B-cell anergy: from transgenic models to naturally occurring anergic B cells?

Dysregulated extracellular signal-regulated kinase signaling associated with impaired B-cell receptor endocytosis in patients with common variable immunodeficiency

BACKGROUND

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by B-cell dysfunction and, in a subgroup, by expansion of CD21(low) B cells. The CD21(low) B cells display defects in early B-cell receptor (BCR) signaling resembling those of anergic B cells.

OBJECTIVE

We sought to investigate whether B cells from patients with CVID, like anergic B cells, have defects in extracellular signal-regulated kinase (ERK) phosphorylation and in endocytic trafficking of the BCR.

METHODS

Using flow cytometry, we evaluated phosphorylated ERK (pERK) expression and internalization of cross-linked BCR in B-cell subsets. The localization of internalized BCR to lysosome-associated membrane protein 1-positive late endosomes was evaluated with confocal microscopy.

RESULTS

Constitutive pERK levels were increased in naive and IgM(+) memory B cells of patients with CVID compared with those of healthy donors, whereas the pERK increment induced by BCR cross-linking was relatively reduced. Intravenous immunoglobulin administration enhanced these anomalies, but they appeared to be intrinsic to B cells from patients with CVID. Cross-linking-induced BCR endocytosis was decreased in the IgM(+) memory B cells, especially in those with a CD21(low) phenotype, but not in the naive B cells of patients with CVID with CD21(low) expansion. Internalized BCR localized normally to late endosomes. Pharmacologic inhibition of ERK phosphorylation suppressed BCR endocytosis in B cells of healthy patients and those with CVID.

CONCLUSIONS

The B cells of patients with CVID with CD21(low) B-cell expansion resemble anergic B cells based on high constitutive pERK expression. The IgM(+) memory B cells of these patients, especially those that are CD21(low), have a defect in BCR endocytosis seemingly caused by dysregulated ERK signaling.

Establishment of murine gammaherpesvirus latency in B cells is not a stochastic event

Murid γ-herpesvirus-4 (MuHV-4) promotes polyclonal B cell activation and establishes latency in memory B cells via unclear mechanisms. We aimed at exploring whether B cell receptor specificity plays a role in B cell susceptibility to viral latency and how this is related to B cell activation. We first observed that MuHV-4-specific B cells represent a minority of the latent population, and to better understand the influence of the virus on non-MuHV-4 specific B cells we used the SW_HEL mouse model, which produce hen egg lysozyme (HEL)-specific B cells. By tracking HEL⁺ and HEL⁻ B cells, we showed that in vivo latency was restricted to HEL⁻ B cells while the two populations were equally sensitive to the virus in vitro. Moreover, MuHV-4 induced two waves of B cell activation. While the first wave was characterized by a general B cell activation, as shown by HEL⁺ and HEL⁻ B cells expansion and upregulation of CD69 expression, the second wave was restricted to the HEL⁻ population, which acquired germinal center (GC) and plasma cell phenotypes. Antigenic stimulation of HEL⁺ B cells led to the development of HEL⁺ GC B cells where latent infection remained undetectable, indicating that MuHV-4 does not benefit from acute B cell responses to establish latency in non-virus specific B cells but relies on other mechanisms of the humoral response. These data support a model in which the establishment of latency in B cells by γ-herpesviruses is not stochastic in terms of BCR specificity and is tightly linked to the formation of GCs.

Murid γ-herpesvirus-4 (MuHV-4) is a good model to study infectious mononucleosis in mice, in which the virus ultimately establishes life-long latency in B cells. Whereas several viral proteins have been shown to modulate B cell behavior, in the present study we aimed at clarifying the parameters that dictate the establishment of viral latency from the B cell perspective. Indeed, the B cell repertoire is highly diverse and it remains unknown whether latency takes place randomly in B cells. To study this question, we isolated latently infected B cells in which we observed a low frequency of virus-specific B cells, suggesting that viral latency is not restricted to this population. To better understand MuHV-4 influence on non-virus specific B cells, we then followed the fate of B cells specific for a foreign antigen, hen egg lysozyme (HEL). While in vitro experiments showed that HEL-specific B cells could be acutely infected by MuHV-4, these cells were resistant to MuHV-4 latent infection in vivo. These results suggest that while establishment of γ-herpesvirus latency is not restricted to virus-specific B cells, it does not take place randomly in B cells and relies on mechanisms that remain to be identified.

B cell receptor signaling-based index as a biomarker for the loss of peripheral immune tolerance in autoreactive B cells in rheumatoid arthritis

This study examines the loss of peripherally induced B cell immune tolerance in Rheumatoid arthritis (RA) and establishes a novel signaling-based measure of activation in a subset of autoreactive B cells - the Induced tolerance status index (ITSI). Naturally occurring naïve autoreactive B cells can escape the “classical” tolerogenic mechanisms of clonal deletion and receptor editing, but remain peripherally tolerized through B cell receptor (BCR) signaling inhibition (postdevelopmental “receptor tuning” or anergy). ITSI is a statistical index that numerically determines the level of homology between activation patterns of BCR signaling intermediaries in B cells that are either tolerized or activated by auto antigen exposure, and thus quantifies the level of peripheral immune tolerance. The index is based on the logistic regression analysis of phosphorylation levels in a panel of BCR signaling proteins. Our results demonstrate a new approach to identifying autoreactive B cells based on their BCR signaling features.

Integrin CD11b negatively regulates BCR signalling to maintain autoreactive B cell tolerance

A variant of the integrin-α-M (CD11b) gene has been linked to the pathogenesis of systemic lupus erythematosus. However, how this genotype results in the lupus phenotype is not fully understood. Here we show that autoreactive B cells lacking CD11b exhibit a hyperproliferative response to B cell receptor (BCR) crosslinking and enhanced survival. In vivo engagement of BCR in CD11b-deficient mice leads to increased autoAb production and kidney Ig deposition. In addition, CD11b-deficient autoreactive B cells have decreased tyrosine phosphorylation including Lyn and CD22 with decreased phosphatase SHP-1 recruitment but increased calcium influx. Results obtained using B cells transfected with the wild type or rs1143679 lupus-associated variant of CD11b suggest that this mutation completely abrogates the regulatory effect of CD11b on BCR signalling. This is through disruption of CD22-CD11b direct binding. These results reveal a previously unrecognized role of CD11b in maintaining autoreactive B cell tolerance.

