Second signals rescue B cells from activation-induced mitochondrial dysfunction and death

Mitochondrial control of lymphocyte homeostasis

Mitochondria play a multitude of essential roles within mammalian cells, and understanding how they control immunity is an emerging area of study. Lymphocytes, as integral cellular components of the adaptive immune system, rely on mitochondria for their function, and mitochondria can dynamically instruct their differentiation and activation by undergoing rapid and profound remodelling. Energy homeostasis and ATP production are often considered the primary functions of mitochondria in immune cells; however, their importance extends across a spectrum of other molecular processes, including regulation of redox balance, signalling pathways, and biosynthesis. In this review, we explore the dynamic landscape of mitochondrial homeostasis in T and B cells, and discuss how mitochondrial disorders compromise adaptive immunity.

Csk restrains BCR-mediated ROS production and contributes to germinal center selection and affinity maturation

Compared with naïve B cells, the B cell receptor (BCR) signal in germinal center (GC) B cells is attenuated; however, the significance of this signaling attenuation has not been well defined. Here, to investigate the role of attenuation of BCR signaling, we employed a Csk mutant mouse model in which Csk deficiency in GC B cells resulted in augmentation of net BCR signaling with no apparent effect on antigen presentation. We found that Csk is required for GC maintenance and efficient antibody affinity maturation. Mechanistically, ROS-induced apoptosis was exacerbated concomitantly with mitochondrial dysfunction in Csk-deficient GC B cells. Hence, our data suggest that attenuation of the BCR signal restrains hyper-ROS production, thereby protecting GC B cells from apoptosis and contributing to efficient affinity maturation.

High recallability of memory B cells requires ZFP318-dependent transcriptional regulation of mitochondrial function

Expression of the transcriptional regulator ZFP318 is induced in germinal center (GC)-exiting memory B cell precursors and memory B cells (MBCs). Using a conditional ZFP318 fluorescence reporter that also enables ablation of ZFP318-expressing cells, we found that ZFP318-expressing MBCs were highly enriched with GC-derived cells. Although ZFP318-expressing MBCs constituted only a minority of the antigen-specific MBC compartment, their ablation severely impaired recall responses. Deletion of Zfp318 did not alter the magnitude of primary responses but markedly reduced MBC participation in recall. CD40 ligation promoted Zfp318 expression, whereas B cell receptor (BCR) signaling was inhibitory. Enforced ZFP318 expression enhanced recall performance of MBCs that otherwise responded poorly. ZFP318-deficient MBCs expressed less mitochondrial genes, had structurally compromised mitochondria, and were susceptible to reactivation-induced cell death. The abundance of ZFP318-expressing MBCs, instead of the number of antigen-specific MBCs, correlated with the potency of prime-boost vaccination. Therefore, ZFP318 controls the MBC recallability and represents a quality checkpoint of humoral immune memory.

A spatiotemporal map of co-receptor signaling networks underlying B cell activation

The B cell receptor (BCR) signals together with a multi-component co-receptor complex to initiate B cell activation in response to antigen binding. Here, we take advantage of peroxidase-catalyzed proximity labeling combined with quantitative mass spectrometry to track co-receptor signaling dynamics in Raji cells from 10 s to 2 h after BCR stimulation. This approach enables tracking of 2,814 proximity-labeled proteins and 1,394 phosphosites and provides an unbiased and quantitative molecular map of proteins recruited to the vicinity of CD19, the signaling subunit of the co-receptor complex. We detail the recruitment kinetics of signaling effectors to CD19 and identify previously uncharacterized mediators of B cell activation. We show that the glutamate transporter SLC1A1 is responsible for mediating rapid metabolic reprogramming and for maintaining redox homeostasis during B cell activation. This study provides a comprehensive map of BCR signaling and a rich resource for uncovering the complex signaling networks that regulate activation.

Conserved role of hnRNPL in alternative splicing of epigenetic modifiers enables B cell activation

The multifunctional RNA-binding protein hnRNPL is implicated in antibody class switching but its broader function in B cells is unknown. Here, we show that hnRNPL is essential for B cell activation, germinal center formation, and antibody responses. Upon activation, hnRNPL-deficient B cells show proliferation defects and increased apoptosis. Comparative analysis of RNA-seq data from activated B cells and another eight hnRNPL-depleted cell types reveals common effects on MYC and E2F transcriptional programs required for proliferation. Notably, while individual gene expression changes are cell type specific, several alternative splicing events affecting histone modifiers like KDM6A and SIRT1, are conserved across cell types. Moreover, hnRNPL-deficient B cells show global changes in H3K27me3 and H3K9ac. Epigenetic dysregulation after hnRNPL loss could underlie differential gene expression and upregulation of lncRNAs, and explain common and cell type-specific phenotypes, such as dysfunctional mitochondria and ROS overproduction in mouse B cells. Thus, hnRNPL is essential for the resting-to-activated B cell transition by regulating transcriptional programs and metabolism, at least in part through the alternative splicing of several histone modifiers.

Multi-omic Analysis of Human B-cell Activation Reveals a Key Lysosomal BCAT1 Role in mTOR Hyperactivation by B-cell receptor and TLR9

B-lymphocytes play major adaptive immune roles, producing antibody and driving T-cell responses. However, how immunometabolism networks support B-cell activation and differentiation in response to distinct receptor stimuli remains incompletely understood. To gain insights, we systematically investigated acute primary human B-cell transcriptional, translational and metabolomic responses to B-cell receptor (BCR), Toll-like receptor 9 (TLR9), CD40-ligand (CD40L), interleukin-4 (IL4) or combinations thereof. T-independent BCR/TLR9 co-stimulation, which drives malignant and autoimmune B-cell states, jointly induced PD-L1 plasma membrane expression, supported by NAD metabolism and oxidative phosphorylation. BCR/TLR9 also highly induced the transaminase BCAT1, which localized to lysosomal membranes to support branched chain amino acid synthesis and mTORC1 hyperactivation. BCAT1 inhibition blunted BCR/TLR9, but not CD40L/IL4-triggered B-cell proliferation, IL10 expression and BCR/TLR pathway-driven lymphoma xenograft outgrowth. These results provide a valuable resource, reveal receptor-mediated immunometabolism remodeling to support key B-cell phenotypes including PD-L1 checkpoint signaling, and identify BCAT1 as a novel B-cell therapeutic target.

CPT2 mediated fatty acid oxidation is dispensable for humoral immunity

B cell activation is accompanied by dynamic metabolic reprogramming, supported by a multitude of nutrients that include glucose, amino acids and fatty acids. While several studies have indicated that fatty acid mitochondrial oxidation is critical for immune cell functions, contradictory findings have been reported. Carnitine palmitoyltransferase II (CPT2) is a critical enzyme for long-chain fatty acid oxidation in mitochondria. Here, we test the requirement of CPT2 for humoral immunity using a mouse model with a lymphocyte specific deletion of CPT2. Stable 13C isotope tracing reveals highly reduced fatty acid-derived citrate production in CPT2 deficient B cells. Yet, CPT2 deficiency has no significant impact on B cell development, B cell activation, germinal center formation, and antibody production upon either thymus-dependent or -independent antigen challenges. Together, our findings indicate that CPT2 mediated fatty acid oxidation is dispensable for humoral immunity, highlighting the metabolic flexibility of lymphocytes.

