Plasma cells: finding new light at the end of B cell development

Rituximab for reduction of anti-HLA antibodies in patients awaiting renal transplantation: 1. Safety, pharmacodynamics, and pharmacokinetics1

BACKGROUND

Preformed HLA antibodies (Ab), reported as panel-reactive antibody (PRA), prolong patient waiting time for kidney transplantation. We hypothesized that rituximab (RTX) could reduce PRA via B-cell depletion. This initial study reports the safety, pharmacokinetics, and pharmacodynamics of RTX in patients with end-stage renal failure.

METHODS

The study was an investigator-initiated single-dose, dose-escalation phase I trial of RTX in chronic dialysis patients (PRA >50%). It was approved by the Institutional Review Board and the Food and Drug Administration. Nine subjects were treated with a single dose of RTX (n=3 per group) at 50, 150, or 375 mg/m. Peripheral lymphocyte cell surface markers and HLA Ab levels (%PRA and titers) were tested using flow cytometry.

RESULTS

There were four significant adverse events: a suspected histoplasmosis infection; two Tenchkoff dialysis catheter infections; and fever (38.7 degrees C) during infusion. At 2 days after RTX therapy, there was depletion of CD19 cells (pre-RTX 181+/-137 vs. post-RTX 12+/-5.6, P =0.006). In 2 (22%) of 9 subjects, there was no appreciable change in PRA. Among the other seven patients, one had a decrease in PRA from 87% to 51% with a concurrent decrease in fluorescence intensity; five patients had changes in histogram architecture suggesting loss of antibody specificity; and one patient had a fourfold decrease in PRA titer from 1:64 to 1:16 at 6 months after treatment. In addition, one of the seven patients converted a donor-specific crossmatch to negative and underwent a successful living donor kidney transplantation.

CONCLUSIONS

RTX can be safely administered and may be an effective agent to reduce high-titer anti-HLA Abs in subjects awaiting kidney transplantation.

The use of rituximab, anti-CD20 monoclonal antibody, in pediatric transplantation

Rituximab, chimeric anti-human CD-20, is approved for treatment of B-cell lymphoma in adults. It is being used experimentally in other various immune-related disease such as immune thrombocytopenic purpura, myasthenia gravis, and rheumatoid arthritis. In transplant recipients it is used for treatment of post-transplant lymphoproliferative disease, and prevention and treatment of acute rejection. There are few data on its use in children. This paper reviews the use of rituximab in these disease states and provides hypotheses for its mode of action.

Infection and inflammation in skeletal muscle from nonhuman primates infected with different genospecies of the Lyme disease spirochete Borrelia burgdorferi

Lyme borreliosis is a multisystemic disease caused by various genospecies of the spirochete Borrelia burgdorferi. To investigate muscle involvement in the nonhuman primate (NHP) model of Lyme disease, 16 adult Macaca mulatta animals inoculated with strain N40 of B. burgdorferi sensu strictu by syringe or by tick bite or with strain Pbi of B. burgdorferi genospecies garinii by syringe were studied. Animals were necropsied while immunosuppressed on day 50 (two animals each inoculated with B. burgdorferi N40 by syringe and with B. garinii Pbi by syringe) or on day 90, 40 days after immunosuppression had been discontinued (four animals each inoculated with strain N40 by syringe, with strain N40 by tick bite, and with strain Pbi by syringe). Skeletal muscles removed at necropsy were studied by (i) microscopic examination of hematoxylin-eosin-stained sections for inflammation and tissue injury; (ii) immunohistochemical and digital image analyses for antibody and complement deposition and cellular inflammation; (iii) Western blot densitometry for the presence of antibodies; and (iv) reverse transcription-PCR for measurement of the spirochetal load or C1q (the first component of the complement cascade) synthesis. The results showed that N40 was more infectious for NHPs than Pbi. NHPs inoculated with N40 but not with Pbi developed myositis. The inflammation in skeletal muscle was more severe in NHPs inoculated with N40 by syringe than in those inoculated by tick bite. The predominant cells in the inflammatory infiltrate were T cells and plasma cells. The deposition of antibody and complement in inflamed muscles from N40-inoculated NHPs was significantly higher than that in Pbi-inoculated NHPs. The spirochetal load was very high in the two N40-inoculated NHPs examined while they were immunosuppressed but decreased to minimal levels in the NHPs when immunocompetence was restored. We conclude that myositis can be a prominent feature of Lyme borreliosis depending on the infecting organism and host immune status.

Aiolos is required for the generation of high affinity bone marrow plasma cells responsible for long-term immunity

Antigenic encounter generates long-term immunity sustained by long-lived high affinity plasma cells resident in the bone marrow (BM). Here we show that the Ikaros family member, Aiolos, is specifically required for the generation of these plasma cells. Failure to generate high affinity plasma cells in the BM and to sustain serum antibody titers is apparent after both primary and secondary immunization of Aiolos(-)(/)(-) mice with a range of hapten concentrations. Chimera reconstitutions demonstrate that the BM plasma cell defect is B cell intrinsic. Lack of Aiolos does not alter expression of any of the previously described factors required for general plasma cell differentiation. No defect in somatic hypermutation, the generation of memory B cells, or short-lived high affinity plasma cells in the spleen was observed upon rechallenge. These studies support a model by which the high affinity plasma cell population in the BM undergoes a unique differentiation program that is dependent on Aiolos.

B cell abnormalities in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic, multisystem autoimmune disease characterized by the differentiation of short- and long-lived immunoglobulin secreting plasma cells that secrete pathogenic autoantibodies. Ectopic germinal centers and plasma cells secreting autoantibodies have been observed in lupus nephritis kidneys. Candidate genetic susceptibility loci for SLE include genes that affect differentiation and survival of plasma cells, such as those that influence activation, proliferation, cytokine and chemokine secretion/responsiveness, and apoptosis of the T and B cells that are involved in humoral immunity generated in germinal centers, as well as genes that are involved in presentation and clearance of apoptotic material and autoantigens by antigen presenting cells and other phagocytes. Emerging data have demonstrated that B lymphocytes are active participants in humoral immune responses that lead to T-dependent and T-independent differentiation of immunoglobulin-secreting plasma cells by homotypic CD154–CD40 interactions as well as continued stimulation by B cell activating factor through B cell maturation antigen, B cell activating factor receptor and transmembrane activater.