The outcome of B-cell receptor signaling in chronic lymphocytic leukemia: proliferation or anergy

Biologists and clinicians agree that the B-cell receptor influences the behavior of chronic lymphocytic leukemia, and promising new drugs are aimed at receptor-associated kinases. Engagement of surface immunoglobulin by antigen is a key driver of malignant cells with outcome influenced by the nature of the cell, the level of stimulation and the microenvironment. Analysis of surface immunoglobulin-mediated signaling in the two major disease subsets defined by IGHV mutational status reveals bifurcation of responses toward proliferation or anergy. Mutated chronic lymphocytic leukemia, generally of relatively good prognosis, is mainly, but not exclusively, driven towards anergy in vivo. In contrast, unmutated chronic lymphocytic leukemia shows less evidence for anergy in vivo retaining more responsiveness to surface immunoglobulin M-mediated signaling, possibly explaining increased tumor progression. Expression and function of surface immunoglobulin M in unmutated chronic lymphocytic leukemia appear rather homogeneous, but mutated chronic lymphocytic leukemia exhibits a highly heterogeneous profile that may relate to further variable clinical behavior within this subset. Anergy should increase susceptibility to apoptosis but, in leukemic cells, this may be countered by overexpression of the B-cell lymphoma-2 survival protein. Maintained anergy spreads to chemokines and adhesion molecules, restraining homing and migration. However, anergy is not necessarily completely benign, being able to reverse and regenerate surface immunoglobulin M-mediated responses. A two-pronged attack on proliferative and anti-apoptotic pathways may succeed. Increased understanding of how chronic lymphocytic leukemia cells are driven to anergy or proliferation should reveal predictive biomarkers of progression and of likely response to kinase inhibitors, which could assist therapeutic decisions.

Chronic Trypanosoma congolense infections in mice cause a sustained disruption of the B-cell homeostasis in the bone marrow and spleen

Trypanosoma congolense is one of the main species responsible for Animal African Trypanosomosis (AAT). As preventive vaccination strategies for AAT have been unsuccessful so far, investigating the mechanisms underlying vaccine failure has to be prioritized. In T. brucei and T. vivax infections, recent studies revealed a rapid onset of destruction of the host B-cell compartment, resulting in the loss of memory recall capacity. To assess such effect in experimental T. congolense trypanosomosis, we performed infections with both the cloned Tc13 parasite, which is considered as a standard model system for T. congolense rodent infections and the noncloned TRT55 field isolate. These infections differ in their virulence level in the C57BL/6 mouse model for trypanosomosis. We show that early on, an irreversible depletion of all developmental B cells stages occur. Subsequently, in the spleen, a detrimental decrease in immature B cells is followed by a significant and permanent depletion of Marginal zone B cells and Follicular B cells. The severity of these events later on in infection correlated with the virulence level of the parasite stock. In line with this, it was observed that later-stage infection-induced IgGs were largely nonspecific, in particular in the more virulent TRT55 infection model.

Differences in cellular function and viral protein expression between IgMhigh and IgMlow B-cells in bovine leukemia virus-infected cattle

Bovine leukemia virus (BLV) induces abnormal B-cell proliferation and B-cell lymphoma in cattle, where the BLV provirus is integrated into the host genome. BLV-infected B-cells rarely express viral proteins in vivo, but short-term cultivation augments BLV expression in some, but not all, BLV-infected B-cells. This observation suggests that two subsets, i.e. BLV-silencing cells and BLV-expressing cells, are present among BLV-infected B-cells, although the mechanisms of viral expression have not been determined. In this study, we examined B-cell markers and viral antigen expression in B-cells from BLV-infected cattle to identify markers that may discriminate BLV-expressing cells from BLV-silencing cells. The proportions of IgM(high) B-cells were increased in blood lymphocytes from BLV-infected cattle. IgM(high) B-cells mainly expressed BLV antigens, whereas IgM(low) B-cells did not, although the provirus load was equivalent in both subsets. Several parameters were investigated in these two subsets to characterize their cellular behaviour. Real-time PCR and microarray analyses detected higher expression levels of some proto-oncogenes (e.g. Maf, Jun and Fos) in IgM(low) B-cells than those in IgM(high) B-cells. Moreover, lymphoma cells obtained from the lymph nodes of 14 BLV-infected cattle contained IgM(low) or IgM(-) B-cells but no IgM(high) B-cells. To our knowledge, this is the first study to demonstrate that IgM(high) B-cells mainly comprise BLV-expressing cells, whereas IgM(low) B-cells comprise a high proportion of BLV-silencing B-cells in BLV-infected cattle.

Variable induction of PRDM1 and differentiation in chronic lymphocytic leukemia is associated with anergy

Despite antigen engagement and intact B-cell-receptor (BCR) signaling, chronic lymphocytic leukemia (CLL) cells fail to undergo terminal differentiation. We hypothesized that such failure may be due to anergy, as CLL cells exhibit variable levels of nonresponsiveness to surface IgM stimulation that is reversible in vitro. Moreover, anergy is associated with reduced differentiation capacity in normal B cells. We investigated responses of CLL cells to two potent differentiation-promoting agents, IL-21 and cytosine guanine dinucleotide-enriched oligo-deoxynucleotides. The induction of PR domain-containing protein 1 (PRDM1; also known as Blimp-1), a critical regulator of plasmacytic differentiation, by these agents was closely correlated but varied between individual cases, despite functionally intact IL-21 receptor- and Toll-like receptor 9-mediated signal transducer and activator of transcription 3, and nuclear factor-κB pathways. PRDM1 induction was inversely correlated with the extent of anergy as measured by the ability to mobilize intracellular Ca(2+) following BCR crosslinking. PRDM1 responsiveness was associated with other markers of differentiation and proliferation but not with differences in apoptosis. The ability to induce PRDM1 did correlate with differential transcriptional and epigenetic regulation of the PRDM1 gene. These studies extend our understanding of CLL pathobiology, demonstrating that reduced differentiation capacity may be a consequence of anergy. Epigenetic drugs may offer possibilities to reactivate PRDM1 expression as part of novel differentiation therapy approaches.

CXCR7 influences the migration of B cells during maturation

The atypical chemokine receptor CXCR7 binds the chemokines CXCL12 and CXCL11. The receptor is widely expressed and was shown to tune CXCR12-induced responses of CXCR4. Here, the function of CXCR7 was examined at late stages of human B-cell maturation, when B cells differentiate into Ab-secreting plasmablasts. We identified two populations of CXCR7(+) cells in tonsillar lymphocytes, one being presumably memory B cells or early plasmablasts (FSC(low) CD19(+) CD38(mid) ) and the other being plasmablasts or early plasma cells (FSC(high) CD19(+) CD38(+) ). CXCR7 is expressed on CD19(+) CD27(+) memory B cells, on CD19(+) CD38(+) CD138(-) and intracellular immunoglobulin high plasmablasts, but not on CD19(+) CD138(+) icIg(high) plasma cells. The differential expression pattern suggests a potential contribution of the scavenger receptor in final B-cell maturation. On in vitro differentiating B cells, we found a marked inverse correlation between CXCR7 and CXCR5 cell surface levels, whereas expression of CXCR4 remained almost constant. Migration assays performed with tonsillar mononuclear cells or in vitro differentiated cells revealed that inhibition of CXCR7 markedly increases chemotaxis toward CXCL12, especially at late stages of B-cell maturation. Chemotaxis was attenuated in the presence of CXCR4 antagonists, confirming that migration is CXCR4 mediated. Our findings unequivocally demonstrate a novel role for CXCR7 in regulating the migration of plasmablasts during B-cell maturation.

