T cell aggregation induced through CD43: intracellular signals and inhibition by the immunomodulatory drug leflunomide

SOX11 inhibits tumor proliferation and promotes cell adhesion mediated-drug resistance via a CD43 dependent manner in mantle cell lymphoma

SOX11 is a critical biomarker for mantle cell lymphoma (MCL) diagnosis; however, its role remains unclear in MCL. Here, clinical-pathological analysis showed Ki67 index was negatively relevant to SOX11 expression only in CD43 positive cases. Coexpression of SOX11/CD43 indicated longer overall survival. In vitro, knockout/overexpression of SOX11 or CD43 promoted/inhibited cell proliferation respectively. CD43 overexpression reversed tumor proliferation induced by SOX11 knockdown. Furthermore, overexpressing/silencing the SOX11/CD43 gene affects phosphorylation of p38-MAPK while p38 inhibitor reversed proliferation induced by si-SOX11 or si-CD43, respectively. In CAM-DR model, both SOX11 and CD43 in MCL cells were elevated when co-cultured with M2-10B4 bone marrow fibroblasts or fibronectin. Knockdown/overexpression of SOX11 decreased/increased cell adhesion, respectively, and the effect induced by silencing SOX11 was reversed by overexpression of CD43. Collectively, SOX11 could inhibit tumor proliferation and promote CAM-DR in a CD43 dependent manner.

TRPP2 dysfunction decreases ATP-evoked calcium, induces cell aggregation and stimulates proliferation in T lymphocytes

Background

Autosomal dominant polycystic kidney disease (ADPKD) is mainly characterised by the development and enlargement of renal cysts that lead to end-stage renal disease (ESRD) in adult patients. Other clinical manifestations of this pathology include hypertension, haematuria, abdominal pain, cardiovascular system alterations and intracranial aneurysms. ADPKD is linked to mutations in either PKD1 or PKD2 that codifies polycystin-1 (PC1) and polycystin-2 (PC2 or TRPP2), respectively. PC1 and TRPP2 are membrane proteins that function as receptor-channel elements able to regulate calcium homeostasis. The function of polycystins has been mainly studied in kidney cells; but the role of these proteins in T lymphocytes is not well defined.

Methods

T lymphocytes were produced from ADPKD1 and ADPKD2 patients as well as from non-ADPKD subjects undergoing renal replacement therapy (RRT) and healthy controls. Protein expression and phosphorylation levels were analysed by western blotting, cell proliferation was calculated by direct counting using trypan blue assay and intracellular calcium concentration was measured by Fura-2 method.

Results

PKD2 mutations lead to the significant reduction of TRPP2 expression in T lymphocytes derived from ADPKD patients. Furthermore, a smaller TRPP2 truncated protein in T lymphocytes of patients carrying the mutation R872X in PKD2 was also observed, suggesting that TRPP2 mutated proteins may be stably expressed. The silencing or mutation of PKD2 causes a strong reduction of ATP-evoked calcium in Jurkat cells and ADPKD2 T lymphocytes, respectively. Moreover, T lymphocytes derived from both ADPKD1 and ADPKD2 patients show increased cell proliferation, basal chemotaxis and cell aggregation compared with T lymphocytes from non-ADPKD subjects. Similarly to observations made in kidney cells, mutations in PKD1 and PKD2 dysregulate ERK, mTOR, NFkB and MIF pathways in T lymphocytes.

Conclusions

Because the alteration of ERK, mTOR, NFkB and MIF signalling found in T lymphocytes of ADPKD patients may contribute to the development of interstitial inflammation promoting cyst growth and kidney failure (ESRD), the targeting of inflammasome proteins could be an intriguing option to delay the progression of ADPKD.

Electronic supplementary material

The online version of this article (10.1186/s12882-019-1540-6) contains supplementary material, which is available to authorized users.

Vedolizumab-mediated integrin α4β7 blockade does not control HIV-1SF162 rebound after combination antiretroviral therapy interruption in humanized mice

OBJECTIVE

The combined combination antiretroviral therapy (cART) and anti-α4β7 RM-Act-1 antibody therapy allows macaques to durably control simian immunodeficiency virus (SIV) rebound after withdrawal of the interventions. Here, we aimed to investigate whether vedolizumab (VDZ), a clinical-grade humanized anti-α4β7 antibody, would have similar effects in controlling live HIV-1 infection in humanized mice.

