown that TDP-43 proteinopathy noticeably occurs after repetitive head trauma. A relatively small number of available preclinical research on single brain injury are not in complete agreement with the results from the human samples, which makes it difficult to draw specific conclusions. Also, as TBI is considered a heterogeneous type of injury, different experimental trauma models and injury intensities may cause differences in the cascade of secondary injury, which should be considered in future studies. Experimental and post-mortem studies of TDP-43 pathobiology should be carried out, preferably in the same laboratories, to determine its involvement in the development of neurodegenerative conditions after one and repetitive TBI, especially in the context of the development of new therapeutic options.

Glia as antigen-presenting cells in the central nervous system

The contribution of the cells within the central nervous system (CNS) toward adaptive immune responses is emerging and incompletely understood. Recent findings indicate important functional interactions between T-cells and glial cells within the CNS that may contribute to disease and neuropathology through antigen presentation. Although glia are not classically considered antigen-presenting cell (APC) types, there is growing evidence indicating that glial antigen presentation plays an important role in several neurological diseases. This review discusses these findings which incriminate microglia, astrocytes, and oligodendrocyte lineage cells as CNS-resident APC types with implications for understanding disease.

The predictive role of systemic inflammation response index in the prognosis of traumatic brain injury: A propensity score matching study

BACKGROUND

We aimed to evaluate the predictive power of systemic inflammation response index (SIRI), a novel biomarker, to predict all-cause mortality in patients with traumatic brain injury (TBI) in the intensive care unit (ICU).

METHODS

Clinical data were retrieved from the Medical Information Mart for Intensive Care-IV (MIMIC-IV) database. Kaplan-Meier (KM) methods and cox proportional hazard models were performed to examine the association between SIRI and all-cause mortality. The predictive power of SIRI was evaluated compared to other leukocyte-related indexes including neutrophils, lymphocytes, monocytes and white blood cells (WBC) by the Receiver Operating Characteristic (ROC)curve for 30-day mortality. In addition, propensity score matching (PSM) was conducted to reduce confounding.

RESULTS

A total of 350 TBI patients were enrolled overall in our study. The optimal cutoff point of SIRI was determined at 11.24 × 109/L. After 1:1 PSM, 66 matched pairs (132 patients) were generated. During the 30-day, in-hospital and 365-day follow-up periods, patients with low SIRI level were associated with improved survival (p < 0.05) compared with patients with high SIRI level. Cox regression analysis identified that higher SIRI values was an independent risk factor for all-cause mortality and results were stable on multiple subgroup analyses. Furthermore, ROC analysis indicated that the area under the curve of SIRI [0.6658 (95% Confidence Interval, 0.5630-0.7687)] was greater than that of neutrophils, monocytes, lymphocytes and WBC. The above results were also observed in the matched cohort.

CONCLUSION

It was suggested that TBI patients with high SIRI level would suffer from a high risk of 30-day, in-hospital and 365-day mortality. SIRI is a promising inflammatory biomarker for predicting TBI patients' prognosis with relatively better predictive power than other single indicators related to peripheral differential leukocyte counts.

Mesenchymal stromal cell secretome for traumatic brain injury: Focus on immunomodulatory action

The severity and long-term consequences of brain damage in traumatic brain injured (TBI) patients urgently calls for better neuroprotective/neuroreparative strategies for this devastating disorder. Mesenchymal stromal cells (MSCs) hold great promise and have been shown to confer neuroprotection in experimental TBI, mainly through paracrine mechanisms via secreted bioactive factors (i.e. secretome), which indicates significant potential for a cell-free neuroprotective approach. The secretome is composed of cytokines, chemokines, growth factors, proteins, lipids, nucleic acids, metabolites, and extracellular vesicles; it may offer advantages over MSCs in terms of delivery, safety, and variability of therapeutic response for brain injury. Immunomodulation by molecular factors secreted by MSCs is considered to be a key mechanism involved in their multi-potential therapeutic effects. Regulated neuroinflammation is required for healthy remodeling of central nervous system during development and adulthood. Moreover, immune cells and their secreted factors can also contribute to tissue repair and neurological recovery following acute brain injury. However, a chronic and maladaptive neuroinflammatory response can exacerbate TBI and contribute to progressive neurodegeneration and long-term neurological impairments. Here, we review the evidence for MSC-derived secretome as a therapy for TBI. Our framework incorporates a detailed analysis of in vitro and in vivo studies investigating the effects of the secretome on clinically relevant neurological and histopathological outcomes. We also describe the activation of immune cells after TBI and the immunomodulatory properties exerted by mediators released in the secretome. We then describe how ageing modifies central and systemic immune responses to TBI and discuss challenges and opportunities of developing secretome based neuroprotective therapies for elderly TBI populations. Finally, strategies aimed at modulating the secretome in order to boost its efficacy for TBI will also be discussed.

Current concepts on communication between the central nervous system and peripheral immunity via lymphatics: what roles do lymphatics play in brain and spinal cord disease pathogenesis?

The central nervous system (CNS) lacks conventional lymphatics within the CNS parenchyma, yet still maintains fluid homeostasis and immunosurveillance. How the CNS communicates with systemic immunity has thus been a topic of interest for scientists in the past century, which has led to several theories of CNS drainage routes. In addition to perineural routes, rediscoveries of lymphatics surrounding the CNS in the meninges revealed an extensive network of lymphatics, which we now know play a significant role in fluid homeostasis and immunosurveillance. These meningeal lymphatic networks exist along the superior sagittal sinus and transverse sinus dorsal to the brain, near the cribriform plate below the olfactory bulbs, at the base of the brain, and surrounding the spinal cord. Inhibition of one or all of these lymphatic networks can reduce CNS autoimmunity in a mouse model of multiple sclerosis (MS), while augmenting these lymphatic networks can improve immunosurveillance, immunotherapy, and clearance in glioblastoma, Alzheimer's disease, traumatic brain injury, and cerebrovascular injury. In this review, we will provide historical context of how CNS drainage contributes to immune surveillance, how more recently published studies fit meningeal lymphatics into the context of CNS homeostasis and neuroinflammation, identify the complex dualities of lymphatic function during neuroinflammation and how therapeutics targeting lymphatic function may be more complicated than currently appreciated, and conclude by identifying some unresolved questions and controversies that may guide future research.

T-cell infiltration, contribution and regulation in the central nervous system post-traumatic injury

T cells participate in the repair process and immune response in the CNS post-traumatic injury and play both a beneficial and harmful role. Together with nerve cells and other immune cells, they form a microenvironment in the CNS post-traumatic injury. The repair of traumatic CNS injury is a long-term process. T cells contribute to the repair of the injury site to influence the recovery. Recently, with the advance of new techniques, such as mass spectrometry-based flow cytometry, modern live-cell imaging, etc, research focusing on T cells is becoming one of the valuable directions for the future therapy of traumatic CNS injury. In this review, we summarized the infiltration, contribution and regulation of T cells in post-traumatic injury, discussed the clinical significance and predicted the future research direction.

