Reduced production of B-1-specified common lymphoid progenitors results in diminished potential of adult marrow to generate B-1 cells

The immunology of B-1 cells: from development to aging

B-1 cells have intricate biology, with distinct function, phenotype and developmental origin from conventional B cells. They generate a B cell receptor with conserved germline characteristics and biased V(D)J recombination, allowing this innate-like lymphocyte to spontaneously produce self-reactive natural antibodies (NAbs) and become activated by immune stimuli in a T cell-independent manner. NAbs were suggested as "rheostats" for the chronic diseases in advanced age. In fact, age-dependent loss of function of NAbs has been associated with clinically-relevant diseases in the elderly, such as atherosclerosis and neurodegenerative disorders. Here, we analyzed comprehensively the ontogeny, phenotypic characteristics, functional properties and emerging roles of B-1 cells and NAbs in health and disease. Additionally, after navigating through the complexities of B-1 cell biology from development to aging, therapeutic opportunities in the field are discussed.

Transient PU.1 low fetal progenitors generate lymphoid progeny that contribute to adult immunity

Hematopoietic stem cells and multipotential progenitors emerge in multiple, overlapping waves of fetal development. Some of these populations seed the bone marrow and sustain adult B- and T-cell development long-term after birth. However, others are present transiently, but whether they are vestigial or generate B and T cells that contribute to the adult immune system is not well understood. We now report that transient fetal progenitors distinguished by expression of low levels of the PU.1 transcription factor generated activated and memory T and B cells that colonized and were maintained in secondary lymphoid tissues. These included the small and large intestines, where they may contribute to the maintenance of gut homeostasis through at least middle age. At least some of the activated/memory cells may have been the progeny of B-1 and marginal zone B cells, as transient PU.1low fetal progenitors efficiently generated those populations. Taken together, our data demonstrate the potential of B- and T-cell progeny of transient PU.1low fetal progenitors to make an early and long-term contribution to the adult immune system.

Post-transcriptional (re)programming of B lymphocyte development: From bench to bedside?

Hematopoiesis, a process which generates blood and immune cells, changes significantly during mammalian development. Definitive hematopoiesis is marked by the emergence of long-term hematopoietic stem cells (HSCs). Here, we will focus on the post-transcriptional differences between fetal liver (FL) and adult bone marrow (ABM) HSCs. It remains unclear how or why exactly FL HSCs transition to ABM HSCs, but we aim to leverage their differences to revive an old idea: in utero HSC transplantation. Unexpectedly, the expression of certain RNA-binding proteins (RBPs) play an important role in HSC specification, and can be employed to convert or reprogram adult HSCs back to a fetal-like state. Among other features, FL HSCs have a broad differentiation capacity that includes the ability to regenerate both conventional B and T cells, as well as innate-like or unconventional lymphocytes such as B-1a and marginal zone B (MzB) cells. This chapter will focus on RNA binding proteins, namely LIN28B and IGF2BP3, that are expressed during fetal life and how they promote B-1a cell development. Furthermore, this chapter considers a potential clinical application of synthetic co-expression of LIN28B and IGF2BP3 in HSCs.

B Cells Dynamic in Aging and the Implications of Nutritional Regulation

Aging negatively affects B cell production, resulting in a decrease in B-1 and B-2 cells and impaired antibody responses. Age-related B cell subsets contribute to inflammation. Investigating age-related alterations in the B-cell pool and developing targeted therapies are crucial for combating autoimmune diseases in the elderly. Additionally, optimal nutrition, including carbohydrates, amino acids, vitamins, and especially lipids, play a vital role in supporting immune function and mitigating the age-related decline in B cell activity. Research on the influence of lipids on B cells shows promise for improving autoimmune diseases. Understanding the aging B-cell pool and considering nutritional interventions can inform strategies for promoting healthy aging and reducing the age-related disease burden.

Borrelia burgdorferi Infection-Induced Persistent IgM Secretion Controls Bacteremia, but Not Bacterial Dissemination or Tissue Burden

Infection with Borrelia burgdorferi causes Lyme disease in humans. In small rodents, the natural reservoir species of this spirochete, infections lead to only modest disease manifestations, despite causing persistence infection. Although B cell responses are central for controlling bacterial tissue burden and disease manifestations, they lack classical aspects of T-dependent responses, such as sustained IgG affinity maturation and longevity, corresponding with a rapid collapse of germinal centers. Instead, the Ab response is characterized by strong and ongoing secretion of IgM, whose origins and impact on protective immunity to B. burgdorferi remain unknown. In this article, we demonstrate that B. burgdorferi infection-induced IgM in mice was produced continuously, mainly by conventional B, not B-1 cells, in a T-independent manner. Although IgM was passively protective and restricted early bacteremia, its production had no effects on bacterial dissemination into solid tissues, nor did it affect Borrelia tissue burden. The latter was controlled by the induction of bactericidal IgG, as shown comparing infections in wild type mice with those of mice lacking exclusively secreted IgM-/-, all class-switched Abs via deletion of aicda (AID-/-), and all secreted Abs (secreted IgM-/- × AID-/-). Consistent with the notion that B. burgdorferi infection drives production of IgM over more tissue-penetrable IgG, we demonstrated increased short- and long-term IgM Ab responses also to a coadministered, unrelated Ag. Thus, the continued production of IgM may explain the absence of B. burgdorferi in the blood.

ROS fine-tunes the function and fate of immune cells

The redox state is essential to the process of cell life, which determines cell fate. As an important signaling molecule of the redox state, reactive oxygen species (ROS) are crucial for the homeostasis of immune cells and participate in the pathological processes of different diseases. We discuss the underlying mechanisms and possible signaling pathways of ROS to fine-tune the proliferation, differentiation, polarization and function of immune cells, including T cells, B cells, neutrophils, macrophages, myeloid-derived inhibitory cells (MDSCs) and dendritic cells (DCs). We further emphasize how excessive ROS lead to programmed immune cell death such as apoptosis, ferroptosis, pyroptosis, NETosis and necroptosis, providing valuable insights for future therapeutic strategies in human diseases.

