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

Metabolic Swifts Govern Normal and Malignant B Cell Lymphopoiesis

B lymphocytes are an indispensable part of the human immune system. They are the effective mediators of adaptive immunity and memory. To accomplish specificity against an antigen, and to establish the related immunologic memory, B cells differentiate through a complicated and strenuous training program that is characterized by multiple drastic genomic modifications. In order to avoid malignant transformation, these events are tightly regulated by multiple checkpoints, the vast majority of them involving bioenergetic alterations. Despite this stringent control program, B cell malignancies are amongst the top ten most common worldwide. In an effort to better understand malignant pathobiology, in this review, we summarize the metabolic swifts that govern normal B cell lymphopoiesis. We also review the existent knowledge regarding malignant metabolism as a means to unravel new research goals and/or therapeutic targets.

Regulation of humoral immune response by HIF-1α-dependent metabolic reprogramming of the germinal center reaction

Hypoxia-inducible factor-1α (HIF-1α) has been implicated in the regulation of many genes responsible for aerobic glycolysis; however, the role of HIF-1α in B-cell metabolism has not been well defined. Here, we analyzed patterns of gene expression and oxygen consumption rates in B-cell subpopulations from humans and mice and described a model of HIF-1α-mediated B-cell metabolic reprogramming during the germinal center (GC) reaction. Importantly, we found that HIF-1α was highly expressed in GC B-cells, and HIF-1α deficiency in B-cells impaired a functional GC reaction, resulting in defective class-switch recombination and generation of high-affinity plasma cells. These results identified an important role of HIF-1α in regulating humoral immunity through metabolic reprogramming during the GC response. This newly discovered metabolic character of GC B-cells will advance our understanding of GC biology and B-cell lymphomagenesis.

Metabolic determinants of B-cell selection

B-cells are antibody-producing cells of the adaptive immune system. Approximately 75% of all newly generated B-cells in the bone marrow are autoreactive and express potentially harmful autoantibodies. To prevent autoimmune disease, the immune system has evolved a powerful mechanism to eliminate autoreactive B-cells, termed negative B-cell selection. While designed to remove autoreactive clones during early B-cell development, our laboratory recently discovered that transformed B-cells in leukemia and lymphoma are also subject to negative selection. Indeed, besides the risk of developing autoimmune disease, B-cells are inherently prone to malignant transformation: to produce high-affinity antibodies, B-cells undergo multiple rounds of somatic immunoglobulin gene recombination and hypermutation. Reflecting high frequencies of DNA-breaks, adaptive immune protection by B-cells comes with a dramatically increased risk of development of leukemia and lymphoma. Of note, B-cells exist under conditions of chronic restriction of energy metabolism. Here we discuss how these metabolic gatekeeper functions during B-cell development provide a common mechanism for the removal of autoreactive and premalignant B-cells to safeguard against both autoimmune diseases and B-cell malignancies.

Expression Level of Membrane Markers CD5, CD19, and CD20 in B Cells after UV-Irradiation and Incubation in the Presence of Autologous Plasma

The effect of UV-light (240-390 nm) in doses of 151 and 755 J/m² on the expression of membrane markers CD5, CD19, CD20 in human peripheral blood B cells was studied by flow cytometry. In 24 h after exposure to UV light, we observed activation of processes accompanied by structural rearrangements of B-cell membranes leading to changes in the expression of receptor molecules: the content of of CD19 and CD20 increased due to activation of the synthesis of these proteins, while the content of CD5 decreased. The percentage of CD5⁺ cells decreased over 24 h after UV-irradiation of lymphocytes, while addition of autologous plasma to the incubation medium produced a photoprotective effect on CD5⁺ cells.

Reduced Energy Metabolism Impairs T Cell-Dependent B Cell Responses in Patients With Advanced HBV-Related Cirrhosis

Background and Aims

Patients with decompensated HBV-related liver cirrhosis (HBV D-LC) showed compromised immune responses, which manifested as a proneness to develop infections and hyporesponsiveness to vaccines, resulting in accelerated disease progression. The alterations in T cell-dependent B cell responses in this pathophysiological process were not well understood. This study aimed to investigate T cell-dependent B cell responses in this process and discuss the mechanism from the perspective of metabolism.

Methods

Changes in phenotypes and subsets of peripheral B cells between HBV D-LC patients and healthy controls (HCs) were compared by flow cytometry. Isolated B cells were activated by coculture with circulating T follicular (cTfh) cells. After coculture, the frequencies of plasmablasts and plasma cells and immunoglobin levels were analyzed. Oxidative phosphorylation (OXPHOS) and glycolysis were analyzed by a Seahorse analyzer. Mitochondrial function and the AKT/mTOR pathway were analyzed by flow cytometry.

Results

The proliferation and differentiation capacities of B cells after T cell stimulation were impaired in D-LC. Furthermore, we found that B cells from D-LC patients showed reductions in OXPHOS and glycolysis after activation, which may result from reduced glucose uptake, mitochondrial dysfunction and attenuated activation of the AKT/mTOR pathway.

Conclusions

B cells from HBV D-LC patients showed dysfunctional energy metabolism after T cell-dependent activation. Understanding the regulations of B cell metabolic pathway and their changes may provide a new direction to rescue B cell hyporesponsiveness in patients with HBV D-LC, preventing these patients be infected and improving sensitivity to vaccines.

