Immunoproteasome inhibition induces plasma cell apoptosis and preserves kidney allografts by activating the unfolded protein response and suppressing plasma cell survival factors

Diagnosis of light‑chain deposition disease after renal transplantation: A case report and literature review

Light chain deposition disease (LCDD) is a rare, clonal plasma cell proliferative condition. The deposition of nonamyloid monoclonal immunoglobulin light chains predominantly affects the kidneys, which may lead to end-stage renal disease, eventually requiring renal replacement therapy. The present study reported a rare case of LCDD that was confirmed after renal transplantation. A 49-year-old man initially presented with heavy proteinuria, hypoproteinemia, hyperlipidemia and renal insufficiency. The patient was diagnosed with nephrotic syndrome and pathological examination revealed fibrillary glomerulonephritis in 2014. Treatment was started with prednisolone. About 5 years later, the patient began to receive continuous hemodialysis due to worsening serum creatinine levels. Renal allograft transplantation was performed in 2020 and dialysis independence was achieved. Laboratory findings before renal transplantation revealed that serum and urine immunofixation electrophoresis was negative. Allograft kidney biopsy established the pathological diagnosis of LCDD at >1 year after renal transplantation for renal dysfunction. The treatment is challenging due to the lack of generally accepted standard treatment practices. Administration of bortezomib combined with dexamethasone was started. As anemia and renal failure developed progressively, the treatment was switched to anti-CD38 antibody and continuous hemodialysis was restarted. The best response achieved was hematological partial response and relief of anemia. However, the patient's renal function did not improve and he remains to have end-stage kidney disease. LCDD is easily missed in cases in which serum and urine immunofixation electrophoresis is negative. Hence, early recognition of LCCD based on kidney biopsy is important. To the best of our knowledge, the use of anti-CD38 antibody therapy in patients with LCDD is rarely reported. Anti-CD38 antibody is effective in treating LCDD, but it may not reverse the marked deterioration of renal function.

The role of the immunoproteasome in cardiovascular disease

The ubiquitinproteasome system (UPS) is the main mechanism responsible for the intracellular degradation of misfolded or damaged proteins. Under inflammatory conditions, the immunoproteasome, an isoform of the proteasome, can be induced, enhancing the antigen-presenting function of the UPS. Furthermore, the immunoproteasome also serves nonimmune functions, such as maintaining protein homeostasis and regulating signalling pathways, and is involved in the pathophysiological processes of various cardiovascular diseases (CVDs). This review aims to provide a comprehensive summary of the current research on the involvement of the immunoproteasome in cardiovascular diseases, with the ultimate goal of identifying novel strategies for the treatment of these conditions.

Immunoproteasomal Inhibition With ONX-0914 Attenuates Atherosclerosis and Reduces White Adipose Tissue Mass and Metabolic Syndrome in Mice

BACKGROUND

Atherosclerosis is the major underlying pathology of cardiovascular disease and is driven by dyslipidemia and inflammation. Inhibition of the immunoproteasome, a proteasome variant that is predominantly expressed by immune cells and plays an important role in antigen presentation, has been shown to have immunosuppressive effects.

METHODS

We assessed the effect of ONX-0914, an inhibitor of the immunoproteasomal catalytic subunits LMP7 (proteasome subunit β5i/large multifunctional peptidase 7) and LMP2 (proteasome subunit β1i/large multifunctional peptidase 2), on atherosclerosis and metabolism in LDLr-/- and APOE*3-Leiden.CETP mice.

RESULTS

ONX-0914 treatment significantly reduced atherosclerosis, reduced dendritic cell and macrophage levels and their activation, as well as the levels of antigen-experienced T cells during early plaque formation, and Th1 cells in advanced atherosclerosis in young and aged mice in various immune compartments. Additionally, ONX-0914 treatment led to a strong reduction in white adipose tissue mass and adipocyte progenitors, which coincided with neutrophil and macrophage accumulation in white adipose tissue. ONX-0914 reduced intestinal triglyceride uptake and gastric emptying, likely contributing to the reduction in white adipose tissue mass, as ONX-0914 did not increase energy expenditure or reduce total food intake. Concomitant with the reduction in white adipose tissue mass upon ONX-0914 treatment, we observed improvements in markers of metabolic syndrome, including lowered plasma triglyceride levels, insulin levels, and fasting blood glucose.

CONCLUSIONS

We propose that immunoproteasomal inhibition reduces 3 major causes underlying cardiovascular disease, dyslipidemia, metabolic syndrome, and inflammation and is a new target in drug development for atherosclerosis treatment.

