Immune Response-Dependent Assembly of IMP Dehydrogenase Filaments.

Inosine monophosphate dehydrogenase (IMPDH) catalyzes the conversion of IMP to xanthosine monophosphate, the rate-limiting step in de novo guanosine monophosphate (GMP) synthesis. In cultured cells, IMPDH polymerizes into micron-scale filamentous structures when GMP synthesis is inhibited by depletion of purine precursors or by various drugs, including mycophenolic acid, ribavirin, and methotrexate. IMPDH filaments also spontaneously form in undifferentiated mouse embryonic stem cells and induced pluripotent stem cells, hinting they might function in various highly proliferative cell types. Therefore, we investigated IMPDH filament formation in human and murine T cells, which rely heavily on de novo guanine nucleotide synthesis to rapidly proliferate in response to antigenic challenge. We discovered extensive in vivo IMPDH filament formation in mature T cells, B cells, and other proliferating splenocytes of normal, adult B6 mice. Both cortical and medullary thymocytes in young and old mice also showed considerable assembly of IMPDH filaments. We then stimulated primary human peripheral blood mononuclear cells ex vivo with T cell mitogens phytohemagglutinin (PHA), concanavalin A (ConA), or antibodies to CD3 and CD28 for 72 h. We detected IMPDH filaments in 40-60% of T cells after activation compared to 0-10% of unstimulated T cells. Staining of activated T cells for the proliferation marker Ki-67 also showed an association between IMPDH filament formation and proliferation. Additionally, we transferred ovalbumin-specific CD4+ T cells from B6.OT-II mice into B6.Ly5a recipient mice, challenged these mice with ovalbumin, and harvested spleens 6 days later. In these spleens, we identified abundant IMPDH filaments in transferred T cells by immunofluorescence, indicating that IMPDH also polymerizes during in vivo antigen-specific T cell activation. Overall, our data indicate that IMPDH filament formation is a novel aspect of T cell activation and proliferation, and that filaments might be useful morphological markers for T cell activation. The data also suggest that in vivo IMPDH filament formation could be occurring in a variety of proliferating cell types throughout the body. We propose that T cell activation will be a valuable model for future experiments probing the molecular mechanisms that drive IMPDH polymerization, as well as how IMPDH filament formation affects cell function.

Epitopes of Immunoreactive Proteins of Streptococcus Agalactiae: Enolase, Inosine 5'-Monophosphate Dehydrogenase and Molecular Chaperone GroEL.

Three Streptococcus agalactiae (group B streptococci, GBS) immunoreactive proteins: enolase (47.4 kDa), inosine 5'-monophosphate dehydrogenase (IMPDH) (53 kDa) and molecular chaperone GroEL (57 kDa) were subjected to investigation. Enolase protein was described in our previous paper, whereas IMPDH and GroEL were presented for the first time. The aim of our paper was to provide mapping of specific epitopes, highly reactive with umbilical cord blood serum. Bioinformatic analyses allowed to select 32 most likely epitopes for enolase, 36 peptides for IMPDH and 41 immunoreactive peptides for molecular chaperone GroEL, which were synthesized by PEPSCAN. Ten peptides: two in enolase, one in IMPDH and seven in molecular chaperone GroEL have been identified as potentially highly selective epitopes that can be used as markers in rapid immunological diagnostic tests or constitute a component of an innovative vaccine against GBS infections.

Mycophenolic acid concentrations in peripheral blood mononuclear cells are associated with the incidence of rejection in renal transplant recipients.

