Ubiquitin ligases in T cell activation and autoimmunity

Cbl-b restrains priming of pathogenic Th17 cells via the inhibition of IL-6 production by macrophages

E3 ubiquitin ligase Cbl-b is involved in the maintenance of a balance between immunity and tolerance. Mice lacking Cbl-b are highly susceptible to experimental autoimmune encephalomyelitis (EAE), a Th17-mediated autoimmune disease. However, how Cbl-b regulates Th17 cell responses remains unclear. In this study, utilizing adoptive transfer and cell type-specific Cblb knockout strains, we show that Cbl-b expression in macrophages, but not T cells or dendritic cells (DCs), restrains the generation of pathogenic Th17 cells and the development of EAE. Cbl-b inhibits IL-6 production by macrophages that is induced by signaling from CARD9-dependent C-type lectin receptor (CLR) pathways, which directs T cells to generate pathogenic Th17 cells. Therefore, our data unveil a previously unappreciated function for Cbl-b in the regulation of pathogenic Th17 responses.

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E3 ubiquitin ligase Cbl-b inhibits EAE disease progression

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Cbl-b dampens pathogenic Th17 response via inhibiting macrophage-derived IL-6

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Cbl-b controls macrophage-derived IL-6 via a CARD9-dependent manner

Modulation of B cell activation threshold mediated by BCR/CD40 costimulation by targeting Cbl-b for ubiquitination

It has been shown that CD40 is required for optimal B cell activation. Casitas-B-lineage lymphoma-b (Cbl-b), a RING finger E3 ubiquitin ligase, inhibits B cell activation. In this report, we demonstrate that CD40 stimulation markedly enhances IgM-induced B cell proliferation in wild-type (WT) mice, whereas this cell proliferation was reduced in CD40-deficient (Cd40 ^-/-) mice. Interestingly, CD40 ligation strongly augments IgM-induced Cbl-b ubiquitination and degradation in primary mouse B cells, which closely correlates with their proliferation capacity. Cbl-b deficiency uncouples BCR-induced B cell proliferation from CD40 costimulation. Our results indicate that Cbl-b negatively regulates costimulation of BCR and CD40, possibly by setting the threshold for B cell activation via controlling Cbl-b expression.

Regulation of C-Type Lectin Receptor-Mediated Antifungal Immunity

Of all the pathogen recognition receptor families, C-type lectin receptor (CLR)-induced intracellular signal cascades are indispensable for the initiation and regulation of antifungal immunity. Ongoing experiments over the last decade have elicited diverse CLR functions and novel regulatory mechanisms of CLR-mediated-signaling pathways. In this review, we highlight novel insights in antifungal innate and adaptive-protective immunity mediated by CLRs and discuss the potential therapeutic strategies against fungal infection based on targeting the mediators in the host immune system.

Protein Tyrosine Phosphatase SHP-1 Modulates T Cell Responses by Controlling Cbl-b Degradation

Previously, we demonstrated that CD28 and CTLA-4 signaling control Casitas-B-lineage lymphoma (Cbl)-b protein expression, which is critical for T cell activation and tolerance induction. However, the molecular mechanism(s) of this regulation remains to be elucidated. In this study, we found that Cbl-b fails to undergo tyrosine phosphorylation upon CD3 stimulation because SHP-1 is recruited to and dephosphorylates Cbl-b, whereas CD28 costimulation abrogates this interaction. In support of this finding, T cells lacking SHP-1 display heightened tyrosine phosphorylation and ubiquitination of Cbl-b upon TCR stimulation, which correlates with decreased levels of Cbl-b protein. The aberrant Th2 phenotype observed in T cell-specific Shp1(-/-) mice is reminiscent of heightened Th2 response in Cblb(-/-) mice. Indeed, overexpressing Cbl-b in T cell-specific Shp1(-/-) T cells not only inhibits heightened Th2 differentiation in vitro, but also Th2 responses and allergic airway inflammation in vivo. Therefore, SHP-1 regulates Cbl-b-mediated T cell responses by controlling its tyrosine phosphorylation and ubiquitination.

E3 Ubiquitin Ligase Cbl-b Regulates Thymic-Derived CD4+CD25+ Regulatory T Cell Development by Targeting Foxp3 for Ubiquitination

CD28 costimulation is essential for the development of thymic-derived CD4(+)CD25(+)Foxp3(+) regulatory T cells ("tTregs"). E3 ubiquitin ligase Cbl-b has been shown to regulate CD28 dependence of T cell activation. In this paper, we report that the loss of Cbl-b partially but significantly rescues the defective development of tTregs in Cd28(-/-) mice. This partial rescue is independent of IL-2. Mechanistically, Cbl-b binds to Foxp3 upon TCR stimulation and, together with Stub1, targets Foxp3 for ubiquitination and subsequently degradation in the proteasome. As Cbl-b self-ubiquitination and proteasomal degradation is impaired in Cd28(-/-) T cells, the defective development of tTregs in Cd28(-/-) mice may in part be due to increased Foxp3 ubiquitination and degradation targeted by Stub1 and Cbl-b. Treating Cd28(-/-) mice with a proteasome inhibitor completely rescues defective tTreg development in these mice. Therefore, Cbl-b, together with Stub1, ubiquitinate Foxp3, and regulate tTreg development.