Evidence from the generation of immunoglobulin G–secreting cells that stochastic mechanisms regulate lymphocyte differentiation

Naive B lymphocytes undergo isotype switching and develop into immunoglobulin-secreting cells to generate the appropriate class and amount of antibody necessary for effective immunity. Although this seems complex, we report here that the generation of immunoglobulin G-secreting cells from naive precursors is highly predictable. The probabilities of isotype switching and development into secreting cells change with successive cell divisions and interleave independently. Cytokines alter the probability of each differentiation event, while leaving intact their independent assortment. As a result, cellular heterogeneity arises automatically as the cells divide. Stochastic division-linked regulation of heterogeneity challenges the conventional paradigms linking distinct phenotypes to unique combinations of signals and has the potential to simplify our concept of immune complexity considerably.

Characterization of clonogenic multiple myeloma cells

The identity of the cells responsible for the initiation and maintenance of multiple myeloma (MM) remains unclear largely because of the difficulty growing MM cells in vitro and in vivo. MM cell lines and clinical specimens are characterized by malignant plasma cells that express the cell surface antigen syndecan-1 (CD138); however, CD138 expression is limited to terminally differentiated plasma cells during B-cell development. Moreover, circulating B cells that are clonally related to MM plasma cells have been reported in some patients with MM. We found that human MM cell lines contained small (< 5%) subpopulations that lacked CD138 expression and had greater clonogenic potential in vitro than corresponding CD138+ plasma cells. CD138- cells from clinical MM samples were similarly clonogenic both in vitro and in nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice, whereas CD138+ cells were not. Furthermore, CD138- cells from both cell lines and clinical samples phenotypically resembled postgerminal center B cells, and their clonogenic growth was inhibited by the anti-CD20 monoclonal antibody rituximab. These data suggest that MM "stem cells" are CD138- B cells with the ability to replicate and subsequently differentiate into malignant CD138+ plasma cells.

Plasma-cell homing

Recent studies indicate that chemoattractant cytokines (chemokines), together with tissue-specific adhesion molecules, coordinate the migration of antibody-secreting cells (ASCs) from their sites of antigen-driven differentiation in lymphoid tissues to target effector tissues. Developing ASCs downregulate the expression of receptors for lymphoid tissue chemokines and selectively upregulate the expression of chemokine receptors that might target the migration of IgA ASCs to mucosal surfaces, IgG ASCs to sites of tissue inflammation and both types of ASC to the bone marrow - an important site for serum antibody production. By directing plasma-cell homing, chemokines might help to determine the character and efficiency of mucosal, inflammatory and systemic antibody responses.

EBV-encoded latent membrane protein 1 cooperates with BAFF/BLyS and APRIL to induce T cell-independent Ig heavy chain class switching

By substituting the H chain C region of IgM with that of IgG, IgA, or IgE, class switching enables Abs to acquire new effector functions that are crucial for the neutralization of invading pathogens. Class switching occurs through class switch DNA recombination (CSR) and usually requires engagement of CD40 on B cells by CD40 ligand on Ag-activated CD4(+) T cells. CSR must be tightly regulated because abnormal IgG and IgA production favors the onset of autoimmunity, whereas increased switching to IgE leads to atopy. These inflammatory disorders can be triggered or exacerbated by EBV infection. In this study, we show that EBV induces CD40-independent CSR from C( micro ) to multiple downstream C(gamma), C(alpha), and C(epsilon) genes through latent membrane protein 1 (LMP1), a CD40-like viral protein that signals in a ligand-independent fashion. LMP1-induced CSR is associated with transcriptional activation of germline C(gamma), C(alpha), and C(epsilon) genes and triggers the up-regulation of activation-induced cytidine deaminase, a crucial component of the CSR machinery. In addition, LMP1 induces B cells to express B cell-activating factor of the TNF family and a proliferation-inducing ligand, two molecules that mediate B cell survival and T cell-independent Ab production. B cell-activating factor of the TNF family and a proliferation-inducing ligand cooperate with LMP1 to induce Ig class switching because their neutralization by appropriate soluble decoy receptors attenuates CSR in LMP1-expressing B cells. By showing that LMP1 triggers T cell-independent CSR, our findings suggest that EBV could play an important role in the pathogenesis of disorders with aberrant IgG, IgA, and/or IgE production.

Blimp-1 is required for the formation of immunoglobulin secreting plasma cells and pre-plasma memory B cells

Blimp-1 is a transcriptional repressor able to drive the terminal differentiation of B cells into Ig-secreting plasma cells. We have created mice with a B cell-specific deletion of prdm1, the gene encoding Blimp-1. B cell development and the number of B cells responding to antigen appear to be normal in these mice. However, in response to either TD or TI antigen, serum Ig, short-lived plasma cells, post-GC plasma cells, and plasma cells in a memory response are virtually absent, demonstrating that Blimp-1 is required for plasmacytic differentiation and Ig secretion. In the absence of Blimp-1, CD79b(+)B220(-) pre-plasma memory B cell development is also defective, providing evidence that this subset is an intermediate in plasma cell development. B cells lacking Blimp-1 cannot secrete Ig or induce muS mRNA when stimulated ex vivo. Furthermore, although prdm1-/- B cells fail to induce XBP-1, XBP-1 cannot rescue plasmacytic differentiation without Blimp-1.

B-cell and plasma-cell splicing differences: a potential role in regulated immunoglobulin RNA processing

The immunoglobulin micro pre-mRNA is alternatively processed at its 3' end by competing splice and cleavage-polyadenylation reactions to generate mRNAs encoding the membrane-associated or secreted forms of the IgM protein, respectively. The relative use of the competing processing pathways varies during B-lymphocyte development, and it has been established previously that cleavage-polyadenylation activity is higher in plasma cells, which secrete IgM, than in B cells, which produce membrane-associated IgM. To determine whether RNA-splicing activity varies during B-lymphocyte development to contribute to micro RNA-processing regulation, we first demonstrate that micro pre-mRNA processing is sensitive to artificial changes in the splice environment by coexpressing SR proteins with the micro gene. To explore differences between the splice environments of B cells and plasma cells, we analyzed the splicing patterns from two different chimeric non-Ig genes that can be alternatively spliced but have no competing cleavage-polyadenylation reaction. The ratio of intact exon splicing to cryptic splice site use from one chimeric gene differs between several B-cell and several plasma-cell lines. Also, the amount of spliced RNA is higher in B-cell than plasma-cell lines from a set of genes whose splicing is dependent on a functional exonic splice enhancer. Thus, there is clear difference between the B-cell and plasma-cell splicing environments. We propose that both general cleavage-polyadenylation and general splice activities are modulated during B-lymphocyte development to ensure proper regulation of the alternative micro RNA processing pathways.

