T-independent activation-induced cytidine deaminase expression, class-switch recombination, and antibody production by immature/transitional 1 B cells

Control of adaptive immunity by pattern recognition receptors

One of the most significant conceptual advances in immunology in recent history is the recognition that signals from the innate immune system are required for induction of adaptive immune responses. Two breakthroughs were critical in establishing this paradigm: the identification of dendritic cells (DCs) as the cellular link between innate and adaptive immunity and the discovery of pattern recognition receptors (PRRs) as a molecular link that controls innate immune activation as well as DC function. Here, we recount the key events leading to these discoveries and discuss our current understanding of how PRRs shape adaptive immune responses, both indirectly through control of DC function and directly through control of lymphocyte function. In this context, we provide a conceptual framework for how variation in the signals generated by PRR activation, in DCs or other cell types, can influence T cell differentiation and shape the ensuing adaptive immune response.

Gut microbiota-gonadal axis: the impact of gut microbiota on reproductive functions

The influence of gut microbiota on physiological processes is rapidly gaining attention globally. Despite being under-studied, there are available data demonstrating a gut microbiota-gonadal cross-talk, and the importance of this axis in reproduction. This study reviews the impacts of gut microbiota on reproduction. In addition, the possible mechanisms by which gut microbiota modulates male and female reproduction are presented. Databases, including Embase, Google scholar, Pubmed/Medline, Scopus, and Web of Science, were explored using relevant key words. Findings showed that gut microbiota promotes gonadal functions by modulating the circulating levels of steroid sex hormones, insulin sensitivity, immune system, and gonadal microbiota. Gut microbiota also alters ROS generation and the activation of cytokine accumulation. In conclusion, available data demonstrate the existence of a gut microbiota-gonadal axis, and role of this axis on gonadal functions. However, majority of the data were compelling evidences from animal studies with a great dearth of human data. Therefore, human studies validating the reports of experimental studies using animal models are important.

An integrated proteome and transcriptome of B cell maturation defines poised activation states of transitional and mature B cells

During B cell maturation, transitional and mature B cells acquire cell-intrinsic features that determine their ability to exit quiescence and mount effective immune responses. Here we use label-free proteomics to quantify the proteome of B cell subsets from the mouse spleen and map the differential expression of environmental sensing, transcription, and translation initiation factors that define cellular identity and function. Cross-examination of the full-length transcriptome and proteome identifies mRNAs related to B cell activation and antibody secretion that are not accompanied by detection of the encoded proteins. In addition, proteomic data further suggests that the translational repressor PDCD4 restrains B cell responses, in particular those from marginal zone B cells, to a T-cell independent antigen. In summary, our molecular characterization of B cell maturation presents a valuable resource to further explore the mechanisms underpinning the specialized functions of B cell subsets, and suggest the presence of 'poised' mRNAs that enable expedited B cell responses.

Does congenital cytomegalovirus infection contribute to the development of acute lymphoblastic leukemia in children?

Cytomegalovirus (CMV) is a ubiquitous herpesvirus that has a profound impact on the host immune system. Congenital cytomegalovirus (cCMV) infection modulates neonatal immune cell compartments, yet the full impact of in utero exposure on developing fetal immune cells remains poorly characterized. A series of recent studies have identified a potential link between cCMV infection and the development of acute lymphoblastic leukemia (ALL) in childhood. Here, we review the emerging evidence linking CMV and ALL risk, discuss what is known about the causes of childhood ALL, and propose how CMV infection in early life may confer increased ALL risk.

E6020, a TLR4 Agonist Adjuvant, Enhances Both Antibody Titers and Isotype Switching in Response to Immunization with Hapten-Protein Antigens and Is Diminished in Mice with TLR4 Signaling Insufficiency

The mechanisms by which TLR4-based adjuvants enhance immunogenicity are not fully understood. We have taken advantage of a novel knock-in mouse strain that homozygously expresses two single-nucleotide polymorphisms (SNPs) that are homologous to human TLR4 (rs4986790 and rs4986791) and have been associated with LPS hyporesponsiveness in vivo and in vitro. TLR4-SNP (coexpressing mutations D298G/N397I in TLR4) mice that recapitulate the human phenotype were compared with wild-type (WT) mice for their hapten-specific Ab responses after immunization with hapten 4-hydroxy-3-nitrophenyl acetyl (NP) NP-Ficoll or NP-OVA in the absence or presence of a water-soluble TLR4 analog adjuvant, E6020. IgM and IgG anti-NP responses were comparable in WT and TLR4-SNP mice after immunization with either NP-Ficoll or NP-OVA only. E6020 significantly yet transiently improved the IgM and IgG anti-NP responses of both WT and TLR4-SNP mice to NP-Ficoll (T-independent), with modestly enhanced Ab production in WT mice. In contrast, T-dependent (NP-OVA), adjuvant-enhanced responses showed sustained elevation of NP-specific Ab titers in WT mice, intermediate responses in TLR4-SNP mice, and negligible enhancement in TLR4-/- mice. E6020-enhanced early humoral responses in WT and TLR4-SNP mice to NP-OVA favored an IgG1 response. After a second immunization, however, the immune responses of TLR4-SNP mice remained IgG1 dominant, whereas WT mice reimmunized with NP-OVA and E6020 exhibited increased anti-NP IgG2c titers and a sustained increase in the IgG1 and IgG2c production by splenocytes. These findings indicate that E6020 increases and sustains Ab titers and promotes isotype class switching, as evidenced by reduced titers and IgG1-dominant immune responses in mice with TLR4 insufficiency.