Neuroinflammation-Driven Lymphangiogenesis in CNS Diseases

The central nervous system (CNS) undergoes immunosurveillance despite the lack of conventional antigen presenting cells and lymphatic vessels in the CNS parenchyma. Additionally, the CNS is bathed in a cerebrospinal fluid (CSF). CSF is continuously produced, and consequently must continuously clear to maintain fluid homeostasis despite the lack of conventional lymphatics. During neuroinflammation, there is often an accumulation of fluid, antigens, and immune cells to affected areas of the brain parenchyma. Failure to effectively drain these factors may result in edema, prolonged immune response, and adverse clinical outcome as observed in conditions including traumatic brain injury, ischemic and hypoxic brain injury, CNS infection, multiple sclerosis (MS), and brain cancer. Consequently, there has been renewed interest surrounding the expansion of lymphatic vessels adjacent to the CNS which are now thought to be central in regulating the drainage of fluid, cells, and waste out of the CNS. These lymphatic vessels, found at the cribriform plate, dorsal dural meninges, base of the brain, and around the spinal cord have each been implicated to have important roles in various CNS diseases. In this review, we discuss the contribution of meningeal lymphatics to these processes during both steady-state conditions and neuroinflammation, as well as discuss some of the many still unknown aspects regarding the role of meningeal lymphatics in neuroinflammation. Specifically, we focus on the observed phenomenon of lymphangiogenesis by a subset of meningeal lymphatics near the cribriform plate during neuroinflammation, and discuss their potential roles in immunosurveillance, fluid clearance, and access to the CSF and CNS compartments. We propose that manipulating CNS lymphatics may be a new therapeutic way to treat CNS infections, stroke, and autoimmunity.

Autoimmunity After Ischemic Stroke and Brain Injury

Ischemic Stroke is a major cause of morbidity and mortality worldwide. Sterile inflammation occurs after both stroke subtypes and contributes to neuronal injury and damage to the blood-brain barrier with release of brain antigens and a potential induction of autoimmune responses that escape central and peripheral tolerance mechanisms. In stroke patients, the detection of T cells and antibodies specific to neuronal antigens suggests a role of humoral adaptive immunity. In experimental models stroke leads to a significant increase of autoreactive T and B cells to CNS antigens. Lesion volume and functional outcome in stroke patients and murine stroke models are connected to antigen-specific responses to brain proteins. In patients with traumatic brain injury (TBI) a range of antibodies against brain proteins can be detected in serum samples. In this review, we will summarize the role of autoimmunity in post-lesional conditions and discuss the role of B and T cells and their potential neuroprotective or detrimental effects.

FDC:TFH Interactions within Cervical Lymph Nodes of SIV-Infected Rhesus Macaques

Cerebrospinal fluid (CSF) drains via the lymphatic drainage pathway. This lymphatic pathway connects the central nervous system (CNS) to the cervical lymph node (CLN). As the CSF drains to CLN via the dural and nasal lymphatics, T cells and antigen presenting cells pass along the channels from the subarachnoid space through the cribriform plate. Human immunodeficiency virus (HIV) may also egress from the CNS along this pathway. As a result, HIV egressing from the CNS may accumulate within the CLN. Towards this objective, we analyzed CLNs isolated from rhesus macaques that were chronically-infected with simian immunodeficiency virus (SIV). We detected significant accumulation of SIV within the CLNs. SIV virion trapping was observed on follicular dendritic cells (FDCs) localized within the follicular regions of CLNs. In addition, SIV antigens formed immune complexes when FDCs interacted with B cells within the germinal centers. Subsequent interaction of these B cells with CD4+ T follicular helper cells (TFHs) resulted in infection of the latter. Of note, 73% to 90% of the TFHs cells within CLNs were positive for SIV p27 antigen. As such, it appears that not only do the FDCs retain SIV they also transmit them (via B cells) to TFHs within these CLNs. This interaction results in infection of TFHs in the CLNs. Based on these observations, we infer that FDCs within the CLNs have a novel role in SIV entrapment with implications for viral trafficking.

Mycobacterium-Infected Dendritic Cells Disseminate Granulomatous Inflammation

The disappearance and reformation of granulomas during tuberculosis has been described using PET/CT/X-ray in both human clinical settings and animal models, but the mechanisms of granuloma reformation during active disease remains unclear. Granulomas can recruit inflammatory dendritic cells (iDCs) that can regulate local T-cell responses and can carry bacteria into the lymph nodes, which is crucial for generating systemic T-cell responses against mycobacteria. Here, we report that a subset of mycobacterium-infected iDCs are associated with bacteria-specific T-cells in infected tissue, outside the granuloma, and that this results in the formation of new and/or larger multi-focal lesions. Mycobacterium-infected iDCs express less CCR7 and migrate less efficiently compared to the non-infected iDCs, which may support T-cell capture in granulomatous tissue. Capture may reduce antigen availability in the lymph node, thereby decreasing systemic priming, resulting in a possible regulatory loop between systemic T-cell responses and granuloma reformation. T-cell/infected iDCs clusters outside the granuloma can be detected during the acute and chronic phase of BCG and Mtb infection. Our studies suggest a direct role for inflammatory dendritic cells in the dissemination of granulomatous inflammation.

Lymphangiogenesis is induced by mycobacterial granulomas via vascular endothelial growth factor receptor-3 and supports systemic T-cell responses against mycobacterial antigen

Granulomatous inflammation is characteristic of many autoimmune and infectious diseases. The lymphatic drainage of these inflammatory sites remains poorly understood, despite an expanding understanding of lymphatic role in inflammation and disease. Here, we show that the lymph vessel growth factor Vegf-c is up-regulated in Bacillus Calmette-Guerin- and Mycobacterium tuberculosis-induced granulomas, and that infection results in lymph vessel sprouting and increased lymphatic area in granulomatous tissue. The observed lymphangiogenesis during infection was reduced by inhibition of vascular endothelial growth factor receptor 3. By using a model of chronic granulomatous infection, we also show that lymphatic remodeling of tissue persists despite resolution of acute infection and a 10- to 100-fold reduction in the number of bacteria and tissue-infiltrating leukocytes. Inhibition of vascular endothelial growth factor receptor 3 decreased the growth of new vessels, but also reduced the proliferation of antigen-specific T cells. Together, our data show that granuloma-up-regulated factors increase granuloma access to secondary lymph organs by lymphangiogenesis, and that this process facilitates the generation of systemic T-cell responses to granuloma-contained antigens.

Dealing with Danger in the CNS: The Response of the Immune System to Injury

Fighting pathogens and maintaining tissue homeostasis are prerequisites for survival. Both of these functions are upheld by the immune system, though the latter is often overlooked in the context of the CNS. The mere presence of immune cells in the CNS was long considered a hallmark of pathology, but this view has been recently challenged by studies demonstrating that immunological signaling can confer pivotal neuroprotective effects on the injured CNS. In this review, we describe the temporal sequence of immunological events that follow CNS injury. Beginning with immediate changes at the injury site, including death of neural cells and release of damage-associated molecular patterns (DAMPs), and progressing through innate and adaptive immune responses, we describe the cascade of inflammatory mediators and the implications of their post-injury effects. We conclude by proposing a revised interpretation of immune privilege in the brain, which takes beneficial neuro-immune communications into account.

Inflammation and neuroprotection in traumatic brain injury

IMPORTANCE

Traumatic brain injury (TBI) is a significant public health concern that affects individuals in all demographics. With increasing interest in the medical and public communities, understanding the inflammatory mechanisms that drive the pathologic and consequent cognitive outcomes can inform future research and clinical decisions for patients with TBI.