AIMS: Although therapeutic drug monitoring of plasma mycophenolic acid (MPA) concentrations has been recommended to individualize dosage in transplant recipients, little is known regarding lymphocyte concentrations of MPA, where MPA inhibits inosine monophosphate dehydrogenase (IMPDH). This study investigated the utility of measuring predose MPA concentrations in peripheral blood mononuclear cells (C0C ) and predose IMPDH activity, as predictors of graft rejection in renal transplant recipients. METHODS: Forty-eight patients commencing mycophenolate mofetil (1 g twice daily) in combination with tacrolimus and prednisolone were recruited. Blood was collected for determination of trough total (C0P ) and unbound (C0u ) plasma MPA concentrations. Peripheral blood mononuclear cells were isolated for determination of C0C and IMPDH activity. The incidence of rejection within 2 days of sample collection was determined histologically and classified according to the Banff 2007 criteria. RESULTS: There was no association between MPA C0C and C0P (rs  = 0.28, P = 0.06), however, MPA C0C were weakly correlated with MPA C0u (rs  = 0.42, P = 0.013). Multivariate analysis indicated that MPA C0C was the only covariate independently associated with rejection (FDR-adjusted P = 0.033). The receiver operating characteristic area under the curve (AUC) for the prediction of severe rejection using MPA C0C was 0.75 (P = 0.013), with 73% sensitivity and specificity at a C0C threshold of 0.5 ng 10-7 cells. However, predose IMPDH activity was not a predictor of rejection (P > 0.15). CONCLUSIONS: MPA C0C measurement within the early post-transplant period may be useful to facilitate early titration of MPA dosing to significantly reduce rejection.

Mycophenolic mofetil, an alternative antiviral and immunomodulator for the highly pathogenic avian influenza H5N1 virus infection.

Infection with the highly pathogenic avian influenza H5N1 virus results in a high incidence of mortality in humans. Severe complications from infection are often associated with hypercytokinemia. However, current neuraminidase inhibitors (NAIs) have several limitations including the appearance of oseltamivir-resistant H5N1 virus and the inability to completely ameliorate hyper-immune responses. To overcome these limitations, we evaluated the anti-viral activity of mycophenolic mofetil (MMF) against A/Vietnam/1194/2004 (H5N1) virus infection using MDCK cells and mice. The IC50 of MMF (0.94 µM) was comparable to that of zanamivir (0.87 µM) in H5N1 virus-infected MDCK cells based on ELISA. Time-course assays demonstrated that MMF completely inhibited H5N1 viral mRNA replication and protein expression for approximately 8 h after the initiation of treatment. In addition, MMF treatment protected 100% of mice, and lung viral titers were substantially reduced. The anti-viral mechanism of MMF against H5N1 virus infection was further confirmed to depend on the inhibition of cellular inosine monophosphate dehydrogenase (IMPDH) by exogenous guanosine, which inhibits viral mRNA and protein expression. Moreover, IL-1ß, IFN-ß, IL-6, and IP-10 mRNA expression levels were significantly downregulated in MDCK cells with MMF treatment. These results indicated that MMF could represent a novel inhibitor of viral replication and a potent immunomodulator for the treatment of H5N1 virus infection.

Autoantibodies against "rods and rings"-related IMPDH2 in hepatitis C genotype 1 and DAA therapy in a "real life" cohort.

Autoantibodies against inosine-5'-monophosphate-dehydrogenase-2 (IMPDH2; "rods and rings" pattern) develop in chronic hepatitis C (CHC) patients under treatment with peg-interferon (IFN) and ribavirin (RBV), an inhibitor of IMPDH2. We investigated the influence of the alternative therapy with direct-acting antivirals (DAA)/ribavirin on anti-IMPDH2 autoantibody generation and the use of anti-IMPDH2 development as a marker for therapy outcome (sustained virologic response, SVR). We analyzed a "real life" cohort of 104 unselected CHC genotype 1 (GT1) patients treated with IFN/first-generation DAA/RBV prospectively compared to a historic cohort of 59 IFN/RBV-treated CHC GT1 patients. First-generation DAA were boceprevir (BOC) or telaprevir (TPR). Serum autoantibodies were tested by indirect immunofluorescence (IFA) using recombinant IMPDH2 expressing HEK293 cells and native HEp2-cells as substrates. 64/163 (39%) CHC patients turned anti-IMPDH2 positive during therapy, but only 43/163 (26%) showed also "rods and rings" structures. 99/163 (61%) were tested as anti-IMPDH2 negative. 53/104 (51%) CHC patients undergoing IFN/DAA/RBV therapy were anti-IMPDH2 positive and 38/104 (37%) were in parallel anti-"rods and rings" positive. HCV clearance/SVR rate after IFN/DAA/RBV therapy and anti-IMPDH2 status were not significantly dependent. CHC GT1 patients treated with IFN/first-generation DAA/RBV developed anti-IMPDH2 autoantibodies comparable to previous studies including patients under IFN/RBV therapy. Anti-IMPDH2 titers show no use as a marker for therapy outcome in CHC GT1 patients.

