Myelin oligodendrocyte glycoprotein peptide-induced experimental allergic encephalomyelitis and T cell responses are unaffected by immunoproteasome deficiency

Lactate Dehydrogenase-Elevating Virus Infection Inhibits MOG Peptide Presentation by CD11b+CD11c+ Dendritic Cells in a Mouse Model of Multiple Sclerosis

Infections may affect the course of autoimmune inflammatory diseases of the central nervous system (CNS), such as multiple sclerosis (MS). Infections with lactate dehydrogenase-elevating virus (LDV) protected mice from developing experimental autoimmune encephalomyelitis (EAE), a mouse counterpart of MS. Uninfected C57BL/6 mice immunized with the myelin oligodendrocyte glycoprotein peptide (MOG35-55) experienced paralysis and lost weight at a greater rate than mice who had previously been infected with LDV. LDV infection decreased the presentation of the MOG peptide by CD11b+CD11c+ dendritic cells (DC) to pathogenic T lymphocytes. When comparing non-infected mice to infected mice, the histopathological examination of the CNS showed more areas of demyelination and CD45+ and CD3+, but not Iba1+ cell infiltration. These results suggest that the protective effect of LDV infection against EAE development is mediated by a suppression of myelin antigen presentation by a specific DC subset to autoreactive T lymphocytes. Such a mechanism might contribute to the general suppressive effect of infections on autoimmune diseases known as the hygiene hypothesis.

Astrocytic TIMP-1 regulates production of Anastellin, an inhibitor of oligodendrocyte differentiation and FTY720 responses

Astrocyte activation is associated with neuropathology and the production of tissue inhibitor of metalloproteinase-1 (TIMP1). TIMP1 is a pleiotropic extracellular protein that functions both as a protease inhibitor and as a growth factor. Astrocytes that lack expression of Timp1 do not support rat oligodendrocyte progenitor cell (rOPC) differentiation, and adult global Timp1 knockout (Timp1KO) mice do not efficiently remyelinate following a demyelinating injury. Here, we performed an unbiased proteomic analysis and identified a fibronectin-derived peptide called Anastellin (Ana) that was unique to the Timp1KO astrocyte secretome. Ana was found to block rOPC differentiation in vitro and enhanced the inhibitory influence of fibronectin on rOPC differentiation. Ana is known to act upon the sphingosine-1-phosphate receptor 1, and we determined that Ana also blocked the pro-myelinating effect of FTY720 (or fingolimod) on rOPC differentiation in vitro. Administration of FTY720 to wild-type C57BL/6 mice during MOG35-55-experimental autoimmune encephalomyelitis ameliorated clinical disability while FTY720 administered to mice lacking expression of Timp1 (Timp1KO) had no effect. Analysis of Timp1 and fibronectin (FN1) transcripts from primary human astrocytes from healthy and multiple sclerosis (MS) donors revealed lower TIMP1 expression was coincident with elevated FN1 in MS astrocytes. Last, analyses of proteomic databases of MS samples identified Ana peptides to be more abundant in the cerebrospinal fluid (CSF) of human MS patients with high disease activity. A role for Ana in MS as a consequence of a lack of astrocytic TIMP-1 production could influence both the efficacy of fingolimod responses and innate remyelination potential in the MS brain.

Astrocytic TIMP-1 regulates production of Anastellin, a novel inhibitor of oligodendrocyte differentiation and FTY720 responses

