Peritoneal CD5+ B-1 cells have signaling properties similar to tolerant B cells

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.

IgM Deficiency in Autoimmune Blistering Mucocutaneous Diseases Following Various Treatments: Long Term Follow-Up and Relevant Observations

IgM deficiency has been reported in patients with many autoimmune diseases treated with Rituximab (RTX). It has not been studied, in detail, in autoimmune mucocutaneous blistering diseases (AIMBD). Our objectives were: (i) Examine the dynamics of IgM levels in patients with and without RTX. (ii) Influence of reduced serum IgM levels on clinical and laboratory parameters. (iii) Explore the possible molecular and cellular basis for reduced serum IgM levels. This retrospective study that was conducted in a single-center from 2000 to 2020. Serial IgM levels were studied in 348 patients with five AIMBD (pemphigus vulgaris, pemphigus foliaceus, bullous pemphigoid, mucous membrane pemphigoid, and ocular cicatricial pemphigoid) and found decreased in 55 patients treated with RTX, IVIG, and conventional immunosuppressive therapy (CIST). Hence the incidence of decreased serum IgM is low. The incidence of decreased IgM in patients treated with RTX was 19.6%, in patients treated with IVIG and CIST, it was 52.8% amongst the 55 patients. IgM levels in the post-RTX group were statistically significantly different from the IVIG group (p<0.018) and CIST group (p<0.001). There were no statistically significant differences between the groups in other clinical and laboratory measures. Decreased serum IgM did not affect depletion or repopulation of CD19+ B cells. Patients in the three groups achieved clinical and serological remission, in spite of decreased IgM levels. Decrease in IgM was isolated, since IgG and IgA were normal throughout the study period. Decreased IgM persisted at the same level, while the patients were in clinical remission, for several years. In spite of persistent decreased IgM levels, the patients did not develop infections, tumors, other autoimmune diseases, or warrant hospitalization. Studies on IgM deficiency in knockout mice provided valuable insights. There is no universally accepted mechanism that defines decreased IgM levels in AIMBD. The data is complex, multifactorial, sometimes contradictory, and not well understood. Nonetheless, data in this study provides novel information that enhances our understanding of the biology of IgM in health and disease.

Cord-Blood-Derived Professional Antigen-Presenting Cells: Functions and Applications in Current and Prospective Cell Therapies

Human umbilical cord blood (UCB) represents a valuable source of hematopoietic stem cells, particularly for patients lacking a matching donor. UCB provides practical advantages, including a lower risk of graft-versus-host-disease and permissive human leukocyte antigen mismatching. These advantageous properties have so far been applied for stem cell, mesenchymal stromal cell, and chimeric antigen receptor T cell therapies. However, UCB-derived professional antigen-presenting cells are increasingly being utilized in the context of immune tolerance and regenerative therapy. Here, we review the cell-specific characteristics as well as recent advancements in UCB-based cell therapies focusing on dendritic cells, monocytes, B lymphocytes, innate lymphoid cells, and macrophages.

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.

Evaluation of the frequency of CD5+ B cells as natural immunoglobulin M producers and circulating soluble CD5 in patients with bladder cancer

Bladder cancer is the most common malignancy of the genitourinary tract. It is the fourth most common malignancy in men and the fifth most common malignancy in the general population, with a high recurrence rate. CD5⁺ B lymphocytes are a subset of B lymphocytes, which contribute to innate immune responses. These cells are involved in the spontaneous production of self-reactive natural antibodies. On the other hand, natural antibodies can recognize tumor-associated antigens, including proteins or carbohydrates, and eliminate these cells in a complement-dependent manner or via induction of apoptosis. Besides surface CD5, the soluble form of this molecule is involved in the regulation of immune system. Considering the role of CD5⁺ B cells in the production of natural immunoglobulin M (IgM) and role of these antibodies in antitumor responses, in this study, we aimed to investigate the frequency of CD5 in B cells and to evaluate the diagnostic potential of these cells and also soluble CD5 (sCD5) in patients with bladder cancer. Blood specimens were collected from 40 patients with bladder cancer, who were referred to Sina Hospital in Tehran, IRAN. The levels of CD5⁺ and CD5^- B lymphocytes were measured in the peripheral blood via flow cytometry, and the levels of sCD5 and total IgM were investigated in the serum by ELISA and nephlometry techniques, respectively. The frequency of CD5⁺ and CD5^- B cells was significantly lower in patients, compared with the healthy controls. Detectable levels of sCD5 were found in two patients (5%), while total IgM showed no significant difference between the patient and control groups. The present results suggest that B cell subsets may be affected by malignancy. Therefore, further research is needed to identify B cells and their soluble markers for diagnosis of patients with bladder cancer.

