Activation-induced deaminase-mediated class switch recombination is blocked by anti-IgM signaling in a phosphatidylinositol 3-kinase-dependent fashion

BCR ligation selectively inhibits IgE class switch recombination

Mechanisms that restrict class switch recombination (CSR) to IgE limit the subsequent production of IgE antibodies and therefore the development of allergic disease. Mice with impaired B cell receptor (BCR) signaling have significantly increased IgE responses, consistent with a role for BCR signaling in IgE regulation. While prior work focused on BCR signaling in IgE-expressing cells to explain these findings, it has been reported that BCR signaling can reduce CSR. Therefore, we investigated the possibility that IgE CSR might be particularly sensitive to inhibition by BCR signaling in unswitched B cells. We found that immunization of mice with high-affinity antigen resulted in reduced representation of IgE-expressing cells among germinal center B cells and plasma cells relative to a low-affinity antigen. Mechanistic experiments with cultured mouse B cells demonstrated that BCR ligands selectively inhibited IgE CSR in a dose-, affinity-, and avidity-dependent manner. Signaling via Syk was required for the inhibition of IgE CSR following BCR stimulation, whereas inhibition of the PI3K subunit p110δ increased IgE CSR independently of BCR ligation. The inhibition of IgE CSR by BCR ligands synergized with IL-21 or TGFβ1. BCR ligation also inhibited CSR to IgE in human tonsillar B cells, and this inhibition was also synergistic with IL-21. These findings establish that IgE CSR is uniquely susceptible to inhibition by BCR signaling in mouse and human B cells, with important implications for the regulation and pathogenesis of allergic disease.

How the Signaling Crosstalk of B Cell Receptor (BCR) and Co-Receptors Regulates Antibody Class Switch Recombination: A New Perspective of Checkpoints of BCR Signaling

Mature B cells express B cell antigen receptor (BCR), toll-like receptors (TLR) and TNF family receptors including CD40 and B-cell activating factor receptor (BAFFR). These receptors transduce cellular signals to govern the physiological and pathological processes in B cells including B cell development and differentiation, survival, proliferation, and antibody-mediated immune responses as well as autoimmune diseases and B cell lymphomagenesis. Effective antibody-mediated immune responses require class switch recombination (CSR), a somatic DNA recombination event occurring at the immunoglobulin heavy chain (Igh) gene locus. Mature B cells initially express IgM as their BCR, and CSR enables the B cells to switch from expressing IgM to expressing different classes of antibodies including IgG, IgA or IgE that exhibit distinct effector functions. Here, we briefly review recent findings about how the signaling crosstalk of the BCR with TLRs, CD40 and BAFFR regulates CSR, antibody-mediate immune responses, and B cell anergy.

TRAF3 Acts as a Checkpoint of B Cell Receptor Signaling to Control Antibody Class Switch Recombination and Anergy

The BCR recognizes foreign Ags to initiate humoral immunity that needs isotype-switched Abs generated via class switch recombination (CSR); however, stimulating the BCR in the absence of costimulation (e.g., CD40) does not induce CSR; thus, it remains elusive whether and how the BCR induces CSR mechanistically. Autoreactive B cells can maintain anergy via unresponsiveness of their BCRs to self-antigens. However, it remains unknown what molecule(s) restrict BCR signaling strength for licensing BCR-induced CSR and whether deficiency of such molecule(s) disrupts autoreactive B cell anergy and causes B cell-mediated diseases by modulating BCR signaling. In this study, we employ mouse models to show that the BCR's capacity to induce CSR is restrained by B cell-intrinsic checkpoints TRAF3 and TRAF2, whose deletion in B cells enables the BCR to induce CSR in the absence of costimulation. TRAF3 deficiency permits BCR-induced CSR by elevating BCR-proximal signaling intensity. Furthermore, NF-κB2 is required for BCR-induced CSR in TRAF3-deficient B cells but not for CD40-induced or LPS-induced CSR, suggesting that TRAF3 restricts NF-κB2 activation to specifically limit the BCR's ability to induce CSR. TRAF3 deficiency also disrupts autoreactive B cell anergy by elevating calcium influx in response to BCR stimulation, leading to lymphoid organ disorders and autoimmune manifestations. We showed that TRAF3 deficiency-associated autoimmune phenotypes can be rectified by limiting BCR repertoires or attenuating BCR signaling strength. Thus, our studies highlight the importance of TRAF3-mediated restraint on BCR signaling strength for controlling CSR, B cell homeostasis, and B cell-mediated disorders.