Gene therapy delivery of myelin oligodendrocyte glycoprotein (MOG) via hematopoietic stem cell transfer induces MOG-specific B cell deletion

The various mechanisms that have been described for immune tolerance govern our ability to control self-reactivity and minimize autoimmunity. However, the capacity to genetically manipulate the immune system provides a powerful avenue to supplement this natural tolerance in an Ag-specific manner. We have previously shown in the mouse model of experimental autoimmune encephalomyelitis that transfer of bone marrow (BM) transduced with retrovirus encoding myelin oligodendrocyte glycoprotein (MOG) promotes disease resistance and CD4(+) T cell deletion within the thymus. However, the consequence of this strategy on B cell tolerance is not known. Using BM from IgH(MOG) mice that develop MOG-specific B cell receptors, we generated mixed chimeras together with BM-encoding MOG. In these animals, the development of MOG-specific B cells was abrogated, resulting in a lack of MOG-specific B cells in all B cell compartments examined. This finding adds a further dimension to our understanding of the mechanisms of tolerance that are associated with this gene therapy approach to treating autoimmunity and may have important implications for Ab-mediated autoimmune disorders.

Negative selection of self-reactive chicken B cells requires B cell receptor signaling and is independent of the bursal microenvironment

Although the negative selection of self-reactive B cells in the bone marrow of mammals has been clearly demonstrated, it remains unclear in models of gut-associated B cell lymphopoiesis, such as that of the chicken (Gallus gallus). We have generated chicken surface IgM-related receptors in which the diversity region of the lamprey variable lymphocyte receptor (VLR) has been fused to the C region of chicken surface IgM (Tμ). Expression of a VLR:Tμ receptor with specificity for PE supported normal development of B cells, whereas a VLR:Tμ receptor specific to hen egg lysozyme (a self-antigen with respect to chicken B cells) induced, in vivo, complete deletion of VLR(HEL)Tμ-expressing B cells. In ovo i.v. injection of PE resulted in deletion of VLR(PE)Tμ-expressing Β cells in the embryo spleen, demonstrating that negative selection was independent of the bursal microenvironment. Although chickens transduced with a murine CD8α:chicken Igα fusion protein contained B cells expressing mCD8α:chIgα, cotransfection of the mCD8α:chIgα construct, together with thymus leukemia Ag (a natural ligand for mCD8α), resulted in reduced levels of mCD8α:chIgα-expressing B cells in inverse proportion to the levels of thymus leukemia Ag-expressing cells. Deletion of mCD8α:chIgα-expressing cells was specific for B cells and required active signaling downstream of the mCD8α:chIgα receptor. Ag-mediated negative selection of developing chicken B cells can therefore occur independently of the bursal microenvironment and is dependent on signaling downstream of the BCR.

Protein kinase Cδ promotes transitional B cell-negative selection and limits proximal B cell receptor signaling to enforce tolerance

Protein kinase Cδ (PKCδ) deficiency causes autoimmune pathology in humans and mice and is crucial for the maintenance of B cell homeostasis. However, the mechanisms underlying autoimmune disease in PKCδ deficiency remain poorly defined. Here, we address the antigen-dependent and -independent roles of PKCδ in B cell development, repertoire selection, and antigen responsiveness. We demonstrate that PKCδ is rapidly phosphorylated downstream of both the B cell receptor (BCR) and the B cell-activating factor (BAFF) receptor. We found that PKCδ is essential for antigen-dependent negative selection of splenic transitional B cells and is required for activation of the proapoptotic Ca(2+)-Erk pathway that is selectively activated during B cell-negative selection. Unexpectedly, we also identified a previously unrecognized role for PKCδ as a proximal negative regulator of BCR signaling that substantially impacts survival and proliferation of mature follicular B cells. As a consequence of these distinct roles, PKCδ deficiency leads to the survival and development of a B cell repertoire that is not only aberrantly autoreactive but also hyperresponsive to antigen stimulation.

Monovalent engagement of the BCR activates ovalbumin-specific transnuclear B cells

Monovalent engagement can trigger BCR signal transduction, and fine-tuning of BCR-ligand recognition can lead to B cell nonresponsiveness, activation, or inhibition.

Valency requirements for B cell activation upon antigen encounter are poorly understood. OB1 transnuclear B cells express an IgG1 B cell receptor (BCR) specific for ovalbumin (OVA), the epitope of which can be mimicked using short synthetic peptides to allow antigen-specific engagement of the BCR. By altering length and valency of epitope-bearing synthetic peptides, we examined the properties of ligands required for optimal OB1 B cell activation. Monovalent engagement of the BCR with an epitope-bearing 17-mer synthetic peptide readily activated OB1 B cells. Dimers of the minimal peptide epitope oriented in an N to N configuration were more stimulatory than their C to C counterparts. Although shorter length correlated with less activation, a monomeric 8-mer peptide epitope behaved as a weak agonist that blocked responses to cell-bound peptide antigen, a blockade which could not be reversed by CD40 ligation. The 8-mer not only delivered a suboptimal signal, which blocked subsequent responses to OVA, anti-IgG, and anti-kappa, but also competed for binding with OVA. Our results show that fine-tuning of BCR-ligand recognition can lead to B cell nonresponsiveness, activation, or inhibition.

B cells and type 1 diabetes ...in mice and men

Nearly 70% of newly produced B cells express autoreactive antigen receptors and must be silenced to prevent autoimmunity. Failure of silencing mechanisms is apparent in type 1 diabetes (T1D), where islet antigen-specific B cells appear critical for development of disease. Evidence for a B cell role in T1D includes success of B cell targeted anti-CD20 therapy, which delays T1D progression in both NOD mice and new onset patients. Demonstrating the importance of specificity, NOD mice whose B cell repertoire is biased toward insulin reactivity show increased disease development, while bias away from insulin reactivity largely prevents disease. Finally, though not required for illness, high affinity insulin autoantibodies are often the first harbingers of T1D. B cell cytokine production and auto-antigen presentation to self-reactive T cells are likely important in pathogenesis. Here we review B cell function, as described above, in T1D in humans and the non-obese diabetic (NOD) mouse. We will discuss recent broad-based B cell depletion studies and how they may provide the basis for refinement of future treatments for the disorder.

