Pathogenic long-lived plasma cells and their survival niches in autoimmunity, malignancy, and allergy

Bortezomib for treatment of therapy-refractory anti-NMDA receptor encephalitis

OBJECTIVE

We assessed the therapeutic potential of the plasma-cell-depleting proteasome inhibitor bortezomib in severe and therapy-refractory cases of anti-NMDA receptor (anti-NMDAR) encephalitis.

METHODS

Five severely affected patients with anti-NMDAR encephalitis with delayed treatment response or resistance to standard immunosuppressive and B-cell-depleting drugs (corticosteroids, IV immunoglobulins, plasma exchange, immunoadsorption, rituximab, cyclophosphamide) who required medical treatment and artificial ventilation on intensive care units were treated with 1-6 cycles of 1.3 mg/m² bortezomib. Occurrence of adverse events was closely monitored.

RESULTS

Bortezomib treatment showed clinical improvement or disease remission, which was accompanied by a partial NMDAR antibody titer decline in 4 of 5 patients. With respect to disease severity, addition of bortezomib to the multimodal immunosuppressive treatment regimen was associated with an acceptable safety profile.

CONCLUSIONS

Our study identifies bortezomib as a promising escalation therapy for severe and therapy-refractory anti-NMDAR encephalitis.

CLASSIFICATION OF EVIDENCE

This retrospective case series provides Class IV evidence that bortezomib reduces antibody titers and improves the clinical course of patients with severe anti-NMDAR encephalitis.

Network Analysis Identifies Proinflammatory Plasma Cell Polarization for Secretion of ISG15 in Human Autoimmunity

Plasma cells (PCs) as effectors of humoral immunity produce Igs to match pathogenic insult. Emerging data suggest more diverse roles exist for PCs as regulators of immune and inflammatory responses via secretion of factors other than Igs. The extent to which such responses are preprogrammed in B-lineage cells or can be induced in PCs by the microenvironment is unknown. In this study, we dissect the impact of IFNs on the regulatory networks of human PCs. We show that core PC programs are unaffected, whereas PCs respond to IFNs with distinctive transcriptional responses. The IFN-stimulated gene 15 (ISG15) system emerges as a major transcriptional output induced in a sustained fashion by IFN-α in PCs and linked both to intracellular conjugation and ISG15 secretion. This leads to the identification of ISG15-secreting plasmablasts/PCs in patients with active systemic lupus erythematosus. Thus, ISG15-secreting PCs represent a distinct proinflammatory PC subset providing an Ig-independent mechanism of PC action in human autoimmunity.

Recent advances and current state of immunotherapy in systemic lupus erythematosus

INTRODUCTION

Systemic lupus erythematosus (SLE) is an autoimmune syndrome that poses significant challenges in diagnosis and treatment. Dysregulated innate and adaptive immune systems are involved in its pathogenesis. A plethora of novel immunotherapies have been developed for the treatment of SLE but many have failed early clinical trials.

AREAS COVERED

This review summarizes immunotherapies under recent development with relevance to the targeted cellular or soluble factors involved in the pathogenesis of SLE.

EXPERT OPINION

SLE is a complicated disease with much heterogeneity. Novel immunotherapies with different mechanisms of action that are currently under development include biologic agents targeting co-stimulatory molecules, cytokines or their receptors and signaling molecules and B cells, cell-based therapy and peptide therapy. Together with good scientific rationale and advanced biological engineering techniques, optimization of clinical trial design, patient selection and disease outcome measures are essential to demonstrate the clinical efficacy and safety of these agents.

Strategies to target long-lived plasma cells for treating hemophilia A inhibitors

Long-lived plasma cells (LLPCs) can persistently produce anti-factor VIII (FVIII) antibodies which disrupt therapeutic effect of FVIII in hemophilia A patients with inhibitors. The migration of plasma cells to BM where they become LLPCs is largely controlled by an interaction between the chemokine ligand CXCL12 and its receptor CXCR4. AMD3100 combined with G-CSF inhibit their interactions, thus facilitating the mobilization of CD34(+) cells and blocking the homing of LLPCs. These reagents were combined with anti-CD20 to reduce B-cells and the specific IL-2/IL-2mAb (JES6-1) complexes to induce Treg expansion for targeting anti-FVIII immune responses. Groups of mice primed with FVIII plasmid and protein respectively were treated with the combined regimen for six weeks, and a significant reduction of anti-FVIII inhibitor titers was observed, associated with the dramatic decrease of circulating and bone marrow CXCR4(+) plasma cells. The combination regimens are highly promising in modulating pre-existing anti-FVIII antibodies in FVIII primed subjects.