The raspberry Gene Is Involved in the Regulation of the Cellular Immune Response in Drosophila melanogaster.

Drosophila is an extremely useful model organism for understanding how innate immune mechanisms defend against microbes and parasitoids. Large foreign objects trigger a potent cellular immune response in Drosophila larva. In the case of endoparasitoid wasp eggs, this response includes hemocyte proliferation, lamellocyte differentiation and eventual encapsulation of the egg. The encapsulation reaction involves the attachment and spreading of hemocytes around the egg, which requires cytoskeletal rearrangements, changes in adhesion properties and cell shape, as well as melanization of the capsule. Guanine nucleotide metabolism has an essential role in the regulation of pathways necessary for this encapsulation response. Here, we show that the Drosophila inosine 5'-monophosphate dehydrogenase (IMPDH), encoded by raspberry (ras), is centrally important for a proper cellular immune response against eggs from the parasitoid wasp Leptopilina boulardi. Notably, hemocyte attachment to the egg and subsequent melanization of the capsule are deficient in hypomorphic ras mutant larvae, which results in a compromised cellular immune response and increased survival of the parasitoid.

Anti-rods/rings autoantibody generation in hepatitis C patients during interferon-α/ribavirin therapy.

Chronic inflammation associated with hepatitis C virus (HCV) infection can lead to disabling liver diseases with progression to liver cirrhosis and hepatocellular carcinoma. Despite the recent availability of more effective and less toxic therapeutic options, in most parts of the world the standard treatment consists of a weekly injection of pegylated interferon α (IFN-α) together with a daily dose of ribavirin. HCV patients frequently present circulating non-organ-specific autoantibodies demonstrating a variety of staining patterns in the indirect immunofluorescence assay for antinuclear antibodies (ANA). Between 20% to 40% of HCV patients treated with IFN-α and ribavirin develop autoantibodies showing a peculiar ANA pattern characterized as rods and rings (RR) structures. The aim of this article is to review the recent reports regarding RR structures and anti-rods/rings (anti-RR) autoantibody production by HCV patients after IFN-α/ribavirin treatment. Anti-RR autoantibodies first appear around the sixth month of treatment and reach a plateau around the twelfth month. After treatment completion, anti-RR titers decrease/disappear in half the patients and remain steady in the other half. Some studies have observed a higher frequency of anti-RR antibodies in relapsers, i.e., patients in which circulating virus reappears after initially successful therapy. The main target of anti-RR autoantibodies in HCV patients is inosine-5'-monophosphate dehydrogenase 2 (IMPDH2), the rate-limiting enzyme involved in the guanosine triphosphate biosynthesis pathway. Ribavirin is a direct IMPDH2 inhibitor and is able to induce the formation of RR structures in vitro and in vivo. In conclusion, these observations led to the hypothesis that anti-RR autoantibody production is a human model of immunologic tolerance breakdown that allows us to explore the humoral autoimmune response from the beginning of the putative triggering event: exposure to ribavirin and interferon.

Interferon-ribavirin therapy induces serum antibodies determining 'rods and rings' pattern in hepatitis C patients.