Insights into extramedullary tumour cell growth revealed by expression profiling of human plasmacytomas and multiple myeloma

Malignant plasma cells generally grow within the bone marrow microenvironment; however, they can also grow at extramedullary sites. To identify the tumour-specific alterations required for extramedullary growth, we analysed the expression profiles of a series of plasma cell neoplasms including primary multiple myeloma (MM), plasma cell leukaemia (PCL) and extramedullary plasmacytoma (EPC). Hierarchical clustering analysis segregated the EPCs from the remaining samples, and revealed an expression pattern associated with angiogenesis in the EPCs, involving higher expression of the genes TIE2, NOTCH3, CD31 and endoglin. Direct comparison of EPC samples with the MM samples identified 156 genes significantly upregulated and 85 genes significantly downregulated (P < 0.005, t-test) in the EPCs, including several genes involved in angiogenesis and adhesion that were upregulated (including angiopoietin 1, SPARC, Notch3 and fibronectin 1). Immunohistochemical staining demonstrated CD31 and endoglin protein expression in the EPC tumour cells, which are both angiogenesis related and could confer malignant plasma cells with the ability to grow outside the normal bone marrow environment. Defining how malignant plasma cell growth is regulated in the bone marrow versus at extramedullary sites will help to delineate the mechanisms underlying the dependence of tumour cell growth on angiogenesis and cell adhesion.

New insights into the pathophysiology of multiple myeloma

For understanding of the pathophysiology of multiple myeloma, features of the malignant clone and changes induced by the bone-marrow microenvironment are equally important. Multiple myeloma plasma cells, which originate from postfollicular B cells, are characterised by complex chromosomal aberrations. Among the earliest genetic events are translocations of the immunoglobulin heavy-chain gene locus, which leads to dysregulation of oncogenes at translocation partner regions (cyclin D1 at 11q13, FGFR3/MMSET at 4p16.3, c-MAF at 16q23, and cyclin D3 at 6p21), and deletions of 13q14, the site of a putative tumour suppressor gene, which is an adverse prognostic indicator. Additional molecular events include epigenetic changes and activation of oncogenes (mutations of N-RAS and K-RAS, and changes in c-MYC), which are usually associated with disease progression. Bone-marrow stromal cells support growth and survival of multiple myeloma cells via various cytokines. Osteoclast activity factors (in particular MIP1alpha) and imbalances between RANKL and osteoprotegerin are major factors for the development of myeloma bone disease. Further characterisation of crucial events in the development of monoclonal gammopathies by novel techniques such as global gene expression profiling will contribute to a molecular classification of multiple myeloma and foster future therapeutic approaches.

The biology of IGE and the basis of allergic disease

Allergic individuals exposed to minute quantities of allergen experience an immediate response. Immediate hypersensitivity reflects the permanent sensitization of mucosal mast cells by allergen-specific IgE antibodies bound to their high-affinity receptors (FcepsilonRI). A combination of factors contributes to such long-lasting sensitization of the mast cells. They include the homing of mast cells to mucosal tissues, the local synthesis of IgE, the induction of FcepsilonRI expression on mast cells by IgE, the consequent downregulation of FcgammaR (through an insufficiency of the common gamma-chains), and the exceptionally slow dissociation of IgE from FcepsilonRI. To understand the mechanism of the immediate hypersensitivity phenomenon, we need explanations of why IgE antibodies are synthesized in preference to IgG in mucosal tissues and why the IgE is so tenaciously retained on mast cell-surface receptors. There is now compelling evidence that the microenvironment of mucosal tissues of allergic disease favors class switching to IgE; and the exceptionally high affinity of IgE for FcepsilonRI can now be interpreted in terms of the recently determined crystal structures of IgE-FcepsilonRI and IgG-FcgammaR complexes. The rate of local IgE synthesis can easily compensate for the rate of the antibody dissociation from its receptors on mucosal mast cells. Effective mechanisms ensure that allergic reactions are confined to mucosal tissues, thereby minimizing the risk of systemic anaphylaxis.

The rise and fall of long-lived humoral immunity: terminal differentiation of plasma cells in health and disease

Long-lived humoral immune responses are a hallmark of thymus-dependent immunity. The cellular basis for enduring antibody-mediated immunity is long-lived memory B cells and plasma cells (PCs). Both of these cell populations acquire longevity as a result of antigen-specific, CD40-dependent, cognate interactions with helper T cells within germinal centers (GCs). At the molecular level, defined functional domains of CD40 control the post-GC fate of B cells. PC precursors that emerge from these GC reactions are highly proliferative and terminally differentiate to end-stage cells within the bone marrow (BM). The striking phenotypic similarities between the PC precursors and the putative malignant cell in multiple myeloma (MM) suggests that MM may result from the transformation of PC precursors. Within the domain of autoimmune disease, recent studies have shown that dysregulated migration of PCs to the BM may impact immune homeostasis and the development of lupus. Understanding the processes of normal PC differentiation will provide strategic insights into identifying therapeutic targets for the treatment of differentiated B-cell disorders.

Homing of antibody secreting cells

As activated B cells differentiate into plasma cells they complete a final series of migration steps that take them to locations where they can efficiently carry out their effector function, secreting immunoglobulin (Ig) M or IgG into circulation or releasing dimeric IgA adjacent to the epithelium for transcytosis. Recent experiments have established a key role for chemokines in directing antibody secreting cell (ASC) movement within the secondary lymphoid organs where they are generated, as well as in guiding the cells to the bone marrow or mucosal surfaces. This review discusses the chemokines involved in directing ASC movements, particularly focusing on the role of CXCR4 and CXCL12/SDF1. The function of CCR9 and CCR10 in IgA ASC homing and contributions made by integrins and lectins are also discussed.

Interpreting the molecular biology and clinical behavior of multiple myeloma in the context of global gene expression profiling

Multiple myeloma (MM) is a rare but uniformly fatal malignancy of antibody-secreting plasma cells (PCs). Although several key molecular events in disease initiation or progression have been confirmed (e.g. FGFR3/MMSET activation) or implicated (e.g. chromosome 13 deletion), the mechanisms of MM development remain enigmatic. Importantly, although generally being indistinguishable morphologically, MM exhibits a tremendous degree of variability in clinical course, with some patients surviving only months and others many years. However, current laboratory parameters can account for no more than 20% of this outcome variability. Furthermore, the means by which current drugs impart their anti-MM effect are also mostly unknown. In addition, the mechanisms by which MM cells contribute to serious comorbidities, such as osteopenia and/or focal lytic lesions of bone, are also poorly understood. Finally, very little knowledge exists concerning the molecular events leading to benign hyperplasia and/or overt malignancy of PCs. Given that abnormal gene expression lies at the heart of most, if not all, cancers, high-throughput global gene expression profiling has become a powerful tool for investigating molecular biology and clinical behavior of diseases. Here, we discuss recent progress made in addressing many of the above issues through the molecular dissection of the transcriptome of normal PCs and MM.

