Continual signaling is responsible for constitutive ERK phosphorylation in B-1a cells

Human VH4-34 antibodies derived from B1 cells are more frequently autoreactive than VH4-34 antibodies derived from memory cells

Human B1 cells produce natural antibodies characterized by overutilization of heavy chain variable region VH4-34 in comparison to other B cell populations. VH4-34-containing antibodies have been reported to be autoreactive and to be associated with lupus and other autoimmune dyscrasias. However, it has been unclear to what extent VH4-34 antibodies manifest autoreactivity in B1 cells or other B cell populations-in other words, are VH4-34 containing antibodies autoreactive wherever found, or mainly within the B1 cell population? To address this issue we sort purified single human B1 and memory B cells and then amplified, sequenced, cloned and expressed VH4-34-containing antibodies from 76 individual B cells. Each of these antibodies was tested for autoreactivity by HEp-2 IFA and autoantigen ELISA. Antibodies were scored as autoreactive if positive by either assay. We found VH4-34 antibodies rescued from B1 cells were much more frequently autoreactive (14/48) than VH4-34 antibodies rescued from memory B cells (2/28). Among B1 cell antibodies, 4 were HEp-2+, 6 were dsDNA+ and 4 were positive for both. Considering only HEp-2+ antibodies, again these were found more frequently among B1 cell VH4-34 antibodies (8/48) than memory B cell VH4-34 antibodies (1/28). We found autoreactivity was associated with greater CDR3 length, as expected; however, we found no association between autoreactivity and a previously described FR1 "hydrophobic patch". Our results indicate that autoreactive VH4-34-containing antibodies tend to reside within the human B1 cell population.

Homeostatic role of B-1 cells in tissue immunity

To date, studies of tissue-resident immunity have mainly focused on innate immune cells and T cells, with limited data on B cells. B-1 B cells are a unique subset of B cells with innate-like properties, enriched in murine pleural and peritoneal cavities and distinct from conventional B-2 cells in their ontogeny, phenotype and function. Here we discuss how B-1 cells represent exemplar tissue-resident immune cells, summarizing the evidence for their long-term persistence & self-renewal within tissues, differential transcriptional programming shaped by organ-specific environmental cues, as well as their tissue-homeostatic functions. Finally, we review the emerging data supporting the presence and homeostatic role of B-1 cells across non-lymphoid organs (NLOs) both in mouse and human.

Sex Influences Age-Related Changes in Natural Antibodies and CD5+ B-1 Cells

Natural Abs are primarily produced by B-1 cells and are essential for protection against Streptococcus pneumoniae The incidence and mortality rate for pneumococcal infection increases dramatically after age 65, disproportionately affecting males in both human and murine systems. To date, there is a significant gap in our understanding of the relationship among sex, aging, natural IgM efficacy, and the natural IgM repertoire. Our investigation demonstrates that the protective capacity of serum IgM against pneumococcal infection is maintained in IgM obtained from aged female mice but absent in IgM from aged male mice. To understand this difference in protective capacity, we examined serum Ig, discovering that the protective change was not associated with shifts in levels of phosphorylcholine (PC)- or pneumococcal capsular polysaccharide serotype 3-specific IgM. Interestingly, we observed that aged females have an increase in the total number of CD5⁺ B-1 cells, higher serum IL-5 levels, and a larger percentage of aged female CD5⁺ B-1 cells that express CD86 as compared with aged males. Furthermore, single-cell IgM repertoire analysis from peritoneal PC⁺, splenic PC⁺, and bone marrow CD5⁺ B-1 cell subsets demonstrated greater diversity with age and a higher level of germline status in female mice than previously observed in studies of aged male mice. Aged female CD5⁺ B-1 cells also expressed higher levels of transcripts associated with cell activity and self-renewal, such as Nanog and Hmga2 Taken together, these data indicate that females maintain a more diverse and active CD5⁺ B-1 cell pool and natural IgM repertoire, which has implications for sex-related susceptibility to infection and disease.

