Th1 polarization of T cells injected into the cerebrospinal fluid induces brain immunosurveillance.

Although CD4 T cells reside within the cerebrospinal fluid, it is yet unclear whether and how they enter the brain parenchyma and migrate to target specific Ags. We examined the ability of Th1, Th2, and Th17 CD4 T cells injected intracerebroventricularly to migrate from the lateral ventricles into the brain parenchyma in mice. We show that primarily Th1 cells cross the ependymal layer of the ventricle and migrate within the brain parenchyma by stimulating an IFN-γ-dependent dialogue with neural cells, which maintains the effector function of the T cells. When injected into a mouse model of Alzheimer's disease, amyloid-ß (Aß)-specific Th1 cells target Aß plaques, increase Aß uptake, and promote neurogenesis with no evidence of pathogenic autoimmunity or neuronal loss. Overall, we provide a mechanistic insight to the migration of cerebrospinal fluid CD4 T cells into the brain parenchyma and highlight implications on brain immunity and repair.

The role of IL-1ß in the early tumor cell-induced angiogenic response.

In this study, we assessed the involvement of IL-1ß in early angiogenic responses induced by malignant cells using Matrigel plugs supplemented with B16 melanoma cells. We found that during the angiogenic response, IL-1ß and vascular endothelial growth factor (VEGF) interact in a newly described autoinduction circuit, in which each of these cytokines induces the other. The IL-1ß and VEGF circuit acts through interactions between bone marrow-derived VEGF receptor 1(+)/IL-1R1(+) immature myeloid cells and tissue endothelial cells. Myeloid cells produce IL-1ß and additional proinflammatory cytokines, which subsequently activate endothelial cells to produce VEGF and other proangiogenic factors and provide the inflammatory microenvironment for angiogenesis and tumor progression. These mechanisms were also observed in a nontumor early angiogenic response elicited in Matrigel plugs by either rIL-1ß or recombinant VEGF. We have shown that IL-1ß inhibition stably reduces tumor growth by limiting inflammation and inducing the maturation of immature myeloid cells into M1 macrophages. In sharp contrast, only transient inhibition of tumor growth was observed after VEGF neutralization, followed by tumor recurrence mediated by rebound angiogenesis. This occurs via the reprogramming of VEGF receptor 1(+)/IL-1R1(+) cells to express hypoxia inducible factor-1α, VEGF, and other angiogenic factors, thereby directly supporting proliferation of endothelial cells and blood vessel formation in a paracrine manner. We suggest using IL-1ß inhibition as an effective antitumor therapy and are currently optimizing the conditions for its application in the clinic.

TGF-ß signaling through SMAD2/3 induces the quiescent microglial phenotype within the CNS environment.

Microglia are myeloid-derived cells that colonize the central nervous system (CNS) at early stages of development and constitute up to 20% of the glial populations throughout life. While extensive progress has been recently made in identifying the cellular origin of microglia, the mechanism whereby the cells acquire the unique ramified and quiescent phenotype within the CNS milieu remains unknown. Here, we show that upon co-culturing of either CD117(+) /Lin(-) hematopoietic progenitors or CD11c(+) bone marrow derived cells with organotypic hippocampal slices or primary glia, the cells acquire a ramified morphology concomitant with reduced levels of CD86, MHCII, and CD11c and up-regulation of the microglial cell-surface proteins CX(3) CR1 and Iba-1. We further demonstrate that the transforming growth factor beta (TGF-ß) signaling pathway via SMAD2/3 phosphorylation is essential for both primary microglia and myeloid-derived cells in order to acquire their quiescent phenotype. Our study suggests that the abundant expression of TGF-ß within the CNS during development and various inflammatory processes plays a key role in promoting the quiescent phenotype of microglia and may thus serve as a target for therapeutic strategies aimed at modulating the function of microglia in neurodegenerative diseases such as Alzheimer's and prion.

