IL-2 gene knockout affects T lymphocyte trafficking and the microglial response to regenerating facial motor neurons

IL2RA rs2104286 and IL2 rs2069762 polymorphisms may be associated with bipolar disorder and its clinical findings

Study results supported that immuno-inflammatory pathways in the brain and environment contribute to the etiopathogenesis of bipolar disorder (BD), a chronic affective disease. Our study aimed to assess the relationship between BD risk and interleukin 2 (IL2) and interleukin 2 receptor subunit alpha (IL2RA) variants in a Turkish population. Genomic DNA from 86 diagnosed BD patients and 100 healthy blood donors was extracted. IL2RA rs2104286, IL2 rs2069762, and IL2 rs2069763 variants were genotyped using the polymerase chain reaction-based restriction fragment length polymorphism (PCR-RFLP) method. It was compared to the relationship between the genotype distributions of these variants and clinical characteristics. Results were evaluated statistically. A statistically significant difference in the genotype distribution of the IL2RA rs2104286 variant was found between patients and controls. There was no GG genotype in the patient group. The IL2RA rs2104286 AA genotype was more common in the patient group than the controls, and the AG genotype was higher in the controls compared to the patients (p = 0.001, p = 0.001, respectively). The IL2 rs2069762 and IL2 rs2069763 genotype distributions did not differ between the patient and control groups (p > 0.05). We found that the clinical global impression severity (CGI-S) score was higher in those with IL2 rs2069762 TG and GG genotypes. In this study, we showed for the first time that the genotype distribution of IL2RA rs2104286 and IL2 rs2069762 is associated with BD susceptibility and CGI-S score in a Turkish population.

CD4+ T cell expression of the IL-10 receptor is necessary for facial motoneuron survival after axotomy

Background

After peripheral nerve transection, facial motoneuron (FMN) survival depends on an intact CD4+ T cell population and a central source of interleukin-10 (IL-10). However, it has not been determined previously whether CD4+ T cells participate in the central neuroprotective IL-10 cascade after facial nerve axotomy (FNA).

Methods

Immunohistochemical labeling of CD4+ T cells, pontine vasculature, and central microglia was used to determine whether CD4+ T cells cross the blood-brain barrier and enter the facial motor nucleus (FMNuc) after FNA. The importance of IL-10 signaling in CD4+ T cells was assessed by performing adoptive transfer of IL-10 receptor beta (IL-10RB)-deficient CD4+ T cells into immunodeficient mice prior to injury. Histology and qPCR were utilized to determine the impact of IL-10RB-deficient T cells on FMN survival and central gene expression after FNA. Flow cytometry was used to determine whether IL-10 signaling in T cells was necessary for their differentiation into neuroprotective subsets.

Results

CD4+ T cells were capable of crossing the blood-brain barrier and associating with reactive microglial nodules in the axotomized FMNuc. Full induction of central IL-10R gene expression after FNA was dependent on CD4+ T cells, regardless of their own IL-10R signaling capability. Surprisingly, CD4+ T cells lacking IL-10RB were incapable of mediating neuroprotection after axotomy and promoted increased central expression of genes associated with microglial activation, antigen presentation, T cell co-stimulation, and complement deposition. There was reduced differentiation of IL-10RB-deficient CD4+ T cells into regulatory CD4+ T cells in vitro.

Conclusions

These findings support the interdependence of IL-10- and CD4+ T cell-mediated mechanisms of neuroprotection after axotomy. CD4+ T cells may potentiate central responsiveness to IL-10, while IL-10 signaling within CD4+ T cells is necessary for their ability to rescue axotomized motoneuron survival. We propose that loss of IL-10 signaling in CD4+ T cells promotes non-neuroprotective autoimmunity after FNA.

Astrocyte-targeted production of IL-10 induces changes in microglial reactivity and reduces motor neuron death after facial nerve axotomy

Interleukin-10 (IL-10) is a cytokine that plays a crucial role in regulating the inflammatory response and immune reactions. In the central nervous system (CNS), IL-10 is mainly produced by astrocytes and microglia and it is upregulated after various insults, such as experimental autoimmune encephalomyelitis, middle cerebral artery occlusion, excitotoxicity and traumatic brain injury. To better understand the effects of IL-10 in the normal and injured CNS, we generated transgenic mice (termed GFAP-IL-10Tg) that expressed the murine IL-10 gene under the transcriptional control of the glial fibrillary acidic protein (GFAP) promoter. Previous studies demonstrated marked changes in the microglial phenotype in these mice under basal conditions. The objective of the present study was to investigate the effects of local astrocyte-targeted IL-10 production on glial activation, neuronal degeneration and leukocyte recruitment after axotomy. GFAP-IL-10Tg mice had marked changes in the phenotype of activated microglial cells, as well as in the number of microglial clusters and in microglial cell density. These microglial changes are accompanied by a twofold increase in lymphocyte infiltration in GFAP-IL-10Tg mice and around twofold decrease in neuronal cell death at 21 dpi. Altogether, our findings suggested that astrocyte-targeted production of IL-10 impacted the microglial response and lymphocyte recruitment and culminated in a beneficial effect on neuronal survival.