Innate B Cells: the Archetype of Protective Immune Cells

The innate B cell (IBC) population is heterogeneous and involved in the primary immune response. IBC functions include a high ability to produce natural antibodies with IgM isotype, the elimination of apoptotic cells, and a capacity to be cognate help to T cells. Among IBC subsets, B-1 cells and marginal zone B cells are the main producers of IgM, act as rapid immune responders that may relocate to follicular lymphoid and differentiate to cytokine and antibody-secreting cells shortly after infection. IBCs functions are highly dependent on their localization site and the nature of their B cell receptor repertoire, suggesting a high plasticity range of different immune responses. In this review, we will describe the nature and functions of the different innate-like B cell subsets, first in mice and then in humans. Besides this, we will emphasize the strong ability of these cells to undertake different protective functions from the first line of defense against pathogens to the regulatory role of the broader immune response.

Essential role for Cmtm7 in cell-surface phenotype, BCR signaling, survival and Igμ repertoire of splenic B-1a cells

B-1a cells represent a distinct B cell population with unique phenotype, self-renewing capacity and restricted Igμ repertoire. They primarily locate in body cavity and also exist in spleen. The different subpopulations of B-1a cells are heavily affected by local environment. Our previous studies revealed that MARVEL-domain-containing membrane protein, CMTM7, was involved in B-1a cell development. Here, we focused its influence on peritoneal and splenic B-1a cells. Unlike peritoneal B-1a cells, we found that splenic Cmtm7^-/- B-1a cells expressed higher level of CD5, CD80 and CD86 compared with WT counterparts. They also exhibited an enhanced tonic BCR signals in steady state. Though the cell viability was unaffected in vitro, Cmtm7 knockout markedly promoted splenic B-1a cell apoptosis in situ, which was likely associated with down-regulation of Il-5rα. With regard to Igμ repertoire, peritoneal and splenic Cmtm7^-/- B-1a cells exhibit similar changes exemplified by the loss of V_H11 and gain of V_H12, whereas an increase in V_H1 usage and skewed J segments from J_H1 to J_H2 and J_H4 families could only be detected within splenic Cmtm7^-/- B-1a cells. Overall, these data indicate that Cmtm7 functions differently in peritoneal and splenic B-1a cells and plays a more important role in splenic cells.

Expansion and Sub-Classification of T Cell-Dependent Antibody Responses to Encompass the Role of Innate-Like T Cells in Antibody Responses

In addition to T cell-dependent (TD) Ab responses, T cells can also regulate T cell-independent (TI) B cell responses in the absence of a specific major histocompatibility complex (MHC) class II and antigenic peptide-based interaction between T and B cells. The elucidation of T cells capable of supporting TI Ab responses is important for understanding the cellular mechanism of different types of TI Ab responses. Natural killer T (NKT) cells represent 1 type of helper T cells involved in TI Ab responses and more candidate helper T cells responsible for TI Ab responses may also include γδ T cells and recently reported B-1 helper CD4⁺ T cells. Marginal zone (MZ) B and B-1 cells, 2 major innate-like B cell subsets considered to function independently of T cells, interact with innate-like T cells. Whereas MZ B and NKT cells interact mutually for a rapid response to blood-borne infection, peritoneal memory phenotype CD49d^highCD4⁺ T cells support natural Ab secretion by B-1 cells. Here the role of innate-like T cells in the so-called TI Ab response is discussed. To accommodate the involvement of T cells in the TI Ab responses, we suggest an expanded classification of TD Ab responses that incorporate cognate and non-cognate B cell help by innate-like T cells.

Bruton's Tyrosine Kinase Is Not Essential for B Cell Survival beyond Early Developmental Stages

Bruton's tyrosine kinase (Btk) is a crucial regulator of B cell signaling and is a therapeutic target for lymphoma and autoimmune disease. BTK-deficient patients suffer from humoral immunodeficiency, as their B cells fail to progress beyond the bone marrow. However, the role of Btk in fully developed, mature peripheral B cells is not well understood. Analysis using BTK inhibitors is complicated by suboptimal inhibition, off-target effects, or failure to eliminate BTK's adaptor function. Therefore a Btk^flox/Cre-ER^T2 mouse model was developed and used to excise Btk after B cell populations were established. Mice lacking Btk from birth are known to have reduced follicular (FO) compartments, with expanded transitional populations, suggesting a block in development. In adult Btk^flox/Cre-ER^T2 mice, Btk excision did not reduce FO B cells, which persisted for weeks. Autoimmune-prone B1 cells also survived conditional Btk excision, contrasting their near absence in global Btk-deficient mice. Therefore, Btk supports BCR signaling during selection into the FO and B1 compartments, but is not needed to maintain these cell populations. B1-related natural IgM levels remained normal, contrasting global Btk deficiency, but B cell proliferation and T-independent type II immunization responses were blunted. Thus, B cells have nuanced signaling responses that are differentially regulated by Btk for development, survival, and function. These findings raise the possibility that Btk may also be expendable for survival of mature human B cells, therefore requiring prolonged dosing to be effective, and that success of BTK inhibitors may depend in part on off-target effects.