Transcriptional regulatory cascades controlling plasma cell differentiation

Plasma cells are the terminally differentiated effector cells of the B lymphocyte lineage. Recently, studies using genetically altered mice and analyses of global gene expression programs have significantly expanded our understanding of the molecular mechanisms regulating plasmacytic differentiation. Specific molecular components of a multistep cascade of transcriptional regulators have been identified. Furthermore, two transcriptional regulators, X box binding protein-1 (XBP-1) and B lymphocyte induced maturation protein-1 (Blimp-1), have been shown to be necessary for plasmacytic differentiation. In addition to providing a mechanistic basis for the induction of genes necessary for immunoglobulin secretion, cessation of cell cycle and other phenotypic changes characteristic of terminally differentiated plasma cells, these studies have led to the important concept that plasmacytic differentiation involves repression of regulators, such as Bcl-6 and Pax5, that are necessary to maintain the earlier developmental phenotype of activated, germinal center B cells. This review describes our current understanding of the transcriptional cascades regulating terminal differentiation of B cells.

Neoplastic development in plasma cells

An increasing number of model systems of plasma cell tumor (PCT) formation have been and are being developed. Discussed here are six models in mice and multiple myeloma (MM) in humans. Each model illustrates a unique set of biological factors. There are two general types of model systems: those that depend upon naturally arising mutagenic changes (pristane-induced PCTs, 5TMM, and MM) and those that are associated with oncogenes (Emu-v-abl), growth factors [interleukin-6 (IL-6)], and anti-apoptotic factors (Bcl-xL/Bcl-2). PCTs develop in several special tissue microenvironments that provide essential cytokines (IL-6) and cell-cell interactions. In mice, the activation and deregulation of c-myc by chromosomal translocations is a major feature in many of the models. This mechanism is much less a factor in MM and the 5T model in mice. Genetically determined susceptibility is involved in many of the mouse models, but only a few genes have been implicated thus far.

The X-box binding protein-1 transcription factor is required for plasma cell differentiation and the unfolded protein response

X-box binding protein-1 (XBP-1) is a transcription factor essential for plasma cell differentiation. XBP-1 transcripts are found at high levels in plasma cells from rheumatoid synovium and myeloma cell lines. Lymphoid chimeras deficient in XBP-1 have a profound defect in plasma cell differentiation, with few plasma cells in their periphery and severely reduced serum immunoglobulin levels. When introduced into B-lineage cells, XBP-1 initiates plasma cell differentiation. XBP-1 is also the mammalian homologue of the yeast transcription factor Hac1p, an important component of the unfolded protein response (UPR). The UPR allows cells to tolerate conditions of endoplasmic reticulum (ER) stress caused by misfolded proteins. Studies examining the relationship between plasma cell differentiation, XBP-1, and the UPR demonstrate that this novel signaling system is vital for plasma cell differentiation. Signals that induce plasma cell differentiation and the UPR cooperate via XBP-1 to induce terminal B-cell differentiation. Additionally, XBP-1 plays an important role in the regulation of interleukin-6 production, a cytokine essential for plasma cell survival.

Gene expression profiling of plasma cells and plasmablasts: toward a better understanding of the late stages of B-cell differentiation

Plasma cells (PCs), the end point of B-cell differentiation, are a heterogeneous cell compartment comprising several cell subsets from short-lived highly proliferative plasmablasts to long-lived nondividing fully mature PCs. Whereas the major transcription factors driving the differentiation of B cells to PCs were recently identified, the subtle genetic changes that underlie the transition from plasmablasts to mature PCs are poorly understood. We recently described an in vitro model making it possible to obtain a large number of cells with the morphologic, phenotypic, and functional characteristics of normal polyclonal plasmablastic cells (PPCs). Using Affymetrix microarrays we compared the gene expression profiles of these PPCs with those of mature PCs isolated from tonsils (TPCs) and bone marrow (BMPCs), and with those of B cells purified from peripheral blood (PBB cells) and tonsils (TBCs). Unsupervised principal component analysis clearly distinguished the 5 cell populations on the basis of their differentiation and proliferation status. Detailed statistical analysis allowed the identification of 85 PC genes and 40 B-cell genes, overexpressed, respectively, in the 3 PC subsets or in the 2 B-cell subsets. In addition, several signaling molecules and antiapoptotic proteins were found to be induced in BMPCs compared with PPCs and could be involved in the accumulation and prolonged survival of BMPCs in close contact with specialized stromal microenvironment. These data should help to better understand the molecular events that regulate commitment to a PC fate, mediate PC maintenance in survival niches, and could facilitate PC immortalization in plasma cell dyscrasias.

Short term administration of costimulatory blockade and cyclophosphamide induces remission of systemic lupus erythematosus nephritis in NZB/W F1 mice by a mechanism downstream of renal immune complex deposition

NZB/W F(1) mice with established nephritis were treated with a single dose of cyclophosphamide with or without a 2-wk course of murine CTLA4Ig, either alone or in combination with anti-CD154. Sixty to 80% of treated mice entered remission, and remission could be reinduced following relapse. A decrease in the frequency of anti-DNA-producing B cells and activated T cells was observed in treated mice, but this effect lasted only 3-6 wk, while remissions were sustained for up to 20 wk. Light microscopy of the kidneys of mice in remission revealed less glomerular inflammation, less tubular damage, and less infiltration of inflammatory cells. By immunofluorescence, however, IgG and C3 staining of glomeruli was no different in treated mice vs controls. Since chemokines and their receptors play an important role in inflammatory cell infiltration of affected organs in autoimmune diseases, we examined chemokine expression in the kidneys. Decreases in the expression of inflammatory cytokines and chemokines were evident in mice in the early stages of remission, but these differences were no longer present in late remission. Increased expression of CXCL13 was detected in the inflammatory infiltrates of the control NZB/NZW mice. Strikingly, we could not detect any CXCL13 in the kidneys of the treated group even in late remission. These findings suggest that costimulatory blockade together with cyclophosphamide influence the activation state of renal CD11c-positive cells and therefore lead to less B and T cell infiltration and nephritis.

Lowering anti-dsDNA antibodies--what's new?