Proteasome composition in immune cells implies special immune‐cell‐specific immunoproteasome function

Immunoproteasomes are a special class of proteasomes, which can be induced with IFN-γ in an inflammatory environment. In recent years, it became evident that certain immune cell types constitutively express high levels of immunoproteasomes. However, information regarding the basal expression of proteolytically active immunoproteasome subunits in different types of immune cells is still rare. Hence, we quantified standard proteasome subunits (β1c, β2c, β5c) and immunoproteasome subunits (LMP2, MECL-1, LMP7) in the major murine (CD4+ T cells, CD8+ T cells, CD19+ B cells, CD11c+ dendritic cells, CD49d+ natural killer cells, Ly-6G+ neutrophils) and human immune cell (CD4+ T cells, CD8+ T cells, CD19+ B cells, CD1c+CD141+ myeloid dendritic cells, CD56+ natural killer cells, granulocytes) subsets. The different human immune cell types were isolated from peripheral blood and the murine immune cell subsets from spleen. We found that proteasomes of most immune cell subsets mainly consist of immunoproteasome subunits. Our data will serve as a reference and guideline for immunoproteasome expression and imply a special role of immunoproteasomes in immune cells.

Translating B cell immunology to the treatment of antibody-mediated allograft rejection

Antibody-mediated rejection (AMR), including chronic AMR (cAMR), causes ~50% of kidney allograft losses each year. Despite attempts to develop well-tolerated and effective therapeutics for the management of AMR, to date, none has obtained FDA approval, thereby highlighting an urgent unmet medical need. Discoveries over the past decade from basic, translational and clinical studies of transplant recipients have provided a foundation for developing novel therapeutic approaches to preventing and treating AMR and cAMR. These interventions are aimed at reducing donor-specific antibody levels, decreasing graft injury and fibrosis, and preserving kidney function. Innovative approaches emerging from basic science findings include targeting interactions between alloreactive T cells and B cells, and depleting alloreactive memory B cells, as well as donor-specific antibody-producing plasmablasts and plasma cells. Therapies aimed at reducing the cytotoxic antibody effector functions mediated by natural killer cells and the complement system, and their associated pro-inflammatory cytokines, are also undergoing evaluation. The complexity of the pathogenesis of AMR and cAMR suggest that multiple approaches will probably be required to treat these disease processes effectively. Definitive answers await results from large, double-blind, multicentre, randomized controlled clinical trials.

A synthetic thiol molecule releasing N-acetyl-l-cysteine and cysteamine drives early up-regulation of immunoproteasome subunits in the lymph nodes of mice infected with LP-BM5 leukemia retrovirus

Thiol molecules have been recently re-considered as drug candidates in viral infections because of their ability to induce redox changes which interfere with virus life cycle and modulate the host immune response. Little is known about the molecular mechanisms of their immunomodulatory properties. Here we show that I-152, a thiol molecule metabolized to release N-acetyl-l-cysteine and cysteamine and acting as a pro-glutathione agent, causes early up-regulation of immunoproteasome subunits in the lymph nodes of murine leukemia virus infected mice. This evidence suggests that the immunoproteasome may be modulated by thiol-based compounds with important implications in understanding redox-controlled immunoregulation.

Zetomipzomib (KZR-616) attenuates lupus in mice via modulation of innate and adaptive immune responses

Zetomipzomib (KZR-616) is a selective inhibitor of the immunoproteasome currently undergoing clinical investigation in autoimmune disorders. Here, we characterized KZR-616 in vitro and in vivo using multiplexed cytokine analysis, lymphocyte activation and differentiation, and differential gene expression analysis. KZR-616 blocked production of >30 pro-inflammatory cytokines in human peripheral blood mononuclear cells (PBMCs), polarization of T helper (Th) cells, and formation of plasmablasts. In the NZB/W F1 mouse model of lupus nephritis (LN), KZR-616 treatment resulted in complete resolution of proteinuria that was maintained at least 8 weeks after the cessation of dosing and was mediated in part by alterations in T and B cell activation, including reduced numbers of short and long-lived plasma cells. Gene expression analysis of human PBMCs and tissues from diseased mice revealed a consistent and broad response focused on inhibition of T, B, and plasma cell function and the Type I interferon pathway and promotion of hematopoietic cell lineages and tissue remodeling. In healthy volunteers, KZR-616 administration resulted in selective inhibition of the immunoproteasome and blockade of cytokine production following ex vivo stimulation. These data support the ongoing development of KZR-616 in autoimmune disorders such as systemic lupus erythematosus (SLE)/LN.