B Cell Metabolism and Autophagy in Autoimmunity

B cells are central to the pathogenesis of multiple autoimmune diseases, through antigen presentation, cytokine secretion, and the production of autoantibodies. During development and differentiation, B cells undergo drastic changes in their physiology. It is emerging that these are accompanied by equally significant shifts in metabolic phenotype, which may themselves also drive and enforce the functional properties of the cell. The dysfunction of B cells during autoimmunity is characterised by the breaching of tolerogenic checkpoints, and there is developing evidence that the metabolic state of B cells may contribute to this. Determining the metabolic phenotype of B cells in autoimmunity is an area of active study, and is important because intervention by metabolism-altering therapeutic approaches may represent an attractive treatment target.

Recent Progress in the Design and Medical Application of In Situ Self-Assembled Polypeptide Materials

Inspired by molecular self-assembly, which is ubiquitous in natural environments and biological systems, self-assembled peptides have become a research hotspot in the biomedical field due to their inherent biocompatibility and biodegradability, properties that are afforded by the amide linkages forming the peptide backbone. This review summarizes the biological advantages, principles, and design strategies of self-assembled polypeptide systems. We then focus on the latest advances in in situ self-assembly of polypeptides in medical applications, such as oncotherapy, materials science, regenerative medicine, and drug delivery, and then briefly discuss their potential challenges in clinical treatment.

Immunometabolism at the Nexus of Cancer Therapeutic Efficacy and Resistance

Constitutive activity of the immune surveillance system detects and kills cancerous cells, although many cancers have developed strategies to avoid detection and to resist their destruction. Cancer immunotherapy entails the manipulation of components of the endogenous immune system as targeted approaches to control and destroy cancer cells. Since one of the major limitations for the antitumor activity of immune cells is the immunosuppressive tumor microenvironment (TME), boosting the immune system to overcome the inhibition provided by the TME is a critical component of oncotherapeutics. In this article, we discuss the main effects of the TME on the metabolism and function of immune cells, and review emerging strategies to potentiate immune cell metabolism to promote antitumor effects either as monotherapeutics or in combination with conventional chemotherapy to optimize cancer management.

The Metabolic Clock Model of B Cell Activation and Differentiation

Recent advancements in the field of B cell immunometabolism have provided mechanistic insights to B cell activation and fate determination. Here, in this short article, I will explain the main principles of our novel metabolic clock model and how it may reshape our perspective on longstanding immunological questions related to pathologies arising from out of context B cell activation.

CpG ODN as an adjuvant arouses the vigor of B cells by relieving the negative regulation of surface TLR9 to enhance the antibody response to vaccine

The surface Toll-like receptor 9 (sTLR9) has been identified on the surface of the B cells and was presumed to be a negative regulator of B cell responses. CpG ODN, a TLR9 agonist, has been successfully used as an adjuvant of hepatitis B vaccine to enhance antibody responses. However, it is unknown whether the sTLR9 is involved in regulating the activation and maturation of B cells in the antibody responses induced by CpG ODN-adjuvanted vaccines. In this study, we immunized mice with hepatitis B vaccine adjuvanted by CpG ODN (CpG 5805) and found that CpG 5805 enhanced the antibody response to vaccine and meanwhile down-regulated the sTLR9 levels on B cells. With antibody feeding assay and flow cytometry analysis, we further found that CpG 5805 induced a movement of the sTLR9 in B cells, internalized first and then mobilized to endosomes. Accompanied with the movement, CD80, CD86, CD40, and MHC II molecules were significantly up-regulated on the B cells. Interestingly, the B cells with internalized sTLR9 enlarged morphologically, and the sTLR9 levels were obviously lower and CD40 levels were obviously higher on the enlarged B cells. Together, the data presented here uncover that CpG ODN can induce the mobilization and relocation of sTLR9 in B cells, thereby triggering the B cell vigor by relieving the negative regulatory effect of sTLR9 on B cells, which may be one of the mechanisms for CpG ODN acting as a vaccine adjuvant to enhance the antibody response.Key points• CpG ODN-enhanced antibody response positively associates with B cell sTLR9 reduction.• CpG ODN reduces the sTLR9 levels by relocating it from B cell surface to endosomes.• sTLR9 reduction arouses B cell vigor via promoting B cell maturation and activation. Graphical Abstract.

Calcium flux control by Pacs1-Wdr37 promotes lymphocyte quiescence and lymphoproliferative diseases

Endoplasmic reticulum (ER) calcium (Ca²⁺ ) stores are critical to proteostasis, intracellular signaling, and cellular bioenergetics. Through forward genetic screening in mice, we identified two members of a new complex, Pacs1 and Wdr37, which are required for normal ER Ca²⁺ handling in lymphocytes. Deletion of Pacs1 or Wdr37 caused peripheral lymphopenia that was linked to blunted Ca²⁺ release from the ER after antigen receptor stimulation. Pacs1-deficient cells showed diminished inositol triphosphate receptor expression together with increased ER and oxidative stress. Mature Pacs1^-/- B cells proliferated and died in vivo under lymphocyte replete conditions, indicating spontaneous loss of cellular quiescence. Disruption of Pacs1-Wdr37 did not diminish adaptive immune responses, but potently suppressed lymphoproliferative disease models by forcing loss of quiescence. Thus, Pacs1-Wdr37 plays a critical role in stabilizing lymphocyte populations through ER Ca²⁺ handling and presents a new target for lymphoproliferative disease therapy.