Antibodies to dsDNA are specific to SLE and are pathogenic, both due to their ability to deposit in tissues through a variety of mechanisms, and to their ability, when present in immune complexes, to activate inflammatory cells. The relationship of serum anti-dsDNA antibody levels to disease activity is a complex one and the factors that determine whether or not such antibodies will be pathogenic in an individual SLE patient are incompletely understood. Although anti-dsDNA antibodies can be made by naïve B cells and B cells belonging to the B1 and marginal zone subsets, pathogenic anti-dsDNA antibodies have the hallmarks of germinal center development and exposure to T cell help, including accumulation of somatic mutations and class switching to the IgG isotype. Epitope spreading may result in aquisition of cross-reactivities with multiple target organ antigens and aquisition of a memory phenotype will allow these B cells to acquire antigen presentation functions that amplify the autoreactive response. In the early stages of disease, or after remission induction protocols, autoreactive B cells may be susceptible to treatments that target T cell costimulation or that deplete or tolerize naïve and mature B cells. Therapeutic approaches targeting innate immune responses or regulatory T cells are starting to be tested in pre-clinical models. In later disease stages, memory and plasma cell accumulation may render patients more resistant to this type of therapeutic approach. Deposition of anti-dsDNA antibodies in target tissues can stimulate an inflammatory cascade that leads to tissue damage. A number of murine models have now been developed that show that interruption of this cascade can prevent or reverse such damage. This type of approach may be beneficial for individuals with established disease. As we learn more about the specific defects that cause SLE, it may become possible to individualize therapy based on patient specific biologic markers.

B cells as effectors

B cells act as immune effectors, primarily through antigen-specific clonal expansion and plasma-cell differentiation. B1 (CD5(+)) B cells and marginal zone B cells dominate T-cell independent humoral responses under the molecular control of activated dendritic cells. Helper T cell-regulated B-cell responses draw on follicular B cells as precursors and rely on qualitatively different patterns of immune synapse formation to regulate B-cell fate. These activities culminate in the germinal center reaction, during which somatic hypermutation and antigen-driven selection produce and preserve high-affinity plasma cells with extended longevity and memory B cells as the sensitized precursors for antigen recall.

Gene expression profiling reveals a highly specialized genetic program of plasma cells

The formation of terminally differentiated plasma cells represents the critical final step in B-cell differentiation. In this study, utilizing oligonucleotide microarray analysis, we describe the highly specialized genetic profile exhibited by terminally differentiated plasma cells. A total of 1476 known genes were differentially expressed by plasma cells compared with B cells. Plasma cells displayed an up-regulation, induction, or a selective retention of a unique constellation of transcription factors, including members of the AP-1, nuclear factor-kappaB (NF-kappaB), nuclear factor of activated T cells (NFAT), and octamer binding factor families. Interestingly, plasma cells also displayed a down-regulation of several RNA polymerase I- related factors, consistent with terminal differentiation, and exhibited a down-regulation of the TATA box binding protein. Furthermore, plasma cells displayed alterations in multiple components of the Wnt and Notch signaling pathways and showed a unique pattern of apoptosis and proliferation-associated genes. Unexpectedly, plasma cells displayed an up-regulation of 2 factors normally associated with microenvironmental positioning of neuronal cells, reelin and neuropilin-1. These results supply insight into the developmental genetics of plasma cell differentiation and provide a foundation for further analysis of plasma cell biology.

Gene expression profiles of cutaneous B cell lymphoma

We studied gene expression profiles of 17 cutaneous B cell lymphomas that were collected with 4-6 mm skin punch biopsies. We also included tissue from two cases of mycosis fungoides, three normal skin biopsies, and three tonsils to create a framework for further interpretation. A hierarchical cluster algorithm was applied for data analysis. Our results indicate that small amounts of skin tissue can be used successfully to perform microarray analysis and result in distinct gene expression patterns. Duplicate specimens clustered together demonstrating a reproducible technique. Within the cutaneous B cell lymphoma specimens two specific B cell differentiation stage signatures of germinal center B cells and plasma cells could be identified. Primary cutaneous follicular and primary cutaneous diffuse large B cell lymphomas had a germinal center B cell signature, whereas a subset of marginal zone lymphomas demonstrated a plasma cell signature. Primary and secondary follicular B cell lymphoma of the skin were closely related, despite previously reported genetic and phenotypic differences. In contrast primary and secondary cutaneous diffuse large B cell lymphoma were less related to each other. This pilot study allows a first glance into the complex and unique microenvironment of B cell lymphomas of the skin and provides a basis for future studies, which may lead to the identification of potential histologic and prognostic markers as well as therapeutic targets.

CCR10 expression is a common feature of circulating and mucosal epithelial tissue IgA Ab-secreting cells

The dissemination of IgA-dependent immunity between mucosal sites has important implications for mucosal immunoprotection and vaccine development. Epithelial cells in diverse gastrointestinal and nonintestinal mucosal tissues express the chemokine MEC/CCL28. Here we demonstrate that CCR10, a receptor for MEC, is selectively expressed by IgA Ab-secreting cells (large s/cIgA(+)CD38(hi)CD19(int/-)CD20(-)), including circulating IgA(+) plasmablasts and almost all IgA(+) plasma cells in the salivary gland, small intestine, large intestine, appendix, and tonsils. Few T cells in any mucosal tissue examined express CCR10. Moreover, tonsil IgA plasmablasts migrate to MEC, consistent with the selectivity of CCR10 expression. In contrast, CCR9, whose ligand TECK/CCL25 is predominantly restricted to the small intestine and thymus, is expressed by a fraction of IgA Ab-secreting cells and almost all T cells in the small intestine, but by only a small percentage of plasma cells and plasmablasts in other sites. These results point to a unifying role for CCR10 and its mucosal epithelial ligand MEC in the migration of circulating IgA plasmablasts and, together with other tissue-specific homing mechanisms, provides a mechanistic basis for the specific dissemination of IgA Ab-secreting cells after local immunization.

Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1

The transcription factor X-box binding protein 1 (XBP-1) is essential for the differentiation of plasma cells and the unfolded protein response (UPR). Here we show that UPR-induced splicing of XBP-1 by the transmembrane endonuclease IRE1 is required to restore production of immunoglobulin in XBP-1-/- mouse B cells, providing an integral link between XBP-1, the UPR and plasma cell differentiation. Signals involved in plasma cell differentiation, specifically interleukin-4, control the transcription of XBP-1, whereas its post-transcriptional processing is dependent on synthesis of immunoglobulins during B cell differentiation. We also show that XBP-1 is involved in controlling the production of interleukin-6, a cytokine that is essential for plasma cell survival. Thus, signals upstream and downstream of XBP-1 integrate plasma cell differentiation with the UPR.