Blocking Orai1 constitutive activity inhibits B-cell cancer migration and synergistically acts with drugs to reduce B-CLL cell survival

Orai1 channel capacity to control store-operated Ca2+ entry (SOCE) and B-cell functions is poorly understood and more specifically in B-cell cancers, including human lymphoma and leukemia. As compared to normal B-cells, Orai1 is overexpressed in B-chronic lymphocytic leukemia (B-CLL) and contributes in resting B-CLL to mediate an elevated basal Ca2+ level through a constitutive Ca2+ entry, and in BCR-activated B-cell to regulate the Ca2+ signaling response. Such observations were confirmed in human B-cell lymphoma and leukemia lines, including RAMOS, JOK-1, MEC-1 and JVM-3 cells. Next, the use of pharmacological Orai1 inhibitors (GSK-7975 A and Synta66) blocks constitutive Ca2+ entry and in turn affects B-cell cancer (primary and cell lines) survival and migration, controls cell cycle, and induces apoptosis through a mitochondrial and caspase-3 independent pathway. Finally, the added value of Orai1 inhibitors in combination with B-CLL drugs (ibrutinib, idelalisib, rituximab, and venetoclax) on B-CLL survival was tested, showing an additive/synergistic effect including in the B-cell cancer lines. To conclude, this study highlights the pathophysiological role of the Ca2+ channel Orai1 in B-cell cancers, and pave the way for the use of ORAI1 modulators as a plausible therapeutic strategy.

IL-4 Licenses B Cell Activation Through Cholesterol Synthesis

Lymphocyte activation involves a transition from quiescence and associated catabolic metabolism to a metabolic state with noted similarities to cancer cells such as heavy reliance on aerobic glycolysis for energy demands and increased nutrient requirements for biomass accumulation and cell division 1-3 . Following antigen receptor ligation, lymphocytes require spatiotemporally distinct "second signals". These include costimulatory receptor or cytokine signaling, which engage discrete programs that often involve remodeling of organelles and increased nutrient uptake or synthesis to meet changing biochemical demands 4-6 . One such signaling molecule, IL-4, is a highly pleiotropic cytokine that was first identified as a B cell co-mitogen over 30 years ago 7 . However, how IL-4 signaling mechanistically supports B cell proliferation is incompletely understood. Here, using single cell RNA sequencing we find that the cholesterol biosynthetic program is transcriptionally upregulated following IL-4 signaling during the early B cell response to influenza virus infection, and is required for B cell activation in vivo . By limiting lipid availability in vitro , we determine cholesterol to be essential for B cells to expand their endoplasmic reticulum, progress through cell cycle, and proliferate. In sum, we demonstrate that the well-known ability of IL-4 to act as a B cell growth factor is through a previously unknown rewiring of specific lipid anabolic programs, relieving sensitivity of cells to environmental nutrient availability.

Oxidative phosphorylation regulates B cell effector cytokines and promotes inflammation in multiple sclerosis

Dysregulated B cell cytokine production contributes to pathogenesis of immune-mediated diseases including multiple sclerosis (MS); however, the underlying mechanisms are poorly understood. In this study we investigated how cytokine secretion by pro-inflammatory (GM-CSF-expressing) and anti-inflammatory (IL-10-expressing) B cells is regulated. Pro-inflammatory human B cells required increased oxidative phosphorylation (OXPHOS) compared with anti-inflammatory B cells. OXPHOS reciprocally modulated pro- and anti-inflammatory B cell cytokines through regulation of adenosine triphosphate (ATP) signaling. Partial inhibition of OXPHOS or ATP-signaling including with BTK inhibition resulted in an anti-inflammatory B cell cytokine shift, reversed the B cell cytokine imbalance in patients with MS, and ameliorated neuroinflammation in a myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalitis mouse model. Our study identifies how pro- and anti-inflammatory cytokines are metabolically regulated in B cells and identifies ATP and its metabolites as a "fourth signal" that shapes B cell responses and is a potential target for restoring the B cell cytokine balance in autoimmune diseases.

Therapeutic vaccine-induced plasma cell differentiation is defective in the presence of persistently high HBsAg levels

BACKGROUND & AIMS

Mechanisms behind the impaired response of antigen-specific B cells to therapeutic vaccination in chronic hepatitis B virus (HBV) infection remain unclear. The development of vaccines or strategies to overcome this obstacle is vital for advancing the management of chronic hepatitis B.

METHODS

A mouse model, denominated as E6F6-B, was engineered to feature a knock-in of a B-cell receptor (BCR) that specifically recognizes HBsAg. This model served as a valuable tool for investigating the temporal and spatial dynamics of humoral responses following therapeutic vaccination under continuous antigen exposure. Using a suite of immunological techniques, we elucidated the differentiation trajectory of HBsAg-specific B cells post-therapeutic vaccination in HBV carrier mice.

RESULTS

Utilizing the E6F6-B transfer model, we observed a marked decline in antibody-secreting cells 2 weeks after vaccination. A dysfunctional and atypical pre-plasma cell population (BLIMP-1+ IRF4+ CD40- CD138- BCMA-) emerged, manifested by sustained BCR signaling. By deploying an antibody to purge persistent HBsAg, we effectively prompted the therapeutic vaccine to provoke conventional plasma cell differentiation. This resulted in an enhanced anti-HBs antibody response and facilitated HBsAg clearance.

CONCLUSIONS

Sustained high levels of HBsAg limit the ability of therapeutic hepatitis B vaccines to induce the canonical plasma cell differentiation necessary for anti-HBs antibody production. Employing a strategy combining antibodies with vaccines can surmount this altered humoral response associated with atypical pre-plasma cells, leading to improved therapeutic efficacy in HBV carrier mice.

IMPACT AND IMPLICATIONS

Therapeutic vaccines aimed at combatting HBV encounter suboptimal humoral responses in clinical settings, and the mechanisms impeding their effectiveness have remained obscure. Our research, utilizing the innovative E6F6-B mouse transfer model, reveals that the persistence of HBsAg can lead to the emergence of an atypical pre-plasma cell population, which proves to be relevant to the potency of therapeutic HBV vaccines. Targeting the aberrant differentiation process of these atypical pre-plasma cells stands out as a critical strategy to amplify the humoral response elicited by HBV therapeutic vaccines in carrier mouse models. This discovery suggests a compelling avenue for further study in the context of human chronic hepatitis B. Encouragingly, our findings indicate that synergistic therapy combining HBV-specific antibodies with vaccines offers a promising approach that could significantly advance the pursuit of a functional cure for HBV.

Single-B cell analysis correlates high-lactate secretion with stress and increased apoptosis

While cellular metabolism was proposed to be a driving factor of the activation and differentiation of B cells and the function of the resulting antibody-secreting cells (ASCs), the study of correlations between cellular metabolism and functionalities has been difficult due to the absence of technologies enabling the parallel measurement. Herein, we performed single-cell transcriptomics and introduced a direct concurrent functional and metabolic flux quantitation of individual murine B cells. Our transcriptomic data identified lactate metabolism as dynamic in ASCs, but antibody secretion did not correlate with lactate secretion rates (LSRs). Instead, our study of all splenic B cells during an immune response linked increased lactate metabolism with acidic intracellular pH and the upregulation of apoptosis. T cell-dependent responses increased LSRs, and added TLR4 agonists affected the magnitude and boosted LSRhigh B cells in vivo, while resulting in only a few immunoglobulin-G secreting cells (IgG-SCs). Therefore, our observations indicated that LSRhigh cells were not differentiating into IgG-SCs, and were rather removed due to apoptosis.

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.

