Transcription factor IRF4 regulates germinal center cell formation through a B cell-intrinsic mechanism

Arid1a-dependent canonical BAF complex suppresses inflammatory programs to drive efficient germinal center B cell responses

The mammalian Brg1/Brm-associated factor (BAF) complexes are major regulators of nucleosomal remodeling that are commonly mutated in several cancers, including germinal center (GC)-derived B cell lymphomas. However, the specific roles of different BAF complexes in GC B cell biology are not well understood. Here we show that the AT-rich interaction domain 1a (Arid1a) containing canonical BAF (cBAF) complex is required for maintenance of GCs and high-affinity antibody responses. While Arid1a-deficient B cells undergo initial activation, they fail to sustain the GC program. Arid1a establishes permissive chromatin landscapes for B cell activation and is concomitantly required to suppress inflammatory gene programs. The inflammatory signatures instigated by Arid1a deficiency promoted the recruitment of neutrophils and inflammatory monocytes. Dampening of inflammatory cues through interleukin-1β blockade or glucocorticoid receptor agonist partially rescued Arid1a-deficient GCs, highlighting a critical role for inflammation in impeding GCs. Our work reveals essential functions of Arid1a-dependent cBAF in promoting efficient GC responses.

A multiscale spatial modeling framework for the germinal center response

The germinal center response or reaction (GCR) is a hallmark event of adaptive humoral immunity. Unfolding in the B cell follicles of the secondary lymphoid organs, a GC culminates in the production of high-affinity antibody-secreting plasma cells along with memory B cells. By interacting with follicular dendritic cells (FDC) and T follicular helper (Tfh) cells, GC B cells exhibit complex spatiotemporal dynamics. Driving the B cell dynamics are the intracellular signal transduction and gene regulatory network that responds to cell surface signaling molecules, cytokines, and chemokines. As our knowledge of the GC continues to expand in depth and in scope, mathematical modeling has become an important tool to help disentangle the intricacy of the GCR and inform novel mechanistic and clinical insights. While the GC has been modeled at different granularities, a multiscale spatial simulation framework - integrating molecular, cellular, and tissue-level responses - is still rare. Here, we report our recent progress toward this end with a hybrid stochastic GC framework developed on the Cellular Potts Model-based CompuCell3D platform. Tellurium is used to simulate the B cell intracellular molecular network comprising NF-κB, FOXO1, MYC, AP4, CXCR4, and BLIMP1 that responds to B cell receptor (BCR) and CD40-mediated signaling. The molecular outputs of the network drive the spatiotemporal behaviors of B cells, including cyclic migration between the dark zone (DZ) and light zone (LZ) via chemotaxis; clonal proliferative bursts, somatic hypermutation, and DNA damage-induced apoptosis in the DZ; and positive selection, apoptosis via a death timer, and emergence of plasma cells in the LZ. Our simulations are able to recapitulate key molecular, cellular, and morphological GC events, including B cell population growth, affinity maturation, and clonal dominance. This novel modeling framework provides an open-source, customizable, and multiscale virtual GC simulation platform that enables qualitative and quantitative in silico investigations of a range of mechanistic and applied research questions on the adaptive humoral immune response in the future.

Regulation of BCR-mediated Ca2+ mobilization by MIZ1-TMBIM4 safeguards IgG1+ GC B cell-positive selection

The transition from immunoglobulin M (IgM) to affinity-matured IgG antibodies is vital for effective humoral immunity. This is facilitated by germinal centers (GCs) through affinity maturation and preferential maintenance of IgG+ B cells over IgM+ B cells. However, it is not known whether the positive selection of the different Ig isotypes within GCs is dependent on specific transcriptional mechanisms. Here, we explored IgG1+ GC B cell transcription factor dependency using a CRISPR-Cas9 screen and conditional mouse genetics. We found that MIZ1 was specifically required for IgG1+ GC B cell survival during positive selection, whereas IgM+ GC B cells were largely independent. Mechanistically, MIZ1 induced TMBIM4, an ancestral anti-apoptotic protein that regulated inositol trisphosphate receptor (IP3R)-mediated calcium (Ca2+) mobilization downstream of B cell receptor (BCR) signaling in IgG1+ B cells. The MIZ1-TMBIM4 axis prevented mitochondrial dysfunction-induced IgG1+ GC cell death caused by excessive Ca2+ accumulation. This study uncovers a unique Ig isotype-specific dependency on a hitherto unidentified mechanism in GC-positive selection.

Defective mitochondria remodelling in B cells leads to an aged immune response

The B cell response in the germinal centre (GC) reaction requires a unique bioenergetic supply. Although mitochondria are remodelled upon antigen-mediated B cell receptor stimulation, mitochondrial function in B cells is still poorly understood. To gain a better understanding of the role of mitochondria in B cell function, here we generate mice with B cell-specific deficiency in Tfam, a transcription factor necessary for mitochondrial biogenesis. Tfam conditional knock-out (KO) mice display a blockage of the GC reaction and a bias of B cell differentiation towards memory B cells and aged-related B cells, hallmarks of an aged immune response. Unexpectedly, blocked GC reaction in Tfam KO mice is not caused by defects in the bioenergetic supply but is associated with a defect in the remodelling of the lysosomal compartment in B cells. Our results may thus describe a mitochondrial function for lysosome regulation and the downstream antigen presentation in B cells during the GC reaction, the dysruption of which is manifested as an aged immune response.

Multiple myeloma in dogs: Use of the cell block technique as a new diagnostic tool

BACKGROUND

The diagnosis of multiple myeloma (MM) in dogs may be challenging and complex. The cell blocks are a diagnostic technique that allows the characterization of neoplastic cells and, therefore, might help in the diagnosis of atypical MM.

OBJECTIVE

The objective of the present work is to describe three clinical cases in which the cell blocks and immunohistochemistry contributed to the definitive diagnosis of canine MM.

METHODS

Three dogs, one female and two males, with different clinical signs, were presented for consultation with anemia, hyperproteinemia with monoclonal gammopathy, and the presence of plasmacytosis in the bone marrow. Cytologic analysis of the spleen was performed in two dogs and was suggestive of the presence of lymphocytes or plasma cells of a neoplastic nature in one of the cases and plasma cell hyperplasia associated with extramedullary hematopoiesis in the other. Given the hypotheses of lymphoid neoplasms with a plasma cell phenotype, cell blocks from aspiration punctures were performed for immunohistochemical analysis with anti-CD3, CD20, CD79αcy, PAX5, and MUM1 antibodies.

RESULTS

The results revealed positive staining for MUM1 in 80% of the cells in the spleen cell block and for CD20 and MUM1 in 70% of the cells in the bone marrow cell blocks, with negative staining for the other antibodies. The immunophenotyping results allowed the diagnosis of MM in the three cases and excluded other lymphoid neoplasms.

CONCLUSIONS

This work reinforces the importance of using cell blocks in the diagnosis of neoplasms by demonstrating their potential to aid the diagnosis of MM.