IL-6-independent expression of Mcl-1 in human multiple myeloma

Mcl-1 is a critical antiapoptotic survival factor for human multiple myeloma (MM). We examined the importance of IL-6 for Mcl-1 expression in five MM cell lines and in primary MM cells from 14 patients. While culture of MM.1S cells in IL-6 did induce Mcl-1 expression, four other MM cell lines exhibited high levels of Mcl-1 expression that were unaffected by IL-6. Similarly, Mcl-1 expression in 10 of 14 primary MM isolates was found to be IL-6-independent. An analysis of the mechanisms responsible for IL-6-independent Mcl-1 expression was undertaken. ERK1/2 activity did not influence Mcl-1 expression, distinct from Mcl-1 regulation that occurs during myeloid differentiation from hematopoietic progenitor cells. Inhibition of the PI3K pathway led to growth inhibition of 8226 and ANBL-6 cells without reduction of Mcl-1 levels, and high level Mcl-1 expression was maintained in the absence of activated STAT3. Analysis of the transcriptional activity of 5'-regulatory sequences from the human Mcl-1 gene in MM cells demonstrated high levels of IL-6-independent indicator gene activation as predicted. These data demonstrate that the mechanisms regulating Mcl-1 levels in MM cells are heterogeneous, and are often independent from IL-6 signaling pathways.

Transcriptional analysis of the B cell germinal center reaction

The germinal center (GC) reaction is crucial for T cell-dependent immune responses and is targeted by B cell lymphomagenesis. Here we analyzed the transcriptional changes that occur in B cells during GC transit (naive B cells --> centroblasts --> centrocytes --> memory B cells) by gene expression profiling. Naive B cells, characterized by the expression of cell cycle-inhibitory and antiapoptotic genes, become centroblasts by inducing an atypical proliferation program lacking c-Myc expression, switching to a proapoptotic program, and down-regulating cytokine, chemokine, and adhesion receptors. The transition from GC to memory cells is characterized by a return to a phenotype similar to that of naive cells except for an apoptotic program primed for both death and survival and for changes in the expression of cell surface receptors including IL-2 receptor beta. These results provide insights into the dynamics of the GC reaction and represent the basis for the analysis of B cell malignancies.

The snRNP-associated U1A levels change following IL-6 stimulation of human B-cells

The U1A protein can be found both in a small-ribonucleoprotein particle (snRNP) that contains U1 RNA, or in a distinctive fraction, free of the snRNP, the SF-A complex. Both components have been shown to influence post- or co-transcriptional RNA processing reactions in HeLa cells. Since U1A may influence the processing of the immunoglobulin heavy chain pre-mRNA in B-cells, we wanted to see if the levels of U1A in either of its two forms changed following IL-6 stimulation to IgM secretion. Using antibodies that specifically recognize the two forms of U1A, snRNP-associated and snRNP-free, we found that approximately 16% of U1A is in the SF-A form in B-cells. We measured the levels of U1A protein in its two states in human B-cell lines both by flow cytometry and exhaustive immunoprecipitations. We found a significant decrease in the amount of snRNP-associated U1A following cytokine stimulation that correlates with the change-over to the secretory-specific poly(A) site use in the SKW 6.4 cell line. Meanwhile, the number of U1A molecules in the SF-A fraction of the pool remains nearly constant following induction to secretion. Our results suggest that the changing level of U1A in the snRNP fraction may be important for influencing Ig heavy chain mRNA processing.

Gene expression profiling of human plasma cell differentiation and classification of multiple myeloma based on similarities to distinct stages of late-stage B-cell development

To identify genes linked to normal plasma cell (PC) differentiation and to classify multiple myeloma (MM) with respect to the expression patterns of these genes, we analyzed global mRNA expression in CD19-enriched B cells (BCs) from 7 tonsils, CD138-enriched PCs from 11 tonsils, 31 normal bone marrow samples, and 74 MM bone marrow samples using microarrays interrogating 6800 genes. Hierarchical clustering analyses with 3288 genes clearly segregated the 4 cell types, and chi-square and Wilcoxin rank sum tests (P <.0005) identified 359 and 500 previously defined and novel genes that distinguish tonsil BCs from tonsil PCs (early differentiation genes [EDGs]), and tonsil PCs from bone marrow PCs (late differentiation genes [LDGs]), respectively. MM as a whole was found to have dramatically variable expression of EDGs and LDGs, and one-way analysis of variance (ANOVA) was used to identify the most variable EDGs (vEDGs) and LDGs (v1LDG and v2LDG). Hierarchical cluster analysis with these genes revealed that previously defined MM gene expression subgroups (MM1-MM4) could be linked to one of the 3 normal cell types. Clustering with 30 vEDGs revealed that 13 of 18 MM4 cases clustered with tonsil BCs (P =.000 05), whereas 14 of 15 MM3 cases clustered with tonsil PCs when using 50 v1LDG (P =.000 008), and 14 of 20 MM2 cases clustered with bone marrow PCs when using 50 v2LDG (P =.000 09). MM1 showed no significant linkage with normal cell types studied. Thus, genes whose expression is linked to distinct transitions in late-stage B-cell differentiation can be used to classify MM.

3. Lymphocytes

The fundamental task of the immune system is to defend "self" from "nonself." Lymphocytes are the primary cells of the immune system that developed one of the most sophisticated and comprehensive defense mechanisms in the biological system. T cells play a central role in orchestrating the immune response. Further, they are instrumental in eliminating intracellular pathogens (viruses, some bacteria) through the generation of cytotoxic T cells. B cells defend against extracellular pathogens by producing antibodies. Natural killer cells are an important component of innate immunity. Dendritic cells play a key role in initiating the immune response by presenting foreign antigens to T cells. The interaction among T cells, B cells, dendritic cells, and natural killer cells constitute the fundamental defense network of the host. The failure of any of these components severely jeopardizes the integrity of the immune system and its ability to mount the most appropriate immune response.

Intrinsic differences in the proliferation of naive and memory human B cells as a mechanism for enhanced secondary immune responses

Humoral immune responses elicited after secondary exposure to immunizing Ag are characterized by robust and elevated reactivity of memory B cells that exceed those of naive B cells during the primary response. The mechanism underlying this difference in responsiveness of naive vs memory B cells remains unclear. We have quantitated the response of naive and memory human B cells after in vitro stimulation with T cell-derived stimuli. In response to stimulation with CD40 ligand alone or with IL-10, both IgM-expressing and Ig isotype-switched memory B cells entered their first division 20-30 h earlier than did naive B cells. In contrast, the time spent traversing subsequent divisions was similar. Consistent with previous studies, only memory cells differentiated to CD38(+) blasts in a manner that increased with consecutive division number. These differentiated CD38(+) B cells divided faster than did CD38(-) memory B cell blasts. Proliferation of CD40 ligand-stimulated naive B cells as well as both CD38(+) and CD38(-) cells present in cultures of memory B cells was increased by IL-10. In contrast, IL-2 enhanced proliferation of CD38(-) and CD38(+) memory B cell blasts, but not naive cells. Thus, memory B cells possess an intrinsic advantage over naive B cells in both the time to initiate a response and in the division-based rate of effector cell development. These differences help explain the accelerated Ab response exhibited by memory B cells after secondary challenge by an invading pathogen, a hallmark of immunological memory.

