Targeting plasma cells in autoimmune diseases

Novel treatment strategies for acetylcholine receptor antibody-positive myasthenia gravis and related disorders

The presence of autoantibodies directed against the muscle nicotinic acetylcholine receptor (AChR) is the most common cause of myasthenia gravis (MG). These antibodies damage the postsynaptic membrane of the neuromuscular junction and cause muscle weakness by depleting AChRs and thus impairing synaptic transmission. As one of the best-characterized antibody-mediated autoimmune diseases, AChR-MG has often served as a reference model for other autoimmune disorders. Classical pharmacological treatments, including broad-spectrum immunosuppressive drugs, are effective in many patients. However, complete remission cannot be achieved in all patients, and 10% of patients do not respond to currently used therapies. This may be attributed to production of autoantibodies by long-lived plasma cells which are resistant to conventional immunosuppressive drugs. Hence, novel therapies specifically targeting plasma cells might be a suitable therapeutic approach for selected patients. Additionally, in order to reduce side effects of broad-spectrum immunosuppression, targeted immunotherapies and symptomatic treatments will be required. This review presents established therapies as well as novel therapeutic approaches for MG and related conditions, with a focus on AChR-MG.

Experimental modeling of desensitization: What have we learned about preventing AMR?

During the past 5 decades, short-term outcomes in kidney transplant have significantly improved, in large part due to reduced rates and severity of acute rejection. Development of better immunosuppressive maintenance agents, as well as new induction therapies, helped make these advances. Nonhuman primate models provided a rigorous testing platform to evaluate candidate biologics during this process. However, antibody-mediated rejection remains a major cause of late failure of kidney allografts despite advances made in pharmacologic immunosuppression and strategies developed to facilitate improved donor-recipient matching. Our laboratory has been actively working to develop strategies to prevent and treat antibody-mediated rejection and immunologic sensitization in organ transplant, relying largely on a nonhuman primate model of kidney transplant. In this review, we will cover outcomes achieved by managing antibody-mediated rejection or sensitization in nonhuman primate models and discuss promises, limitations, and future directions for this model.

Multiple Functions of B Cells in the Pathogenesis of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by excessive autoantibody production and multi-organ involvement. Although the etiology of SLE still remains unclear, recent studies have characterized several pathogenic B cell subsets and regulatory B cell subsets involved in the pathogenesis of SLE. Among pathogenic B cell subsets, age-associated B cells (ABCs) are a newly identified subset of autoreactive B cells with T-bet-dependent transcriptional programs and unique functional features in SLE. Accumulation of T-bet⁺ CD11c⁺ ABCs has been observed in SLE patients and lupus mouse models. In addition, innate-like B cells with the autoreactive B cell receptor (BCR) expression and long-lived plasma cells with persistent autoantibody production contribute to the development of SLE. Moreover, several regulatory B cell subsets with immune suppressive functions have been identified, while the impaired inhibitory effects of regulatory B cells have been indicated in SLE. Thus, further elucidation on the functional features of B cell subsets will provide new insights in understanding lupus pathogenesis and lead to novel therapeutic interventions in the treatment of SLE.

Reduced peripheral blood regulatory B cell levels are not associated with the Expanded Disability Status Scale score in multiple sclerosis

Objective To investigate levels of regulatory B (Breg) cells, plasma cells, and memory B cells in the peripheral blood, and interleukin (IL)-10 in the serum of multiple sclerosis (MS) patients, and to determine the correlation between Breg cell levels and the Expanded Disability Status Scale (EDSS) score. Methods Levels of Breg cells, plasma cells, and memory B cells in the peripheral blood of 12 MS patients were measured using flow cytometry. IL-10 serum levels were measured by enzyme-linked immunosorbent assay. The correlation between Breg cell levels and MS EDSS score was measured using Pearson's correlation coefficient. Results Compared with healthy controls, MS patients had decreased levels of CD19+CD24hiCD38hi Breg cells in their peripheral blood and reduced serum levels of IL-10; however, the ratios of CD19+CD27hiCD38hi plasma cells and CD19+CD27+CD24hi memory B cells to total B cells did not differ significantly between healthy controls and MS patients. CD19+CD24hiCD38hi Breg cell levels in the peripheral blood of MS patients were not significantly correlated with MS EDSS score. Conclusion Peripheral blood CD19+CD24hiCD38hi Breg cell levels and serum IL-10 levels were reduced in MS patients compared with controls, but Breg cell levels were not correlated with MS EDSS score.