Lymphoid cell development from fetal hematopoietic progenitors and human pluripotent stem cells

Lymphoid cells encompass the adaptive immune system, including T and B cells and Natural killer T cells (NKT), and innate immune cells (ILCs), including Natural Killer (NK) cells. During adult life, these lineages are thought to derive from the differentiation of long-term hematopoietic stem cells (HSCs) residing in the bone marrow. However, during embryogenesis and fetal development, the ontogeny of lymphoid cells is both complex and multifaceted, with a large body of evidence suggesting that lymphoid lineages arise from progenitor cell populations antedating the emergence of HSCs. Recently, the application of single cell RNA-sequencing technologies and pluripotent stem cell-based developmental models has provided new insights into lymphoid ontogeny during embryogenesis. Indeed, PSC differentiation platforms have enabled de novo generation of lymphoid immune cells independently of HSCs, supporting conclusions drawn from the study of hematopoiesis in vivo. Here, we examine lymphoid development from non-HSC progenitor cells and technological advances in the differentiation of human lymphoid cells from pluripotent stem cells for clinical translation.

Layered immunity and layered leukemogenicity: Developmentally restricted mechanisms of pediatric leukemia initiation

Hematopoietic stem cells (HSCs) and multipotent progenitor cells (MPPs) arise in successive waves during ontogeny, and their properties change significantly throughout life. Ontological changes in HSCs/MPPs underlie corresponding changes in mechanisms of pediatric leukemia initiation. As HSCs and MPPs progress from fetal to neonatal, juvenile and adult stages of life, they undergo transcriptional and epigenetic reprogramming that modifies immune output to meet age-specific pathogenic challenges. Some immune cells arise exclusively from fetal HSCs/MPPs. We propose that this layered immunity instructs cell fates that underlie a parallel layered leukemogenicity. Indeed, some pediatric leukemias, such as juvenile myelomonocytic leukemia, myeloid leukemia of Down syndrome, and infant pre-B-cell acute lymphoblastic leukemia, are age-restricted. They only present during infancy or early childhood. These leukemias likely arise from fetal progenitors that lose competence for transformation as they age. Other childhood leukemias, such as non-infant pre-B-cell acute lymphoblastic leukemia and acute myeloid leukemia, have mutation profiles that are common in childhood but rare in morphologically similar adult leukemias. These differences could reflect temporal changes in mechanisms of mutagenesis or changes in how progenitors respond to a given mutation at different ages. Interactions between leukemogenic mutations and normal developmental switches offer potential targets for therapy.

Multiple waves of fetal-derived immune cells constitute adult immune system

It has been over three decades since Drs. Herzenberg and Herzenberg proposed the layered immune system hypothesis, suggesting that different types of stem cells with distinct hematopoietic potential produce specific immune cells. This layering of immune system development is now supported by recent studies showing the presence of fetal-derived immune cells that function in adults. It has been shown that various immune cells arise at different embryonic ages via multiple waves of hematopoiesis from special endothelial cells (ECs), referred to as hemogenic ECs. However, it remains unknown whether these fetal-derived immune cells are produced by hematopoietic stem cells (HSCs) during the fetal to neonatal period. To address this question, many advanced tools have been used, including lineage-tracing mouse models, cellular barcoding techniques, clonal assays, and transplantation assays at the single-cell level. In this review, we will review the history of the search for the origins of HSCs, B-1a progenitors, and mast cells in the mouse embryo. HSCs can produce both B-1a and mast cells within a very limited time window, and this ability declines after embryonic day (E) 14.5. Furthermore, the latest data have revealed that HSC-independent adaptive immune cells exist in adult mice, which implies more complicated developmental pathways of immune cells. We propose revised road maps of immune cell development.

Surfing on the waves of the human γδ T cell ontogenic sea

While γδ T cells are present virtually in all vertebrates, there is a remarkable lack of conservation of the TRG and TRD loci underlying the generation of the γδ T cell receptor (TCR), which is associated with the generation of species-specific γδ T cells. A prominent example is the human phosphoantigen-reactive Vγ9Vδ2 T cell subset that is absent in mice. Murine γδ thymocyte cells were among the first immune cells identified to follow a wave-based layered development during embryonic and early life, and since this initial observation, in-depth insight has been obtained in their thymic ontogeny. By contrast, less is known about the development of human γδ T cells, especially regarding the generation of γδ thymocyte waves. Here, after providing an overview of thymic γδ wave generation in several vertebrate classes, we review the evidence for γδ waves in the human fetal thymus, where single-cell technologies have allowed the breakdown of human γδ thymocytes into functional waves with important TCR associations. Finally, we discuss the possible mechanisms contributing to the generation of waves of γδ thymocytes and their possible significance in the periphery.

The layered development of mouse B and T Cells

Traditional models of lymphopoiesis present B and T cell development as a linear process that initiates in the fetus and continues after birth in the bone marrow and thymus, respectively. However, this view of lymphocyte development is not in accord with reports, dating back several decades, indicating that the types of lymphocytes generated before and after birth differ. In this regard, selected γδ T cells, and those that utilize the Vγ3 receptor in particular, and innate-like B-1 B cells preferentially arise during fetal blood cell development. This review synthesizes data from multiple laboratories, with an emphasis on our own work using mouse models, demonstrating that innate and conventional B and T cells emerge in hematopoietic stem cell independent and dependent waves of development that are differentially regulated. This layering of lymphocyte development has implications for understanding the composition of the adult immune system and may provide insights into the origin of various lymphocytic leukemias.

The Dynamics of B Cell Aging in Health and Disease

Aging is considered to be an important risk factor for several inflammatory diseases. B cells play a major role in chronic inflammatory diseases by antibody secretion, antigen presentation and T cell regulation. Different B cell subsets have been implicated in infections and multiple autoimmune diseases. Since aging decreases B cell numbers, affects B cell subsets and impairs antibody responses, the aged B cell is expected to have major impacts on the development and progression of these diseases. In this review, we summarize the role of B cells in health and disease settings, such as atherosclerotic disease. Furthermore, we provide an overview of age-related changes in B cell development and function with respect to their impact in chronic inflammatory diseases.

Combined immunodeficiency with autoimmunity caused by a homozygous missense mutation in inhibitor of nuclear factor 𝛋B kinase alpha (IKKα)

[Figure: see text].

