New Methods To Analyze B Cell Immune Responses to Thymus-Dependent Antigen Sheep Red Blood Cells

B-cell intrinsic regulation of antibody mediated immunity by histone H2A deubiquitinase BAP1

Introduction

BAP1 is a deubiquitinase (DUB) of the Ubiquitin C-terminal Hydrolase (UCH) family that regulates gene expression and other cellular processes, through its direct catalytic activity on the repressive epigenetic mark histone H2AK119ub, as well as on several other substrates. BAP1 is also a highly important tumor suppressor, expressed and functional across many cell types and tissues. In recent work, we demonstrated a cell intrinsic role of BAP1 in the B cell lineage development in murine bone marrow, however the role of BAP1 in the regulation of B cell mediated humoral immune response has not been previously explored.

Methods and results

In the current study, we demonstrate that a B-cell intrinsic loss of BAP1 in activated B cells in the Bap1 fl/fl Cγ1-cre murine model results in a severe defect in antibody production, with altered dynamics of germinal centre B cell, memory B cell, and plasma cell numbers. At the cellular and molecular level, BAP1 was dispensable for B cell immunoglobulin class switching but resulted in an impaired proliferation of activated B cells, with genome-wide dysregulation in histone H2AK119ub levels and gene expression.

Conclusion and discussion

In summary, our study establishes the B-cell intrinsic role of BAP1 in antibody mediated immune response and indicates its central role in the regulation of the genome-wide landscapes of histone H2AK119ub and downstream transcriptional programs of B cell activation and humoral immunity.

Arid1a-dependent canonical BAF complex suppresses inflammatory programs to drive efficient Germinal Center B cell responses

Differentiating B cells in germinal centers (GC) require tightly coordinated transcriptional and epigenetic transitions to generate efficient humoral immune responses. The mammalian Brg1/Brm-associated factor (BAF) complexes are major regulators of nucleosomal remodeling, crucial for cellular differentiation and development, and are commonly mutated in several cancers, including GC-derived B cell lymphomas. However, the specific roles of distinct BAF complexes in GC B cell biology and generation of functional humoral immune responses are not well understood. Here, we show that the A-T Rich Interaction Domain 1a (Arid1a) containing canonical BAF (cBAF) complex is required for maintenance of GCs and therefore high affinity antibody responses. While Arid1a-deficient B cells undergo activation to initiate GC responses, they fail to sustain the GC program resulting in premature GC collapse. We discovered that Arid1a-dependent cBAF activity establishes permissive chromatin landscapes during B cell activation and is concomitantly required to suppress inflammatory gene programs to maintain transcriptional fidelity in early GC B cells. Interestingly, the inflammatory signatures instigated by Arid1a deficiency in early GC B cells recruited neutrophils and inflammatory monocytes and eventually disrupted GC homeostasis. Dampening of inflammatory cues with anti-inflammatory glucocorticoid receptor signaling rescued GC B cell differentiation of Arid1a-deficient B cells, thus highlighting a critical role of inflammation in impeding GC responses. In sum, our work identifies essential functions of Arid1a-dependent BAF activity in promoting efficient GC responses. These findings further support an emerging paradigm in which unrestrained inflammation limits GC-derived humoral responses, as reported in the context of severe bacterial and viral infections.

Temporal Tracking of Plasma Cells in vivo Using J-chain CreERT2 Reporter System

Plasma cells (PCs) are essential for humoral immunity, as they are responsible for the production of antibodies and contribute to immunological memory. Despite their importance, differentiating between long-lived and short-lived PCs in vivo remains a challenge due to a lack of specific markers to distinguish these populations. Addressing this gap, our study introduces a novel J-chain CreERT2 GFP allele (IgJCreERT2) for precise genetic studies of PCs. This model takes advantage of PC-restricted expression of the J-chain gene, enabling temporal and cell-specific tracking of PCs utilizing a tamoxifen-inducible Cre recombinase. Our in vitro and in vivo validation studies of the inducible Cre allele confirmed the fidelity and utility of this model and demonstrated the model's ability to trace the long-lived PC population in vivo following immunization. The IgJCreERT2 model allowed for detailed analysis of surface marker expression on PCs, revealing insights into PC heterogeneity and characteristics. Our findings not only validate the IgJCreERT2 mouse as a reliable tool for studying PCs but also facilitate the investigation of PC dynamics and longevity, particularly in the context of humoral immunity and vaccine responses. This model represents a significant advancement for the in-depth study of PCs in health and disease, offering a new avenue for the exploration of PC biology and immunological memory.