Pharmacologically Inferred Glycolysis and Glutaminolysis Requirement of B Cells in Lupus-Prone Mice

Several studies have shown an enhanced metabolism in the CD4⁺ T cells of lupus patients and lupus-prone mice. Little is known about the metabolism of B cells in lupus. In this study, we compared the metabolism of B cells between lupus-prone B6.Sle1.Sle2.Sle3 triple-congenic mice and C57BL/6 controls at steady state relative to autoantibody production, as well as during T cell-dependent (TD) and T cell-independent (TI) immunizations. Starting before the onset of autoimmunity, B cells from triple-congenic mice showed an elevated glycolysis and mitochondrial respiration, which were normalized in vivo by inhibiting glycolysis with a 2-deoxy-d-glucose (2DG) treatment. 2DG greatly reduced the production of TI-Ag-specific Abs, but showed minimal effect with TD-Ags. In contrast, the inhibition of glutaminolysis with 6-diazo-5-oxo-l-norleucine had a greater effect on TD than TI-Ag-specific Abs in both strains. Analysis of the TI and TD responses in purified B cells in vitro suggests, however, that the glutaminolysis requirement is not B cell-intrinsic. Thus, B cells have a greater requirement for glycolysis in TI than TD responses, as inferred from pharmacological interventions. B cells from lupus-prone and control mice have different intrinsic metabolic requirements or different responses toward 2DG and 6-diazo-5-oxo-l-norleucine, which mirrors our previous results obtained with follicular Th cells. Overall, these results predict that targeting glucose metabolism may provide an effective therapeutic approach for systemic autoimmunity by eliminating both autoreactive follicular Th and B cells, although it may also impair TI responses.

Diversity upon diversity: linking DNA double-strand break repair to blood cancer health disparities

Chromosomal translocations arising from aberrant repair of multiple DNA double-strand breaks (DSBs) are a defining characteristic of many cancers. DSBs are an essential part of physiological processes in antibody-producing B cells. The B cell environment is poised to generate genome instability leading to translocations relevant to the pathology of blood cancers. These are a diverse set of cancers, but limited data from under-represented groups have pointed to health disparities associated with each. We focus on the DSBs that occur in developing B cells and propose the most likely mechanism behind the formation of translocations. We also highlight specific cancers in which these rearrangements occur and address the growing concern of health disparities associated with them.

The mechanisms of human lymphoid chromosomal translocations and their medical relevance

The most common human lymphoid chromosomal translocations involve concurrent failures of the recombination activating gene (RAG) complex and Activation-Induced Deaminase (AID). These are two enzymes that are normally expressed for purposes of the two site-specific DNA recombination processes: V(D)J recombination and class switch recombination (CSR). First, though it is rare, a low level of expression of AID can introduce long-lived T:G mismatch lesions at 20-600 bp fragile zones. Second, the V(D)J recombination process can occasionally fail to rejoin coding ends, and this failure may permit an opportunity for Artemis:DNA-dependent kinase catalytic subunit (DNA-PKcs) to convert the T:G mismatch sites at the fragile zones into double-strand breaks. The 20-600 bp fragile zones must be, at least transiently, in a single-stranded DNA (ssDNA) state for the first step to occur, because AID only acts on ssDNA. Here we discuss the key DNA sequence features that lead to AID action at a fragile zone, which are (a) the proximity and density of strings of cytosine nucleotides (C-strings) that cause a B/A-intermediate DNA conformation; (b) overlapping AID hotspots that contain a methyl CpG (WRCG), which AID converts to a long-lived T:G mismatch; and (c) transcription, which, though not essential, favors increased ssDNA in the fragile zone. We also summarize chromosomal features of the focal fragile zones in lymphoid malignancies and discuss the clinical relevance of understanding the translocation mechanisms. Many of the key principles covered here are also relevant to chromosomal translocations in non-lymphoid somatic cells as well.

Mechanistic basis for chromosomal translocations at the E2A gene and its broader relevance to human B cell malignancies

Analysis of translocation breakpoints in human B cell malignancies reveals that DNA double-strand breaks at oncogenes most frequently occur at CpG sites located within 20-600 bp fragile zones and depend on activation-induced deaminase (AID). AID requires single-stranded DNA (ssDNA) to act, but it has been unclear why or how this region transiently acquires a ssDNA state. Here, we demonstrate the ssDNA state in the 23 bp E2A fragile zone using several methods, including native bisulfite DNA structural analysis in live human pre-B cells. AID deamination within the E2A fragile zone does not require but is increased upon transcription. High C-string density, nascent RNA tails, and direct DNA sequence repeats prolong the ssDNA state of the E2A fragile zone and increase AID deamination at overlapping AID hotspots that contain the CpG sites at which breaks occur in patients. These features provide key insights into lymphoid fragile zones generally.

CD27-CD38+ B cells accumulated in early HIV infection exhibit transitional profile and promote HIV disease progression

Although peripheral B cell dysfunction in early HIV infection is established, how B cell subsets are altered by HIV infection is poorly understood. While investigating B cell subsets among individuals recently infected with HIV, we observe an accumulation of CD27^-CD38⁺ B cells and find that these cells can directly facilitate HIV infection of primary CD4⁺ T cells in vitro. Comprehensive analyses of the phenotype, function, and transcriptome of the CD27^-CD38⁺ B cell subset is conducted compared with memory and naive B cells. We find that the CD27^-CD38⁺ B cells exhibit a transitional B cell phenotype and an extremely high turnover rate. Importantly, individuals with higher proportions of CD27^-CD38⁺ B cells during early HIV infection tend to become rapid progressors in the chronic infection stage. In this study, we identify a peripheral transitional B cell subset that accumulates during early HIV infection and may contribute to disease progression.

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.

miR-15a/16-1 deletion in activated B cells promotes plasma cell and mature B-cell neoplasms

Chromosome 13q deletion [del(13q)], harboring the miR-15a/16-1 cluster, is one of the most common genetic alterations in mature B-cell malignancies, which originate from germinal center (GC) and post-GC B cells. Moreover, miR-15a/16 expression is frequently reduced in lymphoma and multiple myeloma (MM) cells without del(13q), suggesting important tumor-suppressor activity. However, the role of miR-15a/16-1 in B-cell activation and initiation of mature B-cell neoplasms remains to be determined. We show that conditional deletion of the miR-15a/16-1 cluster in murine GC B cells induces moderate but widespread molecular and functional changes including an increased number of GC B cells, percentage of dark zone B cells, and maturation into plasma cells. With time, this leads to development of mature B-cell neoplasms resembling human extramedullary plasmacytoma (EP) as well as follicular and diffuse large B-cell lymphomas. The indolent nature and lack of bone marrow involvement of EP in our murine model resembles human primary EP rather than MM that has progressed to extramedullary disease. We corroborate human primary EP having low levels of miR-15a/16 expression, with del(13q) being the most common genetic loss. Additionally, we show that, although the mutational profile of human EP is similar to MM, there are some exceptions such as the low frequency of hyperdiploidy in EP, which could account for different disease presentation. Taken together, our studies highlight the significant role of the miR-15a/16-1 cluster in the regulation of the GC reaction and its fundamental context-dependent tumor-suppression function in plasma cell and B-cell malignancies.