Evaluation of the frequency of CD5+ B cells as natural immunoglobulin M producers and circulating soluble CD5 in patients with bladder cancer

Bladder cancer is the most common malignancy of the genitourinary tract. It is the fourth most common malignancy in men and the fifth most common malignancy in the general population, with a high recurrence rate. CD5⁺ B lymphocytes are a subset of B lymphocytes, which contribute to innate immune responses. These cells are involved in the spontaneous production of self-reactive natural antibodies. On the other hand, natural antibodies can recognize tumor-associated antigens, including proteins or carbohydrates, and eliminate these cells in a complement-dependent manner or via induction of apoptosis. Besides surface CD5, the soluble form of this molecule is involved in the regulation of immune system. Considering the role of CD5⁺ B cells in the production of natural immunoglobulin M (IgM) and role of these antibodies in antitumor responses, in this study, we aimed to investigate the frequency of CD5 in B cells and to evaluate the diagnostic potential of these cells and also soluble CD5 (sCD5) in patients with bladder cancer. Blood specimens were collected from 40 patients with bladder cancer, who were referred to Sina Hospital in Tehran, IRAN. The levels of CD5⁺ and CD5^- B lymphocytes were measured in the peripheral blood via flow cytometry, and the levels of sCD5 and total IgM were investigated in the serum by ELISA and nephlometry techniques, respectively. The frequency of CD5⁺ and CD5^- B cells was significantly lower in patients, compared with the healthy controls. Detectable levels of sCD5 were found in two patients (5%), while total IgM showed no significant difference between the patient and control groups. The present results suggest that B cell subsets may be affected by malignancy. Therefore, further research is needed to identify B cells and their soluble markers for diagnosis of patients with bladder cancer.

IgM Natural Autoantibodies in Physiology and the Treatment of Disease

Antibodies are vital components of the adaptive immune system for the recognition and response to foreign antigens. However, some antibodies recognize self-antigens in healthy individuals. These autoreactive antibodies may modulate innate immune functions. IgM natural autoantibodies (IgM-NAAs) are a class of primarily polyreactive immunoglobulins encoded by germline V-gene segments which exhibit low affinity but broad specificity to both foreign and self-antigens. Historically, these autoantibodies were closely associated with autoimmune disease. Nevertheless, not all human autoantibodies are pathogenic and compelling evidence indicates that IgM-NAAs may exert a spectrum of effects from injurious to protective depending upon cellular and molecular context. In this chapter, we review the current state of knowledge regarding the potential physiological and therapeutic roles of IgM-NAAs in different disease conditions such as atherosclerosis, cancer, and autoimmune disease. We also describe the discovery of two reparative IgM-NAAs by our laboratory and delineate their proposed mechanisms of action in central nervous system (CNS) disease.

Identification of the First Teleost CD5 Molecule: Additional Evidence on Phenotypical and Functional Similarities between Fish IgM+ B Cells and Mammalian B1 Cells

Despite teleost fish being the first animal group in which all elements of adaptive immunity are present, the lack of follicular structures, as well as the fact that systemic Ab responses rely exclusively on unswitched low-affinity IgM responses, strongly suggests that fish B cell responses resemble mammalian B1 cell responses rather than those of B2 cells. In line with this hypothesis, in the current study, we have identified a homolog of CD5 in teleost fish. This pan-T marker belonging to the scavenger receptor cysteine-rich family of receptors is commonly used in mammals to distinguish a subset of B1 cells. Subsequently, we have demonstrated that a very high percentage of teleost IgM⁺ B cells express this marker, in contrast to the limited population of CD5-expressing B1 cells found in most mammals. Furthermore, we demonstrate that fish IgM⁺ B cells share classical phenotypic features of mammalian B1 cells such as large size, high complexity, high surface IgM, and low surface IgD expression, regardless of CD5 expression. Additionally, fish IgM⁺ B cells, unlike murine B2 cells, also displayed extended survival in cell culture and did not proliferate after BCR engagement. Altogether, our results demonstrate that although fish are evolutionarily the first group in which all the elements of acquired immunity are present, in the absence of follicular structures, most teleost IgM⁺ B cells have retained phenotypical and functional characteristics of mammalian B1 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.

Analysis of Signaling Events in B-1a Cells

B-1a cells are a unique subset of B cells that are phenotypically and functionally distinguishable from conventional (B-2) cells. Here we detail methods to analyze signaling events in B-1a cells. We demonstrate by flow cytometry and Western blot that mouse peritoneal B-1a but not splenic B-2 cells have constitutive activation of signal transducers and activators of transcription-3 (Stat3) and extracellular signal-regulated kinase (Erk).

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.

CD6 Receptor Regulates Intestinal Ischemia/Reperfusion-induced Injury by Modulating Natural IgM-producing B1a Cell Self-renewal

Intestinal ischemia/reperfusion (I/R) injury is a relatively common pathological condition that can lead to multi-organ failure and mortality. Regulatory mechanism for this disease is poorly understood, although it is established that circulating pathogenic natural IgM, which is primarily produced by B1a cells outside of the peritoneal cavity, are integrally involved. CD6 was originally identified as a marker for T cells and was later found to be present on some subsets of B cells in humans; however, whether CD6 plays any role in intestinal I/R-induced injury and, if so, the underlying mechanisms, remain unknown. Here we report that CD6^-/- mice were significantly protected from intestinal inflammation and mucosal damage compared with WT mice in a model of intestinal I/R-induced injury. Mechanistically, we found that CD6 was selectively expressed on B1 cells outside of the bone marrow and peritoneal cavity and that pathogenic natural IgM titers were reduced in the CD6^-/- mice in association with significantly decreased B1a cell population. Our results reveal an unexpected role of CD6 in the pathogenesis of intestinal IR-induced injury by regulating the self-renewal of B1a cells.