Effects of Brain-Derived IL-2 Deficiency and the Development of Autoimmunity on Spatial Learning and Fear Conditioning

Interleukin-2 (IL-2) has been implicated in neurological disorders including multiple sclerosis and Alzheimer’s disease. Peripheral IL-2 deficiency in gene-deleted mice results in T cell mediated autoimmunity that begins to develop slowly after weaning and progressively increases through adulthood. Loss of brain-derived IL-2 results in neurobiological and behavioral abnormalities, and may contribute to the development of CNS autoimmunity by modifying the neuroimmunological milieu of the brain. We have shown previously that IL-2 knockout (KO) mice have altered learning acquisition in the Morris water-maze. Hypothesizing that the learning acquisition deficits in IL-2KO would be associated largely with the loss of brain-derived IL-2, the present study sought to determine if these cognitive alterations are due to the loss the IL-2 gene in the brain and/or autoimmunity resulting from loss of the gene in the peripheral immune system. We found that SCID congenic mice (mice free of IL-2 deficiency induced peripheral autoimmunity) without brain IL-2 (two IL-2KO alleles) did not differ from SCID congenic mice with normal brain IL-2 (two WT IL-2 alleles); thus, contrary to our hypothesis, loss of brain-derived IL-2 did not affect learning acquisition in the water-maze. Compared to adult WT littermates (9 weeks), adult IL-2KO mice with autoimmunity exhibited alterations in learning acquisition in the Morris water-maze whereas younger pre-autoimmune IL-2KO mice (5 weeks) had performance comparable to younger WT littermates, suggesting that the water-maze learning deficits in IL-2KO mice were associated with the development of peripheral autoimmunity. As IL-2KO mice have cytoarchitectural alterations in the dentate gyrus, circuitry involved in the differentiation of contexts (versus places), we also compared IL-2KO mice and littermates in a contextual fear discrimination paradigm. IL-2KO mice were found to have reduced conditioned fear discrimination that was not related to age-associated autoimmunity. Together, these findings suggest that complex interactions between IL-2 deficiency in the brain and immune system may modify brain processes involved in different modalities of learning and memory.

The expression of IL-2 and IL-4 in CD4(+) T cells from mouse lymph nodes and spleen during HSV-1-induced facial palsy

OBJECTIVE

Herpes simplex virus 1 (HSV-1) is regarded as an important underlying cause of Bell's palsy, but the immunologic mechanism remains unknown. Here, we employed a mouse facial paralysis model to investigate the expressions of CD4(+) T lymphocytes and interleukin (IL)-2 and -4 in the left draining cervical lymph nodes (LCLN) and spleen, as well as the inhibitory effects of glucocorticoids (GCs).

METHODS

HSV-1 was inoculated into the surface of the posterior auricle to generate the facial paralysis model. The paralyzed mice were divided into three groups; in one group without any treatment, mice were killed at different time points, and those in the other two groups were injected with methylprednisolone sodium succinate (MPSS) or with a combination of MPSS and GC receptor blocker (RU486). The expression levels of CD4(+) T lymphocytes and CD4(+)-IL-2(+) and CD4(+)-IL-4(+) cells in the LCLN and spleen were detected by fluorescence-activated cell sorting.

RESULTS

Expression levels of CD4(+), IL-2, and IL-4 first increased then decreased in LCLN and spleen and peaked 5 and 7 days, respectively, after the manifestation of facial paralysis. All the data at the peak points were significantly different compared with control (p < 0.05), and these effects were inhibited by MPSS.

CONCLUSION

Our results suggest that CD4(+), IL-2, and IL-4 participate in the HSV-1-induced facial paralysis immune response. MPSS can effectively attenuate HSV-1-mediated nervous system damage, which is associated with its inhibitory effect on expression of these inflammatory markers.

Selective Neuronal and Brain Regional Expession of IL-2 in IL2P 8-GFP Transgenic Mice: Relation to Sensorimotor Gating

Brain-derived interleukin-2 (IL-2) has been implicated in diseases processes that arise during CNS development (e.g., autism) to neurodegenerative alterations involving neuroinflammation (e.g., Alzheimer’s disease). Progress has been limited, however, because the vast majority of current knowledge of IL-2’s actions on brain function and behavior is based on the use exogenously administered IL-2 to make inferences about the function of the endogenous cytokine. Thus, to identify the cell-type(s) and regional circuitry that express brain-derived IL-2, we used B6.Cg-Tg/ IL2-EGFP17Evr (IL2p8-GFP) transgenic mice, which express green fluorescent protein (GFP) in peripheral immune cells known to produce IL-2. We found that the IL2-GFP transgene was localized almost exclusively to NeuN-positive cells, indicating that the IL-2 is produced primarily by neurons. The IL2-GFP transgene was expressed in discrete nuclei throughout the rostral-caudal extent of the brain and brainstem, with the highest levels found in the cingulate, dorsal endopiriform nucleus, lateral septum, nucleus of the solitary tract, magnocellular/gigantocellular reticular formation, red nucleus, entorhinal cortex, mammilary bodies, cerebellar fastigial nucleus, and posterior interposed nucleus. Having identified IL-2 gene expression in brain regions associated with the regulation of sensorimotor gating (e.g., lateral septum, dorsal endopiriform nucleus, entorhinal cortex, striatum), we compared prepulse inhibition (PPI) of the acoustic startle response in congenic mice bred in our lab that have selective loss of the IL-2 gene in the brain versus the peripheral immune system, to test the hypothesis that brain-derived IL-2 plays a role in modulating PPI. We found that congenic mice devoid of IL-2 gene expression in both the brain and the peripheral immune system, exhibited a modest alteration of PPI. These finding suggest that IL2p8-GFP transgenic mice may be a useful tool to elucidate further the role of brain-derived IL-2 in normal CNS function and disease.