Identification of human plasma cells with a lamprey monoclonal antibody

Ab-producing plasma cells (PCs) serve as key participants in countering pathogenic challenges as well as being contributors to autoimmune and malignant disorders. Thus far, only a limited number of PC-specific markers have been identified. The characterization of the unique variable lymphocyte receptor (VLR) Abs that are made by evolutionarily distant jawless vertebrates prompted us to investigate whether VLR Abs could detect novel PC antigens that have not been recognized by conventional Abs. Here, we describe a monoclonal lamprey Ab, VLRB MM3, that was raised against primary multiple myeloma cells. VLRB MM3 recognizes a unique epitope of the CD38 ectoenzyme that is present on plasmablasts and PCs from healthy individuals and on most, but not all, multiple myelomas. Binding by the VLRB MM3 Ab coincides with CD38 dimerization and NAD glycohydrolase activity. Our data demonstrate that the lamprey VLRB MM3 Ab is a unique reagent for the identification of plasmablasts and PCs, with potential applications in the diagnosis and therapeutic intervention of PC or autoimmune disorders.

C9orf72 ablation causes immune dysregulation characterized by leukocyte expansion, autoantibody production, and glomerulonephropathy in mice

The expansion of a hexanucleotide (GGGGCC) repeat in C9ORF72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Both the function of C9ORF72 and the mechanism by which the repeat expansion drives neuropathology are unknown. To examine whether C9ORF72 haploinsufficiency induces neurological disease, we created a C9orf72-deficient mouse line. Null mice developed a robust immune phenotype characterized by myeloid expansion, T cell activation, and increased plasma cells. Mice also presented with elevated autoantibodies and evidence of immune-mediated glomerulonephropathy. Collectively, our data suggest that C9orf72 regulates immune homeostasis and an autoimmune response reminiscent of systemic lupus erythematosus (SLE) occurs in its absence. We further imply that haploinsufficiency is unlikely to be the causative factor in C9ALS/FTD pathology.

Inhibition of B lymphocyte-induced maturation protein-1 reduces the production of autoantibody and alleviates symptoms of systemic lupus erythematosus

The B lymphocyte-induced maturation protein-1 (Blimp-1) is an important transcription factor for the maintenance of antigen-specific immune responses, and it is crucial in the development of systemic lupus erythematosus (SLE). This study aimed to investigate the role of Blimp-1 in the development of SLE and autoimmune-like symptoms. Lentivirus-mediated Blimp-1 siRNA was constructed and injected into MRL-Fas(lpr) lupus mice. The expression levels of Blimp-1, J-chain, C-myc, XBP-1 and BCMA in peripheral blood mononuclear cells (PMBCs) were determined by RT-PCR. Anti-dsDNA autoantibody levels were detected using ELISA. The expression levels of Blimp-1 in liver, kidney, spleen and lymph nodes of mice were also detected by Western blot. The 24-h urinary protein was monitored weekly. Our results demonstrated that in MRL-Fas(lpr) lupus mice, Blimp-1 was upregulated in PMBCs, liver, kidney, spleen and lymph nodes. Administration of Blimp-1 siRNA reduced the expression of Blimp-1 and the anti-dsDNA level by 78 and 28%, respectively, in the peripheral blood, and the expression of XBP-1, J-chain and BCMA was also decreased. Although the Blimp-1 level in liver showed no significant changes, the levels of Blimp-1 in kidney, spleen and lymph nodes were dramatically decreased by 95, 72 and 47%, respectively. Kidney diseases induced by SLE in lupus mice were mitigated, and urinary protein levels were significantly decreased. These results indicate that Blimp-1 plays an important role in promoting the progression of SLE. Therefore, Blimp-1 may provide a new therapeutic target in the treatment of SLE.

Memory B cells in transplantation

Much of the research on the humoral response to allografts has focused on circulating serum antibodies and the long-lived plasma cells that produce these antibodies. In contrast, the interrogation of the quiescent memory B cell compartment is technically more challenging and thus has not been incorporated into the clinical diagnostic or prognostic toolkit. In this review, we discuss new technologies that have allowed this heretofore enigmatic subset of B cells to be identified at quiescence and during a recall response. These technologies in experimental models are providing new insights into memory B cell heterogeneity with respect to their phenotype, cellular function, and the antibodies they produce. Similar technologies are also allowing for the identification of comparable memory alloreactive B cells in transplant recipients. Although much of the focus in transplant immunology has been on controlling the alloreactive B cell population, long-term transplant patient survival is also critically dependent on protection by pathogen-specific memory B cells. Techniques are available that allow the interrogation of memory B cell response to pathogen re-encounter. Thus, we are poised in our ability to investigate how immunosuppression affects allospecific and pathogen-specific memory B cells, and reason that these investigations can yield new insights that will be beneficial for graft and patient survival.