Immunoproteasome inhibition prevents progression of castration-resistant prostate cancer

BACKGROUND

Castration-resistant prostate cancer (CRPC) is refractory to hormone treatment. This study aims to explore the effect and underlying mechanisms of immunoproteasome inhibition, a novel immunotherapy, on the progression of CRPC.

METHODS

The immunoproteasome subunit LMP7 was silenced by using gene knockout or inhibited by the epoxyketone inhibitor ONX 0914 in a mouse CRPC tumour graft model and in interferon-γ-pretreated human CRPC cell lines in vitro.

RESULTS

CRPC tissues reveal a significant "tumour-elicited" Th17-type inflammatory response which induces immunoproteasome subunit expression. LMP7 deficiency in host mice or in CRPC tumour grafts had no effect on the "tumour-elicited" Th17-type inflammatory response and tumour progression. However, the selective LMP7 inhibitor ONX 0914 strongly suppressed the "tumour-elicited" Th17-type inflammatory response and CRPC tumour progression. Treatment of wild-type mice receiving LMP7-deficient CRPC tumour grafts with ONX 0914 further suggested that immunoproteasome inhibition prevents CRPC progression through suppressing IL-17-induced angiogenesis and epithelial-mesenchymal transition via inactivation of COX-2/VEGF-A signalling and β-catenin/Snail signalling. Treatment of LMP7-deficient mice receiving wild-type CRPC tumour grafts with ONX 0914 and inhibition of LMP7 in PC3 and 22Rv.1 cells with ONX 0914 showed that immunoproteasome inhibition also prevents CRPC progression through inducing CRPC cell apoptosis via activation of the unfolded protein response.

CONCLUSIONS

We define a critical role of the immunoproteasome in CRPC and propose immunoproteasome inhibition as a promising therapeutic approach to suppress CRPC progression.

Suppression of prostate cancer and amelioration of the immunosuppressive tumor microenvironment through selective immunoproteasome inhibition

New treatment options to battle hormone-refractory prostate carcinoma (PC) are a pressing medical need. Chronic inflammation has been implicated in PC etiology. The pro-inflammatory cytokines IL-6, IL-23 and IL-17 are key mediators to promote growth of PC. Here, we evaluate the potential of immunoproteasome inhibition for anti-inflammatory and direct anti-tumorigenic therapy of PC. The anti-tumor effect of immunoproteasome inhibitor ONX 0914 was tested in mouse and human PC cells and the in vivo therapeutic efficacy of immunoproteasome inhibition was analyzed in transgenic adenocarcinoma of the mouse prostate (TRAMP) mice in preventive and therapeutic settings and in castration-resistant (CR)PC after castration. Inhibition of the immunoproteasome subunit LMP7 induced apoptotic cell death in PC cell lines. In TRAMP mice, ONX 0914-treatment resulted in significant inhibition of PC growth with a decreased frequency of malignant prostatic lesions and inhibition of metastasis formation. The number of immunosuppressive myeloid cells in PC was greatly reduced in response to ONX 0914. Thus, immunoproteasome inhibition shows remarkable efficacy against PC progression in vivo and impedes tumor recurrence in CRPC-TRAMP mice by blocking the immunosuppressive inflammatory response in the tumor microenvironment. In conclusion, we show that the immunoproteasome is a promising drug target for the treatment of PC.

Immunoproteasome Inhibition Reduces the T Helper 2 Response in Mouse Models of Allergic Airway Inflammation

Background

Allergic asthma is a chronic disease and medical treatment often fails to fully control the disease in the long term, leading to a great need for new therapeutic approaches. Immunoproteasome inhibition impairs T helper cell function and is effective in many (auto-) inflammatory settings but its effect on allergic airway inflammation is unknown.

Methods

Immunoproteasome expression was analyzed in in vitro polarized T helper cell subsets. To study Th2 cells in vivo acute allergic airway inflammation was induced in GATIR (GATA-3-vYFP reporter) mice using ovalbumin and house dust mite extract. Mice were treated with the immunoproteasome inhibitor ONX 0914 or vehicle during the challenge phase and the induction of airway inflammation was analyzed.

Results

In vitro polarized T helper cell subsets (Th1, Th2, Th17, and Treg) express high levels of immunoproteasome subunits. GATIR mice proved to be a useful tool for identification of Th2 cells. Immunoproteasome inhibition reduced the Th2 response in both airway inflammation models. Furthermore, T cell activation and antigen-specific cytokine secretion was impaired and a reduced infiltration of eosinophils and professional antigen-presenting cells into the lung and the bronchoalveolar space was observed in the ovalbumin model.