Distinctive High Expression of Antiretroviral APOBEC3 Protein in Mouse Germinal Center B Cells

Tissue and subcellular localization and its changes upon cell activation of virus-restricting APOBEC3 at protein levels are important to understanding physiological functions of this cytidine deaminase, but have not been thoroughly analyzed in vivo. To precisely follow the possible activation-induced changes in expression levels of APOBEC3 protein in different mouse tissues and cell populations, genome editing was utilized to establish knock-in mice that express APOBEC3 protein with an in-frame FLAG tag. Flow cytometry and immunohistochemical analyses were performed prior to and after an immunological stimulation. Cultured B cells expressed higher levels of APOBEC3 protein than T cells. All differentiation and activation stages of freshly prepared B cells expressed significant levels of APOBEC3 protein, but germinal center cells possessed the highest levels of APOBEC3 protein localized in their cytoplasm. Upon immunological stimulation with sheep red blood cells in vivo, germinal center cells with high levels of APOBEC3 protein expression increased in their number, but FLAG-specific fluorescence intensity in each cell did not change. T cells, even those in germinal centers, did not express significant levels of APOBEC3 protein. Thus, mouse APOBEC3 protein is expressed at distinctively high levels in germinal center B cells. Antigenic stimulation did not affect expression levels of cellular APOBEC3 protein despite increased numbers of germinal center cells.

Preclinical Analysis of Candidate Anti-Human CD79 Therapeutic Antibodies Using a Humanized CD79 Mouse Model

The BCR comprises a membrane-bound Ig that is noncovalently associated with a heterodimer of CD79A and CD79B. While the BCR Ig component functions to sense extracellular Ag, CD79 subunits contain cytoplasmic ITAMs that mediate intracellular propagation of BCR signals critical for B cell development, survival, and Ag-induced activation. CD79 is therefore an attractive target for Ab and chimeric Ag receptor T cell therapies for autoimmunity and B cell neoplasia. Although the mouse is an attractive model for preclinical testing, due to its well-defined immune system, an obstacle is the lack of cross-reactivity of candidate therapeutic anti-human mAbs with mouse CD79. To overcome this problem, we generated knockin mice in which the extracellular Ig-like domains of CD79A and CD79B were replaced with human equivalents. In this study, we describe the generation and characterization of mice expressing chimeric CD79 and report studies that demonstrate their utility in preclinical analysis of anti-human CD79 therapy. We demonstrate that human and mouse CD79 extracellular domains are functionally interchangeable, and that anti-human CD79 lacking Fc region effector function does not cause significant B cell depletion, but induces 1) decreased expression of plasma membrane-associated IgM and IgD, 2) uncoupling of BCR-induced tyrosine phosphorylation and calcium mobilization, and 3) increased expression of PTEN, consistent with the levels observed in anergic B cells. Finally, anti-human CD79 treatment prevents disease development in two mouse models of autoimmunity. We also present evidence that anti-human CD79 treatment may inhibit Ab secretion by terminally differentiated plasmablasts and plasma cells in vitro.

Protection of Quiescence and Longevity of IgG Memory B Cells by Mitochondrial Autophagy

The development of long-lived immune memory cells against pathogens is critical for the success of vaccines to establish protection against future infections. However, the mechanisms governing the long-term survival of immune memory cells remain to be elucidated. In this article, we show that the maintenance mitochondrial homeostasis by autophagy is critical for restricting metabolic functions to protect IgG memory B cell survival. Knockout of mitochondrial autophagy genes, Nix and Bnip3, leads to mitochondrial accumulation and increases in oxidative phosphorylation and fatty acid synthesis, resulting in the loss of IgG⁺ memory B cells in mice. Inhibiting fatty acid synthesis or silencing necroptosis gene Ripk3 rescued Nix^-/-Bnip3^-/- IgG memory B cells, indicating that mitochondrial autophagy is important for limiting metabolic functions to prevent cell death. Our results suggest a critical role for mitochondrial autophagy in the maintenance of immunological memory by protecting the metabolic quiescence and longevity of memory B cells.

Reduced eIF4E function impairs B-cell leukemia without altering normal B-lymphocyte function

The cap-binding protein eukaryotic initiation factor 4E (eIF4E) promotes translation of mRNAs associated with proliferation and survival and is an attractive target for cancer therapeutics. Here, we used Eif4e germline and conditional knockout models to assess the impact of reduced Eif4e gene dosage on B-cell leukemogenesis compared to effects on normal pre-B and mature B-cell function. Using a BCR-ABL-driven pre-B-cell leukemia model, we find that loss of one allele of Eif4e impairs transformation and reduces fitness in competition assays in vitro and in vivo. In contrast, reduced Eif4e gene dosage had no significant effect on development of pre-B and mature B cells or on survival or proliferation of non-transformed B lineage cells. These results demonstrate that inhibition of eIF4E could be a new therapeutic tool for pre-B-cell leukemia while preserving development and function of normal B cells.