Overcoming challenges in the diagnosis and treatment of myasthenia gravis

In recent years, the discovery of new autoantigens and the use of sensitive assays have expanded the clinical spectrum of myasthenia gravis (MG). In particular, antibodies binding to clustered acetylcholine receptors and to the low-density lipoprotein receptor-related protein 4 have not only bridged a significant gap in diagnosis but also have relevant clinical implications. MG management includes different therapeutic options, from symptomatic agents as the only therapy in mildly affected cases to combined long-term immunosuppression and thymectomy in patients with severe disabling disease. MG biological diversity can influence the response to therapies and should be taken into account when planning treatment. Biologic agents are promising, though their use is currently limited to patients with refractory disease.

[Plasma cells]

Plasma cells are specialized terminally differentiated B cells that synthesize and secrete antibodies to maintain humoral immunity. By the production of pathogenic antibodies, plasma cells contribute to the development of many conditions, such as autoimmune disorders, transplant rejection and allergies. Two different plasma cell compartments can independently generate different types of pathogenic antibodies: (1) short-lived plasmablasts (proliferating precursors of mature plasma cells) and plasma cells, which live only as long as B cells are activated. Consequently, these cells cause disease flares that respond to immunosuppressive drugs and B cell targeting therapies. (2) Long-lived non-proliferating memory plasma cells, which survive in niches in bone marrow and inflamed tissues for months, years or a lifetime independent of B or T cell help or antigen contact. Because they do not respond to immunosuppressants or treatment targeting B cells, they are responsible for refractory chronic conditions. Therefore, long-lived memory plasma cells in particular have emerged as important therapeutic targets and strategies to target these cells are discussed in this article. So far long-lived plasma cells can only be depleted by immunoablative therapy with antithymocyte globulin in the setting of stem cell transplantation or by treatment with proteasome inhibitors approved for multiple myeloma. These strategies provide options for treating refractory autoantibody-mediated diseases. One interesting approach aims at an antigen-specific elimination of target plasma cells without depleting the protective plasma cells responsible for maintaining humoral immunity.

Differential Plasma-cell evolution is linked with Dermatophagoides pteronyssinus immunotherapy response

Allergic rhinitis is highly prevalent worldwide. Immunotherapy has been shown to control its symptoms, however, up to 30% of patients may not respond. Previous studies of the immunological mechanisms involved in allergen-immunotherapy (AIT) have focused on the humoral and T-cell response and several studies have evaluated some B-cell subpopulations during AIT and their role in immunological tolerance. However, although B and plasma-cell subpopulations are two of the most important cellular subtypes involved in allergic reactions, their relation with AIT efficacy remains unelucidated. The objective was to analyze the effects of immunotherapy on different B and plasma-cell subpopulations and whether these changes correlate with the clinical response to the treatment. Although no changes are found in B-cell subpopulations, responder patients show increased levels of memory B-cells even before the beginning of treatment. Changes in plasma-cell subpopulations are found, mainly in circulating inflammatory plasma-cells that could affect the response to the allergen. Moreover, an early increase of specific-IgG4 and IgG4 secreting-cells was found. All these suggest that the determination of the memory B-cells before the initiation of the treatment, and the quantification of IgG4 and IgG4-secreting-cells in the first months of immunotherapy, could serve as markers for the clinical response to treatment.