Modulation of nonneutralizing HIV-1 gp41 responses by an MHC-restricted TH epitope overlapping those of membrane proximal external region broadly neutralizing antibodies

A goal of HIV-1 vaccine development is to elicit broadly neutralizing Abs (BnAbs), but current immunization strategies fail to induce BnAbs, and for unknown reasons, often induce nonneutralizing Abs instead. To explore potential host genetic contributions controlling Ab responses to the HIV-1 Envelope, we have used congenic strains to identify a critical role for MHC class II restriction in modulating Ab responses to the membrane proximal external region (MPER) of gp41, a key vaccine target. Immunized H-2(d)-congenic strains had more rapid, sustained, and elevated MPER(+) Ab titers than those bearing other haplotypes, regardless of immunogen, adjuvant, or prime or boost regimen used, including formulations designed to provide T cell help. H-2(d)-restricted MPER(+) serum Ab responses depended on CD4 TH interactions with class II (as revealed in immunized intra-H-2(d/b) congenic or CD154(-/-) H-2(d) strains, and by selective abrogation of MPER restimulated, H-2(d)-restricted primed splenocytes by class II-blocking Abs), and failed to neutralize HIV-1 in the TZM-b/l neutralization assay, coinciding with lack of specificity for an aspartate residue in the neutralization core of BnAb 2F5. Unexpectedly, H-2(d)-restricted MPER(+) responses functionally mapped to a core TH epitope partially overlapping the 2F5/z13/4E10 BnAb epitopes as well as nonneutralizing B cell-Ab binding residues. We propose that class II restriction contributes to the general heterogeneity of nonneutralizing gp41 responses induced by Envelope. Moreover, the proximity of TH and B cell epitopes in this restriction may have to be considered in redesigning minimal MPER immunogens aimed at exclusively binding BnAb epitopes and triggering MPER(+) BnAbs.

Functionally responsive self-reactive B cells of low affinity express reduced levels of surface IgM

Somatic gene rearrangement generates a diverse repertoire of B cells, many which have receptors possessing a range of affinities for self-Ag. Newly generated B cells express high and relatively uniform amounts of surface IgM (sIgM), while follicular (FO) B cells express sIgM at widely varying levels. It is plausible, therefore, that downmodulation of sIgM serves as a mechanism to maintain weakly self-reactive B cells in a responsive state by decreasing their avidity for self-Ag. We tested this hypothesis by performing comparative functional tests with FO IgM(hi) and IgM(lo) B cells from the unrestricted repertoire of WT C57BL/6 mice. We found that FO IgM(lo) B cells mobilized Ca(2+) equivalently to IgM(hi) B cells when the same number of sIgM molecules was engaged. In agreement, FO IgM(lo) B cells were functionally competent to produce an antibody response following adoptive transfer. The FO IgM(lo) cell population had elevated levels of Nur77 transcript, and was enriched with nuclear-reactive specificities. Hybridoma sampling revealed that these B-cell receptors were of low affinity. Collectively, these results suggest that sIgM downmodulation by low-affinity, self-reactive B cells preserves their immunocompetence and circumvents classical peripheral tolerance mechanisms that would otherwise reduce diversity within the B cell compartment.

The role of BCR isotype in B-cell development and activation

The development and function of B lymphocytes critically depend on the non-germline B-cell antigen receptor (BCR). In addition to the diverse antigen-recognition regions, whose coding sequences are generated by the somatic DNA rearrangement, the variety of the constant domains of the Heavy Chain (HC) portion contributes to the multiplicity of the BCR types. The functions of particular classes of the HC, particularly in the context of the membrane BCR, are not completely understood. The expression of the various classes of the HC correlates with the distinct stages of B-cell development, types of B-cell subsets, and their effector functions. In this chapter, we summarize and discuss the accumulated knowledge on the role of the μ, δ, and γ HC isotypes of the conventional and precursor BCR in B-cell differentiation, selection, and engagement with (auto)antigens.

Role of B lymphocytes in the pathogenesis of type 1 diabetes

Though type 1 diabetes (T1D) is considered a T cell-mediated autoimmune disorder, recent evidence indicates that B cells play a critical role in disease. This conclusion is based in part on the success of anti-CD20 (rituximab) therapy, which by broadly depleting B cells delays disease progression in non-obese diabetic (NOD) mice and new-onset patients. B cell receptor (BCR) specificity to islet autoantigen is key. NOD mice whose B cell repertoire is biased toward insulin reactivity show increased disease development, while bias away from insulin reactivity largely prevents disease. Although the operative disease-promoting B cell effector function remains undefined, islet-antigen reactive B cells function in antigen presentation to diabetogenic CD4 T cells. Other studies implicate B cells in antigen presentation to CD8 T cells. B cell participation in TID appears predicated on faulty B cell tolerance. Here, we review extant findings implicating B cells in T1D in mice and men.

CCR7 is mainly expressed in teleost gills, where it defines an IgD+IgM- B lymphocyte subset

Chemokine receptor CCR7, the receptor for both CCL19 and CCL21 chemokines, regulates the recruitment and clustering of circulating leukocytes to secondary lymphoid tissues, such as lymph nodes and Peyer's patches. Even though teleost fish do not have either of these secondary lymphoid structures, we have recently reported a homolog to CCR7 in rainbow trout (Oncorhynchus mykiss). In the present work, we have studied the distribution of leukocytes bearing extracellular CCR7 in naive adult tissues by flow cytometry, observing that among the different leukocyte populations, the highest numbers of cells with membrane (mem)CCR7 were recorded in the gill (7.5 ± 2% CCR7(+) cells). In comparison, head kidney, spleen, thymus, intestine, and peripheral blood possessed <5% CCR7(+) cells. When CCR7 was studied at early developmental stages, we detected a progressive increase in gene expression and protein CCR7 levels in the gills throughout development. Surprisingly, the majority of the CCR7(+) cells in the gills were not myeloid cells and did not express membrane CD8, IgM, nor IgT, but expressed IgD on the cell surface. In fact, most IgD(+) cells in the gills expressed CCR7. Intriguingly, the IgD(+)CCR7(+) population did not coexpress memIgM. Finally, when trout were bath challenged with viral hemorrhagic septicemia virus, the number of CCR7(+) cells significantly decreased in the gills while significantly increased in head kidney. These results provide evidence of the presence of a novel memIgD(+)memIgM(-) B lymphocyte subset in trout that expresses memCCR7 and responds to viral infections. Similarities with IgD(+)IgM(-) subsets in mammals are discussed.

Contributions of B cells to lupus pathogenesis

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the production of autoantibodies. This review summarizes first the results obtained in the mouse that have revealed how B cell tolerance is breached in SLE. We then review the B cell subsets, in addition to the autoAb producing cells, which contribute to SLE pathogenesis, focusing on marginal zone B cells, B-1 cells and regulatory B cells. Finally, we review the interactions between B cells and other immune cells that have been implicated in SLE, such as dendritic cells, macrophages, neutrophils and T cells.

Encoding immunological memory in the initiation of B-cell receptor signaling

In one of the earliest events in the initiation of antigen-driven antibody responses, naïve, IgM-, and IgD-expressing B cells enter germinal centers where they irreversibly isotype switch to the expression of predominately IgG B-cell receptors (BCRs). The IgG-expressing B cells then undergo rounds of antigen-driven selection, ultimately exiting germinal centers as IgG-expressing memory B cells or plasma blast. This early switch from IgM to IgG begs the question: Of what advantage to the memory response is the B cell's expression of an IgG BCR? Despite convincing evidence that the expression of IgG BCRs is essential for antibody memory responses in vivo, the molecular basis of this requirement is only incompletely understood. Here we describe intrinsic features of IgG BCRs that endow memory B cells with the ability to rapidly and efficiently initiate signaling. Remarkably, efficient signaling is mediated through the cytoplasmic tail of the membrane IgG that binds to synapse associated protein 97, a member of a large family of proteins that are best studied for their role in regulating receptor signaling in neuronal synapses. These findings underscore an interesting parallel in the mechanisms at play in encoding immunological memory and memory in the nervous system.