DESIGN AND METHODS

The integrin α4β7 blockade by VDZ was evaluated on human primary memory CD4+ T (MEMT) cells and retinoic acid-induced gut-homing α4β7+MEMT cells (α4β7+MEMT) in vitro. The antiretroviral activity of VDZ was determined using pseudotyped R5-tropic HIV-1SF162, which displays binding activity to α4β7. The preventive and immunotherapeutic efficacies of VDZ were further investigated in humanized mice using the live HIV-1SF162 strain compared with RM-Act-1.

RESULTS

VDZ effectively and dose-dependently blocked the binding of mucosal vascular addressin cell adhesion molecule-1 (MAdCAM-1), the native ligand of α4β7, to α4β7+MEMT cells. HIV-1SF162 not only displayed binding capacity to α4β7-expressing cells, but also showed an increased infectivity in retinoic acid-induced α4β7+MEMT cells pretreated with VDZ. Moreover, VDZ failed to prevent live HIV-1SF162 infection and did not reduce the peak viral load when administrated prior to viral challenge in humanized mice. Lastly, in immunotherapeutic settings, the withdrawal of combined cART with either VDZ or RM-Act-1 resulted in an uncontrolled HIV-1SF162 rebound in humanized mice, whereas the α4β7 molecules remained significantly blocked on circulating CD4+ T cells.

CONCLUSION

VDZ neither prevents nor controls HIV-1SF162 infection both in vitro and in humanized mice. Our findings have significant implications to the clinical application of VDZ in HIV-1 preventive and immunotherapeutic interventions.

Primed polymorphonuclear leukocytes from hemodialysis patients enhance monocyte transendothelial migration

Increased counts and priming of peripheral polymorphonuclear leukocytes (PMNLs) are associated with future or ongoing atherosclerosis; however, the role of PMNLs in enhancing monocyte transendothelial migration is still unclear. Our aims were to examine endothelial and monocyte activation, transmigration, and posttransmigration activation induced ex vivo by in vivo primed PMNLs and the effect of antioxidants on the activation. A unique ex vivo coculture system of three cell types was developed in this study, enabling interactions among the following: primary human umbilical vein endothelial cells (HUVECs), monocytes (THP-1 cell line), and in vivo primed PMNLs from hemodialysis (HD) patients and healthy control (HC) subjects. The interactions among these cells were examined, and an intervention with superoxide dismutase and catalase was performed. Preexposed HUVECs to HD/HC PMNLs showed a significant monocyte transmigration yield, 120–170% above HCs. Monocyte exposure to HD PMNLs induced pre- and posttransmigration activation. When the three cell types were cocultivated at the same time, monocyte chemoattractant protein-1 protein levels released from HUVECs, and activation markers on HUVECs [CD54 and chemokine (C-X3-C motif) ligand 1] and monocytes [chemokine (C-X3-C) receptor 1 and chemokine (C-C motif) receptor 2] were increased. Monocyte transmigration yield decreased to 70% (compared with HC subjects) due to adherence and accumulation of monocytes to HUVECs. When superoxide dismutase and catalase were used, reduced HUVEC and monocyte activation markers brought the transmigration yields to control levels and abolished accumulation of monocytes, emphasizing the role of superoxide in this process. We conclude that peripheral primed PMNLs play a pivotal role in enhancing monocyte transendotelial migration, the hallmark of the atherosclerotic process. Primed PMNLs can be used as a mediator and a biomarker of atherosclerosis even before plaque formation.

NEW & NOTEWORTHY Primed polymorphonuclear leukocytes are key mediators in monocyte transendothelial migration, a new understanding of the initiation of endothelial dysfunction and monocyte activation, transmigration, and accumulation in the subendothelial layer.

An alternative mode of CD43 signal transduction activates pro-survival pathways of T lymphocytes