Conclusion

These results show the importance of the immunoproteasome in Th2 cells and airway inflammation. Our data provides first insight into the potential of using immunoproteasome inhibition to target the aberrant Th2 response, e.g. in allergic airway inflammation.

Romidepsin (FK228) improves the survival of allogeneic skin grafts through downregulating the production of donor-specific antibody via suppressing the IRE1α-XBP1 pathway

Antibody-mediated rejection (AMR) is one of the major causes of graft loss after transplantation. Recently, the regulation of B cell differentiation and the prevention of donor-specific antibody (DSA) production have gained increased attention in transplant research. Herein, we established a secondary allogeneic in vivo skin transplant model to study the effects of romidepsin (FK228) on DSA. The survival of grafted skins was monitored daily. The serum levels of DSA and the number of relevant immunocytes in the recipient spleens were evaluated by flow cytometry. Then, we isolated and purified B cells from B6 mouse spleens in vitro by magnetic bead sorting. The B cells were cultured with interleukin-4 (IL-4) and anti-clusters of differentiation 40 (CD40) antibody with or without FK228 treatment. The immunoglobulin G1 (IgG1) and IgM levels in the supernatant were evaluated by enzyme-linked immunosorbent assay (ELISA). Quantitative reverse transcription-polymerase chain reaction (RT-qPCR) and western blotting were conducted to determine the corresponding levels of messenger RNA (mRNA) and protein expression in cultured cells and the recipient spleens. The results showed that FK228 significantly improved the survival of allogeneic skin grafts. Moreover, FK228 inhibited DSA production in the serum along with the suppression of histone deacetylase 1 (HADC1) and HDAC2 and the upregulation of the acetylation of histones H2A and H3. It also inhibited the differentiation of B cells to plasma cells, decreased the transcription of positive regulatory domain-containing 1 (Prdm1) and X-box-binding protein 1 (Xbp1), and decreased the expression of phosphorylated inositol-requiring enzyme 1 α (p-IRE1α), XBP1, and B lymphocyte-induced maturation protein-1 (Blimp-1). In conclusion, FK228 could decrease the production of antibodies by B cells via inhibition of the IRE1α-XBP1 signaling pathway. Thus, FK228 is considered as a promising therapeutic agent for the clinical treatment of AMR.

Discovery and Early Clinical Development of Selective Immunoproteasome Inhibitors

Inhibitors of the proteolytic activity of the 20S proteasome have transformed the treatment of multiple B-cell malignancies. These agents have also been employed with success in the treatment of patients with autoimmune diseases and immune-mediated disorders. However, new agents are needed to fully unlock the potential of proteasome inhibitors as immunomodulatory drugs. The discovery that selective inhibitors of the immunoproteasome possess broad anti-inflammatory activity in preclinical models has led to the progression of multiple compounds to clinical trials. This review focuses on the anti-inflammatory potential of immunoproteasome inhibition and the early development of KZR-616, the first selective inhibitor of the immunoproteasome to reach clinical testing.

On the Role of the Immunoproteasome in Protein Homeostasis

Numerous cellular processes are controlled by the proteasome, a multicatalytic protease in the cytosol and nucleus of all eukaryotic cells, through regulated protein degradation. The immunoproteasome is a special type of proteasome which is inducible under inflammatory conditions and constitutively expressed in hematopoietic cells. MECL-1 (β2i), LMP2 (β1i), and LMP7 (β5i) are the proteolytically active subunits of the immunoproteasome (IP), which is known to shape the antigenic repertoire presented on major histocompatibility complex (MHC) class I molecules. Furthermore, the immunoproteasome is involved in T cell expansion and inflammatory diseases. In recent years, targeting the immunoproteasome in cancer, autoimmune diseases, and transplantation proved to be therapeutically effective in preclinical animal models. However, the prime function of standard proteasomes and immunoproteasomes is the control of protein homeostasis in cells. To maintain protein homeostasis in cells, proteasomes remove proteins which are not properly folded, which are damaged by stress conditions such as reactive oxygen species formation, or which have to be degraded on the basis of regular protein turnover. In this review we summarize the latest insights on how the immunoproteasome influences protein homeostasis.