OBJECTIVES

To review known inflammatory mechanisms in TBI and to highlight clinical trials and neuroprotective therapeutic manipulations of pathologic and inflammatory mechanisms of TBI.

EVIDENCE REVIEW

We searched articles in PubMed published between 1960 and August 1, 2014, using the following keywords: traumatic brain injury, sterile injury, inflammation, astrocytes, microglia, monocytes, macrophages, neutrophils, T cells, reactive oxygen species, alarmins, danger-associated molecular patterns, purinergic receptors, neuroprotection, and clinical trials. Previous clinical trials or therapeutic studies that involved manipulation of the discussed mechanisms were considered for inclusion. The final list of selected studies was assembled based on novelty and direct relevance to the primary focus of this review.

FINDINGS

Traumatic brain injury is a diverse group of sterile injuries induced by primary and secondary mechanisms that give rise to cell death, inflammation, and neurologic dysfunction in patients of all demographics. Pathogenesis is driven by complex, interacting mechanisms that include reactive oxygen species, ion channel and gap junction signaling, purinergic receptor signaling, excitotoxic neurotransmitter signaling, perturbations in calcium homeostasis, and damage-associated molecular pattern molecules, among others. Central nervous system resident and peripherally derived inflammatory cells respond to TBI and can provide neuroprotection or participate in maladaptive secondary injury reactions. The exact contribution of inflammatory cells to a TBI lesion is dictated by their anatomical positioning as well as the local cues to which they are exposed.

CONCLUSIONS AND RELEVANCE

The mechanisms that drive TBI lesion development as well as those that promote repair are exceedingly complex and often superimposed. Because pathogenic mechanisms can diversify over time or even differ based on the injury type, it is important that neuroprotective therapeutics be developed and administered with these variables in mind. Due to its complexity, TBI has proven particularly challenging to treat; however, a number of promising therapeutic approaches are now under pre-clinical development, and recent clinical trials have even yielded a few successes. Given the worldwide impact of TBI on the human population, it is imperative that research remains active in this area and that we continue to develop therapeutics to improve outcome in afflicted patients.

Regulatory T cells in central nervous system injury: a double-edged sword

Previous research investigating the roles of T effector (T(eff)) and T regulatory (T(reg)) cells after injury to the CNS has yielded contradictory conclusions, with both protective and destructive functions being ascribed to each of these T cell subpopulations. In this work, we study this dichotomy by examining how regulation of the immune system affects the response to CNS trauma. We show that, in response to CNS injury, T(eff) and T(reg) subsets in the CNS-draining deep cervical lymph nodes are activated, and surgical resection of these lymph nodes results in impaired neuronal survival. Depletion of T(reg), not surprisingly, induces a robust T(eff) response in the draining lymph nodes and is associated with impaired neuronal survival. Interestingly, however, injection of exogenous T(reg) cells, which limits the spontaneous beneficial immune response after CNS injury, also impairs neuronal survival. We found that no T(reg) accumulate at the site of CNS injury, and that changes in T(reg) numbers do not alter the amount of infiltration by other immune cells into the site of injury. The phenotype of macrophages at the site, however, is affected: both addition and removal of T(reg) negatively impact the numbers of macrophages with alternatively activated (tissue-building) phenotype. Our data demonstrate that neuronal survival after CNS injury is impaired when T(reg) cells are either removed or added. With this exacerbation of neurodegeneration seen with both addition and depletion of T(reg), we recommend exercising extreme caution when considering the therapeutic targeting of T(reg) cells after CNS injury, and possibly in chronic neurodegenerative conditions.

Traumatic brain injury causes selective, CD74-dependent peripheral lymphocyte activation that exacerbates neurodegeneration

Introduction

Traumatic brain injury (TBI), a significant cause of death and disability, causes, as in any injury, an acute, innate immune response. A key component in the transition between innate and adaptive immunity is the processing and presentation of antigen by professional antigen presenting cells (APCs). Whether an adaptive immune response to brain injury is beneficial or detrimental is not known. Current efforts to understand the contribution of the immune system after TBI have focused on neuroinflammation and brain-infiltrating immune cells. Here, we characterize and target TBI-induced expansion of peripheral immune cells that may act as potential APCs. Because MHC Class II-associated invariant peptide (CLIP) is important for antigen processing and presentation, we engineered a competitive antagonist (CAP) for CLIP, and tested the hypothesis that peptide competition could reverse or prevent neurodegeneration after TBI.

Results

We show that after fluid percussion injury (FPI), peripheral splenic lymphocytes, including CD4+ and CD8+ T cells, regulatory T cells (Tregs), and γδ T cells, are increased in number within 24 hours after FPI. These increases were reversed by CAP treatment and this antagonism of CLIP also reduced neuroinflammation and neurodegeneration after TBI. Using a mouse deficient for the precursor of CLIP, CD74, we observed decreased peripheral lymphocyte activation, decreased neurodegeneration, and a significantly smaller lesion size following TBI.

Conclusion

Taken together, the data support the hypothesis that neurodegeneration following TBI is dependent upon antigen processing and presentation that requires CD74.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-014-0143-5) contains supplementary material, which is available to authorized users.

Activated CD8+ T lymphocytes inhibit neural stem/progenitor cell proliferation: role of interferon-gamma

The ability of neural stem/progenitor cells (NSCs) to self-renew, migrate to damaged sites, and differentiate into neurons has renewed interest in using them in therapies for neurodegenerative disorders. Neurological diseases, including viral infections of the brain, are often accompanied by chronic inflammation, whose impact on NSC function remains unexplored. We have previously shown that chronic neuroinflammation, a hallmark of experimental herpes simplex encephalitis (HSE) in mice, is dominated by brain-infiltrating activated CD8 T-cells. In the present study, activated CD8 lymphocytes were found to suppress NSC proliferation profoundly. Luciferase positive (luc⁺) NSCs co-cultured with activated, MHC-matched, CD8⁺ lymphocytes (luc⁻) showed two- to five-fold lower luminescence than co-cultures with un-stimulated lymphocytes. On the other hand, similarly activated CD4⁺ lymphocytes did not suppress NSC growth. This differential lymphocyte effect on proliferation was confirmed by decreased BrdU uptake by NSC cultured with activated CD8 T-cells. Interestingly, neutralizing antibodies to interferon-gamma (IFN-γ) reversed the impact of CD8 lymphocytes on NSCs. Antibodies specific to the IFN-γ receptor-1 subunit complex abrogated the inhibitory effects of both CD8 lymphocytes and IFN-γ, indicating that the inhibitory effect of these cells was mediated by IFN-γ in a receptor-specific manner. In addition, activated CD8 lymphocytes decreased levels of nestin and Sox2 expression in NSCs while increasing GFAP expression, suggesting possible induction of an altered differentiation state. Furthermore, NSCs obtained from IFN-γ receptor-1 knock-out embryos were refractory to the inhibitory effects of activated CD8⁺ T lymphocytes on cell proliferation and Sox2 expression. Taken together, the studies presented here demonstrate a role for activated CD8 T-cells in regulating NSC function mediated through the production of IFN-γ. This cytokine may influence neuro-restorative processes and ultimately contribute to the long-term sequelae commonly seen following herpes encephalitis.