CD43 is one of the most abundant co-stimulatory molecules on a T-cell surface; it transduces activation signals through its cytoplasmic domain, contributing to modulation of the outcome of T-cell responses. The aim of this study was to uncover new signalling pathways regulated by this sialomucin. Analysis of changes in protein abundance allowed us to identify pyruvate kinase isozyme M2 (PKM2), an enzyme of the glycolytic pathway, as an element potentially participating in the signalling cascade resulting from the engagement of CD43 and the T-cell receptor (TCR). We found that the glycolytic activity of this enzyme was not significantly increased in response to TCR+CD43 co-stimulation, but that PKM2 was tyrosine phosphorylated, suggesting that it was performing moonlight functions. We report that phosphorylation of both Y¹⁰⁵ of PKM2 and of Y⁷⁰⁵ of signal transducer and activator of transcription 3 was induced in response to TCR+CD43 co-stimulation, resulting in activation of the mitogen-activated protein kinase kinase 5/extracellular signal-regulated kinase 5 (MEK5/ERK5) pathway. ERK5 and the cAMP response element binding protein (CREB) were activated, and c-Myc and nuclear factor-κB (p65) nuclear localization, as well as Bad phosphorylation, were augmented. Consistent with this, expression of human CD43 in a murine T-cell hybridoma favoured cell survival. Altogether, our data highlight novel signalling pathways for the CD43 molecule in T lymphocytes, and underscore a role for CD43 in promoting cell survival through non-glycolytic functions of metabolic enzymes.

Engagement of distinct epitopes on CD43 induces different co-stimulatory pathways in human T cells

Co‐receptors, being either co‐stimulatory or co‐inhibitory, play a pivotal role in T‐cell immunity. Several studies have indicated that CD43, one of the abundant T‐cell surface glycoproteins, acts not only as a potent co‐receptor but also as a negative regulator for T‐cell activation. Here we demonstrate that co‐stimulation of human peripheral blood (PB) T cells through two distinct CD43 epitopes recognized by monoclonal antibodies (mAb) CD43‐6E5 (T_6E5‐act) and CD43‐10G7 (T_10G7‐act) potently induced T‐cell proliferation. However, T‐cell co‐stimulation through two CD43 epitopes differentially regulated activation of nuclear factor of activated T cells (NFAT) and nuclear factor‐κB (NF‐κB) transcription factors, T‐cell cytokine production and effector function. T_6E5‐act produced high levels of interleukin‐22 (IL‐22) and interferon‐γ (IFN‐γ) similar to T cells activated via CD28 (T_CD28‐act), whereas T_10G7‐act produced low levels of inflammatory cytokines but higher levels of regulatory cytokines transforming growth factor‐β (TGF‐β) and interleukin‐35 (IL‐35). Compared with T_6E5‐act or to T_CD28‐act, T_10G7‐act performed poorly in response to re‐stimulation and further acquired a T‐cell suppressive function. T_10G7‐act did not directly inhibit proliferation of responder T cells, but formed stable heterotypic clusters with dendritic cells (DC) via CD2 to constrain activation of responder T cells. Together, our data demonstrate that CD43 is a unique and polarizing regulator of T‐cell function.

Pseudorabies virus triggers glycoprotein gE-mediated ERK1/2 activation and ERK1/2-dependent migratory behavior in T cells

The interaction between viruses and immune cells of the host may lead to modulation of intracellular signaling pathways and to subsequent changes in cellular behavior that are of benefit for either virus or host. ERK1/2 (extracellular signal regulated kinase 1/2) signaling represents one of the key cellular signaling axes. Here, using wild-type and gE null virus, recombinant gE, and gE-transfected cells, we show that the gE glycoprotein of the porcine Varicellovirus pseudorabies virus (PRV) triggers ERK1/2 phosphorylation in Jurkat T cells and primary porcine T lymphocytes. PRV-induced ERK1/2 signaling resulted in homotypic T cell aggregation and increased motility of T lymphocytes. Our study reveals a new function of the gE glycoprotein of PRV and suggests that PRV, through activation of ERK1/2 signaling, has a substantial impact on T cell behavior.

IMPORTANCE

Herpesviruses are known to be highly successful in evading the immune system of their hosts, subverting signaling pathways of the host to their own advantage. The ERK1/2 signaling pathway, being involved in many cellular processes, represents a particularly attractive target for viral manipulation. Glycoprotein E (gE) is an important virulence factor of alphaherpesviruses, involved in viral spread. In this study, we show that gE has the previously uncharacterized ability to trigger ERK1/2 phosphorylation in T lymphocytes. We also show that virus-induced ERK1/2 signaling leads to increased migratory behavior of T cells and that migratory T cells can spread the infection to susceptible cells. In conclusion, our results point to a novel function for gE and suggest that virus-induced ERK1/2 activation may trigger PRV-carrying T lymphocytes to migrate and infect other cells susceptible to PRV replication.

