The Bw cells, a novel B cell population conserved in the whole genus Mus

Immune cell subset differentiation and tissue inflammation

Immune cells were traditionally considered as major pro-inflammatory contributors. Recent advances in molecular immunology prove that immune cell lineages are composed of different subsets capable of a vast array of specialized functions. These immune cell subsets share distinct duties in regulating innate and adaptive immune functions and contribute to both immune activation and immune suppression responses in peripheral tissue. Here, we summarized current understanding of the different subsets of major immune cells, including T cells, B cells, dendritic cells, monocytes, and macrophages. We highlighted molecular characterization, frequency, and tissue distribution of these immune cell subsets in human and mice. In addition, we described specific cytokine production, molecular signaling, biological functions, and tissue population changes of these immune cell subsets in both cardiovascular diseases and cancers. Finally, we presented a working model of the differentiation of inflammatory mononuclear cells, their interaction with endothelial cells, and their contribution to tissue inflammation. In summary, this review offers an updated and comprehensive guideline for immune cell development and subset differentiation, including subset characterization, signaling, modulation, and disease associations. We propose that immune cell subset differentiation and its complex interaction within the internal biological milieu compose a “pathophysiological network,” an interactive cross-talking complex, which plays a critical role in the development of inflammatory diseases and cancers.

Control of B-1a cell development by instructive BCR signaling

B-1a cells remain one of the most enigmatic lymphocyte subsets. In this review, we discuss recent advances in our understanding of the development of these cells and their regulation by the transcription factors Bhlhe41 and Arid3a as well as by the RNA-binding protein Lin28b. A large body of literature supports an instructive role of BCR signaling in B-1a cell development and lineage commitment, which is initiated only after signaling from an autoreactive BCR. While both fetal and adult hematopoiesis can generate B-1a cells, the contribution of adult hematopoiesis to the B-1a cell compartment is low under physiological conditions. We discuss several models that can reconcile the instructive role of BCR signaling with this fetal bias in B-1a cell development.

Porcine B Cell Subset Responses to Toll-like Receptor Ligands

Toll-like receptors (TLR) triggering of B cells are known to promote B cell expansion, differentiation of B cells into antibody-producing and memory cells, but the TLR responses of porcine B cells is poorly characterized. Therefore, this study investigated the response pattern of porcine B cell subsets to a large collection of TLR ligands and demonstrates that the TLR2 ligand Pam3Cys-SK4 and the TLR7/8 ligands gardiquimod and resiquimod are particularly efficient at inducing proliferation, CD25 and CCR7. This activation was also determined in B-cell subpopulations including a CD21⁺IgM⁺ subset, an IgG⁺ subset and two putative B1-like subsets, defined as CD21^-IgM^highCD11R1⁺CD11c⁺CD14⁺ and CD21^-IgM^high CD11R1^-CD11c⁺CD14^- B cells. The latter two were larger and expressed higher levels of CD80/86 and spontaneous phospholipase C-γ2 phosphorylation. All porcine B-cell subsets were activated by TLR2, TLR7, and TLR9 ligands. Naïve and memory conventional B cells responded similar to TLR ligands. The CD11R1⁺ B1-like subset had the highest proliferative responses. While both B1-like subsets did not spontaneously secrete IgM, they were the only subsets to produce high level of TLR-induced IgM. Similar to polyclonal IgM responses, memory B cells were efficiently induced to produce specific antibodies by CpG oligodinucleotide, resiquimod, and to a weaker extend by Pam3Cys-SK4. Depletion of plasmacytoid dendritic cells (pDCs) enhanced TLR-induced antibodies. The same set of TLR ligands also induced CD40 on cDCs, pDCs, and monocytes with the exception of TLR4 ligand being unable to activate pDCs. Gardiquimod and resiquimod were particularly efficient at inducing CCR7 on pDCs. Porcine B cells expressed high levels of TLR7, but relatively little other TLR mRNA. Nevertheless, TLR2 on B cells was rapidly upregulated following stimulation, explaining the strong responses following stimulation. Subset-specific analysis of TLR expression demonstrated a comparable expression of TLR2, TLR7, and TLR9 in all B cell subsets, but TLR3 was restricted to B1-like cells, whereas TLR4 was only expressed on conventional B cells, although both at low levels. Altogether, our data describe porcine innate B1-like cells, and how different B cell subsets are involved in innate sensing.