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.

Essential role for the transcription factor Bhlhe41 in regulating the development, self-renewal and BCR repertoire of B-1a cells

Innate-like B-1a cells provide a first line of defense against pathogens, yet little is known about their transcriptional control. Here we identified an essential role of the transcription factor Bhlhe41, with a lesser contribution of Bhlhe40, in controlling late stages of B-1a cell differentiation. Bhlhe41^–/–Bhlhe40^–/– B-1a cells were severely reduced as compared to their wild-type counterparts. Mutant B-1a cells exhibited an abnormal cell-surface phenotype and altered B-cell receptor (BCR) repertoire exemplified by loss of the phosphatidylcholine-specific V_H12/Vκ4 BCR. Expression of a pre-rearranged V_H12/Vκ4 BCR failed to rescue the mutant phenotype and revealed enhanced proliferation accompanied with increased cell death. Bhlhe41 directly repressed the expression of cell cycle regulators and inhibitors of BCR signaling, while enabling pro-survival cytokine signaling. Thus, Bhlhe41 controls the development, BCR repertoire and self-renewal of B-1a cells.

Developmental expression of B cell molecules in equine lymphoid tissues

Identification and classification of B cell subpopulations has been shown to be challenging and inconsistent among different species. Our study tested aspects of ontogeny, phenotype, tissue distribution, and function of equine CD5^hi B cells, which represented a greater proportion of B cells early in development and in the peritoneal cavity. CD5^hi and CD5^lo B cells differentially expressed B cell markers (CD2, CD21, IgM) measured using flow cytometry, but similar mRNA expression of signature genes (DGKA, FGL2, PAX5, IGHM, IL10) measured using quantitative RT-PCR. Sequencing lambda light chain segments revealed that CD5^hi B cells generated diverse immunoglobulin repertoires, and more frequently bound to fluorescence-labeled phosphorylcholine. This study shows developmental characteristics and tissue distribution of a newly described subpopulation of B cells in the horse.

Expansion of B-1a Cells with Germline Heavy Chain Sequence in Lupus Mice

B6.Sle1.Sle2.Sle3 (B6.TC) lupus-prone mice carrying the NZB allele of Cdkn2c, encoding for the cyclin-dependent kinase inhibitor P18(INK4), accumulate B-1a cells due to a higher rate of proliferative self-renewal. However, it is unclear whether this affects primarily early-appearing B-1a cells of fetal origin or later-appearing B-1a cells that emerge from bone marrow. B-1a cells are the major source of natural autoantibodies, and it has been shown that their protective nature is associated with a germline-like sequence, which is characterized by few N-nucleotide insertions and a repertoire skewed toward rearrangements predominated during fetal life, VH11 and VH12. To determine the nature of B-1a cells expanded in B6.TC mice, we amplified immunoglobulin genes by PCR from single cells in mice. Sequencing showed a significantly higher proportion of B-1a cell antibodies that display fewer N-additions in B6.TC mice than in B6 control mice. Following this lower number of N-insertions within the CDR-H3 region, the B6.TC B-1a cells display shorter CDR-H3 length than B6 B-1a cells. The absence of N-additions is a surrogate for fetal origin, as TdT expression starts after birth in mice. Therefore, our results suggest that the B-1a cell population is not only expanded in autoimmune B6.TC mice but also qualitatively different with the majority of cells from fetal origin. Accordingly, our sequencing results also demonstrated the overuse of VH11 and VH12 in autoimmune B6.TC mice as compared to B6 controls. These results suggest that the development of lupus autoantibodies in these mice is coupled with skewing of the B-1a cell repertoire and possible retention of protective natural antibodies.