Development of a coordinated allo T cell and auto B cell response against autosomal PTK2B after allogeneic hematopoietic stem cell transplantation

It is well known that allo-reactive T cells play a crucial role in graft-versus-leukemia and graft-versus-host disease after allogeneic hematopoietic stem cell transplantation (alloSCT). Allo-reactive CD4(+) T cells can mediate direct cytolysis, but may also stimulate production of IgG antibodies as helper cells. Immune complexes may subsequently be processed and presented by professional antigen presenting cells and stimulate induction of specific CD8(+) T cells. As such, proteins targeted in coordinated T- and B-cell responses may represent a class of immunodominant antigens in clinical responses after alloSCT. We previously identified LB-PTK2B-1T as HLA class II restricted polymorphic antigen in a patient treated with donor lymphocyte infusion for relapsed chronic myeloid leukemia after HLA-matched alloSCT. Since PTK2B has also been described as antibody target, we here investigated whether a coordinated T- and B-cell response against PTK2B was induced. Patient serum before and after alloSCT and donor lymphocyte infusion (DLI) was screened for antibodies, and we indeed observed development of a humoral immune response against PTK2B. Antibodies against PTK2B were only found after DLI and, in contrast to the CD4(+) T cells, recognized a monomorphic region of the protein. To our knowledge, this is the first description of a coordinated allo-reactive CD4(+) T-cell and auto-reactive antibody response against an autosomal antigen.

RasGRP Ras guanine nucleotide exchange factors in cancer

RasGRP proteins are activators of Ras and other related small GTPases by the virtue of functioning as guanine nucleotide exchange factors (GEFs). In vertebrates, four RasGRP family members have been described. RasGRP-1 through -4 share many structural domains but there are also subtle differences between each of the different family members. Whereas SOS RasGEFs are ubiquitously expressed, RasGRP proteins are expressed in distinct patterns, such as in different cells of the hematopoietic system and in the brain. Most studies have concentrated on the role of RasGRP proteins in the development and function of immune cell types because of the predominant RasGRP expression profiles in these cells and the immune phenotypes of mice deficient for Rasgrp genes. However, more recent studies demonstrate that RasGRPs also play an important role in tumorigenesis. Examples are skin- and hematological-cancers but also solid malignancies such as melanoma or prostate cancer. These novel studies bring up many new and unanswered questions related to the molecular mechanism of RasGRP-driven oncogenesis, such as new receptor systems that RasGRP appears to respond to as well as regulatory mechanism for RasGRP expression that appear to be perturbed in these cancers. Here we will review some of the known aspects of RasGRP biology in lymphocytes and will discuss the exciting new notion that RasGRP Ras exchange factors play a role in oncogenesis downstream of various growth factor receptors.

Induction of HIV-1 broad neutralizing antibodies in 2F5 knock-in mice: selection against membrane proximal external region-associated autoreactivity limits T-dependent responses

A goal of HIV-1 vaccine development is to elicit broadly neutralizing Abs (BnAbs). Using a knock-in (KI) model of 2F5, a human HIV-1 gp41 membrane proximal external region (MPER)-specific BnAb, we previously demonstrated that a key obstacle to BnAb induction is clonal deletion of BnAb-expressing B cells. In this study of this model, we provide a proof-of-principle that robust serum neutralizing IgG responses can be induced from pre-existing, residual, self-reactive BnAb-expressing B cells in vivo using a structurally compatible gp41 MPER immunogen. Furthermore, in CD40L-deficient 2F5 KI mice, we demonstrate that these BnAb responses are elicited via a type II T-independent pathway, coinciding with expansion and activation of transitional splenic B cells specific for 2F5's nominal gp41 MPER-binding epitope (containing the 2F5 neutralization domain ELDKWA). In contrast, constitutive production of nonneutralizing serum IgGs in 2F5 KI mice is T dependent and originates from a subset of splenic mature B2 cells that have lost their ability to bind 2F5's gp41 MPER epitope. These results suggest that residual, mature B cells expressing autoreactive BnAbs, like 2F5 as BCR, may be limited in their ability to participate in T-dependent responses by purifying selection that selectively eliminates reactivity for neutralization epitope-containing/mimicked host Ags.

Aberrant B-lymphocyte responses in lupus: inherent or induced and potential therapeutic targets

BACKGROUND

Lupus is a prototype autoimmune disease of unknown aetiology. The disease is complex; manifest diverse clinical symptoms and disease mechanisms. This complexity has provided many leads to explore: from disease mechanisms to approaches for therapy. B-lymphocytes play a central role in the pathogenesis of the disease. However, the cause of aberrant B-lymphocyte responses in patients and, indeed, its causal relationship with the disease remain unclear.

DESIGN

This article provides a synopsis of current knowledge of immunological abnormalities in lupus with an emphasis on abnormalities in the B-lymphocyte compartment.

RESULTS

There is evidence for abnormalities in most compartments of the immune system in animal models and patients with lupus including an ever expanding list of abnormalities within the B-lymphocyte compartment. In addition, recent genome-wide linkage analyses in large cohorts of patients have identified new sets of genetic association factors some with potential links with defective B-lymphocyte responses although their full pathophysiological effects remain to be determined. The accumulating knowledge may help in the identification and application of new targeted therapies for treating lupus disease.

CONCLUSIONS

Cellular, molecular and genetic studies have provided significant insights into potential causes of immunological defects associated with lupus. Most of this insight relate to defects in B- and T-lymphocyte tolerance, signalling and responses. For B-lymphocytes, there is evidence for altered regulation of inter and intracellular signalling pathways at multiple levels. Some of these abnormalities will be discussed within the context of potential implications for disease pathogenesis and targeted therapies.

Common tolerance mechanisms, but distinct cross-reactivities associated with gp41 and lipids, limit production of HIV-1 broad neutralizing antibodies 2F5 and 4E10

Developing an HIV-1 vaccine has been hampered by the inability of immunogens to induce broadly neutralizing Abs (BnAbs) that protect against infection. Previously, we used knockin (KI) mice expressing a prototypical gp41-specific BnAb, 2F5, to demonstrate that immunological tolerance triggered by self-reactivity of the 2F5 H chain impedes BnAb induction. In this study, we generate KI models expressing H chains from two other HIV-1 Abs, 4E10 (another self-/polyreactive, anti-gp41 BnAb) and 48d (an anti-CD4 inducible, nonpolyreactive Ab), and find a similar developmental blockade consistent with central B cell deletion in 4E10, but not in 48d VH KI mice. Furthermore, in KI strains expressing the complete 2F5 and 4E10 Abs as BCRs, we find that residual splenic B cells arrest at distinct developmental stages, yet exhibit uniformly low BCR densities, elevated basal activation, and profoundly muted responses to BCR ligation and, when captured as hybridoma mAb lines, maintain their dual (gp41/lipid) affinities and capacities to neutralize HIV-1, establishing a key role for anergy in suppressing residual 2F5- or 4E10-expressing B cells. Importantly, serum IgGs from naive 2F5 and 4E10 KI strains selectively eliminate gp41 and lipid binding, respectively, suggesting B cells expressing 2F5 or 4E10 as BCRs exhibit specificity for a distinct spectrum of host Ags, including selective interactions by 2F5 BCR(+) B cells (i.e., and not 4E10 BCR(+) B cells) with those mimicked by its gp41 neutralization epitope.