Adaptive immunity at the crossroads of autophagy and metabolism

The function of lymphocytes is dependent on their plasticity, particularly their adaptation to energy availability and environmental stress, and their protein synthesis machinery. Lymphocytes are constantly under metabolic stress, and macroautophagy/autophagy is the primary metabolic pathway that helps cells overcome stressors. The intrinsic role of autophagy in regulating the metabolism of adaptive immune cells has recently gained increasing attention. In this review, we summarize and discuss the versatile roles of autophagy in regulating cellular metabolism and the implications of autophagy for immune cell function and fate, especially for T and B lymphocytes.

Intravital quantification reveals dynamic calcium concentration changes across B cell differentiation stages

Calcium is a universal second messenger present in all eukaryotic cells. The mobilization and storage of Ca²⁺ ions drives a number of signaling-related processes, stress-responses, or metabolic changes, all of which are relevant for the development of immune cells and their adaption to pathogens. Here, we introduce the Förster resonance energy transfer (FRET)-reporter mouse YellowCaB expressing the genetically encoded calcium indicator TN-XXL in B lymphocytes. Calcium-induced conformation change of TN-XXL results in FRET-donor quenching measurable by two-photon fluorescence lifetime imaging. For the first time, using our novel numerical analysis, we extract absolute cytoplasmic calcium concentrations in activated B cells during affinity maturation in vivo. We show that calcium in activated B cells is highly dynamic and that activation introduces a persistent calcium heterogeneity to the lineage. A characterization of absolute calcium concentrations present at any time within the cytosol is therefore of great value for the understanding of long-lived beneficial immune responses and detrimental autoimmunity.

Noncoding RNAs in B cell responses

B cells constitute a main branch adaptive immune system. They mediate host defence through the production of high-affinity antibodies against an enormous diversity of foreign antigens. Remarkably, B cells undergo multiple types of somatic DNA mutation to achieve this effector function, including class switch recombination (CSR) and somatic hypermutation (SHM). These processes occur in response to antigen recognition and inflammatory signals, and require strict biological control at multiple levels. Transcription within the locus that encodes antibodies plays direct roles in CSR. Additional non-coding RNAs (ncRNAs), including both microRNAs (miRNAs) and long ncRNAs (lncRNAs), also play pivotal roles in B cell activation and terminal effector function through post-transcriptional gene regulation and chromatin remodelling, respectively.

Transcriptional and Metabolic Control of Memory B Cells and Plasma Cells

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

Population-Specific Metabolic Alterations in Professional Antigen-Presenting Cells Contribute to Sepsis-Associated Immunosuppression

Sepsis is a complex host response triggered by an infection, with the patient's immune system between hyper- and hypo-responsiveness being the main reason for the syndromes' development and propagation. Studies conducted in peripheral blood mononuclear cells uncovered an association between an impaired immunometabolism and the severity and outcome of the disease. With this prospective observational study, we aimed to evaluate the immunometabolic phenotype of monocytes and B cells and its association with the cell function.Monocytes and B cells were isolated from patients with sepsis (n = 10; onset, days 4 and 8) and healthy volunteers (n = 10) and subsequently analyzed for metabolic changes and human leukocyte antigen-DR (HLA-DR) expression. Contemporaneously, immune checkpoints on monocytes and the ex vivo cytokine responses (interleukins 6 and 8) upon lipopolysaccharide or zymosan stimulation were analyzed. The distribution of B cell subsets was assessed, and plasma levels of immunoglobulins and tricarboxylic acid cycle intermediates were quantified.Both monocytes and B cells exhibited decreased HLA-DR expression in patients with sepsis. Monocytes displayed a stable upregulated glycolysis while B cells augmented glycolysis and respiration over time. The monocytes' ability to respond to stimulation was stimuli-dependently reduced but recovered over time. The B cell compartment shifted toward antibody-producing subsets and elevated immunoglobulins within the first days.Our results provide evidence for the induction of a state of trained immunity in monocytes and an early but transient immunosuppressive phenotype accounting for peripheral sepsis-induced vulnerability to infections. B cells exhibit an unsustainable activation contributing to adaptive immunosuppression.

Metabolic Gatekeepers of Pathological B Cell Activation

Unlike other cell types, B cells undergo multiple rounds of V(D)J recombination and hypermutation to evolve high-affinity antibodies. Reflecting high frequencies of DNA double-strand breaks, adaptive immune protection by B cells comes with an increased risk of malignant transformation. In addition, the vast majority of newly generated B cells express an autoreactive B cell receptor (BCR). Thus, B cells are under intense selective pressure to remove autoreactive and premalignant clones. Despite stringent negative selection, B cells frequently give rise to autoimmune disease and B cell malignancies. In this review, we discuss mechanisms that we term metabolic gatekeepers to eliminate pathogenic B cell clones on the basis of energy depletion. Chronic activation signals from autoreactive BCRs or transforming oncogenes increase energy demands in autoreactive and premalignant B cells. Thus, metabolic gatekeepers limit energy supply to levels that are insufficient to fuel either a transforming oncogene or hyperactive signaling from an autoreactive BCR.

Central Role of T Follicular Helper Cells in Myasthenia Gravis

Myasthenia gravis (gMG) is a critical autoimmune disease, which has a serious impact on the life and survival of patients. Ocular Myasthenia Gravis (oMG) is often the initial manifestation of MG and has the potential to progress to gMG. However, to date no distinct mechanism has been found to clarify the pathogenesis of conversion from oMG to gMG. Recent studies have shown that the development and clinical progression of MG is closely associated with the abnormal function of follicular helper T (Tfh) cells. Thus, this article reviews the recently achieved research progress on the involvement of Tfh cells in MG immunopathogenesis and focuses on the role of Tfh cells and related-factors (IL-21, CXCL13, CXCR5, bcl-6 etc.) in germinal center formation and antibody production in MG immune response.