Evolutionary implication of B-1 lineage cells from innate to adaptive immunity

The paradigm that B cells mainly play a central role in adaptive immunity may have to be reevaluated because B-1 lineage cells have been found to exhibit innate-like functions, such as phagocytic and bactericidal activities. Therefore, the evolutionary connection of B-1 lineage cells between innate and adaptive immunities have received much attention. In this review, we summarized various innate-like characteristics of B-1 lineage cells, such as natural antibody production, antigen-presenting function in primary adaptive immunity, and T cell-independent immune responses. These characteristics seem highly conserved between fish B cells and mammalian B-1 cells during vertebrate evolution. We proposed an evolutionary outline of B cells by comparing biological features, including morphology, phenotype, ontogeny, and functional activity between B-1 lineage cells and macrophages or B-2 cells. The B-1 lineage may be a transitional cell type between phagocytic cells (e.g., macrophages) and B-2 cells that functionally connects innate and adaptive immunities. Our discussion would contribute to the understanding on the origination of B cells specialized in adaptive immunity from innate immunity. The results might provide further insight into the evolution of the immune system as a whole.

Human B-1 cells take the stage

B-1 cells play critical roles in defending against microbial invasion and in housekeeping removal of cellular debris. B-1 cells secrete natural antibody and manifest functions that influence T cell expansion and differentiation and in these and other ways differ from conventional B-2 cells. B-1 cells were originally studied in mice where they are easily distinguished from B-2 cells, but their identity in the human system remained poorly defined for many years. Recently, functional criteria for human B-1 cells were established on the basis of murine findings, and reverse engineering resulted in identification of the phenotypic profile, CD20(+)CD27(+)CD43(+)CD70(-), for B-1 cells found in both umbilical cord blood and adult peripheral blood. Human B-1 cells may contribute to multiple disease states through production of autoantibody and stimulation/modulation of T cell activity. Human B-1 cells could be a rich source of antibodies useful in treating diseases present in elderly populations where natural antibody protection may have eroded. Manipulation of human B-1 cell numbers and/or activity may be a new avenue for altering T cell function and treating immune dyscrasias.

Pathogenic and physiological autoantibodies in the central nervous system

In this article, we review the current knowledge on pathological and physiological autoantibodies directed toward structures in the central nervous system (CNS) with an emphasis on their regulation and origin. Pathological autoantibodies in the CNS that are associated with autoimmunity often lead to severe neurological deficits via inflammatory processes such as encephalitis. In some instances, however, autoantibodies function as a marker for diagnostic purposes without contributing to the pathological process and/or disease progression. The existence of naturally occurring physiological autoantibodies has been known for a long time, and their role in maintaining homeostasis is well established. Within the brain, naturally occurring autoantibodies targeting aggregated proteins have been detected and might be promising candidates for new therapeutic approaches for neurodegenerative disorders. Further evidence has demonstrated the existence of naturally occurring antibodies targeting antigens on neurons and oligodendrocytes that promote axonal outgrowth and remyelination. The numerous actions of physiological autoantibodies as well as their regulation and origin are summarized in this review.

A B-cell subset uniquely responsive to innate stimuli accumulates in aged mice

We have discovered a distinct mature B-cell subset that accumulates with age, which we have termed age-associated B cells. These cells comprise up to 30% of mature B cells by 22 months. Despite sharing some features with other mature B-cell subsets, they are refractory to BCR and CD40 stimulation. Instead, they respond to TLR9 or TLR7 stimulation and divide maximally on combined BCR and TLR ligation, leading to Ig production and preferential secretion of IL-10 and IL-4. Although similar to follicular B cells in both B-lymphocyte stimulator (BLyS) receptor expression and BLyS binding capacity, these cells do not rely on BLyS for survival. They are neither cycling nor the result of intrinsically altered B lymphopoiesis in aged BM, but instead appear to be generated from mature B cells that exhaustively expand during the individual's lifetime. Finally, they present Ag effectively and favor polarization to a TH17 profile. Together, these findings reveal that while the magnitude of the mature primary B-cell niche is maintained with age, it is increasingly occupied by cells refractory to BCR-driven activation yet responsive to innate receptor stimulation.

Universal expression and dual function of the atypical chemokine receptor D6 on innate-like B cells in mice

Mouse innate-like B cells are a heterogeneous collection of multifunctional cells that control infection, play housekeeping roles, contribute to adaptive immunity, and suppress inflammation. We show that, among leukocytes, chemokine internalization by the D6 receptor is a unique and universal feature of all known innate-like B-cell populations and, to our knowledge, the most effective unifying marker of these cells. Moreover, we identify novel D6(active) B1-cell subsets, including those we term B1d, which lack CD5 and CD11b but exhibit typical B1-cell properties, including spontaneous ex vivo production of IgM, IL-10, and anti-phosphorylcholine antibody. The unprecedented opportunity to examine D6 on primary cells has allowed us to clarify its ligand specificity and show that, consistent with a scavenging role, D6 internalizes chemokines but cannot induce Ca(2+) fluxes or chemotaxis. Unexpectedly, however, D6 can also suppress the function of CXCR5, a critical chemokine receptor in innate-like B-cell biology. This is associated with a reduction in B1 cells and circulating class-switched anti-phosphorylcholine antibody in D6-deficient mice. Therefore, in the present study, we identify a unifying marker of innate-like B cells, describe novel B1-cell subsets, reveal a dual role for D6, and provide the first evidence of defects in resting D6-deficient mice.