Siglec-G Deficiency Leads to Autoimmunity in Aging C57BL/6 Mice

Siglec-G, a member of the sialic acid-binding Ig-like lectin (Siglec) family, is expressed on B cell and dendritic cell surfaces. It acts as an inhibitory coreceptor and modulates B cell activation, especially on B1 cells, as Siglec-G-deficient mice show mainly a B1 cell-restricted phenotype resulting in increased B1 cell numbers. Although higher B1 cell numbers are discussed to be associated with autoimmunity, loss of Siglec-G does not result in autoimmune disease in BALB/c mice. However, there is evidence from Siglec-G × CD22 double-deficient mice and Siglec-G(-/-) mice on an autoimmune-prone MRL/lpr background that Siglec-G is important to maintain tolerance in B cells. In this study, we analyzed the role of Siglec-G in induction and maintenance of B cell tolerance on C57BL/6 background and in the FcγRIIb-deficient background. We find that aging Siglec-G-deficient and Siglec-G × FcγRIIb double-deficient mice develop an autoimmune phenotype with elevated autoantibody levels and mild glomerulonephritis. Aging Siglec-G-deficient mice have elevated numbers of plasma cells and germinal center B cells, as well as a higher number of activated CD4 T cells, which likely all contribute to autoantibody production. Additional loss of the inhibitory receptor FcγRIIb in Siglec-G(-/-) mice does not result in exacerbation of disease. These results indicate that Siglec-G is important to maintain tolerance in B cells and prevent autoimmunity.

The challenging definition of naïve patient for biological drug use

Biosimilar is defined by The European Medical Agency as a biological medicinal product, which is similar but not identical to the biological drug already authorized. The biosimilar and its reference product are expected to display the same safety and efficacy profile and are generally used to treat the same conditions. The Italian Medicines Agency considers biosimilars as a valid therapeutic option with an economic advantage, especially in primary naïve patients with no previous exposure to the originator or with a sufficiently long wash-out period ("secondary naïve"). The identification of "secondary naïve" is not well defined and can be subjected to different variables, mainly the drug biologic effect and its immunogenicity. The first one depends on the type of biologics and on their mechanism of action. The second one is related to the fact that biologicals may be immunogenic and can trigger an anti-drug antibody response (ADA). ADA may behave as neutralizing antibodies blocking the active site of the biological but can also recognize other epitopes favoring the formation of immune-complexes that eventually affect the pharmacodynamics. Moreover, the concomitant immune-suppressive treatment can affect the immunogenicity, even if the exact mechanism remains unknown. In conclusion, the development and use of biosimilars represent a tool for increasing health system sustainability. However it is of paramount importance to distinguish between the pharmacodynamics of a given drug and its immunogenicity being the two aspects unrelated. Thus a detailed definition of "secondary naïve" patients is challenging, and may be related to both the two parameters.

Intrathecal IgG synthesis: a resistant and valuable target for future multiple sclerosis treatments

Intrathecal IgG synthesis is a key biological feature of multiple sclerosis (MS). When acquired early, it persists over time. A growing body of evidence suggests that intrathecal Ig-secreting cells may be pathogenic either by a direct action of toxic IgG or by locally secreting bystander toxic products. Intrathecal IgG synthesis depends on the presence of CNS lymphoid organs, which are strongly linked at anatomical level to cortical subpial lesions and at clinical level to the impairment slope in progressive MS. As a consequence, targeting CNS lymphoid lesions could be a valuable new target in MS, especially during the progressive phase. As intrathecal IgGs are end-products of these lymphoid lesions, intrathecal IgG synthesis may be considered as a specific marker of the persistence of these inflammatory lesions. Here we review the effect upon intrathecal IgG synthesis of all drugs ever used in MS. Except for steroids, all these therapeutic strategies, including rituximab, failed to decrease intrathecal IgG synthesis, with the exception of a questionable incomplete action of natalizumab. Thus, IgG synthesis is a robust marker of persistent intrathecal inflammation and its complete normalization should be one of the goals in future therapeutic strategies.

