Polyclonal expansion of regulatory T cells interferes with effector cell migration in a model of multiple sclerosis

A CD28 superagonistic antibody elicits 2 functionally distinct waves of T cell activation in rats

Administration of the CD28 superagonistic antibody JJ316 is an efficient means to treat autoimmune diseases in rats, but the humanized antibody TGN1412 caused devastating side effects in healthy volunteers during a clinical trial. Here we show that JJ316 treatment of rats induced a dramatic redistribution of T lymphocytes from the periphery to the secondary lymphoid organs, resulting in severe T lymphopenia. Live imaging of secondary lymphoid organs revealed that JJ316 administration almost instantaneously (<2 minutes) arrested T cells in situ. This reduction in T cell motility was accompanied by profound cytoskeletal rearrangements and increased cell size. In addition, surface expression of lymphocyte function-associated antigen-1 was enhanced, endothelial differentiation sphingolipid G protein-coupled receptor 1 and L selectin levels were downregulated, and the cells lost their responsiveness to sphingosine 1-phosphate-directed migration. These proadhesive alterations were accompanied by signs of strong activation, including upregulation of CD25, CD69, CD134, and proinflammatory mediators. However, this did not lead to a cytokine storm similar to the clinical trial. While most of the early changes disappeared within 48 hours, we observed that CD4+CD25+FoxP3+ regulatory T cells experienced a second phase of activation, which resulted in massive cell enlargement, extensive polarization, and increased motility. These data suggest that CD28 superagonists elicit 2 qualitatively distinct waves of activation.

TWIK-related acid-sensitive K+ channel 1 (TASK1) and TASK3 critically influence T lymphocyte effector functions

Two major K(+) channels are expressed in T cells, (i) the voltage-dependent K(V)1.3 channel and (ii) the Ca(2+)-activated K(+) channel KCa 3.1 (IKCa channel). Both critically influence T cell effector functions in vitro and animal models in vivo. Here we identify and characterize TWIK-related acid-sensitive potassium channel 1 (TASK1) and TASK3 as an important third K(+) conductance on T lymphocytes. T lymphocytes constitutively express TASK1 and -3 protein. Application of semi-selective TASK blockers resulted in a significant reduction of cytokine production and cell proliferation. Interference with TASK channels on CD3(+) T cells revealed a dose-dependent reduction ( approximately 40%) of an outward current in patch clamp recordings indicative of TASK channels, a finding confirmed by computational modeling. In vivo relevance of our findings was addressed in an experimental model of multiple sclerosis, adoptive transfer experimental autoimmune encephalomyelitis. Pretreatment of myelin basic protein-specific encephalitogenic T lymphocytes with TASK modulators was associated with significant amelioration of the disease course in Lewis rats. These data introduce K(2)P channels as novel potassium conductance on T lymphocytes critically influencing T cell effector function and identify a possible molecular target for immunomodulation in T cell-mediated autoimmune disorders.

Silencing of the mineralocorticoid receptor by ribonucleic acid interference in transgenic rats disrupts endocrine homeostasis

Currently, gene disruption by homologous recombination in embryonic stem cells is only feasible in mice. To circumvent this problem, we silenced mineralocorticoid receptor (MR) expression by RNA interference in knockdown rats generated through lentiviral transgenesis. Analysis of the F1 progeny at 3 wk of age revealed strongly decreased MR levels. This was specific for the targeted gene and related to the abundance of the short interfering RNA. Reminiscent of MR knockout mice, the transgenic rats showed a reduced body weight, elevated serum aldosterone levels, increased plasma renin activity, and altered expression of MR target genes. Some of these effects correlated with the degree to which MR mRNA expression was reduced. Whereas disruption of the MR by gene targeting in mice leads to postnatal death, our strategy also allowed obtaining adult knockdown rats with defects in hormone and electrolyte homeostasis resembling pseudohypoaldosteronism. In conclusion, this is the first example of a human disease model based on RNA interference in rats.