DNAJA3 regulates B cell development and immune function

BACKGROUND

DnaJ homolog subfamily A member 3 (DNAJA3), also known as the tumorous imaginal disc (Tid1), is shown to be crucial in T cell development. DNAJA3 functions as a tumor suppressor implicated in lymphocyte development and survival. However, the role of DNAJA3 in B cell development and immune function remains unknown. In this study, we utilized a mouse model of B cell-specific DNAJA3 knockout (CD19-Cre/+; DNAJA3flx/flx) to investigate the physiological function of DNAJA3 in B cell development and immune function.

METHODS

We characterized B cell populations in various developmental stages and examined mitochondrial content and function between control and DNAJA3 KO using flow cytometry analysis. DNAJA3 and OXPHOS protein complexes in sorted B cells between mice groups were compared using immunoblot techniques. The activity of B cell blastogenesis in splenocytes was measured by performing CFSE and MTT assays. Furthermore, immunoglobulin production was detected using the ELISA method.

RESULTS

DNAJA3 deficiency decreases from pro B cells to immature B cells. The overall B220+ population in the bone marrow and secondary immune organs also decreased. B cell subpopulations B1 (B1b) and B2 significantly decrease. The B cell blastogenesis activity and immunoglobulin production decreased in DNAJA3 KO mice. Mechanistically, DNAJA3 deficiency significantly increases dysfunctional mitochondria activity and decreases mitochondrial mass, membrane potential, and mitochondria respiratory complex proteins. These factors could have influenced B cell differentiation during development, differentiation to antibody-secreting cells, and immune activation.

CONCLUSION

Overall, our study provides supportive evidence for the role of DNAJA3 in B cell development and function.

Factors Governing B Cell Recognition of Autoantigen and Function in Type 1 Diabetes

Islet autoantibodies predict type 1 diabetes (T1D) but can be transient in murine and human T1D and are not thought to be directly pathogenic. Rather, these autoantibodies signal B cell activity as antigen-presenting cells (APCs) that present islet autoantigen to diabetogenic T cells to promote T1D pathogenesis. Disrupting B cell APC function prevents T1D in mouse models and has shown promise in clinical trials. Autoantigen-specific B cells thus hold potential as sophisticated T1D biomarkers and therapeutic targets. B cell receptor (BCR) somatic hypermutation is a mechanism by which B cells increase affinity for islet autoantigen. High-affinity B and T cell responses are selected in protective immune responses, but immune tolerance mechanisms are known to censor highly autoreactive clones in autoimmunity, including T1D. Thus, different selection rules often apply to autoimmune disease settings (as opposed to protective host immunity), where different autoantigen affinity ceilings are tolerated based on variations in host genetics and environment. This review will explore what is currently known regarding B cell signaling, selection, and interaction with T cells to promote T1D pathogenesis.

Monophosphoryl Lipid A-based Adjuvant to Promote the Immunogenicity of Multivalent Meningococcal Polysaccharide Conjugate Vaccines

Activation of the adaptive immune system requires the engagement of costimulatory pathways in addition to B and T cell Ag receptor signaling, and adjuvants play a central role in this process. Many Gram-negative bacterial polysaccharide vaccines, including the tetravalent meningococcal conjugate vaccines (MCV4) and typhoid Vi polysaccharide vaccines, do not incorporate adjuvants. The immunogenicity of typhoid vaccines is due to the presence of associated TLR4 ligands in these vaccines. Because the immunogenicity of MCV4 is poor and requires boosters, I hypothesized that TLR4 ligands are absent in MCV4 and that incorporation of a TLR4 ligand-based adjuvant would improve their immunogenicity. Consistent with this hypothesis, two Food and Drug Administration-approved MCV4 vaccines, MENVEO and MenQuadfi, lack TLR4 ligands. Admixing monophosphoryl lipid A, a TLR4 ligand-based adjuvant formulation named "Turbo" with MCV4 induced significantly improved IgM and IgG responses to all four meningococcal serogroup polysaccharides in adult and aged mice after a single immunization. Furthermore, in infant mice, a single booster was sufficient to promote a robust IgG response and 100% seroconversion when MCV4 was adjuvanted with Turbo. Turbo upregulated the expression of the costimulatory molecules CD40 and CD86 on B cells, and Turbo-driven adjuvanticity is lost in mice deficient in CD40 and CD86. These data suggest that Turbo induces the required costimulatory molecules for its adjuvant activity and that incorporation of Turbo could make bacterial polysaccharide vaccines more immunogenic, minimize booster requirements, and be cost-effective, particularly for those individuals in low- and middle-income and disease-endemic countries.

Rag-GTPase-TFEB/TFE3 axis controls B cell mitochondrial fitness and humoral immunity independent of mTORC1

During the humoral immune response, B cells undergo rapid metabolic reprogramming with a high demand for nutrients, which are vital to sustain the formation of the germinal centers (GCs). Rag-GTPases sense amino acid availability to modulate the mechanistic target of rapamycin complex 1 (mTORC1) pathway and suppress transcription factor EB (TFEB) and transcription factor enhancer 3 (TFE3), members of the microphthalmia (MiT/TFE) family of HLH-leucine zipper transcription factors. However, how Rag-GTPases coordinate amino acid sensing, mTORC1 activation, and TFEB/TFE3 activity in humoral immunity remains undefined. Here, we show that B cell-intrinsic Rag-GTPases are critical for the development and activation of B cells. RagA/RagB deficient B cells fail to form GCs, produce antibodies, and generate plasmablasts in both T-dependent (TD) and T-independent (TI) humoral immune responses. Deletion of RagA/RagB in GC B cells leads to abnormal dark zone (DZ) to light zone (LZ) ratio and reduced affinity maturation. Mechanistically, the Rag-GTPase complex constrains TFEB/TFE3 activity to prevent mitophagy dysregulation and maintain mitochondrial fitness in B cells, which are independent of canonical mTORC1 activation. TFEB/TFE3 deletion restores B cell development, GC formation in Peyer's patches and TI humoral immunity, but not TD humoral immunity in the absence of Rag-GTPases. Collectively, our data establish Rag-GTPase-TFEB/TFE3 axis as an mTORC1 independent mechanism to coordinating nutrient sensing and mitochondrial metabolism in B cells.

RNA modifications in cellular metabolism: implications for metabolism-targeted therapy and immunotherapy

Cellular metabolism is an intricate network satisfying bioenergetic and biosynthesis requirements of cells. Relevant studies have been constantly making inroads in our understanding of pathophysiology, and inspiring development of therapeutics. As a crucial component of epigenetics at post-transcription level, RNA modification significantly determines RNA fates, further affecting various biological processes and cellular phenotypes. To be noted, immunometabolism defines the metabolic alterations occur on immune cells in different stages and immunological contexts. In this review, we characterize the distribution features, modifying mechanisms and biological functions of 8 RNA modifications, including N6-methyladenosine (m6A), N6,2'-O-dimethyladenosine (m6Am), N1-methyladenosine (m1A), 5-methylcytosine (m5C), N4-acetylcytosine (ac4C), N7-methylguanosine (m7G), Pseudouridine (Ψ), adenosine-to-inosine (A-to-I) editing, which are relatively the most studied types. Then regulatory roles of these RNA modification on metabolism in diverse health and disease contexts are comprehensively described, categorized as glucose, lipid, amino acid, and mitochondrial metabolism. And we highlight the regulation of RNA modifications on immunometabolism, further influencing immune responses. Above all, we provide a thorough discussion about clinical implications of RNA modification in metabolism-targeted therapy and immunotherapy, progression of RNA modification-targeted agents, and its potential in RNA-targeted therapeutics. Eventually, we give legitimate perspectives for future researches in this field from methodological requirements, mechanistic insights, to therapeutic applications.