The magnitude of germinal center reactions is restricted by a fixed number of preexisting niches

Antibody affinity maturation occurs in the germinal center (GC), a highly dynamic structure that arises upon antigen stimulation and recedes after infection is resolved. While the magnitude of the GC reaction is highly fluctuating and depends on antigens or pathological conditions, it is unclear whether GCs are assembled ad hoc in different locations or in preexisting niches within B cell follicles. We show that follicular dendritic cells (FDCs), the essential cellular components of the GC architecture, form a predetermined number of clusters. The total number of FDC clusters is the same on several different genetic backgrounds and is not altered by immunization or inflammatory conditions. In unimmunized and germ-free mice, a few FDC clusters contain GC B cells; in contrast, immunization or autoimmune milieu significantly increases the frequency of FDC clusters occupied by GC B cells. Excessive occupancy of GC niches by GC B cells after repeated immunizations or in autoimmune conditions suppresses subsequent antibody responses to new antigens. These data indicate that the magnitude of the GC reaction is restricted by a fixed number of permissive GC niches containing preassembled FDC clusters. This finding may help in the future design of vaccination strategies and in the modulation of antibody-mediated autoimmunity.

BCL6 BTB-specific inhibitor reversely represses T Cell activation, Tfh cells differentiation and germinal center reaction in vivo

Inhibition of the BCL6 BTB domain results in killing Diffuse Large B-cell Lymphoma (DLBL) cells, reducing the T-cell dependent germinal center (GC) reaction in mice, and reversing GC hyperplasia in nonhuman primates. The available BCL6 BTB-specific inhibitors are poorly water soluble thus limiting their absorption in vivo and our understanding of therapeutic strategy targeting GC. We synthesized a prodrug (AP-4-287) from a potent BCL6 BTB inhibitor (FX1) with improved aqueous solubility and pharmacokinetics (PK) in mice. We also evaluated its in vivo biological activity on humoral immune responses using the sheep red blood cells (SRBC)-vaccination mouse model. AP-4-287 had a significant higher aqueous solubility and was readily converted to FX1 in vivo after intraperitoneally (i.p.) administration, but a shorter half-life in vivo. Importantly, AP-4-287 treatment led to a reversible effect on (1) the reduction in the frequency of splenic Ki67⁺ CD4⁺ T cells, Tfh cells, and GC B cells; (2) lower GC formation following vaccination; and (3) a decrease in the titers of antigen-specific IgG and IgM antibodies. Our study advances the pre-clinical development of drug targeting BCL6 BTB domain for the treatment of diseases that are associated with abnormal BCL6 elevation. This article is protected by copyright. All rights reserved.

Antioxidant and immune responses of broiler chickens supplemented with Rhazya stricta extract in drinking water

Background and Aim

Rhazya stricta is a herbal plant widely used in traditional medicine due to its proficiency and naturalness with few side effects. In this study, we investigated the impact of using an R. stricta extract supplement on broiler chickens' performance, especially the immune system.

Materials and Methods

In addition to the control group, one group received the methanol extract of R. stricta in drinking water for the first 2 weeks before being challenged with sheep erythrocytes (SRBCs), while the other group was challenged with SRBCs without receiving the R. stricta treatment. We evaluated cellular immunity by determining the phagocytic activity and lymphocyte (L) proliferation and assessed humoral immunity by quantification of the serum total IgM and IgG. We measured the serum levels of antioxidant enzymes and performed a histological examination of the spleen and the bursa of Fabricius (BF).

Results

Our results indicate a significant enhancement in cellular immunity in the group supplemented with R. stricta as demonstrated by a significant increase in the phagocytic activity, L proliferation, and percentages of circulating L (p<0.05). The chickens treated with R. stricta exhibit an enhanced humoral response shown by a significant elevation in the serum levels of the total antibodies of the IgM and IgG isotypes, along with a notable increase in BF activity. Furthermore, R. stricta supplementation is associated with a significant increase in the serum levels of catalase and superoxide dismutase (p<0.05), along with a significant improvement in broilers' general performance, body weight, and feed efficiency.

Conclusion

Our results suggest an immunomodulatory effect for the methanol extract of R. stricta and highlight the potential use of this plant in preventive and therapeutic medicine.