Dissecting the effects of endogenous brain IL-2 and normal versus autoreactive T lymphocytes on microglial responsiveness and T cell trafficking in response to axonal injury

IL-2 is essential for T-helper regulatory (Treg) cell function and self-tolerance, and dysregulation of both endogenous brain and peripheral IL-2 gene expression may have important implications for neuronal injury and repair. We used an experimental approach combining mouse congenic breeding and immune reconstitution to test the hypothesis that the response of motoneurons to injury is modulated by the combined effects of IL2-mediated processes in the brain that modulate its endogenous neuroimmunological milieu, and IL2-mediated processes in the peripheral immune system that regulate T cell function (i.e., normal versus autoreactive Treg-deficient T cells). This experimental strategy enabled us to test our hypothesis by disentangling the effect of normal versus autoreactive T lymphocytes from the effect of endogenous brain IL-2 on microglial responsiveness (microglial phagocytic clusters normally associated with dead motoneurons and MHC2(+) activated microglia) and T cell trafficking, using the facial nerve axotomy model of injury. The results demonstrate that the loss of both brain and peripheral IL-2 had an additive effect on numbers of microglial phagocytic clusters at day 14 following injury, whereas the autoreactive status of peripheral T cells was the primary factor that determined the degree to which T cells entered the injured brain and contributed to increased microglial phagocytic clusters. Changes in activated MHC2(+) microglial in the injured FMN were associated with loss of endogenous brain IL-2 and/or peripheral IL-2. This model may provide greater understanding of the mechanisms involved in determining if T cells entering the injured central nervous system (CNS) have damaging or proregenerative effects.

Using comparative anatomy in the axotomy model to identify distinct roles for microglia and astrocytes in synaptic stripping

The synaptic terminals' withdrawal from the somata and proximal dendrites of injured motoneuron by the processes of glial cells following facial nerve axotomy has been the subject of research for many years. This phenomenon is referred to as synaptic stripping, which is assumed to help survival and regeneration of neurons via reduction of synaptic inputs. Because there is no disruption of the blood-brain barrier or infiltration of macrophages, the axotomy paradigm has the advantage of being able to selectively investigate the roles of resident glial cells in the brain. Although there have been numerous studies of synaptic stripping, the detailed mechanisms are still under debate. Here we suggest that the species and strain differences that are often present in previous work might be related to the current controversies of axotomy studies. For instance, the survival ratios of axotomized neurons were generally found to be higher in rats than in mice. However, some studies have used the axotomy paradigm to follow the glial reactions and did not assess variations in neuronal viability. In the first part of this article, we summarize and discuss the current knowledge on species and strain differences in neuronal survival, glial augmentation and synaptic stripping. In the second part, we focus on our recent findings, which show the differential involvement of microglia and astrocytes in synaptic stripping and neuronal survival. This article suggests that the comparative study of the axotomy paradigm across various species and strains may provide many important and unexpected discoveries on the multifaceted roles of microglia and astrocytes in injury and repair.

Mechanistic explanations how cell-mediated immune activation, inflammation and oxidative and nitrosative stress pathways and their sequels and concomitants play a role in the pathophysiology of unipolar depression

This paper reviews that cell-mediated-immune (CMI) activation and inflammation contribute to depressive symptoms, including anhedonia; anxiety-like behaviors; fatigue and somatic symptoms, e.g. illness behavior or malaise; and mild cognitive impairment (MCI). These effects are in part mediated by increased levels of pro-inflammatory cytokines (PICs), e.g. interleukin-1 (IL-1), IL-6 and tumor necrosis factor (TNF)α, and Th-1-derived cytokines, such as IL-2 and interferon (IFN)γ. Moreover, new pathways, i.e. concomitants and sequels of CMI activation and inflammation, were detected in depression: (1) Induction of indoleamine 2,3-dioxygenase (IDO) by IFNγ and some PICs is associated with depleted plasma tryptophan, which may interfere with brain 5-HT synthesis, and increased production of anxiogenic and depressogenic tryptophan catabolites. (2) Increased bacterial translocation may cause depression-like behaviors by activating the cytokine network, oxidative and nitrosative stress (O&NS) pathways and IDO. (3) Induction of O&NS causes damage to membrane ω3 PUFAs, functional proteins, DNA and mitochondria, and autoimmune responses directed against intracellular molecules that may cause dysfunctions in intracellular signaling. (4) Decreased levels of ω3 PUFAs and antioxidants, such as coenzyme Q10, glutathione peroxidase or zinc, are associated with an increased inflammatory potential; more oxidative damage; the onset of specific symptoms; and changes in the expression or functions of brain 5-HT and N-methyl-d-aspartate receptors. (5) All abovementioned factors cause neuroprogression, that is a combination of neurodegeneration, neuronal apoptosis, and lowered neurogenesis and neuroplasticity. It is concluded that depression may be the consequence of a complex interplay between CMI activation and inflammation and their sequels/concomitants which all together cause neuroprogression that further shapes the depression phenotype. Future research should employ high throughput technologies to collect genetic and gene expression and protein data from patients with depression and analyze these data by means of systems biology methods to define the dynamic interactions between the different cell signaling networks and O&NS pathways that cause depression.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Loss of CNS IL-2 gene expression modifies brain T lymphocyte trafficking: response of normal versus autoreactive Treg-deficient T cells