Transcriptional reprogramming in neonatal hematopoietic stem and progenitor cells

Hematopoietic stem cells (HSCs) and lineage-committed hematopoietic progenitor cells (HPCs) undergo profound shifts in gene expression during the neonatal and juvenile stages of life. Temporal changes in HSC/HPC gene expression underlie concomitant changes in self-renewal capacity, lineage biases, and hematopoietic output. Moreover, they can modify disease phenotypes. For example, childhood leukemias have distinct driver mutation profiles relative to adult leukemias, and they may arise from distinct cells of origin. The putative relationship between neonatal HSC/HPC ontogeny and childhood blood disorders highlights the importance of understanding how, at a mechanistic level, HSCs transition from fetal to adult transcriptional states. In this perspective piece, we summarize recent work indicating that the transition is uncoordinated and imprecisely timed. We discuss implications of these findings, including mechanisms that might enable neonatal HSCs and HPCs to acquire adultlike properties over a drawn-out period, in lieu of precise gene regulatory networks. The transition from fetal to adult transcriptional programs coincides with a pulse of type I interferon signaling that activates many genes associated with the adultlike state. This pulse may sensitize HSCs/HPCs to mutations that drive leukemogenesis shortly after birth. If we can understand how developmental switches modulate HSC and HPC fate after birth-both under normal circumstances and in the setting of disease-causing mutations-we can potentially reprogram these switches to treat or prevent childhood leukemias.

The Fetal-to-Adult Hematopoietic Stem Cell Transition and its Role in Childhood Hematopoietic Malignancies

As with most organ systems that undergo continuous generation and maturation during the transition from fetal to adult life, the hematopoietic and immune systems also experience dynamic changes. Such changes lead to many unique features in blood cell function and immune responses in early childhood. The blood cells and immune cells in neonates are a mixture of fetal and adult origin due to the co-existence of both fetal and adult types of hematopoietic stem cells (HSCs) and progenitor cells (HPCs). Fetal blood and immune cells gradually diminish during maturation of the infant and are almost completely replaced by adult types of cells by 3 to 4 weeks after birth in mice. Such features in early childhood are associated with unique features of hematopoietic and immune diseases, such as leukemia, at these developmental stages. Therefore, understanding the cellular and molecular mechanisms by which hematopoietic and immune changes occur throughout ontogeny will provide useful information for the study and treatment of pediatric blood and immune diseases. In this review, we summarize the most recent studies on hematopoietic initiation during early embryonic development, the expansion of both fetal and adult types of HSCs and HPCs in the fetal liver and fetal bone marrow stages, and the shift from fetal to adult hematopoiesis/immunity during neonatal/infant development. We also discuss the contributions of fetal types of HSCs/HPCs to childhood leukemias.

B1 lymphocytes develop independently of Notch signaling during mouse embryonic development

B1 lymphocytes are a small but unique component of the innate immune-like cells. However, their ontogenic origin is still a matter of debate. Although it is widely accepted that B1 cells originate early in fetal life, whether or not they arise from hematopoietic stem cells (HSCs) is still unclear. In order to shed light on the B1 cell origin, we set out to determine whether their lineage specification is dependent on Notch signaling, which is essential for the HSC generation and, therefore, all derivatives lineages. Using mouse embryonic stem cells (mESCs) to recapitulate murine embryonic development, we have studied the requirement for Notch signaling during the earliest B-cell lymphopoiesis and found that Rbpj-deficient mESCs are able to generate B1 cells. Their Notch independence was confirmed in ex vivo experiments using Rbpj-deficient embryos. In addition, we found that upregulation of Notch signaling induced the emergence of B2 lymphoid cells. Taken together, these findings indicate that control of Notch signaling dose is crucial for different B-cell lineage specification from endothelial cells and provides pivotal information for their in vitro generation from PSCs for therapeutic applications.

This article has an associated ‘The people behind the papers’ interview.

Summary: During development, the first B lymphocytes are produced in the absence of Notch signaling and, thus, independently from HSCs.

SLy2-overexpression impairs B-cell development in the bone marrow and the IgG response towards pneumococcal conjugate-vaccine

Background

Infections with Streptococcus pneumoniae can cause severe diseases in humans including pneumonia. Although guidelines for vaccination have been established, S. pneumoniae is still responsible for a serious burden of disease around the globe. Currently, two pneumococcal immunizations are available, namely the pure polysaccharide vaccine Pneumovax23 (P23) and the conjugate‐vaccine Prevenar13 (PCV13). We recently reported impaired thymus‐independent antibody responses towards P23 in mice overexpressing the immunoinhibitory adapter SLy2. The purpose of this study was to evaluate adaptive B‐cell responses towards the thymus‐dependent vaccine PCV13 in SLy2‐overexpressing mice and to study their survival rate during pneumococcal lung infection. Moreover, we investigated B‐cell developmental stages within the bone marrow (BM) in the context of excessive SLy2‐expression.

Methods

B‐cell subsets and their surface immune globulins were investigated by flow cytometry. For class‐switch assays, isolated splenic B cells were stimulated in vitro with lipopolysaccharide and interleukin‐4 and antibody secretion was quantified via LEGENDplex. To study PCV13‐specific responses, mice were immunized and serum antibody titers (immunoglobulin M, immunoglobulins IgG₁, IgG₂, and IgG₃) were examined by enzyme‐linked immunosorbent assay. Survival rates of mice were assessed within 7 days upon intranasal challenge with S. pneumoniae.

Results

Our data demonstrate impaired IgG₁ and IgG₃ antibody responses towards the pneumococcal conjugate‐vaccine PCV13 in SLy2‐overexpressing mice. This was accompanied by reduced frequencies and numbers of BM‐resident plasmablasts. In addition, we found drastically reduced counts of B‐cell precursors in the BM of SLy2‐Tg mice. The survival rate upon intranasal challenge with S. pneumoniae was mostly comparable between the genotypes.

Conclusion

Our findings demonstrate an important role of the adapter protein SLy2 in the context of adaptive antibody responses against pneumococcal conjugate‐vaccine. Interestingly, deficits in humoral immunity seemed to be compensated by cellular immune effectors upon bacterial challenge. Our study further shows a novel relevance of SLy2 for plasmablasts and B‐cell progenitors in the BM.

Lipid Metabolism in Tumor-Associated B Cells

Breakthroughs have been made in the cancer immunotherapy field focusing on utilizing T cells' antitumor immunity, and the lipid metabolism of tumor-associated B cells is not well studied compared to T cells. Accumulating evidence suggested that B cells also play important roles in tumor biology and antitumor immunity, especially the germinal center B cells that present in the tumor-related tertiary lymphoid structures. Due to scarce studies on lipid metabolisms of tumor-associated B cells, this chapter mainly summarized findings on B cell lipid metabolism and discussed B cell development and major transcription factors, tumor-associated B cell populations and their potential functions in antitumor immunity, fatty acid oxidation in germinal center B cells, and tumor microenvironment factors that potentially affect B cell lipid metabolism, focusing on hypoxia and nutrients competition, as well as lipid metabolites that affect B cell function, including cholesterol, geranylgeranyl pyrophosphate, oxysterols, and short-chain fatty acids.