A cytoplasmic antigen associated to inosine-5'-monophosphatedehydrogenase 2 eliciting specific antibodies (antirods and rings, RR) has been identified in patients with chronic hepatitis C who were exposed to pegylated interferon (PI) and ribavirin (RBV). The significance of anti-RR in these patients merits to be investigated. Sera from 88 chronic hepatitis C virus (HCV)-infected patients undergoing PI-RBV therapy were analysed for the presence of RR pattern by indirect immunofluorescence on HEp-2 substrate (Inova Diagnostics, San Diego, CA, USA). Anti-RR antibodies developed de novo in 32 patients independently of any demographic and virological feature, but with a significant association with cumulative exposure to PI-RBV (P = 0.0089; chi-square test). RR pattern was significantly more frequent in relapsers than in patients achieving sustained virological response (56% vs 30%; P = 0.0282, chi-square test). Anti-RR titre ranged from 1:80 to 1:1280, but significantly declined following treatment cessation. Anti-RR develop de novo in a substantial proportion of patients exposed to PI-RBV in relation to the duration of treatment exposure. Further investigations are necessary to unravel the mechanisms leading to the formation of these autoantibodies.

Temporal evolution of human autoantibody response to cytoplasmic rods and rings structure during anti-HCV therapy with ribavirin and interferon-α.

Autoantibodies to inosine monophosphate dehydrogenase-2 (IMPDH2), an enzyme involved in de novo biosynthesis of guanine nucleotides, are observed in a subset of hepatitis C virus (HCV) patients receiving interferon alpha (IFN-α) plus ribavirin. Anti-IMPDH2 antibodies display a peculiar cytoplasmic "rod/ring" (RR) pattern in IIF-HEp-2. We examined the dynamics of anti-RR autoimmune response with respect to immunoglobulin isotypes, titer, avidity, and protein targets in 80 sequential samples from 15 HCV patients (plus 12 randomly selected anti-RR-positive, totalizing 92 samples) collected over an 18-month period, including samples collected before, during, and after IFN-α + ribavirin treatment. Immunoprecipitation showed reactivity with the 55 kDa IMPDH2 protein in 12/15 patients (80 %) and 11/15 (73 %) reacted with IMPDH2 in a sandwich ELISA. During treatment, anti-IMPDH2 autoantibodies hit their highest levels after 6-12 months of treatment and decreased post-treatment, while anti-HCV antibodies levels were stable over time. Anti-IMPDH2 IgM levels increased up until the sixth month of treatment and remained stable thereafter, while IgG levels increased steadily up to the twelfth month. Both IgG and IgM decreased during the post-treatment period. IgG avidity increased steadily up to the twelfth month of treatment. In conclusion, this study showed that the temporal kinetics of IFN-α + ribavirin-induced humoral autoimmune response to IMPDH2 exhibited a considerably delayed pace of increase in antibody levels and avidity as well as in isotype class switch in comparison with a conventional humoral response to infectious agents. These unique findings uncover intriguing differences between the autoimmune response and the immune response to exogenous agents in humans.

Clinical associations and potential novel antigenic targets of autoantibodies directed against rods and rings in chronic hepatitis C infection.