Emerging data from our lab and others suggested that dysregulation of the brain's endogenous neuroimmunological milieu may occur with the loss of brain IL-2 gene expression and be involved in initiating processes that lead to CNS autoimmunity. We sought to test our working hypothesis that IL-2 deficiency induces endogenous changes in the CNS that play a key role in eliciting T cell homing into the brain. To accomplish this goal, we used an experimental approach that combined mouse congenic breeding and immune reconstitution. In congenic mice without brain IL-2 (two IL-2 KO alleles) that were reconstituted with a normal wild-type immune system, the loss of brain IL-2 doubled the number of T cells that trafficked into the brain in all regions quantified (hippocampus, septum, and cerebellum) compared to mice with two wild-type brain IL-2 alleles and a wild-type peripheral immune system. Congenic mice with normal brain IL-2 (two wild-type IL-2 alleles) that were immune reconstituted with autoreactive Treg-deficient T cells from IL-2 KO mice developed the expected peripheral autoimmunity (splenomegaly) and had a comparable doubling of T cell trafficking into the hippocampus and septum, whereas they exhibited an additional twofold proclivity for the cerebellum over the septohippocampal regions. Unlike brain trafficking of wild-type T cells, the increased homing of IL-2 KO T cells to the cerebellum was independent of brain IL-2 gene expression. These findings demonstrate that brain IL-2 deficiency induces endogenous CNS changes that may lead to the development of brain autoimmunity, and that autoreactive Treg-deficient IL-2 KO T cells trafficking to the brain could have a proclivity to induce cerebellar neuropathology.

Age and facial nerve axotomy-induced T cell trafficking: relation to microglial and motor neuron status

Following peripheral axotomy of the facial nerve in mice, T lymphocytes cross the blood-brain-barrier (BBB) into the central nervous system (CNS), where they home to the neuronal cell bodies of origin in the facial motor nucleus (FMN) and act in concert with microglial cells to support the injured motor neurons. Several lines of evidence suggested normal aging may alter the injury-related responses of T cells, microglia, and motor neurons in this model. In this study, we therefore sought to test the hypothesis that compared to 8-week-old mice (young adult), 52-week-old mice (advanced middle age) would exhibit more neuronal damage and increased T cell trafficking into the injured FMN following facial nerve resection. Comparison of 8- and 52-week-old mice at 7, 14, 21, and 28 days post-resection of the facial nerve, confirmed our hypothesis that age influences the kinetics of CD3(+) T lymphocyte trafficking in the axotomized FMN. The peak T cell response was significantly higher, occurred later, and remained elevated longer in the injured FMN of mice in the 52 week age group. Although the kinetics of motor neuron death (identified by quantifying CD11b(+) perineuronal microglial phagocytic clusters engulfing the dead neurons at 7, 14, 21, and 28 days post-resection) differed between the age groups, motor neuron profile counts at day 28 showed that levels of cumulative motor neuron loss did not differ between the age groups. Compared to 8-week-old mice, however, there was small reduction in the mean cell size of the surviving motor neurons in the 52 week age group. Since T lymphocyte function decreases with normal aging, it will be important to determine if increased T cell trafficking into the injured CNS is a compensatory response to the decreased function of older T cells, and if these and related neuroimmunological changes are more pronounced in mice in the late stages of the life cycle.

Microglial responses around intrinsic CNS neurons are correlated with axonal regeneration

Background

Microglia/macrophages and lymphocytes (T-cells) accumulate around motor and primary sensory neurons that are regenerating axons but there is little or no microglial activation or T-cell accumulation around axotomised intrinsic CNS neurons, which do not normally regenerate axons. We aimed to establish whether there was an inflammatory response around the perikarya of CNS neurons that were induced to regenerate axons through a peripheral nerve graft.

Results

When neurons of the thalamic reticular nucleus (TRN) and red nucleus were induced to regenerate axons along peripheral nerve grafts, a marked microglial response was found around their cell bodies, including the partial enwrapping of some regenerating neurons. T-cells were found amongst regenerating TRN neurons but not rubrospinal neurons. Axotomy alone or insertion of freeze-killed nerve grafts did not induce a similar perineuronal inflammation. Nerve grafts in the corticospinal tracts did not induce axonal regeneration or a microglial or T-cell response in the motor cortex.

Conclusions

These results strengthen the evidence that perineuronal microglial accumulation (but not T-cell accumulation) is involved in axonal regeneration by intrinsic CNS and other neurons.

Interleukin-2 deficiency-induced T cell autoimmunity in the mouse brain

Interleukin-2 (IL-2) has been implicated in the pathogenesis of neurodevelopmental and neurodegenerative disorders. Studies from our lab have shown that adult IL-2 knockout (KO) mice exhibit septohippocampal pathology and related behavioral deficits. Compared to IL-2 wild-type (WT) mice, IL-2 KO mice have a marked and selective loss of septal cholinergic neurons that occurs between the third postnatal week and adulthood. Given that the development of septal neurons is completed by embryonic day 17 and that IL-2 KO mice exhibit peripheral autoimmunity that develops progressively post-weaning, our data and others led us to postulate that the loss of septal neurons in adult IL-2 KO mice is due to selective autoimmune neurodegeneration that coincides with increasing levels of peripheral autoimmunity. Thus, the present study tested the hypotheses: (1) that T cells selectively target the septum, and; (2) that T lymphocyte infiltration to the septum would correlate with peripheral autoimmune disease. We quantified CD3(+) T cells in the septum, hippocampus, and cerebellum of IL-2 KO and IL-2 WT mice at ages ranging from 2 to 14 weeks. T cells infiltrated the brains of IL-2 deficient mice, but were not selective for the septum. Brain T lymphocyte levels in IL-2 KO mice correlated positively with the degree of peripheral autoimmunity. We did not detect CD19(+) B lymphocytes, IgG-positive lymphocytes or IgG deposition indicative of autoantibodies in the brains of IL-2 KO mice. Further study is needed to understand how IL-2 deficiency-induced autoimmune T lymphocytes interact with endogenous brain cells to alter function and promote disease.