Regnase-1 is essential for B cell homeostasis to prevent immunopathology

Regnase-1 is an emerging regulator of immune responses with essential roles in the posttranscriptional control of immune cell activation. Regnase-1 is expressed in B cells; however, its B cell-specific functions remain unknown. Here, we demonstrate that Regnase-1 prevents severe autoimmune pathology and show its essential role in maintaining B cell homeostasis. Using Cre driver mice for ablation of Regnase-1 at various stages of B cell development, we demonstrate that loss of Regnase-1 leads to aberrant B cell activation and differentiation, resulting in systemic autoimmunity and early morbidity. The basis of these findings was informed by gene expression data revealing a regulatory role for Regnase-1 in the suppression of a transcriptional program that promotes B cell activation, survival, and differentiation. Overall, our study shows that Regnase-1 exerts critical control of B cell activation, which is required for prevention of immunopathology.

Effect of ozonation on resistance of ovine and human erythrocytes to hypothermic storage

Long-term hypothermic storage of animal blood can lead to the loss of its quality and can cause complications in recipient animals after transfusion, so the search for new methods of increasing the preservation of erythrocytes after hypothermic storage continues. The article presents the data of the ozonation effect on the preservation rate of ovine and human erythrocytes during hypothermic storage with Alsever’s solution and mannitol medium. Hemolysis, osmotic fragility and distribution density of ovine and human erythrocytes by the sphericity index were determined at different stages of hypothermic storage. The ovine erythrocytes in the control had a lower osmotic resistance compared to human erythrocytes; however, their osmotic fragility did not change significantly after hypothermic storage for 8 weeks, and for human erythrocytes, it significantly increased. Storage of ovine and human erythrocytes longer than 8 weeks led to a sharp hemolysis, while the level of hemolysis of ovine erythrocytes was lower than that of human erythrocytes. Preservation of ozonated erythrocytes is higher than non-ozonated ones during prolonged hypothermic storage. The effect of ozonation on the preservation rate of red blood cells depended on the composition of the preservation media. Hypothermal storage of human erythrocytes in Alsever’s solution for up to 8 weeks led to a shift in the density of distribution according to the sphericity index towards spheroidization of cells; in a medium with mannitol, the number of flattened cell forms increased. After 8 weeks of hypothermic storage of ovine erythrocytes, most of the cells had high sphericity indices. Pretreatment of human and sheep erythrocytes with ozone allows the distribution of cells to be kept closer to the control during long-term storage, which is especially pronounced in mannitol medium. Ovine erythrocytes retained the ability to form rosettes with human T-lymphocytes after hypothermic storage for up to 12 weeks. Thus, ozone pretreatment and the use of mannitol as part of hypothermic storage medium could be an approach to improve the quality of preserved ovine erythrocytes.

Gut Microbiota Composition Modulates the Magnitude and Quality of Germinal Centers during Plasmodium Infections

Gut microbiota composition is associated with human and rodent Plasmodium infections, yet the mechanism by which gut microbiota affects the severity of malaria remains unknown. Humoral immunity is critical in mediating the clearance of Plasmodium blood stage infections, prompting the hypothesis that mice with gut microbiota-dependent decreases in parasite burden exhibit better germinal center (GC) responses. In support of this hypothesis, mice with a low parasite burden exhibit increases in GC B cell numbers and parasite-specific antibody titers, as well as better maintenance of GC structures and a more targeted, qualitatively different antibody response. This enhanced humoral immunity affects memory, as mice with a low parasite burden exhibit robust protection against challenge with a heterologous, lethal Plasmodium species. These results demonstrate that gut microbiota composition influences the biology of spleen GCs as well as the titer and repertoire of parasite-specific antibodies, identifying potential approaches to develop optimal treatments for malaria.

Research has shown that gut microbiota composition influences malaria severity, but the mechanism has remained unclear. Waide et al. show that microbiota composition drives differences in the humoral immune response, including differences in germinal center cell numbers and parasite-specific antibodies, ultimately affecting the memory response to subsequent infection.