Up-regulation of transcription factor Blimp1 in systemic lupus erythematosus

PURPOSE

B lymphocyte induced maturation protein 1 (Blimp1) is a transcription factor that is critical for differentiation and antibody production in plasma cells. In order to understand the mechanism of systemic lupus erythematosus (SLE), the role of Blimp1 expression was studied in patients with SLE and in healthy control subjects. And Blimp1 tissue distribution in MRL/lpr lupus mice was also investigated.

METHODS

The mRNA expression level of Blimp1 was analyzed by fluorescent real time PCR and compared between the 40 SLE patients and 30 control subjects. Expression of CD138, CD27 and CD19 in peripheral blood cells was analyzed by flow cytometry. Blimp1 mRNA and protein expression levels and tissue distribution in the kidneys, spleen and lymph nodes of MRL/lpr lupus and normal mice were analyzed.

RESULTS

Blimp1 mRNA expression level was 2.1 times greater in the SLE group as compared to the control group. The increased mRNA expression of Blimp1 seemed to be related to SLE disease activity and anti-nuclear antibody (ANA) titer. In SLE patients, the CD138+ plasma cells increased as the CD27+ cells decreased. Compared with normal mice, Blimp1 was strongly expressed in the kidneys, lymph nodes and spleen of MRL/lpr lupus mice. The expression level of Blimp1 mRNA in the kidneys, lymph nodes and spleen of MRL/lpr lupus mice was much higher than normal mice (1.76, 2.02, and 2.05 times greater, respectively, P<0.05). Similarly, protein levels in the above mentioned organs were also much higher (1.54, 1.99, and 2.21 times greater, respectively, P<0.05).

CONCLUSIONS

The elevated expression of Blimp1 in SLE patients and in the lupus mouse model is correlated with increases in plasma cells, autoantibodies and disease activity. It is closely related to differentiation of B-lymphocytes, antibody production and renal lesions. Blimp1 may play a role in SLE disease development.

A double-edged sword in B-cell-targeted therapy for inflammatory diseases

Cells of the immune system, including B cells, perform inflammatory functions against microbial invasion, accompanied by anti-inflammatory responses to avoid host damage. B-cell-depletion therapy using anti-CD20 monoclonal antibodies against inflammatory diseases has beneficial or adverse effects depending on the timing and/or microenvironment in which they are used. To achieve effective B-cell-targeted therapy, it is necessary to identify and understand the modes of action of pathogenic and regulatory B cells, which include antibody production, formation of immune complexes, cytokine and chemokine production, cytotoxic killing, lymphoid neogenesis and antigen presentation. B cells interact with multiple cells, including dendritic cells, T cells and natural killer T cells, creating a complex regulatory network. Specific targeting of B-cell subsets and/or their interaction partners might lead to clinical benefits with minimal host damage.

B-lymphocyte-induced maturation protein1 up-regulates the expression of B-cell maturation antigen in mouse plasma cells

B-lymphocyte-induced maturation protein1(Blimp-1) and B-cell maturation antigen (BCMA) are essential factors in the development and survival of plasma cells. However, whether Blimp-1 could regulate the expression of BCMA is unknown. We found that the BCMA promoter region did not have typical "TATA" and "CAAT" box, but contained several potential binding sites of transcription factors, including the consensus sequences for Blimp-1, located in the "-31 to -21" and "-46 to -36" from the potential transcription initiation site of the mouse BCMA gene, respectively. Furthermore, induction of Blimp-1 over-expression significantly up-regulated the expression of BCMA and increased the BCMA promoter activity in mouse J558L plasma cells. In parallel, knockdown of Blimp-1 expression by the Blimp-1-specific shRNA significantly reduced the BCMA mRNA transcription and protein expression in J558L cells in vitro. Substitution mutation of the "-38 to -42" sequence, but not the "-23 to -27", in the BCMA promoter abolished the regulatory effect of Blimp-1 on the expression of BCMA. Importantly, Blimp-1 bound to the "GAAAC", but not its mutant "GATTC", contained BCMA promoter, as determined by competitive electrophoretic mobility shift assay (EMSA). Therefore, our data clearly suggest that Blimp-s a positive regulator of the expression of BCMA gene in mouse plasma cells.