T cells in the central nervous system: messengers of destruction or purveyors of protection?

Although the destructive effects of an overactive adaptive immune system have been well established, especially in the context of autoimmune diseases, recently an understanding of the beneficial effects of the adaptive immunity in central nervous system (CNS) injuries has emerged. CD4(+) T cells have been shown to benefit injured CNS tissue through various mechanisms; both traditional cytokine signalling and by modulating the phenotype of neural cells in the injury site. One of the major targets of the cytokine signalling in the CNS are myeloid cells, both resident microglia and monocytes, that infiltrate the tissue after injury and whose phenotype; protective or destructive, appears to be directly influenced by T cells. This cross-talk between the adaptive and innate immune systems presents potential new targets that could provide tangible benefits in pathologies that currently have few treatment options.

Immune cell trafficking from the brain maintains CNS immune tolerance

In the CNS, no pathway dedicated to immune surveillance has been characterized for preventing the anti-CNS immune responses that develop in autoimmune neuroinflammatory disease. Here, we identified a pathway for immune cells to traffic from the brain that is associated with the rostral migratory stream (RMS), which is a forebrain source of newly generated neurons. Evaluation of fluorescently labeled leukocyte migration in mice revealed that DCs travel via the RMS from the CNS to the cervical LNs (CxLNs), where they present antigen to T cells. Pharmacologic interruption of immune cell traffic with the mononuclear cell-sequestering drug fingolimod influenced anti-CNS T cell responses in the CxLNs and modulated experimental autoimmune encephalomyelitis (EAE) severity in a mouse model of multiple sclerosis (MS). Fingolimod treatment also induced EAE in a disease-resistant transgenic mouse strain by altering DC-mediated Treg functions in CxLNs and disrupting CNS immune tolerance. These data describe an immune cell pathway that originates in the CNS and is capable of dampening anti-CNS immune responses in the periphery. Furthermore, these data provide insight into how fingolimod treatment might exacerbate CNS neuroinflammation in some cases and suggest that focal therapeutic interventions, outside the CNS have the potential to selectively modify anti-CNS immunity.

Molecular dialogs between the ischemic brain and the peripheral immune system: dualistic roles in injury and repair

Immune and inflammatory responses actively modulate the pathophysiological processes of acute brain injuries such as stroke. Soon after the onset of stroke, signals such as brain-derived antigens, danger-associated molecular patterns (DAMPs), cytokines, and chemokines are released from the injured brain into the systemic circulation. The injured brain also communicates with peripheral organs through the parasympathetic and sympathetic branches of the autonomic nervous system. Many of these diverse signals not only activate resident immune cells in the brain, but also trigger robust immune responses in the periphery. Peripheral immune cells then migrate toward the site of injury and release additional cytokines, chemokines, and other molecules, causing further disruptive or protective effects in the ischemic brain. Bidirectional communication between the injured brain and the peripheral immune system is now known to regulate the progression of stroke pathology as well as tissue repair. In the end, this exquisitely coordinated crosstalk helps determine the fate of animals after stroke. This article reviews the literature on ischemic brain-derived signals through which peripheral immune responses are triggered, and the potential impact of these peripheral responses on brain injury and repair. Pharmacological strategies and cell-based therapies that target the dialog between the brain and peripheral immune system show promise as potential novel treatments for stroke.

The resolution of neuroinflammation in neurodegeneration: leukocyte recruitment via the choroid plexus

Inflammation is an integral part of the body's physiological repair mechanism, unless it remains unresolved and becomes pathological, as evident in the progressive nature of neurodegeneration. Based on studies from outside the central nervous system (CNS), it is now understood that the resolution of inflammation is an active process, which is dependent on well-orchestrated innate and adaptive immune responses. Due to the immunologically privileged status of the CNS, such resolution mechanism has been mostly ignored. Here, we discuss resolution of neuroinflammation as a process that depends on a network of immune cells operating in a tightly regulated sequence, involving the brain's choroid plexus (CP), a unique neuro-immunological interface, positioned to integrate signals it receives from the CNS parenchyma with signals coming from circulating immune cells, and to function as an on-alert gate for selective recruitment of inflammation-resolving leukocytes to the inflamed CNS parenchyma. Finally, we propose that functional dysregulation of the CP reflects a common underlying mechanism in the pathophysiology of neurodegenerative diseases, and can thus serve as a potential novel target for therapy.

Localisation and role of activin receptor-interacting protein 1 in mouse brain

Activin A, a stimulator of follicle-stimulating hormone secretion from the pituitary, acts as a neurotrophic and neuroprotective factor in the central nervous system. Activin receptor-interacting protein 1 (ARIP1) has been identified as a cytoplasmic protein that interacts with the type II receptor of activin (ActRII). However, the distribution pattern and function of ARIP1 are not well characterised in the brain. In the present study, we confirmed the existence of mRNA and protein of ARIP1 in the mouse brain, and found that ARIP1 was mainly localised at the hippocampus and hypothalamus in the cerebrum, granular layers in the cerebellum (especially in Purkinje cells of the cerebellum) and choroid epithelial cells by immunohistochemical staining. Furthermore, in contrast to the significant increase of activin A mRNA, ARIP1 mRNA and protein expression decreased in the mechanically lesioned brain of the mouse. Using neuroblastoma-derived Neuro-2a cells to investigate the function of ARIP1, we found that overexpression of ARIP1 down-regulated the activin A-induced signal transduction and significantly decreased the voltage-gated Na(+) current (I(Na) ). These data indicate that ARIP1 is a key molecule for the regulation of the action of activin in neurones, and also that decreased ARIP1 expression in the lesioned brain may be beneficial to the neurotrophic and neuroprotective roles of activin A in recovery after brain injury.

Dendritic cells and multiple sclerosis: disease, tolerance and therapy

Multiple sclerosis (MS) is a devastating neurological disease that predominantly affects young adults resulting in severe personal and economic impact. The majority of therapies for this disease were developed in, or are beneficial in experimental autoimmune encephalomyelitis (EAE), the animal model of MS. While known to target adaptive anti-CNS immune responses, they also target, the innate immune arm. This mini-review focuses on the role of dendritic cells (DCs), the professional antigen presenting cells of the innate immune system. The evidence for a role for DCs in the appropriate regulation of anti-CNS autoimmune responses and their role in MS disease susceptibility and possible therapeutic utility are discussed. Additionally, the current controversy regarding the evidence for the presence of functional DCs in the normal CNS is reviewed. Furthermore, the role of CNS DCs and potential routes of their intercourse between the CNS and cervical lymph nodes are considered. Finally, the future role that this nexus between the CNS and the cervical lymph nodes might play in site directed molecular and cellular therapy for MS is outlined.