RasGRP1 Is an Essential Signaling Molecule for Development of B1a Cells with Autoantigen Receptors

B1a cells, particularly the PD-L2(+) B1a cell subset, are enriched with autoantigen-specific receptors. However, the underlying molecular mechanism responsible for the skewed selection of autoreactive B1a cells remains unclear. In this study, we find that B1 cells express only Ras guanyl nucleotide-releasing protein (RasGRP) 1, whereas B2 cells express mostly RasGRP3 and little RasGRP1. RasGRP1 is indispensable for transduction of weak signals. RasGRP1 deficiency markedly impairs B1a cell development and reduces serum natural IgM production; in particular, B1a cells that express autoantigen receptors, such as anti-phosphatidylcholine B1a cells, are virtually eliminated. Thus, unlike Btk and other signalosome components, RasGRP1 deficiency selectively affects only the B1a cell population with autoantigen receptors rather than the entire pool of B1a cells.

IL-17A Promotes Pulmonary B-1a Cell Differentiation via Induction of Blimp-1 Expression during Influenza Virus Infection

B-1 cells play a critical role in early protection during influenza infections by producing natural IgM antibodies. However, the underlying mechanisms involved in regulating this process are largely unknown. Here we found that during influenza infection pleural cavity B-1a cells rapidly infiltrated lungs, where they underwent plasmacytic differentiation with enhanced IgM production. This process was promoted by IL-17A signaling via induction of Blimp-1 expression and NF-κB activation in B-1a cells. Deficiency of IL-17A led to severely impaired B-1a-derived antibody production in the respiratory tract, resulting in a deficiency in viral clearance. Transfer of B-1a-derived natural antibodies rescued Il17a^-/- mice from otherwise lethal infections. Together, we identify a critical function of IL-17A in promoting the plasmacytic differentiation of B-1a cells. Our findings provide new insights into the mechanisms underlying the regulation of pulmonary B-1a cell response against influenza infection.

Influenza infection is highly localized in respiratory tract where immune response is triggered to provide protection from primary infection. Although natural IgM antibodies produced by B-1a cells have long been recognized as first-line protection against influenza, it remains unclear whether B-1a cell response occurs in the lung and what molecular mechanisms regulate this process. We show that airway exposure to influenza causes migration of B-1a cells to lungs for further differentiation into plasma cells with enhanced production of protective IgM antibodies. IL-17A critically regulates this process by driving differentiation of B-1a cells to high-rate IgM producing plasma cells in situ. Thus, IL-17A is a key factor in the local inflammatory milieu that modulates early humoral immunity afforded by B-1a cells.

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.

FCRL regulation in innate-like B cells

Coelomic cavity–derived B-1 and splenic marginal zone (MZ) B lymphocytes play principal roles in frontline host protection at homeostasis and during primary humoral immune responses. Although they share many features that enable rapid and broad-based defense against pathogens, these innate-like subsets have disparate B cell receptor (BCR) signaling features. Members of the Fc receptor–like (FCRL) family are preferentially expressed by B cells and possess tyrosine-based immunoregulatory function. An unusual characteristic of many of these cell surface proteins is the presence of both inhibitory (ITIM) and activating (ITAM-like) motifs in their cytoplasmic tails. In mice, FCRL5 is a discrete marker of splenic MZ and peritoneal B-1 B cells and has both ITIM and ITAM-like sequences. Recent work explored its signaling properties and identified that FCRL5 differentially influences innate-like BCR function. Closer scrutiny of these differences disclosed the ability of FCRL5 to counter-regulate BCR activation by recruiting SHP-1 and Lyn to its cytoplasmic motifs. Furthermore, the disparity in FCRL5 regulation between MZ and B-1 B cells correlated with relative intracellular concentrations of SHP-1. These findings validate and extend our understanding of the unique signaling features in innate-like B cells and provide new insight into the complexity of FCRL modulation.

Dysregulated extracellular signal-regulated kinase signaling associated with impaired B-cell receptor endocytosis in patients with common variable immunodeficiency

BACKGROUND

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by B-cell dysfunction and, in a subgroup, by expansion of CD21(low) B cells. The CD21(low) B cells display defects in early B-cell receptor (BCR) signaling resembling those of anergic B cells.