Clinical Relevance of Interferon Regulatory Family-4 (IRF4) Expression in Newly Diagnosed Patients with Multiple Myeloma

Multiple myeloma (MM) is a malignant plasma cell neoplasm with complex biology and heterogenous course. Interferon regulatory factor 4 (IRF4) transcription factor, important key developmental stages of hematopoiesis, represents an excellent potential therapeutic target. The present work aimed to investigate the expression status of IRF4 in the diagnostic bone marrow biopsy (BMB) cores of MM patients. This prospective study included 62 newly diagnosed MM patients. The expression of IRF4 was assessed in the BMB by immunohistochemistry (IHC). The data were correlated to the patients' clinico-pathological features, response to treatment and survival rates. IRF4 expression was observed in 50% of MM patients (31/62). IRF-4 positive patients were more frequently male patients (P = 0.018), have immunoglobulin heavy chain (IgH) translocations (P = 0.05) and tended to present with a higher platelets count (P = 0.07). Multiple myeloma patients presenting with urine M-protein had worse overall survival (OS) than negative cases (P = 0.012). Normocellular BM aspirate (BMA) was associated with better OS than hypercellular and hypocellular BMA (P = 0.006). Patchy distribution of plasma cells in BMB was associated with better disease-free survival (DFS) while diffuse infiltration had the worst (P = 0.019). Of note, after treatment, MM patients had significantly lower percentage of BMA plasma cells, platelet count, β2 microglobulin and creatinine levels (P = 0.037, < 0.001, 0.022 and 0.026, respectively). Had higher albumin level (P = 0.007), compared to initial investigations. No significant association was found between IRF4 expression and the patients'clinical outcomes. Patterns of plasma cells distribution in BMB, BMA cellularity and urine M-protein are prognostically relevant in MM.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12288-023-01628-3.

The miR-17∼92 miRNAs promote plasma cell differentiation by suppressing SOCS3-mediated NIK degradation

The miR-17∼92 family microRNAs (miRNAs) play a key role in germinal center (GC) reaction through promoting T follicular helper (TFH) cell differentiation. It remains unclear whether they also have intrinsic functions in B cell differentiation and function. Here we show that mice with B cell-specific deletion of the miR-17∼92 family exhibit impaired GC reaction, plasma cell differentiation, and antibody production in response to protein antigen immunization and chronic viral infection. Employing CRISPR-mediated functional screening, we identify Socs3 as a key functional target of miR-17∼92 in regulating plasma cell differentiation. Mechanistically, SOCS3, whose expression is elevated in miR-17∼92 family-deficient B cells, interacts with NIK and promotes its ubiquitination and degradation, thereby impairing NF-κB signaling and plasma cell differentiation. This moderate increase in SOCS3 expression has little effect on IL-21-STAT3 signaling. Our study demonstrates differential sensitivity of two key signaling pathways to alterations in the protein level of an miRNA target gene.

T-bet-expressing B cells contribute to the autoreactive plasma cell pool in Lyn-/- mice

Systemic Lupus Erythematosus (SLE) is characterized by pathogenic autoantibodies against nucleic acid-containing antigens. Understanding which B-cell subsets give rise to these autoantibodies may reveal therapeutic approaches for SLE that spare protective responses. Mice lacking the tyrosine kinase Lyn, which limits B and myeloid cell activation, develop lupus-like autoimmune diseases characterized by increased autoreactive plasma cells (PCs). We used a fate-mapping strategy to determine the contribution of T-bet+ B cells, a subset thought to be pathogenic in lupus, to the accumulation of PCs and autoantibodies in Lyn-/- mice. Approximately, 50% of splenic PCs in Lyn-/- mice originated from T-bet+ cells, a significant increase compared to WT mice. In vitro, splenic PCs derived from T-bet+ B cells secreted both IgM and IgG anti-dsDNA antibodies. To determine the role of these cells in autoantibody production in vivo, we prevented T-bet+ B cells from differentiating into PCs or class switching in Lyn-/- mice. This resulted in a partial reduction in splenic PCs and anti-dsDNA IgM and complete abrogation of anti-dsDNA IgG. Thus, T-bet+ B cells make an important contribution to the autoreactive PC pool in Lyn-/- mice.

Leishmaniasis: Immune Cells Crosstalk in Macrophage Polarization

Leishmaniasis is a complex infectious parasitic disease caused by protozoa of the genus Leishmania, belonging to a group of neglected tropical diseases. It establishes significant global health challenges, particularly in socio-economically disadvantaged regions. Macrophages, as innate immune cells, play a crucial role in initiating the inflammatory response against the pathogens responsible for this disease. Macrophage polarization, the process of differentiating macrophages into pro-inflammatory (M1) or anti-inflammatory (M2) phenotypes, is essential for the immune response in leishmaniasis. The M1 phenotype is associated with resistance to Leishmania infection, while the M2 phenotype is predominant in susceptible environments. Notably, various immune cells, including T cells, play a significant role in modulating macrophage polarization by releasing cytokines that influence macrophage maturation and function. Furthermore, other immune cells can also impact macrophage polarization in a T-cell-independent manner. Therefore, this review comprehensively examines macrophage polarization's role in leishmaniasis and other immune cells' potential involvement in this intricate process.

Profiling Germinal Center-like B Cell Responses to Conjugate Vaccines Using Synthetic Immune Organoids

Glycoengineered bacteria have emerged as a cost-effective platform for rapid and controllable biosynthesis of designer conjugate vaccines. However, little is known about the engagement of such conjugates with naïve B cells to induce the formation of germinal centers (GC), a subanatomical microenvironment that converts naïve B cells into antibody-secreting plasma cells. Using a three-dimensional biomaterials-based B-cell follicular organoid system, we demonstrate that conjugates triggered robust expression of hallmark GC markers, B cell receptor clustering, intracellular signaling, and somatic hypermutation. These responses depended on the relative immunogenicity of the conjugate and correlated with the humoral response in vivo. The occurrence of these mechanisms was exploited for the discovery of high-affinity antibodies against components of the conjugate on a time scale that was significantly shorter than for typical animal immunization-based workflows. Collectively, these findings highlight the potential of synthetic organoids for rapidly predicting conjugate vaccine efficacy as well as expediting antigen-specific antibody discovery.

A multimorphic mutation in IRF4 causes human autosomal dominant combined immunodeficiency

Interferon regulatory factor 4 (IRF4) is a transcription factor (TF) and key regulator of immune cell development and function. We report a recurrent heterozygous mutation in IRF4, p.T95R, causing an autosomal dominant combined immunodeficiency (CID) in seven patients from six unrelated families. The patients exhibited profound susceptibility to opportunistic infections, notably Pneumocystis jirovecii, and presented with agammaglobulinemia. Patients' B cells showed impaired maturation, decreased immunoglobulin isotype switching, and defective plasma cell differentiation, whereas their T cells contained reduced TH17 and TFH populations and exhibited decreased cytokine production. A knock-in mouse model of heterozygous T95R showed a severe defect in antibody production both at the steady state and after immunization with different types of antigens, consistent with the CID observed in these patients. The IRF4T95R variant maps to the TF's DNA binding domain, alters its canonical DNA binding specificities, and results in a simultaneous multimorphic combination of loss, gain, and new functions for IRF4. IRF4T95R behaved as a gain-of-function hypermorph by binding to DNA with higher affinity than IRF4WT. Despite this increased affinity for DNA, the transcriptional activity on IRF4 canonical genes was reduced, showcasing a hypomorphic activity of IRF4T95R. Simultaneously, IRF4T95R functions as a neomorph by binding to noncanonical DNA sites to alter the gene expression profile, including the transcription of genes exclusively induced by IRF4T95R but not by IRF4WT. This previously undescribed multimorphic IRF4 pathophysiology disrupts normal lymphocyte biology, causing human disease.