CD93 is required for maintenance of antibody secretion and persistence of plasma cells in the bone marrow niche

Plasma cells represent the end stage of B-cell development and play a key role in providing an efficient antibody response, but they are also involved in numerous pathologies. Here we show that CD93, a receptor expressed during early B-cell development, is reinduced during plasma-cell differentiation. High CD93/CD138 expression was restricted to antibody-secreting cells both in T-dependent and T-independent responses as naive, memory, and germinal-center B cells remained CD93-negative. CD93 was expressed on (pre)plasmablasts/plasma cells, including long-lived plasma cells that showed decreased cell cycle activity, high levels of isotype-switched Ig secretion, and modification of the transcriptional network. T-independent and T-dependent stimuli led to re-expression of CD93 via 2 pathways, either before or after CD138 or Blimp-1 expression. Strikingly, while humoral immune responses initially proceeded normally, CD93-deficient mice were unable to maintain antibody secretion and bone-marrow plasma-cell numbers, demonstrating that CD93 is important for the maintenance of plasma cells in bone marrow niches.

The myasthenia gravis thymus: a rare source of human autoantibody-secreting plasma cells for testing potential therapeutics

In early-onset myasthenia gravis (EOMG), the thymus is colonized by lymph node-like infiltrates including T cell areas and germinal centers. Our Group(1) showed (1978) spontaneous anti-acetylcholine receptor (AChR) autoantibody production by EOMG thymic cells. Especially after enzymic dispersal, these are enriched in plasma cells that are evidently autonomous, long-lived, terminally differentiated and radio-resistant. Radiolabeled AChR is highly sensitive both for localizing them in situ and detecting their ongoing antibody production in culture at limiting cell numbers. Thus EOMG thymi are a readily available source of specific autoimmune human plasma cells suitable for studying their biology and testing new therapies.

OBF-1 is essential for the generation of antibody-secreting cells and the development of autoimmunity in MRL-lpr mice

As reported previously, the lack of the transcriptional co-activator OBF-1 prevented development of autoimmunity in Aiolos knockout mice. To further investigate the role and mechanism of OBF-1 in autoimmunity, we crossed OBF-1 null mice with MRL-lpr mice and generated OBF-1-deficent MRL-lpr mice. OBF-1 deletion abrogated all autoantibodies in the MRL-lpr mice, including anti-dsDNA Ab and anti-Sm Ab. The failure to produce autoantibodies was not related to development of immature or mature B cells, but correlated with severely reduced antibody-secreting cells (ASCs). The loss of OBF-1 protected against hypergammaglobulinemia, immune complex deposition, glomerulonephritis, and early mortality in MRL-lpr mice. In addition, accumulation of CD4(-)CD8(-)B220(+)CD3(+) T cells that characteristically develop in Fas mutation mice were markedly reduced in MRL-lpr mice without OBF-1. These results identify OBF-1 as a critical gene in the development of autoantibodies and reveal an essential role for OBF-1 in the generation of antibody/autoantibody-secreting cells in vivo.

Immunity and immunological memory following smallpox vaccination

The smallpox vaccine consists of live vaccinia virus and is generally considered the gold standard of vaccines, since it is the only one that has led to the complete eradication of an infectious disease from the human population. Renewed fears that smallpox might be deliberately released in an act of bioterrorism have led to resurgence in the study of immunity and immunological memory to vaccinia virus and other poxviruses. Here we review our current understanding of memory T-cell, memory B-cell, and antibody responses to vaccinia and related poxviruses, both in animal models and human subjects. Of particular interest are recent advances in understanding protective immunity to poxviruses, quantifying immunological memory to the smallpox vaccine in humans, and identifying major vaccinia-specific T-cell and B-cell epitopes. In addition, potential mechanisms for maintenance of immunological memory are discussed.

Hypogammaglobulinemia and exacerbated CD8 T-cell–mediated immunopathology in SAP-deficient mice with chronic LCMV infection mimics human XLP disease

The human genetic disease X-linked lymphoproliferative disease (XLP), which is caused by mutations in SH2D1A/SAP that encode SLAM-associated protein (SAP), is characterized by an inability to control Epstein-Barr virus (EBV) and hypogammaglobulinemia. It is unclear which aspects of XLP disease are specific to herpesvirus infection and which reflect general immunologic functions performed by SAP. We examined SAP– mice during a chronic LCMV infection, specifically to address the following question: Which SAP deficiency immunologic problems are general, and which are EBV specific? Illness, weight loss, and prolonged viral replication were much more severe in SAP– mice. Aggressive immunopathology was observed. This inability to control chronic LCMV was associated with both CD8 T-cell and B-cell response defects. Importantly, we demonstrate that SAP– CD8 T cells are the primary cause of the immunopathology and clinical illness, because depletion of CD8 T cells blocked disease. This is the first direct demonstration of SAP– CD8 T-cell–mediated immunopathology, confirming 30 years of XLP clinical observations and indirect experimentation. In addition, germinal center formation was extremely defective in chronically infected SAP– animals, and hypogammaglobulinemia was observed. These findings in a chronic viral infection mouse model recapitulate key features of human XLP and clarify SAP's critical role regulating both cellular and humoral immunity.