HLA-DR alleles in amyloid beta-peptide autoimmunity: a highly immunogenic role for the DRB1*1501 allele.

Active amyloid beta-peptide (Abeta) immunization of patients with Alzheimer's disease (AD) caused meningoencephalitis in approximately 6% of immunized patients in a clinical trial. In addition, long-term studies of AD patients show varying degrees of Abeta Ab responses, which correlate with the extent of Abeta clearance from the brain. In this study, we examined the contribution of various HLA-DR alleles to these immune-response variations by assessing Abeta T cell reactivity, epitope specificity, and immunogenicity. Analysis of blood samples from 133 individuals disclosed that the abundant DR haplotypes DR15 (found in 36% of subjects), DR3 (in 18%), DR4 (12.5%), DR1 (11%), and DR13 (8%) were associated with Abeta-specific T cell responses elicited via distinct T cell epitopes within residues 15-42 of Abeta. Because the HLA-DRB1*1501 occurred most frequently, we examined the effect of Abeta challenge in humanized mice bearing this allele. The observed T cell response was remarkably strong, dominated by secretion of IFN-gamma and IL-17, and specific to the same T cell epitope as that observed in the HLA-DR15-bearing humans. Furthermore, following long-term therapeutic immunization of an AD mouse model bearing the DRB1*1501 allele, Abeta was effectively cleared from the brain parenchyma and brain microglial activation was reduced. The present study thus characterizes HLA-DR alleles directly associated with specific Abeta T cell epitopes and demonstrates the highly immunogenic properties of the abundant allele DRB1*1501 in a mouse model of AD. This new knowledge enables us to explore the basis for understanding the variations in naturally occurring Abeta-reactive T cells and Abeta immunogenicity among humans.

IFN-gamma enhances neurogenesis in wild-type mice and in a mouse model of Alzheimer's disease.

The generation of new neurons and glia from a precursor stem cell appears to take place in the adult brain. However, new neurons generated in the dentate gyrus decline sharply with age and to an even greater extent in neurodegenerative diseases. Here we raise the question whether peripheral immune mechanisms can generate immunity to such deficits in neuronal repair. We demonstrate that in contrast to primarily innate immunity cytokines, such as interleukin-6 and tumor necrosis factor-alpha, the adaptive immunity cytokine IFN-gamma enhances neurogenesis in the dentate gyrus of adult mice and improves the spatial learning and memory performance of the animals. In older mice, the effect of IFN-gamma is more pronounced in both wild-type mice and mice with Alzheimer's-like disease and is associated with neuroprotection. In addition, IFN-gamma reverses the increase in oligodendrogenesis observed in a mouse model of Alzheimer's disease. We demonstrate that limited amounts of IFN-gamma in the brain shape the neuropoietic milieu to enhance neurogenesis, possibly representing the normal function of the immune system in controlling brain inflammation and repair.

Immunity and neuronal repair in the progression of Alzheimer's disease: a brief overview.

Alzheimer's disease (AD) is an age-related progressive neurodegenerative disorder characterized by memory loss and severe cognitive decline. The etiology of the disease has not been explored, although a significant body of evidence suggests that neuronal dysfunction is caused by hyperphosphorylation and intracellular accumulation of the Tau protein, extracellular accumulation of the amyloid beta-peptide (Abeta), and the associated chronic activation of glial cells. Clearance of toxic Abeta, apoptotic cells and debris from the brain together with induction of neuronal repair mechanisms may all take place partially throughout the progression of AD, but therapeutic approaches based on knowledge of these processes have been unsuccessfully developed. Here, we address the question of whether autoimmune mechanisms can be boosted to safely facilitate the above-mentioned clearance and neuronal repair in the AD brain. We have previously demonstrated that depending on genetic background, autoimmunity targeted to Abeta is already induced in elderly individuals and in patients with AD. We have shown in a mouse model of AD that given a preexisting proinflammatory milieu in the brain, immune cells can enter the brain tissue and participate in clearance of Abeta. Furthermore, the decline in cognitive functions and neurogenesis throughout the progression of AD may also be affected by autoimmune mechanisms operating in the periphery and in the brain. In light of the so-far unsuccessful anti-inflammatory approaches to treating AD, we suggest that boosting - rather than suppressing - the endogenous immune mechanisms induced in AD may enhance repair pathways in the brain, provided that this approach can be safely applied.