The spleen: "epicenter" in malaria infection and immunity

The spleen is a complex secondary lymphoid organ that plays a crucial role in controlling blood‐stage infection with Plasmodium parasites. It is tasked with sensing and removing parasitized RBCs, erythropoiesis, the activation and differentiation of adaptive immune cells, and the development of protective immunity, all in the face of an intense inflammatory environment. This paper describes how these processes are regulated following infection and recognizes the gaps in our current knowledge, highlighting recent insights from human infections and mouse models.

An Autochthonous Mouse Model of Myd88- and BCL2-Driven Diffuse Large B-cell Lymphoma Reveals Actionable Molecular Vulnerabilities

Based on gene expression profiles, diffuse large B cell lymphoma (DLBCL) is sub-divided into germinal center B cell-like (GCB) and activated B cell-like (ABC) DLBCL. Two of the most common genomic aberrations in ABC-DLBCL are mutations in MYD88, as well as BCL2 copy number gains. Here, we employ immune phenotyping, RNA-Seq and whole exome sequencing to characterize a Myd88 and Bcl2-driven mouse model of ABC-DLBCL. We show that this model resembles features of human ABC-DLBCL. We further demonstrate an actionable dependence of our murine ABC-DLBCL model on BCL2. This BCL2 dependence was also detectable in human ABC-DLBCL cell lines. Moreover, human ABC-DLBCLs displayed increased PD-L1 expression, compared to GCB-DLBCL. In vivo experiments in our ABC-DLBCL model showed that combined venetoclax and RMP1-14 significantly increased the overall survival of lymphoma bearing animals, indicating that this combination may be a viable option for selected human ABC-DLBCL cases harboring MYD88 and BCL2 aberrations.

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.

Deep Characterization of the Human Antibody Response to Natural Infection Using Longitudinal Immune Repertoire Sequencing

Human antibody response studies are largely restricted to periods of high immune activity (e.g. vaccination). To comprehensively understand the healthy B cell immune repertoire and how this changes over time and through natural infection, we conducted immune repertoire RNA sequencing on flow cytometry-sorted B cell subsets to profile a single individual's antibodies over 11 months through two periods of natural viral infection. We found that 1) a baseline of healthy variable (V) gene usage in antibodies exists and is stable over time, but antibodies in memory cells consistently have a different usage profile relative to earlier B cell stages; 2) a single complementarity-determining region 3 (CDR3) is potentially generated from more than one VJ gene combination; and 3) IgG and IgA antibody transcripts are found at low levels in early human B cell development, suggesting that class switching may occur earlier than previously realized. These findings provide insight into immune repertoire stability, response to natural infections, and human B cell development.

Regulatory functions of B cells and regulatory plasma cells

B cells critically contribute to health through the production of antibodies that provide a vital line of defence against infectious agents. In addition, B cells are known to play an integrative role in immunity, acting as crucial antigen-presenting cells for T cells, and being an important source of cytokines that can target multiple cell types including stromal cells, innate cells, and adaptive lymphocytes. This review focuses on the role of B cells as negative regulators of immunity through the production of interleukin-10 (IL-10) in autoimmune, infectious, and malignant diseases. It discusses the phenotypes of the B cell subsets most competent to produce IL-10 in vitro and to exert suppressive functions in vivo upon adoptive transfer in recipient mice, the signals and transcription factors regulating IL-10 expression in B cells, and the recent identification of plasmocytes, including short-lived plasmablasts and long-lived plasma cells, as an important source of IL-10 in secondary lymphoid organs and inflamed tissues in vivo during mouse and human diseases. With our increasing knowledge of this non-canonical B cell function a coherent framework starts emerging that will help monitoring and targeting this B cell function in health and disease.

Targeting Antigens to CD180 but Not CD40 Programs Immature and Mature B Cell Subsets to Become Efficient APCs

Targeting Ags to the CD180 receptor activates both B cells and dendritic cells (DCs) to become potent APCs. After inoculating mice with Ag conjugated to an anti-CD180 Ab, B cell receptors were rapidly internalized. Remarkably, all B cell subsets, including even transitional 1 B cells, were programed to process, present Ag, and stimulate Ag-specific CD4⁺ T cells. Within 24-48 hours, Ag-specific B cells were detectable at T-B borders in the spleen; there, they proliferated in a T cell-dependent manner and induced the maturation of T follicular helper (T_FH) cells. Remarkably, immature B cells were sufficient for the maturation of T_FH cells after CD180 targeting: T_FH cells were induced in BAFFR^-/- mice (with only transitional 1 B cells) and not in μMT mice (lacking all B cells) following CD180 targeting. Unlike CD180 targeting, CD40 targeting only induced DCs but not B cells to become APCs and thus failed to efficiently induce T_FH cell maturation, resulting in slower and lower-affinity IgG Ab responses. CD180 targeting induces a unique program in Ag-specific B cells and to our knowledge, is a novel strategy to induce Ag presentation in both DCs and B cells, especially immature B cells and thus has the potential to produce a broad range of Ab specificities. This study highlights the ability of immature B cells to present Ag to and induce the maturation of cognate T_FH cells, providing insights toward vaccination of mature B cell-deficient individuals and implications in treating autoimmune disorders.

Understanding regulatory B cells in autoimmune diseases: the case of multiple sclerosis

The suppressive function of B cells is mediated mostly through their provision of cytokines with anti-inflammatory properties, in particular interleukin-10. This B cell activity has been convincingly described in mice with autoimmune, infectious, as well as malignant diseases, and evidence is accumulating of its relevance in human. This review provides a personal view of this B cell function using multiple sclerosis and its animal model experimental autoimmune encephalomyelitis as representative examples, in an attempt to bridge observations obtained in mice and human, with the goal of providing a coherent transversal framework to further explore this field, and eventually manipulate this B cell function therapeutically.