Regulatory network of BLIMP1, IRF4, and XBP1 triad in plasmacytic differentiation and multiple myeloma pathogenesis

Antibody secreting plasma cell plays an indispensable role in humoral immunity. As activated B cell undergoes germinal center reaction and develops into plasma cell, it gradually loses B cell characteristics and embraces functional changes associated with immunoglobulins production. Differentiation of B cell into plasma cell involves drastic changes in cell structure, granularity, metabolism, gene expression and epigenetic regulation that couple with the mounting capacity for synthesis of a large quantity of antigen-specific antibodies. The interplay between three hallmark transcriptional regulators IRF4, BLIMP1, and XBP1, is critical for supporting the cellular reprograming activities during B to plasma cell transition. IRF4 promotes plasma cell generation by directing immunoglobulin class switching, proliferation and survival; BLIMP1 serves as a transcriptional repressor that extinguishes B cell features; whereas XBP1 controls unfolded protein response that relieves endoplasmic reticulum stress and permits antibody release during terminal differentiation. Intriguingly, high expression of IRF4, BLIMP1, and XBP1 molecules have been reported in myeloma cells derived from multiple myeloma patients, which negatively impact treatment outcome, prognosis, and relapse frequency. Despite the introduction of immunomodulatory drugs in recent years, multiple myeloma is still an incurable disease with poor survival rate. An in-depth review of IRF4, BLIMP1, and XBP1 triad molecules in plasma cell generation and multiple myeloma tumorigenesis may provide clues to the possibility of targeting these molecules in disease management.

B cell-intrinsic changes with age do not impact antibody-secreting cell formation but delay B cell participation in the germinal centre reaction

Vaccines typically protect against (re)infections by generating pathogen-neutralising antibodies. However, as we age, antibody-secreting cell formation and vaccine-induced antibody titres are reduced. Antibody-secreting plasma cells differentiate from B cells either early post-vaccination through the extrafollicular response or from the germinal centre (GC) reaction, which generates long-lived antibody-secreting cells. As the formation of both the extrafollicular antibody response and the GC requires the interaction of multiple cell types, the impaired antibody response in ageing could be caused by B cell intrinsic or extrinsic factors, or a combination of the two. Here, we show that B cells from older people do not have intrinsic defects in their proliferation and differentiation into antibody-secreting cells in vitro compared to those from the younger donors. However, adoptive transfer of B cells from aged mice to young recipient mice showed that differentiation into extrafollicular plasma cells was favoured at the expense of B cells entering the GC during the early stages of GC formation. In contrast, by the peak of the GC response, GC B cells derived from the donor cells of aged mice had expanded to the same extent as those from the younger donors. This indicates that age-related intrinsic B cell changes delay the GC response but are not responsible for the impaired antibody-secreting response or smaller peak GC response in ageing. Collectively, this study shows that B cells from aged individuals are not intrinsically defective in responding to stimulation and becoming antibody-secreting cells, implicating B cell-extrinsic factors as the primary cause of age-associated impairment in the humoral immunity.

Lineage- and Stage-specific Oncogenicity of IRF4

Dysregulation of transcription factor genes represents a unique molecular etiology of hematological malignancies. A number of transcription factors that play a role in hematopoietic cell development, lymphocyte activation or their maintenance have been identified as oncogenes or tumor suppressors. Many of them exert oncogenic abilities in a context-dependent manner by governing the key transcriptional program unique to each cell type. IRF4, a member of the interferon regulatory factor (IRF) family, acts as an essential regulator of the immune system and is a prime example of a stage-specific oncogene. The expression and oncogenicity of IRF4 are restricted to mature lymphoid neoplasms, while IRF4 potentially serves as a tumor suppressor in other cellular contexts. This is in marked contrast to its immediate downstream target, MYC, which can cause cancers in a variety of tissues. In this review article, we provide an overview of the roles of IRF4 in the development of the normal immune system and lymphoid neoplasms and discuss the potential mechanisms of lineage- and stage-specific oncogenicity of IRF4.

Atypical multiple myeloma in 3 young dogs

Three dogs under 12 months old were diagnosed with atypical multiple myeloma (MM), having an aggressive multifocal anaplastic round cell sarcoma in bone marrow, viscera, and/or peripheral blood, which were confirmed by cytology and immunohistochemistry to be of plasma cell origin. The intramedullary sarcomas caused myelophthisis, osteolysis, and hypercalcemia. Complete or free light chain monoclonal gammopathy in the serum and/or urine was demonstrated by protein electrophoresis and immunofixation. The polymerase chain reaction for antigen receptor rearrangement assay performed on 2 cases identified a clonally rearranged immunoglobulin gene. Neoplastic cells lacked expression of CD45, CD3, CD18, CD21, CD34, and MHCII by flow cytometry. Immunohistochemistry revealed MUM1 immunoreactivity of the neoplastic cells. Combining all data, the diagnosis was MM. An aggressive form of MM in young dogs should be a differential diagnosis for patients with an immunoglobulin-productive, B cell-clonal, CD45-negative, MUM1-positive discrete cell neoplasm arising from the bone marrow.

B cell-intrinsic TBK1 is essential for germinal center formation during infection and vaccination in mice

The germinal center (GC) is a site where somatic hypermutation and clonal selection are coupled for antibody affinity maturation against infections. However, how GCs are formed and regulated is incompletely understood. Here, we identified an unexpected role of Tank-binding kinase-1 (TBK1) as a crucial B cell–intrinsic factor for GC formation. Using immunization and malaria infection models, we show that TBK1-deficient B cells failed to form GC despite normal Tfh cell differentiation, although some malaria-infected B cell–specific TBK1-deficient mice could survive by GC-independent mechanisms. Mechanistically, TBK1 phosphorylation elevates in B cells during GC differentiation and regulates the balance of IRF4/BCL6 expression by limiting CD40 and BCR activation through noncanonical NF-κB and AKT^T308 signaling. In the absence of TBK1, CD40 and BCR signaling synergistically enhanced IRF4 expression in Pre-GC, leading to BCL6 suppression, and therefore failed to form GCs. As a result, memory B cells generated from TBK1-deficient B cells fail to confer sterile immunity upon reinfection, suggesting that TBK1 determines B cell fate to promote long-lasting humoral immunity.

Bhlhe40: Gatekeeper of the GC

The generation of high-affinity antibodies in the germinal center (GC) requires interplay between GC B cells and T follicular helper cells. Rauschmeier et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20211406) report that Bhlhe40 restrains GC output through distinct regulatory roles in both arms of the response.