The Role of Microbial Translocation and Immune Activation in AIDS-Associated Non-Hodgkin Lymphoma Pathogenesis: What Have We Learned?

Human immunodeficiency virus (HIV) infection is associated with greatly increased risk for development of non-Hodgkin lymphoma (NHL). Nearly all acquired immunodeficiency syndrome (AIDS)-associated NHL (AIDS-NHL) is of B-cell origin. Two major mechanisms are believed to contribute to the genesis of AIDS-NHL: (1) loss of immunoregulation of Epstein-Barr virus (EBV)+ B cells, resulting from impaired T-cell function late in the course of HIV disease and (2) chronic B-cell activation, leading to DNA-modifying events that contribute to oncogene mutations/ translocations. HIV infection has long been known to be associated with chronic inflammation and polyclonal B-cell activation, and more recently, microbial translocation. Microbial translocation is bacterial product leakage from gut lumen into the peripheral circulation, resulting in high levels of lipopolysaccharide (LPS) in the peripheral circulation, leading to chronic immune activation and inflammation. We review recent literature linking microbial translocation to lymphom-agenesis. This includes epidemiological studies of biomarkers of microbial translocation with risk of AIDS-NHL and emerging data on the mechanisms by which microbial translocation may lead to AIDS-NHL development.

Oxaliplatin Treatment Alters Systemic Immune Responses

Purpose

Oxaliplatin is a platinum-based chemotherapeutic agent demonstrating significant antitumor efficacy. Unlike conventional anticancer agents which are immunosuppressive, oxaliplatin has the capacity to stimulate immunological effects in response to the presentation of damage associated molecular patterns (DAMPs) elicited upon cell death. However, the effects of oxaliplatin treatment on systemic immune responses remain largely unknown. Aims of this study were to investigate the effects of oxaliplatin treatment on the proportions of (1) splenic T cells, B cells, macrophages, pro-/anti-inflammatory cytokines, gene expression of splenic cytokines, chemokines, and mediators; (2) double-positive and single-positive CD4⁺ and CD8⁺ T thymocytes; (3) bone-marrow hematopoietic stem and progenitor cells.

Methods

Male BALB/c mice received intraperitoneal injections of oxaliplatin (3mg/kg/d) or sterile water tri-weekly for 2 weeks. Leukocyte populations within the spleen, thymus, and bone-marrow were assessed using flow cytometry. RT-PCR was performed to characterise changes in splenic inflammation-associated genes.

Results

Oxaliplatin treatment reduced spleen size and cellularity (CD45⁺ cells), increased the proportion of CD4⁺, CD8⁺, and Treg cells, and elevated TNF-α expression. Oxaliplatin was selectively cytotoxic to B cells but had no effect on splenic macrophages. Oxaliplatin treatment altered the gene expression of several cytokines, chemokines, and cell mediators. Oxaliplatin did not deplete double-positive thymocytes but increased the single-positive CD8⁺ subset. There was also an increase in activated (CD69⁺) CD8⁺ T cells. Bone-marrow hematopoietic progenitor pool was demonstrably normal following oxaliplatin treatment when compared to the vehicle-treated cohort.

Conclusion

Oxaliplatin does not cause systemic immunosuppression and, instead, has the capacity to induce beneficial antitumor immune responses.