Human immunodeficiency virus type 1 infection increases the in vivo capacity of peripheral monocytes to cross the blood-brain barrier into the brain and the in vivo sensitivity of the blood-brain barrier to disruption by lipopolysaccharide

Human immunodeficiency virus type 1 (HIV-1), introduced into the brain by HIV-1-infected monocytes which migrate across the blood-brain barrier (BBB), infects resident macrophages and microglia and initiates a process that causes HIV-1-associated neurocognitive disorders. The mechanism by which HIV-1 infection circumvents the BBB-restricted passage of systemic leukocytes into the brain and disrupts the integrity of the BBB is not known. Circulating lipopolysaccharide (LPS), which can compromise the integrity of the BBB, is significantly increased in HIV-1-infected individuals. We hypothesized that HIV-1 infection increases monocyte capacity to migrate across the BBB, which is further facilitated by a compromise of BBB integrity mediated by the increased systemic LPS levels present in HIV-1-infected individuals. To investigate this possibility, we examined the in vivo BBB migration of monocytes derived from our novel mouse model, JR-CSF/EYFP mice, which are transgenic for both a long terminal repeat-regulated full-length infectious HIV-1 provirus and ROSA-26-regulated enhanced yellow fluorescent protein. We demonstrated that JR-CSF/EYFP mouse monocytes displayed an increased capacity to enter the brain by crossing either an intact BBB or a BBB whose integrity was partially compromised by systemic LPS. We also demonstrated that the JR-CSF mouse BBB was more susceptible to disruption by systemic LPS than the control wild-type mouse BBB. These results demonstrated that HIV-1 infection increased the ability of monocytes to enter the brain and increased the sensitivity of the BBB to disruption by systemic LPS, which is elevated in HIV-1-infected individuals. These mice represent a new in vivo system for studying the mechanism by which HIV-1-infected monocytes migrate into the brain.

Modulation of microglial/macrophage activation by macrophage inhibitory factor (TKP) or tuftsin (TKPR) attenuates the disease course of experimental autoimmune encephalomyelitis

Background

Myelin Oligodendrocyte Glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE) is the most commonly used mouse model for multiple sclerosis (MS). During the of progression of EAE, microglia, the immunocompetent cells of the brain, become activated and accumulate around demyelinated lesions. Microglial activation is mediated by the extracellular protease tissue Plasminogen Activator (tPA), and mice lacking tPA display altered EAE progression. In this study, we have used pharmacological inhibitors and stimulators of microglial/macrophage activation to examine the temporal requirement for microglial activation in EAE progression and to determine whether such approaches might potentially be of therapeutic value.

Results

Intervention using the tripeptide macrophage/microglia inhibitory factor MIF (TKP) and the tetrapeptide macrophage/microglial stimulator tuftsin (TKPR) attenuated EAE symptoms and revealed that the timing of macrophage/microglial activation is critical for the clinical outcome of EAE. We show that the disease progression can potentially be manipulated favorably at early stages by altering the timing of microglial activation, which in turn alters the systemic immune response to favor upregulation of T helper cell 2 genes that promote recovery from EAE.

Conclusion

Preventative and therapeutic modulation of macrophage/microglial activity significantly alters the outcome of EAE at symptomatic stages. Specific molecular targets have been identified that represent potential avenues of exploration for the treatment and prevention of MS.

IL-15 and IL-15R alpha gene deletion: effects on T lymphocyte trafficking and the microglial and neuronal responses to facial nerve axotomy

IL-15 is a potent T cell chemoattractant, and this cytokine and its unique alpha subunits, IL-15R alpha, can modify immune cell expression of several T cell chemokines and their receptors. Facial nerve axotomy in mice leads to T cell migration across an intact blood-brain-barrier (BBB), and under certain conditions T cells can provide neuroprotection to injured neurons in the facial motor nucleus (FMN). Although chemokines and chemoattractant cytokines are thought to be responsible for T cell migration to the injured cell bodies, data addressing this question are lacking. This study tested the hypothesis that T cell homing to the axotomized FMN would be impaired in knockout (KO) mice with the IL-15 and IL-15R alpha genes deleted, and sought to determine if microglial responsiveness and motoneuron death are affected. Both IL-15KO and IL-15R alpha KO mice exhibited a marked reduction in CD3(+) T cells and had fewer MHC2(+) activated microglia in the injured FMN than their respective WT controls at day 14 post-axotomy. Although there was a relative absence of T cell recruitment into the axotomized FMN in both knockout strains, IL-15R alpha KO mice had five times more motoneuron death (characterized by perineuronal microglial clusters engulfing dead motoneurons) than their WT controls, whereas dead neurons in IL-15KO did not differ from their WT controls. Further studies are needed to dissect the mechanisms that underlie these observations (e.g., central vs. peripheral immune contributions).