Dysfunctional mitochondria, disrupted levels of reactive oxygen species, and autophagy in B cells from common variable immunodeficiency patients

Introduction

Common Variable Immunodeficiency (CVID) patients are characterized by hypogammaglobulinemia and poor response to vaccination due to deficient generation of memory and antibody-secreting B cells. B lymphocytes are essential for the development of humoral immune responses, and mitochondrial function, hreactive oxygen species (ROS) production and autophagy are crucial for determining B-cell fate. However, the role of those basic cell functions in the differentiation of human B cells remains poorly investigated.

Methods

We used flow cytometry to evaluate mitochondrial function, ROS production and autophagy processes in human naïve and memory B-cell subpopulations in unstimulated and stimulated PBMCs cultures. We aimed to determine whether any alterations in these processes could impact B-cell fate and contribute to the lack of B-cell differentiation observed in CVID patients.

Results

We described that naïve CD19+CD27- and memory CD19+CD27+ B cells subpopulations from healthy controls differ in terms of their dependence on these processes for their homeostasis, and demonstrated that different stimuli exert a preferential cell type dependent effect. The evaluation of mitochondrial function, ROS production and autophagy in naïve and memory B cells from CVID patients disclosed subpopulation specific alterations. Dysfunctional mitochondria and autophagy were more prominent in unstimulated CVID CD19+CD27- and CD19+CD27+ B cells than in their healthy counterparts. Although naïve CD19+CD27- B cells from CVID patients had higher basal ROS levels than controls, their ROS increase after stimulation was lower, suggesting a disruption in ROS homeostasis. On the other hand, memory CD19+CD27+ B cells from CVID patients had both lower ROS basal levels and a diminished ROS production after stimulation with anti-B cell receptor (BCR) and IL-21.

Conclusion

The failure in ROS cell signalling could impair CVID naïve B cell activation and differentiation to memory B cells. Decreased levels of ROS in CVID memory CD19+CD27+ B cells, which negatively correlate with their in vitro cell death and autophagy, could be detrimental and lead to their previously demonstrated premature death. The final consequence would be the failure to generate a functional B cell compartment in CVID patients.

Isotype switching in human memory B cells sets intrinsic antigen-affinity thresholds that dictate antigen-driven fates

Memory B cells (MBCs) play a critical role in protection against homologous and variant pathogen challenge by either differentiating to plasma cells (PCs) or to germinal center (GC) B cells. The human MBC compartment contains both switched IgG+ and unswitched IgM+ MBCs; however, whether these MBC subpopulations are equivalent in their response to B cell receptor cross-linking and their resulting fates is incompletely understood. Here, we show that IgG+ and IgM+ MBCs can be distinguished based on their response to κ-specific monoclonal antibodies of differing affinities. IgG+ MBCs responded only to high-affinity anti-κ and differentiated almost exclusively toward PC fates. In contrast, IgM+ MBCs were eliminated by apoptosis by high-affinity anti-κ but responded to low-affinity anti-κ by differentiating toward GC B cell fates. These results suggest that IgG+ and IgM+ MBCs may play distinct yet complementary roles in response to pathogen challenge ensuring the immediate production of high-affinity antibodies to homologous and closely related challenges and the generation of variant-specific MBCs through GC reactions.

The nutrient-sensing Rag-GTPase complex in B cells controls humoral immunity via TFEB/TFE3-dependent mitochondrial fitness

During the humoral immune response, B cells undergo rapid metabolic reprogramming with a high demand for nutrients, which are vital to sustain the formation of the germinal centers (GCs). Rag-GTPases sense amino acid availability to modulate the mechanistic target of rapamycin complex 1 (mTORC1) pathway and suppress transcription factor EB (TFEB) and transcription factor enhancer 3 (TFE3), members of the microphthalmia (MiT/TFE) family of HLH-leucine zipper transcription factors. However, how Rag-GTPases coordinate amino acid sensing, mTORC1 activation, and TFEB/TFE3 activity in humoral immunity remains undefined. Here, we show that B cell-intrinsic Rag-GTPases are critical for the development and activation of B cells. RagA/RagB deficient B cells fail to form GCs, produce antibodies, and generate plasmablasts in both T-dependent (TD) and T-independent (TI) humoral immune responses. Deletion of RagA/RagB in GC B cells leads to abnormal dark zone (DZ) to light zone (LZ) ratio and reduced affinity maturation. Mechanistically, the Rag-GTPase complex constrains TFEB/TFE3 activity to prevent mitophagy dysregulation and maintain mitochondrial fitness in B cells, which are independent of canonical mTORC1 activation. TFEB/TFE3 deletion restores B cell development, GC formation in Peyer's patches and TI humoral immunity, but not TD humoral immunity in the absence of Rag-GTPases. Collectively, our data establish Rag-GTPase-TFEB/TFE3 pathway as an mTORC1 independent mechanism to coordinating nutrient sensing and mitochondrial metabolism in B 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.

Metabolic reprogramming in the CLL TME; potential for new therapeutic targets

Chronic lymphocytic leukemia (CLL) cells circulate between peripheral (PB) blood and lymph node (LN) compartments, and strictly depend on microenvironmental factors for proliferation, survival and drug resistance. All cancer cells display metabolic reprogramming and CLL is no exception - though the inert status of the PB CLL cells has hampered detailed insight into these processes. We summarize previous work on reactive oxygen species (ROS), oxidative stress, and hypoxia, as well as the important roles of Myc, and PI3K/Akt/mTor pathways. In vitro co-culture systems and gene expression analyses have provided a partial picture of CLL LN metabolism. New broad omics techniques allow to obtain molecular and also single-cell level understanding of CLL plasticity and metabolic reprogramming. We summarize recent developments and describe the new concept of glutamine addiction for CLL, which may hold therapeutic promise.

New insights into the ontogeny, diversity, maturation and survival of long-lived plasma cells

Plasma cells are unique immune effectors, capable of producing large amounts of high-affinity antibodies that protect against pathogenic infections. Although most plasma cells have short lifespans, certain conditions or vaccinations can give rise to long-lived plasma cells (LLPCs) that provide individuals with lifelong protection against pathogen exposure. The nature of these LLPCs is poorly understood; however, recent studies have shed new light on the ontogeny, diversity, maturation and survival of these unique cells. Whereas LLPCs had been thought to arise preferentially from germinal centres, novel genetic tools have revealed that they can originate from various stages throughout the humoral response. Furthermore, new single-cell analyses have shown that mouse and human plasma cells are heterogeneous and may undergo further maturation in situ in the bone marrow niche. Finally, plasma cells were previously considered to be sessile cells maintained in fixed survival niches, but new data show that plasma cell subsets can differentially migrate and organize into clusters that may be associated with survival niches. These descriptive findings provide new insights into how cell-intrinsic programmes and extrinsic factors may regulate the longevity of plasma cells in various contexts, which suggest new research avenues for their functional validation.

In vitro B cell experiments explore the role of CD24, CD38, and energy metabolism in ME/CFS

Introduction

Disturbances of energy metabolism contribute to the clinical manifestations of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Previously, we found that B cells from ME/CFS patients have an increased expression of CD24, a modulator of many cellular functions including those of cell stress. The relative ability of B cells from ME/CFS patients and healthy controls (HC) to respond to rapid changes in energy demand was compared.