A conserved enhancer element differentially regulates developmental expression of CD5 in B and T cells

We previously identified an enhancer element upstream of the mouse cd5 gene that was required in reporter assays for the induction of cd5 promoter activity by BCR cross-linking. This element is highly conserved in placental mammals. To determine its physiological role, we have now generated mice with a targeted deletion of the enhancer. The result is the loss of CD5 expression in peritoneal and splenic B-1a cells of adult mice and an inability to induce CD5 by cross-linking of the BCR on splenic B-2 cells. Surprisingly, CD5 expression on B-1a cells of neonatal mice was only minimally compromised. Cd5 enhancer deletion also had only a modest effect on CD5 expression in the T lineage. Thus, this enhancer provides age- and tissue-specific regulation of CD5 expression and is an example of the utilization of different modes of regulation of expression in T and B cells.

B-cell development: one problem, multiple solutions

Interspecies variations in the processes of B-cell development and repertoire generation contrast with the greater consistency of T-cell development. B-cell development in mice and humans, with postnatal B-cell generation of new repertoire in the bone marrow throughout life, is regarded as the 'standard' pattern. In contrast, accounts of B cells in birds, sheep, cattle, rabbits and pigs (the 'other' species) describe cessation of gene diversification in the perinatal period, with the gut-associated lymphoid tissue (GALT) functioning as the primary lymphoid organ thereafter. It has become customary to regard the developmental pathways of T and B cells within any individual species as being as dissimilar as the functions of the two mature cell types. Reinterpretation of B-cell development patterns in different species is overdue in response to two types of reports. The first of these describe T-B 'crossover', specifically the intrathymic production of B cells and the extrathymic production of T cells. The second attests to the extent of sharing of B-cell developmental features across the two groups of species. We propose that, as is a feature of other haematopoietic cells, a menu of alternative B- and T-cell pathways has been retained and shared across species. A single pathway usually predominates in any species, masking alternatives. The observed predominance of any pathway is determined by factors such as placental permeability, extent of maturation of the immune system by birth and the feasibility of direct experimental intervention in development.

Wild-derived mouse strains, a valuable model to study B cell responses

In the present report, we revisited the B cell responsiveness of 7 wild-derived mouse strains to various toll-like receptor ligands (TLR-L). We found that 2 of them, namely PWK and STF presented profound defects in B cell proliferative responses to most of the TLR-L. Yet, their macrophage responses were largely unaffected, suggesting that regulation of TLR pathways are distinct in B cells and macrophages. We also showed that, anti-CD40 mAbs rescued the low proliferative responses to CpG in both PWK and STF B cells. In the other hand, CpG synergized with LPS to induce high levels of proliferation in STF B cells, which did not respond to LPS alone. Cytokine or immunoglobulin (Ig) productions, in vitro, were less impaired than the proliferative responses to LPS or CpG alone. In STF B cells, both ERK, P38 and JNK pathways were affected following in vitro TLR4 or TLR9 signaling. Moreover, while the basal levels of Ig secreting cells and of serum Igs were similar to that of control mice, antibody responses to both TI and TD antigens were severely affected, mainly in STF mice. Our findings therefore highlight the relevance of wild-derived mouse strains and TLR-L to study B cell physiology.

Role of retinoic acid in the imprinting of gut-homing IgA-secreting cells

Antibody-secreting cells (ASCs) lodging in the mucosa of the small intestine are derived from activated B cells that are thought to arise in gut-associated lymphoid tissues (GALT). Upon leaving the GALT, B cells return to the blood where they must express the gut-homing receptors alpha4beta7 and CCR9 in order to emigrate into the small bowel. Recent evidence indicates that gut-associated dendritic cells (DCs) in GALT induce gut-homing receptors on B cells via a mechanism that depends on the vitamin A metabolite retinoic acid (RA). In addition, although ASC associated with other mucosal tissues secrete IgA in an RA-independent fashion, the presence of high levels of RA in intestine and GALT can promote B cell class switching to IgA and thus, boost the production of IgA in the intestinal mucosa. Here, we discuss the role of RA in the imprinting of gut-homing ASC and the evidence linking RA with the generation of intestinal IgA-ASCs.

Peripheral B cell subsets

Our understanding of the origins and the biological functions of different peripheral B cell subsets continues to evolve. Some understanding has been obtained regarding the synergy between BCR-derived signals and other receptors and signaling pathways that drive the development of follicular, marginal zone, and B-1 B cells, but this remains a complex and poorly understood issue. More recent information regarding the origins of B-1 and B-2 B cells, the ability of follicular B cells to mature both in the bone marrow and the spleen, the existence of a definable precursor for MZ B cells, and the ability of follicular B cells to occupy two distinct niches are all highlighted in this review.