Accelerated microglial pathology is associated with Aß plaques in mouse models of Alzheimer's disease.

Microglia integrate within the neural tissue with a distinct ramified morphology through which they scan the surrounding neuronal network. Here, we used a digital tool for the quantitative morphometric characterization of fine cortical microglial structures in mice, and the changes they undergo with aging and in Alzheimer's-like disease. We show that, compared with microglia in young mice, microglia in old mice are less ramified and possess fewer branches and fine processes along with a slightly increased proinflammatory cytokine expression. A similar microglial pathology appeared 6-12 months earlier in mouse models of Alzheimer's disease (AD), along with a significant increase in brain parenchyma lacking coverage by microglial processes. We further demonstrate that microglia near amyloid plaques acquire unique activated phenotypes with impaired process complexity. We thus show that along with a chronic proinflammatory reaction in the brain, aging causes a significant reduction in the capacity of microglia to scan their environment. This type of pathology is markedly accelerated in mouse models of AD, resulting in a severe microglial process deficiency, and possibly contributing to enhanced cognitive decline.

Active Aß vaccination fails to enhance amyloid clearance in a mouse model of Alzheimer's disease with Aß42-driven pathology.

Aß vaccination has been shown to induce remarkable clearance of brain amyloid plaques in mouse models of Alzheimer's disease (AD). However, the extent to which antibody-mediated Aß clearance is affected by predominant formation of Aß42 over Aß40 is unclear. Here we demonstrate for the first time that in a mouse model carrying the human APP mutations KM670/671NL and the human PS1 mutation P166L, Aß vaccination does not result in plaque clearance. This was in spite of the strong T- and B-cell immune responses evoked under the DR1501 genetic background and the activation of microglia at sites of Aß plaques. Our findings suggest the existence of antibody-resistant forms of Aß deposits in the brain consisting of primarily Aß42, and shed light on the mechanisms of antibody-dependent amyloid clearance as well as novel therapeutic strategies for AD.

Dendritic cells regulate amyloid-ß-specific T-cell entry into the brain: the role of perivascular amyloid-ß.

Amyloid-ß (Aß) accumulation in the brain is one of the hallmarks of Alzheimer's disease (AD). T-cell entry into vascular and parenchymal brain areas loaded with Aß has been observed with both beneficial as well as detrimental effects. Using a new AD mouse model, we studied the molecular mechanisms allowing CD4 T cells to specifically target Aß-loaded brain areas. We observed that following Aß immunization, CD11c+ dendritic cells (DCs) and CD4 T cells occurred primarily in the perivascular and leptomeningial spaces of cerebral vessels deposited with Aß. CD11c+ cells expressed high levels of the DC maturation markers DEC-205, MHC class II and CD86. Notably, the majority of cerebral blood vessels were found adjacent to Aß plaques, expressing high levels of the ICAM-1 and VCAM-1 adhesion molecules. We propose that the drainage of Aß to the leptomeningeal and perivascular spaces and its deposition there provide the antigenic source for DCs to stimulate Aß-specific T cells on their way to target amyloid plaques within the brain tissue.

Amyloid beta-HSP60 peptide conjugate vaccine treats a mouse model of Alzheimer's disease.