Plasma cell biology: Foundations for targeted therapeutic development in transplantation

Solid organ transplantation is a life-saving procedure for patients with end-stage organ disease. Over the past 70 years, tremendous progress has been made in solid organ transplantation, particularly in T-cell-targeted immunosuppression and organ allocation systems. However, humoral alloimmune responses remain a major challenge to progress. Patients with preexisting antibodies to human leukocyte antigen (HLA) are at significant disadvantages in regard to receiving a well-matched organ, moreover, those who develop anti-HLA antibodies after transplantation face a significant foreshortening of renal allograft survival. Historical therapies to desensitize patients prior to transplantation or to treat posttransplant AMR have had limited effectiveness, likely because they do not significantly reduce antibody levels, as plasma cells, the source of antibody production, remain largely unaffected. Herein, we will discuss the significance of plasma cells in transplantation, aspects of their biology as potential therapeutic targets, clinical challenges in developing strategies to target plasma cells in transplantation, and lastly, novel approaches that have potential to advance the field.

Emerging New Approaches in Desensitization: Targeted Therapies for HLA Sensitization

There is an urgent need for therapeutic interventions for desensitization and antibody-mediated rejection (AMR) in sensitized patients with preformed or de novo donor-specific HLA antibodies (DSA). The risk of AMR and allograft loss in sensitized patients is increased due to preformed DSA detected at time of transplant or the reactivation of HLA memory after transplantation, causing acute and chronic AMR. Alternatively, de novo DSA that develops post-transplant due to inadequate immunosuppression and again may lead to acute and chronic AMR or even allograft loss. Circulating antibody, the final product of the humoral immune response, has been the primary target of desensitization and AMR treatment. However, in many cases these protocols fail to achieve efficient removal of all DSA and long-term outcomes of patients with persistent DSA are far worse when compared to non-sensitized patients. We believe that targeting multiple components of humoral immunity will lead to improved outcomes for such patients. In this review, we will briefly discuss conventional desensitization methods targeting antibody or B cell removal and then present a mechanistically designed desensitization regimen targeting plasma cells and the humoral response.

Role of Proteasomes in Inflammation

The ubiquitin–proteasome system (UPS) is involved in multiple cellular functions including the regulation of protein homeostasis, major histocompatibility (MHC) class I antigen processing, cell cycle proliferation and signaling. In humans, proteasome loss-of-function mutations result in autoinflammation dominated by a prominent type I interferon (IFN) gene signature. These genomic alterations typically cause the development of proteasome-associated autoinflammatory syndromes (PRAAS) by impairing proteasome activity and perturbing protein homeostasis. However, an abnormal increased proteasomal activity can also be found in other human inflammatory diseases. In this review, we cast a light on the different clinical aspects of proteasomal activity in human disease and summarize the currently studied therapeutic approaches.

Immunoproteasome Upregulation Is Not Required to Control Protein Homeostasis during Viral Infection

The prime function of proteasomes is the control of protein homeostasis in cells (i.e., the removal of proteins that are not properly folded, damaged by stress conditions like reactive oxygen species formation, or degraded on the basis of regular protein turnover). During viral infection, the standard proteasome is replaced by the so-called immunoproteasome (IP) in an IFN-γ-dependent manner. It has been proposed that the IP is required to protect cell viability under conditions of IFN-induced oxidative stress. In this study, we investigated the requirement for IP to cope with the enhanced need for protein degradation during lymphocytic choriomeningitis virus (LCMV) infection in mice lacking the IP subunit LMP7. We found that IP are upregulated in the liver but not in the spleen during LCMV infection, although the total proteasome content was not altered. The expression of standard proteasome subunits is not induced in LMP7-deficient mice, indicating that enhanced proteasomal activity is not required during viral infection. Furthermore, ubiquitin accumulation, apoptosis induction, and viral titers were similar in LCMV-infected mice lacking LMP7 compared with wild-type mice. Taken together, these data indicate that the IP is not required to regulate protein homeostasis during LCMV infection.