Stromal niches, plasma cell differentiation and survival

Contacts made with other cells and stroma have a major impact on proliferation, differentiation, survival, migration and immunoglobulin class switching of plasma cell precursors as well as on the lifespan of the antibody-secreting cells. Induction of tissue-specific chemokine receptors and adhesion molecules directs migratory plasma cell precursors to tissues close to those in which the original immune stimulation occurred. This mechanism focuses the production of specific antibodies within a particular type of tissue, thus providing a means for the most efficient protection against tissue-specific pathogens. Relocation does not apply to long-lived plasma cells responsible for sustained titers of high-affinity systemic antibody. These are formed in germinal centers and migrate to specific niches in the bone marrow that support their further differentiation and long-term survival.

A mimic of p21WAF1/CIP1 ameliorates murine lupus

Systemic lupus erythematosus (SLE) is a progressive autoimmune disease characterized by the production of high levels of affinity-matured IgG autoantibodies to dsDNA and, possibly, visceral involvement. Pathogenic autoantibodies result from the activation and proliferation of autoreactive T and B lymphocytes stimulated by epitopes borne by nucleosomal histones. To inhibit the proliferation of autoreactive cells and abrogate the development of SLE, a novel tool, cell cycle inhibiting peptide therapy, was used. Thus, a peptidyl mimic of p21WAF1/CIP1 that inhibits the interaction between cyclin-dependent kinase 4 and type D cyclins abrogated the in vitro proliferative response of T cells to histones and T-independent and T-dependent proliferative responses of B cells. The WAF1/CIP1 mimic also abrogated the in vitro production of total and anti-dsDNA IgG Abs by B cells. Similarly, the p21WAF1/CIP1 construct inhibited the ex vivo T and B cell proliferative responses to histones and decreased the numbers of activated/memory B and T spleen cells. The alterations in the balance of spleen cell subsets resulted from proapoptotic effects of the p21WAF1/CIP1)construct on activated splenocytes. Finally, in vivo, four i.v. injections of the p21WAF1/CIP1 mimic were sufficient to inhibit the progression of the lupus-like syndrome in (NZB x NZW)F1 mice. The levels of anti-dsDNA IgG autoantibodies and the incidence and severity of renal involvement were lower in treated mice than in nontreated mice. Those observations open new avenues for the treatment of SLE and prompt us to evaluate the potential interest of peptidic therapy in human SLE.

BAFF supports human B cell differentiation in the lymphoid follicles through distinct receptors

B cell-activating factor of the tumor necrosis factor family (BAFF/BLys) plays a critical role in B cell survival and immune responses through its three receptors: BAFF receptor (BAFF-R/BR3), transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) and B cell maturation antigen (BCMA). Using specific antibodies, we have investigated the expression of BAFF-R on human tonsillar B cells and their functional roles in naive and germinal center (GC) B cell differentiation. Our studies show that BAFF-R is the dominant receptor on naive B cells. However, three receptors are differentially modulated during in vitro GC-B cell differentiation. BAFF-R expression increased initially and then decreased with a corresponding induction of TACI and BCMA expression during differentiation to plasma cells (PCs). Consistently, blocking of BAFF-R alone with specific mAb inhibited GC-B cell proliferation and PC generation in the early period of their differentiation, whereas depletion of BAFF with TACI-Ig exhibited consistent inhibition throughout the differentiation. Finally, histological and molecular analyses of human tonsil tissue revealed that follicular dendritic cells produce BAFF. In conclusion, BAFF in the GC plays an important role through more than one receptor, and the three known receptors are differentially modulated as GC-B cells differentiate to PCs.

How do follicular dendritic cells interact intimately with B cells in the germinal centre?

The germinal centre is a dynamic microenvironment where antigen-activated B cells rapidly expand and differentiate, generating plasma cells and memory B cells. These cellular events are accompanied by dramatic changes in the antibody molecules that undergo somatic hypermutation and isotype switching. Follicular dendritic cells (FDCs) are the stromal cells located in the germinal centre. Although the capacity of FDCs to present antigen to B cells through antigen-antibody complexes has been recognized for many years, additional critical functions of FDCs have only recently been recognized. FDCs prevent apoptosis of germinal centre B cells and stimulate cellular interaction and proliferation. Here, we review the FDC signalling molecules that have recently been identified, some of which offer potential therapeutic targets for autoimmune diseases and B-cell lymphomas.