CD4+ T cells and CD40 participate in selection and homeostasis of peripheral B cells

Transcriptomic Analysis Reveals Sixteen Potential Genes Associated with the Successful Differentiation of Antibody-Secreting Cells through the Utilization of Unfolded Protein Response Mechanisms in Robust Responders to the Influenza Vaccine

The seasonal influenza vaccine remains one of the vital recommended infection control measures for the elderly with chronic illnesses. We investigated the immunogenicity of a single dose of influenza vaccine in 123 seronegative participants and classified them into four distinct groups, determined by the promptness of vaccine response, the longevity of humoral immunity, and the likelihood of exhibiting cross-reactivity. Subsequently, we used transcriptional profiling and differential gene expression analysis to identify potential genes directly associated with the robust response to the vaccine. The group of exemplary vaccine responders differentially expressed 16 genes, namely: MZB1, MYDGF, TXNDC5, TXNDC11, HSP90B1, FKBP11, PDIA5, PRDX4, CD38, SDC1, TNFRSF17, TNFRSF13B, PAX5, POU2AF1, IRF4, and XBP1. Our findings point out a list of expressed proteins that are related to B cell proliferation, unfolded protein response, and cellular haemostasis, as well as a linkage of these expressions to the survival of long-lived plasma cells.

B-cell maturation antigen targeting strategies in multiple myeloma treatment, advantages and disadvantages

B cell maturation antigen (BCMA), a transmembrane glycoprotein member of the tumor necrosis factor receptor superfamily 17 (TNFRSF17), highly expressed on the plasma cells of Multiple myeloma (MM) patients, as well as the normal population. BCMA is used as a biomarker for MM. Two members of the TNF superfamily proteins, including B-cell activating factor (BAFF) and A proliferation-inducing ligand (APRIL), are closely related to BCMA and play an important role in plasma cell survival and progression of MM. Despite the maximum specificity of the monoclonal antibody technologies, introducing the tumor-specific antigen(s) is not applicable for all malignancies, such as MM that there plenty of relatively specific antigens such as GPCR5D, MUC1, SLAMF7 and etc., but higher expression of BCMA on these cells in comparison with normal ones can be regarded as a relatively exclusive marker. Currently, different monoclonal antibody (mAb) technologies applied in anti-MM therapies such as daratuzumab, SAR650984, GSK2857916, and CAR-T cell therapies are some of these tools that are reviewed in the present manuscript. By the way, the structure, function, and signaling of the BCMA and related molecule(s) role in normal plasma cells and MM development, evaluated as well as the potential side effects of its targeting by different CAR-T cells generations. In conclusion, BCMA can be regarded as an ideal molecule to be targeted in immunotherapeutic methods, regarding lower potential systemic and local side effects.

Activated interleukin-7 receptor signaling drives B-cell acute lymphoblastic leukemia in mice

Philadelphia chromosome-like acute lymphoblastic leukemia (Ph-like ALL) is a high-risk subtype of B-ALL often associated with genetic variants that alter cytokine receptor signaling, including mutations in the interleukin-7 receptor (IL7R). To investigate whether IL7R variants are leukemia-initiating, we built mouse models expressing activated Il7r (aIL7R). B-cell intrinsic aIL7R mice developed spontaneous B-ALL, demonstrating sufficiency of Il7r activating mutations in leukemogenesis. Concomitant introduction of a knock-out allele in the associated adapter protein Lnk (encoded by Sh2b3) or a dominant-negative variant of the transcription factor Ikaros (Ikzf1) increased disease penetrance. The resulting murine leukemias displayed monoclonality and recurrent somatic Kras mutations and efficiently engrafted into immunocompetent mice. Phosphoproteomic analyses of aIL7R leukemic cells revealed constitutive Stat5 signaling and B cell receptor (BCR)-like signaling despite the absence of surface pre-BCR. Finally, in vitro treatment of aIL7R leukemic B-cells with Jak, mTOR, or Syk inhibitors blocked growth, confirming that each pathway is active in this mouse model of IL7R-driven B-ALL.