T cells contribute to lysophosphatidylcholine-induced macrophage activation and demyelination in the CNS

We have previously shown that intraspinal microinjection of lysophosphatidylcholine (LPC), a potent demyelinating agent, results in a rapid but brief influx of T cells (between 6 and 12 h). This is accompanied by a robust activation of macrophages/microglia that leads to demyelination by 48 h. In the present study, we examined whether this brief influx of T cells contributes to the activation of macrophages/microglia and demyelination by injecting LPC into the dorsal column white matter of athymic Nude mice that lack T cells. We show that there is a significant reduction in macrophage/microglial activation and myelin clearance after LPC injection in Nude mice as compared with wildtype controls. We also show that there is no difference in the recruitment of hematogenous macrophages into the spinal cord after LPC injection in the two mouse strains. Of the T cell cytokines assessed, there was a marked reduction in the mRNA expression of interleukin-2 (IL-2) in Nude mice compared with wildtype animals. Neutralizing IL-2 with function-blocking antibodies in wildtype animals resulted in a significant decrease in the number of phagocytic macrophages/microglia and a reduction in demyelination induced by LPC. While there may be other defects in Nude mice that might contribute to the effects shown here, these data suggest that the brief influx of T cells in this model of chemically-induced demyelination could play a role in macrophage/microglial activation and demyelination. These results may also have implications for remyelination in this and other types of CNS damage.

Prior facial motor neuron injury elicits endogenous T cell memory: relation to neuroregeneration

We tested the hypotheses that prior injury to the facial motor nucleus (FMN) would elicit a more robust T cell response in the opposite FMN when the contralateral facial nerve was injured later in life, and that this would result in improved neuroregeneration. Measures of T cell, neuronal and microglial status were compared in sensitized mice (right facial nerve transection followed by contralateral facial nerve transection 9.5 weeks later) and naïve mice (sham surgery of the right facial nerve followed by contralateral facial nerve transection 9.5 weeks later) following axotomy of the contralateral facial nerve. At day 14 post-axotomy, sensitized mice exhibited nearly a two-fold increase in T cells in the FMN compared to naïve mice. There were no differences between the groups in levels of dead neurons and NeuN expression by surviving motor neurons at day 14, or motor neuron survival and cell area at day 49 post-axotomy. Measures of microglial responsiveness did not differ between the groups. Further study is needed to delineate the role of endogenous T cell memory in neuronal injury and regeneration.

The injury response of oligodendrocyte precursor cells is induced by platelets, macrophages and inflammation-associated cytokines

Oligodendrocyte precursor cells recognized with the NG2 antibody respond rapidly to CNS injuries with hypertrophy and upregulation of the NG2 chondroitin sulfate proteoglycan within 24 h. These cells participate in glial scar formation, remaining around the injury site for several weeks. After injury, reactive oligodendrocyte precursor cells increase their production of several chondroitin sulfate proteoglycans, including NG2: this cell type thus represents a component of the inhibitory environment that prevents regeneration of axons in the injured CNS. This study analyzes factors that activate oligodendrocyte precursor cells. Both microglia and astrocytes become reactive around motor neurons following peripheral nerve lesions. We show that oligodendrocyte precursor cells do not hypertrophy or increase NG2 levels after these lesions. Those lesions that cause an oligodendrocyte precursor cell reaction generally open the blood-brain barrier. We therefore opened the blood-brain barrier with microinjections of vascular endothelial growth factor or lipopolysaccharide to the rat and mouse brain, and examined oligodendrocyte precursor cell reactivity after 24 h. Both treatments led to increases in NG2 and hypertrophy of oligodendrocyte precursor cells. Of directly injected blood components serum and thrombin were without effect, while platelets and macrophages activated oligodendrocyte precursor cells. We tested the effects of a range of injury-related cytokines, of which tumor necrosis factor alpha; interleukin-1; transforming growth factor beta; interferon gamma had effects on oligodendrocyte precursor cells. Oligodendrocyte precursor cell chemokines, and mitogens did not increase NG2 levels.

Changes in hippocampal IL-15, related cytokines, and neurogenesis in IL-2 deficient mice

Previous studies have demonstrated that interleukin-2 knockout (KO) mice exhibit alterations in hippocampal cytoarchitecture. Several lines of evidence suggest that these variations may result from immune dysregulation and/or autoimmunity. Thus, this study sought to compare adult IL-2 KO mice and wild-type littermates (8-12 weeks of age), the age where differences in hippocampal cytoarchitecture have previously been observed, for differences in measures of neuroimmunological status in the hippocampus. Furthermore, because IL-15 shares the same receptor subunits for signal transduction as IL-2 (IL-2/15Rbeta and gammac) that are enriched in the hippocampus and may induce inflammatory processes in IL-2 KO mice, we sought to test the hypothesis that IL-15 is elevated in the hippocampus of IL-2 KO mice. Compared to wild-type mice, IL-2 KO mice exhibited increased hippocampal protein concentrations of IL-15 as well as IL-12, IP-10, and MCP-1. These cytokine changes, however, did not correlate with levels in the peripheral circulation, and there were no T cells or an increase in MHCII-positive microglia in the hippocampus of IL-2 KO mice. Since elevated levels of certain inflammatory cytokines may impair hippocampal neurogenesis, we also tested the hypothesis that changes in neuroimmunological status would be associated with reductions in neurogenesis of neurons in the dentate gyrus of IL-2 KO mice. Contrary to this hypothesis, compared to wild-type mice, male IL-2 KO mice exhibited increased neurogenesis in both the infrapyramidal and suprapyramidal limbs of the granule cell layer of the dentate gyrus, differences that were not observed between females. These findings indicate that IL-2 gene deletion alters the neuroimmunological status of the mouse hippocampus through a dysregulation of cytokines produced by CNS cells, and in males, these changes are associated with increased hippocampal neurogenesis.