Integrin CD11b provides a new marker of pre-germinal center IgA + B cells in murine Peyer's patches

Activated B cells can enter germinal centers (GCs) for affinity maturation to produce high-affinity antibodies. However, which activated B cells will enter GCs remains unknown. Here, we found a small population of CD11b⁺IgA⁺ B cells located outside of GCs in murine Peyer’s patches (PPs). After injection of the CD11b⁺IgA⁺ PP B cells into a PP of a recipient mouse, they entered GCs forty hours later. They expressed GC surface markers and pre-GC B cell genes, suggesting that CD11b provides a novel surface marker of pre-GC IgA⁺ B cells in murine PPs. Furthermore, independently of dendritic cell activation, CD11b expression on B cells can be induced by bacterial antigens, such as pam3CSK4 and heat-killed Escherichia coli in vitro. In addition, mice orally administered with pam3CSK4 or heat-killed E. coli increased the number of PP GC B cells within two days, and enhanced the mucosal antigen-specific IgA response. Our results demonstrate that the induction of CD11b on B cells is a promising marker for selecting an effective mucosal vaccine adjuvant.

Bhlhe40 function in activated B and TFH cells restrains the GC reaction and prevents lymphomagenesis

The generation of high-affinity antibodies against pathogens and vaccines requires the germinal center (GC) reaction, which relies on a complex interplay between specialized effector B and CD4 T lymphocytes, the GC B cells and T follicular helper (T_FH) cells. Intriguingly, several positive key regulators of the GC reaction are common for both cell types. Here, we report that the transcription factor Bhlhe40 is a crucial cell-intrinsic negative regulator affecting both the B and T cell sides of the GC reaction. In activated CD4 T cells, Bhlhe40 was required to restrain proliferation, thus limiting the number of T_FH cells. In B cells, Bhlhe40 executed its function in the first days after immunization by selectively restricting the generation of the earliest GC B cells but not of early memory B cells or plasmablasts. Bhlhe40-deficient mice with progressing age succumbed to a B cell lymphoma characterized by the accumulation of monoclonal GC B-like cells and polyclonal T_FH cells in various tissues.

IRF4 Has a Unique Role in Early B Cell Development and Acts Prior to CD21 Expression to Control Marginal Zone B Cell Numbers

Strict control of B lymphocyte development is required for the ability to mount humoral immune responses to diverse foreign antigens while remaining self-tolerant. In the bone marrow, B lineage cells transit through several developmental stages in which they assemble a functional B cell receptor in a stepwise manner. The immunoglobulin heavy chain gene is rearranged at the pro-B stage. At the large pre-B stage, cells with a functional heavy chain expand in response to signals from IL-7 and the pre-BCR. Cells then cease proliferation at the small pre-B stage and rearrange the immunoglobulin light chain gene. The fully formed BCR is subsequently expressed on the surface of immature B cells and autoreactive cells are culled by central tolerance mechanisms. Once in the periphery, transitional B cells develop into mature B cell subsets such as marginal zone and follicular B cells. These developmental processes are controlled by transcription factor networks, central to which are IRF4 and IRF8. These were thought to act redundantly during B cell development in the bone marrow, with their functions diverging in the periphery where IRF4 limits the number of marginal zone B cells and is required for germinal center responses and plasma cell differentiation. Because of IRF4's unique role in mature B cells, we hypothesized that it may also have functions earlier in B cell development that cannot be compensated for by IRF8. Indeed, we find that IRF4 has a unique role in upregulating the pre-B cell marker CD25, limiting IL-7 responsiveness, and promoting migration to CXCR4 such that IRF4-deficient mice have a partial block at the pre-B cell stage. We also find that IRF4 acts in early transitional B cells to restrict marginal zone B cell development, as deletion of IRF4 in mature B cells with CD21-cre impairs plasma cell differentiation but has no effect on marginal zone B cell numbers. These studies highlight IRF4 as the dominant IRF family member in early B lymphopoiesis.

OCT2 pre-positioning facilitates cell fate transition and chromatin architecture changes in humoral immunity

During the germinal center (GC) reaction, B cells undergo profound transcriptional, epigenetic and genomic architectural changes. How such changes are established remains unknown. Mapping chromatin accessibility during the humoral immune response, we show that OCT2 was the dominant transcription factor linked to differential accessibility of GC regulatory elements. Silent chromatin regions destined to become GC-specific super-enhancers (SEs) contained pre-positioned OCT2-binding sites in naive B cells (NBs). These preloaded SE 'seeds' featured spatial clustering of regulatory elements enriched in OCT2 DNA-binding motifs that became heavily loaded with OCT2 and its GC-specific coactivator OCAB in GC B cells (GCBs). SEs with high abundance of pre-positioned OCT2 binding preferentially formed long-range chromatin contacts in GCs, to support expression of GC-specifying factors. Gain in accessibility and architectural interactivity of these regions were dependent on recruitment of OCAB. Pre-positioning key regulators at SEs may represent a broadly used strategy for facilitating rapid cell fate transitions.

Continuous Culture of Mouse Primary B Lymphocytes by Forced Expression of Bach2

Stable, long-term culture of primary B lymphocytes has many potential scientific and medical applications, but remains an elusive feat. A major obstacle to long-term culture is that in vitro mitogens quickly drive B cells to differentiate into short-lived plasma cells (PCs). PC differentiation is governed by opposing teams of transcription factors: Pax5, Bach2, and Bcl6 suppress PC commitment, whereas IFN regulatory factor 4 and Blimp1 promote it. To determine whether transcriptional programming could prolong B cell culture by blocking PC commitment, we generated mouse primary B cells harboring gain- or loss-of-function in the key transcription factors, continuously stimulated these cells with CD154 and IL-21, and determined growth potential and phenotypes in vitro. We found that transgenic expression of Bach2 prohibits PC commitment and endows B cells with extraordinary growth potential in response to external proliferation and survival cues. Long-term Bach2-transgenic B cell lines have genetically stable BCRs [i.e., do not acquire V(D)J mutations], express high levels of MHC class II and molecules for costimulation of T cells, and transduce intracellular signals when incubated with BCR ligands. Silencing the Bach2 transgene in an established transgenic cell line causes the cells to secrete large quantities of Ig. This system has potential applications in mAb production, BCR signaling studies, Ag presentation to T cells, and ex vivo clonal expansion for adoptive cell transfer. Additionally, our results provide insight into molecular control over activated B cell fate and suggest that forced Bach2 expression in vivo may augment germinal center B cell or memory B cell differentiation at the expense of PC commitment.

IRF4 ablation in B cells abrogates allogeneic B cell responses and prevents chronic transplant rejection

BACKGOUND

B cells contribute to chronic transplant rejection by producing donor-specific antibodies and promoting T cell response, but how these processes are regulated at the transcriptional level remains unclear. Herein, we investigate the role of transcription factor interferon regulatory factor 4 (IRF4) in controlling B cell response during chronic transplant rejection.

METHODS

We generated the Irf4^gfp reporter mice to determine IRF4 expression in B cell lineage. We then used mice with B cell-specific IRF4 deletion to define the role of IRF4 in B cell response after NP-KLH immunization or allogeneic heart transplantation. In particular, graft survival and histology, as well as B and T cell responses, were evaluated after transplantation.