The E3 ubiquitin ligase Itch deficiency promotes antigen-driven B-cell responses in mice

Deficiency of Itch, an E3 ubiquitin ligase, usually induced severe systemic and progressive autoimmune disease. The Itch function is well studied in T cells but not in B cells. We hypothesize that B-cell-specific Itch deficiency promoted antigen-induced B-cell activation and antibody-expressing plasma cell (PC) production. We found that unlike Itch KO, Itch cKO (CD19^cre Itch^f/f ) mice did not demonstrated a significant increase in the sizes of spleens and LNs, antibody level, and base mutation of antibody gene. However, in line with the fact that Itch expression decreased in GC B cells, PCs, and plasmablast (PB)-like SP 2/0 cells, Itch deficiency promoted B-cell activation and antibody production induced by antigens including lipopolysaccharide (LPS) and sheep red blood cells (SRBCs). Mechanistically, we found that Itch deficiency promotes antigen-induced cytokine production because Itch controls the proteins (e.g., eIF3a, eIF3c, eIF3h) with translation initiation factor activity. Altogether, our data suggest that Itch deficiency promotes antigen-driven B-cell response. This may provide hints for Itch-targeted treatment of patients with autoimmune disease.

The AKT isoforms 1 and 2 drive B cell fate decisions during the germinal center response

The PI3K pathway is integral for the germinal center (GC) response. However, the contribution of protein kinase B (AKT) as a PI3K effector in GC B cells remains unknown. Here, we show that mice lacking the AKT1 and AKT2 isoforms in B cells failed to form GCs, which undermined affinity maturation and antibody production in response to immunization. Upon B-cell receptor stimulation, AKT1/2-deficient B cells showed poor survival, reduced proliferation, and impaired mitochondrial and metabolic fitness, which collectively halted GC development. By comparison, Foxo1 T24A mutant, which cannot be inactivated by AKT1/2 phosphorylation and is sequestered in the nucleus, significantly enhanced antibody class switch recombination via induction of activation-induced cytidine deaminase (AID) expression. By contrast, repression of FOXO1 activity by AKT1/2 promoted IRF4-driven plasma cell differentiation. Last, we show that T-cell help via CD40, but not enforced expression of Bcl2, rescued the defective GC response in AKT1/2-deficient animals by restoring proliferative expansion and energy production. Overall, our study provides mechanistic insights into the key role of AKT and downstream pathways on B cell fate decisions during the GC response.

Self-reactive B cells in the GALT are actively curtailed to prevent gut inflammation

Immune homeostasis in the gut associated lymphoid tissues (GALT) is critical to prevent the development of inadvertent pathologies. B cells as the producers of antibodies and cytokines plays an important role in maintaining the GALT homeostasis. However, the mechanism by which B cells specifically direct their responses towards non-self-antigens and become ignorant to self-antigens in the GALT is not known. Therefore, we developed a novel mouse model by expressing Duck Egg Lysozyme (DEL) in gut epithelial cells in presence of HEL reactive B cells. Notably, we observed a transient activation and rapid deletion of self-reactive B cells in Peyers Patches and Mesenteric lymph nodes upon self-antigen exposure. The survival of self-reactive B cells upon exposure to their self-antigen was partially rescued by blocking receptor editing but could be completely rescued by stronger survival signal like ectopic expression of BCL2. Importantly, rescuing the self-reactive B cells promoted production of auto-antibodies and gut inflammation. Mechanistically, we identify a specific activation of TGFβ signaling in self-reactive B cells in the gut and a critical role of this pathway in maintaining peripheral tolerance. Collectively, our studies describe functional consequences and fate of self-reactive B cells in GALT and provide novel mechanistic insights governing self-tolerance of B cells in the gut.

The mTORC1/4E-BP/eIF4E Axis Promotes Antibody Class Switching in B Lymphocytes

During an adaptive immune response, activated mature B cells give rise to Ab-secreting plasma cells to fight infection. B cells undergo Ab class switching to produce different classes of Abs with varying effector functions. The mammalian/mechanistic target of rapamycin (mTOR) signaling pathway is activated during this process, and disrupting mTOR complex 1 (mTORC1) in B cells impairs class switching by a poorly understood mechanism. In particular, it is unclear which mTORC1 downstream substrates control this process. In this study, we used an in vitro murine model in which the mTORC1 inhibitor rapamycin, when added after a B cell has committed to divide, suppresses class switching while preserving proliferation. Investigation of mTORC1 substrates revealed a role for eukaryotic translation initiation factor 4E (eIF4E) and eIF4E-binding proteins in class switching. Mechanistically, we show that genetic or pharmacological disruption of eIF4E binding to eIF4G reduced cap-dependent translation, which specifically affected the expression of activation-induced cytidine deaminase protein but not Aicda mRNA. This translational impairment decreased Ab class switching independently of proliferation. These results uncover a previously undescribed role for mTORC1 and the eIF4E-binding proteins/eIF4E axis in activation-induced cytidine deaminase protein expression and Ab class switching in mouse B cells, suggesting that cap-dependent translation regulates key steps in B cell differentiation.