Indications for use and safety of rituximab in childhood renal diseases

Rituximab was initially developed for the treatment of patients with B cell lymphoma but has during the last decade proven to be quite effective in treating a range of kidney diseases including lupus nephritis, nephrotic syndrome, and also in different situations before and after a renal transplant. We will here review the scientific basis for the use of rituximab in children with renal diseases and give recommendations both regarding its clinical use and need for further research.

Modulation of B-cell tolerance by murine gammaherpesvirus 68 infection: requirement for Orf73 viral gene expression and follicular helper T cells

Viruses such as Epstein-Barr virus (EBV) have been linked to mechanisms that support autoantibody production in diseases such as systemic lupus erythematosus. However, the mechanisms by which viruses contribute to autoantibody production remain poorly defined. This stems in part, from the high level of seropositivity for EBV (> 95%) and the exquisite species specificity of EBV. In this study we overcame these problems by using murine gammaherpesvirus 68 (MHV68), a virus genetically and biologically related to EBV. We first showed that MHV68 drives autoantibody production by promoting a loss of B-cell anergy. We next showed that MHV68 infection resulted in the expansion of follicular helper T (Tfh) cells in vivo, and that these Tfh cells supported autoantibody production and a loss of B-cell anergy. Finally, we showed that the expansion of Tfh cells and autoantibody production was dependent on the establishment of viral latency and expression of a functional viral gene called Orf73. Collectively, our studies highlighted an unexpected role for viral latency in the development of autoantibodies following MHV68 infection and suggest that virus-induced expansion of Tfh cells probably plays a key role in the loss of B-cell anergy.

Censoring of self-reactive B cells by follicular dendritic cell-displayed self-antigen

In the secondary lymphoid organs, intimate contact with follicular dendritic cells (FDCs) is required for B cell retention and Ag-driven selection during the germinal center response. However, selection of self-reactive B cells by Ag on FDCs has not been addressed. To this end, we generated a mouse model to conditionally express a membrane-bound self-antigen on FDCs and to monitor the fate of developing self-reactive B cells. In this article, we show that self-antigen displayed on FDCs mediates effective elimination of self-reactive B cells at the transitional stage. Notwithstanding, some self-reactive B cells persist beyond this checkpoint, showing evidence of Ag experience and intact proximal BCR signaling, but they are short-lived and unable to elicit T cell help. These results implicate FDCs as an important component of peripheral B cell tolerance that prevents the emergence of naive B cells capable of responding to sequestered self-antigens.

Chemokine unresponsiveness of chronic lymphocytic leukemia cells results from impaired endosomal recycling of Rap1 and is associated with a distinctive type of immunological anergy

Trafficking of malignant lymphocytes is fundamental to the biology of chronic lymphocytic leukemia (CLL). Transendothelial migration (TEM) of normal lymphocytes into lymph nodes requires the chemokine-induced activation of Rap1 and αLβ2 integrin. However, in most cases of CLL, Rap1 is refractory to chemokine stimulation, resulting in failed αLβ2 activation and TEM unless α4β1 is coexpressed. In this study, we show that the inability of CXCL12 to induce Rap1 GTP loading in CLL cells results from failure of Rap1-containing endosomes to translocate to the plasma membrane. Furthermore, failure of chemokine-induced Rap1 translocation/GTP loading was associated with a specific pattern of cellular IgD distribution resembling that observed in normal B cells anergized by DNA-based Ags. Anergic features and chemokine unresponsiveness could be simultaneously reversed by culturing CLL cells ex vivo, suggesting that these two features are coupled and driven by stimuli present in the in vivo microenvironment. Finally, we show that failure of Rap1 translocation/GTP loading is linked to defective activation of phospholipase D1 and its upstream activator Arf1. Taken together, our findings indicate that chemokine unresponsiveness in CLL lymphocytes results from failure of Arf1/phospholipase D1-mediated translocation of Rap1 to the plasma membrane for GTP loading and may be a specific feature of anergy induced by DNA Ags.

B-lymphocyte signalling abnormalities and lupus immunopathology

Lupus is a complex autoimmune rheumatic disease of unknown aetiology. The disease is associated with diverse features of immunological abnormality in which B-lymphocytes play a central role. However, the cause of atypical B-lymphocyte responses remains unclear. In this article, we provide a synopsis of current knowledge on intracellular signalling abnormalities in B-lymphocytes in lupus and their potential effects on the response of these cells in mouse models and in patients. There are numerous reported defects in the regulation of intracellular signalling proteins and pathways in B-lymphocytes in lupus that, potentially, affect critical biological responses. Most of the evidence for these defects comes from studies of disease models and genetically engineered mice. However, there is also increasing evidence from studying B-lymphocytes from patients and from genome-wide linkage analyses for parallel defects to those observed in mice. These studies provide molecular and genetic explanations for the key immunological abnormalities associated with lupus. Most of the new information appears to relate to defects in intracellular signalling that impact B-lymphocyte tolerance, cytokine production and responses to infections. Some of these abnormalities will be discussed within the context of disease pathogenesis.

Aberrant B cell selection and activation in systemic lupus erythematosus

The detrimental role of B lymphocytes in systemic lupus erythematosus (SLE) is evident from the high levels of pathogenic antinuclear autoantibodies (ANAs) found in SLE patients. Affirming this causative role, additional antibody-independent roles of B cells in SLE were appreciated. In recent years, many defects in B cell selection and activation have been identified in murine lupus models and SLE patients that explain the increased emergence and persistence of autoreactive B cells and their lowered activation threshold. Therefore, clinical trials with B cell depletion regimens in SLE patients were initiated but disappointingly the efficacy of B cell depleting agents proved to be limited. Remarkably however, a major breakthrough in SLE therapy was accomplished by blocking B cell survival factors rather then eliminating B cells. This surprising finding indicates that although SLE is a B cell-driven disease, the amplifying crosstalk between B cells and other cells of the immune system likely evokes the observed tolerance breakdown in B cells. Moreover, this implies that intelligent interception of pro-inflammatory loops rather then selectively silencing B cells will be key to the development of new SLE therapies. In this review, we will not only highlight the intrinsic B cell defects that facilitate the persistence of autoreactive B cells and their activation, but in addition we will focus on B cell extrinsic signals derived from T cells and innate immune cells that lower the activation threshold for B cells.