OBJECTIVE

We sought to investigate whether B cells from patients with CVID, like anergic B cells, have defects in extracellular signal-regulated kinase (ERK) phosphorylation and in endocytic trafficking of the BCR.

METHODS

Using flow cytometry, we evaluated phosphorylated ERK (pERK) expression and internalization of cross-linked BCR in B-cell subsets. The localization of internalized BCR to lysosome-associated membrane protein 1-positive late endosomes was evaluated with confocal microscopy.

RESULTS

Constitutive pERK levels were increased in naive and IgM(+) memory B cells of patients with CVID compared with those of healthy donors, whereas the pERK increment induced by BCR cross-linking was relatively reduced. Intravenous immunoglobulin administration enhanced these anomalies, but they appeared to be intrinsic to B cells from patients with CVID. Cross-linking-induced BCR endocytosis was decreased in the IgM(+) memory B cells, especially in those with a CD21(low) phenotype, but not in the naive B cells of patients with CVID with CD21(low) expansion. Internalized BCR localized normally to late endosomes. Pharmacologic inhibition of ERK phosphorylation suppressed BCR endocytosis in B cells of healthy patients and those with CVID.

CONCLUSIONS

The B cells of patients with CVID with CD21(low) B-cell expansion resemble anergic B cells based on high constitutive pERK expression. The IgM(+) memory B cells of these patients, especially those that are CD21(low), have a defect in BCR endocytosis seemingly caused by dysregulated ERK signaling.

RGMb is a novel binding partner for PD-L2 and its engagement with PD-L2 promotes respiratory tolerance

We report that programmed death ligand 2 (PD-L2), a known ligand of PD-1, also binds to repulsive guidance molecule b (RGMb), which was originally identified in the nervous system as a co-receptor for bone morphogenetic proteins (BMPs). PD-L2 and BMP-2/4 bind to distinct sites on RGMb. Normal resting lung interstitial macrophages and alveolar epithelial cells express high levels of RGMb mRNA, whereas lung dendritic cells express PD-L2. Blockade of the RGMb-PD-L2 interaction markedly impaired the development of respiratory tolerance by interfering with the initial T cell expansion required for respiratory tolerance. Experiments with PD-L2-deficient mice showed that PD-L2 expression on non-T cells was critical for respiratory tolerance, but expression on T cells was not required. Because PD-L2 binds to both PD-1, which inhibits antitumor immunity, and to RGMb, which regulates respiratory immunity, targeting the PD-L2 pathway has therapeutic potential for asthma, cancer, and other immune-mediated disorders. Understanding this pathway may provide insights into how to optimally modulate the PD-1 pathway in cancer immunotherapy while minimizing adverse events.

B-1a cell diversity: nontemplated addition in B-1a cell Ig is determined by progenitor population and developmental location

Natural Abs produced by B-1a cells are required for immediate protection against infection. The protective capacity of natural Abs is attributed to germline-like structure, which includes the relative absence of N-region addition. Previous studies have shown B-1a cell Ig from aged mice contains abundant nontemplated (N)-additions. B-1a cells have been shown to derive from a specific lineage-negative (Lin(-))CD45R(low/-)CD19(+) progenitor found both in fetal liver and adult bone marrow. In this study, we report identification of a fetal liver population characterized phenotypically as Lin(-)CD45R(-)CD19(-), which gives rise to IgM(+)IgD(low)CD45R(low)CD5(+)Mac-1(+)CD19(high)CD43(+)CD23(low) B-1a cells upon adoptive transfer to SCID recipients. These B-1a cells derived from the Lin(-)CD45R(-)CD19(-) fetal liver population produce natural Ab that binds pneumococcal Ags, but this Ig contains substantial N-addition despite initial absence of TdT. Furthermore, we show extensive N-addition is also present in B-1a cells derived from the Lin(-)CD45R(low/-)CD19(+) B-1 progenitor found in the bone marrow. Together these results demonstrate B-1a cell N-addition depends on the type of progenitor and the location of the progenitor during its development. These findings have implications for how regulation of different progenitors from fetal liver and bone marrow may play a role in the age-related increase in N-region addition by B-1a cells in normal animals.