BACKGROUND: Chronic hepatitis C virus (HCV) infection is frequently associated with extrahepatic autoimmune disorders while interferon (IFN) and ribavirin treatment may exacerbate these conditions. Autoantibodies from HCV patients identify a novel indirect immunofluorescence (IIF) pattern on HEp-2 cells characterized by cytoplasmic rods and rings (RR). Our objectives were to determine the prevalence and clinical associations of RR autoantibodies in HCV patients, and identify related novel autoantibody targets. METHODS: Sera from 315 patients with HCV (301 treatment naive, 14 treated with interferon and/or ribavirin) were analyzed for the presence of RR antibodies by IIF on commercially available HEp-2 cell substrates. Antibodies to inosine monophosphate dehydrogenase 2 (IMPDH2) and cytidine triphosphate synthase 1 (CTPS1) were detected by addressable laser bead assay and other potential targets were identified by immunoscreening a protein microarray. Clinical and demographic data including HCV genotype, mode of infection, prior antiviral therapy, and histological findings were compared between RR antibody positive (RR+) and negative (RR-) patients. RESULTS: The median age of the HCV cohort was 51 years, 61% were male, and 76% were infected with HCV genotype 1 (G1). Four percent (n=14) had been treated with IFN-based therapy (IFN monotherapy, n=3; IFN/ribavirin, n=11); all had a sustained virologic response. In total, 15 patients (5% of the cohort) were RR+. RR+ and RR- patients had similar demographic and clinical characteristics including age, sex, mode of HCV infection, prevalence of the G1 HCV genotype, and moderate to severe fibrosis. Nevertheless, RR+ patients were significantly more likely than RR- cases to have been treated with IFN-based therapy (33% vs. 3%; adjusted odds ratio 20.5 [95% confidence interval 5.1-83.2]; P<0.0005). Only 1/10 RR positive sera had detectable antibodies to IMPHD2 and none had antibodies to CTPS1. Potentially important autoantibody targets identified on protein arrays included Myc-associated zinc finger protein (MAZI) and ankyrin repeat motif. CONCLUSION: The majority of HCV patients with RR autoantibodies previously received IFN/ribavirin antiviral therapy. Further studies are necessary to determine the genesis of intracellular RR and elucidate the clinically relevant autoantigens as well as the clinical and prognostic significance of their cognate autoantibodies.

Development of a recombinant cell-based indirect immunofluorescence assay (RC-IFA) for the determination of autoantibodies against "rings and rods"-associated inosine-5'-monophosphate dehydrogenase 2 in viral hepatitis C.

BACKGROUND AND AIMS: Autoantibodies against so-called "rings and rods" structures, as determined by indirect immunofluorescence assay (IFA) using the human cell line HEp-2, have been described in chronic hepatitis C virus (HCV) infected patients treated with interferon/ribavirin. Recently, cytidine triphosphate synthase (CTPS) and inosine-5'-monophosphate dehydrogenase 2 (IMPDH2), the enzyme inhibited by ribavirin, were proposed as the target antigens. We wanted to confirm the identification and setup a robust system for autoantibody testing in routine laboratories. METHODS: CTPS and IMPDH2 were individually expressed in HEK293 cells and the recombinant cells were used in IFA (RC-IFA) to analyze sera from 33 anti-"rings and rods" antibody positive individuals with unknown diagnosis, 50 patients with chronic HCV infection, 100 with autoimmune hepatitis, 50 with primary biliary cirrhosis and 50 healthy blood donors. RESULTS: We found that all sera with anti-"rings and rods" reacted with recombinant IMPDH2 but none with CTPS. In western blot or ELISA, anti-IMPDH2 positive sera reacted only weakly, if at all, with Escherichia coli derived recombinant IMPDH2 indicating that the autoantibody reaction probably depends on the 3-dimensional conformation of the antigen. A rabbit hyperimmune serum raised against bacterially expressed IMPDH2 produced the ring/rods pattern in IFA using HEp-2. CONCLUSIONS: We conclude, that IMPDH2 is indeed the main target of anti-"rings and rods" while CTPS is an unlikely target. Moreover, the novel RC-IFA test system allows a standardized semi-quantitative determination of anti-IMPDH2 basing on a defined recombinant antigen.

Differential reactivity to IMPDH2 by anti-rods/rings autoantibodies and unresponsiveness to pegylated interferon-alpha/ribavirin therapy in US and Italian HCV patients.