Metallothionein (MT) -I and MT-II expression are induced and cause zinc sequestration in the liver after brain injury

Experiments with transgenic over-expressing, and null mutant mice have determined that metallothionein-I and -II (MT-I/II) are protective after brain injury. MT-I/II is primarily a zinc-binding protein and it is not known how it provides neuroprotection to the injured brain or where MT-I/II acts to have its effects. MT-I/II is often expressed in the liver under stressful conditions but to date, measurement of MT-I/II expression after brain injury has focused primarily on the injured brain itself. In the present study we measured MT-I/II expression in the liver of mice after cryolesion brain injury by quantitative reverse-transcriptase PCR (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) with the UC1MT antibody. Displacement curves constructed using MT-I/II knockout (MT-I/II(-/-)) mouse tissues were used to validate the ELISA. Hepatic MT-I and MT-II mRNA levels were significantly increased within 24 hours of brain injury but hepatic MT-I/II protein levels were not significantly increased until 3 days post injury (DPI) and were maximal at the end of the experimental period, 7 DPI. Hepatic zinc content was measured by atomic absorption spectroscopy and was found to decrease at 1 and 3 DPI but returned to normal by 7DPI. Zinc in the livers of MT-I/II(-/-) mice did not show a return to normal at 7 DPI which suggests that after brain injury, MT-I/II is responsible for sequestering elevated levels of zinc to the liver.

CONCLUSION

MT-I/II is up-regulated in the liver after brain injury and modulates the amount of zinc that is sequestered to the liver.

Increased circulating leukocyte numbers and altered macrophage phenotype correlate with the altered immune response to brain injury in metallothionein (MT)-I/II null mutant mice

BACKGROUND

Metallothionein-I and -II (MT-I/II) is produced by reactive astrocytes in the injured brain and has been shown to have neuroprotective effects. The neuroprotective effects of MT-I/II can be replicated in vitro which suggests that MT-I/II may act directly on injured neurons. However, MT-I/II is also known to modulate the immune system and inflammatory processes mediated by the immune system can exacerbate brain injury. The present study tests the hypothesis that MT-I/II may have an indirect neuroprotective action via modulation of the immune system.

METHODS

Wild type and MT-I/II(-/-) mice were administered cryolesion brain injury and the progression of brain injury was compared by immunohistochemistry and quantitative reverse-transcriptase PCR. The levels of circulating leukocytes in the two strains were compared by flow cytometry and plasma cytokines were assayed by immunoassay.

RESULTS

Comparison of MT-I/II(-/-) mice with wild type controls following cryolesion brain injury revealed that the MT-I/II(-/-) mice only showed increased rates of neuron death after 7 days post-injury (DPI). This coincided with increases in numbers of T cells in the injury site, increased IL-2 levels in plasma and increased circulating leukocyte numbers in MT-I/II(-/-) mice which were only significant at 7 DPI relative to wild type mice. Examination of mRNA for the marker of alternatively activated macrophages, Ym1, revealed a decreased expression level in circulating monocytes and brain of MT-I/II(-/-) mice that was independent of brain injury.

CONCLUSIONS

These results contribute to the evidence that MT-I/II(-/-) mice have altered immune system function and provide a new hypothesis that this alteration is partly responsible for the differences observed in MT-I/II(-/-) mice after brain injury relative to wild type mice.

Inflammatory dendritic cells migrate in and out of transplanted chronic mycobacterial granulomas in mice

An estimated one-third of the world's population is infected with Mycobacterium tuberculosis, although most affected individuals maintain a latent infection. This control is attributed to the formation of granulomas, cell masses largely comprising infected macrophages with T cells aggregated around them. Inflammatory DCs, characterized as CD11c+CD11b+Ly6C+, are also found in granulomas and are an essential component of the acute immune response to mycobacteria. However, their function during chronic infection is less well understood. Here, we report that CD11c+ cells dynamically traffic in and out of both acute and chronic granulomas induced by Mycobacterium bovis strain bacillus Calmette-Guérin (BCG) in mice. By transplanting Mycobacterium-induced granulomas containing fluorescently labeled CD11c+ cells and bacteria into unlabeled mice, we were able to follow CD11c+ cell trafficking and T cell activation. We found that half of the CD11c+ cells in chronic granulomas were exchanged within 1 week. Compared with tissue-resident DC populations, CD11c+ cells migrating out of granuloma-containing tissue had an unexpected systemic dissemination pattern. Despite low antigen availability, systemic CD4+ T cell priming still occurred during chronic infection. These data demonstrate that surveillance of granulomatous tissue by CD11c+ cells is continuous and that these cells are distinct from tissue-resident DC populations and support T cell priming during both stages of Mycobacterium infection. This intense DC surveillance may also be a feature of Mycobacterium tuberculosis infection and other granuloma-associated diseases.

Migration of cytotoxic lymphocytes in cell cycle permits local MHC I-dependent control of division at sites of viral infection

Virus-specific cytotoxic CD8⁺ T cells are in cell cycle as they transit from lymphoid tissues to sites of infection.

After virus infection, cytotoxic T lymphocytes (CTLs) divide rapidly to eradicate the pathogen and prevent the establishment of persistence. The magnitude of an antiviral CTL response is thought to be controlled by the initiation of a cell cycle program within lymphoid tissues. However, it is presently not known whether this division program proceeds during migration or is influenced locally at sites of viral infection. We demonstrate that antiviral CTLs remain in cell cycle while transiting to infected tissues. Up to one third of virus-specific CTLs within blood were found to be in cell cycle after infection with lymphocytic choriomeningitis virus or vesicular stomatitis virus. Using two-photon microscopy, we found that effector CTL divided rapidly upon arrest in the virus-infected central nervous system as well as in meningeal blood vessels. We also observed that MHC I–dependent interactions, but not costimulation, influenced the division program by advancing effector CTL through stages of the cell cycle. These results demonstrate that CTLs are poised to divide in transit and that their numbers can be influenced locally at the site of infection through interactions with cells displaying cognate antigen.

Dendritic cells in chronic mycobacterial granulomas restrict local anti-bacterial T cell response in a murine model

BACKGROUND

Mycobacterium-induced granulomas are the interface between bacteria and host immune response. During acute infection dendritic cells (DCs) are critical for mycobacterial dissemination and activation of protective T cells. However, their role during chronic infection in the granuloma is poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We report that an inflammatory subset of murine DCs are present in granulomas induced by Mycobacteria bovis strain Bacillus Calmette-guerin (BCG), and both their location in granulomas and costimulatory molecule expression changes throughout infection. By flow cytometric analysis, we found that CD11c(+) cells in chronic granulomas had lower expression of MHCII and co-stimulatory molecules CD40, CD80 and CD86, and higher expression of inhibitory molecules PD-L1 and PD-L2 compared to CD11c(+) cells from acute granulomas. As a consequence of their phenotype, CD11c(+) cells from chronic lesions were unable to support the reactivation of newly-recruited, antigen 85B-specific CD4(+)IFNgamma(+) T cells or induce an IFNgamma response from naïve T cells in vivo and ex vivo. The mechanism of this inhibition involves the PD-1:PD-L signaling pathway, as ex vivo blockade of PD-L1 and PD-L2 restored the ability of isolated CD11c(+) cells from chronic lesions to stimulate a protective IFNgamma T cell response.

CONCLUSIONS/SIGNIFICANCE

Our data suggest that DCs in chronic lesions may facilitate latent infection by down-regulating protective T cell responses, ultimately acting as a shield that promotes mycobacterium survival. This DC shield may explain why mycobacteria are adapted for long-term survival in granulomatous lesions.