Single-Cell Analysis of Neonatal HSC Ontogeny Reveals Gradual and Uncoordinated Transcriptional Reprogramming that Begins before Birth

Fetal and adult hematopoietic stem cells (HSCs) have distinct proliferation rates, lineage biases, gene expression profiles, and gene dependencies. Although these differences are widely recognized, it is not clear how the transition from fetal to adult identity is coordinated. Here we show that murine HSCs and committed hematopoietic progenitor cells (HPCs) undergo a gradual, rather than precipitous, transition from fetal to adult transcriptional states. The transition begins prior to birth and is punctuated by a late prenatal spike in type I interferon signaling that promotes perinatal HPC expansion and sensitizes progenitors to the leukemogenic FLT3ITD mutation. Most other changes in gene expression and enhancer activation are imprecisely timed and poorly coordinated. Thus, heterochronic enhancer elements, and their associated transcripts, are activated independently of one another rather than as part of a robust network. This simplifies the regulatory programs that guide neonatal HSC/HPC ontogeny, but it creates heterogeneity within these populations.

Bmi1 Maintains the Self-Renewal Property of Innate-like B Lymphocytes

The self-renewal ability is a unique property of fetal-derived innate-like B-1a lymphocytes, which survive and function without being replenished by bone marrow (BM) progenitors. However, the mechanism by which IgM-secreting mature B-1a lymphocytes self-renew is poorly understood. In this study, we showed that Bmi1 was critically involved in this process. Although Bmi1 is considered essential for lymphopoiesis, the number of mature conventional B cells was not altered when Bmi1 was deleted in the B cell lineage. In contrast, the number of peritoneal B-1a cells was significantly reduced. Peritoneal cell transfer assays revealed diminished self-renewal ability of Bmi1-deleted B-1a cells, which was restored by additional deletion of Ink4-Arf, the well-known target of Bmi1 Fetal liver cells with B cell-specific Bmi1 deletion failed to repopulate peritoneal B-1a cells, but not other B-2 lymphocytes after transplantation assays, suggesting that Bmi1 may be involved in the developmental process of B-1 progenitors to mature B-1a cells. Although Bmi1 deletion has also been shown to alter the microenvironment for hematopoietic stem cells, fat-associated lymphoid clusters, the reported niche for B-1a cells, were not impaired in Bmi1 ^-/- mice. RNA expression profiling suggested lysine demethylase 5B (Kdm5b) as another possible target of Bmi1, which was elevated in Bmi1^-/- B-1a cells in a stress setting and might repress B-1a cell proliferation. Our work has indicated that Bmi1 plays pivotal roles in self-renewal and maintenance of fetal-derived B-1a cells.

The ontogeny of murine B-1a cells

It has been over 35 years since the discovery of a special subtype of B cells in mice. These IgM⁺ B cells are named B-1 cells, whereas conventional B cells are referred to as B-2 cells. B-1 cells express Ly-1 (CD5) and CD11b antigen, which are usually expressed in T cells and myeloid cells, respectively, reside mainly in the peritoneal and pleural cavities, and secrete natural IgM antibodies in a T cell-independent manner. B-1 cells are further categorized into CD5⁺ B-1a cells and CD5^- B-1b cells. B-1 cells may develop through positive selection and secrete natural antibodies, including low-affinity-binding autoantibodies. Transplantation assays have revealed that the fetal liver, not the bone marrow (BM), is a major site for the production of B-1a cells, leading to the concept of a fetal origin for B-1a cells. This review introduces how the origin of B-1a cells has been explored, and describes the current state of knowledge gained through various approaches.

IL-10-producing B cells are enriched in murine pericardial adipose tissues and ameliorate the outcome of acute myocardial infarction

Acute myocardial infarction (MI) provokes an inflammatory response in the heart that removes damaged tissues to facilitate tissue repair/regeneration. However, overactive and prolonged inflammation compromises healing, which may be counteracted by antiinflammatory mechanisms. A key regulatory factor in an inflammatory response is the antiinflammatory cytokine IL-10, which can be produced by a number of immune cells, including subsets of B lymphocytes. Here, we investigated IL-10-producing B cells in pericardial adipose tissues (PATs) and their role in the healing process following acute MI in mice. We found that IL-10-producing B cells were enriched in PATs compared to other adipose depots throughout the body, with the majority of them bearing a surface phenotype consistent with CD5+ B-1a cells (CD5+ B cells). These cells were detected early in life, maintained a steady presence during adulthood, and resided in fat-associated lymphoid clusters. The cytokine IL-33 and the chemokine CXCL13 were preferentially expressed in PATs and contributed to the enrichment of IL-10-producing CD5+ B cells. Following acute MI, the pool of CD5+ B cells was expanded in PATs. These cells accumulated in the infarcted heart during the resolution of MI-induced inflammation. B cell-specific deletion of IL-10 worsened cardiac function, exacerbated myocardial injury, and delayed resolution of inflammation following acute MI. These results revealed enrichment of IL-10-producing B cells in PATs and a significant contribution of these cells to the antiinflammatory processes that terminate MI-induced inflammation. Together, these findings have identified IL-10-producing B cells as therapeutic targets to improve the outcome of MI.

Reactive oxygen species: The signal regulator of B cell

Reactive oxygen species (ROS) are indispensable for determining the fate of immune cells in both physiological and pathogenic environments, thus stimulating the interest of immunologists and clinicians. B cells are essential in maintaining immune homeostasis, and studies have indicated that ROS affect the maturation, activation and differentiation of B cells by controlling the signaling molecules in various molecular pathways. In the present review, we aimed to summarize the biological properties of ROS and the mechanisms by which ROS regulate B cell signaling pathways. We propose that ROS and their mediated signal transduction can be a new approach for manipulating B cell immune functions.