PURPOSE: Autoantibodies to cytoplasmic structures called rods and rings (RR) are primarily specific to patients with hepatitis C virus (HCV) infection treated with pegylated interferon-alpha/ribavirin (IFN/R). Our aim is to examine anti-RR antibodies specificity and correlation with the response to IFN/R therapy in two independent cohorts (US and Italy) of HCV patients. METHODS: Sera from the US cohort (n = 47) and the Italian cohort (n = 46) pre-selected for anti-RR antibodies were analyzed by immunofluorescence and radioimmunoprecipitation. The prevalence and titers of anti-RR were analyzed for correlation with the response to IFN/R therapy. RESULTS: In the US cohort, anti-RR antibodies were more frequently non-responders to IFN/R (71 % vs 29 % responders). Titers in responder patients (n = 11) were ≤1:3200, whereas titers in non-responder patients (n = 27) reached 1:819,200 (p = 0.0016). In the Italian cohort, anti-RR titers ranged from 1:200 to >1:819,200 and only relapsers had the highest anti-RR titers. Radioimmunoprecipitation demonstrated that anti-RR autoantibodies were mainly anti-inosine monophosphate dehydrogenase 2 (IMPDH2) - 96 % in the Italian cohort vs. 53 % in the US cohort. CONCLUSIONS: In the two cohorts analyzed, the anti-IMPDH2 response as a component of the anti-RR response is much more prominent in the Italian cohort. The reason for the difference between the US and Italian cohorts is unclear but it possibly illustrates the heterogeneity in response and the overall negative correlation between the production of these autoantibodies and response to IFN/R therapy. Patients with high titer anti-RR antibodies are either relapsers (Italian) or non-responders/relapsers (US).

Mizoribine--an inosine monophosphate dehydrogenase inhibitor--acts synergistically with cyclosporine A in prolonging survival of murine islet cell and heart transplants across major histocompatibility barrier.

INTRODUCTION: Mizoribine (MZR) is an inosine monophosphate dehydrogenase inhibitor. It has been widely used in Japan in the treatment of autoimmune diseases and is known to inhibit T and B cell proliferation. The aim of this study was to evaluate the efficacy of MZR as an immunosuppressive agent and determine its ability to synergize with a commonly used calcineurin inhibitor Cyclosporine A (CsA) in prolonging survival of murine islet cells and heart transplanted across major histocompatibility barrier. METHODS: Murine allogeneic islet cell transplantation between Balb/c donor mice and C57BL/6 recipient mice and heterotopic heart transplantation was done between C3H/He donor mice and Balb/c recipient mice. Recipients were divided into groups based on immunosuppression: Group 1-No immunosuppression, Group 2-MZR alone (20 mg/kg/day), Group 3-CsA alone (20 mg/kg/day), Group 4-MZR+CsA (20 mg/kg/day). Donor specific IFN-γ, IL-10, IL-2, IL-4 secreting cells were enumerated by ELISpot. Serum cytokine and chemokine concentration was measured by Luminex. RESULTS: Islet cell allograft recipients treated with CsA and MZR had prolonged islet function compared to other groups [normoglycemia (blood glucose <200 mg/dL) up to 32±4 days, p<0.05]. Similarly, heart allograft survival was significantly improved in mice treated with CsA and MZR compared to other groups (50% 30-day survival, p=0.04). Donor specific IFN-γ, IL-4, IL-2 secreting cells were significantly decreased in recipients treated with CsA and MZR with marked increase in IL-10 secreting cells (p<0.05). There was also an increase in serum IL-10 with decrease in IFN-γ, IL-4, IL-2, MCP-1, and IL-6 in mice treated with CsA and MZR CONCLUSION: MZR and CsA when used in combination are potent immunosuppressive agents in murine islet cell and heart transplantation models. These agents lead to a decrease in donor specific IFN-γ with increase in IL-10 secreting cells leading to improved allograft survival and function.

Autoantibodies against inosine-5'-monophosphate dehydrogenase 2--characteristics and prevalence in patients with HCV-infection.