Ubiquitination regulates expression of the serine/arginine-rich splicing factor 1 (SRSF1) in normal and systemic lupus erythematosus (SLE) T cells

T cells from patients with systemic lupus erythematosus (SLE) exhibit reduced expression of the critical T cell receptor (TCR)-associated CD3ζ signaling chain and are poor producers of the vital cytokine IL-2. By oligonucleotide pulldown and mass spectrometry discovery approaches, we identified the splicing regulator serine/arginine-rich splicing factor (SRSF) 1 or splicing factor 2/alternative splicing factor (SF2/ASF) to be important in the expression of CD3ζ chain. Importantly, increases in the expression of SRSF1 rescued IL-2 production in T cells from patients with SLE. In this study, we investigated the regulation of SRSF1 expression in resting and activated human T cells. We found that T cell stimulation induced a rapid and significant increase in mRNA expression of SRSF1; however, protein expression levels did not correlate with this increase. Co-engagement of CD28 induced a similar mRNA induction and reduction in protein levels. Proteasomal but not lysosomal degradation was involved in this down-regulation as evidenced by blocking with specific inhibitors MG132 and bafilomycin, respectively. Immunoprecipitation studies showed increased ubiquitination of SRSF1 in activated T cells. Interestingly, T cells from patients with SLE showed increased ubiquitination of SRSF1 when compared with those from healthy individuals. Our results demonstrate a novel mechanism of regulation of the splicing factor SRSF1 in human T cells and a potential molecular mechanism that controls its expression in SLE.

T cell activation threshold regulated by E3 ubiquitin ligase Cbl-b determines fate of inducible regulatory T cells

E3 ubiquitin ligase Casitas-B-lineage lymphoma protein-b (Cbl-b) is critical for establishing the threshold for T cell activation and is essential for induction of T cell anergy. Recent studies suggest that Cbl-b is involved in the development of CD4(+)CD25(+) inducible regulatory T cells (iTregs). In this study, we report that the optimal induction of Foxp3 by naive CD4(+)CD25(-) T cells requires suboptimal TCR triggering. In the absence of Cbl-b, the TCR strength for optimal Foxp3 induction is downregulated in vitro. Using TCR-transgenic Rag(-/-) mice in combination with Cbl-b deficiency, we show that in vivo iTreg development is also controlled by Cbl-b via tuning the TCR strength. Furthermore, we show that Akt-2 but not Akt-1 regulates Foxp3 expression downstream of Cbl-b. Therefore, we demonstrate that Cbl-b regulates the fate of iTregs via controlling the threshold for T cell activation.

E3 ubiquitin ligase Cbl-b regulates Pten via Nedd4 in T cells independently of its ubiquitin ligase activity

E3 ubiquitin ligase Cbl-b plays a crucial role in T cell activation and tolerance induction. However, the molecular mechanism by which Cbl-b inhibits T cell activation remains unclear. Here, we report that Cbl-b does not inhibit PI3K but rather suppresses TCR/CD28-induced inactivation of Pten. The elevated Akt activity in Cbl-b(-/-) T cells is therefore due to heightened Pten inactivation. Suppression of Pten inactivation in T cells by Cbl-b is achieved by impeding the association of Pten with Nedd4, which targets Pten K13 for K63-linked polyubiquitination. Consistent with this finding, introducing Nedd4 deficiency into Cbl-b(-/-) mice abrogates hyper-T cell responses caused by the loss of Cbl-b. Hence, our data demonstrate that Cbl-b inhibits T cell activation by suppressing Pten inactivation independently of its ubiquitin ligase activity.

Syk mediates BCR- and CD40-signaling integration during B cell activation

CD40 is essential for optimal B cell activation. It has been shown that CD40 stimulation can augment BCR-induced B cell responses, but the molecular mechanism(s) by which CD40 regulates BCR signaling is poorly understood. In this report, we attempted to characterize the signaling synergy between BCR- and CD40-mediated pathways during B cell activation. We found that spleen tyrosine kinase (Syk) is involved in CD40 signaling, and is synergistically activated in B cells in response to BCR/CD40 costimulation. CD40 stimulation alone also activates B cell linker (BLNK), Bruton tyrosine kinase (Btk), and Vav-2 downstream of Syk, and significantly enhances BCR-induced formation of complex consisting of, Vav-2, Btk, BLNK, and phospholipase C-gamma2 (PLC-γ2) leading to activation of extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase, Akt, and NF-κB required for optimal B cell activation. Therefore, our data suggest that CD40 can strengthen BCR-signaling pathway and quantitatively modify BCR signaling during B cell activation.