Chemokine Profiles Are Affected in Serum of Patients with Acute Rejection of Kidney Allograft

Kidney allograft transplantation improved the prognosis and quality of life of patients with end-stage renal diseases but the occurrence of acute rejection represents a limitation of the final outcome. Noninvasive biomarkers are needed as well as further advancements in the understanding of immune mechanisms of reaction to the allograft. Our study of 138 patients focused on one-year monitoring of serum concentrations of 12 chemokines regulating the recruitment of different immune cells into transplanted allograft and on in vitro regulation of the same chemokines release by interactions of renal proximal epithelial cells with monocyte/macrophage cell line stimulated with TNF alpha. In a group of 44 patients with acute rejection, higher serum pretransplant levels of CXCL1, CXCL5, CXCL6, CCL2, CCL21, and particularly CXCL10 and CX3CL1(both p < 0.001) were found suggesting their higher proinflammatory status as compared to subjects with the uncomplicated outcome. In samples collected at the day of biopsy positive for acute rejection, chemokines CXCL9 and CXCL11 attracting preferentially Th1 lymphocytes were found to be upregulated. In our in vitro model with TNF alpha induction, renal proximal epithelial cells seemed to be a more potent source of chemokines attracting neutrophils as compared to monocyte/macrophage cell line but the coculture of these cells potentiated release of neutrophilic chemokines CXCL5 and CXCL6. Similar augmentation of chemokine production was found also in the case of CCL2. On the other hand, adding of monocytes/macrophages to a culture of renal epithelial cells suppressed the release of CXCL10 and CXCL11 attracting T lymphocytes. We assume from our data that in kidney allograft transplantation, chemokines attracting neutrophils, T lymphocytes, and monocytes are induced simultaneously and measurement some of them in combination might be used as biomarkers of acute rejection. Mutual cell-cell interactions of immune cells with renal parenchyma seem to be important for fine regulation of chemokine release.

Noncytotoxic Inhibition of the Immunoproteasome Regulates Human Immune Cells In Vitro and Suppresses Cutaneous Inflammation in the Mouse

Inhibitors of the immunoproteasome (i-20S) have shown promise in mouse models of autoimmune diseases and allograft rejection. In this study, we used a novel inhibitor of the immunoproteasome, PKS3053, that is reversible, noncovalent, tight-binding, and highly selective for the β5i subunit of the i-20S to evaluate the role that i-20S plays in regulating immune responses in vitro and in vivo. In contrast to irreversible, less-selective inhibitors, PKS3053 did not kill any of the primary human cell types tested, including plasmacytoid dendritic cells, conventional dendritic cells, macrophages, and T cells, all of which expressed genes encoding both the constitutive proteasome (c-20S) and i-20S. PKS3053 reduced TLR-dependent activation of plasmacytoid dendritic cells, decreasing their maturation and IFN-α response and reducing their ability to activate allogenic T cells. In addition, PKS3053 reduced T cell proliferation directly and inhibited TLR-mediated activation of conventional dendritic cells and macrophages. In a mouse model of skin injury that shares some features of cutaneous lupus erythematosus, blocking i-20S decreased inflammation, cellular infiltration, and tissue damage. We conclude that the immunoproteasome is involved in the activation of innate and adaptive immune cells, that their activation can be suppressed with an i-20S inhibitor without killing them, and that selective inhibition of β5i holds promise as a potential therapy for inflammatory skin diseases such as psoriasis, cutaneous lupus erythematosus, and systemic sclerosis.

Novel strategies to target the humoral alloimmune response

Antibody-mediated rejection (ABMR) represents a major cause of late allograft loss in solid organ transplantation worldwide. This process is driven by donor-specific antibodies (DSA), which develop either de-novo or, in sensitized patients, are preformed at the time of transplantation. Effective targeting of ABMR has been hampered by a lack of robust randomized controlled trials (RCT), required for the regulatory approval of new therapeutics. In this review, we discuss the evidence behind the present "standard" of care and recent progress in the development of novel strategies targeting different aspects of the alloimmune humoral response, including naïve and memory B-cell activation, the germinal centre reaction, plasma cell survival and antibody effector functions. In particular, we focus on co-stimulation blockade and its combination with next-generation proteasome inhibitors, new depleting monoclonal antibodies (anti-CD19, anti-BCMA, anti-CD38, anti-CD138), interleukin-6 blockade, complement inhibition and DSA degradation. These treatment modalities, when used in the appropriate clinical context and combination, have the potential to finally improve long-term allograft survival.

Recent insights how combined inhibition of immuno/proteasome subunits enables therapeutic efficacy

The proteasome is a multicatalytic protease in the cytosol and nucleus of all eukaryotic cells that controls numerous cellular processes through regulated protein degradation. Proteasome inhibitors have significantly improved the survival of multiple myeloma patients. However, clinically approved proteasome inhibitors have failed to show efficacy against solid tumors, neither alone nor in combination with other therapies. Targeting the immunoproteasome with selective inhibitors has been therapeutically effective in preclinical models for several autoimmune diseases and colon cancer. Moreover, immunoproteasome inhibitors prevented the chronic rejection of allogeneic organ transplants. In recent years, it has become apparent that inhibition of one single active center of the proteasome is insufficient to achieve therapeutic benefits. In this review we summarize the latest insights how targeting multiple catalytically active proteasome subunits can interfere with disease progression in autoimmunity, growth of solid tumors, and allograft rejection.