Targeting Bruton's Tyrosine Kinase in Inflammatory and Autoimmune Pathologies

Bruton's tyrosine kinase (BTK) was discovered due to its importance in B cell development, and it has a critical role in signal transduction downstream of the B cell receptor (BCR). Targeting of BTK with small molecule inhibitors has proven to be efficacious in several B cell malignancies. Interestingly, recent studies reveal increased BTK protein expression in circulating resting B cells of patients with systemic autoimmune disease (AID) compared with healthy controls. Moreover, BTK phosphorylation following BCR stimulation in vitro was enhanced. In addition to its role in BCR signaling, BTK is involved in many other pathways, including pattern recognition, Fc, and chemokine receptor signaling in B cells and myeloid cells. This broad involvement in several immunological pathways provides a rationale for the targeting of BTK in the context of inflammatory and systemic AID. Accordingly, numerous in vitro and in vivo preclinical studies support the potential of BTK targeting in these conditions. Efficacy of BTK inhibitors in various inflammatory and AID has been demonstrated or is currently evaluated in clinical trials. In addition, very recent reports suggest that BTK inhibition may be effective as immunosuppressive therapy to diminish pulmonary hyperinflammation in coronavirus disease 2019 (COVID-19). Here, we review BTK's function in key signaling pathways in B cells and myeloid cells. Further, we discuss recent advances in targeting BTK in inflammatory and autoimmune pathologies.

AID in Chronic Lymphocytic Leukemia: Induction and Action During Disease Progression

The enzyme activation-induced cytidine deaminase (AID) initiates somatic hypermutation (SHM) and class switch recombination (CSR) of immunoglobulin (Ig) genes, critical actions for an effective adaptive immune response. However, in addition to the benefits generated by its physiological roles, AID is an etiological factor for the development of human and murine leukemias and lymphomas. This review highlights the pathological role of AID and the consequences of its actions on the development, progression, and therapeutic refractoriness of chronic lymphocytic leukemia (CLL) as a model disease for mature lymphoid malignancies. First, we summarize pertinent aspects of the expression and function of AID in normal B lymphocytes. Then, we assess putative causes for AID expression in leukemic cells emphasizing the role of an activated microenvironment. Thirdly, we discuss the role of AID in lymphomagenesis, in light of recent data obtained by NGS analyses on the genomic landscape of leukemia and lymphomas, concentrating on the frequency of AID signatures in these cancers and correlating previously described tumor-gene drivers with the presence of AID off-target mutations. Finally, we discuss how these changes could affect tumor suppressor and proto-oncogene targets and how they could be associated with disease progression. Collectively, we hope that these sections will help to better understand the complex paradox between the physiological role of AID in adaptive immunity and its potential causative activity in B-cell malignancies.

Mycobacterium tuberculosis PPE18 protein inhibits MHC class II antigen presentation and B cell response in mice

PPE18 protein belongs to PE/PPE family of Mycobacterium tuberculosis. We reported earlier that PPE18 protein provides survival advantage to M. tuberculosis during infection. In the current study, we found that PPE18 inhibits MHC class II-mediated antigen presentation by macrophages in a dose-dependent manner without affecting the surface level of MHC class II or co-stimulatory molecules. PPE18 does not affect antigen uptake or presentation of preprocessed peptide by macrophages. Antigen degradation was found to be inhibited by PPE18 protein due to perturbation in phagolysosomal acidification. PPE18-mediated inhibition of MHC class II antigen presentation caused poorer activation of CD4 T cells. Mice infected with M. smegmatis expressing PPE18 exhibited reduced maturation and activation of B cells and had decreased Mycobacteria-specific antibody titers. Thus M. tuberculosis probably utilizes PPE18 to inhibit MHC class II antigen presentation causing poorer activation of adaptive immune responses. This study may be useful in understanding host-pathogen interaction and open up directions of designing novel therapeutics targeting PPE18 to tackle this nefarious pathogen.

The Plasticity of Newly Formed B Cells

Newly formed B cells (NF-B cells) that emerge from the bone marrow to the periphery have often been referred to as immature or transitional B cells. However, NF-B cells have several striking characteristics, including a distinct BCR repertoire, high expression of AID, high sensitivity to PAMPs, and the ability to produce cytokines. A number of findings do not support their designation as immature because NF-B cells have the potential to become Ab-producing cells and to undergo class-switch recombination. In this review, we provide a fresh perspective on NF-B cell functions and describe some of the signals driving their activation. We summarize growing evidence supporting a role for NF-B cells in protection against infections and as a potential source of autoantibody-producing cells in autoimmune diseases such as systemic lupus erythematosus.