Trafficking of immune cells in the central nervous system

The CNS is an immune-privileged environment, yet the local control of multiple pathogens is dependent on the ability of immune cells to access and operate within this site. However, inflammation of the distinct anatomical sites (i.e., meninges, cerebrospinal fluid, and parenchyma) associated with the CNS can also be deleterious. Therefore, control of lymphocyte entry and migration within the brain is vital to regulate protective and pathological responses. In this review, several recent advances are highlighted that provide new insights into the processes that regulate leukocyte access to, and movement within, the brain.

Two-photon laser scanning microscopy imaging of intact spinal cord and cerebral cortex reveals requirement for CXCR6 and neuroinflammation in immune cell infiltration of cortical injury sites

The mouse spinal cord is an important site for autoimmune and injury models. Skull thinning surgery provides a minimally invasive window for microscopy of the mouse cerebral cortex, but there are no parallel methods for the spinal cord. We introduce a novel, facile and inexpensive method for two-photon laser scanning microscopy of the intact spinal cord in the mouse by taking advantage of the naturally accessible intervertebral space. These are powerful methods when combined with gene-targeted mice in which endogenous immune cells are labeled with green fluorescent protein (GFP). We first demonstrate that generation of the intervertebral window does not elicit a reaction of GFP(+) microglial cells in CX3CR1(gfp/+) mice. We next demonstrate a distinct rostrocaudal migration of GFP(+) immune cells in the spinal cord of CXCR6(gfp/+) mice during active experimental autoimmune encephalomyelitis (EAE). Interestingly, infiltration of the cerebral cortex by GFP(+) cells in these mice required three conditions: EAE induction, cortical injury and expression of CXCR6 on immune cells.

Inflammation on the mind: visualizing immunity in the central nervous system

The central nervous system (CNS) is a remarkably complex structure that utilizes electrochemical signaling to coordinate activities throughout the entire body. Because the nervous system contains nonreplicative cells, it is postulated that, through evolutionary pressures, this compartment has acquired specialized mechanisms to limit damage. One potential source of damage comes from our immune system, which has the capacity to survey the CNS and periphery for the presence of foreign material. The immune system is equipped with numerous effector mechanisms and can greatly alter the homeostasis and function of the CNS. Degeneration, autoimmunity, and pathogen infection can all result in acute, and sometimes chronic, inflammation within the CNS. Understanding the specialized functionality of innate and adaptive immune cells within the CNS is critical to the design of more efficacious treatments to mitigate CNS inflammatory conditions. Much of our knowledge of CNS-immune interactions stems from seminal studies that have used static and dynamic imaging approaches to visualize inflammatory cells responding to different CNS conditions. This review will focus on how imaging techniques have elevated our understanding of CNS inflammation as well as the exciting prospects that lie ahead as we begin to pursue investigation of the inflamed CNS in real time.

Enhanced microglial clearance of myelin debris in T cell-infiltrated central nervous system

Acute multiple sclerosis lesions are characterized by accumulation of T cells and macrophages, destruction of myelin and oligodendrocytes, and axonal damage. There is, however, limited information on neuroimmune interactions distal to sites of axonal damage in the T cell-infiltrated central nervous system. We investigated T-cell infiltration, myelin clearance, microglial activation, and phagocytic activity distal to sites of axonal transection through analysis of the perforant pathway deafferented dentate gyrus in SJL mice that had received T cells specific for myelin basic protein (TMBP) or ovalbumin (TOVA). The axonal lesion of TMBP-recipient mice resulted in lesion-specific recruitment of large numbers of T cells in contrast to very limited T-cell infiltration in TOVA-recipient and -naïve perforant pathway-deafferented mice. By double immunofluorescence and confocal microscopy, infiltration with TMBP but not TOVA enhanced the microglial response to axonal transection and microglial phagocytosis of myelin debris associated with the degenerating axons. Because myelin antigen-specific immune responses may provoke protective immunity, increased phagocytosis of myelin debris might enhance regeneration after a neural antigen-specific T cell-mediated immune response in multiple sclerosis.

The level of B7 homologue 1 expression on brain DC is decisive for CD8 Treg cell recruitment into the CNS during EAE

DC in the CNS have emerged as the major rate-limiting factor for immune invasion and subsequent neuroinflammation during EAE. The mechanism of how this is regulated by brain-localized DC remains unknown. Here, we describe the ability of brain-localized DC expressing B7-H1 molecules to recruit CD8(+) T cells to the site of inflammation. Using intracerebral microinjections of B7-homologue 1-deficient DC, we demonstrate a substantial brain infiltration of CD8(+) T cells displaying a regulatory phenotype (CD122(+)) and function, resulting in a decrease of EAE peak clinical values. The recruitment of regulatory-type CD8(+) T cells into the CNS and the role of brain DC expressing B7-homologue 1 molecules in this process open up the possibility of DC-targeted therapeutic manipulation of neuroinflammatory diseases.

Intracerebral dendritic cells critically modulate encephalitogenic versus regulatory immune responses in the CNS

Dendritic cells (DCs) appear in higher numbers within the CNS as a consequence of inflammation associated with autoimmune disorders, such as multiple sclerosis, but the contribution of these cells to the outcome of disease is not yet clear. Here, we show that stimulatory or tolerogenic functional states of intracerebral DCs regulate the systemic activation of neuroantigen-specific T cells, the recruitment of these cells into the CNS and the onset and progression of experimental autoimmune encephalomyelitis (EAE). Intracerebral microinjection of stimulatory DCs exacerbated the onset and clinical course of EAE, accompanied with an early T-cell infiltration and a decreased proportion of regulatory FoxP3-expressing cells in the brain. In contrast, the intracerebral microinjection of DCs modified by tumor necrosis factor alpha induced their tolerogenic functional state and delayed or prevented EAE onset. This triggered the generation of interleukin 10 (IL-10)-producing neuroantigen-specific lymphocytes in the periphery and restricted IL-17 production in the CNS. Our findings suggest that DCs are a rate-limiting factor for neuroinflammation.

The role of macrophages in optic nerve regeneration

Following injury to the nervous system, the activation of macrophages, microglia, and T-cells profoundly affects the ability of neurons to survive and to regenerate damaged axons. The primary visual pathway provides a well-defined model system for investigating the interactions between the immune system and the nervous system after neural injury. Following damage to the optic nerve in mice and rats, retinal ganglion cells, the projection neurons of the eye, normally fail to regenerate their axons and soon begin to die. Induction of an inflammatory response in the vitreous strongly enhances the survival of retinal ganglion cells and enables these cells to regenerate lengthy axons beyond the injury site. T cells modulate this response, whereas microglia are thought to contribute to the loss of retinal ganglion cells in this model and in certain ocular diseases. This review discusses the complex and sometimes paradoxical actions of blood-borne macrophages, resident microglia, and T-cells in determining the outcome of injury in the primary visual pathway.

Parasite stage-specific recognition of endogenous Toxoplasma gondii-derived CD8+ T cell epitopes

BACKGROUND

BALB/c mice control infection with the obligate intracellular parasite Toxoplasma gondii and develop a latent chronic infection in the brain, as do immunocompetent humans. Interferon-gamma-producing CD8+ T cells provide essential protection against T. gondii infection, but the epitopes recognized have so far remained elusive.

METHODS

We employed caged major histocompatibility complex molecules to generate approximately 250 H-2L(d) tetramers and to distinguish T. gondii-specific CD8+ T cells in BALB/c mice.