Hematopoietic stem cell-independent hematopoiesis and the origins of innate-like B lymphocytes

The current paradigm that a single long-term hematopoietic stem cell can regenerate all components of the mammalian immune system has been challenged by recent findings in mice. These findings show that adult tissue-resident macrophages and innate-like lymphocytes develop early in fetal hematopoiesis from progenitors that emerge prior to, and apparently independently of, conventional long-term hematopoietic stem cells. Here, we discuss these recent findings, which show that an early and distinct wave of hematopoiesis occurs for all major hematopoietic lineages. These data provide evidence that fetal hematopoietic progenitors not derived from the bona fide long-term hematopoietic stem cells give rise to tissue-resident immune cells that persist throughout adulthood. We also discuss recent insights into B lymphocyte development and attempt to synthesize seemingly contradictory recent findings on the origins of innate-like B-1a lymphocytes during fetal hematopoiesis.

Chasing Mavericks: The quest for defining developmental waves of hematopoiesis

Hematopoiesis is the process by which mature blood and immune cells are produced from hematopoietic stem and progenitor cells (HSCs and HSPCs). The last several decades of research have shed light on the origin of HSCs, as well as the heterogeneous pools of fetal progenitors that contribute to lifelong hematopoiesis. The overarching concept that hematopoiesis occurs in dynamic, overlapping waves throughout development, with each wave contributing to both continuous and developmentally limited cell types, has been solidified over the years. However, recent advances in our ability to track the production of hematopoietic cells in vivo have challenged several long-held dogmas on the origin and persistence of distinct hematopoietic cell types. In this review, we highlight emerging concepts in hematopoietic development and identify unanswered questions.

B-1 cell responses to infections

B-1 cells represent an innate-like early-developing B cell population, whose existence as an independent lymphocyte subset has been questioned in the past. Recent molecular and lineage tracing studies have not only confirmed their unique origins and differentiation paths, they have also provided a rationale for their distinctive functionalities compared to conventional B cells. This review summarizes our current understanding of B-1 cell development, and the activation events that regulate B-1 cell responses to self and foreign antigens. We discuss the unresolved question to what extent BCR engagement, that is, antigen-specificity versus innate signaling contributes to B-1 cell's participation in tissue homeostasis and immune defense as providers of 'natural' and antigen-induced antibody responses, and as cytokine-producing immune regulators.

Innate-like B cell subsets during immune responses: Beyond antibody production

B lymphocytes are recognized for their crucial role in the adaptive immunity since they represent the only leukocyte lineage capable of differentiating into Ab-secreting cells. However, it has been demonstrated that these lymphocytes can exert several Ab-independent functions, including engulfing and processing Ags for presentation to T cells, secreting soluble mediators, providing co-stimulatory signals, and even participating in lymphoid tissues development. Beyond that, several reports claiming the existence of multiple B cell subsets contributing directly to innate immune responses have appeared. These "innate-like" B lymphocytes, whose phenotype, development pathways, tissue distribution, and functions are in most cases notoriously different from those of conventional B cells, are crucial to early protective responses against pathogens by exerting "crossover" defensive strategies that blur the established boundaries of innate and adaptive branches of immunity. Examples of these mechanisms include the rapid secretion of the polyspecific natural Abs, increased susceptibility to innate receptors-mediated activation, cytokine secretion, downstream priming of other innate cells, usage of specific variable immunoglobulin gene-segments, and other features. As these new insights emerge, it is becoming preponderant to redefine the functionality of B cells beyond their classical adaptive-immune tasks.

Peripheral PDGFRα+gp38+ mesenchymal cells support the differentiation of fetal liver-derived ILC2

Group 2 innate lymphoid cells (ILC2s) are derived from common lymphoid progenitors (CLPs) via several specific precursors, and the transcription factors essential for ILC2 differentiation have been extensively studied. However, the external factors regulating commitment to the ILC lineage as well as the sites and stromal cells that constitute the optimal microenvironment for ILC2-specific differentiation are not fully defined. In this study, we demonstrate that three key external factors, the concentration of interleukin 7 (IL-7) and strength and duration of Notch signaling, coordinately determine the fate of CLP toward the T, B, or ILC lineage. Additionally, we identified three stages of ILC2 in the fetal mesentery that require STAT5 signals for maturation: ILC progenitors, CCR9⁺ ILC2 progenitors, and KLRG1^- immature ILC2. We further demonstrate that ILC2 development is supported by mesenteric platelet-derived growth factor receptor α (PDGFRα)⁺ glycoprotein 38 (gp38)⁺ mesenchymal cells. Collectively, our results suggest that early differentiation of ILC2 occurs in the fetal liver via IL-7 and Notch signaling, whereas final differentiation occurs in the periphery with the aid of PDGFRα⁺gp38⁺ cells.

B1a B cells require autophagy for metabolic homeostasis and self-renewal

Clarke et al. demonstrate that the innate-like B1 B cell subset has a distinct metabolic phenotype, characterized by high levels of glycolysis, pentose phosphate pathway, and TCA cycle activity, and depends on autophagy for metabolic homeostasis and self-renewal.

Specific metabolic programs are activated by immune cells to fulfill their functional roles, which include adaptations to their microenvironment. B1 B cells are tissue-resident, innate-like B cells. They have many distinct properties, such as the capacity to self-renew and the ability to rapidly respond to a limited repertoire of epitopes. The metabolic pathways that support these functions are unknown. We show that B1 B cells are bioenergetically more active than B2 B cells, with higher rates of glycolysis and oxidative phosphorylation, and depend on glycolysis. They acquire exogenous fatty acids and store lipids in droplet form. Autophagy is differentially activated in B1a B cells, and deletion of the autophagy gene Atg7 leads to a selective loss of B1a B cells caused by a failure of self-renewal. Autophagy-deficient B1a B cells down-regulate critical metabolic genes and accumulate dysfunctional mitochondria. B1 B cells, therefore, have evolved a distinct metabolism adapted to their residence and specific functional properties.

Many layers of embryonic hematopoiesis: new insights into B-cell ontogeny and the origin of hematopoietic stem cells

In adult hematopoiesis, the hematopoietic stem cell (HSC) sits at the top of a hierarchy of hematopoietic progenitors responsible for generating the diverse repertoire of blood and immune cells. During embryonic development, however, the initial waves of hematopoiesis provide the first functioning blood cells of the developing embryo, such as primitive erythrocytes arising in the yolk sac, independently of HSCs. In the field of developmental immunology, it has been recognized that some components of the immune system, such as B-1a lymphocytes, are uniquely produced during the embryonic and neonatal period, suggesting a "layered" development of immunity. Several recent studies have shed new light on the developmental origin of the layered immune system, suggesting complex and sometimes multiple contributions to unique populations of innate-like immune cells from both fetal HSCs and earlier HSC-independent progenitors. In this review, we will attempt to synthesize these studies to provide an integrated model of developmental hematopoiesis and layered immunity that may offer new insights into the origin of HSCs.