Homeostasis and regulation of autoreactive B cells

In contrast to the previous belief that autoreactive B cells are eliminated from the normal repertoire of B cells, many autoreactive B cells actually escape clonal deletion and develop into mature B cells. These autoreactive B cells in healthy individuals perform some beneficial functions in the host and are homeostatically regulated by regulatory T and B cells or other mechanisms to prevent autoimmune diseases. Autoreactive B-1 cells constitutively produce polyreactive natural antibodies for tissue homeostasis. Recently, autoreactive follicular B cells were reported to participate actively in the germinal center reaction. Furthermore, the selection and usefulness of autoreactive marginal zone (MZ) B cells found in autoimmune diseases are not well understood, although the repertoire of MZ B-cell receptors (BCRs) is presumed to be biased to detect bacterial antigens. In this review, we discuss the autoreactive B-cell populations among all three major B-cell subsets and their regulation in immune responses and diseases.

Sex Differences in Older Adults' Immune Responses to Seasonal Influenza Vaccination

Background: Sex differences in immune responses to influenza vaccine may impact efficacy across populations. Methods: In a cohort of 138 older adults (50-74 years old), we measured influenza A/H1N1 antibody titers, B-cell ELISPOT response, PBMC transcriptomics, and PBMC cell compositions at 0, 3, and 28 days post-immunization with the 2010/11 seasonal inactivated influenza vaccine. Results: We identified higher B-cell ELISPOT responses in females than males. Potential mechanisms for sex effects were identified in four gene clusters related to T, NK, and B cells. Mediation analysis indicated that sex-dependent expression in T and NK cell genes can be partially attributed to higher CD4+ T cell and lower NK cell fractions in females. We identified strong sex effects in 135 B cell genes whose expression correlates with ELISPOT measures, and found that cell subset differences did not explain the effect of sex on these genes' expression. Post-vaccination expression of these genes, however, mediated 41% of the sex effect on ELISPOT responses. Conclusions: These results improve our understanding of sexual dimorphism in immunity and influenza vaccine response.

Costimulatory blockade molecules and B-cell-mediated immune response: current knowledge and perspectives

There is an urgent need for therapeutic agents that target humoral alloimmunity in solid organ transplantation. This includes sensitized patients with preformed donor-specific human leukocyte antigen antibodies and patients who develop de novo donor-specific antibodies, both of which are associated with acute and chronic antibody-mediated rejection and allograft loss. In the last decade, both experimental and clinical studies highlighted the major impact of costimulation molecules in the control of immune responses both in the field of transplantation and autoimmune disease. Although these molecules have been initially developed to control the early steps of T-cell activation, recent evidence also supports their influence at several steps of the humoral response. In this review, we aim to provide an overview of the current knowledge of the effects of costimulatory blockade agents on humoral responses in both autoimmune and allogeneic contexts. We first present the effects of costimulatory molecules on the different steps of alloantibody production. We then summarize mechanisms and clinical results observed using cytotoxic T lymphocyte antigen-4 (CTLA4)-Ig molecules both in transplantation and autoimmunity. Finally, we present the potential interest and implications of other costimulatory family members as therapeutic targets, with emphasis on combinatorial approaches, for the optimal control of the alloantigen-specific humoral response.

Activated PIK3CD drives innate B cell expansion yet limits B cell-intrinsic immune responses

B cell–intrinsic expression of activated PIK3CD (aPIK3CD) restricts immature BM B cell development while promoting the expansion of MZ and B1a B cells via enhanced survival. aPIK3CD is counter-productive during both T cell–independent and –dependent responses, limiting antigen-specific antibodies and class-switch recombination.

Activated PI3K-delta syndrome (APDS) is an immunodeficiency caused by gain-of-function mutations in PIK3CD. This disease exhibits complex immune phenotypes including increased IgM, recurrent infection, and impaired vaccine responses. To better understand the impact of B cells in this disease, we generated an inducible model of the common APDS mutation (hPIK3CD-E1021K; referred to as aPIK3CD) and intercrossed these mice with B cell–specific Cre models. Mb1-aPIK3CD mice exhibited bone marrow B lymphopenia and, conversely, expansion of the peripheral innate B1a and MZ B cell compartments. aPIK3CD B cells manifest increased pS6 and increased survival at several stages, without alterations in cycling, and baseline increases in plasma cells, natural IgM, and IgG3. Finally, Mb1-aPIK3CD mice exhibited blunted T cell–independent immune responses, and both AID- and CD21-aPIK3CD mice displayed reduced class-switched antibodies following T cell–dependent immunization. Thus, aPIK3CD alters B cell development and function and is counter-productive during immune responses, providing insight into B cell–intrinsic contributions to the APDS phenotype.