The direct and indirect regulation of follicular T helper cell differentiation in inflammation and cancer

Follicular T helper (Tfh) cells play important roles in facilitating B-cell differentiation and inducing the antibody response in humoral immunity and immune-associated inflammatory diseases, including infections, autoimmune diseases, and cancers. However, Tfh cell differentiation is mainly achieved through self-directed differentiation regulation and the indirect regulation mechanism of antigen-presenting cells (APCs). During the direct intrinsic differentiation of naïve CD4⁺ T cells into Tfh cells, Bcl-6, as the characteristic transcription factor, plays the core role of transcriptional regulation. APCs indirectly drive Tfh cell differentiation mainly by changing cytokine secretion mechanisms. Altered metabolic signaling is also critically involved in Tfh cell differentiation. This review summarizes the recent progress in understanding the direct and indirect regulatory signals and metabolic mechanisms of Tfh cell differentiation and function in immune-associated diseases.

Enhanced BCR signaling inflicts early plasmablast and germinal center B cell death

It is still not clear how B cell receptor (BCR) signaling intensity affects plasma cell (PC) and germinal center (GC) B cell differentiation. We generated Cγ1 ^Cre/wt Ptpn6 ^fl/fl mice where SHP-1, a negative regulator of BCR signaling, is deleted rapidly after B cell activation. Although immunization with T-dependent antigens increased BCR signaling, it led to PC reduction and increased apoptosis. Dependent on the antigen, the early GC B cell response was equally reduced and apoptosis increased. At the same time, a higher proportion of GC B cells expressed cMYC, suggesting GC B cell-Tfh cell interactions may be increased. GC B cell numbers returned to normal at later stages, whereas affinity maturation was suppressed in the long term. This confirms that BCR signaling not only directs affinity-dependent B cell selection but also, without adequate further stimulation, can inflict cell death, which may be important for the maintenance of B cell tolerance.

Length difference of multi-walled carbon nanotubes generates differential cytotoxic responses

Carbon nanotubes have recently been rated as an effective biomaterial owing to their functionalization ability. However, the safety of multi-walled carbon nanotubes (MWCNTs) has yet to be clearly understood. To investigate how cells differentially react to minor geometric differences, we prepared well-dispersed and stable long and short MWCNTs showing an approximately 100-nm length difference in an in vitro system. Through an optimal combination of bovine serum albumin (BSA) and fetal bovine serum (FBS) biosurfactants and ultrasonication, we first confirmed that the MWCNTs were maintained without aggregation throughout the experiments. Internalized MWCNTs in human coronary artery smooth muscle cells were then quantified in a label-free manner using coherent anti-Stokes Raman scattering, followed by an analysis of their localization via two-photon excitation fluorescence. Intracellular MWCNTs were found to primarily localize in mitochondria with abnormal morphologies. Mitochondrial dysfunction, which was found to result from early stages of oxidative stress that consequently lead to cell death, was then proved via decreasing mitochondrial membrane potentials, with short MWCNTs showing significantly greater cytotoxicity than long MWCNTs. Our results suggest that even small length differences of MWCNTs may lead to differential responses in cells.

Stearoyl-CoA Desaturase-Mediated Monounsaturated Fatty Acid Availability Supports Humoral Immunity

Immune cells can metabolize glucose, amino acids, and fatty acids (FAs) to generate energy. The roles of different FA species and their impacts on humoral immunity remain poorly understood. Here, we report that proliferating B cells require monounsaturated FAs (MUFAs) to maintain mitochondrial metabolism and mTOR activity and to prevent excessive autophagy and endoplasmic reticulum (ER) stress. Furthermore, B cell-extrinsic stearoyl-CoA desaturase (SCD) activity generates MUFA to support early B cell development and germinal center (GC) formation in vivo during immunization and influenza infection. Thus, SCD-mediated MUFA production is critical for humoral immunity.

Zhou et al. show that monounsaturated fatty acids (MUFAs), generated by stearoyl-CoA desaturase (SCD), support B cell mitochondrial metabolism and mTOR activity and promote B cell development and humoral immune responses. These data establish MUFA availability as a key regulator for humoral immunity and a potential therapeutic target.

Eliciting B cell immunity against infectious diseases using nanovaccines

Infectious diseases, including the coronavirus disease 2019 (COVID-19) pandemic that has brought the world to a standstill, are emerging at an unprecedented rate with a substantial impact on public health and global economies. For many life-threatening global infectious diseases, such as human immunodeficiency virus (HIV) infection, malaria and influenza, effective vaccinations are still lacking. There are numerous roadblocks to developing new vaccines, including a limited understanding of immune correlates of protection to these global infections. To induce a reproducible, strong immune response against difficult pathogens, sophisticated nanovaccine technologies are under investigation. In contrast to conventional vaccines, nanovaccines provide improved access to lymph nodes, optimal packing and presentation of antigens, and induction of a persistent immune response. This Review provides a perspective on the global trends in emerging nanoscale vaccines for infectious diseases and describes the biological, experimental and logistical problems associated with their development, and how immunoengineering can be leveraged to overcome these challenges.

Effects of cellular senescence on metabolic pathways in non-immune and immune cells

Many cellular stresses induce cellular senescence and the irreversible arrest of cell proliferation in different cell types. Although blocked in their capacity to divide, senescent cells are metabolically active and are characterized by a different metabolic phenotype as compared to non-senescent cells. Changes observed in senescent cells depend from the cell type and lead to an adaptative flexibility in the type of metabolism. This metabolic reprogramming is needed to cope with survival and with the energetic demands of the senescent program that include the increased secretion of senescence-associated secretory phenotype factors.