A division-linked mechanism for the rapid generation of Ig-secreting cells from human memory B cells

Memory B cells, when re-exposed to Ag and T cell help, differentiate into Ig-secreting cells (ISC) at the same time as maintaining a residual pool of non-Ig-secreting cells with memory capabilities. To investigate the mechanism underlying this dual process, we followed the fate of human B cells activated in vitro with the T cell-derived signals CD40 ligand (CD40L), IL-2, and IL-10 using CFSE to monitor cell division. A substantial number of ISCs detected by ELISPOT, intracellular Ig staining, and Ig secretion could be generated from memory but not naive B cells. The proportion of ISCs increased with successive cell divisions and was markedly enhanced by IL-10 at each division. Within ISCs, two distinct populations were detected after withdrawal of CD40L. The first had acquired the plasma cell marker CD38 and continued to proliferate despite the absence of CD40L. In contrast, the second population remained CD38(-), ceased dividing, and underwent rapid apoptosis. The former most likely represent the immediate precursors of long-lived plasma cells, which preferentially home to the bone marrow in vivo, whereas the latter contain short-lived ISCs responsible for the initial Ab response to stimulation with Ag and T cell help. Taken together, the results point to a division-based mechanism responsible not only for regulating differentiation of short- and long-lived ISCs from memory B cells, but for preserving the memory B cell pool for reactivation upon subsequent Ag exposure.

Serial analysis of gene expression in circulating gamma delta T cell subsets defines distinct immunoregulatory phenotypes and unexpected gene expression profiles

Gene expression profiles were compared in circulating bovine GD3.5+ (CD8-) and GD3.5- (predominantly CD8+) gammadelta T cells using serial analysis of gene expression (SAGE). Approximately 20,000 SAGE tags were generated from each library. A comparison of the two libraries demonstrated 297 and 173 tags representing genes with 5-fold differential expression in GD3.5+ and GD3.5- gammadelta T cells, respectively. Consistent with their localization into sites of inflammation, GD3.5+ gammadelta T cells appeared transcriptionally and translationally more active than GD3.5- gammadelta cells. GD3.5- gammadelta T cells demonstrated higher expression of the cell proliferation inhibitor BAP 37, which was associated with their less activated gene expression phenotype. The immune regulatory and apoptosis-inducing molecule, galectin-1, was identified as a highly abundant molecule and was higher in GD3.5+gammadelta T cells. Surface molecules attributed to myeloid cells, such as CD14, CD68, and scavenger receptor-1, were identified in both populations. Furthermore, expression of B lymphocyte-induced maturation protein, a master regulator of B cell and myeloid cell differentiation, was identified by SAGE analysis and was confirmed at the RNA level to be selectively expressed in gammadelta T cells vs alphabeta T cells. These results provide new insights into the inherent differences between circulating gammadelta T cell subsets.

Activation of an unfolded protein response during differentiation of antibody-secreting B cells

The unfolded protein response pathway (UPR) is believed to detect and compensate for excessive protein accumulation in the endoplasmic reticulum (ER). The UPR can be induced by pharmacological agents that perturb ER functions, but may also occur during cellular developmental processes such as the transition of B-lymphocytes into antibody-secreting plasma cells. Here we show that major UPR components are activated in B cells stimulated to secrete antibody. Increased expression of UPR targets including the ER chaperones BiP and GRP94 and the transcription factor XBP-1 initiates early in the differentiation program prior to up-regulated synthesis of Ig chains. Furthermore, these same kinetics are observed during differentiation for cleavage of the ER-localized ATF6alpha protein and splicing of XBP-1 mRNA to generate p50ATF6alpha and p54XBP-1, the two known UPR transcriptional activators. All of these UPR events reach maximal levels once Ig synthesis and secretion are markedly induced. Interestingly, these events are not accompanied by expression of CHOP, a transcription factor induced by ER stress agents commonly used to investigate the UPR. These results suggest that a physiological UPR elicited during differentiation of B-lymphocytes into high-rate secretory cells may be distinct from the UPR defined by agents that disrupt protein maturation in the ER.

Progressive differentiation and selection of the fittest in the immune response

T cells are stimulated by stochastic exposure to antigen-presenting cells and cytokines. We review evidence that the level of signal that is accumulated determines progression through hierarchical thresholds for proliferation and differentiation, leading to the generation of various intermediates and effector T cells. These cells are then selected to enter the memory pool according to their fitness--that is, their capacity to access and use survival signals. We suggest that the intermediates that are generated by antigenic stimulation of T and B cells persist as central memory cells, which can mount secondary responses to antigen and maintain appropriate levels of effector cells and antibodies throughout the lifetime of an individual.

An interfering form of Blimp-1 increases IgM secreting plasma cells and blocks maturation of peripheral B cells

B lymphocyte-induced maturation protein-1 (Blimp-1) can drive plasmacytic differentiation in cultured cell models. To determine the role of Blimp-1 in B cell development in vivo, we have generated transgenic mice expressing an interfering truncated form of Blimp-1 (TBlimp) under the control of an immunoglobulin heavy chain promoter and intronic (E) enhancer. TBlimp-transgenic mice have elevated serum IgM and a prolonged IgM response. This effect is due to an increased number of short-lived, IgM-secreting plasma cells resulting from increased proliferation and prolonged survival. In addition, TBlimp-transgenic mice have a developmental defect in the generation of mature B cells in the spleen. These results show that in vivo Blimp-1 plays a fundamental role in the control of the life span and exit from the cell cycle of IgM secreting plasma cells.

Novel B cell population producing functional IgG in the absence of membrane IgM expression

Surface expression of IgM is a characteristic feature of the development of most B cells. Only pre-B cells bearing functional IgM heavy chains mu chains) are selected for clonal expansion and differentiation. Cells lacking mu chains are normally eliminated. muMT mice carrying a deletion of the first exon coding for the transmembrane domain of the immunoglobulin mu chain gene were described as mice deficient for mature B cells, plasma cells and immunoglobulins in serum. In this study, we describe in muMT/BALB/c mice the presence of a novel B cell population, producing IgG, IgA and IgE in the absence of IgM membrane expression. Moreover, this small population of B cells is able to recognize antigens and to differentiate into plasma cells. These "non-conventional" mu(- / -) B cells produce functional immunoglobulins after immunization, undergo germinal center reactions, and maintain B cell memory. Our findings support the concept, that a small percentage of mu -non-expressing pre-B cells can escape elimination, switch to downstream immunoglobulin heavy chains and respond to antigens. It remains an open question how the reactivity of these B cells is regulated and in which extent such B cells play a role in physiological and pathological processes such as autoantibody production and autoimmunity.