Integrated B Cell, Toll-like, and BAFF Receptor Signals Promote Autoantibody Production by Transitional B Cells

The B cell survival cytokine BAFF has been linked with the pathogenesis of systemic lupus erythematosus (SLE). BAFF binds distinct BAFF-family surface receptors, including the BAFF-R and transmembrane activator and CAML interactor (TACI). Although originally characterized as a negative regulator of B cell activation, TACI signals are critical for class-switched autoantibody (autoAb) production in BAFF transgenic mice. Consistent with this finding, a subset of transitional splenic B cells upregulate surface TACI expression and contribute to BAFF-driven autoAb. In the current study, we interrogated the B cell signals required for transitional B cell TACI expression and Ab production. Surprisingly, despite established roles for dual BCR and TLR signals in autoAb production in SLE, signals downstream of these receptors exerted distinct impacts on transitional B cell TACI expression and autoAb titers. Whereas loss of BCR signals prevented transitional B cell TACI expression and resulted in loss of serum autoAb across all Ig isotypes, lack of TLR signals exerted a more limited impact restricted to autoAb class-switch recombination without altering transitional B cell TACI expression. Finally, in parallel with the protective effect of TACI deletion, loss of BAFF-R signaling also protected against BAFF-driven autoimmunity. Together, these findings highlight how multiple signaling pathways integrate to promote class-switched autoAb production by transitional B cells, events that likely impact the pathogenesis of SLE and other BAFF-dependent autoimmune diseases.

Vivax infection alters peripheral B-cell profile and induces persistent serum IgM

B cell-mediated humoral responses are essential for controlling malarial infection. Studies have addressed the effects of Plasmodium falciparum infection on peripheral B-cell subsets but not much is known for P. vivax infection. Furthermore, majority of the studies investigate changes during acute infection, but not after parasite clearance. In this prospective study, we analysed peripheral B-cell profiles and antibody responses during acute P. vivax infection and upon recovery (30 days post-treatment) in a low-transmission area in India. Dengue patients were included as febrile-condition controls. Both dengue and malaria patients showed a transient increase in atypical memory B cells during acute infection. However, transient B cell-activating factor (BAFF)-independent increase in the percentage of total and activated immature B cells was observed in malaria patients. Naïve B cells from malaria patients also showed increased TLR4 expression. Total IgM levels remained unchanged during acute infection but increased significantly at recovery. Serum antibody profiling showed a parasite-specific IgM response that persisted at recovery. A persistent IgM autoantibody response was also observed in malaria but not dengue patients. Our data suggest that in hypoendemic regions acute P. vivax infection skews peripheral B-cell subsets and results in a persistent parasite-specific and autoreactive IgM response.

Investigation of tumor-tumor interactions in a double human cervical carcinoma xenograft model in nude mice

Tumor-tumor distant interactions within one organism are of major clinical relevance determining clinical outcome. To investigate this poorly understood phenomenon, a double human cervical xenograft model in nude mice was developed. A first tumor was induced subcutaneously by injection of human papillomavirus positive cervical carcinoma cells into the mouse lower right flank and 3 weeks later, animals were challenged with the same tumor cell line injected subcutaneously into the upper left flank. These tumors had no direct physical contact and we found no systemic changes induced by the primary tumor affecting the growth of a secondary tumor. However, ablation of the primary tumor by local treatment with cidofovir, a nucleotide analogue with known antiviral and antiproliferative properties, resulted not only in a local antitumor effect but also in a temporary far-reaching effect leading to retarded growth of the challenged tumor. Cidofovir far-reaching effects were linked to a reduced tumor-driven inflammation, to increased anti-tumor immune responses, and could not be enhanced by co-administration with immune stimulating adjuvants. Our findings point to the potential use of cidofovir in novel therapeutic strategies aiming to kill tumor cells as well as to influence the immune system to fight cancer.

Chronic Lymphocytic Leukemia B-Cell Normal Cellular Counterpart: Clues From a Functional Perspective

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of small mature-looking CD19+ CD23+ CD5+ B-cells that accumulate in the blood, bone marrow, and lymphoid organs. To date, no consensus has been reached concerning the normal cellular counterpart of CLL B-cells and several B-cell types have been proposed. CLL B-cells have remarkable phenotypic and gene expression profile homogeneity. In recent years, the molecular and cellular biology of CLL has been enriched by seminal insights that are leading to a better understanding of the natural history of the disease. Immunophenotypic and molecular approaches (including immunoglobulin heavy-chain variable gene mutational status, transcriptional and epigenetic profiling) comparing the normal B-cell subset and CLL B-cells provide some new insights into the normal cellular counterpart. Functional characteristics (including activation requirements and propensity for plasma cell differentiation) of CLL B-cells have now been investigated for 50 years. B-cell subsets differ substantially in terms of their functional features. Analysis of shared functional characteristics may reveal similarities between normal B-cell subsets and CLL B-cells, allowing speculative assignment of a normal cellular counterpart for CLL B-cells. In this review, we summarize current data regarding peripheral B-cell differentiation and human B-cell subsets and suggest possibilities for a normal cellular counterpart based on the functional characteristics of CLL B-cells. However, a definitive normal cellular counterpart cannot be attributed on the basis of the available data. We discuss the functional characteristics required for a cell to be logically considered to be the normal counterpart of CLL B-cells.

Association of Defective Regulation of Autoreactive Interleukin-6-Producing Transitional B Lymphocytes With Disease in Patients With Systemic Sclerosis

OBJECTIVE

Systemic sclerosis (SSc) has the highest case-specific mortality of any rheumatic disease, and no effective therapy is available. A clear manifestation of SSc is the presence of autoantibodies. However, the origin of autoantibody-producing B lymphocytes, their mechanisms of activation and autoantibody production, and their role remain unclear. This study was undertaken to identify mechanisms that contribute to pathogenic B cell generation and involvement in SSc and to assess the altered distribution and function of B cells in SSc patients.

METHODS

Multicolor flow cytometry was performed to determine B cell subset distribution, cytokine production, and tolerance induction in SSc patients and healthy controls. Cytokine production following stimulation of the cells ex vivo was determined by multiplex assay.