Signaling control of antibody isotype switching

Class switch recombination (CSR) generates isotype-switched antibodies with distinct effector functions essential for mediating effective humoral immunity. CSR is catalyzed by activation-induced deaminase (AID) that initiates DNA lesions in the evolutionarily conserved switch (S) regions at the immunoglobulin heavy chain (Igh) locus. AID-initiated DNA lesions are subsequently converted into DNA double stranded breaks (DSBs) in the S regions of Igh locus, repaired by non-homologous end-joining to effect CSR in mammalian B lymphocytes. While molecular mechanisms of CSR are well characterized, it remains less well understood how upstream signaling pathways regulate AID expression and CSR. B lymphocytes express multiple receptors including the B cell antigen receptor (BCR) and co-receptors (e.g., CD40). These receptors may share common signaling pathways or may use distinct signaling elements to regulate CSR. Here, we discuss how signals emanating from different receptors positively or negatively regulate AID expression and CSR.

Immunization Elicits Antigen-Specific Antibody Sequestration in Dorsal Root Ganglia Sensory Neurons

The immune and nervous systems are two major organ systems responsible for host defense and memory. Both systems achieve memory and learning that can be retained, retrieved, and utilized for decades. Here, we report the surprising discovery that peripheral sensory neurons of the dorsal root ganglia (DRGs) of immunized mice contain antigen-specific antibodies. Using a combination of rigorous molecular genetic analyses, transgenic mice, and adoptive transfer experiments, we demonstrate that DRGs do not synthesize these antigen-specific antibodies, but rather sequester primarily IgG1 subtype antibodies. As revealed by RNA-seq and targeted quantitative PCR (qPCR), dorsal root ganglion (DRG) sensory neurons harvested from either naïve or immunized mice lack enzymes (i.e., RAG1, RAG2, AID, or UNG) required for generating antibody diversity and, therefore, cannot make antibodies. Additionally, transgenic mice that express a reporter fluorescent protein under the control of Igγ1 constant region fail to express Ighg1 transcripts in DRG sensory neurons. Furthermore, neural sequestration of antibodies occurs in mice rendered deficient in neuronal Rag2, but antibody sequestration is not observed in DRG sensory neurons isolated from mice that lack mature B cells [e.g., Rag1 knock out (KO) or μMT mice]. Finally, adoptive transfer of Rag1-deficient bone marrow (BM) into wild-type (WT) mice or WT BM into Rag1 KO mice revealed that antibody sequestration was observed in DRG sensory neurons of chimeric mice with WT BM but not with Rag1-deficient BM. Together, these results indicate that DRG sensory neurons sequester and retain antigen-specific antibodies released by antibody-secreting plasma cells. Coupling this work with previous studies implicating DRG sensory neurons in regulating antigen trafficking during immunization raises the interesting possibility that the nervous system collaborates with the immune system to regulate antigen-mediated responses.

Intrinsic properties of germinal center-derived B cells promote their enhanced class switching to IgE

BACKGROUND

Research on the origins and development of human IgE-expressing (IgE(+) ) cells is required for understanding the pathogenesis of allergy and asthma. These studies have been thwarted by the rarity of IgE(+) cells in vivo and the low frequency of class switch recombination (CSR) to IgE ex vivo. To determine the main source of IgE(+) cells, we investigated the relation between the phenotypic composition of tonsil B cells and the CSR to IgE ex vivo.

METHODS

Human tonsil B cells were analyzed by flow cytometry (FACS) and cultured with IL-4 and anti-CD40 to induce CSR to IgE. Naïve, germinal center (GC), early GC (eGC), and memory tonsil B cells were isolated by FACS, and their capacities for IL-4 and anti-CD40 signaling, cell proliferation, and de novo class switching to IgE were analyzed by RT-PCR and FACS.

RESULTS

B cells from different tonsils exhibited varying capacities for CSR to IgE ex vivo. This was correlated with the percentage of eGC B cells in the tonsil at the outset of the culture. Despite relatively poor cell viability, eGC and GC B-cell cultures produced the highest yields of IgE(+) cells compared to naïve and memory B-cell cultures. The main factors accounting for this result were the strength of IL-4R and CD40 signaling and relative rates of cell proliferation.

CONCLUSIONS

This study shows that the maturation state of tonsil B cells determines their capacity to undergo class switching to IgE ex vivo, with the GC-derived B cells yielding the highest percentage of IgE(+) cells.