RESULTS

We identified 2 T. gondii-specific H-2L(d)-restricted T cell epitopes, one from dense granule protein GRA4 and the other from rhoptry protein ROP7. H-2L(d)/GRA4 reactive T cells from multiple organ sources predominated 2 weeks after infection, while the reactivity of the H-2L(d)/ROP7 T cells peaked 6-8 weeks after infection. BALB/c animals infected with T. gondii mutants defective in establishing a chronic infection showed altered levels of antigen-specific T cells, depending on the T. gondii mutant used.

CONCLUSIONS

Our results shed light on the identity and the parasite stage-specificity of 2 CD8+ T cell epitopes recognized in the acute and chronic phase of infection with T. gondii.

Application of glatiramer acetate to neurodegenerative diseases beyond multiple sclerosis: the need for disease-specific approaches

Adaptive and innate immunity, if well controlled, contribute to the maintenance of the CNS, as well as to downregulation of adverse acute and chronic neurological conditions. T cells that recognize CNS antigens are needed to activate resident immune cells and to recruit blood-borne monocytes, which act to restore homeostasis and facilitate repair. However, boosting such a T-cell response in a risk-free way requires a careful choice of the antigen, carrier, and regimen. A single vaccination with CNS-derived peptides or their weak agonists reduces neuronal loss in animal models of acute neurodegeneration. Repeated injections are needed to maintain a long-lasting effect in chronic neurodegenerative conditions, yet the frequency of the injections seems to have a critical effect on the outcome. An example is glatiramer acetate, a compound that is administered in a daily regimen to patients with multiple sclerosis. A single injection of glatiramer acetate, with or without an adjuvant, is neuroprotective in some animal models of acute CNS injuries. However, in an animal model of amyotrophic lateral sclerosis, a single injection of adjuvant-free glatiramer acetate is insufficient, while daily injections are not only ineffective but can carry an increased risk of mortality in female mice.Thus, considering immune-based therapies as a single therapy, rather than as a family of therapies that are regimen dependent, may be misleading. Moreover, the vaccination regimen and administration of a compound, even one shown to be safe in humans for the treatment of a particular neurodegenerative disease, must be studied in preclinical experiments before it is tested in a clinical trial for a novel indication; otherwise, an effective drug in a certain regimen for one disease may be ineffective or even carry risks when used for another disorder.

In situ activation of antigen-specific CD8+ T cells in the presence of antigen in organotypic brain slices

The activation of Ag-specific T cells locally in the CNS could potentially contribute to the development of immune-mediated brain diseases. We addressed whether Ag-specific T cells could be stimulated in the CNS in the absence of peripheral lymphoid tissues by analyzing Ag-specific T cell responses in organotypic brain slice cultures. Organotypic brain slice cultures were established 1 h after intracerebral OVA Ag microinjection. We showed that when OVA-specific CD8(+) T cells were added to Ag-containing brain slices, these cells became activated and migrated into the brain to the sites of their specific Ags. This activation of OVA-specific T cells was abrogated by the deletion of CD11c(+) cells from the brain slices of the donor mice. These data suggest that brain-resident CD11c(+) cells stimulate Ag-specific naive CD8(+) T cells locally in the CNS and may contribute to immune responses in the brain.

Adaptive immunity affects learning behavior in mice

Regulation of neuronal plasticity by the immune system is an evolving field of modern neuroscience. Here we employ immune deficient mice to examine the role of the immune system in learning behavior of mice in a variety of cognitive tasks. While no motivation or motor function deficits are evident in severe combined immune deficient (scid) mice, there was significant impairment in acquisition of cognitive tasks as compared to wild-type (WT) control mice. Moreover, acute depletion of adaptive immunity in adult WT mice significantly impaired learning behavior. Passive transfer of autologous T cells into WT mice following ablation of adaptive immunity restored previously impaired cognitive function. These results suggest that throughout lifetime, immune system supports cognitive function and may therefore have far-reaching therapeutic implications for cognitive disorders.

Influence of macrophages and lymphocytes on the survival and axon regeneration of injured retinal ganglion cells in rats from different autoimmune backgrounds

The immune response after neural injury influences the survival and regenerative capacity of neurons. In the primary visual pathway, previous studies have described beneficial effects of macrophages and T-cells in promoting neural survival and axonal regeneration in some rat strains. However, the contributions of specific cell populations to these responses have been unclear. In adult Fischer (F344) rats, we confirm prior reports that intravitreal macrophage activation promotes the survival of retinal ganglion cells (RGCs) and greatly enhances axonal regeneration through a peripheral nerve graft. Neonatal thymectomy that results in elimination of T-cell production enhanced RGC survival after axotomy, but diminished the effect of intravitreal macrophage activation on axon regeneration. Thus, in F344 rats, lymphocytes appear to suppress RGC survival but augment the pro-regenerative effects of macrophages. The cytotoxic effect of lymphocytes on RGCs was confirmed in in vitro studies; coculture of retinal explants with lymphocytes led to a 60% reduction in viable RGCs. Similar in vivo results were obtained in Sprague Dawley rats. By comparison, in adult Lewis rats, neither RGC survival nor axonal regeneration was increased after intravitreal macrophage activation. Neonatal thymectomy had only a small beneficial effect on RGC survival, and although Lewis lymphocytes reduced RGC viability in culture, they did so to a lesser extent. Thus, in addition to a complex role of lymphocytes, particularly T-cells, after central nervous system injury, the present results demonstrate that the impact of macrophages is also influenced by genetic background.

Central nervous system and non-central nervous system antigen vaccines exacerbate neuropathology caused by nerve injury

Previously, we showed that autoimmune (central nervous system myelin-reactive) T cells exacerbate tissue damage and impair neurological recovery after spinal cord injury. Conversely, independent studies have shown T cell-mediated neuroprotection after spinal cord injury or facial nerve axotomy (FNAx). The antigen specificity of the neuroprotective T cells has not been investigated after FNAx. Here, we compared the neuroprotective capacity of autoimmune and non-autoimmune lymphocytes after FNAx. Prior to axotomy, C57BL/6 mice were immunized with myelin basic protein, myelin oligodendrocyte glycoprotein (MOG) or ovalbumin (a non-self antigen) emulsified in complete Freund's adjuvant (CFA). FNAx mice receiving injections of phosphate-buffered saline (PBS) only (unimmunized) or PBS/CFA emulsions served as controls. At 4 weeks after axotomy, bilateral facial motor neuron counts were obtained throughout the facial motor nucleus using unbiased stereology (optical fractionator). The data show that neuroantigen immunizations and 'generic' lymphocyte activation (e.g. PBS/CFA or ovalbumin/CFA immunizations) exacerbated neuron loss above that caused by FNAx alone. We also found that nerve injury potentiated the effector potential of autoimmune lymphocytes. Indeed, prominent forelimb and hindlimb motor deficits were accompanied by disseminated neuroinflammation and demyelination in FNAx mice receiving subencephalitogenic immunization with MOG. FNAx or neuroantigen (MOG or myelin basic protein) immunization alone did not cause these pathological changes. Thus, irrespective of the antigens used to trigger an immune response, neuropathology was enhanced when the immune system was primed in parallel with nerve injury. These data have important implications for therapeutic vaccination in clinical neurotrauma and neurodegeneration.