CD147 and CD98 complex-mediated homotypic aggregation attenuates the CypA-induced chemotactic effect on Jurkat T cells

Homotypic cell aggregation plays important roles in physiological and pathological processes, including embryogenesis, immune responses, angiogenesis, tumor cell invasion and metastasis. CD147 has been implicated in most of these phenomena, and it was identified as a T cell activation-associated antigen due to its obvious up-regulation in activated T cells. However, the explicit function and mechanism of CD147 in T cells have not been fully elucidated. In this study, large and compact aggregates were observed in Jurkat T cells after treatment with the specific CD147 monoclonal antibody HAb18 or after the expression of CD147 was silenced by RNA interference, which indicated an inhibitory effect of CD147 in T cell homotypic aggregation. Knocking down CD147 expression resulted in a significant decrease in CD98, along with prominent cell aggregation, similar to that treated by CD98 and CD147 monoclonal antibodies. Furthermore, decreased cell chemotactic activity was observed following CD147- and CD98-mediated cell aggregation, and increased aggregation was correlated with a decrease in the chemotactic ability of the Jurkat T cells, suggesting that CD147- and CD98-mediated homotypic cell aggregation plays a negative role in T cell chemotaxis. Our data also showed that p-ERK, p-ZAP70, p-CD3ζ and p-LCK were significantly decreased in the CD147- and CD98-knocked down Jurkat T cells, which suggested that decreased CD147- and/or CD98-induced homotypic T cell aggregation and aggregation-inhibited chemotaxis might be associated with these signaling pathways. A role for CD147 in cell aggregation and chemotaxis was further indicated in primary CD4(+) T cells. Similarly, low expression of CD147 in primary T cells induced prominent cell aggregation and this aggregation attenuated primary T cell chemotactic ability in response to CypA. Our results have demonstrated the correlation between homotypic cell aggregation and the chemotactic response of T cells to CypA, and these data indicate that CD147 and CD98 might play important roles in cyclophilin-induced cell migration.

Molecular pharmacodynamics of new oral drugs used in the treatment of multiple sclerosis

New oral drugs have considerably enriched the therapeutic armamentarium for the treatment of multiple sclerosis. This review focuses on the molecular pharmacodynamics of fingolimod, dimethyl fumarate (BG-12), laquinimod, and teriflunomide. We specifically comment on the action of these drugs at three levels: 1) the regulation of the immune system; 2) the permeability of the blood-brain barrier; and 3) the central nervous system. Fingolimod phosphate (the active metabolite of fingolimod) has a unique mechanism of action and represents the first ligand of G-protein-coupled receptors (sphingosine-1-phosphate receptors) active in the treatment of multiple sclerosis. Dimethyl fumarate activates the nuclear factor (erythroid-derived 2)-related factor 2 pathway of cell defense as a result of an initial depletion of reduced glutathione. We discuss how this mechanism lies on the border between cell protection and toxicity. Laquinimod has multiple (but less defined) mechanisms of action, which make the drug slightly more effective on disability progression than on annualized relapse rate in clinical studies. Teriflunomide acts as a specific inhibitor of the de novo pyrimidine biosynthesis. We also discuss new unexpected mechanisms of these drugs, such as the induction of brain-derived neurotrophic factor by fingolimod and the possibility that laquinimod and teriflunomide regulate the kynurenine pathway of tryptophan metabolism.

Teriflunomide for oral therapy in multiple sclerosis

Teriflunomide, the active metabolite of an approved antirheumatic drug, is an emerging oral therapy for multiple sclerosis (MS). Next to the inhibition of pyrimidine biosynthesis and proliferation of activated lymphocytes, it seems to have multiple anti-inflammatory and immunomodulating effects. Phase II and III clinical trials in relapsing MS demonstrated favorable safety and tolerability of the drug, as well as clinical efficacy, with a significant reduction of relapse rate, comparable with those of the available injectable immunomodulatory agents. While multiple other studies with teriflunomide are currently ongoing, its exact place in future treatment algorithms for MS is difficult to predict. It may be a good alternative for patients wishing to have an oral treatment with relatively large data regarding long-term safety.

