Igal Ifergan

Human TH17 lymphocytes promote blood-brain barrier disruption and central nervous system inflammation

TH17 lymphocytes appear to be essential in the pathogenesis of numerous inflammatory diseases. We demonstrate here the expression of IL-17 and IL-22 receptors on blood-brain barrier endothelial cells (BBB-ECs) in multiple sclerosis lesions, and show that IL-17 and IL-22 disrupt BBB tight junctions in vitro and in vivo. Furthermore, TH17 lymphocytes transmigrate efficiently across BBB-ECs, highly express granzyme B, kill human neurons and promote central nervous system inflammation through CD4+ lymphocyte recruitment.

The hedgehog pathway promotes blood-brain barrier integrity and CNS immune quiescence

Hedgehog signaling is required for maintaining the integrity of the blood-brain barrier. The blood-brain barrier (BBB) is composed of tightly bound endothelial cells (ECs) and perivascular astrocytes that regulate central nervous system (CNS) homeostasis. We showed that astrocytes secrete Sonic hedgehog and that BBB ECs express Hedgehog (Hh) receptors, which together promote BBB formation and integrity during embryonic development and adulthood. Using pharmacological inhibition and genetic inactivation of the Hh signaling pathway in ECs, we also demonstrated a critical role of the Hh pathway in promoting the immune quiescence of BBB ECs by decreasing the expression of proinflammatory mediators and the adhesion and migration of leukocytes, in vivo and in vitro. Overall, the Hh pathway provides a barrier-promoting effect and an endogenous anti-inflammatory balance to CNS-directed immune attacks, as occurs in multiple sclerosis.

Activated leukocyte cell adhesion molecule promotes leukocyte trafficking into the central nervous system

Adhesion molecules of the immunoglobulin superfamily are crucial effectors of leukocyte trafficking into the central nervous system. Using a lipid raft-based proteomic approach, we identified ALCAM as an adhesion molecule involved in leukocyte migration across the blood-brain barrier (BBB). ALCAM expressed on BBB endothelium localized together with CD6 on leukocytes and with BBB endothelium transmigratory cups. ALCAM expression on BBB cells was upregulated in active multiple sclerosis and experimental autoimmune encephalomyelitis lesions. Moreover, ALCAM blockade restricted the transmigration of CD4+ lymphocytes and monocytes across BBB endothelium in vitro and in vivo and reduced the severity and delayed the time of onset of experimental autoimmune encephalomyelitis. Our findings indicate an important function for ALCAM in the recruitment of leukocytes into the brain and identify ALCAM as a potential target for the therapeutic dampening of neuroinflammation.

Type 2 Monocyte and Microglia Differentiation Mediated by Glatiramer Acetate Therapy in Patients with Multiple Sclerosis

Glatiramer acetate (GA) therapy of patients with multiple sclerosis (MS) represents a unique setting in which in vivo Th2 deviation of T cells is consistently observed and associated with clinical benefit in a human autoimmune disease. We postulated that APCs are important targets of GA therapy and demonstrate that treatment of MS patients with GA reciprocally regulates the IL-10/IL-12 cytokine network of monocytes in vivo. We further show that Th1- or Th2-polarized GA-reactive T cells isolated from untreated or treated MS patients mediate type 1 and 2 APC differentiation of human monocytes, based on their ability to efficiently induce subsequent Th1 and Th2 deviation of naive T cells, respectively. These observations are extended to human microglia, providing the first demonstration of type 2 differentiation of CNS-derived APCs. Finally, we confirm that the fundamental capacity of polarized T cells to reciprocally modulate APC function is not restricted to GA-reactive T cells, thereby defining a novel and dynamic positive feedback loop between human T cell and APC responses. In the context of MS, we propose that GA therapy results in the generation of type 2 APCs, contributing to Th2 deviation both in the periphery and in the CNS of MS patients. In addition to extending insights into the therapeutic mode of action of GA, our findings revisit the concept of bystander suppression and underscore the potential of APCs as attractive targets for therapeutic immune modulation.