Expansion of autoreactive unresponsive CD21-/low B cells in Sjögren's syndrome-associated lymphoproliferation

OBJECTIVE

Primary Sjögren's syndrome (SS) is an autoimmune disease associated with a high risk of developing non-Hodgkin's lymphoma. This study was undertaken to determine the nature of B cells driving lymphoproliferation in primary SS.

METHODS

B cell subsets and function were analyzed in peripheral blood from 66 adult patients with primary SS (including 14 patients with B cell lymphoproliferative disease [LPD]) and 30 healthy donors, using flow cytometry, calcium mobilization, and gene array analysis. The reactivity of recombinant antibodies isolated from single B cells from patients with primary SS and LPD was tested using an enzyme-linked immunosorbent assay.

RESULTS

We observed an expansion of an unusual CD21-/low B cell population that correlated with lymphoproliferation in patients with primary SS. A majority of CD21-/low B cells from patients with primary SS expressed autoreactive antibodies, which recognized nuclear and cytoplasmic structures. These B cells belonged to the memory compartment, since their Ig genes were mutated. They were unable to induce calcium flux, become activated, or proliferate in response to B cell receptor and/or CD40 triggering, suggesting that these autoreactive B cells may be anergic. However, CD21-/low B cells from patients with primary SS remained responsive to Toll-like receptor (TLR) stimulation. Molecules specifically expressed in CD21-/low B cells that are likely to induce their unresponsive stage were detected in gene array analyses.

CONCLUSION

Patients with primary SS who display high frequencies of autoreactive and unresponsive CD21-/low B cells are susceptible to developing lymphoproliferation. These cells remain in peripheral blood controlled by functional anergy instead of being eliminated, and chronic antigenic stimulation through TLR stimulation may create a favorable environment for breaking tolerance and activating these cells.

Phosphatase regulation of immunoreceptor signaling in T cells, B cells and mast cells

Recent progress has begun to reveal the often complex and changing roles of phosphotyrosine and phosphoinositide phosphatases in regulation of immunoreceptor signaling. The resultant confusion has been further increased by discoveries of new players. Here we provide a review of recent progress in defining the roles of these enzymes in immunoreceptor-dependent mast cell, T cell and B cell activation.

[Auto-vaccines: an immunological alternative to gene silencing]

Auto-vaccination is a procedure that recently attracted the interest of a growing number of investigators as an alternative to gene inactivation for functional studies of cytokines or other mediators. It is based on the observation that autologous cytokines cross-linked to a foreign protein or peptide are recognized by self-reactive B cells that present foreign peptides, and by doing so attract illicit help from helper T cells that recognize the foreign peptide on the self-reactive B cell MHC Class II complex. This leads to the production of antibodies reacting with self-proteins and thus to neutralization of the targeted factor. Here, we summarize the different techniques that were successful in breaking this self-tolerance and provide several examples of the functional consequences of these auto-vaccines. An additional output of auto-vaccination is the production of mouse monoclonal antibodies against mouse factors. Such antibodies have obvious advantages for long-term use in vivo.

Autoimmunity risk alleles: hotspots in B cell regulatory signaling pathways

Autoimmunity is the consequence of the combination of genetic predisposition and environmental effects, such as infection, injury, and constitution of the gut microbiome. In this edition of the JCI, Dai et al. describe the use of knockin technology to test the mechanism of action of a polymorphism in the protein tyrosine phosphatase nonreceptor 22 (PTPN22) (LYP) that is associated with susceptibility to multiple autoimmune diseases. The function of this allele, and that of a disproportionate number of autoimmune disease risk alleles, suggests that inhibitory signaling pathways that maintain B lymphocyte immune tolerance may represent an Achilles' heel in the prevention of autoimmunity.

Targeting B-cell anergy in chronic lymphocytic leukemia

B-cell receptor (BCR) triggering and responsiveness have a crucial role in the survival and expansion of chronic lymphocytic leukemia (CLL) clones. Analysis of in vitro response of CLL cells to BCR triggering allowed the definition of 2 main subsets of patients and lack of signaling capacity was associated with constitutive activation of extracellular-regulated kinases 1/2 (ERK1/2) and nuclear factor of activated T cells c1 (NF-ATc1), consistent with the idea that at least one group of CLL patients derives from the abnormal expansion of anergic B cells. In the present work, we further investigated the anergic subset of CLL (defined as the one with constitutive ERK1/2 phosphorylation) and found that it is characterized by low levels of surface immunoglobulin M and impairment of calcium mobilization after BCR engagement in vitro. Chronic BCR triggering promoted CLL cell survival selectively in phosphorylated ERK1/2 samples and the use of mitogen-activated protein kinase and NF-AT signaling inhibitors specifically induced apoptosis in this group of patients. Apoptosis induction was preceded by an initial phase of anergy reversal consisting in the loss of ERK phosphorylation and NF-AT nuclear translocation and by the restoration of BCR responsiveness, reinforcing the idea that the anergic program favors the survival of leukemic lymphocytes.

Tolerant anti-insulin B cells are effective APCs

Autoreactive B lymphocytes that are not culled by central tolerance in the bone marrow frequently enter the peripheral repertoire in a state of functional impairment, termed anergy. These cells are recognized as a liability for autoimmunity, but their contribution to disease is not well understood. Insulin-specific 125Tg B cells support T cell-mediated type 1 diabetes in NOD mice, despite being anergic to B cell mitogens and T cell-dependent immunization. Using this model, the potential of anergic, autoreactive B cells to present Ag and activate T cells was investigated. The data show that 1) insulin is captured and rapidly internalized by 125Tg BCRs, 2) these Ag-exposed B cells are competent to activate both experienced and naive CD4(+) T cells, 3) anergic 125Tg B cells are more efficient than naive B cells at activating T cells when Ag is limiting, and 4) 125Tg B cells are competent to generate low-affinity insulin B chain epitopes necessary for activation of diabetogenic anti-insulin BDC12-4.1 T cells, indicating the pathological relevance of anergic B cells in type 1 diabetes. Thus, phenotypically tolerant B cells that are retained in the repertoire may promote autoimmunity by driving activation and expansion of autoaggressive T cells via Ag presentation.

L chain allelic inclusion does not increase autoreactivity in lupus-prone New Zealand Black/New Zealand White mice

L chain allelic inclusion has been proposed as a B cell tolerance mechanism in addition to clonal deletion, clonal anergy, and receptor editing. It is said to rescue autoreactive B cells from elimination by diluting out the self-reactive BCR through the expression of a second innocuous L chain. In autoimmune animals, such as lupus-prone mice, allelically included B cells could be activated and produce pathogenic autoantibodies. We have previously shown that anti-DNA hybridomas from diseased New Zealand Black/New Zealand White F1 mice exhibit nearly perfect allelic exclusion. In the current study, we have analyzed single B cells from these and from nonautoimmune mice. In addition, we have cloned and expressed the Ig variable regions of several L chain-included B cells in cell culture. We find that although the number of L chain-included B cells increases as a result of receptor editing, the majority of such cells do not retain an autoreactive HxL chain combination and, therefore, allelic inclusion in itself does not serve as a B cell tolerance mechanism in these autoimmune mice.