RESULTS

A range of defects in B lymphocyte tolerance and cytokine production in SSc were noted. There was evidence of altered distribution of transitional B cell subsets, increased production of interleukin-6 (IL-6) and IL-8, and defective tolerance induction in SSc B cells. In addition, B cells from SSc patients had a reduced ability to produce IL-10 when stimulated through innate immune pathways. In contrast to healthy individuals, tolerance checkpoints in SSc patients failed to suppress the emergence of B cells that produce autoantibodies with specificity to the Scl-70 antigen, which is strongly associated with SSc. These defects were paralleled by altered intracellular signaling and apoptosis following B cell receptor engagement.

CONCLUSION

Our findings provide new insights into mechanisms underlying defective B lymphocyte responses in patients with SSc and their contribution to disease.

BCR and Endosomal TLR Signals Synergize to Increase AID Expression and Establish Central B Cell Tolerance

Activation-induced cytidine deaminase (AID) is required to purge autoreactive immature and transitional-1 (immature/T1) B cells at the first tolerance checkpoint, but how AID selectively removes self-reactive B cells is unclear. We now show that B cell antigen receptor (BCR) and endosomal Toll-like receptor (TLR) signals synergize to elicit high levels of AID expression in immature/T1 B cells. This synergy is restricted to ligands for endocytic TLR and requires phospholipase-D activation, endosomal acidification, and MyD88. The first checkpoint is significantly impaired in AID- or MyD88-deficient mice and in mice doubly heterozygous for AID and MyD88, suggesting interaction of these factors in central B cell tolerance. Moreover, administration of chloroquine, an inhibitor of endosomal acidification, results in a failure to remove autoreactive immature/T1 B cells in mice. We propose that a BCR/TLR pathway coordinately establishes central tolerance by hyper-activating AID in immature/T1 B cells that bind ligands for endosomal TLRs.

Protection of mice deficient in mature B cells from West Nile virus infection by passive and active immunization

B cell activating factor receptor (BAFFR)-/- mice have a profound reduction in mature B cells, but unlike μMT mice, they have normal numbers of newly formed, immature B cells. Using a West Nile virus (WNV) challenge model that requires antibodies (Abs) for protection, we found that unlike wild-type (WT) mice, BAFFR-/- mice were highly susceptible to WNV and succumbed to infection within 8 to 12 days after subcutaneous virus challenge. Although mature B cells were required to protect against lethal infection, infected BAFFR-/- mice had reduced WNV E-specific IgG responses and neutralizing Abs. Passive transfer of immune sera from previously infected WT mice rescued BAFFR-/- and fully B cell-deficient μMT mice, but unlike μMT mice that died around 30 days post-infection, BAFFR-/- mice survived, developed WNV-specific IgG Abs and overcame a second WNV challenge. Remarkably, protective immunity could be induced in mature B cell-deficient mice. Administration of a WNV E-anti-CD180 conjugate vaccine 30 days prior to WNV infection induced Ab responses that protected against lethal infection in BAFFR-/- mice but not in μMT mice. Thus, the immature B cells present in BAFFR-/- and not μMT mice contribute to protective antiviral immunity. A CD180-based vaccine may promote immunity in immunocompromised individuals.

Accelerated Systemic Autoimmunity in the Absence of Somatic Hypermutation in 564Igi: A Mouse Model of Systemic Lupus with Knocked-In Heavy and Light Chain Genes

564Igi mice have knocked-in immunoglobulin (Ig) heavy (H) and light (L) chain genes that encode an autoantibody recognizing RNA. Previously, we showed that these mice produce pathogenic IgG autoantibodies when activation-induced deaminase (AID) is expressed in pre-B and immature B cells but not when it is expressed only in mature B cells. AID has two functions; it is necessary for somatic hypermutation (SHM) and class switch recombination (CSR). To determine the role of each of these functions in the generation of pathogenic autoantibodies, we generated 564Igi mice that carry a mutant AID-encoding gene, Aicda (Aicda^G23S), which is capable of promoting CSR but not SHM. We found that 564Igi Aicda^G23S mice secreted class-switched antibodies (Abs) at levels approximately equal to 564Igi mice. However, compared to 564Igi mice, 564Igi Aicda^G23S mice had increased pathogenic IgG Abs and severe systemic lupus erythematosus-like disease, including, glomerulonephritis, and early death. We suggest that in 564Igi mice SHM by AID changes Ig receptors away from self reactivity, thereby mitigating the production of autoantibody, providing a novel mechanism of tolerance.

Activation-induced cytidine deaminase prevents pro-B cell acute lymphoblastic leukemia by functioning as a negative regulator in Rag1 deficient pro-B cells

Activation-induced cytidine deaminase (AID) is essential for somatic hypermutation and class switch recombination in mature B-cells, while AID was also shown to play a role in developing pre-BCR/BCR-positive B-cells of the bone marrow. To further elucidate a potential function of Aid in the bone marrow prior to V(D)J-recombination, we utilized an in vivo model which exerts a B-cell developmental arrest at the pro-B cell stage with low frequencies of pro-B cell acute lymphoblastic leukemia (pro-B ALL) development. Therefore, p19Arf^-/-Rag1^-/- (AR) mice were crossed with Aid-deficient mice (ARA). Surprisingly, loss of Aid expression in pro-B cells accelerated pro-B ALL incidence from 30% (AR) to 98% (ARA). This effect was Aid dose dependent, since Aid^+/- animals of the same background displayed a significantly lower incidence (83%). Furthermore, B-cell-specific Aid up-regulation was observed in Aid-competent pro-B ALLs. Additional whole exome/sanger sequencing of murine pro-B ALLs revealed an accumulation of recurrent somatic Jak3 (p.R653H, p.V670A) and Dnm2 (p.G397R) mutations, which highlights the importance of active IL7R signaling in the pro-B ALL blast cells. These findings were further supported by an enhanced proliferative potential of ARA pro-B cells compared to Aid-competent cells from the same genetic background. In summary, we show that both Aid and Rag1 act as a negative regulators in pro-B cells, preventing pro-B ALL.