αv Integrins regulate germinal center B cell responses through noncanonical autophagy

Germinal centers (GCs) are major sites of clonal B cell expansion and generation of long-lived, high-affinity antibody responses to pathogens. Signaling through TLRs on B cells promotes many aspects of GC B cell responses, including affinity maturation, class switching, and differentiation into long-lived memory and plasma cells. A major challenge for effective vaccination is identifying strategies to specifically promote GC B cell responses. Here, we have identified a mechanism of regulation of GC B cell TLR signaling, mediated by αv integrins and noncanonical autophagy. Using B cell-specific αv-KO mice, we show that loss of αv-mediated TLR regulation increased GC B cell expansion, somatic hypermutation, class switching, and generation of long-lived plasma cells after immunization with virus-like particles (VLPs) or antigens associated with TLR ligand adjuvants. Furthermore, targeting αv-mediated regulation increased the magnitude and breadth of antibody responses to influenza virus vaccination. These data therefore identify a mechanism of regulation of GC B cells that can be targeted to enhance antibody responses to vaccination.

The Autoimmune Risk Variant PTPN22 C1858T Alters B Cell Tolerance at Discrete Checkpoints and Differentially Shapes the Naive Repertoire

A common genetic variant in the gene encoding the protein tyrosine phosphatase nonreceptor type 22 (PTPN22 C1858T) has been linked to a wide range of autoimmune disorders. Although a B cell-intrinsic role in promoting disease has been reported, the mechanism(s) through which this variant functions to alter the preimmune B cell repertoire remains unknown. Using a series of polyclonal and transgenic self-reactive models harboring the analogous mutation in murine Ptpn22, we show evidence for enhanced BCR, B cell-activating factor receptor, and CD40 coreceptor programs, leading to broadly enhanced positive selection of B cells at two discrete checkpoints in the bone marrow and spleen. We further identified a bias for selection of B cells into the follicular mature versus marginal zone B cell compartment. Using a biomarker to track a self-reactive H chain in peripheral blood, we found evidence of similarly enhanced positive selection in human carriers of the PTPN22 C1858T variant. Our combined data support a model whereby the risk variant augments the BCR and coreceptor programs throughout B cell development, promoting enrichment of self-reactive specificities into the follicular mature compartment and thereby likely increasing the risk for seeding of autoimmune B cell responses.

Comparative Evaluation of αCD40 (2C10R4) and αCD154 (5C8H1 and IDEC-131) in a Nonhuman Primate Cardiac Allotransplant Model

BACKGROUND

Specific blockade of T cell costimulation pathway is a promising immunomodulatory approach being developed to replace our current clinical immunosuppression therapies. The goal of this study is to compare results associated with 3 monoclonal antibodies directed against the CD40/CD154 T cell costimulation pathway.

METHODS

Cynomolgus monkey heterotopic cardiac allograft recipients were treated with either IDEC-131 (humanized αCD154, n = 9), 5C8H1 (mouse-human chimeric αCD154, n = 5), or 2C10R4 (mouse-rhesus chimeric αCD40, n = 6) monotherapy using a consistent, comparable dosing regimen for 3 months after transplant.

RESULTS

Relative to the previously reported IDEC-131-treated allografts, median survival time (35 ± 31 days) was significantly prolonged in both 5C8H1-treated (142 ± 26, P < 0.002) and 2C10R4-treated (124 ± 37, P < 0.020) allografts. IDEC-131-treated grafts had higher cardiac allograft vasculopathy severity scores during treatment relative to either 5C8H1 (P = 0.008) or 2C10R4 (P = 0.0002). Both 5C8H1 (5 of 5 animals, P = 0.02) and 2C10R4 (6/6, P = 0.007), but not IDEC-131 (2/9), completely attenuated IgM antidonor alloantibody (alloAb) production during treatment; 5C8H1 (5/5) more consistently attenuated IgG alloAb production compared to 2C10R4 (4/6) and IDEC-131 (0/9). All evaluable explanted grafts experienced antibody-mediated rejection. Only 2C10R4-treated animals exhibited a modest, transient drop in CD20 lymphocytes from baseline at day 14 after transplant (-457 ± 152 cells/μL) compared with 5C8H1-treated animals (16 ± 25, P = 0.037), and the resurgent B cells were primarily of a naive phenotype.