RESULTS

IRF4 is dynamically expressed at different stages of B cell development and is absent in germinal center (GC) B cells. However, IRF4 ablation in the B cell lineage primarily eliminates GC B cells in both naïve and NP-KLH immunized mice. In the transplantation setting, IRF4 functions intrinsically in B cells and governs allogeneic B cell responses at multiple levels, including GC B cell generation, plasma cell differentiation, donor-specific antibody production, and support of T cell response. B cell-specific IRF4 deletion combined with transient CTLA4-Ig treatment abrogates acute and chronic cardiac allograft rejection in naïve recipient mice but not in donor skin-sensitized recipients.

CONCLUSIONS

B cells require IRF4 to mediate chronic transplant rejection. IRF4 ablation in B cells abrogates allogeneic B cell responses and may also inhibit the ability of B cells to prime allogenic T cells. Targeting IRF4 in B cells represents a potential therapeutic strategy for eliminating chronic transplant rejection.

The Immunity-malignancy equilibrium in multiple myeloma: lessons from oncogenic events in plasma cells

Multiple myeloma (MM) is a malignancy of plasma cells (PC) that grow within the bone marrow and maintain massive immunoglobulin (Ig) production. Disease evolution is driven by genetic lesions, whose effects on cell biology and fitness underlie addictions and vulnerabilities of myeloma cells. Several genes mutated in myeloma are strictly involved in dictating PC identity and antibody factory function. Here, we evaluate the impact of mutations in IRF4, PRDM1, and XBP1, essential transcription factors driving the B to PC differentiation, on MM cell biology and homeostasis. These factors are highly specialized, with limited overlap in their downstream transcriptional programs. Indeed, IRF4 sustains metabolism, survival, and proliferation, while PRDM1 and XBP1 are mainly responsible for endoplasmic reticulum expansion and sustained Ig secretion. Interestingly, IRF4 undergoes activating mutations and translocations, while PRDM1 and XBP1 are hit by loss-of-function events, raising the hypothesis that containment of the secretory program, but not its complete extinction, may be beneficial to malignant PCs. Finally, recent studies unveiled that also the PRDM1 target, FAM46C/TENT5C, an onco-suppressor uniquely and frequently mutated or deleted in myeloma, is directly and potently involved in orchestrating ER homeostasis and secretory activity. Inactivating mutations found in this gene and its interactors strengthen the notion that reduced secretory capacity confers advantage to myeloma cells. We believe that dissection of the evolutionary pressure on genes driving PC-specific functions in myeloma will disclose the cellular strategies by which myeloma cells maintain an equilibrium between antibody production and survival, thus unveiling novel therapeutic targets.

The gene regulatory network controlling plasma cell function

Antibodies are an essential element of the immune response to infection, and in long-term protection upon re-exposure to the same micro-organism. Antibodies are produced by plasmablasts and plasma cells, the terminally differentiated cells of the B lymphocyte lineage. These relatively rare populations, collectively termed antibody secreting cells (ASCs), have developed highly specialized transcriptional and metabolic pathways to facilitate their extraordinarily high rates of antibody synthesis and secretion. In this review, we discuss the gene regulatory network that controls ASC identity and function, with a particular focus on the processes that influence the transcription, translation, folding, modification and secretion of antibodies. We will address how ASCs have adapted their transcriptional, metabolic and protein homeostasis pathways to sustain such high rates of antibody production, and the roles that the major ASC regulators, the transcription factors, Irf4, Blimp-1 and Xbp1, play in co-ordinating these processes.

Biology of Germinal Center B Cells Relating to Lymphomagenesis

The germinal center (GC) reaction is a key feature of adaptive humoral immunity. GCs represent the site where mature B cells refine their B-cell receptor (BCR) and are selected based on the newly acquired affinity for the antigen. In the GC, B cells undergo multiple cycles of proliferation, BCR remodeling by immunoglobulin somatic hypermutation (SHM), and affinity-based selection before emerging as effector memory B cells or antibody-secreting plasma cells. At least 2 histologically and functionally distinct compartments are identified in the GC: the dark zone (DZ) and the light zone (LZ). The proliferative burst and immunoglobulin remodeling by SHM occur prevalently in the DZ compartment. In the LZ, GC B cells undergo an affinity-based selection process that requires the interaction with the antigen and accessory cells. GC B cells are also targeted by class switch recombination, an additional mechanism of immunoglobulin remodeling that ensures the expression of diverse isotype classes. These processes are regulated by a complex network of transcription factors, epigenetic modifiers, and signaling pathways that act in concert with mechanisms of intra-GC B-cell trafficking. The same mechanisms underlying the unique ability of GC B cells to generate high affinity antibodies and ensure immunological memory are hijacked during lymphomagenesis and become powerful weapons for malignant transformation. This review will summarize the main processes and transcriptional networks that drive GC B-cell development and are relevant for human B-cell lymphomagenesis.

Transcriptional regulation of memory B cell differentiation

Memory B cells (MBCs) are critical for the rapid development of protective immunity following re-infection. MBCs capable of neutralizing distinct subclasses of pathogens, such as influenza and HIV, have been identified in humans. However, efforts to develop vaccines that induce broadly protective MBCs to rapidly mutating pathogens have not yet been successful. Better understanding of the signals regulating MBC development and function are essential to overcome current challenges hindering successful vaccine development. Here, we discuss recent advancements regarding the signals and transcription factors regulating germinal centre-derived MBC development and function.

B Cell Response to Vaccination

As one of the most important weapons against infectious diseases, vaccines have saved countless lives since their first use in the late eighteenth century. Antibodies produced by effector B cells upon vaccination play a critical role in mediating protection. The past several decades of research have led to a revolution in our understanding of B cell response to vaccination. Vaccines against SARS-CoV-2 coronavirus were developed at an unprecedented speed to power our global fight against COVID-19 pandemic. Nevertheless, we still face many challenges in the development of vaccines against many other deadly viruses, such as human immunodeficiency virus (HIV) and influenza virus. In this review, we summarize the latest findings on B cell response to vaccination and pathogen infection. We also discuss the current challenges in the field and the potential strategies targeting B cell response to improve vaccine efficacy.Key abbreviations box: BCR: B cell receptor; bNAb: broadly neutralizing antibody; DC: dendritic cells; DZ: dark zone; EF response: extrafollicular response; FDC: follicular dendritic cell; GC: germinal center; HIV: human immunodeficiency virus; IC: immune complex; LLPC: long-lived plasma cell; LZ: light zone; MBC: memory B cell; SLPB: short-lived plasmablast; TFH: T follicular helper cells; TLR: Toll-like receptor.