Methods

CD24, the ectonucleotidases CD39 and CD73, the NAD-degrading enzyme CD38, and mitochondrial mass (MM) were measured following cross-linking of the B cell receptor and costimulation with either T-cell-dependent or Toll-like-receptor-9-dependent agonists. The levels of metabolites consumed/produced were measured using 1H-NMR spectroscopy and analyzed in relation to cell growth and immunophenotype.

Results

Proliferating B cells from patients with ME/CFS showed a lower mitochondrial mass and a significantly increased usage of essential amino acids compared with those from HC, with a significantly delayed loss of CD24 and an increased expression of CD38 following stimulation.

Discussion

The immunophenotype results suggested the triggering of a stress response in ME/CFS B cells associated with the increased usage of additional substrates to maintain necessary ATP levels. Disturbances in energy metabolism in ME/CFS B cells were thus confirmed in a dynamic in vitro model, providing the basis for further mechanistic investigations.

Sending positive signals and good (calcium) vibes

In this issue of JEM, Yada et al. (https://doi.org/10.1084/jem.20222178) demonstrate that effective antibody affinity selection in germinal centers relies on the store-operated calcium entry (SOCE) component of the B cell receptor (BCR) signaling network. Therefore, active BCR signaling is as relevant to positive selection as the function of BCRs as endocytic receptors, answering a question that had puzzled experts for a while. These findings transform our understanding of the mechanisms supporting adaptive immune responses (to vaccines, for example) and have important implications for interpreting the genomics and pathogenesis of germinal center-derived B cell lymphomas.

Glucose metabolism in B cell malignancies: a focus on glycolysis branching pathways

Glucose catabolism, one of the essential pathways sustaining cellular bioenergetics, has been widely studied in the context of tumors. Nevertheless, the function of various branches of glucose metabolism that stem from 'classical' glycolysis have only been partially explored. This review focuses on discussing general mechanisms and pathological implications of glycolysis and its branching pathways in the biology of B cell malignancies. We summarize here what is known regarding pentose phosphate, hexosamine, serine biosynthesis, and glycogen synthesis pathways in this group of tumors. Despite most findings have been based on malignant B cells themselves, we also discuss the role of glucose metabolism in the tumor microenvironment, with a focus on T cells. Understanding the contribution of glycolysis branching pathways and how they are hijacked in B cell malignancies will help to dissect the role they have in sustaining the dissemination and proliferation of tumor B cells and regulating immune responses within these tumors. Ultimately, this should lead to deciphering associated vulnerabilities and improve current therapeutic schedules.

Molecular Mechanisms Underpinning Immunometabolic Reprogramming: How the Wind Changes during Cancer Progression

Metabolism and the immunological state are intimately intertwined, as defense responses are bioenergetically expensive. Metabolic homeostasis is a key requirement for the proper function of immune cell subsets, and the perturbation of the immune-metabolic balance is a recurrent event in many human diseases, including cancer, due to nutrient fluctuation, hypoxia and additional metabolic changes occurring in the tumor microenvironment (TME). Although much remains to be understood in the field of immunometabolism, here, we report the current knowledge on both physiological and cancer-associated metabolic profiles of immune cells, and the main molecular circuits involved in their regulation, highlighting similarities and differences, and emphasizing immune metabolic liabilities that could be exploited in cancer therapy to overcome immune resistance.

COMO: a pipeline for multi-omics data integration in metabolic modeling and drug discovery

Identifying potential drug targets using metabolic modeling requires integrating multiple modeling methods and heterogeneous biological datasets, which can be challenging without efficient tools. We developed Constraint-based Optimization of Metabolic Objectives (COMO), a user-friendly pipeline that integrates multi-omics data processing, context-specific metabolic model development, simulations, drug databases and disease data to aid drug discovery. COMO can be installed as a Docker Image or with Conda and includes intuitive instructions within a Jupyter Lab environment. It provides a comprehensive solution for the integration of bulk and single-cell RNA-seq, microarrays and proteomics outputs to develop context-specific metabolic models. Using public databases, open-source solutions for model construction and a streamlined approach for predicting repurposable drugs, COMO enables researchers to investigate low-cost alternatives and novel disease treatments. As a case study, we used the pipeline to construct metabolic models of B cells, which simulate and analyze them to predict metabolic drug targets for rheumatoid arthritis and systemic lupus erythematosus, respectively. COMO can be used to construct models for any cell or tissue type and identify drugs for any human disease where metabolic inhibition is relevant. The pipeline has the potential to improve the health of the global community cost-effectively by providing high-confidence targets to pursue in preclinical and clinical studies. The source code of the COMO pipeline is available at https://github.com/HelikarLab/COMO. The Docker image can be pulled at https://github.com/HelikarLab/COMO/pkgs/container/como.

MAVS Positively Regulates Mitochondrial Integrity and Metabolic Fitness in B Cells

Activated B cells experience metabolic changes that require mitochondrial remodeling, in a process incompletely defined. In this study, we report that mitochondrial antiviral signaling protein (MAVS) is involved in BCR-initiated cellular proliferation and prolonged survival. MAVS is well known as a mitochondrial-tethered signaling adaptor with a central role in viral RNA-sensing pathways that induce type I IFN. The role of MAVS downstream of BCR stimulation was recognized in absence of IFN, indicative of a path for MAVS activation that is independent of viral infection. Mitochondria of BCR-activated MAVS-deficient mouse B cells exhibited a damaged phenotype including disrupted mitochondrial morphology, excess mitophagy, and the temporal progressive blunting of mitochondrial oxidative capacity with mitochondrial hyperpolarization and cell death. Costimulation of MAVS-deficient B cells with anti-CD40, in addition to BCR stimulation, partially corrected the mitochondrial structural defects and functionality. Our data reveal a (to our knowledge) previously unrecognized role of MAVS in controlling the metabolic fitness of B cells, most noticeable in the absence of costimulatory help.

'Persistent germinal center responses: slow-growing trees bear the best fruits'

Germinal centers (GCs) are key microanatomical sites in lymphoid organs where responding B cells mature and undergo affinity-based selection. The duration of the GC reaction has long been assumed to be relatively brief, but recent studies in humans, nonhuman primates, and mice indicate that GCs can last for weeks to months after initial antigen exposure. This review examines recent studies investigating the factors that influence GC duration, including antigen persistence, T-follicular helper cells, and mode of immunization. Potential mechanisms for how persistent GCs influence the B-cell repertoire are considered. Overall, these studies provide a blueprint for how to design better vaccines that elicit persistent GC responses.

Immunology of hepatitis D virus infection: General concepts and present evidence

Infection with the hepatitis D virus induces the most severe form of chronic viral hepatitis, affecting over 12 million people worldwide. Chronic HDV infection leads to rapid development of liver cirrhosis and hepatocellular carcinoma in ~70% of patients within 15 years of infection. Recent evidence suggests that an interplay of different components of the immune system are contributing to viral control and may even be implicated in liver disease pathogenesis. This review will describe general concepts of antiviral immune response and elicit the present evidence concerning the interplay of the hepatitis D virus with the immune system.