An update of teriflunomide for treatment of multiple sclerosis

There are a number of oral agents emerging as potential disease-modifying agents in multiple sclerosis (MS). Among these investigational agents, teriflunomide has shown promise in large, multicenter, phase III clinical trials with respect to safety and efficacy in relapsing MS patients, and is the latest disease-modifying agent approved for use in MS patients in the United States. This review will summarize teriflunomide’s historical development, clinical pharmacology, studies in animals, clinical trials, and safety data, and will end with a discussion of the role of teriflunomide in MS in the context of existing treatment options.

The active metabolite of leflunomide, A77 1726, protects rat hepatocytes against bile acid-induced apoptosis

BACKGROUND/AIMS

Leflunomide is used in the treatment of autoimmune diseases as an anti-inflammatory agent. Leflunomide and its active metabolite A77 1726 modulate mitogen-activated protein kinases (MAPK), Src kinases, the phosphoinositide-3 kinase (PI3K)/Akt-pathway and nuclear factor (NF)-kappaB activation. Both cell protective and cytotoxic effects of leflunomide have been described. Since leflunomide affects pathways involved in hepatocyte cell survival, we examined the effects of A77 1726 on hepatocyte cell death.

METHODS

Primary rat hepatocytes were exposed to the bile acid glycochenodeoxycholic acid (GCDCA), the superoxide anion donor menadione, or tumor necrosis factor (TNF) alpha in combination with actinomycin D. Activation of MAP-kinases was determined by Western blot analysis. Apoptosis and necrosis were analyzed by acridine orange staining and caspase activity and Sytox Green staining, respectively.

RESULTS

A77 1726 dose-dependently reduces GCDCA-induced apoptosis and necrosis, but not menadione- or TNFalpha/ActD-induced apoptosis. The hepatoprotective effect of A77 1726 does not involve ERK1/2, p38 or PI3K/Akt activation. A77 1726 does not inhibit NF-kappaB activation in hepatocytes.

CONCLUSIONS

Since A77 1726 inhibits bile acid-induced apoptosis and does not sensitize hepatocytes to TNFalpha, our results suggest that A77 1726 could be considered for the treatment of chronic liver diseases accompanied by elevated bile acid levels and inflammation.

CD43 promotes cell growth and helps to evade FAS-mediated apoptosis in non-hematopoietic cancer cells lacking the tumor suppressors p53 or ARF

CD43 is a highly glycosylated transmembrane protein expressed on the surface of most hematopoietic cells. Expression of CD43 has also been demonstrated in many human tumor tissues, including colon adenomas and carcinomas, but not in normal colon epithelium. The potential contribution of CD43 to tumor development is still not understood. Here, we show that overexpression of CD43 increases cell growth and colony formation in mouse and human cells lacking expression of either p53 or ARF (alternative reading frame) tumor-suppressor proteins. In addition, CD43 overexpression also lowers the detection of the FAS death receptor on the cell surface of human cancer cells, and thereby helps to evade FAS-mediated apoptosis. However, when both p53 and ARF proteins are present, CD43 overexpression activates p53 and suppresses colony formation due to induction of apoptosis. These observations suggest CD43 as a potential contributor to tumor development and the functional ARF-p53 pathway is required for the elimination of cells with aberrant CD43 expression.

Disruption of the interaction of T cells with antigen-presenting cells by the active leflunomide metabolite teriflunomide: involvement of impaired integrin activation and immunologic synapse formation

OBJECTIVE

Leflunomide, a potent disease-modifying antirheumatic drug of the isoxazole class, exhibits antiinflammatory, antiproliferative, and immunosuppressive effects by largely unknown mechanisms, although alterations of pyrimidine synthesis have been proposed. Successful immune responsiveness requires T cell activation by interaction with antigen-presenting cells (APCs), and integrin activation and formation of an immunologic synapse (IS). In this study, we evaluated the impact of the active leflunomide metabolite teriflunomide on T cell integrin activation, evolution of the IS, and antigen-specific formation of stable T cell/APC conjugates.

METHODS

Effects of pharmacologic concentrations of teriflunomide on CD3/CD28- and lymphocyte function-associated antigen 1-induced signal transduction and activation of primary human T cells were investigated. Furthermore, T cells were stimulated with superantigen- and antigen-pulsed APCs to study relocalization of molecules to the IS and T cell/APC conjugate formation.

RESULTS

Teriflunomide inhibited T cell receptor (TCR)/CD3-mediated calcium mobilization, but other critical T cell signaling events, including activation of MAPK and NF-kappaB, remained unaltered. In contrast, inhibition of TCR/CD3-triggered beta1,2 integrin avidity and integrin-mediated costimulation (outside-in signaling) by teriflunomide revealed a striking interference with integrin function that was independent of altered pyrimidine synthesis. Moreover, teriflunomide abolished molecule relocalization to the IS and induction of T cell/APC conjugates.