A Multi-Omics Analysis of Metastatic Melanoma Identifies a Germinal Center-Like Tumor Microenvironment in HLA-DR-Positive Tumor Areas

The emergence of immune checkpoint inhibitors has dramatically changed the therapeutic landscape for patients with advanced melanoma. However, relatively low response rates and a high incidence of severe immune-related adverse events have prompted the search for predictive biomarkers. A positive predictive value has been attributed to the aberrant expression of Human Leukocyte Antigen-DR (HLA-DR) by melanoma cells, but it remains unknown why this is the case. In this study, we have examined the microenvironment of HLA-DR positive metastatic melanoma samples using a multi-omics approach. First, using spatial, single-cell mapping by multiplexed immunohistochemistry, we found that the microenvironment of HLA-DR positive melanoma regions was enriched by professional antigen presenting cells, including classical dendritic cells and macrophages, while a more general cytotoxic T cell exhaustion phenotype was present in these regions. In parallel, transcriptomic analysis on micro dissected tissue from HLA-DR positive and HLA-DR negative areas showed increased IFNγ signaling, enhanced leukocyte adhesion and mononuclear cell proliferation in HLA-DR positive areas. Finally, multiplexed cytokine profiling identified an increased expression of germinal center cytokines CXCL12, CXCL13 and CCL19 in HLA-DR positive metastatic lesions, which, together with IFNγ and IL4 could serve as biomarkers to discriminate tumor samples containing HLA-DR overexpressing tumor cells from HLA-DR negative samples. Overall, this suggests that HLA-DR positive areas in melanoma attract the anti-tumor immune cell infiltration by creating a dystrophic germinal center-like microenvironment where an enhanced antigen presentation leads to an exhausted microenvironment, nevertheless representing a fertile ground for a better efficacy of anti-PD-1 inhibitors due to simultaneous higher levels of PD-1 in the immune cells and PD-L1 in the HLA-DR positive melanoma cells.

Single-cell analysis of human B cell maturation predicts how antibody class switching shapes selection dynamics

Protective humoral memory forms in secondary lymphoid organs where B cells undergo affinity maturation and differentiation into memory or plasma cells. Here, we provide a comprehensive roadmap of human B cell maturation with single-cell transcriptomics matched with bulk and single-cell antibody repertoires to define gene expression, antibody repertoires, and clonal sharing of B cell states at single-cell resolution, including memory B cell heterogeneity that reflects diverse functional and signaling states. We reconstruct gene expression dynamics during B cell activation to reveal a pre-germinal center state primed to undergo class switch recombination and dissect how antibody class-dependent gene expression in germinal center and memory B cells is linked with a distinct transcriptional wiring with potential to influence their fate and function. Our analyses reveal the dynamic cellular states that shape human B cell-mediated immunity and highlight how antibody isotype may play a role during their antibody-based selection.

Transcriptional and Metabolic Control of Memory B Cells and Plasma Cells

For many infections and almost all vaccines, neutralizing-antibody-mediated immunity is the primary basis and best functional correlate of immunological protection. Durable long-term humoral immunity is mediated by antibodies secreted by plasma cells that preexist subsequent exposures and by memory B cells that rapidly respond to infections once they have occurred. In the midst of the current pandemic of coronavirus disease 2019, it is important to define our current understanding of the unique roles of memory B cells and plasma cells in immunity and the factors that control the formation and persistence of these cell types. This fundamental knowledge is the basis to interpret findings from natural infections and vaccines. Here, we review transcriptional and metabolic programs that promote and support B cell fates and functions, suggesting points at which these pathways do and do not intersect.

The Histone Methyltransferase DOT1L Is Essential for Humoral Immune Responses

Histone modifiers are essential for the ability of immune cells to reprogram their gene expression during differentiation. The recruitment of the histone methyltransferase DOT1L (disruptor of telomeric silencing 1-like) induces oncogenic gene expression in a subset of B cell leukemias. Despite its importance, its role in the humoral immune system is unclear. Here, we demonstrate that DOT1L is a critical regulator of B cell biology. B cell development is defective in Dot1l^f/fMb1^Cre/+ mice, culminating in a reduction of peripheral mature B cells. Upon immunization or influenza infection of Dot1l^f/fCd23^Cre/+ mice, class-switched antibody-secreting cells are significantly attenuated and germinal centers fail to form. Consequently, DOT1L is essential for B cell memory formation. Transcriptome, pathway, and histological analyses identified a role for DOT1L in reprogramming gene expression for appropriate localization of B cells during the initial stage of the response. Together, these results demonstrate an essential role for DOT1L in generating an effective humoral immune response.

IRF4 Activity Is Required in Established Plasma Cells to Regulate Gene Transcription and Mitochondrial Homeostasis

The transcription factor interferon regulatory factor 4 (IRF4) is critical for the development, maintenance, and function of plasma cells. The mechanism by which IRF4 exerts its action in mature plasma cells has been elusive due to the death of all such cells upon IRF4 loss. While we identify apoptosis as a critical pathway for the death of plasma cells caused by IRF4 loss, we also determine that IRF4 did not regulate the intrinsic apoptotic pathway directly. By using an inducible IRF4 deletion system in the presence of the overexpression of anti-apoptotic BCL2, we identify genes whose expression is coordinated by IRF4 and that in turn specify plasma cell identity and mitochondrial homeostasis.

In vivo Screen Identifies Zdhhc2 as a Critical Regulator of Germinal Center B Cell Differentiation

Germinal center (GC) B cell differentiation is critical for the production of affinity-matured pathogen-specific antibodies, the dysregulation of which may lead to humoral immunodeficiency or autoimmunity. The development of an in vivo screening system for factors regulating GC B cell differentiation has been a challenge. Here we describe a small-scale in vivo screening system with NP-specific B1-8^hi cells and a retroviral shRNA library targeting 78 candidate genes to search for B cell-intrinsic factors that specifically regulate GC B cell differentiation. Zdhhc2, a gene encoding palmitoyltransferase ZDHHC2 and highly expressed in GC B cells, is identified as a strong positive regulator of GC B cell differentiation. B1-8^hi cells transduced with Zdhhc2-shRNA are severely compromised in differentiating into GC B cells. A further analysis of in vitro differentiated B cells transduced with Zdhhc2-shRNA shows that Zdhhc2 is critical for the proliferation and the survival of B cells stimulated by CD40L, BAFF, and IL-21 and consequently impacts on their differentiation into GC B cells and post-GC B cells. These studies not only identify Zdhhc2 as a novel regulator of GC B cell differentiation but also represent a proof of concept of in vivo screen for regulators of GC B cell differentiation.

A Synchronous IRF4-Dependent Gene Regulatory Network in B and Helper T Cells Orchestrating the Antibody Response

Control of diverse pathogens requires an adaptive antibody response, dependent on cellular division of labor to allocate antigen-dependent B- and CD4⁺ T-cell fates that collaborate to control the quantity and quality of antibody. This is orchestrated by the dynamic action of key transcriptional regulators mediating gene expression programs in response to pathogen-specific environmental inputs. We describe a conserved, likely ancient, gene regulatory network that intriguingly operates contemporaneously in B and CD4⁺ T cells to control their cell fate dynamics and thus, the character of the antibody response. The remarkable output of this network derives from graded expression, designated by antigen receptor signal strength, of a pivotal transcription factor that regulates alternate cell fate choices.