Contribution of secondary Igkappa rearrangement to primary immunoglobulin repertoire diversification

Abs reactive to DNA and DNA/histone complexes are a distinguished characteristic of primary immunoglobulin repertoires in autoimmune B6.MRL-Fas^lpr and MRL/MpJ-Fas^lpr mice. These mice are defective in Fas receptor, which is critical for the apoptosis of autoreactive B cells by an extrinsic pathway. In the present study, we explored the possibility that bone marrow small pre-B and immature B cells from adult B6.MRL-Fas^lpr mice and MRL/MpJ-Fas^lpr mice respectively, which contain autoreactive B-cell antigen receptors (BCR) and manifest autoimmune syndromes, exhibit enhanced receptor editing patterns. Indeed, FAS^lpr pre B and immature B cells were shown to possess more ongoing replacements of non-productive (nP) than productive (P) primary VκJκ rearrangements. Significantly, the P vs nP ratios of these replaced primary rearrangements were 1:2, thus indicating that κ light-chain production appears not to inhibit secondary rearrangements. In addition, we identified multiple atypical rearrangements, such as Vκ cRS (cryptic recombination signals) cleavages. These results suggest that the onset of light chain secondary rearrangements persists similarly as a non-selected mode and independent of BCR autoreactivity during certain developmental windows of bone marrow B cells in lupus-prone mice and control, and leads us to propose the function of secondary, de novo Igκ rearrangements to increase BCR diversity.

Mechanisms of human lymphoid chromosomal translocations

Analysis of chromosomal translocation sequence locations in human lymphomas has provided valuable clues about the mechanism of the translocations and when they occur. Biochemical analyses on the mechanisms of DNA breakage and rejoining permit formulation of detailed models of the human chromosomal translocation process in lymphoid neoplasms. Most human lymphomas are derived from B cells in which a DNA break at an oncogene is initiated by activation-induced deaminase (AID). The partner locus in many cases is located at one of the antigen receptor loci, and this break is generated by the recombination activating gene (RAG) complex or by AID. After breakage, the joining process typically occurs by non-homologous DNA end-joining (NHEJ). Some of the insights into this mechanism also apply to translocations that occur in non-lymphoid neoplasms.

Chronic lymphocytic leukemia cells diversify and differentiate in vivo via a nonclassical Th1-dependent, Bcl-6-deficient process

Xenografting primary tumor cells allows modeling of the heterogeneous natures of malignant diseases and the influences of the tissue microenvironment. Here, we demonstrate that xenografting primary chronic lymphocytic leukemia (CLL) B lymphocytes with activated autologous T cells into alymphoid mice results in considerable CLL B cell division and sizable T cell expansion. Nevertheless, most/all CD5⁺CD19⁺ cells are eventually lost, due in part to differentiation into antibody-secreting plasmablasts/plasma cells. CLL B cell differentiation is associated with isotype class switching and development of new IGHV-D-J mutations and occurs via an activation-induced deaminase-dependent pathway that upregulates IRF4 and Blimp-1 without appreciable levels of the expected Bcl-6. These processes were induced in IGHV-unmutated and IGHV-mutated clones by Th1-polarized T-bet⁺ T cells, not classical T follicular helper (Tfh) cells. Thus, the block in B cell maturation, defects in T cell action, and absence of antigen-receptor diversification, which are often cardinal characteristics of CLL, are not inherent but imposed by external signals and the microenvironment. Although these activities are not dominant features in human CLL, each occurs in tissue proliferation centers where the mechanisms responsible for clonal evolution operate. Thus, in this setting, CLL B cell diversification and differentiation develop by a nonclassical germinal center-like reaction that might reflect the cell of origin of this leukemia.

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.

Activation-Induced Cytidine Deaminase Expression in Human B Cell Precursors Is Essential for Central B Cell Tolerance

Activation-induced cytidine deaminase (AID), the enzyme-mediating class-switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes, is essential for the removal of developing autoreactive B cells. How AID mediates central B cell tolerance remains unknown. We report that AID enzymes were produced in a discrete population of immature B cells that expressed recombination-activating gene 2 (RAG2), suggesting that they undergo secondary recombination to edit autoreactive antibodies. However, most AID+ immature B cells lacked anti-apoptotic MCL-1 and were deleted by apoptosis. AID inhibition using lentiviral-encoded short hairpin (sh)RNA in B cells developing in humanized mice resulted in a failure to remove autoreactive clones. Hence, B cell intrinsic AID expression mediates central B cell tolerance potentially through its RAG-coupled genotoxic activity in self-reactive immature B cells.

High affinity IgM(+) memory B cells are generated through a germinal center-dependent pathway

During a T cell-dependent immune response, B cells undergo clonal expansion and selection and the induction of isotype switching and somatic hypermutation (SHM). Although somatically mutated IgM(+) memory B cells have been reported, it has not been established whether they are really high affinity B cells. We tracked (4-hydroxy-3-nitrophenyl) acetyl hapten-specific GC B cells from normal immunized mice based on affinity of their B cell receptor (BCR) and performed BCR sequence analysis. SHM was evident by day 7 postimmunization and increased with time, such that high affinity IgM(+) as well as IgG(+) memory B cells continued to be generated up to day 42. In contrast, class-switch recombination (CSR) was almost completed by day 7 and then the ratio of IgG1(+)/IgM(+) GC B cells remained unchanged. Together these findings suggest that IgM(+) B cells undergo SHM in the GC to generate high affinity IgM(+) memory cells and that this process continues even after CSR is accomplished.

Toll-like receptors and B cells: functions and mechanisms

Numerous reports have described Toll-like receptor (TLR) functions in myeloid cells such as dendritic cells (DCs) and macrophages, but relatively fewer studies have examined TLR responses in B lymphocytes. B cells express a wide variety of TLRs and are highly activated after TLR ligation, leading to enhancements in B cell survival, surface molecule expression, cytokine and antibody production, and antigen presentation. During an immune response, B cells can receive signals through TLRs as well as the B cell antigen receptor (BCR) and/or CD40. TLR ligation synergizes with signals through these receptors and augments both innate and adaptive immune functions of B lymphocytes. Additionally, targeting B cell TLRs may provide new therapies against certain types of cancer as well as autoimmune diseases. Here, we summarize TLR expression and contributions to both normal and pathogenic functions in mouse and human B cells.