CONCLUSIONS

In this model, CD154/CD40 axis blockade using IDEC-131 is an inferior immunomodulatory treatment than 5C8H1 or 2C10R4, which have similar efficacy to prolong graft survival and to delay cardiac allograft vasculopathy development and antidonor alloAb production during treatment.

Role of Sex Hormone Levels and Psychological Stress in the Pathogenesis of Autoimmune Diseases

The aim of this review article is to assess the connection between psychological stress and sex hormones and their effect on the development of autoimmune diseases. Psychological stress describes what people feel when they are under mental, physical, or emotional pressure. We searched for online articles using MEDLINE®, Embase, Cochrane Library and Google Scholar. Our research yielded a total of 165 articles out of which 30 articles were considered for further perusal. The articles were reviewed from February 2016 to February 2017. Case reports and patients suffering from hematolymphoid malignancies and active infections were excluded from the review. Estrogen and testosterone are potential physiological regulatory factors for the peripheral development of CD4+CD25+ T regulatory cells. Stress at any age leads to the depletion of estrogen and testosterone stores in the body, leading to the loss of expansion of T regulatory cells, making the immature B cells evade the negative selection at the germinal center, or in other words, leading to the loss of central tolerance, a triggering event in autoimmune diseases like systemic lupus erythematosus. Autoimmune diseases in women are most likely due to changes in estrogen levels during mental, physical, pre-menopausal, post-menopausal, and pregnancy-induced stress. We conclude that modulating estrogen in females (pre-menopausal and post-menopausal) and testosterone in males can be used to treat stress-related immune imbalance resulting in autoimmune diseases in both sexes.

Enhanced Bruton's Tyrosine Kinase Activity in Peripheral Blood B Lymphocytes From Patients With Autoimmune Disease

OBJECTIVE

Bruton's tyrosine kinase (BTK) transmits crucial survival signals from the B cell receptor (BCR) in B cells. Pharmacologic BTK inhibition effectively diminishes disease symptoms in mouse models of autoimmunity; conversely, transgenic BTK overexpression induces systemic autoimmunity in mice. We undertook this study to investigate BTK expression and activity in human B cells in the context of autoimmune disease.

METHODS

Using intracellular flow cytometry, we quantified BTK expression and phosphorylation in subsets of peripheral blood B cells from 30 patients with rheumatoid arthritis (RA), 26 patients with primary Sjögren's syndrome (SS), and matched healthy controls.

RESULTS

In circulating B cells, BTK protein expression levels correlated with BTK phosphorylation. BTK expression was up-regulated upon BCR stimulation in vitro and was significantly higher in CD27+ memory B cells than in CD27-IgD+ naive B cells. Importantly, BTK protein and phospho-BTK were significantly increased in B cells from anti-citrullinated protein antibody (ACPA)-positive RA patients but not in B cells from ACPA-negative RA patients. BTK was increased both in naive B cells and in memory B cells and correlated with frequencies of circulating CCR6+ Th17 cells. Likewise, BTK protein was increased in B cells from a major fraction of patients with primary SS and correlated with serum rheumatoid factor levels and parotid gland T cell infiltration. Interestingly, targeting T cell activation in patients with primary SS using the CTLA-4Ig fusion protein abatacept restored BTK protein expression in B cells to normal levels.

CONCLUSION

These data indicate that autoimmune disease in humans is characterized by enhanced BTK activity, which is linked not only to autoantibody formation but also to T cell activity.

Altered B cell signalling in autoimmunity

Recent work has provided new insights into how altered B cell-intrinsic signals - through the B cell receptor (BCR) and key co-receptors - function together to promote the pathogenesis of autoimmunity. These combined signals affect B cells at two distinct stages: first, in the selection of the naive repertoire; and second, during extrafollicular or germinal centre activation responses. Thus, dysregulated signalling can lead to both an altered naive BCR repertoire and the generation of autoantibody-producing B cells. Strikingly, high-affinity autoantibodies predate and predict disease in several autoimmune disorders, including type 1 diabetes and systemic lupus erythematosus. This Review summarizes how, rather than being a downstream consequence of autoreactive T cell activation, dysregulated B cell signalling can function as a primary driver of many human autoimmune diseases.