Immune responses to human factor IX in haemophilia B mice of different genetic backgrounds are distinct and modified by TLR4

Our laboratory develops protocols to prevent or reverse ongoing anti-hFIX IgG inhibitors in haemophilia B mice with a F9 gene deletion on BALB/c and C3H/HeJ backgrounds. C3H/HeJ F9^−/Y mice develop high titre anti-hFIX IgG1 inhibitors and anaphylaxis, whereas most BALB/c F9^−/Y mice have mild anti-hFIX IgG1 inhibitors and no anaphylaxis. Our aim was to determine if hFIX-specific B- and T-cell responses in BALB/c and C3H/HeJ F9^−/Y mice trigger the difference in anti-hFIX immune responses. BALB/c and C3H/HeJ F9^−/Y mice were challenged weekly with recombinant hFIX protein. Humoral immune responses were determined by IgG1 and IgG2a anti-hFIX ELISA, Bethesda assay for inhibitors and B-cell ELISpot on bone marrow and spleen cells. T-cell studies measured the T_H1 (IFN-γ) and T_H2 (IL-4) cytokine responses in splenocytes at the mRNA and protein level in response to hFIX protein. Antibody responses were also measured in C3H/HeJ/OuJ F9^−/Y mice with restored toll-like receptor 4 (TLR4) function. BALB/c F9^−/Y mice have a T_H2 skewed response and a reduction in anti-hFIX secreting plasma cells in the bone marrow. Independent antigen challenge revealed both strains generated equivalent IgG1 antibody titres to an intravenously delivered antigen. C3H/HeJ F9^−/Y mice have a mixed T_H1 and T_H2 response (mainly T_H2). Importantly, TLR4 signalling has a modulatory role in the C3H background on the levels of anti-hFIX IgG1 and incidence of anaphylaxis. The background strain strongly impacts the immune response to hFIX, which can be significantly impacted by mutations in innate immune sensors.

Efficacy and safety of rituximab for myasthenia gravis: a systematic review and meta-analysis

Myasthenia gravis is an autoimmune disorder of the neuromuscular junction caused by circulating antibodies specific for the post-synaptic acetylcholine receptor or, in a minority of cases, for the muscle-specific tyrosine-kinase and the low-density lipoprotein receptor-related protein 4. A wide range of symptomatic and immunosuppressive treatments is currently available for MG patients with variable outcome. However, most immunosuppressive treatments are characterized by delayed onset of action and in some cases are not sufficient to induce stable remission of the disease. Rituximab (RTX) is a chimaeric monoclonal antibody specific for the CD20 B-cell surface antigen. Recent studies have provided evidence that RTX may be an effective treatment for patients with myasthenia gravis (MG) who are refractory to standardized immunosuppressive therapy. We performed a systematic review and a meta-analysis of the efficacy and safety of RTX in myasthenia gravis considering the potential predictive factors related to patients' response to RTX in this disease.

Neuronal surface antibody-mediated autoimmune encephalitis

In the past few years, many autoimmune encephalitides have been identified, with specific clinical syndromes and associated antibodies against neuronal surface antigens. There is compelling evidence that many of these antibodies are pathogenic and most of these encephalitides are highly responsive to immunotherapies. The clinical spectra of some of these antibody-mediated syndromes, especially those reported in only a few patients, are evolving. Others, such as anti-N-methyl-D-aspartate (NMDA) receptor encephalitis, are well characterized. Diagnosis involves recognizing the specific syndromes and identifying the antibody in a patient's cerebrospinal fluid (CSF) and/or serum. These syndromes are associated with variable abnormalities in CSF, magnetic resonance imaging, and electroencephalography. Treatment is often multidisciplinary and should be focused upon neutralizing the effects of antibodies and eliminating their source. Overlapping disorders have been noted, with some patients having more than one neurologic autoimmune disease. In other patients, viral infections such as herpes simplex virus encephalitis trigger robust antineuronal autoimmune responses.

Lymphocyte-based model systems for allergy research: a historic overview

During the last decades, a multitude of studies applying distinct in vitro and in vivo model systems have contributed greatly to our better understanding of the initiation and regulation of inflammatory processes leading to allergic diseases. Over the years, it has become evident that among lymphocytes, not only IgE-producing B cells and allergy-orchestrating CD4(+) helper cells but also cytotoxic CD8(+) T cells, γδ-T cells and innate lymphoid cells, as well as regulatory lymphocytes, might critically shape the immune response towards usually innocuous allergens. In this review, we provide a historic overview of pioneering work leading to the establishment of important lymphocyte-based model systems for allergy research. Moreover, we contrast the original findings with our currently more refined knowledge to appreciate the actual validity of the respective models and to reassess the conclusions obtained from them. Conflicting studies and interpretations are identified and discussed. The tables are intended to provide an easy overview of the field not only for scientists newly entering the field but also for the broader readership interested in updating their knowledge. Along those lines, herein we discuss in vitro and in vivo approaches to the investigation of lymphocyte effector cell activation, polarization and regulation, and describe depletion and adoptive transfer models along with gene knockout and transgenic (tg) methodologies. In addition, novel attempts to establish humanized T cell antigen receptor tg mouse models for allergy research are described and discussed.