Immunoproteasome in IgA Nephropathy: State-of-Art and Future Perspectives

IgA nephropathy (IgAN) is a leading cause of chronic kidney disease and renal failure. The exact pathogenesis of IgAN is not well defined, but some genetic studies have led to a novel discovery that the immunoproteasome probably plays an important role in IgAN. The immunoproteasome is a proteasome variant that is expressed when cells are stressed or receive inflammatory signals. While immunoproteasome is suggested to be mainly involved in major histocompatibility complex-I (MHC-I) antigen presentation, recent studies indicate that it may assert broad functions in trafficking events that activate both innate and adaptive immunity. In this review, we first summarize new insights into its functions in immunity, and discuss how it underlies its associations with IgAN. We also highlight its potential as a therapeutic target for the future.

Proteasome Inhibitors Diminish c-Met Expression and Induce Cell Death in Non-Small Cell Lung Cancer Cells

Non-small cell lung cancer (NSCLC) is the most common type of lung cancer and accounts for 85% of all lung carcinomas. The hepatocyte growth factor receptor (c-Met) has been considered as a potential therapeutic target for NSCLC. Proteasome inhibition induces cell apoptosis and has been used as a novel therapeutic approach for treating diseases including NSCLC; however, the effects of different proteasome inhibitors on NSCLC have not been fully investigated. The aim of this study is to determine a precise strategy for treating NSCLC by targeting c-Met using different proteasome inhibitors. Three proteasome inhibitors, bortezomib, MG132, and ONX 0914, were used in this study. Bortezomib (50 nM) significantly reduced c-Met levels and cell viability in H1299 and H441 cells, while similar effects were observed in H460 and A549 cells when a higher concentration (∼100 nM) was used. Bortezomib decreased c-Met gene expression in H1299 and H441 cells, but it had no effect in A549 and H460 cells. MG-132 at a low concentration (0.5 μM) diminished c-Met levels in H441 cells, while neither a low nor a high concentration (∼20 μM) altered c-Met levels in A549 and H460 cells. A higher concentration of MG-132 (5 μM) was required for decreasing c-Met levels in H1299 cells. Furthermore, MG-132 induced cell death in all four cell types. Among all the four cell lines, H441 cells expressed higher levels of c-Met and appeared to be the most susceptible to MG-132. MG-132 decreased c-Met mRNA levels in both H1299 and H441 cells. ONX 0914 reduced c-Met levels in H460, H1299, and H441 cells but not in A549 cells. c-Met levels were decreased the most in H441 cells treated with ONX 0914. ONX 0914 did not alter cell viability in H441; however, it did induce cell death among H460, A549, and H1299 cells. This study reveals that different proteasome inhibitors produce varied inhibitory effects in NSCLS cell lines.

Plasma cell targeting to prevent antibody-mediated rejection

Plasma cells (PCs) are the major source of pathogenic allo- and autoantibodies and have historically demonstrated resistance to therapeutic targeting. However, significant recent clinical progress has been made with the use of second-generation proteasome inhibitors (PIs). PIs provide efficient elimination of plasmablast-mediated humoral responses; however, long-lived bone marrow (BM) resident PCs (LLPCs) demonstrate therapeutic resistance, particularly to first-generation PIs. In addition, durability of antibody (Ab) reduction still requires improvement. More recent clinical trials have focused on conditions mediated by LLPCs and have included mechanistic studies of LLPCs from PI-treated patients. A recent clinical trial of carfilzomib (a second-generation irreversible PI) demonstrated improved efficacy in eliminating BM PCs and reducing anti-HLA Abs in chronically HLA-sensitized patients; however, Ab rebound was observed over several weeks to months following PI therapy. Importantly, recent murine studies have provided substantial insights into PC biology, thereby further enhancing our understanding of PC populations. It is now clear that BMPC populations, where LLPCs are thought to primarily reside, are heterogeneous and have distinct gene expression, metabolic, and survival signatures that enable identification and characterization of PC subsets. This review highlights recent advances in PC biology and clinical trials in transplant populations.