Mediation of transitional B cell maturation in the absence of functional Bruton's tyrosine kinase

X-linked immune-deficient (Xid) mice, carrying a mutation in Bruton’s tyrosine kinase (Btk), have multiple B cell lineage differentiation defects. We now show that, while Xid mice showed only mild reduction in the frequency of the late transitional (T2) stage of peripheral B cells, the defect became severe when the Xid genotype was combined with either a CD40-null, a TCRbeta-null or an MHC class II (MHCII)-null genotype. Purified Xid T1 and T2 B cells survived poorly in vitro compared to wild-type (WT) cells. BAFF rescued WT but not Xid T1 and T2 B cells from death in culture, while CD40 ligation equivalently rescued both. Xid transitional B cells ex vivo showed low levels of the p100 protein substrate for non-canonical NF-kappaB signalling. In vitro, CD40 ligation induced equivalent activation of the canonical but not of the non-canonical NF-kappaB pathway in Xid and WT T1 and T2 B cells. CD40 ligation efficiently rescued p100-null T1 B cells from neglect-induced death in vitro. These data indicate that CD40-mediated signals, likely from CD4 T cells, can mediate peripheral transitional B cell maturation independent of Btk and the non-canonical NF-kappaB pathway, and thus contribute to the understanding of the complexities of peripheral B cell maturation.

BCR and co-receptor crosstalk facilitate the positive selection of self-reactive transitional B cells

The establishment of a diverse B cell repertoire requires fine-tuning of antigen receptor selection during development in order to permit sufficient diversity while reducing the potential for autoimmunity. In this review, we highlight recent studies demonstrating the central role of the B cell antigen receptor (BCR), in coordination with other key pro-survival signals mediated by CD40, BAFF-R, TACI and/or TLRs, in regulating both negative and positive selection of autoreactive B cells. In particular, we show how altered antigen or co-stimulatory signaling can facilitate positive selection of transitional B cells with self-reactive BCRs, ultimately leading to their entry into the mature, naive B cell compartment. We propose a model wherein altered receptor signals (due to inherited genetic changes) leads: first, to enhanced positive selection of autoreactive cells into the naïve B cell repertoire; subsequently, to an increased probability of pathogenic germinal center responses in individuals with a broad range of autoimmune disorders.

Somatically Hypermutated Plasmodium-Specific IgM(+) Memory B Cells Are Rapid, Plastic, Early Responders upon Malaria Rechallenge

Humoral immunity consists of pre-existing antibodies expressed by long-lived plasma cells and rapidly reactive memory B cells (MBC). Recent studies of MBC development and function after protein immunization have uncovered significant MBC heterogeneity. To clarify functional roles for distinct MBC subsets during malaria infection, we generated tetramers that identify Plasmodium-specific MBCs in both humans and mice. Long-lived murine Plasmodium-specific MBCs consisted of three populations: somatically hypermutated immunoglobulin M(+) (IgM(+)) and IgG(+) MBC subsets and an unmutated IgD(+) MBC population. Rechallenge experiments revealed that high affinity, somatically hypermutated Plasmodium-specific IgM(+) MBCs proliferated and gave rise to antibody-secreting cells that dominated the early secondary response to parasite rechallenge. IgM(+) MBCs also gave rise to T cell-dependent IgM(+) and IgG(+)B220(+)CD138(+) plasmablasts or T cell-independent B220(-)CD138(+) IgM(+) plasma cells. Thus, even in competition with IgG(+) MBCs, IgM(+) MBCs are rapid, plastic, early responders to a secondary Plasmodium rechallenge and should be targeted by vaccine strategies.