CONCLUSION

These data show that the active metabolite of leflunomide prevents the interaction of T cells with APCs to form an IS. Since IS formation is crucial for eliciting an immune response, this novel mechanism could underlie the beneficial effects of leflunomide in immune-mediated disorders such as rheumatoid arthritis.

PKCtheta is required for the activation of human T lymphocytes induced by CD43 engagement

The turnover of phosphoinositides leading to PKC activation constitutes one of the principal axes of intracellular signaling. In T lymphocytes, the enhanced and prolonged PKC activation resulting from the engagement of the TcR and co-receptor molecules ensures a productive T cell response. The CD43 co-receptor promotes activation and proliferation, by inducing IL-2 secretion and CD69 expression. CD43 engagement has been shown to promote phosphoinositide turnover and DAG production. Moreover, PKC activation was found to be required for the activation of the MAP kinase pathway in response to CD43 ligation. Here we show that CD43 engagement led to the membrane translocation and enzymatic activity of specific PKC isoenzymes: cPKC (alpha/beta), nPKC (epsilon and theta;), aPKC (zeta) and PKCmu. We also show that activation of PKCtheta; resulting from CD43 ligation induced CD69 expression through an ERK-dependent pathway leading to AP-1, NF-kappaB activation and an ERK independent pathway promoting NFAT activation. Together, these data suggest that PKCtheta; plays a critical role in the co-stimulatory functions of CD43 in human T cells.

Comparative analysis of T-cell costimulation and CD43 activation reveals novel signaling pathways and target genes

The CD43 lymphocyte surface receptor is involved in the regulation of lymphocyte adhesion and activation. Many CD43 functions remain controversial or unclear, and it is not known to which extent CD43 signaling pathways are shared with or distinct from those used by the T-cell receptor (TCR). Here, we systematically compared signaling events and target gene expression induced by CD43 or T-cell costimulation in primary human peripheral T cells. These studies identify nuclear factor-κB (NF-κB) p65 serine 468 as a novel inducible phosphorylation site strongly induced by T-cell costimulation and only weakly triggered by CD43 ligation. We also identified CD43 as a novel Jun N-terminal kinase (JNK) activator and a comprehensive analysis of further signaling events suggests that both stimuli use overlapping but also distinct signaling pathways. Microarray analysis of inflammatory genes shows 1 group of genes coregulated by both stimuli and 2 further groups of target genes affected solely by costimulation or primarily by CD43. (Blood. 2004;104:3302-3304)

Modulation of effector cell functions in experimental autoimmune encephalomyelitis by leflunomide--mechanisms independent of pyrimidine depletion

Leflunomide inhibits de novo pyrimidine synthesis and is a novel, immunosuppressive agent that has been successfully used to treat rheumatoid arthritis. Here, we investigated the efficacy of leflunomide and its mode of action in experimental autoimmune encephalomyelitis (EAE), which is a T helper cell type 1 cell-borne disease model to simulate inflammatory aspects of multiple sclerosis and was induced in Lewis rats by adoptive transfer of myelin basic protein (MBP)-specific T line cells. Given in vivo for 7 days after cell transfer, leflunomide suppressed clinical signs of disease even in uridine-substituted animals. MBP-specific T line cells that had been antigen-activated in vitro in the presence of A77 1726 (active metabolite of leflunomide) produced less interferon-gamma, whereas interleukin (IL)-10 secretion had a tendency to be increased without changes in signal transducer and activator of transcription 6 trafficking. Furthermore, these T cells exhibited reduced chemotaxis and induced a significantly mitigated disease course upon transfer into naive rats. The effects of leflunomide on MBP-specific memory type T line cells in vitro may not be mediated by pyrimidine depletion, as they were not reversible by exogenous uridine. Moreover, A77 1726 led to increased expression of CD86 (B7-2) and secretion of IL-10 in cultured microglial cells in vitro, strengthening their down-modulatory impact on activated, autoantigen-specific T cells. In conclusion, our observations underline that the immunomodulatory potential of leflunomide in effector cells of EAE is clinically relevant and is not exclusively dependent on the depletion of cellular pyrimidine pools.