Astrocyte activation is associated with neuropathology and the production of tissue inhibitor of metalloproteinase-1 (TIMP1). TIMP1 is a pleiotropic extracellular protein that functions both as a protease inhibitor and as a growth factor. We have previously demonstrated that murine astrocytes that lack expression of Timp1 do not support rat oligodendrocyte progenitor cell (rOPC) differentiation, and adult global Timp1 knockout ( Timp1 KO ) mice do not efficiently remyelinate following a demyelinating injury. To better understand the basis of this, we performed unbiased proteomic analyses and identified a fibronectin-derived peptide called anastellin that is unique to the murine Timp1 KO astrocyte secretome. Anastellin was found to block rOPC differentiation in vitro and enhanced the inhibitory influence of fibronectin on rOPC differentiation. Anastellin is known to act upon the sphingosine-1-phosphate receptor 1 (S1PR1), and we determined that anastellin also blocked the pro-myelinating effect of FTY720 (or fingolimod) on rOPC differentiation in vitro . Further, administration of FTY720 to wild-type C57BL/6 mice during MOG 35-55 -EAE ameliorated clinical disability while FTY720 administered to mice lacking expression of Timp1 in astrocytes ( Timp1 cKO ) had no effect. Analysis of human TIMP1 and fibronectin ( FN1 ) transcripts from healthy and multiple sclerosis (MS) patient brain samples revealed an inverse relationship where lower TIMP1 expression was coincident with elevated FN1 in MS astrocytes. Lastly, we analyzed proteomic databases of MS samples and identified anastellin peptides to be more abundant in the cerebrospinal fluid (CSF) of human MS patients with high versus low disease activity. The prospective role for anastellin generation in association with myelin lesions as a consequence of a lack of astrocytic TIMP-1 production could influence both the efficacy of fingolimod responses and the innate remyelination potential of the the MS brain.

Significance Statement

Astrocytic production of TIMP-1 prevents the protein catabolism of fibronectin. In the absence of TIMP-1, fibronectin is further digested leading to a higher abundance of anastellin peptides that can bind to sphingosine-1-phosphate receptor 1. The binding of anastellin with the sphingosine-1-phosphate receptor 1 impairs the differentiation of oligodendrocytes progenitor cells into myelinating oligodendrocytes in vitro , and negates the astrocyte-mediated therapeutic effects of FTY720 in the EAE model of chronic CNS inflammation. These data indicate that TIMP-1 production by astrocytes is important in coordinating astrocytic functions during inflammation. In the absence of astrocyte produced TIMP-1, elevated expression of anastellin may represent a prospective biomarker for FTY720 therapeutic responsiveness.

The Functional and Mechanistic Roles of Immunoproteasome Subunits in Cancer

Cell-mediated immunity is driven by antigenic peptide presentation on major histocompatibility complex (MHC) molecules. Specialized proteasome complexes called immunoproteasomes process viral, bacterial, and tumor antigens for presentation on MHC class I molecules, which can induce CD8 T cells to mount effective immune responses. Immunoproteasomes are distinguished by three subunits that alter the catalytic activity of the proteasome and are inducible by inflammatory stimuli such as interferon-γ (IFN-γ). This inducible activity places them in central roles in cancer, autoimmunity, and inflammation. While accelerated proteasomal degradation is an important tumorigenic mechanism deployed by several cancers, there is some ambiguity regarding the role of immunoproteasome induction in neoplastic transformation. Understanding the mechanistic and functional relevance of the immunoproteasome provides essential insights into developing targeted therapies, including overcoming resistance to standard proteasome inhibition and immunomodulation of the tumor microenvironment. In this review, we discuss the roles of the immunoproteasome in different cancers.

Extracellular vesicle fibrinogen induces encephalitogenic CD8+ T cells in a mouse model of multiple sclerosis

In this report, we show that fibrinogen, identified by proteomics to be present in blood plasma extracellular vesicles (EVs), is sufficient and required for autoimmune-mediated spontaneous relapsing disease activity in a murine model of multiple sclerosis (MS). Unique to this model is that plasma EVs induced CD8-mediated disease. Analysis of human plasma EVs identified fibrinogen in MS patient samples, thereby providing a compelling translational association between our experimental findings and the perpetuation of CD8-mediated autoimmunity in human MS. Hence, these findings provide evidence for EVs as means by which to model an important aspect of spontaneous CD8+ T cell development related to autoimmunity in MS.