The influence of developmental timing on B cell diversity

The adult adaptive immune system is comprised of a wide spectrum of lymphocyte subsets with distinct antigen receptor repertoire profiles, effector functions, turnover times and anatomical locations, acting in concert to provide optimal host protection and self-regulation. While some lymphocyte populations are replenished by bone marrow hematopoietic stem cells (HSCs) through adulthood, others emerge during a limited window of time during fetal and postnatal life and sustain through self-replenishment. Despite fundamental implications in immune regeneration, early life immunity and leukemogenesis, the impact of developmental timing on lymphocyte output remains an under explored frontier in immunology. In this review, we spotlight recent insights into the developmental changes in B cell output in mice and explore how several age specific cellular and molecular factors may shape the formation of a diverse adaptive immune system.

A Hard(y) Look at B-1 Cell Development and Function

A small population of B cells exists in lymphoid tissues and body cavities of mice that is distinct in development, phenotype, and function from the majority (B-2) B cell population. This population, originally termed "Ly-1" and now "B-1," has received renewed interest as an innate-like B cell population of fetal-derived hematopoiesis, responsible for natural Ab production and rapid immune responses. Molecular analyses have begun to define fetal and adult hematopoiesis, while cell-fate mapping studies have revealed complex developmental origins of B-1 cells. Together the studies provide a more detailed understanding of B-1 cell regulation and function. This review outlines studies that defined B-1 cells as natural Ab- and cytokine-producing B cells of fetal origin, with a focus on work conducted by R.R. Hardy, an early pioneer and codiscoverer of B-1 cells, whose seminal contributions enhanced our understanding of this enigmatic B cell population.

Progenitor B-1 B-cell acute lymphoblastic leukemia is associated with collaborative mutations in 3 critical pathways

B-1 and B-2 lymphocytes are derived from distinct developmental pathways and represent layered arms of the innate and adaptive immune systems, respectively. In contrast to a majority of murine B-cell malignancies, which stain positive with the B220 antibody, we discovered a novel form of B-cell leukemia in NUP98-PHF23 (NP23) transgenic mice. The immunophenotype (Lin^- B220^- CD19⁺ AA4.1⁺) was identical to that of progenitor (pro) B-1 cells, and V_H gene usage was skewed toward 3' V regions, similar to murine fetal liver B cells. Moreover, the gene expression profile of these leukemias was most similar to that of fetal liver pro-B fraction BC, a known source of B-1 B cells, further supporting a pro-B-1 origin of these leukemias. The NP23 pro-B-1 acute lymphoblastic leukemias (ALLs) acquired spontaneous mutations in both Bcor and Janus kinase (Jak) pathway (Jak1/2/3 and Stat5a) genes, supporting a hypothesis that mutations in 3 critical pathways (stem-cell self-renewal, B-cell differentiation, and cytokine signaling) collaborate to induce B-cell precursor (BCP) ALL. Finally, the thymic stromal lymphopoietin (Tslp) cytokine is required for murine B-1 development, and chromosomal rearrangements resulting in overexpression of the TSLP receptor (CRLF2) are present in some patients with high-risk BCP-ALL (referred to as CRLF2r ALL). Gene expression profiles of NP23 pro-B-1 ALL were more similar to that of CRLF2r ALL than non-CRLF2r ALL, and analysis of V_H gene usage from patients with CRLF2r ALL demonstrated preferential usage of V_H regions used by human B-1 B cells, leading to the suggestion that this subset of patients with BCP-ALL has a malignancy of B-1, rather than B-2, B-cell origin.

HSC Contribution in Making Steady-State Blood

In homeostasis, whether blood cells are derived from committed progenitor or mutipotent stem cell activity remains controversial. In this issue of Immunity, Sawai et al. (2016) describe murine HSCs as the major contributor to the maintenance of multilineage hematopoiesis, both in the steady state and during cytokine response.

Fetal and adult progenitors give rise to unique populations of CD8+ T cells

During the ontogeny of the mammalian immune system, distinct lineages of cells arise from fetal and adult hematopoietic stem cells (HSCs) during specific stages of development. However, in some cases, the same immune cell type is produced by both HSC populations, resulting in the generation of phenotypically similar cells with distinct origins and divergent functional properties. In this report, we demonstrate that neonatal CD8⁺ T cells preferentially become short-lived effectors and adult CD8⁺ T cells selectively form long-lived memory cells after infection because they are derived from distinct progenitor cells. Notably, we find that naïve neonatal CD8⁺ T cells originate from a progenitor cell that is distinguished by expression of Lin28b. Remarkably, ectopic expression of Lin28b enables adult progenitors to give rise to CD8⁺ T cells that are phenotypically and functionally analogous to those found in neonates. These findings suggest that neonatal and adult CD8⁺ T cells belong to separate lineages of CD8⁺ T cells, and potentially explain why it is challenging to elicit memory CD8⁺ T cells in early life.

To B1a or not to B1a: do hematopoietic stem cells contribute to tissue-resident immune cells?

Hematopoietic stem cells (HSCs) have long been considered the continuous source of all hematopoietic cells for the life of an individual. Recent findings have questioned multiple aspects of this view, including the ability of lifelong HSCs to contribute to tissue-resident immune cells. Here we discuss the most recent findings on the source of B1a cells, innatelike lymphocytes that primarily reside in serous cavities. Powerful experimental approaches including bar coding, single cell transplantation, in vivo lineage tracing, and HSC-specific pulse-chase labeling have provided novel insights on B1a-cell generation during ontogeny. We evaluate the evidence for fetal vs adult B1a-cell production capacity and the identity of putative cells of origin. Integrating these most recent findings with previous work, we propose a working model that encapsulates our current understanding of waves of immune development.