TLR9 promotes tolerance by restricting survival of anergic anti-DNA B cells, yet is also required for their activation

Nucleic acid-reactive B cells frequently arise in the bone marrow but are tolerized by mechanisms including receptor editing, functional anergy, and/or deletion. TLR9, a sensor of endosomal dsDNA, both promotes and regulates systemic autoimmunity in vivo, but the precise nature of its apparently contradictory roles in autoimmunity remained unclear. In this study, using the 3H9 anti-DNA BCR transgene in the autoimmune-prone MRL.Fas(lpr) mouse model of systemic lupus erythematosus, we identify the stages at which TLR9 contributes to establishing and breaking B cell tolerance. Although TLR9 is dispensable for L chain editing during B cell development in the bone marrow, TLR9 limits anti-DNA B cell life span in the periphery and is thus tolerogenic. In the absence of TLR9, anti-DNA B cells have much longer life spans and accumulate in the follicle, neither activated nor deleted. These cells retain some characteristics of anergic cells, in that they have elevated basal BCR signaling but impaired induced responses and downregulate their cell-surface BCR expression. In contrast, whereas TLR9-intact anergic B cells accumulate near the T/B border, TLR9-deficient anti-DNA B cells are somewhat more dispersed throughout the follicle. Nonetheless, in older autoimmune-prone animals, TLR9 expression specifically within the B cell compartment is required for spontaneous peripheral activation of anti-DNA B cells and their differentiation into Ab-forming cells via an extrafollicular pathway. Thus, TLR9 has paradoxical roles in regulating anti-DNA B cells: it helps purge the peripheral repertoire of autoreactive cells, yet is also required for their activation.

Systemic circulation and bone recruitment of osteoclast precursors tracked by using fluorescent imaging techniques

Osteoclasts are bone-resorbing polykaryons differentiated from monocyte/macrophage-lineage hematopoietic precursors. It remains unclear whether osteoclasts originate from circulating blood monocytes or from bone tissue-resident precursors. To address this question, we combined two different experimental procedures: 1) shared blood circulation "parabiosis" with fluorescently labeled osteoclast precursors, and 2) photoconversion-based cell tracking with a Kikume Green-Red protein (KikGR). In parabiosis, CX(3)CR1-EGFP knock-in mice in which osteoclast precursors were labeled with EGFP were surgically connected with wild-type mice to establish a shared circulation. Mature EGFP(+) osteoclasts were found in the bones of the wild-type mice, indicating the mobilization of EGFP(+) osteoclast precursors into bones from systemic circulation. Receptor activator for NF-κB ligand stimulation increased the number of EGFP(+) osteoclasts in wild-type mice, suggesting that this mobilization depends on the bone resorption state. Additionally, KikGR(+) monocytes (including osteoclast precursors) in the spleen were exposed to violet light, and 2 d later we detected photoconverted "red" KikGR(+) osteoclasts along the bone surfaces. These results indicate that circulating monocytes from the spleen entered the bone spaces and differentiated into mature osteoclasts during a certain period. The current study used fluorescence-based methods clearly to demonstrate that osteoclasts can be generated from circulating monocytes once they home to bone tissues.

Endogenous antigen tunes the responsiveness of naive B cells but not T cells

In humans, up to 75% of newly generated B cells and about 30% of mature B cells show some degree of autoreactivity. Yet, how B cells establish and maintain tolerance in the face of autoantigen exposure during and after development is not certain. Studies of model B-cell antigen receptor (BCR) transgenic systems have highlighted the critical role of functional unresponsiveness or ‘anergy’. Unlike T cells, evidence suggests that receptor editing and anergy, rather than deletion, account for much of B-cell tolerance. However, it remains unclear whether the mature diverse B-cell repertoire of mice contains anergic autoreactive B cells, and if so, whether antigen was encountered during or after their development. By taking advantage of a reporter mouse in which BCR signalling rapidly and robustly induces green fluorescent protein expression under the control of the Nur77 regulatory region, antigen-dependent and antigen-independent BCR signalling events in vivo during B-cell maturation were visualized. Here we show that B cells encounter antigen during development in the spleen, and that this antigen exposure, in turn, tunes the responsiveness of BCR signalling in B cells at least partly by downmodulating expression of surface IgM but not IgD BCRs, and by modifying basal calcium levels. By contrast, no analogous process occurs in naive mature T cells. Our data demonstrate not only that autoreactive B cells persist in the mature repertoire, but that functional unresponsiveness or anergy exists in the mature B-cell repertoire along a continuum, a fact that has long been suspected, but never yet shown. These results have important implications for understanding how tolerance in T and B cells is differently imposed, and how these processes might go awry in disease.

Deletion and anergy of polyclonal B cells specific for ubiquitous membrane-bound self-antigen

Both deletion and anergy shape B cell tolerance to membrane-bound antigens.

B cell tolerance to self-antigen is critical to preventing antibody-mediated autoimmunity. Previous work using B cell antigen receptor transgenic animals suggested that self-antigen–specific B cells are either deleted from the repertoire, enter a state of diminished function termed anergy, or are ignorant to the presence of self-antigen. These mechanisms have not been assessed in a normal polyclonal repertoire because of an inability to detect rare antigen-specific B cells. Using a novel detection and enrichment strategy to assess polyclonal self-antigen–specific B cells, we find no evidence of deletion or anergy of cells specific for antigen not bound to membrane, and tolerance to these types of antigens appears to be largely maintained by the absence of T cell help. In contrast, a combination of deleting cells expressing receptors with high affinity for antigen with anergy of the undeleted lower affinity cells maintains tolerance to ubiquitous membrane-bound self-antigens.

Rheumatoid arthritis is associated with signaling alterations in naturally occurring autoreactive B-lymphocytes

Immune tolerance established during the development of B lymphocytes can be subverted in mature cells and lead to autoimmunity. This study focuses on the recently discovered subset of CD19(+)CD27(-)IgD(+)IgM(low/-) B cells that recognize self-antigens and have the capacity to produce autoantibodies, but under normal conditions do not generate autoimmune response due to intrinsic signaling inhibition (a condition known as clonal anergy and characterized by impaired antigen receptor signaling). Phosphorylation of intracellular signaling proteins and Ca(2+) responses in anergic B cells were measured by multicolor flow cytometry. Our results demonstrate a distinct phosphorylation pattern for major signal transduction proteins, which distinguishes anergic B cells. Comparison of B cell signaling properties in Rheumatoid Arthritis patients and healthy controls revealed a reversal of pTyr and Ca(2+) anergic signaling features in patients, accompanied by phosphorylation decreases of Blnk, Syk, SHP2, CD19. We identified BCR signaling pathway alterations associated with the loss of anergic B cell tolerance in Rheumatoid Arthritis.