Autoimmunity in 2012

The initiation and perpetuation of autoimmunity recognize numerous checkpoints, from the genomic susceptibility to the breakdown of tolerance. This latter phenomenon includes the loss of B cell anergy and T regulatory cell failure, as well as the production of autoantibodies and autoreactive T cells. These mechanisms ultimately lead to tissue injury via different mechanisms that span from the production of proinflammatory cytokines to the chemotaxis of immune cells to the target sites. The pathways to autoimmunity have been widely investigated over the past year and resulted in a number of articles in peer-reviewed journals that has increased by nearly 10 % compared to 2011. We herein follow on the attempt to provide a brief discussion of the majority of articles on autoimmune diseases that were published in the major immunology journals in the previous solar year. The selection is necessarily arbitrary and may thus not be seen as comprehensive but reflects current research trends. Indeed, 2012 articles were mostly dedicated to define new and old mechanisms with potential therapeutic implications in autoimmunity in general, though based on specific clinical conditions or animal models. As paradigmatic examples, the environmental influence on autoimmunity, Th17 changes modulating the autoimmune response, serum autoantibodies and B cell changes as biomarkers and therapeutic targets were major issues addressed by experimental articles in 2012. Further, a growing number of studies investigated the sex bias of autoimmunity and supported different working hypotheses to explain the female predominance, including sex chromosome changes and reproductive life factors. In conclusion, the resulting scenario illustrates that common factors may underlie different autoimmune diseases and this is well represented by the observed alterations in interferon-α and TGFβ or by the shared signaling pathways.

Does disease-irrelevant intrathecal synthesis in multiple sclerosis make sense in the light of tertiary lymphoid organs?

Although partly disease-irrelevant, intrathecal immunoglobulins (Ig) synthesis is a typical feature of multiple sclerosis (MS) and is driven by the tertiary lymphoid organs (TLO). A long-known hallmark of this non-specific intrathecal synthesis is the MRZ pattern, an intrathecal synthesis of Ig against measles, rubella, and zoster viruses. This non-specific intrathecal synthesis could also be directed against a wide range of pathogens. However, it is highly problematic since brain TLO should not be able to drive the clonal expansion of lymphocytes against alien antigens that are thought to be absent in MS brain. We propose to explain the paradox of non-specific intrathecal synthesis by discussing the natural properties of TLO. In fact, besides local antigen-driven clonal expansion, circulating plasmablasts and plasma cells (PC) are non-specifically recruited from blood and gain access to survival niches in the inflammatory CNS. This mechanism, which has been described in other inflammatory disorders, takes place in the TLO. As a consequence, PCs recruited in brain mirror the individual's history of immunization and intrathecal synthesis of IgG in MS may target a broad range of common infectious agents, a hypothesis in line with epidemiological data. Moreover, the immunization schedule and its timing may interfere with PC recruitment. If this hypothesis is correct, the reaction against EBV appears paradoxical: although early infection of MS patients is systematic, intrathecal synthesis is far lower than expected, suggesting a crucial interaction between MS onset and timing of EBV infection. A growing body of evidence suggests that the non-specific intrathecal synthesis observed in MS is also common in many chronic CNS inflammatory disorders. Assuming that cortical TLO in MS are associated with typical sub-pial lesions, we have coined the concept of "TLO-pathy" to describe these lesions and take examples of them from non-MS disorders. Lastly, we propose that intrathecal synthesis could be considered a strong hallmark of CNS TLO and might be used to monitor future TLO-targeted therapies.