Cutting Edge: BAFF Promotes Autoantibody Production via TACI-Dependent Activation of Transitional B Cells

Mice overexpressing B cell activating factor of the TNF family (BAFF) develop systemic autoimmunity characterized by class-switched anti-nuclear Abs. Transmembrane activator and CAML interactor (TACI) signals are critical for BAFF-mediated autoimmunity, but the B cell developmental subsets undergoing TACI-dependent activation in settings of excess BAFF remain unclear. We report that, although surface TACI expression is usually limited to mature B cells, excess BAFF promotes the expansion of TACI-expressing transitional B cells. TACI(+) transitional cells from BAFF-transgenic mice are characterized by an activated, cycling phenotype, and the TACI(+) cell subset is specifically enriched for autoreactivity, expresses activation-induced cytidine deaminase and T-bet, and exhibits evidence of somatic hypermutation. Consistent with a potential contribution to BAFF-mediated humoral autoimmunity, TACI(+) transitional B cells from BAFF-transgenic mice spontaneously produce class-switched autoantibodies ex vivo. These combined findings highlight a novel mechanism through which BAFF promotes humoral autoimmunity via direct, TACI-dependent activation of transitional B cells.

Altered BCR and TLR signals promote enhanced positive selection of autoreactive transitional B cells in Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency disorder frequently associated with systemic autoimmunity, including autoantibody-mediated cytopenias. WAS protein (WASp)-deficient B cells have increased B cell receptor (BCR) and Toll-like receptor (TLR) signaling, suggesting that these pathways might impact establishment of the mature, naive BCR repertoire. To directly investigate this possibility, we evaluated naive B cell specificity and composition in WASp-deficient mice and WAS subjects (n = 12). High-throughput sequencing and single-cell cloning analysis of the BCR repertoire revealed altered heavy chain usage and enrichment for low-affinity self-reactive specificities in murine marginal zone and human naive B cells. Although negative selection mechanisms including deletion, anergy, and receptor editing were relatively unperturbed, WASp-deficient transitional B cells showed enhanced proliferation in vivo mediated by antigen- and Myd88-dependent signals. Finally, using both BCR sequencing and cell surface analysis with a monoclonal antibody recognizing an intrinsically autoreactive heavy chain, we show enrichment in self-reactive cells specifically at the transitional to naive mature B cell stage in WAS subjects. Our combined data support a model wherein modest alterations in B cell-intrinsic, BCR, and TLR signals in WAS, and likely other autoimmune disorders, are sufficient to alter B cell tolerance via positive selection of self-reactive transitional B cells.

Critical role of CD4 T cells in PF4/heparin antibody production in mice

Antibodies specific for platelet factor 4 (PF4)/heparin complexes are central to the pathogenesis of heparin-induced thrombocytopenia. Marginal zone B cells appear to be the source of such antibodies, but whether T-cell help is required is unclear. Here, we showed that induction of PF4/heparin-specific antibodies by PF4/heparin complexes was markedly impaired in mice depleted of CD4 T cells by anti-CD4 antibodies. Furthermore, Rag1-deficient recipient mice produced PF4/heparin-specific antibodies upon PF4/heparin challenge when reconstituted with a mixture of wild-type splenic B cells and splenocytes from B-cell-deficient (μMT) mice but not splenocytes from T- and B-cell-deficient (Rag1 knockout) mice. Lastly, mice with B cells lacking CD40, a B-cell costimulatory molecule that helps T-cell-dependent B-cell responses, displayed a marked reduction of PF4/heparin-specific antibody production following PF4/heparin challenge. Together, these findings show that helper T cells play a critical role in production of PF4/heparin-specific antibodies.

T cell-B cell thymic cross-talk: maintenance and function of thymic B cells requires cognate CD40-CD40 ligand interaction

Thymic development requires bidirectional interaction or cross-talk between developing T cells and thymic stromal cells, a relationship that has been best characterized for the interaction between thymocytes and thymic epithelial cells. We have characterized in this article the requirement for similar cross-talk in the maintenance and function of thymic B cells, another population that plays a role in selection of developing thymic T cells. We found that maintenance of thymic B cells is strongly dependent on the presence of mature single-positive thymocytes and on the interactions of these T cells with specific Ag ligand. Maintenance of thymic B cell number is strongly dependent on B cell-autonomous expression of CD40, but not MHC class II, indicating that direct engagement of CD40 on thymic B cells is necessary to support their maintenance and proliferation. Thymic B cells can mediate negative selection of superantigen-specific, self-reactive, single-positive thymocytes, and we show that CD40 expression on B cells is critical for this negative selection. Cross-talk with thymic T cells is thus required to support the thymic B cell population through a pathway that requires cell-autonomous expression of CD40, and that reciprocally functions in negative selection of autoreactive T cells.