The B cell response to both protein and nucleic acid antigens displayed on apoptotic cells are dependent on endosomal pattern recognition receptors

In systemic autoimmune diseases such as systemic lupus erythematosus (SLE), B cell tolerance is lost and there is a production of autoantibodies that drive pathology. The specificities of these antibodies are towards a wide range of autoantigens including proteins such as serum factors including cytokines as well as towards nucleic acids and modified glycolipids. It is known that endosomal pattern recognition receptors are involved in specific responses but if they drive specificity towards a specific group of autoantigens is not known. Here, we used syngeneic apoptotic cells alone to break B cell tolerance and investigated the antibody response in Unc93b1 mutant mice that lack signalling from the TLR3, TLR7 and TLR9 receptors. We found that specific B cell responses known from patients with SLE including antibodies towards Ro-52/60, La, cardiolipin as well as DNA were all significantly lower in the knockout mice. Thus, we found that endosomal TLR receptors were involved in break of tolerance and drive B cell responses for protein, nucleic acid and modified lipid antigens. This pinpoints these receptors as key drivers for the full range of antibody driven pathology in SLE and suggests that targeting of endosomal TLR driven responses will quench all B cell driven autoreactivity.

Adipose Tissue Immunomodulation: A Novel Therapeutic Approach in Cardiovascular and Metabolic Diseases

Adipose tissue is a critical regulator of systemic metabolism and bodily homeostasis as it secretes a myriad of adipokines, including inflammatory and anti-inflammatory cytokines. As the main storage pool of lipids, subcutaneous and visceral adipose tissues undergo marked hypertrophy and hyperplasia in response to nutritional excess leading to hypoxia, adipokine dysregulation, and subsequent low-grade inflammation that is characterized by increased infiltration and activation of innate and adaptive immune cells. The specific localization, physiology, susceptibility to inflammation and the heterogeneity of the inflammatory cell population of each adipose depot are unique and thus dictate the possible complications of adipose tissue chronic inflammation. Several lines of evidence link visceral and particularly perivascular, pericardial, and perirenal adipose tissue inflammation to the development of metabolic syndrome, insulin resistance, type 2 diabetes and cardiovascular diseases. In addition to the implication of the immune system in the regulation of adipose tissue function, adipose tissue immune components are pivotal in detrimental or otherwise favorable adipose tissue remodeling and thermogenesis. Adipose tissue resident and infiltrating immune cells undergo metabolic and morphological adaptation based on the systemic energy status and thus a better comprehension of the metabolic regulation of immune cells in adipose tissues is pivotal to address complications of chronic adipose tissue inflammation. In this review, we discuss the role of adipose innate and adaptive immune cells across various physiological and pathophysiological states that pertain to the development or progression of cardiovascular diseases associated with metabolic disorders. Understanding such mechanisms allows for the exploitation of the adipose tissue-immune system crosstalk, exploring how the adipose immune system might be targeted as a strategy to treat cardiovascular derangements associated with metabolic dysfunctions.

Metabolic needs of brain-infiltrating leukocytes and microglia in multiple sclerosis

Metabolism, the umbrella term for complex biochemical pathways that sustain the basic functions of life, has garnered attention in recent years for its role in immune activation. Indeed, metabolic pathways and their intricate and complex connections with immune mechanisms constitute a new area of immunology termed 'immunometabolism'. One highlight is the existence of a switch in the key metabolic programs in immune cells, which executes their effector functions. 'Metabolic reprogramming' is observed in conditions of both peripheral diseases as well as in neurodegenerative conditions associated with inflammation such as multiple sclerosis. Moreover metabolic reprogramming occurs for almost every immune cell type. Whether metabolic changes are cause or effect of immune activation, however, remains to be fully understood. Being central to cellular activation, metabolism has become very topical in terms of exploring therapeutic targets. This review covers the major metabolic programs in immune cells, discuss metabolites as regulators of immune cell functions, and consider metabolic enzymes or pathways as therapeutic targets using examples from multiple sclerosis and its animal models.

PIK3IP1 Promotes Extrafollicular Class Switching in T-Dependent Immune Responses

PI3K plays multiple roles throughout the life of a B cell. As such, its signaling is tightly regulated. The importance of this is illustrated by the fact that both loss- and gain-of-function mutations in PI3K can cause immunodeficiency in humans. PIK3IP1, also known as TrIP, is a transmembrane protein that has been shown to inhibit PI3K in T cells. Results from the ImmGen Consortium indicate that PIK3IP1 expression fluctuates throughout B cell development in a manner inversely correlated with PI3K activity; however, its role in B cells is poorly understood. In this study, we define the consequences of B cell-specific deletion of PIK3IP1. B cell development, basal Ig levels, and T-independent responses were unaffected by loss of PIK3IP1. However, there was a significant delay in the production of IgG during T-dependent responses, and secondary responses were impaired. This is likely due to a role for PIK3IP1 in the extrafollicular response because germinal center formation and affinity maturation were normal, and PIK3IP1 is not appreciably expressed in germinal center B cells. Consistent with a role early in the response, PIK3IP1 was downregulated at late time points after B cell activation, in a manner dependent on PI3K. Increased activation of the PI3K pathway was observed in PIK3IP1-deficient B cells in response to engagement of both the BCR and CD40 or strong cross-linking of CD40 alone. Taken together, these observations suggest that PIK3IP1 promotes extrafollicular responses by limiting PI3K signaling during initial interactions between B and T cells.