Fetal Lymphoid Progenitors Become Restricted to B-1 Fates Coincident with IL-7Rα Expression

B-1 cells represent a sub-fraction of B lymphocytes that participate in T cell-independent antibody production and contribute to innate immunity. While the production of B-1 cells is favored during the fetal waves of lymphopoiesis, it has been unclear when and how that differentiation option is specified. To clarify this, lymphoid and hematopoietic progenitors of fetal liver (FL) and adult bone marrow (ABM) were examined for the B cell differentiation potential. Mouse common lymphoid progenitors (CLPs) and more primitive KSL fraction of FL and ABM were transferred to SCID mice and donor-derived B cell subsets were analyzed 4 weeks later. CLPs were also cultured on ST2 stromal cells for 6 days prior to transplantation. While Lin^- IL-7Rα⁺ CLPs from ABM differentiated to B-1, B-2 and marginal zone B (MZB) cells, equivalent cells from d15 FL differentiated mostly to B-1a cells. We found that fetal CLPs had less ability to colonize the bone marrow than adult CLPs. However, the fetal/adult difference was already present when progenitors were cultured in an identical condition before transplantation. More primitive KSL fraction of FL could generate the same broad spectrum of B cells typical of adults, including splenic MZB cells. In conclusion, we argue that FL and ABM-CLPs are intrinsically different regarding B-1/B-2 fates and the difference is acquired just before or coincident with the acquisition of IL-7Rα expression.

A Transient Developmental Hematopoietic Stem Cell Gives Rise to Innate-like B and T Cells

The generation of distinct hematopoietic cell types, including tissue-resident immune cells, distinguishes fetal from adult hematopoiesis. However, the mechanisms underlying differential cell production to generate a layered immune system during hematopoietic development are unclear. Using an irreversible lineage-tracing model, we identify a definitive hematopoietic stem cell (HSC) that supports long-term multilineage reconstitution upon transplantation into adult recipients but does not persist into adulthood in situ. These HSCs are fully multipotent, yet they display both higher lymphoid cell production and greater capacity to generate innate-like B and T lymphocytes as compared to coexisting fetal HSCs and adult HSCs. Thus, these developmentally restricted HSCs (drHSCs) define the origin and generation of early lymphoid cells that play essential roles in establishing self-recognition and tolerance, with important implications for understanding autoimmune disease, allergy, and rejection of transplanted organs.

B-1 Cell Heterogeneity and the Regulation of Natural and Antigen-Induced IgM Production

A small subset of B cells, termed B-1 cells, with developmental origins, phenotypes, and functions that are distinct from those of conventional B cells exist in mice. It contributes the vast majority of spontaneously produced "natural" IgM. Natural IgM is constitutively produced, even in the absence of microbiota, and fulfills many distinct functions in tissue homeostasis and host defense. B-1 cells also respond with IgM production to innate signals and pathogen exposure, while maintaining steady-state levels natural IgM. Thus, within the B-1 cell pool, cells of distinct and heterogeneous functionality must exist to facilitate these different functions. This review considers three factors that may contribute to this heterogeneity: first, developmental differences regarding the origins of the precursors, second, tissue-specific signals that may differentially affect B-1 cells in the tissue compartments, and finally responsiveness to self-antigens as well as innate and antigen-specific signals. All three are likely to shape the repertoire and responsiveness of B-1 cells to homeostatic- and antigen-induced signals and thus contribute to the functional heterogeneity among these innate-like B cells.

Distinct Genetic Networks Orchestrate the Emergence of Specific Waves of Fetal and Adult B-1 and B-2 Development

B cell development is often depicted as a linear process initiating in the fetus and continuing postnatally. Using a PU.1 hypomorphic mouse model, we found that B-1 and B-2 lymphopoiesis occurred in distinct fetal and adult waves differentially dependent on the Sfpi1 14 kB upstream regulatory element. The initial wave of fetal B-1 development was absent in PU.1 hypomorphic mice, while subsequent fetal and adult waves emerged. In contrast, B-2 lymphopoiesis occurred in distinct fetal and adult waves. Whole-transcriptome profiling of fetal and adult B cell progenitors supported the existence of three waves of B-1 and two waves of B-2 development and revealed that the network of transcription factors governing B lineage specification and commitment was highly divergent between B-1 and B-2 progenitors. These findings support the view that the B-1 and B-2 lineages are distinct and provide a genetic basis for layering of immune system development.

Aging Converts Innate B1a Cells into Potent CD8+ T Cell Inducers

B cell dysregulation in aging is thought to mostly occur in conventional B2 cells without affecting innate B1 cells. Elderly humans and mice also accumulate 4-1BBL(+)MHC class-I(Hi)CD86(Hi)B cells of unknown origin. In this article, we report that these cells, termed 4BL cells, are activated murine and possibly human B1a cells. The activation is mediated by aging human monocytes and murine peritoneal macrophages. They induce expression and activation of 4-1BBL and IFN-γR1 on B1a cells to subsequently upregulate membrane TNF-α and CD86. As a result, activated B1a/4BL cells induce expression of granzyme B in CD8(+)T cells by targeting TNFR2 via membrane TNF-α and providing costimulation with CD86. Thus, for the first time, to our knowledge, these results indicate that aging affects the function of B1a cells. Upon aging, these cells lose their tumor-supporting activity and become inducers of potentially antitumor and autoimmune CD8(+)T cells.

Role of Polycomb RYBP in Maintaining the B-1-to-B-2 B-Cell Lineage Switch in Adult Hematopoiesis

Polycomb chromatin modifiers regulate hematopoietic pluripotent stem and progenitor cell self-renewal and expansion. Polycomb complex redundancy and biochemical heterogeneity complicate the unraveling of the functional contributions of distinct components. We have studied the hematopoietic activity of RYBP, a direct interactor and proposed modulator of RING1A/RING1B-dependent histone H2A monoubiquitylation (H2AUb). Using a mouse model to conditionally inactivate Rybp in adult hematopoiesis, we have found that RYBP deletion results in a reversion of B-1-to-B-2 B-cell progenitor ratios, i.e., of the innate (predominantly fetal) to acquired (mostly adult) immunity precursors. Increased numbers of B-1 progenitors correlated with a loss of pre-proB cells, the B-2 progenitors. RYBP-deficient stem and progenitor cell populations (LKS) and isolated common lymphoid progenitors (CLP) gave rise to increased numbers of B-1 progenitors in vitro. Rybp inactivation, however, did not result in changes of global H2AUb and did not interact genetically with Ring1A or Ring1B deletions. These results show that a sustained regulation of the B-1-to-B-2 switch is needed throughout adult life and that RYBP plays an important role in keeping B-2 dominance, most likely independently of its Polycomb affiliation.