Curiouser and curiouser: the role(s) of AID expression in self-tolerance

Aicda is crucial for antibody diversification by mediating Ig class-switch recombination, V(D)J hypermutation (SHM) and, in some species, gene conversion. Recently, evidence has accumulated to show that Aicda is expressed during B-cell development and that this expression in some unknown way, mediates tolerance in immature and transitional B cells. In this issue of the European Journal of Immunology, Umiker et al. [Eur. J. Immunol. 2014. 44: 3093-3108] show that enforced expression of Aicda during early B-cell development is associated with self-tolerance. Curiously, constitutive Aicda expression that begins early in B cells suppresses the generation of autoreactive IgM but promotes the expression of self-reactive IgG. In contrast, when Aicda is activated later in B-cell development, self-reactive IgM is abundant but IgG is not. These observations suggest pathways for self-tolerance that have been little explored.

Natural IgM is produced by CD5- plasma cells that occupy a distinct survival niche in bone marrow

Natural IgM is constitutively present in the serum, where it aids in the early control of viral and bacterial expansions. Natural IgM also plays a significant role in the prevention of autoimmune disease by promoting the clearance of cellular debris. Nevertheless, the origins of natural IgM have not been precisely defined. Previous studies focused on the role of CD5(+) B1 cells in the production of natural IgM, but we show in this article that a discrete population of CD5(-) IgM plasmablasts and plasma cells in the bone marrow (BM) produces the majority of serum IgM in resting mice. These Ab-secreting cells (ASC) originate from peritoneal cavity-resident cells, because transfer of peritoneal cells completely restores serum IgM and the specific compartment of BM ASC in Rag1-deficient mice. We show that BM natural IgM ASC arise from a fetal-lineage progenitor that is neither B1a nor B1b, and that this IgM ASC compartment contains a substantial fraction of long-lived plasma cells that do not occupy the IgG plasma cell survival niche in the BM; instead, they are supported by IL-5. In summary, we identified the primary source of natural IgM and showed that these ASC are maintained long-term in a unique survival niche within the BM.

Production of IgG autoantibody requires expression of activation-induced deaminase in early-developing B cells in a mouse model of SLE

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the presence of pathogenic IgG antinuclear antibodies. Pathogenic IgG autoantibody production requires B-cell activation, leading to the production of activation-induced deaminase (AID) and class switching of IgM genes to IgG. To understand how and when B cells are activated to produce these IgG autoantibodies, we studied cells from 564Igi, a mouse model of SLE. 564Igi mice develop a disease profile closely resembling that found in human SLE patients, including the presence of IgG antinucleic acid Abs. We have generated 564Igi mice that conditionally express an activation-induced cytidine deaminase transgene (Aicda(tg) ), either in all B cells or only in mature B cells. Here, we show that class-switched pathogenic IgG autoantibodies were produced only in 564Igi mice in which AID was functional in early-developing B cells, resulting in loss of tolerance. Furthermore, we show that the absence of AID in early-developing B cells also results in increased production of self-reactive IgM, indicating that AID, through somatic hypermutation, contributes to tolerance. Our results suggest that the pathophysiology of clinical SLE might also be dependent on AID expression in early-developing B cells.

Naturally occurring human phosphorylcholine antibodies are predominantly products of affinity-matured B cells in the adult

Phosphorylcholine (PC) is a classic T-independent Ag that is exposed on apoptotic cells, oxidized phospholipids, and bacterial polysaccharides. Experimental as well as epidemiological studies have over the past decade implicated Abs against PC (anti-PC) as anti-inflammatory and a strong protective factor in cardiovascular disease. Although clinically important, little is known about the development of anti-PC in humans. This study was conceived to dissect the human anti-PC repertoire and generate human mAbs. We designed a PC-specific probe to identify, isolate, and characterize PC-reactive B cells from 10 healthy individuals. The donors had all mounted somatically mutated Abs toward PC using a broad variety of Ig genes. PC-reactive B cells were primarily found in the IgM(+) memory subset, although significant numbers also were detected among naive, IgG(+), and CD27(+)CD43(+) B cells. Abs from these subsets were clonally related, suggesting a common origin. mAbs derived from the same donors exhibited equivalent or higher affinity for PC than the well-characterized murine T-15 clone. These results provide novel insights into the cellular and molecular ontogeny of atheroprotective PC Abs, thereby offering new opportunities for Ab-based therapeutic interventions.

Enhanced antibody responses to an HIV-1 membrane-proximal external region antigen in mice reconstituted with cultured lymphocytes

We have shown that the protective HIV-1 Ab, 2F5, avidly reacts with a conserved mammalian self-Ag, kynureninase, and that the development of B cells specific for the 2F5 epitope is constrained by immunological tolerance. These observations suggest that the capacity to mount Ab responses to the 2F5 epitope is mitigated by tolerance, but such capacity may be latent in the pretolerance and/or anergic B cell pools. In this study, we use B cell tetramer reagents to track the frequencies of B cells that recognize the HIV-1 2F5 epitope (SP62): in C57BL/6 mice, SP62-binding transitional B cells are readily identified in bone marrow but are lost during subsequent development. Unsurprisingly then, immunization with SP62 immunogen does not elicit significant humoral responses in normal C57BL/6 mice. Reconstitution of Rag1(null) mice with normal congenic B cells that have matured in vitro restores the capacity to mount significant serum Ab and germinal center responses to this HIV-1 epitope. These B cell cultures are permissive for the development of autoreactive B cells and support the development of SP62-specific B cell compartments normally lost in 2F5 Ab knockin mice. The recovery of humoral responses to the 2F5/SP62 epitope of HIV-1 by reconstitution with B cells containing forbidden, autoreactive clones provides direct evidence that normal C57BL/6 mice latently possess the capacity to generate humoral responses to a conserved, neutralizing HIV-1 epitope.

B-type suppression: a role played by "regulatory B cells" or "regulatory plasma cells"?