The role of bone marrow-derived stromal cells in the maintenance of plasma cell longevity

Protective circulating Abs originate primarily from long-lived plasma cells in the bone marrow. However, the molecular and cellular basis of plasma cell longevity is unknown. We investigated the capacity of primary bone marrow-derived stromal cells to maintain plasma cell viability in vitro. Plasma cells purified from the bone marrow or lymph nodes died rapidly when plated in media, but a subpopulation of plasma cells survived and secreted high levels of Ab for up to 4 wk when cocultured with stromal cells. Ab secretion was inhibited by the addition of anti-very late Ag-4 to plasma cell/stromal cell cocultures indicating that direct interactions occur and are necessary between stromal cells and plasma cells. The addition of rIL-6 to plasma cells cultured in media alone partially relieved the sharp decline in Ab secretion observed in the absence of stromal cells. Moreover, when stromal cells from IL-6(-/-) mice were used in plasma cell/stromal cell cocultures, Ab levels decreased 80% after 7 days as compared with wild-type stromal cells. Further, IL-6 mRNA message was induced in stromal cells by coculture with plasma cells. These data indicate that bone marrow plasma cells are not intrinsically long-lived, but rather that plasma cells contact and modify bone marrow stromal cells to provide survival factors.

Generation of polyclonal plasmablasts from peripheral blood B cells: a normal counterpart of malignant plasmablasts

A new way to identify tumor-specific genes is to compare gene expression profiles between malignant cells and their autologous normal counterparts. In patients with multiple myeloma, a major plasma cell disorder, normal plasma cells are not easily attainable in vivo. We report here that in vitro differentiation of peripheral blood B lymphocytes, purified from healthy donors and from patients with multiple myeloma, makes it possible to obtain a homogeneous population of normal plasmablastic cells. These cells were identified by their morphology, phenotype, production of polyclonal immunoglobulins, and expression of major transcription factors involved in B-cell differentiation. Oligonucleotide microarray analysis shows that these polyclonal plasmablastic cells have a gene expression pattern close to that of normal bone marrow–derived plasma cells. Detailed analysis of genes statistically differentially expressed between normal and tumor plasma cells allows the identification of myeloma-specific genes, including oncogenes and genes coding for tumor antigens. These data should help to disclose the molecular mechanisms of myeloma pathogenesis and to define new therapeutic targets in this still fatal malignancy. In addition, the comparison of gene expression between plasmablastic cells and B cells provides a new and powerful tool to identify genes specifically involved in normal plasma cell differentiation.

Repression of AP-1 function: a mechanism for the regulation of Blimp-1 expression and B lymphocyte differentiation by the B cell lymphoma-6 protooncogene

The B cell lymphoma-6 (BCL-6) transcriptional repressor protein is an important regulator of B cell differentiation and is strongly implicated in the development of B cell lymphoma. Expression of the Blimp-1 transcription factor, which is critical for promoting B cell differentiation into plasma cells, is repressed by BCL-6. We have investigated the mechanism for how BCL-6 represses Blimp-1 transcription, and have found that BCL-6 regulates the Blimp-1 promoter through a novel mechanism involving AP-1 elements. Specifically, BCL-6 is a potent repressor of transcriptional activity mediated by AP-1 factors. We found that the zinc-finger region of BCL-6 interacts with c-Jun, JunB, and JunD proteins but does not bind c-Fos or Fra-2 proteins. An estrogen receptor ligand binding domain fusion with the BCL-6 zinc finger domain can act as a estrogen-inducible dominant negative protein and increase AP-1 activity in BCL-6(+) cells but not in BCL-6(-) cells, indicating that endogenous BCL-6 represses AP-1 activity. Additionally, we have confirmed a specific interaction between c-Jun and the zinc finger domain of BCL-6 in vivo using a mammalian two-hybrid assay. Repression of AP-1 function by BCL-6 may be a key mechanism for how BCL-6 regulates gene expression to control inflammation, lymphocyte differentiation, and lymphomagenesis.

Humoral immunity and long-lived plasma cells

A selected fraction of plasmablasts enters the compartment of nondividing, long-lived plasma cells to maintain humoral antibody memory. In accord with a current model for lymphocyte homeostasis, the lifetime of long-lived plasma cells is probably regulated by competition for a limited number of survival niches present in splenic red pulp, bone marrow and inflamed tissue. Plasma cells secreting autoantibodies specific for some, but not all, self-antigens are probably 'allowed' to enter the compartment of long-lived plasma cells and provide antibody-mediated 'autoimmune memory' that is resistant to conventional therapies.

T and B cell recovery in arthritis adoptively transferred to SCID mice: antigen-specific activation is required for restoration of autopathogenic CD4+ Th1 cells in a syngeneic system

T cell homeostasis is a physiological function of the immune system that maintains a balance in the numbers and ratios of T cells at the periphery. A self-MHC/self-peptide ligand can induce weak (covert) signals via the TCR, thus providing an extended lifespan for naive T cells. A similar mechanism is responsible for the restoration of immune homeostasis in severe lymphopenic conditions such as those following irradiation or chemotherapy, or upon transfer of lymphocytes to nu/nu or SCID mice. To date, the genetic backgrounds of donor and recipient SCID mice were unmatched in all autoimmune arthritis transfer experiments, and the recovery of lymphoid cells in the host has not been followed. In this study, we present the adoptive transfer of proteoglycan (PG)-induced arthritis using unseparated and T or B cell-depleted lymphocytes from arthritic BALB/c donors to genetically matched syngeneic SCID recipient mice. We demonstrate that selectively recovered lymphoid subsets determine the clinical and immunological status of the recipient. We found that when T cells were depleted (>98% depleted), B cells did not produce PG-specific anti-mouse (auto) Abs unless SCID mice received a second Ag (PG) injection, which promoted the recovery of Ag-specific CD4(+) Th1 cells. Reciprocally, as a result of B cell recovery, high levels of serum anti-PG Abs were found in SCID mice that received B cell-depleted (>99% depleted) T lymphocytes. Our results indicate a selective and highly effective cooperation between CD4(+) T cells and B lymphocytes that is required for the restoration of pathological homeostasis and development of autoimmune arthritis in SCID mice.