Major histocompatibility complex (MHC2+) perivascular macrophages in the axotomized facial motor nucleus are regulated by receptors for interferon-gamma (IFNgamma) and tumor necrosis factor (TNF)

The major histocompatibility complex (MHC) glycoproteins, MHC1 and MHC2, play a key role in the presentation of antigen and the development of the immune response. In the current study we examined the regulation of the MHC2 in the mouse brain after facial axotomy. The normal facial motor nucleus showed very few slender and elongated MHC2+ cells. Transection of the facial nerve led to a gradual but strong upregulation in the number of MHC2+ cells, beginning at day 2 and reaching a maximum 14 days after axotomy, correlated with the induction of mRNA for tumor necrosis factor (TNF) alpha, interleukin (IL) 1beta and interferon-gamma (IFNgamma) and a peak in neuronal cell death. In almost all cases, MHC2 immunoreactivity was restricted to perivascular macrophages that colocalized with vascular basement membrane laminin and macrophage IBA1-immunoreactivity, with no immunoreactivity on phagocytic microglia, astrocytes or invading T-cells. Heterologous transplantation and systemic injection of endotoxin or IFNgamma did not affect this perivascular MHC2 immunoreactivity, and transgenic deletion of the IL1 receptor type I, or TNF receptor type 1, also had no effect. However, the deletion of IFNgamma receptor subunit 1 caused a significant increase, and that of TNF receptor type 2 a strong reduction in the number of MHC2+ macrophages, pointing to a counter-regulatory role of IFNgamma and TNFalpha in the immune surveillance of the injured nervous system.

Restoration of axotomy-induced PACAP gene induction in SCID mice with CD4+ T-lymphocytes

PACAP is a neuropeptide with putative neuroprotective, regenerative, and immunomodulatory actions. PACAP mRNA is up-regulated in motor neurons following facial nerve axotomy in wild type, but not immunodeficient SCID mice. Because CD4+ lymphocytes appear to be neuroprotective in facial nerve and other injury models, we studied PACAP gene expression in SCID mice preinfused with CD4+ enriched splenocytes. Whereas the mean number of PACAP hybridizing neurons after axotomy was reduced by 75% in uninfused SCID mice, infusion of CD4+ enriched splenocytes restored the number to a value not significantly different than controls. The CD4+ cell-dependent induction of PACAP in motor neurons may thus be a factor in the cascade of events triggered by immune cells that ultimately lead to nerve regeneration.

B7.2 on activated and phagocytic microglia in the facial axotomy model: regulation by interleukin-1 receptor type 1, tumor necrosis factor receptors 1 and 2 and endotoxin

Co-stimulatory factors are involved in different forms of brain pathology and play an important role in the activation of T-cells. In the current study, we explored the regulation of B7.2, a prominent member of the B7 family of costimulatory factors, in the facial motor nucleus (FMN) following facial axotomy and systemic application of lipopolysaccharide (LPS, endotoxin) using light and electron immunohistochemistry and cytokine-receptor-deficient mice. Facial axotomy led to a gradual increase of B7.2 immunoreactivity (IR) on microglial cell surface; similar effects were also observed following application of LPS, but both effects were not additive, suggesting overlapping or saturated signaling pathways. Some B7.2-IR was already present on activated microglia surrounding injured neurons at days 1-4 after injury, but became particularly intense during neuronal cell death, peaking at day 14. Previous studies revealed that these late microglial changes are accompanied by a strong increase in the expression of proinflammatory cytokines such as interleukin-1 beta (IL1beta) tumor necrosis factor-alpha (TNFalpha) and interferon gamma (IFNgamma) [J. Neurosci. 18 (1998a) 5804]. Here, deletion of the receptors for these cytokines-IL1R1, TNFR1 or TNFR2, but not IFNgammaR1-caused a strong and significant reduction in B7.2-IR in reactive microglial cells, compared with their wild type (WT) controls on the same genetic strain background, with a 31% decrease in IL1R1-/- , 39% in TNFR1-/- and 49% in TNFR2-/- mice. These data underscore the significance of IL1beta, TNFalpha and LPS, and their receptors, as potent inflammatory signals that regulate the cellular response in the injured brain as well as the interaction with the rapidly recruited immune system. The broad susceptibility of B7.2 regulation to a wide range of different inflammatory signals also points to its role as a sensor of molecular pathology, and a factor that plays an important accessory role in allowing and shaping the microglia/T-cell interaction in the injured central nervous system.