Long-term B cell depletion in murine lupus eliminates autoantibody-secreting cells and is associated with alterations in the kidney plasma cell niche

Autoantibodies to dsDNA, produced by autoreactive plasma cells (PCs), are a hallmark of systemic lupus erythematosus and play a key role in disease pathogenesis. Recent data suggest that autoreactive PCs accumulate not only in lymphoid tissues, but also in the inflamed kidney in lupus nephritis. We hypothesized that the variable efficacy of anti-CD20 (rituximab)-mediated B cell depletion in systemic lupus erythematosus may be related to the absence of an effect on autoreactive PCs in the kidney. In this article, we report that an enrichment of autoreactive dsDNA Ab-secreting cells (ASCs) in the kidney of lupus-prone mice (up to 40% of the ASCs) coincided with a progressive increase in splenic germinal centers and PCs, and an increase in renal expression for PC survival factors (BAFF, a proliferation-inducing ligand, and IL-6) and PC attracting chemokines (CXCL12). Short-term treatment with anti-CD20 (4 wk) neither decreased anti-dsDNA nor IgG ASCs in different anatomical locations. However, long-term treatment (12 wk) significantly reduced both IgG- and dsDNA-specific ASCs. In addition, long-term treatment substantially decreased splenic germinal center and PC generation, and unexpectedly reduced the expression for PC survival factors in the kidney. These results suggest that prolonged B cell depletion may alter the PC survival niche in the kidney, regulating the accumulation and maintenance of autoreactive PCs.

Plasma cells within granulomatous inflammation display signs pointing to autoreactivity and destruction in granulomatosis with polyangiitis

Introduction

Plasma cells residing in inflamed tissues produce antibodies in chronic inflammatory and systemic autoimmune diseases. This study examined if plasma cells, located within inflamed nasal tissue in granulomatosis with polyangiitis (GPA), express features potentially associated with the autoimmune and destructive character of this disease.

Methods

Ig gene mutation patterns of individual tissue-derived plasma cells from GPA (n = 5) were analyzed, by using laser-assisted microdissection followed by semi-nested polymerase chain reaction (PCR). Signs of B-lymphocyte maturation (ectopic lymphoid structures, ELS) and survival (a proliferation-inducing ligand, APRIL; B-cell maturation antigen, BCMA; transmembrane-activator and calcium modulator and cyclophilin interactor, TACI; receptor activator of nuclear factor κB ligand, RANKL) were examined in nasal tissues or serum, respectively, by using immunohistochemistry/fluorescence and enzyme-linked immunosorbent assay, ELISA.

Results

Plasma-cell derived Ig genes (light- and heavy-chain pairs, n = 4; heavy chains, n = 33) resembled mutation patterns seen in other autoimmune diseases, predominantly displaying selection against replacement mutations within the framework region of Ig genes (10 of 15), which is responsible for structural integrity. Ectopic lymphoid structures were similar between GPA and a disease control (that is, unspecific chronic rhinosinusitis. However, histomorphologic features distinguishing GPA from rhinosinusitis (that is, neutrophilic microabscess and granuloma) expressed considerable amounts of membrane-associated and secreted APRIL, respectively. The latter was co-localized with CD138 and found in close proximity to cells expressing IgG, TACI, and BCMA. Interestingly, plasma cells strongly expressed receptor activator of nuclear factor κB ligand (RANKL), apart from fibroblast-like cells.

Conclusions

Plasma cells within granulomatous inflammation appear to display features that might be required for autoreactivity and, possibly, RANKL-mediated destruction in GPA.