BACKGROUND: Indirect immunofluorescence (IIFT) on in house HEp-2 cell preparations revealed a novel antibody giving a granular cytoplasmic pattern not described before, which on two commercial cell preparations revealed a "rings and rods" pattern. This pattern was also observed in four HCV-RNA carriers and prompted the identification of the reactive antigen and the evaluation of the antibody prevalence in HCV-RNA carriers and control groups. METHODS: The antigen's molecular weight was determined by radioimmunoprecipitation of 35S-methionine labeled cell proteins. Expression library screening and sequencing was performed by standard techniques using an oligo(dT)-primed human HeLa cell cDNA expression library. Antibodies against the novel antigen Inositol-5'-monophosphatdehydrogenase 2 (IMPDH2) were analyzed by IIFT, western blot, line blot, and radioimmunoprecipitation assay (RIPA). IIFT was performed on commercial HEp-2 cells and cells cultivated in house for 24 - 60 hours, with or without the IMPDH2 inhibitors mycophenolic acid (MPA) or ribavirin, and subjected to various fixation conditions. Western and line blots were performed with IMPDH2 synthesized in E. coli, RIPA with 35S-methionine-IMPDH2 from in vitro transcription/translation products. Sera screened were positive for HCV-RNA (108), HBV-DNA (100), anti-mitochondrial (31), anti-actin (42), and anti-nuclear antibodies (51) and negative for HCV-RNA (100) and blood donors (100). RESULTS: IMPDH2 is capable of considerable intracellular rearrangements (upon action of inhibitors like MPA and ribavirin), which explains the contrasting immunofluorescence patterns in cells from different sources. By RIPA, proven to be the sole assay suitable for screening of anti-IMPDH2 in human sera, autoantibodies were found in 35.2% of HCV-RNA carriers and in low concentrations in 31% of anti-actin positive patients suspicious of autoimmune hepatitis. Antibodies reacted preferentially with conformational epitopes. Compared to the low concentration of anti-IMPDH2 found in other disease groups, high antibody concentrations were observed in HCV-RNA carriers. CONCLUSIONS: The common occurrence of anti-IMPDH2 in HCV-RNA carriers may be related to ribavirin therapy, causing intracellular aggregation of IMPDH2 thereby altering its immunogenicity. In this study the "rods and rings" immunofluorescence pattern observed could be ascribed to anti-IMPDH2. Anti-IMPDH2 may cause difficulties in interpretation of immunofluorescence patterns in routine autoantibody testing.

IMPDHII protein inhibits Toll-like receptor 2-mediated activation of NF-kappaB.

Toll-like receptor 2 (TLR2) plays an essential role in innate immunity by the recognition of a large variety of pathogen-associated molecular patterns. It induces its recruitment to lipid rafts induces the formation of a membranous activation cluster necessary to enhance, amplify, and control downstream signaling. However, the exact composition of the TLR2-mediated molecular complex is unknown. We performed a proteomic analysis in lipopeptide-stimulated THP1 and found IMPDHII protein rapidly recruited to lipid raft. Whereas IMPDHII is essential for lymphocyte proliferation, its biologic function within innate immune signal pathways has not been established yet. We report here that IMPDHII plays an important role in the negative regulation of TLR2 signaling by modulating PI3K activity. Indeed, IMPDHII increases the phosphatase activity of SHP1, which participates to the inactivation of PI3K.

The immunosuppressor mycophenolic acid kills activated lymphocytes by inducing a nonclassical actin-dependent necrotic signal.

Mycophenolate mofetil (MMF) is an immunosuppressive agent used in transplantation. Over the last decade, MMF has also emerged as an alternative therapeutic regimen for autoimmune diseases, mainly for patients refractory to other therapies. The active compound of MMF, mycophenolic acid (MPA), depletes the intracellular pool of guanosine tri-phosphate through inosine monophosphate dehydrogenase blockade. The molecular mechanism involved in the elimination of T and B lymphocytes upon inhibition of inosine monophosphate dehydrogenase remains elusive. In this study, we showed that in contrast to the immunosuppressors azathioprine, cyclosporin A, and tacrolimus, MPA killed lymphocytes through the activation of a caspase-independent necrotic signal. Furthermore, the MPA-mediated necrotic signal relied on the transmission of a novel intracellular signal involving Rho-GTPase Cdc42 activity and actin polymerization. In addition to its medical interest, this study sheds light on a novel and atypical molecular mechanism leading to necrotic cell death.