NR4A nuclear receptors restrain B cell responses to antigen when second signals are absent or limiting

Antigen stimulation (signal 1) triggers B cell proliferation, and primes B cells to recruit, engage, and respond to T cell help (signal 2). Failure to receive signal 2 within a defined time window results in B cell apoptosis, yet the mechanisms that enforce dependence upon co-stimulation are incompletely understood. Nr4a1-3 encode a small family of orphan nuclear receptors that are rapidly induced by B cell antigen receptor (BCR) stimulation. Here we showed that Nr4a1 and Nr4a3 play partially redundant roles to restrain B cell responses to antigen in the absence of co-stimulation, and do so in part by repressing expression of BATF and consequently MYC. The NR4A family also restrains B cell access to T cell help by repressing expression of the T cell chemokines CCL3 and CCL4, as well as CD86 and ICAM1. Such NR4A-mediated regulation plays a role specifically under conditions of competition for limiting T cell help.

Amino Assets: How Amino Acids Support Immunity

Amino acids are fundamental building blocks supporting life. Their role in protein synthesis is well defined, but they contribute to a host of other intracellular metabolic pathways, including ATP generation, nucleotide synthesis, and redox balance, to support cellular and organismal function. Immune cells critically depend on such pathways to acquire energy and biomass and to reprogram their metabolism upon activation to support growth, proliferation, and effector functions. Amino acid metabolism plays a key role in this metabolic rewiring, and it supports various immune cell functions beyond increased protein synthesis. Here, we review the mechanisms by which amino acid metabolism promotes immune cell function, and how these processes could be targeted to improve immunity in pathological conditions.

Age-related factors that affect B cell responses to vaccination in mice and humans

Aging significantly changes the ability to respond to vaccinations and infections. In this review, we summarize published results on age-related changes in response to infection with the influenza virus and on the factors known to increase influenza risk infection leading to organ failure and death. We also summarize how aging affects the response to the influenza vaccine with a special focus on B cells, which have been shown to be less responsive in the elderly. We show the cellular and molecular mechanisms contributing to the dysfunctional immune response of the elderly to the vaccine against influenza. These include a defective interaction of helper T cells (CD4+) with B cells in germinal centers, changes in the microenvironment, and the generation of immune cells with a senescence-associated phenotype. Finally, we discuss the effects of aging on metabolic pathways and we show how metabolic complications associated with aging lead to immune dysfunction.

YY1 control of mitochondrial-related genes does not account for regulation of immunoglobulin class switch recombination in mice

Immunoglobulin class switch recombination (CSR) occurs in activated B cells with increased mitochondrial mass and membrane potential. Transcription factor Yin Yang 1 (YY1) is critical for CSR and for formation of the DNA loops involved in this process. We therefore sought to determine if YY1 knockout impacts mitochondrial gene expression and mitochondrial function in murine splenic B cells, providing a potential mechanism for regulating CSR. We identified numerous genes in splenic B cells differentially regulated when cells are induced to undergo CSR. YY1 conditional knockout caused differential expression of 1129 genes, with 59 being mitochondrial-related genes. ChIP-seq analyses showed YY1 was directly bound to nearly half of these mitochondrial-related genes. Surprisingly, at the time when YY1 knockout dramatically reduces DNA loop formation and CSR, mitochondrial mass and membrane potential were not significantly impacted, nor was there a significant change in mitochondrial oxygen consumption, extracellular acidification rate, or mitochondrial complex I or IV activities. Our results indicate that YY1 regulates numerous mitochondrial-related genes in splenic B cells, but this does not account for the impact of YY1 on CSR or long-distance DNA loop formation.

Immunometabolism: From basic mechanisms to translation

Immunometabolism has emerged as a major mechanism central to adaptive and innate immune regulation. From early observations that inflammatory cytokines were induced in obese adipose tissue and that these cytokines contributed to metabolic disease, it was clear that metabolism and the immunological state are inextricably linked. With a second research wave arising from studies in cancer metabolism to also study the intrinsic metabolic pathways of immune cells themselves and how those pathways influence cell fate and function, immunometabolism is a rapidly maturing area of research. Several key themes and goals drive the field. There is abundant evidence that metabolic pathways are closely tied to cell signaling and differentiation which leads different subsets of immune cells to adopt unique metabolic programs specific to their state and environment. In this way, metabolic signaling drives cell fate. It is also apparent that microenvironment greatly influences cell metabolism. Immune cells adopt programs specific for the tissues where they infiltrate and reside. Ultimately, a central goal of the field is to apply immunometabolism findings to the discovery of novel therapeutic strategies in a wide range of diseases, including cancer, autoimmunity, and metabolic syndrome. This review summarizes these facets of immunometabolism and highlights opportunities for clinical translation.

Exaggerated mitophagy: a weapon of striatal destruction in the brain?

Mechanisms responsible for neuronal vulnerability in the brain remain unclear. Striatal neurons are preferentially damaged by 3-nitropropionic acid (3-NP), a mitochondrial complex-II inhibitor, causing striatal damage reminiscent of Huntington's disease (HD), but the mechanisms of the selectivity are not as well understood. We have discovered that Rhes, a protein enriched in the striatum, removes mitochondria via the mitophagy process. The process becomes intensified in the presence of 3-NP, thereby eliminating most of the mitochondria from the striatum. We put forward the hypothesis that Rhes acts as a 'mitophagy ligand' in the brain and promotes mitophagy via NIX, a mitophagy receptor. Since Rhes interacts and promotes toxicity in association with mutant huntingtin (mHTT), the genetic cause of HD, it is tempting to speculate on whether the exaggerated mitophagy may be a contributing factor to the striatal lesion found in HD. Thus, Rhes-mediated exaggerated mitophagy may act as a weapon of striatal destruction in the brain.