Intrathecal immune reset in multiple sclerosis: exploring a new concept

Multiple sclerosis impairment is mainly driven by the progressive phase, whose pathology remains elusive. No drug has yet been able to halt this phase so therapeutic management remains challenging. It was recently demonstrated that late disability correlates with the spreading of cortical subpial lesions, and tertiary lymphoid organs (TLO) were identified in close apposition with these lesions. TLO are of crucial importance since they are able to mount a complete local immune response, as observed in the intrathecal compartment from the moment MS is diagnosed (i.e. oligoclonal bands). This article examines the consequences of this intrathecal response: giving a worst clinical prognostic value and bearing arguments for possible direct brain toxicity, intrathecal secretion should be targeted by drugs abating both B-lymphocytes and plasma cells. Another consequence is that intrathecal secretion has value as a surrogate marker of the persistence of an ongoing intrathecal immune reaction after treatment. Although it is still unsure which mechanism or byproduct secreted by TLO triggers cortical lesions, we propose to target TLO components as a new therapeutic avenue in progressive MS. Whereas it was long considered that the inability of therapies to penetrate the blood-brain-barrier was a crucial obstacle, our proposed strategy will take advantage of the properties of the BBB to safely reset the intrathecal immune system in order to halt the slow axonal burning underlying secondary MS. We review the literature in support of the rationale for treating MS with intrathecal drugs dedicated to clearing the local immune response. Since many targets are involved, achieving this goal may require a combination of monoclonal antibodies targeting each cell sub-type. Hope might be rekindled with a one-shot intrathecal multi-drug treatment in progressive MS.

A replicative self-renewal model for long-lived plasma cells: questioning irreversible cell cycle exit

Plasma cells are heterogenous in terms of their origins, secretory products, and lifespan. A current paradigm is that cell cycle exit in plasma cell differentiation is irreversible, following a pattern familiar in short-lived effector populations in other hemopoietic lineages. This paradigm no doubt holds true for many plasma cells whose lifespan can be measured in days following the completion of differentiation. Whether this holds true for long-lived bone marrow plasma cells that are potentially maintained for the lifespan of the organism is less apparent. Added to this the mechanisms that establish and maintain cell cycle quiescence in plasma cells are incompletely defined. Gene expression profiling indicates that in the transition of human plasmablasts to long-lived plasma cells a range of cell cycle regulators are induced in a pattern that suggests a quiescence program with potential for cell cycle re-entry. Here a model of relative quiescence with the potential for replicative self-renewal amongst long-lived plasma cells is explored. The implications of such a mechanism would be diverse, and the argument is made here that current evidence is not sufficiently strong that the possibility should be disregarded.

STAT-3 activation by differential cytokines is critical for human in vivo-generated plasma cell survival and Ig secretion

Maturation and survival of plasma cells (PCs) depends on extrinsic factors provided in specialized niches. In addition, B lymphocyte differentiation into PCs requires the activation of the JAK-STAT-3 pathway. However, whether STAT-3 is needed only during the transition of B lymphocytes to PC, or it is also involved in the survival and function of PCs at different stages of maturation, has not been unequivocally clarified. This study analyzes the effect of IL-10, IL-21, and IL-6 on human in vivo-generated PCs isolated from secondary lymphoid organs, blood (circulating, recently Ag-induced PCs), and bone marrow. PCs from these different organs show specific profiles of receptors for, and responsiveness to, these cytokines required for their survival and sustained Ab secretion. However, IL-10, IL-21, and IL-6 commonly induce STAT-3 phosphorylation in the three PC subsets, and all of their effects are exerted strictly through the STAT-3 activation. The inhibition or nonactivation of this pathway in the three PC populations impairs not only the effect of STAT-3-activating cytokines, but also the action of other cytokines important at the PC level, including a proliferation-induced ligand, BAFF, insulin-like growth factor 1, vascular endothelial growth factor, and stromal cell-derived factor-1α. These results indicate that STAT-3 activation is critical for human PCs throughout their maturation.

Plasma cell homeostasis: the effects of chronic antigen stimulation and inflammation

Long-lived plasma cells (LLPCs) that maintain humoral immunity to previously encountered Ags occupy a compartment in the bone marrow (BM). The rules and mechanisms by which cells enter (and leave) this compartment are poorly understood. We looked at what happens to the LLPC compartment and to plasma cell lifespan in general, in situations in which Ag stimulation and/or inflammation persist. We find that chronic Ag supply causes the generation of short-lived plasma cells in the local lymphoid organ, at the expense of any LLPC production. Furthermore, we find that inflammation caused by infection (mediated via TNF-α) causes a dramatic mobilization of LLPCs from the BM, with a concomitant reduction in circulating Ab levels against previously immunized Ags. These data are discussed in the context of the capacity of the BM LLPC compartment and competition for entry to it.