IgH chain class switch recombination: mechanism and regulation

IgH class switching occurs rapidly after activation of mature naive B cells, resulting in a switch from expression of IgM and IgD to expression of IgG, IgE, or IgA; this switch improves the ability of Abs to remove the pathogen that induces the humoral immune response. Class switching occurs by a deletional recombination between two switch regions, each of which is associated with a H chain constant region gene. Class switch recombination (CSR) is instigated by activation-induced cytidine deaminase, which converts cytosines in switch regions to uracils. The uracils are subsequently removed by two DNA-repair pathways, resulting in mutations, single-strand DNA breaks, and the double-strand breaks required for CSR. We discuss several aspects of CSR, including how CSR is induced, CSR in B cell progenitors, the roles of transcription and chromosomal looping in CSR, and the roles of certain DNA-repair enzymes in CSR.

ATAD5 deficiency decreases B cell division and Igh recombination

Mammalian ATPase family AAA domain-containing protein 5 (ATAD5) and its yeast homolog enhanced level of genomic instability 1 are responsible for unloading proliferating cell nuclear antigen from newly synthesized DNA. Prior work in HeLa and yeast cells showed that a decrease in ATAD5 protein levels resulted in accumulation of chromatin-bound proliferating cell nuclear antigen, slowed cell division, and increased genomic instability. In this study, B cells from heterozygous (Atad5(+/m)) mice were used to examine the effects of decreased cell proliferation on Ab diversity. ATAD5 haploinsufficiency did not change the frequency or spectrum of somatic hypermutation in Ab genes, indicating that DNA repair and error-prone DNA polymerase η usage were unaffected. However, immunized Atad5(+/m) mice had decreased serum IgG1 Abs, demonstrating a functional effect on class switch recombination. The mechanism of this altered immune response was then examined following ex vivo stimulation of splenic B cells, where Atad5(+/m) cells accumulated in the S phase of the cell cycle and had reduced proliferation compared with wild-type cells. These haploinsufficient cells underwent a significant decline in activation-induced deaminase expression, resulting in decreased switch region DNA double-strand breaks and interchromosomal translocations in the Igh locus. Class switch recombination to several isotypes was also reduced in Atad5(+/m) cells, although the types of end-joining pathways were not affected. These results describe a defect in DNA replication that affects Igh recombination via reduced cell division.

BCR-signalling synergizes with TLR-signalling for induction of AID and immunoglobulin class-switching through the non-canonical NF-κB pathway

By diversifying antibody biological effector functions, class switch DNA recombination has a central role in the maturation of the antibody response. Here we show that BCR-signalling synergizes with Toll-like receptor (TLR) signalling to induce class switch DNA recombination. BCR-signalling activates the non-canonical NF-κB pathway and enhances the TLR-dependent canonical NF-κB pathway, thereby inducing activation-induced cytidine deaminase (AID), which is critical for class switch DNA recombination. Escherichia coli lipopolysaccharide (LPS) triggers dual TLR4/BCR-signalling and induces hallmarks of BCR-signalling, including CD79a phosphorylation and Ca(2+) mobilization, and activates both the NF-κB pathways to induce AID and class switch DNA recombination in a PI(3)K p85α-dependent fashion. CD40-signalling activates the two NF-κB pathways to induce AID and class switch DNA recombination independent of BCR-signalling. Finally, dual BCR/TLR-engaging NP-lipopolysaccharide effectively elicits class-switched NP-specific IgG3 and IgG2b in mice. Thus, by integrating signals of the non-canonical and canonical NF-κB pathways, BCR and TLRs synergize to induce AID and T-cell-independent class switch DNA recombination.