Extracellular vesicles (EVs) are emerging as potent mediators of intercellular communication with roles in inflammation and disease. In this study, we examined the role of EVs from blood plasma (pEVs) in an experimental autoimmune encephalomyelitis mouse model of central nervous system demyelination. We determined that pEVs induced a spontaneous relapsing−remitting disease phenotype in MOG_35–55-immunized C57BL/6 mice. This modified disease phenotype was found to be driven by CD8+ T cells and required fibrinogen in pEVs. Analysis of pEVs from relapsing−remitting multiple sclerosis patients also identified fibrinogen as a significant portion of pEV cargo. Together, these data suggest that fibrinogen in pEVs contributes to the perpetuation of neuroinflammation and relapses in disease.

Lipoprotein Lipase Is a Feature of Alternatively-Activated Microglia and May Facilitate Lipid Uptake in the CNS During Demyelination

Severe demyelinating disorders of the central nervous system (CNS) such as multiple sclerosis (MS), can be devastating for many young lives. To date, the factors resulting in poor remyelination and repair are not well understood, and reparative therapies that benefit MS patients have yet to be developed. We have previously shown that the activity and abundance of Lipoprotein Lipase (LPL)—the rate-limiting enzyme in the hydrolysis of triglyceride-rich lipoproteins—is increased in Schwann cells and macrophages following nerve crush injury in the peripheral nervous system (PNS), suggesting that LPL may help scavenge myelin-derived lipids. We hypothesized that LPL may play a similar role in the CNS. To test this, mice were immunized with MOG_35–55 peptide to induce experimental allergic encephalomyelitis (EAE). LPL activity was increased (p < 0.05) in the brain at 30 days post-injection, coinciding with partial remission of clinical symptoms. Furthermore, LPL abundance and activity was up-regulated (p < 0.05) at the transition between de- and re-myelination in lysolecithin-treated ex vivo cerebellar slices. Since microglia are the key immune effector cells of the CNS we determined the role of LPL in microglia. Lipid uptake was decreased (p < 0.001) in LPL-deficient BV-2 microglial cells compared to WT. In addition, LPL-deficient cells showed dramatically reduced expression of anti-inflammatory markers, YM1 (−22 fold, p < 0.001), and arginase 1 (Arg1; −265 fold, p < 0.001) and increased expression of pro-inflammatory markers, such as iNOS compared to WT cells (+53 fold, p < 0.001). This suggests that LPL is a feature of reparative microglia, further supported by the metabolic and inflammatory profile of LPL-deficient microglia. Taken together, our data strongly suggest that LPL expression is a novel feature of a microglial phenotype that supports remyelination and repair through the clearance of lipid debris. This mechanism may be exploited to develop future reparative therapies for MS and primary neurodegenerative disorders (Alzheimer’s disease (AD) and Parkinson’s disease).

Ubiquitin-independent proteosomal degradation of myelin basic protein contributes to development of neurodegenerative autoimmunity

Recent findings indicate that the ubiquitin–proteasome system is involved in the pathogenesis of cancer as well as autoimmune and several neurodegenerative diseases, and is thus a target for novel therapeutics. One disease that is related to aberrant protein degradation is multiple sclerosis, an autoimmune disorder involving the processing and presentation of myelin autoantigens that leads to the destruction of axons. Here, we show that brain-derived proteasomes from SJL mice with experimental autoimmune encephalomyelitis (EAE) in an ubiquitin-independent manner generate significantly increased amounts of myelin basic protein peptides that induces cytotoxic lymphocytes to target mature oligodendrocytes ex vivo. Ten times enhanced release of immunogenic peptides by cerebral proteasomes from EAE-SJL mice is caused by a dramatic shift in the balance between constitutive and β1i^high immunoproteasomes in the CNS of SJL mice with EAE. We found that during EAE, β1i is increased in resident CNS cells, whereas β5i is imported by infiltrating lymphocytes through the blood–brain barrier. Peptidyl epoxyketone specifically inhibits brain-derived β1i^high immunoproteasomes in vitro (k_obs/[I] = 240 M⁻¹s⁻¹), and at a dose of 0.5 mg/kg, it ameliorates ongoing EAE in vivo. Therefore, our findings provide novel insights into myelin metabolism in pathophysiologic conditions and reveal that the β1i subunit of the immunoproteasome is a potential target to treat autoimmune neurologic diseases.—Belogurov Jr., A., Kuzina, E., Kudriaeva, A., Kononikhin, A., Kovalchuk, S., Surina, Y., Smirnov, I., Lomakin, Y., Bacheva, A., Stepanov, A., Karpova, Y., Lyupina, Y., Kharybin, O., Melamed, D., Ponomarenko, N., Sharova, N., Nikolaev, E., Gabibov, A. Ubiquitin-independent proteosomal degradation of myelin basic protein contributes to development of neurodegenerative autoimmunity.