Immunometabolism changes in fibrosis: from mechanisms to therapeutic strategies

Immune cells are essential for initiating and developing the fibrotic process by releasing cytokines and growth factors that activate fibroblasts and promote extracellular matrix deposition. Immunometabolism describes how metabolic alterations affect the function of immune cells and how inflammation and immune responses regulate systemic metabolism. The disturbed immune cell function and their interactions with other cells in the tissue microenvironment lead to the origin and advancement of fibrosis. Understanding the dysregulated metabolic alterations and interactions between fibroblasts and the immune cells is critical for providing new therapeutic targets for fibrosis. This review provides an overview of recent advances in the pathophysiology of fibrosis from the immunometabolism aspect, highlighting the altered metabolic pathways in critical immune cell populations and the impact of inflammation on fibroblast metabolism during the development of fibrosis. We also discuss how this knowledge could be leveraged to develop novel therapeutic strategies for treating fibrotic diseases.

Mechanotransduction governs CD40 function and underlies X-linked Hyper IgM syndrome

B cell maturation in germinal centers (GCs) depends on cognate interactions between the T and B cells. Upon interaction with CD40 ligand (CD40L) on T cells, CD40 delivers co-stimulatory signals alongside B cell antigen receptor (BCR) signaling to regulate affinity maturation and antibody class-switch during GC reaction. Mutations in CD40L disrupt interactions with CD40, which lead to abnormal antibody responses in immune deficiencies known as X-linked Hyper IgM syndrome (X-HIgM). Assuming that physical interactions between highly mobile T and B cells generate mechanical forces on CD40-CD40L bonds, we set out to study the B cell mechanobiology mediated by CD40-CD40L interaction. Using a suite of biophysical assays we find that CD40 forms catch bond with CD40L where the bond lasts longer at larger forces, B cells exert tension on CD40-CD40L bonds, and force enhances CD40 signaling and antibody class-switch. Significantly, X-HIgM CD40L mutations impair catch bond formation, suppress endogenous tension, and reduce force-enhanced CD40 signaling, leading to deficiencies in antibody class switch. Our findings highlight the critical role of mechanotransduction in CD40 function and provide insights into the molecular mechanisms underlying X-HIgM syndrome.

Mitochondria-related genes and metabolic profiles of innate and adaptive immune cells in primary Sjögren's syndrome

Background

Primary Sjogren's syndrome (pSS) is a prototypical systemic autoimmune disease characterised by lymphocyte infiltration and immune-complex deposition in multiple organs. The specific distribution of immune cell populations and their relationship with mitochondria remain unknown.

Methods

Histological analysis was performed to assess the specific distribution of innate and adaptive immune cell populations in labial salivary gland (LSG) samples from 30 patients with pSS and 13 patients with non-pSS. The ultrastructural morphometric features of mitochondria within immune cells were observed under the transmission electron microscope (TEM). RNA sequencing was performed on LSG samples from 40 patients with pSS and 7 non-pSS patients. The Single-sample Gene Set Enrichment Analysis (ssGSEA), ESTIMATE, and CIBERSORT algorithms and Pearson correlation coefficients were used to examine the relationship between mitochondria-related genes and immune infiltration. Weighted Gene Co-expression Network Analysis (WGCNA) was used to identify the mitochondria-specific genes and the related pathways based on the immune cell types.

Results

HE staining revealed a massive infiltration of plasma cells with abundant immunoglobulin protein distributed around phenotypically normal-appearing acinar and ductal tissues of patients with pSS. Immunohistochemical analyses revealed that innate immune cells (macrophages, eosinophils and NK cells) were distributed throughout the glandular tissue. Dominant adaptive immune cell infiltration composed of B cells, CD4⁺T cells and CD8⁺ T cells or ectopic lymphoid follicle-like structures were observed in the LSGs of patients with pSS. TEM validated the swelling of mitochondria with disorganised cristae in some lymphocytes that had invaded the glandular tissue. Subsequently, bioinformatic analysis revealed that innate and adaptive immune cells were associated with different mitochondrial metabolism pathways. Mitochondrial electron transport and respiratory chain complexes in the glandular microenvironment were positively correlated with innate immune cells, whereas amino acid and nucleic acid metabolism were negatively correlated with adaptive immune cells. In addition, mitochondrial biogenesis and mitochondrial apoptosis in the glandular microenvironment were closely associated with adaptive immune cells.

Conclusion

Innate and adaptive immune cells have distinct distribution profiles in the salivary gland tissues of patients with pSS and are associated with different mitochondrial metabolic pathways, which may contribute to disease progression.

IL-27 enhances peripheral B cell glycolysis of rheumatoid arthritis patients via activating mTOR signaling

Our previous study found that increased serum IL-27 could promote rheumatoid arthritis (RA) B cell dysfunction via activating mTOR signaling pathway. This study aimed to explore the effects of IL-27 on B cell metabolism and clarify the mechanisms via which IL-27 enhancing glycolysis to induce B cells hyperactivation. Peripheral CD19⁺ B cells were purified from healthy controls (HC) and RA patients and then cultured with or without anti-CD40/CpG and glycolysis inhibitor 2-deoxy-D-glucose (2-DG) or mTOR inhibitor rapamycin. Furthermore, the isolated CD19⁺ B cells were treated by HC serum or RA serum in the presence and absence of recombinant human IL-27 or anti-IL-27 neutralizing antibodies or 2-DG or rapamycin. The B cell glycolysis level, proliferation, differentiation and inflammatory actions were detected by qPCR, flow cytometry or ELISA. We found that the glycolysis in RA B cells was increased significantly compared with HC B cells. Glycolysis inhibition downregulated the proliferation, differentiation, and inflammatory actions of RA B cells. RA serum and IL-27 promoted B cell glycolysis, which could be obviously rescued by anti-IL-27 antibodies or mTOR inhibitor rapamycin. Our results suggest that the enhanced cellular glycolysis of RA B cells induced by IL-27 may contribute to B cells hyperactivation through activating the mTOR signaling pathway.

Attempts to evaluate locus suicide recombination and its potential role in B cell negative selection in the mouse

Introduction

In mature B cells, activation-induced deaminase reshapes Ig genes through somatic hypermutation and class switch recombination of the Ig heavy chain (IgH) locus under control of its 3' cis-regulatory region (3'RR). The 3'RR is itself transcribed and can undergo "locus suicide recombination" (LSR), then deleting the constant gene cluster and terminating IgH expression. The relative contribution of LSR to B cell negative selection remains to be determined.

Methods

Here, we set up a knock-in mouse reporter model for LSR events with the aim to get clearer insights into the circumstances triggering LSR. In order to explore the consequences of LSR defects, we reciprocally explored the presence of autoantibodies in various mutant mouse lines in which LSR was perturbed by the lack of Sµ or of the 3'RR.

Results

Evaluation of LSR events in a dedicated reporter mouse model showed their occurrence in various conditions of B cell activation, notably in antigen-experienced B cells Studies of mice with LSR defects evidenced increased amounts of self-reactive antibodies.

Discussion

While the activation pathways associated with LSR are diverse, in vivo as well as in vitro, this study suggests that LSR may contribute to the elimination of self-reactive B cells.