Characterization of the temporo-spatial effects of chronic bilateral intrahippocampal cannulae on interleukin-1beta

The implantation of a foreign object in the brain produces an acute neuroinflammatory state in which glia (astrocytes and microglia) may remain chronically activated in response to the inert foreign object. Activated glia can exhibit a sensitized pro-inflammatory response to immunogenic stimuli. This may be relevant to intracranial cannula implantation, which is commonly used to administer substances directly into the brain. If intracranial cannulation activates glia, a subsequent neuroinflammatory stimulus might induce a potentiated pro-inflammatory response, thereby introducing a potential experimental confound. We tested the temporal and spatial responses of interleukin-1beta (IL-1beta) to an acute immune challenge produced by lipopolysaccharide (LPS) in animals with chronic bilateral intrahippocampal cannulae implants (stainless steel). Cannulation increased the gene expression of the microglia activation antigens MHC II and CD11b, but not the astrocyte antigen GFAP. Moreover, this activation was temporally and spatially dependent. In addition, IL-1beta mRNA, but not IL-1beta protein, was significantly elevated in cannulated animals. Administration of LPS, however, significantly potentiated the brain IL-1beta response in cannulated animals, but not in stab wounded or naïve animals. This IL-1beta response was also temporo-spatially dependent. Thus, the pro-inflammatory sequelae of intracranial cannulation should be considered when designing studies of neuroinflammatory processes.

Strain-specific differences in the effects of cyclosporin A and FK506 on the survival and regeneration of axotomized retinal ganglion cells in adult rats

The immune response can influence neuronal viability and plasticity after injury, effects differing in strains of rats with different susceptibility to autoimmune disease. We assessed the effects of i.p. injections of cyclosporin A (CsA) or FK506 on adult retinal ganglion cell (RGC) survival and axonal regeneration into peripheral nerve (PN) autografted onto the cut optic nerve of rats resistant (Fischer F344) or vulnerable (Lewis) to autoimmune disease. Circulating and tissue CsA and FK506 levels were similar in both strains. Three weeks after autologous PN transplantation the number of viable beta-III tubulin-positive RGCs was significantly greater in CsA- and FK506-treated F344 rats compared with saline-injected controls. RGC survival in Lewis rats was not significantly altered. In F344 rats, retrograde labeling of RGCs revealed that CsA or FK506 treatment significantly increased the number of RGCs that regenerated an axon into a PN autograft; however these agents had no beneficial effect on axonal regeneration in Lewis rats. PN grafts in F344 rats also contained comparatively more pan-neurofilament immunoreactive axons. In both strains, 3 weeks after transplantation CsA or FK506 treatment resulted in increased retinal macrophage numbers, but only in F344 rats was this increase significant. At this time-point PN grafts in both strains contained many macrophages and some T cells. T cell numbers in Lewis rats were significantly greater than in F344 animals. The increased RGC axonal regeneration seen in CsA- or FK506-treated F344 but not Lewis rats shows that modulation of immune responses after neurotrauma has complex and not always predictable outcomes.

Dendritic cell transmigration through brain microvessel endothelium is regulated by MIP-1alpha chemokine and matrix metalloproteinases

Dendritic cells (DCs) accumulate in the CNS during inflammatory diseases, but the exact mechanism regulating their traffic into the CNS remains to be defined. We now report that MIP-1alpha increases the transmigration of bone marrow-derived, GFP-labeled DCs across brain microvessel endothelial cell monolayers. Furthermore, occludin, an important element of endothelial tight junctions, is reorganized when DCs migrate across brain capillary endothelial cell monolayers without causing significant changes in the barrier integrity as measured by transendothelial electrical resistance. We show that DCs produce matrix metalloproteinases (MMP) -2 and -9 and GM6001, an MMP inhibitor, decreases both baseline and MIP-1alpha-induced DC transmigration. These observations suggest that DC transmigration across brain endothelial cell monolayers is partly MMP dependent. The migrated DCs express higher levels of CD40, CD80, and CD86 costimulatory molecules and induce T cell proliferation, indicating that the transmigration of DCs across brain endothelial cell monolayers contributes to the maintenance of DC Ag-presenting function. The MMP dependence of DC migration across brain endothelial cell monolayers raises the possibility that MMP blockers may decrease the initiation of T cell recruitment and neuroinflammation in the CNS.

Kinetics of the cellular immune response following closed head injury

BACKGROUND

The contribution of brain edema to brain swelling in cases of traumatic brain injury (TBI) remains a critical problem. We believe that inflammatory reactions may play a fundamental role in brain swelling following a head injury. Although possible roles of microglia activation and the release of mediators have been suggested, direct evidence of cellular immune reactivity in diffuse brain injury following closed head trauma is lacking. Accordingly, the objective of this study was to assess the temporal pattern of microglia activation and lymphocyte migration in an experimental model of TBI.

METHOD

An impact acceleration TBI model was utilized to induce diffuse brain damage in adult Wistar rats. The animals were separated into three groups: unoperated controls, sham-operated controls and trauma group. At various times after TBI induction (5 min-24 h), rats were perfused transcardially. Sagittal brain sections were analyzed with immunohistochemical markers of CD3 to reveal the presence of T-lymphocytes, and by immunochemistry for the detection of CD11b to reveal microglia activation within the brain parenchyma.

FINDINGS

In the control groups, scattered T-cells were found in the brain parenchyma. In the trauma group, TBI induced microglia activation and a transient biphasic T-cell infiltration of the brain parenchyma in all regions was found, beginning as early as 30 min post injury and reaching its maximum values at 45 min and 3 h after trauma induction.

CONCLUSION

These results lead us to suggest that the acute response to severe head trauma with early edema formation is likely to be associated with inflammatory events which might be triggered by activated microglia and infiltrating lymphocytes. It is difficult to overestimate the clinical significance of these observations, as the early and targeted treatment of patients with severe head injuries with immunosuppressive medication may result in a far more favorable outcome.

Innate response to focal necrotic injury inside the blood-brain barrier

We have studied the initial innate immune response to focal necrotic injury on different sides of the mouse blood-brain barrier by two-photon intravital microscopy. Transgenic mice in which the promoter of the myeloid isoform of lysozyme drives GFP were used to track granulocytes and monocytes. Necrotic injury in the meninges, but not the brain parenchyma, recruited GFP+ cells within minutes that fully surrounded the necrotic site within a day. Recently, it has been suggested that microglial cells and astrocytes cooperate to mount a distinct response to laser injury behind the blood-brain barrier. We followed the microglial response in heterozygous knockin mice in which GFP replaces CX3CR1 coding sequence. Prior to injury, microglial cell bodies were immobile over days, but moved to the laser injury site within 1 day. We followed astrocytes, which have been proposed to cooperate with microglial cells in response to focal injury, using transgenic mice in which glial fibrillary acidic protein promoter drives GFP expression. Before injury fine astrocyte processes permeate the parenchyma. Astrocytes polarized toward the injury in an ATP, connexin hemichannels, and intracellular Ca2+ -dependent process. The astrocytes network established a cytoplasmic Ca2+ gradient that preceded the microglial response. This is consistent with astrocyte-microglial collaboration to mount this innate response that excludes blood leukocytes.