Inhibition of the immunoproteasome ameliorates experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a chronic demyelinating immune mediated disease of the central nervous system. The immunoproteasome is a distinct class of proteasomes found predominantly in monocytes and lymphocytes. Recently, we demonstrated a novel function of immunoproteasomes in cytokine production and T cell differentiation. In this study, we investigated the therapeutic efficacy of an inhibitor of the immunoproteasome (ONX 0914) in two different mouse models of MS. ONX 0914 attenuated disease progression after active and passive induction of experimental autoimmune encephalomyelitis (EAE), both in MOG_35–55 and PLP_139–151-induced EAE. Isolation of lymphocytes from the brain or spinal cord revealed a strong reduction of cytokine-producing CD4⁺ cells in ONX 0914 treated mice. Additionally, ONX 0914 treatment prevented disease exacerbation in a relapsing-remitting model. An analysis of draining lymph nodes after induction of EAE revealed that the differentiation to Th17 or Th1 cells was strongly impaired in ONX 0914 treated mice. These results implicate the immunoproteasome in the development of EAE and suggest that immunoproteasome inhibitors are promising drugs for the treatment of MS.

Immuno- and constitutive proteasomes do not differ in their abilities to degrade ubiquitinated proteins

Immunoproteasomes are alternative forms of proteasomes that have an enhanced ability to generate antigenic peptides. Recently, Seifert and colleagues reported surprising observations concerning the functions of immunoproteasomes and cellular responses to interferon-γ: (1) that immunoproteasomes degrade ubiquitinated proteins faster than the constitutive proteasomes, (2) that polyubiquitin conjugates accumulate after interferon-γ treatment but then are preferentially degraded by immunoproteasomes, and (3) that immunoproteasome deficiency causes the formation of inclusions and more severe experimental autoimmune encephalomyelitis (EAE). In contrast, we find that polyubiquitin conjugates do not transiently accumulate following IFNγ-treatment and that immunoproteasomes do not prevent the formation of intracellular inclusions or protect against EAE. Furthermore, purified 26S constitutive and immunoproteasomes bind ubiquitin conjugates similarly and degrade them at similar rates. We conclude that, although immunoproteasomes can increase the generation of peptides appropriate for MHC class I presentation, they do not degrade ubiquitinated proteins more efficiently than constitutive particles.

The immunoproteasome in antigen processing and other immunological functions

Treatment of cells with interferon-γ leads to the replacement of the constitutive catalytic proteasome subunits β1, β2, and β5 by the inducible subunits LMP2 (β1i), MECL-1 (β2i), and LMP7 (β5i), respectively, building the so-called immunoproteasome. The incorporation of these subunits is required for the production of numerous MHC class-I restricted T cell epitopes. Recently, new evidence for an involvement of the immunoproteasome in other facets of the immune response emerged. Investigations of autoimmune diseases in animal models and a genetic predisposition of β5i in human autoimmune disorders suggest a crucial function of the immunoproteasome in proinflammatory diseases. The recent elucidation of the high-resolution structure of the immunoproteasome will facilitate the design of immunoproteasome selective inhibitors for pharmacological intervention.