The dynamics of effector T cells and Foxp3+ regulatory T cells in the promotion and regulation of autoimmune encephalomyelitis

The Th1/Th2 paradigm of T helper cell subsets had to be revised when IL-17 producing T cells (Th17) were identified as a distinct T helper cell lineage. Th17 cells are very efficient inducers of tissue inflammation and crucial initiators of organ-specific autoimmunity. Whereas Th17 cells promote autoimmune tissue inflammation, Foxp3+ regulatory T cells (T-reg) are necessary and sufficient to prevent autoimmunity throughout the life span of an individual. Here, we review recent findings of how responses of effector T cells and T-reg cells with a defined antigen-specificity develop in autoimmune encephalomyelitis. Moreover, Th17 cells and Foxp3+ T-reg seem to be dichotomously related in that TGF-beta induces Foxp3 in naïve T cells, but TGF-beta and IL-6 together drive the generation of Th17 cells. Thus, we give an overview of how Th17 cells, induced Foxp3+ T-reg, as well as how naturally occurring T-reg cells might cooperate to promote and regulate autoimmune inflammation of the central nervous system (CNS). The monitoring of the population dynamics of these T cell subsets in reporter mice in vivo will enable us to revisit the pathogenic concept of autoimmune inflammation in the CNS and design rational and phase-specific therapeutic interventions.

Glucocorticoids in the control of neuroinflammation

Glucocorticoids are a class of steroid hormones that are endowed with profound anti-inflammatory and immunosuppressive activities. Endogenous glucocorticoids are key players in the modulation of the immune system and establish an endocrine basis of many inflammatory diseases. In addition, synthetic glucocorticoids are amongst the most commonly prescribed drugs worldwide for the treatment of autoimmune disorders. In this review we summarize our present knowledge on the mechanisms by which glucocorticoids impact on multiple sclerosis (MS), a highly prevalent neuroinflammatory disease, and its animal model experimental autoimmune encephalomyelitis (EAE). In spite of the new methodologies that have become available during recent years, we are still far from a comprehensive picture of the mechanism by which glucocorticoids control neuroinflammation.

Termination of inflammation in the nervous system

T cell apoptosis has been studied in animal models for human autoimmune disorders of the nervous system and in other tissues devoid of specialized immune-defense mechanisms. Our data suggest that the central nervous system has a high potential to eliminate T cell inflammation, whereas this mechanism is less effective in the peripheral nervous system, and even more in muscle and skin. In-vitro experiments indicate different scenarios how specific cellular and humoral elements in the nervous system may synergize and sensitize T cells for apoptosis in-vivo. Probably release of TNF-alpha in the nervous system is a central mechanism to limit inflammation in the brain. This is further substantiated since neutralization of TNF-alpha in MS patients increased cellular inflammation and relapses. Therapeutically several conventional and novel approaches like glucocorticosteroids and high-dose antigen therapy induce T cell apoptosis in-situ. We also discuss regulatory, proapoptotic mechanisms such as the Fas/FasL system and counterregulatory mechanisms that have been utilized to limit tissue damage.

Neuroprotective activities of CD4+CD25+ regulatory T cells in an animal model of Parkinson's disease

Progressive loss of dopaminergic neurons in the substantia nigra pars compacta and their terminal connections in the striatum are central features in Parkinson's disease (PD). Emerging evidence supports the notion that microglia neuroinflammatory responses speed neurodegenerative events. We demonstrated previously that this can be slowed by adoptive transfer of T cells from Copolymer-1-immunized mice administered to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) recipients. The cellular basis for this neuroprotective response was the CD4+ T cell population, suggesting involvement of CD4+CD25+ regulatory T cells (Tregs), cells known to suppress immune activation and maintain immune homeostasis and tolerance. We show for the first time that adoptive transfer of CD3-activated Tregs to MPTP-intoxicated mice provides greater than 90% protection of the nigrostriatal system. The response was dose-dependent and paralleled modulation of microglial responses and up-regulation of glial cell-derived neurotrophic factor (CDNF) and TGF-beta. Interestingly, that adoptive transfer of effector T cells showed no significant neuroprotective activities. Tregs were found to mediate neuroprotection through suppression of microglial responses to stimuli, including aggregated, nitrated alpha-synuclein. Moreover, Treg-mediated suppression was also operative following removal of Tregs from culture prior to stimulation. This neuroprotection was achieved through modulation of microglial oxidative stress and inflammation. As Tregs can be modulated in vivo, these data strongly support the use of such immunomodulatory strategies to treat PD.