Dynamic mitochondrial transcription and translation in B cells control germinal center entry and lymphomagenesis

Germinal center (GC) B cells undergo proliferation at very high rates in a hypoxic microenvironment but the cellular processes driving this are incompletely understood. Here we show that the mitochondria of GC B cells are highly dynamic, with significantly upregulated transcription and translation rates associated with the activity of transcription factor A, mitochondrial (TFAM). TFAM, while also necessary for normal B cell development, is required for entry of activated GC precursor B cells into the germinal center reaction; deletion of Tfam significantly impairs GC formation, function and output. Loss of TFAM in B cells compromises the actin cytoskeleton and impairs cellular motility of GC B cells in response to chemokine signaling, leading to their spatial disorganization. We show that B cell lymphoma substantially increases mitochondrial translation and that deletion of Tfam in B cells is protective against the development of lymphoma in a c-Myc transgenic mouse model. Finally, we show that pharmacological inhibition of mitochondrial transcription and translation inhibits growth of GC-derived human lymphoma cells and induces similar defects in the actin cytoskeleton.

Enhanced B Cell Receptor Signaling Partially Compensates for Impaired Toll-like Receptor 4 Responses in LPS-Stimulated IκBNS-Deficient B Cells

Lipopolysaccharide (LPS) stimulates dual receptor signaling by bridging the B cell receptor and Toll-like receptor 4 (BCR/TLR4). B cells from IκBNS-deficient bumble mice treated with LPS display reduced proliferative capacity and impaired plasma cell differentiation. To improve our understanding of the regulatory role of IκBNS in B cell activation and differentiation, we investigated the BCR and TLR4 signaling pathways separately by using dimeric anti-IgM Fab (F(ab')₂) or lipid A, respectively. IκBNS-deficient B cells exhibited reduced survival and defective proliferative capacity in response to lipid A compared to B cells from wildtype (wt) control mice. In contrast, anti-IgM stimulation of bumble B cells resulted in enhanced viability and increased differentiation into CD138⁺ cells compared to control B cells. Anti-IgM-stimulated IκBNS-deficient B cells also showed enhanced cycle progression with increased levels of c-Myc and cyclin D2, and augmented levels of pCD79a, pSyk, and pERK compared to control B cells. These results suggest that IκBNS acts as a negative regulator of BCR signaling and a positive regulator of TLR4 signaling in mouse B cells.

Beyond Antioxidation: Keap1-Nrf2 in the Development and Effector Functions of Adaptive Immune Cells

Ubiquitously expressed in mammalian cells, the Kelch-like ECH-associated protein 1 (Keap1)-NF erythroid 2-related factor 2 (Nrf2) complex forms the evolutionarily conserved antioxidation system to tackle oxidative stress caused by reactive oxygen species. Reactive oxygen species, generated as byproducts of cellular metabolism, were identified as essential second messengers for T cell signaling, activation, and effector responses. Apart from its traditional role as an antioxidant, a growing body of evidence indicates that Nrf2, tightly regulated by Keap1, modulates immune responses and regulates cellular metabolism. Newer functions of Keap1 and Nrf2 in immune cell activation and function, as well as their role in inflammatory diseases such as sepsis, inflammatory bowel disease, and multiple sclerosis, are emerging. In this review, we highlight recent findings about the influence of Keap1 and Nrf2 in the development and effector functions of adaptive immune cells, that is, T cells and B cells, and discuss the knowledge gaps in our understanding. We also summarize the research potential and targetability of Nrf2 for treating immune pathologies.

Spleen fibroblastic reticular cell-derived acetylcholine promotes lipid metabolism to drive autoreactive B cell responses

Autoreactive B cell responses are essential for the development of systemic lupus erythematosus (SLE). Fibroblastic reticular cells (FRCs) are known to construct lymphoid compartments and regulate immune functions. Here, we identify spleen FRC-derived acetylcholine (ACh) as a key factor that controls autoreactive B cell responses in SLE. In SLE, CD36-mediated lipid uptake leads to enhanced mitochondrial oxidative phosphorylation in B cells. Accordingly, the inhibition of fatty acid oxidation results in reduced autoreactive B cell responses and ameliorated diseases in lupus mice. Ablation of CD36 in B cells impairs lipid uptake and differentiation of autoreactive B cells during autoimmune induction. Mechanistically, spleen FRC-derived ACh promotes lipid influx and generation of autoreactive B cells through CD36. Together, our data uncover a novel function of spleen FRCs in lipid metabolism and B cell differentiation, placing spleen FRC-derived ACh in a key position in promoting autoreactive B cells in SLE.

A Spatiotemporal Map of Co-Receptor Signaling Networks Underlying B Cell Activation

The B cell receptor (BCR) signals together with a multi-component co-receptor complex to initiate B cell activation in response to antigen binding. This process underlies nearly every aspect of proper B cell function. Here, we take advantage of peroxidase-catalyzed proximity labeling combined with quantitative mass spectrometry to track B cell co-receptor signaling dynamics from 10 seconds to 2 hours after BCR stimulation. This approach enables tracking of 2,814 proximity-labeled proteins and 1,394 quantified phosphosites and provides an unbiased and quantitative molecular map of proteins recruited to the vicinity of CD19, the key signaling subunit of the co-receptor complex. We detail the recruitment kinetics of essential signaling effectors to CD19 following activation, and then identify new mediators of B cell activation. In particular, we show that the glutamate transporter SLC1A1 is responsible for mediating rapid metabolic reprogramming immediately downstream of BCR stimulation and for maintaining redox homeostasis during B cell activation. This study provides a comprehensive map of the BCR signaling pathway and a rich resource for uncovering the complex signaling networks that regulate B cell activation.

B cell receptor signaling in germinal centers prolongs survival and primes B cells for selection

Germinal centers (GCs) are sites of B cell clonal expansion, diversification, and antibody affinity selection. This process is limited and directed by T follicular helper cells that provide helper signals to B cells that endocytose, process, and present cognate antigens in proportion to their B cell receptor (BCR) affinity. Under this model, the BCR functions as an endocytic receptor for antigen capture. How signaling through the BCR contributes to selection is not well understood. To investigate the role of BCR signaling in GC selection, we developed a tracker for antigen binding and presentation and a Bruton's tyrosine kinase drug-resistant-mutant mouse model. We showed that BCR signaling per se is necessary for the survival and priming of light zone B cells to receive T cell help. Our findings provide insight into how high-affinity antibodies are selected within GCs and are fundamental to our understanding of adaptive immunity and vaccine development.

Transcriptomic analysis of pathways associated with ITGAV/alpha(v) integrin-dependent autophagy in human B cells

Macroautophagy/autophagy proteins have been linked with the development of immune-mediated diseases including lupus, but the mechanisms for this are unclear due to the complex roles of these proteins in multiple immune cell types. We have previously shown that a form of noncanonical autophagy induced by ITGAV/alpha(v) integrins regulates B cell activation by viral and self-antigens, in mice. Here, we investigate the involvement of this pathway in B cells from human tissues. Our data reveal that autophagy is specifically induced in the germinal center and memory B cell subpopulations of human tonsils and spleens. Transcriptomic analysis show that the induction of autophagy is related to unique aspects of activated B cells such as mitochondrial metabolism. To understand the function of ITGAV/alpha(v) integrin-dependent autophagy in human B cells, we used CRISPR-mediated knockdown of autophagy genes. Integrating data from primary B cells and knockout cells, we found that ITGAV/alpha(v)-dependent autophagy limits activation of specific pathways related to B cell responses, while promoting others. These data provide new mechanistic links for autophagy and B-cell-mediated immune dysregulation in diseases such as lupus.