Microglial Expression of the B7 Family Member B7 Homolog 1 Confers Strong Immune Inhibition: Implications for Immune Responses and Autoimmunity in the CNS

Inflammation of the CNS is usually locally limited to avoid devastating consequences. Critical players involved in this immune regulatory process are the resident immune cells of the brain, the microglia. Interactions between the growing family of B7 costimulatory ligands and their receptors are increasingly recognized as important pathways for costimulation and/or inhibition of immune responses. Human and mouse microglial cells constitutively express B7 homolog 1 (B7-H1) in vitro. However, under inflammatory conditions [presence of interferon-γ (IFN-γ) or T-helper 1 supernatants], a significant upregulation of B7-H1 was detectable. Expression levels of B7-H1 protein on microglial cells were substantially higher compared with astrocytes or splenocytes. Coculture experiments of major histocompatibility complex class II-positive antigen-presenting cells (APC) with syngeneic T cells in the presence of antigen demonstrated the functional consequences of B7-H1 expression on T-cell activation. In the presence of a neutralizing anti-B7-H1 antibody, both the production of inflammatory cytokines (IFN-γ and interleukin-2) and the upregulation of activation markers (inducible costimulatory signal) by T cells were markedly enhanced. Interestingly, this effect was clearly more pronounced when microglial cells were used as APC, compared with astrocytes or splenocytes. Furthermore, B7-H1 was highly upregulated during the course of myelin oligodendrocyte glycoprotein-induced and proteolipid protein-induced experimental allergic encephalomyelitis in vivo. Expression was predominantly localized to areas of strongest inflammation and could be colocalized with microglial cells/macrophages as well as T cells. Together, our data propose microglial B7-H1 as an important immune inhibitory molecule capable of downregulating T-cell activation in the CNS and thus confining immunopathological damage.

Statins reduce human blood–brain barrier permeability and restrict leukocyte migration: Relevance to multiple sclerosis

Dysregulation of the blood–brain barrier (BBB) and transendothelial migration of immune cells are among the earliest central nervous system changes partaking in lesion formation in both multiple sclerosis (MS) and its early clinical form, the clinically isolated syndrome. Evidence for the anti‐inflammatory effects of 3‐hydroxy‐3‐methylglutaryl coenzyme A reductase inhibitors within the central nervous system arose from studies demonstrating that statins improve clinical signs in the animal model of MS and reduce the number of gadolinium‐enhancing lesions in MS.

Activation of kinin receptor B1 limits encephalitogenic T lymphocyte recruitment to the central nervous system

Previous proteomic and transcriptional analyses of multiple sclerosis lesions revealed modulation of the renin-angiotensin and the opposing kallikrein-kinin pathways. Here we identify kinin receptor B1 (Bdkrb1) as a specific modulator of immune cell entry into the central nervous system (CNS). We demonstrate that the Bdkrb1 agonist R838 (Sar-[D-Phe]des-Arg9-bradykinin) markedly decreases the clinical symptoms of experimental autoimmune encephalomyelitis (EAE) in SJL mice, whereas the Bdkrb1 antagonist R715 (Ac-Lys-[D-βNal7, Ile8]des-Arg9-bradykinin) resulted in earlier onset and greater severity of the disease. Bdkrb1-deficient (Bdkrb1−/−) C57BL/6 mice immunized with a myelin oligodendrocyte glycoprotein fragment, MOG35–55, showed more severe disease with enhanced CNS-immune cell infiltration. The same held true for mixed bone marrow–chimeric mice reconstituted with Bdkrb1−/− T lymphocytes, which showed enhanced T helper type 17 (TH17) cell invasion into the CNS. Pharmacological modulation of Bdkrb1 revealed that in vitro migration of human TH17 lymphocytes across blood-brain barrier endothelium is regulated by this receptor. Taken together, these results suggest that the kallikrein-kinin system is involved in the regulation of CNS inflammation, limiting encephalitogenic T lymphocyte infiltration into the CNS, and provide evidence that Bdkrb1 could be a new target for the treatment of chronic inflammatory diseases such as multiple sclerosis.