Transcriptional Control of Mature B Cell Fates

The mature naïve B cell repertoire consists of three well-defined populations: B1, B2 (follicular B, FOB), and marginal zone B (MZB) cells. FOB cells are the dominant mature B cell population in the secondary lymphoid organs and blood of both humans and mice. The driving forces behind mature B lineage selection have been linked to B cell receptor (BCR) signaling strength and environmental cues, but how these fate-determination factors are transcriptionally regulated remains poorly understood. We summarize emerging data on the role of transcription factors (TFs) - particularly the ETS and IRF families - in regulating MZB and FOB lineage selection. Indeed, genomic analyses have identified four major groups of target genes that are crucial for FOB differentiation, revealing previously unrecognized pathways that ultimately determine biological responses specific to this lineage.

B Cell Activation and Response Regulation During Viral Infections

Acute viral infections are characterized by rapid increases in viral load, leading to cellular damage and the resulting induction of complex innate and adaptive antiviral immune responses that cause local and systemic inflammation. Successful antiviral immunity requires the activation of many immune cells, including T cells, natural killer cells, and macrophages. B cells play a unique part through their production of antibodies that can both neutralize and clear viral particles before virus entry into a cell. Protective antibodies are produced even before the first exposure of a pathogen, through the regulated secretion of so-called natural antibodies that are generated even in the complete absence of prior microbial exposure. An early wave of rapidly secreted antibodies from extrafollicular (EF) responses draws on the preexisting naive or memory repertoire of B cells to induce a strong protective response that in kinetics tightly follows the clearance of acute infections, such as with influenza virus. Finally, the generation of germinal centers (GCs) provides long-term protection through production of long-lived plasma cells and memory B cells, which shape and broaden the B cell repertoire for more effective responses following repeat exposures. In this study, we review B cell responses to acute viral infections, primarily influenza virus, from the earliest nonspecific B-1 cell to early, antigen-specific EF responses and finally to GC responses. Throughout, we address known factors that lead to distinct B cell response outcomes and discuss how their functions effect viral clearance, highlighting the critical contributions of each response type to the induction of highly protective antiviral humoral immunity.

Immunity and autoantibodies of a mouse strain with autistic-like behavior

Female and male mice of the BTBR T ⁺ Itpr3 ^tf /J (BTBR) strain have behaviors that resemble autism spectrum disorder. In comparison to C57BL/6 (B6) mice, BTBR mice have elevated humoral immunity, in that they have naturally high serum IgG levels and generate high levels of IgG antibodies, including autoantibodies to brain antigens. This study focused on the specificities of autoantibodies and the immune cells and their transcription factors that might be responsible for the autoantibodies. BTBR IgG autoantibodies bind to neurons better than microglia and with highest titer to nuclear antigens. Two of the antigens identified were alpha-enolase (ENO1) and dihydrolipoyllysine-residue succinyltransferase component of 2-oxoglutarate dehydrogenase complex, mitochondrial (DLST). Surprisingly based on IgG levels, the blood and spleens of BTBR mice have more CD4⁺ and CD8⁺ T cells, but fewer B cells than B6 mice. The high levels of autoantibodies in BTBR relates to their splenic T follicular helper (Tfh) cell levels, which likely are responsible for the higher number of plasma cells in BTBR mice than B6 mice. BTBR mice have increased gene expression of interleukin-21 receptor (I l -21 r) and Paired Box 5 (Pax5), which are known to aid B cell differentiation to plasma cells, and an increased Lysine Demethylase 6B (Kdm6b)/DNA Methyltransferase 1 (Dnmt1) ratio, which increases gene expression. Identification of gene expression and immune activities of BTBR mice may aid understanding of mechanisms associated with autism since neuroimmune network interactions have been posited and induction of autoantibodies may drive the neuroinflammation associated with autism.

T and B-cell signaling in activated PI3K delta syndrome: From immunodeficiency to autoimmunity

Phosphatidylinositol 3 kinases (PI3K) are a family of lipid kinases that are activated by a variety of cell-surface receptors, and regulate a wide range of downstream readouts affecting cellular metabolism, growth, survival, differentiation, adhesion, and migration. The importance of these lipid kinases in lymphocyte signaling has recently been highlighted by genetic analyses, including the recognition that both activating and inactivating mutations of the catalytic subunit of PI3Kδ, p110δ, lead to human primary immunodeficiencies. In this article, we discuss how studies on the human genetic disorder "Activated PI3K-delta syndrome" and mouse models of this disease (Pik3cd^E1020K/+ mice) have provided fundamental insight into pathways regulated by PI3Kδ in T and B cells and their contribution to lymphocyte function and disease, including responses to commensal bacteria and the development of autoimmunity and tumors. We highlight critical roles of PI3Kδ in T follicular helper cells and the orchestration of the germinal center reaction, as well as in CD8⁺ T-cell function. We further  present data demonstrating the ability of the AKT-resistant FOXO1^AAA mutant to rescue IgG1 class switching defects in Pik3cd^E1020K/+ B cells, as well as data supporting a role for PI3Kδ in promoting multiple T-helper effector cell lineages.

Sex differences in Tfh cell help to B cells contribute to sexual dimorphism in severity of rat collagen-induced arthritis

The study examined germinal centre (GC) reaction in lymph nodes draining inflamed joints and adjacent tissues (dLNs) in male and female Dark Agouti rat collagen type II (CII)-induced arthritis (CIA) model of rheumatoid arthritis. Female rats exhibiting the greater susceptibility to CIA mounted stronger serum CII-specific IgG response than their male counterparts. This correlated with the higher frequency of GC B cells in female compared with male dLNs. Consistently, the frequency of activated/proliferating Ki-67+ cells among dLN B cells was higher in females than in males. This correlated with the shift in dLN T follicular regulatory (Tfr)/T follicular helper (Tfh) cell ratio towards Tfh cells in females, and greater densities of CD40L and CD40 on their dLN T and B cells, respectively. The higher Tfh cell frequency in females was consistent with the greater dLN expression of mRNA for IL-21/27, the key cytokines involved in Tfh cell generation and their help to B cells. Additionally, in CII-stimulated female rat dLN cell cultures IFN-γ/IL-4 production ratio was shifted towards IFN-γ. Consistently, the serum IgG2a(b)/IgG1 CII-specific antibody ratio was shifted towards an IgG2a(b) response in females. Thus, targeting T-/B-cell interactions should be considered in putative further sex-based translational pharmacology research.

Plasma cells: You are what you eat

Plasma cells are terminally differentiated B lymphocytes that constitutively secrete antibodies. These antibodies can provide protection against pathogens, and their quantity and quality are the best clinical correlates of vaccine efficacy. As such, plasma cell lifespan is the primary determinant of the duration of humoral immunity. Yet dysregulation of plasma cell function can cause autoimmunity or multiple myeloma. The longevity of plasma cells is primarily dictated by nutrient uptake and non-transcriptionally regulated metabolic pathways. We have previously shown a positive effect of glucose uptake and catabolism on plasma cell longevity and function. In this review, we discuss these findings with an emphasis on nutrient uptake and its effects on respiratory capacity, lifespan, endoplasmic reticulum stress, and antibody secretion in plasma cells. We further discuss how some of these pathways may be dysregulated in multiple myeloma, potentially providing new therapeutic targets. Finally, we speculate on the connection between plasma cell intrinsic metabolism and systemic changes in nutrient availability and metabolic diseases.