Atypical Response of B-1 Cells to BCR Ligation: A Speculative Model

Peritoneal B-1a cells manifest unusual signaling characteristics that distinguish them from splenic B-2 cells. These include the failure of BCR engagement to trigger NF-κB activation and DNA replication. Despite extensive study, a clear explanation for these characteristics has not emerged. Here we aim to develop a unified paradigm based on previous reports and recent results, which proposes a central role for phosphatase activity. We hypothesize B-1a cells are unable to induce NF-κB or proliferate after BCR cross-linking due to increased phosphatase abundance or activity. This phosphatase abundance and/or activity may be the result of unique B-1a cell characteristics such as increased levels of HSP70 and/or constitutive secretion of IL-10. We speculate phosphatase activity cannot be overcome by BCR ligation alone due to insufficient Vav protein expression, which does not allow for proper production of reactive oxygen species, which inhibit phosphatases. Furthermore, constitutively active Lyn also plays a negative regulatory role in B-1a. We expect that a new focus on phosphatase activity and its suppression will be revealing for BCR signal transduction in B-1 cells.

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.

FCRL5 exerts binary and compartment-specific influence on innate-like B-cell receptor signaling

Innate-like splenic marginal zone (MZ) and peritoneal cavity B1 B lymphocytes share critical responsibilities in humoral responses but have divergent B-cell receptor (BCR) signaling features. A discrete marker of these subsets with tyrosine-based dual regulatory potential termed "Fc receptor-like 5" (FCRL5) was investigated to explore this discrepancy. Although FCRL5 repressed the robust BCR activity that is characteristic of MZ B cells, it had no influence on antigen receptor stimulation that is blunted in peritoneal cavity-derived B1 B cells. The molecular basis for the receptor's inhibitory function derived from recruitment of the Src homology-2 domain-containing tyrosine phosphatase 1 (SHP-1) to a cytoplasmic immunoreceptor tyrosine-based inhibitory motif. Surprisingly, mutagenesis of this docking site unearthed coactivation properties for FCRL5 that were orchestrated by independent association of the Lyn Src-family kinase with an intracellular immunoreceptor tyrosine-based activation motif-like sequence. FCRL5's unique binary regulation directly correlated with SHP-1 and Lyn activity, which, like BCR function, differed between MZ and B1 B cells. These findings collectively imply a specialized counterregulatory role for FCRL molecules at the intersection of innate and adaptive immunity.

Natural IgM in immune equilibrium and harnessing their therapeutic potential

Natural IgM Abs are the constitutively secreted products of B1 cells (CD5(+) in mice and CD20(+)CD27(+)CD43(+)CD70(-) in humans) that have important and diverse roles in health and disease. Whereas the role of natural IgM as the first line of defense for protection against invading microbes has been extensively investigated, more recent reports have highlighted their potential roles in the maintenance of tissue homeostasis via clearance of apoptotic and altered cells through complement-dependent mechanisms, inhibition of inflammation, removal of misfolded proteins, and regulation of pathogenic autoreactive IgG Abs and autoantibody-producing B cells. These observations have provided the theoretical underpinnings for efforts that currently seek to harness the untapped therapeutic potential of natural IgM either by boosting in vivo natural IgM production or via therapeutic infusions of monoclonal and polyclonal IgM preparations.

Human b1 cell frequency: isolation and analysis of human b1 cells

Controversy over the frequency of human B1 cells in normal individuals has arisen as different labs have begun to employ non-uniform techniques to study this population. The phenotypic profile and relative paucity of circulating human B1 cells place constraints on methodology to identify and isolate this population. Multiple steps must be optimized to insure accurate enumeration and optimal purification. In the course of working with human B1 cells we have developed a successful strategy that provides consistent analysis of B1 cells for frequency determination and efficient isolation of B1 cells for functional studies. Here we discuss issues attendant to identifying human B1 cells and outline a carefully optimized approach that leads to uniform and reproducible data.