B-cell depletion can improve disease in some patients with rheumatoid arthritis or multiple sclerosis, indicating the pathogenic contribution of B cells to autoimmunity. However, studies in mice have demonstrated that B cells have immunosuppressive functions as well, with IL-10 being a critical mediator of B-cell-mediated suppression. IL-10-secreting B cells have been shown to promote disease remission in some mouse models of autoimmune disorders. Human B cells also produce IL-10, and evidence is accumulating that human IL-10-producing B cells might inhibit immunity. There is considerable interest in identifying the phenotype of B cells providing IL-10 in a suppressive manner, which would facilitate the analysis of the molecular mechanisms controlling this B-cell property. Here, we review current knowledge on the B-cell subpopulations found to provide suppressive functions in mice, considering both the pathological context in which they were identified and the signals that control their induction. We discuss the phenotype of B cells that have IL-10-dependent regulatory activities in mice, which leads us to propose that antibody-secreting cells are, in some cases at least, the major source of B-cell-derived regulatory IL-10 in vivo. Anti-inflammatory cytokine production by antibody-secreting cells offers a novel mechanism for the coordination of innate and humoral immune responses.

Flexible ordering of antibody class switch and V(D)J joining during B-cell ontogeny

Omenn syndrome is a severe immunodeficiency disease commonly arising from hypomorphic RAG recombinase gene mutations. RAG recombinase mediates V(D)J joining during early B-lymphocyte development in the bone marrow (BM). V(D)J recombination and class switch recombination are thought to partition between the BM and secondary lymphoid organs. Kenter and colleagues show that V(D)J joining and switching are interchangeably inducible in the BM. This study has important implications for the development of Omenn syndrome, autoimmune diseases, and leukemia.

V(D)J joining is mediated by RAG recombinase during early B-lymphocyte development in the bone marrow (BM). Activation-induced deaminase initiates isotype switching in mature B cells of secondary lymphoid structures. Previous studies questioned the strict ontological partitioning of these processes. We show that pro-B cells undergo robust switching to a subset of immunoglobulin H (IgH) isotypes. Chromatin studies reveal that in pro-B cells, the spatial organization of the Igh locus may restrict switching to this subset of isotypes. We demonstrate that in the BM, V(D)J joining and switching are interchangeably inducible, providing an explanation for the hyper-IgE phenotype of Omenn syndrome.

Expression of an anti-RNA autoantibody in a mouse model of SLE increases neutrophil and monocyte numbers as well as IFN-I expression

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the presence of antinucleic acid autoantibodies, high levels of circulating type I interferon (IFN-I), and an IFN-I-dependent elevated expression of activating FcγR. Increases in neutrophils and monocytes are often observed in clinical SLE, but how these contribute to autoantibody and IFN-I production is poorly understood. Here, we analyzed SLE pathogenesis in 564Igi mice, an SLE-model strain carrying gene-targeted heavy and light chain antibody genes encoding an anti-RNA autoantibody in a C57BL/6 background. Similar to human SLE patients, 564Igi mice produce anti-RNA autoantibodies and expanded neutrophil and monocyte populations. These myeloid cells produced IFN-I and exhibit increased FcγRIV expression induced via an IFN-I autocrine loop. A direct effect of IFN-I on 56 Igi BM B cells and neutrophils was supported by their upregulation of "IFN-I signature genes". In addition, 564Igi developing B cells showed upregulated TLR7 resulting in IgG2a/2b class switch recombination and autoantibody production. Our results indicate that the production of anti-RNA autoantibody is sufficient to induce an increase of BM, blood, and spleen IFN-I-producing neutrophils, and suggest a mechanism by which autoantibody and IFN-I contribute to SLE by activating B lymphocytes, neutrophils, and monocyte effector cells in vivo.

Overexpression of TLR7 promotes cell-intrinsic expansion and autoantibody production by transitional T1 B cells

Transgenic expression of TLR7 results in the expansion and hyperactivation of T1 B cells in response to endogenous RNA complexes, leading to increased autoantibody production.

Toll-like receptor (TLR), a ligand for single-stranded RNA, has been implicated in the development of pathogenic anti-RNA autoantibodies both in systemic lupus erythematous (SLE) patients and in murine models of lupus. It is still unclear, however, where and how TLR7-mediated interactions affect the development of autoreactive B cells. We found that overexpression of TLR7 in transgenic mice (TLR7.1Tg) leads to marked alterations of transitional (T1) B cells, associated with their expansion and proliferation within the splenic red pulp (RP). This phenotype was intrinsic to the T1 subset of B cells and occurred independently of type 1 IFN signals. Overexpression of RNase in TLR7.1Tg mice significantly limited the expansion and proliferation of T1 cells, indicating that endogenous RNA complexes are driving their activation. TLR7.1Tg T1 cells were hyper-responsive to anti-IgM and TLR7 ligand stimulation in vitro and produced high concentrations of class-switched IgG2b and IgG2c, including anti-RNA antibodies. Our results demonstrate that initial TLR7 stimulation of B cells occurs at the T1 stage of differentiation in the splenic RP and suggest that dysregulation of TLR7 expression in T1 cells can result in production of autoantibodies.

Targeting antigens to CD180 rapidly induces antigen-specific IgG, affinity maturation, and immunological memory

Targeting antigen to B cells and dendritic cells via direct conjugation to anti-CD180 antibody promotes robust antigen-specific antibody responses in the absence of adjuvant.

Antigen (Ag) targeting is an efficient way to induce immune responses. Ag is usually coupled to an antibody (Ab) specific for a receptor expressed on dendritic cells (DCs), and then the Ag–anti-receptor is inoculated with an adjuvant. Here we report that targeting Ag to a receptor expressed on both B cells and DCs, the TLR orphan receptor CD180, in the absence of adjuvant rapidly induced IgG responses that were stronger than those induced by Ag in alum. Ag conjugated to anti-CD180 (Ag-αCD180) induced affinity maturation and Ab responses that were partially T cell independent, as Ag-specific IgGs were generated in CD40- and T cell–deficient mice. After preimmunization with Ag-αCD180 and boosting with soluble Ag, both WT and CD40 knockout (KO) mice rapidly produced Ag-specific IgG-forming cells, demonstrating that Ag–anti-CD180 induces immunological memory. The potent adjuvant effect of Ag-αCD180 required Ag to be coupled to anti-CD180 and the responsive B cells to express both CD180 and an Ag-specific B cell receptor. Surprisingly, CD180 Ag targeting also induced IgG Abs in BAFF-R KO mice lacking mature B cells and in mice deficient in interferon signaling. Targeting Ag to CD180 may be useful for therapeutic vaccination and for vaccinating the immune compromised.