Epigenetic regulation of B cell fate and function during an immune response

The humoral immune response requires coordination of molecular programs to mediate differentiation into unique B cell subsets that help clear the infection and form immune memory. Epigenetic modifications are crucial for ensuring that the appropriate genes are transcribed or repressed during B cell differentiation. Recent studies have illuminated the changes in DNA methylation and histone post-translational modifications that accompany the formation of germinal center and antibody-secreting cells during an immune response. In particular, the B cell subset-specific expression and function of DNA methyltransferases and histone-modifying complexes that mediate epigenome changes have begun to be unravelled. This review will discuss the recent advances in this field, as well as highlight critical questions about the relationship between epigenetic regulation and B cell fate and function that are yet to be answered.

Germinal center B cell initiation, GC maturation, and the coevolution of its stromal cell niches

Throughout the developing GC response, B cell survival and fate choices made at the single cell level are dependent on signals received largely through interactions with other cells, often with cognate T cells. The type of signals that a given B cell can encounter is dictated by its location within tissue microarchitecture. The focus of this review is on the initiation and evolution of the GC response at the earliest time points. Here, we review the key factors influencing the progression of GC B cell differentiation that are both stage and context dependent. Finally, we describe the coevolution of niches within and surrounding the GC that influence the outcome of the GC response.

From Loops to Looks: Transcription Factors and Chromatin Organization Shaping Terminal B Cell Differentiation

B lymphopoiesis is tightly regulated at the level of gene transcription. In recent years, investigators have shed light on the transcription factor networks and the epigenetic machinery involved at all differentiation steps of mammalian B cell development. During terminal differentiation, B cells undergo dramatic changes in gene transcriptional programs to generate germinal center B cells, plasma cells and memory B cells. Recent evidence indicates that mature B cell formation involves an essential contribution from 3D chromatin conformations through its interplay with transcription factors and epigenetic machinery. Here, we provide an up-to-date overview of the coordination between transcription factors, epigenetic changes, and chromatin architecture during terminal B cell differentiation, focusing on recent discoveries and technical advances for studying 3D chromatin structures.

Selective Histone Deacetylase 6 Inhibition Normalizes B Cell Activation and Germinal Center Formation in a Model of Systemic Lupus Erythematosus

Autoantibody production by plasma cells (PCs) plays a pivotal role in the pathogenesis of systemic lupus erythematosus (SLE). The molecular pathways by which B cells become pathogenic PC secreting autoantibodies in SLE are incompletely characterized. Histone deactylase 6 (HDAC6) is a unique cytoplasmic HDAC that modifies the interaction of a number of tubulin- associated proteins; inhibition of HDAC6 has been shown to be beneficial in murine models of SLE, but the downstream pathways accounting for the therapeutic benefit have not been clearly delineated. In the current study, we sought to determine whether selective HDAC6 inhibition would abrogate abnormal B cell activation in SLE. We treated NZB/W lupus mice with the selective HDAC6 inhibitor, ACY-738, for 4 weeks beginning at 20 weeks-of age. After only 4 weeks of treatment, manifestation of lupus nephritis (LN) were greatly reduced in these animals. We then used RNAseq to determine the genomic signatures of splenocytes from treated and untreated mice and applied computational cellular and pathway analysis to reveal multiple signaling events associated with B cell activation and differentiation in SLE that were modulated by HDAC6 inhibition. PC development was abrogated and germinal center (GC) formation was greatly reduced. When the HDAC6 inhibitor-treated lupus mouse gene signatures were compared to human lupus patient gene signatures, the results showed numerous immune, and inflammatory pathways increased in active human lupus were significantly decreased in the HDAC6 inhibitor treated animals. Pathway analysis suggested alterations in cellular metabolism might contribute to the normalization of lupus mouse spleen genomic signatures, and this was confirmed by direct measurement of the impact of the HDAC6 inhibitor on metabolic activities of murine spleen cells. Taken together, these studies show HDAC6 inhibition decreases B cell activation signaling pathways and reduces PC differentiation in SLE and suggest that a critical event might be modulation of cellular metabolism.

RNAi Transfection Optimized in Primary Naïve B Cells for the Targeted Analysis of Human Plasma Cell Differentiation

Upon antigen recognition, naïve B cells undergo rapid proliferation followed by differentiation to specialized antibody secreting cells (ASCs), called plasma cells. Increased circulating plasma cells are reported in patients with B cell-associated malignancies, chronic graft-vs.-host disease, and autoimmune disorders. Our aim was to optimize an RNAi-based method that efficiently and reproducibly knocks-down genes of interest in human primary peripheral B cells for the targeted analysis of ASC differentiation. The unique contributions of transcriptional diversity in species-specific regulatory networks and the mechanisms of gene function need to be approached directly in human B cells with tools to hone our basic inferences from animal models to human biology. To date, methods for gene knockdown in human primary B cells, which tend to be more refractory to transfection than immortalized B cell lines, have been limited by losses in cell viability and ineffective penetrance. Our single-step siRNA nucleofector-based approach for human primary naïve B cells demonstrates reproducible knockdown efficiency (~40–60%). We focused on genes already known to play key roles in murine ASC differentiation, such as interferon regulatory factor 4 (IRF4) and AID. This study reports a validated non-viral method of siRNA delivery into human primary B cells that can be applied to study gene regulatory networks that control human ASC differentiation.

Function and dysfunction of plasma cells in intestine

As the main player in humoral immunity, antibodies play indispensable roles in the body's immune system. Plasma cells (PCs), as antibody factories, are important contributors to humoral immunity. PCs, recognized by their unique marker CD138, are always discovered in the medullary cords of spleen and lymph nodes and in bone marrow and mucosal lymphoid tissue. This article will review the origin and differentiation of PCs, characteristics of short- and long-lived PCs, and the secretion of antibodies, such as IgA, IgM, and IgG. PCs play a crucial role in the maintenance of intestinal homeostasis using immunomodulation though complex mechanisms. Clearly, PCs play functional roles in maintaining intestinal health, but more details are needed to fully understand all the other effects of intestinal PCs.

Molecular regulation of peripheral B cells and their progeny in immunity

In this review, Boothby et al. summarize some salient advances toward elucidation of the molecular programming of the fate choices and function of B cells in the periphery. They also note unanswered questions that pertain to differences among subsets of B lymphocytes and plasma cells.

Mature B lymphocytes are crucial components of adaptive immunity, a system essential for the evolutionary fitness of mammals. Adaptive lymphocyte function requires an initially naïve cell to proliferate extensively and its progeny to have the capacity to assume a variety of fates. These include either terminal differentiation (the long-lived plasma cell) or metastable transcriptional reprogramming (germinal center and memory B cells). In this review, we focus principally on the regulation of differentiation and functional diversification of the “B2” subset. An overview is combined with an account of more recent advances, including initial work on mechanisms that eliminate DNA methylation and potential links between intracellular metabolites and chromatin editing.