Fetal Hematopoietic Stem Cell Transplantation Fails to Fully Regenerate the B-Lymphocyte Compartment

B cells are key components of cellular and humoral immunity and, like all lymphocytes, are thought to originate and renew from hematopoietic stem cells (HSCs). However, our recent single-HSC transfer studies demonstrate that adult bone marrow HSCs do not regenerate B-1a, a subset of tissue B cells required for protection against pneumonia, influenza, and other infections. Since B-1a are regenerated by transfers of fetal liver, the question arises as to whether B-1a derive from fetal, but not adult, HSCs. Here we show that, similar to adult HSCs, fetal HSCs selectively fail to regenerate B-1a. We also show that, in humanized mice, human fetal liver regenerates tissue B cells that are phenotypically similar to murine B-1a, raising the question of whether human HSC transplantation, the mainstay of such models, is sufficient to regenerate human B-1a. Thus, our studies overtly challenge the current paradigm that HSCs give rise to all components of the immune system.

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Purified LT-HSC transplantation fails to fully regenerate the murine immune system

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LT-HSC transplants selectively fail to regenerate B-1a cells

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LT-HSC transplantation does not regenerate VH11-encoded natural antibodies

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Human fetal liver regenerate peritoneal B cells that resemble murine B-1a

New insights into heterogeneity of peritoneal B-1a cells

Peritoneal B-1a cells are characterized by their expression of CD5 and enrichment for germline-encoded IgM B cell receptors. Early studies showing expression of a diverse array of VDJ sequences among purified B-1a cells provided a molecular basis for understanding the heterogeneity of the B-1a cell repertoire. Antigen-driven positive selection and the identification of B-1a specific progenitors suggest multiple origins of B-1a cells. The introduction of new markers such as PD-L2, CD25, CD73, and PC1 (plasma cell alloantigen 1, also known as ectonucleotide phosphodiesterase/pyrophosphatase 1) further helped to identify phenotypically and functionally distinct B-1a subsets. Among many B-1a subsets defined by these new markers, PC1 is unique in that it subdivides B-1a cells into PC1(hi) and PC1(lo) subpopulations with distinct functions, such as production of natural IgM and gut IgA, response to the pneumococcal antigen PPS-3, secretion of interleukin-10, and support for T helper 1 (TH 1) cell differentiation. RNA sequencing of these subsets revealed differential expression of genes involved in cellular movement and immune cell trafficking. We will discuss these new insights underlying the heterogeneous nature of the B-1a cell repertoire.

Distinct mechanisms define murine B cell lineage immunoglobulin heavy chain (IgH) repertoires

Processes that define immunoglobulin repertoires are commonly presumed to be the same for all murine B cells. However, studies here that couple high-dimensional FACS sorting with large-scale quantitative IgH deep-sequencing demonstrate that B-1a IgH repertoire differs dramatically from the follicular and marginal zone B cells repertoires and is defined by distinct mechanisms. We track B-1a cells from their early appearance in neonatal spleen to their long-term residence in adult peritoneum and spleen. We show that de novo B-1a IgH rearrangement mainly occurs during the first few weeks of life, after which their repertoire continues to evolve profoundly, including convergent selection of certain V(D)J rearrangements encoding specific CDR3 peptides in all adults and progressive introduction of hypermutation and class-switching as animals age. This V(D)J selection and AID-mediated diversification operate comparably in germ-free and conventional mice, indicating these unique B-1a repertoire-defining mechanisms are driven by antigens that are not derived from microbiota.

DOI:http://dx.doi.org/10.7554/eLife.09083.001

Our immune system protects us by recognizing and destroying invading viruses, bacteria and other microbes. B cells are immune cells that produce protective proteins called antibodies to stop infections. These cells are activated by ‘antigens’, which are fragments of molecules from the microbes or from our own cells. When an antigen binds to a B cell, the cell matures, multiplies and produces proteins called antibodies. These antibodies can bind to the antigen, which marks the microbe for attack and removal by other cells in the immune system.

Each antibody consists of two ‘heavy chain’ and two ‘light chain’ proteins. B cells are able to produce a large variety of different antibodies due to the rearrangement of the gene segments that encode the heavy and light chains. In mice, there are two kinds of B cells – known as B-1a and B-2 cells – that play different roles in immune responses. B-1a cells have long been known to produce the ‘natural’ antibodies that are present in the blood prior to an infection. On the other hand, B-2 cells produce antibodies that are specifically stimulated by an infection and are better adapted to fighting it. Previous studies have shown that both types of antibodies are required to allow animals to successfully fight the flu virus.

Here, Yang, Wang et al. used a technique called fluorescence-activated cell sorting (or FACS) and carried out extensive genomic sequencing to study how the B-1a and B-2 populations rearrange their genes to produce heavy chains. This approach made it possible to separate the different types of B cells and then sequence the gene for the heavy chain within the individual cells. The experiments show that the “repertoire” of heavy chains in the antibodies of the B-1a cells is much less random and more repetitive than that of B-2 populations.

Furthermore, Yang, Wang et al. show that B-1a cells produce and maintain their repertoire of heavy chains in a different way to other B-2 populations. B-1a cells develop earlier and the major genetic rearrangements in the gene that encodes the heavy chain occur within the first few weeks of life. Although the gene rearrangements have mostly stopped by adulthood, the B-1a antibody repertoire continues to evolve profoundly as the B-1a cells divide over the life of the animal. On the other hand, the gene rearrangements that make the heavy chains in the B-2 cells continue throughout the life of the animal to produce the wider repertoire of antibodies found in these cells. In addition, the processes that continue to change the antibody reperotire in the B-1a cells during adulthood do not occur in the B-2 populations.

Importantly, the these reperotire-changing processes in B-1a cells also occur in mice that have been raised in germ-free conditions, which demonstrates that – unlike other B cells – the repertoire of heavy chains in B-1a cells is not influenced by antigens from microbes. Instead, it is mainly driven by antigens that are expressed by normal cells in the body. These findings open the way to future work aimed at understanding how B-1a cells help to protect us against infection, and their role in autoimmune diseases, where immune cells attack the body’s own healthy cells.

DOI:http://dx.doi.org/10.7554/eLife.09083.002

B cells in the aging immune system: time to consider B-1 cells

The investigation of immune senescence has uncovered many changes in B cell development, maintenance, and function with increasing age. However, most of these studies have focused on conventional B cell subsets in the spleen. The B-1 cell subset is an essential arm of the innate immune system, which in general has been understudied in terms of immune senescence. Here, we review what is currently known about B cells during aging and go on to describe why B-1 cell biology is an important component of the aging immune system in the context of diseases that most affect the aged population.