Proteomic identification of immunoproteasome accumulation in formalin-fixed rodent spinal cords with experimental autoimmune encephalomyelitis

Clinically relevant formalin-fixed and paraffin-embedded (FFPE) tissues have not been widely used in neuroproteomic studies because many proteins are presumed to be degraded during tissue preservation. Recent improvements in proteomics technologies, from the 2D gel analysis of intact proteins to the "shotgun" quantification of peptides and the use of isobaric tags for absolute and relative quantification (iTRAQ) method, have made the analysis of FFPE tissues possible. In recent years, iTRAQ has been one of the main methods of choice for high throughput quantitative proteomics analysis, which enables simultaneous comparison of up to eight samples in one experiment. Our objective was to assess the relative merits of iTRAQ analysis of fresh frozen versus FFPE nervous tissues by comparing experimental autoimmune encephalomyelitis (EAE)-induced proteomic changes in FFPE rat spinal cords and frozen tissues. EAE-induced proteomic changes in FFPE tissues were positively correlated with those found in the frozen tissues, albeit with ∼50% less proteome coverage. Subsequent validation of the enrichment of immunoproteasome (IP) activator 1 in EAE spinal cords led us to evaluate other proteasome and IP-specific proteins. We discovered that many IP-specific (as opposed to constitutive) proteasomal proteins were enriched in EAE rat spinal cords, and EAE-induced IP accumulation also occurred in the spinal cords of an independent mouse EAE model in a disability score-dependent manner. Therefore, we conclude that it is feasible to generate useful information from iTRAQ-based neuroproteomics analysis of archived FFPE tissues for studying neurological disease tissues.

Prevention of experimental colitis by a selective inhibitor of the immunoproteasome

The proteasome, a multicatalytic protease, is responsible for the degradation of intracellular proteins. Stimulation of cells with inflammatory cytokines, such as IFN-gamma, leads to the replacement of the constitutive catalytic proteasome subunits by the inducible subunits low molecular mass polypeptide (LMP)2 (beta1i), multicatalytic endopeptidase complex-like-1 (beta2i), and LMP7 (beta5i), which are required for the production of certain MHC class I-restricted T cell epitopes. In this study, we investigated the effect of immunoproteasomes on the development of dextran sulfate sodium-induced colitis. Colitis induction in LMP2-, LMP7-, and multicatalytic endopeptidase complex-like-1-deficient mice caused reduced weight loss compared with wild-type mice. Although colon lengths were shortened in wild-type mice, no reduction was observed in immunoproteasome-deficient mice. In accordance with this, proinflammatory cytokines, such as TNF-alpha and IL-1beta, were not upregulated in these mice. Blockage of LMP7 by a novel LMP7-selective inhibitor (PR-957) strongly reduced pathological symptoms of dextran sulfate sodium-induced colitis. Production of numerous cytokines in PR-957-treated mice was suppressed, resulting in reduced inflammation and tissue destruction. Taken together, these results demonstrate that an immunoproteasome-specific inhibitor can be used to attenuate autoimmune diseases like colitis.

A selective inhibitor of the immunoproteasome subunit LMP7 blocks cytokine production and attenuates progression of experimental arthritis

The immunoproteasome, a distinct class of proteasome found predominantly in monocytes and lymphocytes, is known to shape the antigenic repertoire presented on class I major histocompatibility complexes (MHC-I). However, a specific role for the immunoproteasome in regulating other facets of immune responses has not been established. We describe here the characterization of PR-957, a selective inhibitor of low-molecular mass polypeptide-7 (LMP7, encoded by Psmb8), the chymotrypsin-like subunit of the immunoproteasome. PR-957 blocked presentation of LMP7-specific, MHC-I-restricted antigens in vitro and in vivo. Selective inhibition of LMP7 by PR-957 blocked production of interleukin-23 (IL-23) by activated monocytes and interferon-gamma and IL-2 by T cells. In mouse models of rheumatoid arthritis, PR-957 treatment reversed signs of disease and resulted in reductions in cellular infiltration, cytokine production and autoantibody levels. These studies reveal a unique role for LMP7 in controlling pathogenic immune responses and provide a therapeutic rationale for targeting LMP7 in autoimmune disorders.