Metabolic dialogs between B cells and the tumor microenvironment: Implications for anticancer immunity

Immunometabolism, a branch of biology describing the link between immunity and metabolism, is an emerging topic in cancer immunology. It is currently well accepted that B cells and tertiary lymph structures formed by them are associated with favorable outcomes when patients undergo cancer immunotherapy. Understanding the determinants of B-cell fate and function in cancer patients is necessary for improving cancer immunotherapy. Accumulating evidence points to the tumor microenvironment being a critical metabolic hurdle to an efficient antitumor B-cell response. At the same time, several B-cell-derived metabolites have recently been reported to inhibit anticancer immunity. In this literature review, key B-cell immunometabolism studies and the metabolic life of B cells were summarized. Then, we discussed the intrinsic metabolic pathways of B cells themselves and how the tumor microenvironment and B cells in tumors metabolically influence each other. Finally, we pointed out key questions to provide some inspiration for further study of the role of B-cell immunometabolism in the antitumor immune response.

mTORC1-GLUT1-mediated glucose metabolism drives hyperactivation of B cells in primary Sjogren's syndrome

Primary Sjögren's syndrome (pSS) is a chronic systemic autoimmune disease characterized by B cell hyperactivation and hypergrammaglobulinemia. Currently, the role of metabolic pathways in the B cells of pSS patients is poorly defined. Here, we showed that upon cytosine phosphate-guanine (CpG)/sCD40L/IL-4 stimulation, B cells proportionally increased glycolysis and oxygen consumption, and compared with B cells from healthy controls (HCs), B cells from pSS patients exhibited higher glycolysis capacity and maximal oxidative respiration (OXPHOS). We also found that glucose transporter 1 (GLUT1) expression in B cells from pSS patients was significantly higher than that in B cells from HCs. Treatment with 2-deoxy-d-glucose (2-DG) inhibited the activation of B cells in pSS patients. Both 2-DG and Metformin inhibited the proliferation, formation of plasma/plasmablasts and decreased the IgG and IgM levels in the supernatants of B cells from pSS patients. Furthermore, inhibition of mTORC1 by rapamycin had an effect similar to that of 2-DG, suppressing B cell activation, proliferation and antibody production. Taken together, we demonstrated that B cells from pSS patients are more metabolically active than those from HCs and suggested that the mTORC1-GLUT1 glycolysis pathways were the major drivers of B cell hyperactivation and autoantibody production in pSS patients.

Immune dysregulation in Glycogen Storage Disease 1b - a CyTOF approach

Glycogen Storage Disease type 1b (GSD1b) is a rare disease manifesting as hypoglycemia, recurrent infections and neutropenia, resulting from deleterious mutations in the SLC37A4 gene encoding the glucose-6-phosphate transporter. The susceptibility to infections is thought to be attributed not only to the neutrophil defect, though extensive immunophenotyping characterization is currently missing. Here we apply a systems immunology approach utilizing Cytometry by Time Of Flight (CyTOF) to map the peripheral immune landscape of 6 GSD1b patients. When compared to control subjects, those with GSD1b had a significant reduction in anti-inflammatory macrophages, CD16+ macrophages, and Natural Killer cells. Additionally, there was a preference towards a central versus an effector memory phenotype in multiple T cell populations, which may suggest that these changes stem from an inability of activated immune cell populations to undergo the appropriate switch to glycolytic metabolism in the hypoglycemic conditions associated with GSD1b. Furthermore, we identified a global reduction of CD123, CD14, CCR4, CD24 and CD11b across several populations and a multi-cluster upregulation of CXCR3, hinting at a potential role of impaired immune cell trafficking in the context of GSD1b. Taken together, our data indicates that that the immune impairment observed in GSD1b patients extends far beyond neutropenia and encompasses innate and adaptive compartments, which may provide novel insights into the pathogenesis of this disorder.

Orai3 and Orai1 mediate CRAC channel function and metabolic reprogramming in B cells

The essential role of store-operated Ca2+ entry (SOCE) through Ca2+ release-activated Ca2+ (CRAC) channels in T cells is well established. In contrast, the contribution of individual Orai isoforms to SOCE and their downstream signaling functions in B cells are poorly understood. Here, we demonstrate changes in the expression of Orai isoforms in response to B cell activation. We show that both Orai3 and Orai1 mediate native CRAC channels in B cells. The combined loss of Orai1 and Orai3, but not Orai3 alone, impairs SOCE, proliferation and survival, nuclear factor of activated T cells (NFAT) activation, mitochondrial respiration, glycolysis, and the metabolic reprogramming of primary B cells in response to antigenic stimulation. Nevertheless, the combined deletion of Orai1 and Orai3 in B cells did not compromise humoral immunity to influenza A virus infection in mice, suggesting that other in vivo co-stimulatory signals can overcome the requirement of BCR-mediated CRAC channel function in B cells. Our results shed important new light on the physiological roles of Orai1 and Orai3 proteins in SOCE and the effector functions of B lymphocytes.

Glucose Requirement of Antigen-Specific Autoreactive B Cells and CD4+ T Cells

The activation of lymphocytes in patients with lupus and in mouse models of the disease is coupled with an increased cellular metabolism in which glucose plays a major role. The pharmacological inhibition of glycolysis with 2-deoxy-d-glucose (2DG) reversed the expansion of follicular helper CD4+ T cells and germinal center B cells in lupus-prone mice, as well as the production of autoantibodies. The response of foreign Ags was however not affected by 2DG in these mice, suggesting that B and CD4+ T cell activation by autoantigens is uniquely sensitive to glycolysis. In this study, we tested this hypothesis with monoclonal B cells and CD4+ T cells specific for lupus-relevant autoantigens. AM14 Vκ8R (AM14) transgenic B cells are activated by IgG2a/chromatin immune complexes and they can receive cognate help from chromatin-specific 13C2 CD4+ T cells. We showed that activation of AM14 B cells by their cognate Ag PL2-3 induced glycolysis, and that the inhibition of glycolysis reduced their activation and differentiation into Ab-forming cells, in the absence or presence of T cell help. The dependency of autoreactive B cells on glycolysis is in sharp contrast with the previously reported dependency of 4-hydroxy-3-nitrophenyl acetyl-specific B cells on fatty acid oxidation. Contrary to AM14 B cells, the activation and differentiation of 13C2 T cells into follicular helper CD4+ T cells was not altered by 2DG, which differs from polyclonal CD4+ T cells from lupus-prone mice. These results further define the role of glycolysis in the production of lupus autoantibodies and demonstrate the need to evaluate the metabolic requirements of Ag-specific B and T cells.

Immunometabolic Signature during Respiratory Viral Infection: A Potential Target for Host-Directed Therapies

RNA viruses are known to induce a wide variety of respiratory tract illnesses, from simple colds to the latest coronavirus pandemic, causing effects on public health and the economy worldwide. Influenza virus (IV), parainfluenza virus (PIV), metapneumovirus (MPV), respiratory syncytial virus (RSV), rhinovirus (RhV), and coronavirus (CoV) are some of the most notable RNA viruses. Despite efforts, due to the high mutation rate, there are still no effective and scalable treatments that accompany the rapid emergence of new diseases associated with respiratory RNA viruses. Host-directed therapies have been applied to combat RNA virus infections by interfering with host cell factors that enhance the ability of immune cells to respond against those pathogens. The reprogramming of immune cell metabolism has recently emerged as a central mechanism in orchestrated immunity against respiratory viruses. Therefore, understanding the metabolic signature of immune cells during virus infection may be a promising tool for developing host-directed therapies. In this review, we revisit recent findings on the immunometabolic modulation in response to infection and discuss how these metabolic pathways may be used as targets for new therapies to combat illnesses caused by respiratory RNA viruses.