Missing-in-Metastasis/Metastasis Suppressor 1 Regulates B Cell Receptor Signaling, B Cell Metabolic Potential, and T Cell-Independent Immune Responses

Efficient generation of antibodies by B cells is one of the prerequisites of protective immunity. B cell activation by cognate antigens via B cell receptors (BCRs), or pathogen-associated molecules through pattern-recognition receptors, such as Toll-like receptors (TLRs), leads to transcriptional and metabolic changes that ultimately transform B cells into antibody-producing plasma cells or memory cells. BCR signaling and a number of steps downstream of it rely on coordinated action of cellular membranes and the actin cytoskeleton, tightly controlled by concerted action of multiple regulatory proteins, some of them exclusive to B cells. Here, we dissect the role of Missing-In-Metastasis (MIM), or Metastasis suppressor 1 (MTSS1), a cancer-associated membrane and actin cytoskeleton regulating protein, in B cell-mediated immunity by taking advantage of MIM knockout mouse strain. We show undisturbed B cell development and largely normal composition of B cell compartments in the periphery. Interestingly, we found that MIM-/- B cells are defected in BCR signaling in response to surface-bound antigens but, on the other hand, show increased metabolic activity after stimulation with LPS or CpG. In vivo, MIM knockout animals exhibit impaired IgM antibody responses to immunization with T cell-independent antigen. This study provides the first comprehensive characterization of MIM in B cells, demonstrates its regulatory role for B cell-mediated immunity, as well as proposes new functions for MIM in tuning receptor signaling and cellular metabolism, processes, which may also contribute to the poorly understood functions of MIM in cancer.

BCR-Induced Ca2+ Signals Dynamically Tune Survival, Metabolic Reprogramming, and Proliferation of Naive B Cells

B cell receptor (BCR) engagement induces naive B cells to differentiate and perform critical immune-regulatory functions. Acquisition of functional specificity requires that a cell survive, enter the cell cycle, and proliferate. We establish that quantitatively distinct Ca2+ signals triggered by variations in the extent of BCR engagement dynamically regulate these transitions by controlling nuclear factor κB (NF-κB), NFAT, and mTORC1 activity. Weak BCR engagement induces apoptosis by failing to activate NF-κB-driven anti-apoptotic gene expression. Stronger signals that trigger more robust Ca2+ signals promote NF-κB-dependent survival and NFAT-, mTORC1-, and c-Myc-dependent cell-cycle entry and proliferation. Finally, we establish that CD40 or TLR9 costimulation circumvents these Ca2+-regulated checkpoints of B cell activation and proliferation. As altered BCR signaling is linked to autoimmunity and B cell malignancies, these results have important implications for understanding the pathogenesis of aberrant B cell activation and differentiation and therapeutic approaches to target these responses.

Differential Effects of Reactive Oxygen Species on IgG versus IgM Levels in TLR-Stimulated B Cells

It is becoming increasingly evident that reactive oxygen species (ROS) have critical roles as "second messengers" in cell signaling. In B cells, ROS can be generated either as a byproduct of mitochondrial respiration, as a result of the endoplasmic reticulum stress response induced by high production of Igs, or by the activation of NADPH oxidase (NOX) complexes. Having previously shown that costimulation of B cells via TLR 9 and the TLR-related receptor RP105 drives maturation of human peripheral blood B cells into Ig-producing cells, we aimed to study the role of ROS generated during this vital process. To this end, the ROS levels were either reduced by the NOX inhibitor VAS2870 or by the ROS scavenger N-acetyl cysteine (NAC). We revealed that TLR9/RP105-mediated stimulation of human B cells involved a rapid activation of NOX. Moreover, VAS2870 blocked the TLR9/RP105-induced B cell activation and thereby all Ig production. Importantly, we showed that ROS targeted by NAC was selectively required for IgG but not for IgM production. The endoplasmic reticulum stress response in the TLR9/RP105-stimulated cells was higher in IgG⁺ than in IgG^- cells and was reduced by NAC in IgG⁺ cells only. Of note, we revealed that substantially higher levels of IgG than IgM were produced per cell and that IgG⁺ cells produced significantly higher ROS levels than IgG^- cells. Taken together, our results imply that NAC-targeted ROS may be particularly important for sustaining the high Ig production in IgG⁺ B cells.

The immunologic Warburg effect: Evidence and therapeutic opportunities in autoimmunity

Pro‐inflammatory signals induce metabolic reprogramming in innate and adaptive immune cells of both myeloid and lymphoid lineage, characterized by a shift to aerobic glycolysis akin to the Warburg effect first described in cancer. Blocking the switch to aerobic glycolysis impairs the survival, differentiation, and effector functions of pro‐inflammatory cell types while favoring anti‐inflammatory and regulatory phenotypes. Glycolytic reprogramming may therefore represent a selective vulnerability of inflammatory immune cells, providing an opportunity to modulate immune responses in autoimmune disease without broad toxicity in other tissues of the body. The mechanisms by which aerobic glycolysis and the balance between glycolysis and oxidative phosphorylation regulate immune responses have only begun to be understood, with many additional insights expected in the years to come. Immunometabolic therapies targeting aerobic glycolysis include both pharmacologic inhibitors of key enzymes and glucose‐restricted diets, such as the ketogenic diet. Animal studies support a role for these pharmacologic and dietary therapies for the treatment of autoimmune diseases, and in a few cases proof of concept has been demonstrated in human disease. Nonetheless, much more work is needed to establish the clinical safety and efficacy of these treatments.