ONX 0914 Lacks Selectivity for the Cardiac Immunoproteasome in CoxsackievirusB3 Myocarditis of NMRI Mice and Promotes Virus-Mediated Tissue Damage

: Inhibition of proteasome function by small molecules is highly efficacious in cancer treatment. Other than non-selective proteasome inhibitors, immunoproteasome-specific inhibitors allow for specific targeting of the proteasome in immune cells and the profound anti-inflammatory potential of such compounds revealed implications for inflammatory scenarios. For pathogen-triggered inflammation, however, the efficacy of immunoproteasome inhibitors is controversial. In this study, we investigated how ONX 0914, an immunoproteasome-selective inhibitor, influences CoxsackievirusB3 infection in NMRI mice, resulting in the development of acute and chronic myocarditis, which is accompanied by formation of the immunoproteasome in heart tissue. In groups in which ONX 0914 treatment was initiated once viral cytotoxicity had emerged in the heart, ONX 0914 had no anti-inflammatory effect in the acute or chronic stages. ONX 0914 treatment initiated prior to infection, however, increased viral cytotoxicity in cardiomyocytes, promoting infiltration of myeloid immune cells into the heart. At this stage, ONX 0914 completely inhibited the β5 subunit of the standard cardiac proteasome and less efficiently blocked its immunoproteasome counterpart LMP7. In conclusion, ONX 0914 unselectively perturbs cardiac proteasome function in viral myocarditis of NMRI mice, reduces the capacity of the host to control the viral burden and promotes cardiac inflammation.

Different roles of bortezomib and ONX 0914 in acute kidney injury

Proteasome inhibitor bortezomib offers one more option for acute or chronic antibody-mediated rejection after kidney transplantation, but aggravated acute kidney injury (AKI) in some cases early after surgery using bortezomib bring new problem. Here, we evaluated the effects of bortezomib and ONX-0914 on renal tubule injury in a mouse model of ischemia-reperfusion injury. After treated with bortezomib, serum creatinine, usea nitrogen and tubular necrosis significantly increased compared with vehicle-treated mice, but decreased in ONX-0914 group mildly. Infiltration of neutrophil and macrophage were less in bortezomib and ONX-0914-treated mice than vehicle-treated group, and the same was observed on oxidative stress in the kidneys. Furthermore, the apoptosis of renal tubular epithelial cells increased in bortezomib-treated mice' kidneys compared with ONX-0914 and vehicle-treated controls. In vitro HK2 cell experiments also demonstrated the proapoptotic effect of bortezomib. The mRNA expression of several proapoptotic factors increased in kidneys of bortezomib-treated mice. In brief, bortezomib, as a proteasome inhibitor, shows a certain cytotoxicity to renal tubular epithelial cell during ischemia/reperfusion injury (IRI) through increased apoptosis. ONX-0914, as an immunoproteasome inhibitor, showed equal potency on anti-inflammation and oxidative stress relieving compared with bortezomib, while less cytotoxicity. The results render the immunoproteasome is a better target for anti-rejection and protecting kidney function in the field of organ transplantation.

Immunoproteasome Inhibition Selectively Kills Human CD14+ Monocytes and as a Result Dampens IL-23 Secretion

MECL-1 (β2i), LMP2 (β1i), and LMP7 (β5i) are the proteolytically active subunits of the immunoproteasome (IP), a special type of proteasome mainly expressed in hematopoietic cells. Targeting the IP in autoimmune diseases proved to be therapeutically effective in preclinical mouse models. In endotoxin-stimulated human PBMCs, IP inhibition reduces the secretion of several proinflammatory cytokines, with the suppression of IL-23 being the most prominent. In this study, we investigated why the production of IL-23, a key mediator of inflammation in autoimmunity, is blocked when the IP is inhibited in LPS-stimulated human PBMCs. CD14⁺ monocytes could be identified as the main producers of IL-23 in LPS-stimulated PBMCs. We found that IP inhibition with the irreversible LMP7/LMP2 inhibitor ONX 0914 induced apoptosis in CD14⁺ monocytes, whereas CD4⁺, CD3⁺, CD19⁺, and CD56⁺ cells remained unaffected. A high expression of IPs renders monocytes susceptible to IP inhibition, leading to an accumulation of polyubiquitylated proteins and the induction of the unfolded protein response. Similar to IP inhibition, inducers of the unfolded protein response selectively kill CD14⁺ monocytes in human PBMCs. The blockage of the translation in CD14⁺ monocytes protects these cells from ONX 0914-induced cell death, indicating that the IP is required to maintain protein turnover in monocytes. Taken together, our data reveal why IP inhibition is particularly effective in the suppression of IL-23-driven autoimmunity.