Melanoma cell adhesion molecule identifies encephalitogenic T lymphocytes and promotes their recruitment to the central nervous system.

In multiple sclerosis, encephalitogenic CD4(+) lymphocytes require adhesion molecules to accumulate into central nervous system inflammatory lesions. Using proteomic techniques, we identified expression of melanoma cell adhesion molecule (MCAM) on a subset of human effector memory CD4(+) lymphocytes and on human blood-brain barrier endothelium. Herein, we demonstrate that MCAM is a stable surface marker that refines the identification of interleukin 17(+), interleukin 22(+), RAR-related orphan receptor γ and interleukin 23 receptor(+) cells within the CD161(+)CCR6(+) subset of memory CD4(+) lymphocytes. We also show that MCAM(+) lymphocytes express significantly more granulocyte/macrophage colony stimulating factor and granzyme B than MCAM(-) lymphocytes. Furthermore, the proportion of MCAM(+) CD4(+) lymphocytes is significantly increased in the blood and in the central nervous system of patients with multiple sclerosis and experimental autoimmune encephalomyelitis animals compared with healthy controls or other neurological diseases, and MCAM expression is upregulated at the blood-brain barrier within inflammatory lesions. Moreover, blockade of MCAM or depletion of MCAM(+) CD4(+) T lymphocytes both restrict the migration of T(H)17 lymphocytes across blood-brain barrier endothelial cells and decrease the severity of experimental autoimmune encephalomyelitis. Our findings indicate that MCAM could serve as a potential biomarker for multiple sclerosis and represents a valuable target for the treatment of neuroinflammatory conditions.

Role of Ninjurin-1 in the migration of myeloid cells to central nervous system inflammatory lesions.

Blood‐derived myeloid antigen‐presenting cells (APCs) account for a significant proportion of the leukocytes found within lesions of multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). These APCs along with activated microglia are thought to be pivotal in the initiation of the central nervous system (CNS)‐targeted immune response in MS and EAE. However, the exact molecules that direct the migration of myeloid cells from the periphery across the blood–brain barrier (BBB) remain largely unknown.

IFN-β regulates CD73 and adenosine expression at the blood-brain barrier

IFN‐β treatment reduces the relapse rate in MS but its mechanism of action remains incompletely understood. Our aim was to clarify the beneficial effect of IFN‐β in the treatment of MS. We assessed the influence of IFN‐β treatment on (i) CD73 expression on the surface of primary cultures of human blood–brain barrier endothelial cells (BBB‐EC) and human astrocytes using immunofluorescence staining and flow cytometry, (ii) transmigration of CD4+ T lymphocytes using an in vitro model of BBB and (iii) CD73 enzyme activity, i.e. ecto‐5′‐nucleotidase activity in the serum of MS patients using a radiochemical assay. IFN‐β increases the expression of ecto‐5′‐nucleotidase both on BBB‐EC and astrocytes. As a consequence, lymphocyte transmigration through BBB‐EC is reduced. Importantly, this reduction can be reversed using α,β‐methyleneadenosine‐5′‐diphosphate, a specific inhibitor of ecto‐5′‐nucleotidase. CD73 is strongly expressed in microvasculature in samples of postmortem MS brain and, moreover, in the majority of MS patients there was a clear upregulation both in the soluble serum ecto‐5′‐nucleotidase activity and skin microvascular CD73 expression after IFN‐β treatment. Upregulation of ecto‐5′‐nucleotidase and a subsequent increase in adenosine production might contribute to the beneficial effects of IFN‐β on MS via enhancing the endothelial barrier function.