Dynamic changes in binding of immunoglobulin heavy chain 3' regulatory region to protein factors during class switching

The 3' regulatory region (3' RR) of the Igh locus works at long distances on variable region (V(H)) and switch region (I) region promoters to initiate germ line (non-coding) transcription (GT) and promote class switch recombination (CSR). The 3' RR contains multiple elements, including enhancers (hs3a, hs1.2, hs3b, and hs4) and a proposed insulator region containing CTCF (CCCTC-binding factor) binding sites, i.e. hs5/6/7 and the downstream region ("38"). Notably, deletion of each individual enhancer (hs3a-hs4) has no significant phenotypic consequence, suggesting that the 3' RR has considerable structural flexibility in its function. To better understand how the 3' RR functions, we identified transcription factor binding sites and used chromatin immunoprecipitation (ChIP) assays to monitor their occupancy in splenic B cells that initiate GT and undergo CSR (LPS±IL4), are deficient in GT and CSR (p50(-/-)), or do not undergo CSR despite efficient GT (anti-IgM+IL4). Like 3' RR enhancers, hs5-7 and the 38 region were observed to contain multiple Pax5 binding sites (in addition to multiple CTCF sites). We found that the Pax5 binding profile to the 3' RR dynamically changed during CSR independent of the specific isotype to which switching was induced, and binding focused on hs1.2, hs4, and hs7. CTCF-associated and CTCF-independent cohesin interactions were also identified. Our observations are consistent with a scaffold model in which a platform of active protein complexes capable of facilitating GT and CSR can be formed by varying constellations of 3' RR elements.

AID and somatic hypermutation

In response to an assault by foreign organisms, peripheral B cells can change their antibody affinity and isotype by somatically mutating their genomic DNA. The ability of a cell to modify its DNA is exceptional in light of the potential consequences of genetic alterations to cause human disease and cancer. Thus, as expected, this mechanism of antibody diversity is tightly regulated and coordinated through one protein, activation-induced deaminase (AID). AID produces diversity by converting cytosine to uracil within the immunoglobulin loci. The deoxyuracil residue is mutagenic when paired with deoxyguanosine, since it mimics thymidine during DNA replication. Additionally, B cells can manipulate the DNA repair pathways so that deoxyuracils are not faithfully repaired. Therefore, an intricate balance exists which is regulated at multiple stages to promote mutation of immunoglobulin genes, while retaining integrity of the rest of the genome. Here we discuss and summarize the current understanding of how AID functions to cause somatic hypermutation.

High-affinity B cell receptor ligation by cognate antigen induces cytokine-independent isotype switching

The selection of an appropriate Ig isotype is critical for an effective immune response against pathogens. Isotype regulation is sensitive to external signals, particularly cytokines secreted by Th cells. For example, IL-4 induces isotype switching to IgG1 via a STAT6-dependent signaling pathway. In this study, we show that BCR ligation also induces IgG1 switching in mouse B cells. The extent of switch induction by Ag is affinity-dependent, and high-affinity Ag binding leads to IgG1 switching levels comparable to those induced by saturating IL-4. However, the Ag-induced IgG1 switch does not require additional cytokine signals and occurs in a STAT6-independent manner. Thus, BCR ligation represents a novel pathway for direct isotype switching leading to IgG1 secretion.

AID constrains germinal center size by rendering B cells susceptible to apoptosis

The germinal center (GC) is a transient lymphoid tissue microenvironment that fosters T cell–dependent humoral immunity. Within the GC, the B cell–specific enzyme, activation-induced cytidine deaminase (AID), mutates the immunoglobulin locus, thereby altering binding affinity for antigen. In the absence of AID, larger GC structures are observed in both humans and mice, but the reason for this phenomenon is unclear. Because significant apoptosis occurs within the GC niche to cull cells that have acquired nonproductive mutations, we have examined whether a defect in apoptosis could account for the larger GC structures in the absence of AID. In this report, we reveal significantly reduced death of B cells in AID−/− mice as well as in B cells derived from AID−/− bone marrow in mixed bone marrow chimeric mice. Furthermore, AID-expressing B cells show decreased proliferation and survival compared with AID−/− B cells, indicating an AID-mediated effect on cellular viability. The GC is an etiologic site for B-cell autoimmunity and lymphomagenesis, both of which have been linked to aberrant AID activity. We report a link between AID-induced DNA damage and B-cell apoptosis that has implications for the development of B-cell disorders.