This article is categorized under:

Biological Mechanisms > Metabolism

Translational, Genomic, and Systems Medicine > Translational Medicine

Biological Mechanisms > Cell Signaling

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

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

Women in immunology: 2020 and beyond

Women have been at the forefront of tremendous achievements in immunology in the past decade. However, disparities still exist, limiting upward potential and further advancements. As four NIH intramural women scientists who care deeply about scientific progress and the progress of women in our field, we review ongoing challenges and discuss potential approaches to help advance the promotion of women in the sciences.

Plasma cells: You are what you eat

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

Metabolic control of B cell immune responses

Humoral immunity critically depends on appropriate B cell responses. B cell activation, proliferation, differentiation and antibody secretion are processes carefully orchestrated by a complex network of intracellular signaling pathways and transcription factors. In order to meet the energetic and biosynthetic demands of protein synthesis and cell division, signal transduction pathways reshape the metabolic profile of activated B cells. However, the relationship between signaling and metabolism is by no means unidirectional. Emerging evidence suggests that shifts in available fuel sources and intracellular metabolite concentrations profoundly impact cell fate decisions. The reciprocal regulation of cell signaling and metabolism could potentially be exploited to curb immune dysfunction in metabolic disorders or to antagonize autoimmunity and B cell malignancies.

Induction of metabolic quiescence defines the transitional to follicular B cell switch

Transitional B cells must actively undergo selection for self-tolerance before maturing into their resting follicular B cell successors. We found that metabolic quiescence was acquired at the follicular B cell stage in both humans and mice. In follicular B cells, the expression of genes involved in ribosome biogenesis, aerobic respiration, and mammalian target of rapamycin complex 1 (mTORC1) signaling was reduced when compared to that in transitional B cells. Functional metabolism studies, profiling of whole-cell metabolites, and analysis of cell surface proteins in human B cells suggested that this transition was also associated with increased extracellular adenosine salvage. Follicular B cells increased the abundance of the cell surface ectonucleotidase CD73, which coincided with adenosine 5'-monophosphate-activated protein kinase (AMPK) activation. Differentiation to the follicular B cell stage in vitro correlated with surface acquisition of CD73 on human transitional B cells and was augmented with the AMPK agonist, AICAR. Last, individuals with gain-of-function PIK3CD (PI3Kδ) mutations and increased pS6 activation exhibited a near absence of circulating follicular B cells. Together, our data suggest that mTORC1 attenuation may be necessary for human follicular B cell development. These data identify a distinct metabolic switch during human B cell development at the transitional to follicular stages, which is characterized by an induction of extracellular adenosine salvage, AMPK activation, and the acquisition of metabolic quiescence.

Self-reactivity on a spectrum: A sliding scale of peripheral B cell tolerance

Efficient mechanisms of central tolerance, including receptor editing and deletion, prevent highly self-reactive B cell receptors (BCRs) from populating the periphery. Despite this, modest self-reactivity persists in (and may even be actively selected into) the mature B cell repertoire. In this review, we discuss new insights into mechanisms of peripheral B cell tolerance that restrain mature B cells from mounting inappropriate responses to endogenous antigens, and place recent work into historical context. In particular, we discuss new findings that have arisen from application of a novel in vivo reporter of BCR signaling, Nur77-eGFP, expression of which scales with the degree of self-reactivity in both monoclonal and polyclonal B cell repertoires. We discuss new and historical evidence that self-reactivity is not just tolerated, but actively selected into the peripheral repertoire. We review recent progress in understanding how dual expression of the IgM and IgD BCR isotypes on mature naive follicular B cells tunes responsiveness to endogenous antigen recognition, and discuss how this may be integrated with other features of clonal anergy. Finally, we discuss how expression of Nur77 itself couples chronic antigen stimulation with B cell tolerance.

Antigen Complexed with a TLR9 Agonist Bolsters c-Myc and mTORC1 Activity in Germinal Center B Lymphocytes

The germinal center (GC) is the anatomical site where humoral immunity evolves. B cells undergo cycles of proliferation and selection to produce high-affinity Abs against Ag. Direct linkage of a TLR9 agonist (CpG) to a T-dependent Ag increases the number of GC B cells. We used a T-dependent Ag complexed with CpG and a genetic model for ablating the TLR9 signaling adaptor molecule MyD88 specifically in B cells (B-MyD88^- mice) together with transcriptomics to determine how this innate pathway positively regulates the GC. GC B cells from complex Ag-immunized B-MyD88^- mice were defective in inducing gene expression signatures downstream of c-Myc and mTORC1. In agreement with the latter gene signature, ribosomal protein S6 phosphorylation was increased in GC B cells from wild-type mice compared with B-MyD88^- mice. However, GC B cell expression of a c-Myc protein reporter was enhanced by CpG attached to Ag in both wild-type and B-MyD88^- mice, indicating a B cell-extrinsic effect on c-Myc protein expression combined with a B cell-intrinsic enhancement of gene expression downstream of c-Myc. Both mTORC1 activity and c-Myc are directly induced by T cell help, indicating that TLR9 signaling in GC B cells either enhances their access to T cell help or directly influences these pathways to further enhance the effect of T cell help. Taken together, these findings indicate that TLR9 signaling in the GC could provide a surrogate prosurvival stimulus, "TLR help," thus lowering the threshold for selection and increasing the magnitude of the GC response.