Active vaccination with amyloid beta peptide (Aß) to induce beneficial antibodies was found to be effective in mouse models of Alzheimer's disease (AD), but human vaccination trials led to adverse effects, apparently caused by exuberant T-cell reactivity. Here, we sought to develop a safer active vaccine for AD with reduced T-cell activation. We treated a mouse model of AD carrying the HLA-DR DRB1*1501 allele, with the Aß B-cell epitope (Aß 1-15) conjugated to the self-HSP60 peptide p458. Immunization with the conjugate led to the induction of Aß-specific antibodies associated with a significant reduction of cerebral amyloid burden and of the accompanying inflammatory response in the brain; only a mild T-cell response specific to the HSP peptide but not to the Aß peptide was found. This type of vaccination, evoking a gradual increase in antibody titers accompanied by a mild T-cell response is likely due to the unique adjuvant and T-cell stimulating properties of the self-HSP peptide used in the conjugate and might provide a safer approach to effective AD vaccination.

T cells specifically targeted to amyloid plaques enhance plaque clearance in a mouse model of Alzheimer's disease.

Patients with Alzheimer's disease (AD) exhibit substantial accumulation of amyloid-beta (Abeta) plaques in the brain. Here, we examine whether Abeta vaccination can facilitate the migration of T lymphocytes to specifically target Abeta plaques and consequently enhance their removal. Using a new mouse model of AD, we show that immunization with Abeta, but not with the encephalitogenic proteolipid protein (PLP), results in the accumulation of T cells at Abeta plaques in the brain. Although both Abeta-reactive and PLP-reactive T cells have a similar phenotype of Th1 cells secreting primarily IFN-gamma, the encephalitogenic T cells penetrated the spinal cord and caused experimental autoimmune encephalomyelitis (EAE), whereas Abeta T cells accumulated primarily at Abeta plaques in the brain but not the spinal cord and induced almost complete clearance of Abeta. Furthermore, while a single vaccination with Abeta resulted in upregulation of the phagocytic markers triggering receptors expressed on myeloid cells-2 (TREM2) and signal regulatory protein-beta1 (SIRPbeta1) in the brain, it caused downregulation of the proinflammatory cytokines TNF-alpha and IL-6. We thus suggest that Abeta deposits in the hippocampus area prioritize the targeting of Abeta-reactive but not PLP-reactive T cells upon vaccination. The stimulation of Abeta-reactive T cells at sites of Abeta plaques resulted in IFN-gamma-induced chemotaxis of leukocytes and therapeutic clearance of Abeta.

Abeta-induced meningoencephalitis is IFN-gamma-dependent and is associated with T cell-dependent clearance of Abeta in a mouse model of Alzheimer's disease.

Vaccination against amyloid beta-peptide (Abeta) has been shown to be successful in reducing Abeta burden and neurotoxicity in mouse models of Alzheimer's disease (AD). However, although Abeta immunization did not show T cell infiltrates in the brain of these mice, an Abeta vaccination trial resulted in meningoencephalitis in 6% of patients with AD. Here, we explore the characteristics and specificity of Abeta-induced, T cell-mediated encephalitis in a mouse model of the disease. We demonstrate that a strong Abeta-specific T cell response is critically dependent on the immunizing T cell epitope and that epitopes differ depending on MHC genetic background. Moreover, we show that a single immunization with the dominant T cell epitope Abeta10-24 induced transient meningoencephalitis only in amyloid precursor protein (APP)-transgenic (Tg) mice expressing limited amounts of IFN-gamma under an myelin basic protein (MBP) promoter. Furthermore, immune infiltrates were targeted primarily to sites of Abeta plaques in the brain and were associated with clearance of Abeta. Immune infiltrates were not targeted to the spinal cord, consistent with what was observed in AD patients vaccinated with Abeta. Using primary cultures of microglia, we show that IFN-gamma enhanced clearance of Abeta, microglia, and T cell motility, and microglia-T cell immunological synapse formation. Our study demonstrates that limited expression of IFN-gamma in the brain, as observed during normal brain aging, is essential to promote T cell-mediated immune infiltrates after Abeta immunization and provides a model to investigate both the beneficial and detrimental effects of Abeta-specific T cells.