Age-related alterations of gene expression patterns in human CD8+ T cells

Aging is associated with progressive T-cell deficiency and increased incidence of infections, cancer and autoimmunity. In this comprehensive study, we have compared the gene expression profiles in CD8+ T cells from aged and young healthy subjects using Affymetrix microarray Human Genome U133A-2 GeneChips. A total of 5.2% (754) of the genes analyzed had known functions and displayed statistically significant age-associated expression changes. These genes were involved in a broad array of complex biological processes, mainly in nucleic acid and protein metabolism. Functional groups, in which downregulated genes were overrepresented, were the following: RNA transcription regulation, RNA and DNA metabolism, intracellular (Golgi, endoplasmic reticulum and nuclear) transportation, signaling transduction pathways (T-cell receptor, Ras/MAPK, JNK/Stat, PI3/AKT, Wnt, TGFbeta, insulin-like growth factor and insulin), and the ubiquitin cycle. In contrast, the following functional groups contained more up-regulated genes than expected: response to oxidative stress and cytokines, apoptosis, and the MAPKK signaling cascade. These age-associated gene expression changes may be responsible for impaired DNA replication, RNA transcription, and signal transduction, possibly resulting in instability of cellular and genomic integrity, and alterations of growth, differentiation, apoptosis and anergy in human aged CD8+ T cells.

Basic Science for the Clinician 40

mRNA is made from DNA. Protein is made from mRNA. Although one might say that "DNA is forever," the same cannot be said for mRNA or protein. These molecules are made in response to the cell's present needs; once the cell's circumstances change, a whole new repertoire of proteins may be needed and the previous set of proteins may be unnecessary, perhaps even deleterious. So, the cell must be able to eliminate the characters in the previous act in favor of the actors needed for the current act. In addition, there is good evidence that the DNA to mRNA to protein flow may not be efficient; abnormal proteins, as well as damaged or misfolded proteins, are quite common and must also be eliminated. This process depends on the ability of the cell to tag the protein to be eliminated with a small protein (or chain of these proteins) that targets the protein to a special structure for digestion into its constituent amino acids for recycling into new proteins. This very common protein tag was identified in the 1970s and called "ubiquitin"--it truly was everyplace! In addition, ubiquitin is crucial to targeting normal proteins to their appropriate place in or on the cell and for recycling of proteins. Ubiqutination of proteins and what follows this tagging are crucial to the normal function of cells. The complexity of these processes is being used for therapy in oncology now and perhaps in immunology and rheumatology in the near future.

E3 ubiquitin ligases and their control of T cell autoreactivity

A loss of T cell tolerance underlies the development of most autoimmune diseases. The design of therapeutic strategies to reinstitute immune tolerance, however, is hampered by uncertainty regarding the molecular mechanisms involved in the inactivation of potentially autoreactive T cells. Recently, E3 ubiquitin ligases have been shown to mediate the development of a durable state of unresponsiveness in T cells called clonal anergy. In this review, we will discuss the mechanisms used by E3 ligases to control the activation of T cells and prevent the development of autoimmunity.

Small ubiquitin-related modifiers: A novel and independent class of autoantigens in primary biliary cirrhosis

Serum autoantibodies against components of nuclear dots (anti-NDs), namely PML and Sp100, are specifically detected in 20% to 30% of patients with primary biliary cirrhosis (PBC). Although anti-ND antibodies are nonpathogenic, the mechanisms that lead to this unique reactivity are critical to understanding the loss of immune tolerance in PBC. Importantly, Sp100 and PML are both covalently linked to small ubiquitin-related modifiers (SUMOs). Therefore, we investigated whether SUMO proteins are independent autoantigens in PBC and studied 99 PBC sera samples for reactivity against NDs, PML, and Sp100, as well as against SUMO-2 and SUMO-1 recombinant proteins. Autoantibodies against SUMO-2 and SUMO-1 were found in 42% and 15% of anti-ND-positive PBC sera, respectively. Anti-SUMO reactivity was not observed in anti-ND-negative sera. Anti-SUMO-2 autoantibodies were found in 58% of sera containing autoantibodies against both PML and Sp100 and were detected exclusively in sera containing anti-Sp100 autoantibodies. In conclusion, SUMO proteins constitute a novel and independent class of autoantigens in PBC. Furthermore, we believe our data emphasize the post-translational modification of lysine by either lipoylation in the case of AMA or SUMOylation in the case of specific anti-ND autoantibodies as the pivotal site for autoantibody generation in PBC.