FcγRIIb and BAFF differentially regulate peritoneal B1 cell survival

B1 cells produce most natural Abs in unimmunized mice and play a key role in the response to thymus-independent Ags and microbial infection. Enlargement of B1 cell number in mice is often associated with autoimmunity. However, the factors that control peripheral B1 cell survival remain poorly characterized. Mice lacking the inhibitory receptor FcγRIIb exhibit a massive expansion in peritoneal B1 cells, implicating this receptor in B1 cell homeostasis. In this study, we show that peritoneal B1 cells express the highest levels of FcγRIIb among B cell subsets and are highly susceptible to FcγRIIb-mediated apoptosis. B1 cells upregulate FcγRIIb in response to innate signals, including CpG, and the B cell homeostatic cytokine BAFF efficiently protects activated B1 cells from FcγRIIb-mediated apoptosis via receptor downregulation. BAFF-transgenic mice manifest an expansion of peritoneal B1 cells that express lower levels of FcγRIIb and exhibit reduced susceptibility to apoptosis. Whereas both peritoneal B1 cells from wild-type and BAFF-transgenic mice immunized with CpG exhibit an increase in FcγRIIb levels, this change is blunted in BAFF-transgenic animals. Our combined results demonstrate that FcγRIIb controls peritoneal B1 cell survival and this program can be modulated by the BAFF signaling axis.

A CD25⁻ positive population of activated B1 cells expresses LIFR and responds to LIF

B1 B cells defend against infectious microorganisms by spontaneous secretion of broadly reactive “natural” immunoglobulin that appears in the absence of immunization. Among many distinguishing characteristics, B1 B cells display evidence of activation that includes phosphorylated STAT3. In order to identify the origin of pSTAT3 we examined interleukin-2 receptor (IL-2R) expression on B1 cells. We found that some (about 1/5) B1a cells express the IL-2R α chain, CD25. Although lacking CD122 and unresponsive to IL-2, B1a cells marked by CD25 express increased levels of activated signaling intermediates, interruption of which results in diminished CD25. Further, CD25⁺ B1a cells contain most of the pSTAT3 found in the B1a population as a whole. Moreover, CD25⁺ B1a cells express leukemia inhibitory factor receptor (LIFR), and respond to LIF by upregulating pSTAT3. Together, these results define a new subset of B1a cells that is marked by activation-dependent CD25 expression, expresses substantial amounts of activated STAT3, and contains a functional LIFR.

Human B1 cells in umbilical cord and adult peripheral blood express the novel phenotype CD20+ CD27+ CD43+ CD70-

Human B1 cells consist of CD20⁺CD27⁺CD43⁺CD70⁻ cells bearing a skewed B cell receptor repertoire, and are present in umbilical cord and adult peripheral blood.

B1 cells differ in many ways from conventional B cells, most prominently in the production of natural immunoglobulin, which is vitally important for protection against pathogens. B1 cells have also been implicated in the pathogenesis of autoimmune dyscrasias and malignant diseases. It has been impossible to accurately study B1 cells during health and illness because the nature of human B1 cells has not been successfully defined. This has produced controversy regarding the existence of human B1 cells. Here, we determined the phenotype of human B1 cells by testing sort-purified B cell fractions for three fundamental B1 cell functions based on mouse studies: spontaneous IgM secretion, efficient T cell stimulation, and tonic intracellular signaling. We found that a small population of CD20⁺CD27⁺CD43⁺ cells present in both umbilical cord and adult peripheral blood fulfilled these criteria and expressed a skewed B cell receptor repertoire. These B cells express little or no surface CD69 and CD70, both of which are markedly up-regulated after activation of CD20⁺CD27⁻CD43⁻ (naive) and CD20⁺CD27⁺CD43⁻ (memory) B cells. This work identifies human B1 cells as CD20⁺CD27⁺CD43⁺CD70⁻. We determined that the proportion of B1 cells declines with age, which may contribute to disease susceptibility. Identification of human B1 cells provides a foundation for future studies on the nature and role of these cells in human disease.

The double life of a B-1 cell: self-reactivity selects for protective effector functions

During their development, B and T cells with self-reactive antigen receptors are generally deleted from the repertoire to avoid autoimmune diseases. Paradoxically, innate-like B-1 cells in mice are positively selected for self-reactivity and form a pool of long-lived, self-renewing B cells that produce most of the circulating natural IgM antibodies. This Review provides an overview of the developmental processes that shape the B-1 cell pool in mice, outlines the functions of B-1 cells in both the steady state and during host defence, and discusses possible functional B-1 cell homologues that exist in humans.