Molecular targets for disrupting leukocyte trafficking during multiple sclerosis

Autoimmune diseases of the central nervous system (CNS) involve the migration of abnormal numbers of self-directed leukocytes across the blood–brain barrier that normally separates the CNS from the immune system. The cardinal lesion associated with neuroinflammatory diseases is the perivascular infiltrate, which comprises leukocytes that have traversed the endothelium and have congregated in a subendothelial space between the endothelial-cell basement membrane and the glial limitans. The exit of mononuclear cells from this space can be beneficial, as when virus-specific lymphocytes enter the CNS for pathogen clearance, or might induce CNS damage, such as in the autoimmune disease multiple sclerosis when myelin-specific lymphocytes invade and induce demyelinating lesions. The molecular mechanisms involved in the movement of lymphocytes through these compartments involve multiple signalling pathways between these cells and the microvasculature. In this review, we discuss adhesion, costimulatory, cytokine, chemokine and signalling molecules involved in the dialogue between lymphocytes and endothelial cells that leads to inflammatory infiltrates within the CNS, and the targeting of these molecules as therapies for the treatment of multiple sclerosis.

Dynamics of antigen-specific regulatory T-cells in the context of autoimmunity

Naturally occurring regulatory T-cells (nT-reg) are crucial in preventing generalized autoimmunity. However, antigen-specific T-reg responses during organ-specific autoimmunity have not been characterized until recently when novel tools including MHC class II-tetramers and Foxp3gfp "knock-in" reporter mice (Foxp3gfp.KI) became available. In this review, we introduce the concept of functional compartmentalisation in that T-reg may behave differently when they maintain peripheral tolerance in the secondary lymphoid tissue in a non-inflammatory environment as compared to when they are expanded and targeted to the site of inflammation following an autoantigen-specific, immunogenic stimulus.

Antigen therapy of experimental autoimmune encephalomyelitis selectively induces apoptosis of pathogenic T cells

Administration of high-dose myelin antigen induces massive T cell apoptosis in experimental autoimmune encephalomyelitis (EAE) but the nature of the target cells remains elusive. Here we have used a cell line established in eGFP-transgenic Lewis rats to distinguish between pathogenic and bystander T cells in adoptive transfer EAE. Intravenous application of gpMBP strongly reduced the amount of encephalitogenic cells in spinal cord and spleen while the number of the other T cells remained constant. This could be attributed to their differential sensitivity to apoptosis. Thus, antigen therapy selectively targets pathogenic T cells and should therefore limit potential adverse effects.

Manipulation of Regulatory T‐Cell Number and Function with CD28‐Specific Monoclonal Antibodies

Suppressor or "regulatory" CD4 T cells play a key role in the control of autoimmunity and overshooting immune responses to foreign antigens, but can also obstruct effective anticancer therapies. The homeostasis and activation of these regulatory T cells (Treg cells) is tightly connected to that of effector CD4 T cells via the costimulatory receptor CD28 and the cytokine IL-2: Both subsets require costimulation to be activated by antigen, and Treg cells additionally depend on IL-2 produced by effector CD4 T cells in a costimulation-dependent fashion. Depending on the therapeutic aim, blockade, or stimulation of CD28 with monoclonal antibodies (mAb) can therefore profoundly affect the size and activity of the Treg compartment. In this chapter, experiments performed in rodents with distinct types of CD28-specific mAb, and the recent failure to translate CD28-driven Treg activation into humans, are discussed.

Immune tolerance and control of CNS autoimmunity: from animal models to MS patients

Multiple sclerosis (MS) is a chronic inflammatory disease resulting in demyelination and axonal loss within the CNS. An autoimmune reaction directed against myelin antigens contributes to the disease process. As the CNS has long been considered an immune privileged site, how such an immune response can develop locally has remained enigmatic. Recent data, mostly based on the study of animal models for MS, have shown that the CNS is in fact more permissive to the development of immune responses than previously thought. This observation is counterbalanced by the fact that immune tolerance to myelin antigens can be induced outside the CNS. This review focuses on the mechanisms preventing CNS autoimmunity, which act in three separate tissues. In the thymus, expression of CNS autoantigens promotes partial protection, notably through elimination of autoreactive T cells. In the secondary lymphoid organs, the remaining autoreactive T cells are kept under control by the naturally occurring regulatory T cells of the CD4(+)Foxp3(+) phenotype. In the CNS, multiple mechanisms including the local activation of regulatory T cells further limit autoimmunity. A better understanding of the induction of regulatory T cells, of their mechanisms of action, and of approaches to manipulate them in vivo may offer new therapeutic opportunities for MS patients.