The facial nerve axotomy model

Experimental models such as the facial nerve axotomy paradigm in rodents allow the systematic and detailed study of the response of neurones and their microenvironment to various types of challenges. Well-studied experimental examples include peripheral nerve trauma, the retrograde axonal transport of neurotoxins and locally enhanced inflammation following the induction of experimental autoimmune encephalomyelitis in combination with axotomy. These studies have led to novel insights into the regeneration programme of the motoneurone, the role of microglia and astrocytes in synaptic plasticity and the biology of glial cells. Importantly, many of the findings obtained have proven to be valid in other functional systems and even across species barriers. In particular, microglial expression of major histocompatibility complex molecules has been found to occur in response to various types of neuronal damage and is now regarded as a characteristic component of "glial inflammation". It is found in the context of numerous neurodegenerative disorders including Parkinson's and Alzheimer's disease. The detachment of afferent axonal endings from the surface membrane of regenerating motoneurones and their subsequent displacement by microglia ("synaptic stripping") and long-lasting insulation by astrocytes have also been confirmed in humans. The medical implications of these findings are significant. Also, the facial nerve system of rats and mice has become the best studied and most widely used test system for the evaluation of neurotrophic factors.

Microglial major histocompatibility complex glycoprotein-1 in the axotomized facial motor nucleus: regulation and role of tumor necrosis factor receptors 1 and 2

Presentation of antigen is key to the development of the immune response, mediated by association of antigen with major histocompatibility complex glycoproteins abbreviated as MHC1 and MHC2. In the current study, we examined the regulation of MHC1 in the brain after facial axotomy. The normal facial motor nucleus showed no immunoreactivity for MHC1 (MHC1-IR). Transection of the facial nerve led to a strong and selective up-regulation of MHC1-IR on the microglia in the affected nucleus, beginning at day 2 and reaching a maximum 14 days after axotomy, coinciding with a peak influx of the T lymphocytes that express CD8, the lymphocyte coreceptor for MHC1. Specificity of the MHC1 staining was confirmed in beta2-microglobulin-deficient mice, which lack normal cell surface MHC1-IR. MHC1-IR was particularly strong on phagocytic microglia, induced by delayed neuronal cell death, and correlated with the induction of mRNA for tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and interferon-gamma and the influx of T lymphocytes. Mice with severe combined immunodeficiency (scid), lacking T and B cells, showed an increase in the number of MHC1-positive nodules but no significant effect on overall MHC1-IR. Transgenic deletion of the IL1 receptor type I, or the interferon-gamma receptor type 1 subunit, did not affect the microglial MHC1-IR. However, a combined deletion of TNF receptors 1 and 2 (TNFR1&2-KO) led to a decrease in microglial MHC1-IR and to a striking absence of the phagocytic microglial nodules. Deletion of TNFR2 (p75) did not have an effect; deletion of TNFR1 (p55) reduced the diffuse microglial staining for MHC1-IR but did not abolish the MHC1(+) microglial nodules. In summary, neural injury leads to the induction of MHC1-IR on the activated, phagocytic microglia. This induction of MHC1 precedes the interaction with the immune system, at least in the facial motor nucleus model. Finally, the impaired induction of these molecules, up to now, only in the TNFR-deficient mice underscores the central role of TNF in the immune activation of the injured nervous system.

Induction of neuropeptide gene expression and blockade of retrograde transport in facial motor neurons following local peripheral nerve inflammation in severe combined immunodeficiency and BALB/C mice

Peripheral nerve inflammation is a common clinical problem that accompanies nerve injury and several diseases including Guillain-Barre syndrome and acute and chronic inflammatory demyelinating polyneuropathy. To determine if neuropeptides are induced in motor neurons after inflammation and to study the mechanisms involved, a nerve cuff soaked in complete Freund's adjuvant (CFA) was applied locally to the facial nerve of Balb/C mice. This procedure resulted in an influx of lymphocytes and macrophages to the affected area and a blockade of retrograde axonal transport distal, but not proximal, to the site of application. The same treatment resulted in a strong ipsilateral induction of pituitary adenylyl cyclase activating peptide (PACAP) gene expression in motor neurons in the facial motor nucleus. Because the changes could have occurred due to the loss of target-derived factors or to the production of new factors by immune cells, we studied the effect of the inflammatory stimulus on PACAP mRNA in mice with severe combined immunodeficiency (SCID). As expected, SCID mice showed a severely reduced influx of T-lymphocytes but not macrophages to the peripheral nerve. Moreover, although retrograde transport distal to the inflammation site was blocked similarly in control and SCID mice, the number of motor neurons expressing PACAP mRNA after CFA application was significantly reduced in SCID mice. The data indicate that the induction of PACAP mRNA during nerve inflammation requires the involvement of lymphocytes. However, because the induction of PACAP gene expression was only partially blocked in SCID mice, macrophages, loss of target-derived factors, or other mechanisms may also contribute to the upregulation of PACAP gene expression in motor neurons after nerve inflammation.

Discrete gene loci regulate neurodegeneration, lymphocyte infiltration, and major histocompatibility complex class II expression in the CNS

Neurodegeneration and inflammation are fundamental aspects of many neurological diseases. A genome-wide scan of the response to ventral root avulsion (VRA) in a rat F2 cross discloses specific gene regions that regulate these processes. Two gene loci displayed linkage to neurodegeneration and T cell infiltration, respectively, and a single locus displayed extreme linkage to VRA-induced major histocompatibility complex class II expression on microglia. The demonstration that polymorphic genes in different loci control neurodegeneration and CNS inflammation has implications for various experimental rodent nervous system paradigms and potentially for genetically regulated susceptibility to a variety of human CNS diseases.