CD5 expression promotes IL-10 production through activation of the MAPK/Erk pathway and upregulation of TRPC1 channels in B lymphocytes

CD5 is constitutively expressed on T cells and a subset of mature normal and leukemic B cells in patients with chronic lymphocytic leukemia (CLL). Important functional properties are associated with CD5 expression in B cells, including signal transducer and activator of transcription 3 activation, IL-10 production and the promotion of B-lymphocyte survival and transformation. However, the pathway(s) by which CD5 influences the biology of B cells and its dependence on B-cell receptor (BCR) co-signaling remain unknown. In this study, we show that CD5 expression activates a number of important signaling pathways, including Erk1/2, leading to IL-10 production through a novel pathway independent of BCR engagement. This pathway is dependent on extracellular calcium (Ca²⁺) entry facilitated by upregulation of the transient receptor potential channel 1 (TRPC1) protein. We also show that Erk1/2 activation in a subgroup of CLL patients is associated with TRPC1 overexpression. In this subgroup of CLL patients, small inhibitory RNA (siRNA) for CD5 reduces TRPC1 expression. Furthermore, siRNAs for CD5 or for TRPC1 inhibit IL-10 production. These findings provide new insights into the role of CD5 in B-cell biology in health and disease and could pave the way for new treatment strategies for patients with B-CLL.

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.

JNK regulatory molecule G5PR induces IgG autoantibody-producing plasmablasts from peritoneal B1a cells

Peritoneal B1a cells expressing CD5 and CD11b generate autoantibody-producing precursors in autoimmune-prone mice. Previous studies show reduced JNK signaling in peritoneal B1a cells of female New Zealand Black mice and an abnormal increase of protein phosphatase 2A subunit G5PR that regulates BCR-mediated JNK signaling as a cause of autoimmunity. To investigate the mechanism regulating B1a differentiation into autoantibody-secreting plasmablasts (PBs), we applied an in vitro culture system that supports long-term growth of germinal center (GC) B cells (iGB) with IL-4, CD40L, and BAFF. Compared with spleen B2 cells, B1a cells differentiated into GC-like B cells, but more markedly into PBs, and underwent class switching toward IgG1. During iGB culture, B1a cells expressed GC-associated aicda, g5pr, and bcl6, and markedly PB-associated prdm1, irf4, and xbp1. B1a-derived iGB cells from New Zealand Black × New Zealand White F1 mice highly differentiated into autoantibody-secreting PBs in vitro and localized to the GC area in vivo. In iGB culture, JNK inhibitor SP600125 augmented the differentiation of C57BL/6 B1a cells into PBs. Furthermore, B1a cells from G5PR transgenic mice markedly differentiated into IgM and IgG autoantibody-secreting PBs. In conclusion, JNK regulation is critical to suppress autoantibody-secreting PBs from peritoneal B1a cells.

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.

Autoantigen can promote progression to a more aggressive TCL1 leukemia by selecting variants with enhanced B-cell receptor signaling

(Auto)antigen engagement by the B-cell receptor (BCR) and possibly the sites where this occurs influence the outcome of chronic lymphocytic leukemia (CLL). To test if selection for autoreactivity leads to increased aggressiveness and if this selection plays out equally in primary and secondary tissues, we used T-cell leukemia (TCL)1 cells reactive with the autoantigen phosphatidylcholine (PtC). After repeated transfers of splenic lymphocytes from a single mouse with oligoclonal PtC-reactive cells, outgrowth of cells expressing a single IGHV-D-J rearrangement and superior PtC-binding and disease virulence occurred. In secondary tissues, increased PtC-binding correlated with enhanced BCR signaling and cell proliferation, whereas reduced signaling and division of cells from the same clone was documented in cells residing in the bone marrow, blood, and peritoneum, even though cells from the last site had highest surface membrane IgM density. Gene-expression analyses revealed reciprocal changes of genes involved in BCR-, CD40-, and PI3K-signaling between splenic and peritoneal cells. Our results suggest autoantigen-stimulated BCR signaling in secondary tissues promotes selection, expansion, and disease progression by activating pro-oncogenic signaling pathways, and that--outside secondary lymphoid tissues--clonal evolution is retarded by diminished BCR-signaling. This transferrable, antigenic-specific murine B-cell clone (TCL1-192) provides a platform to study the types and sites of antigen-BCR interactions and genetic alterations that result and may have relevance to patients.

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.

Mouse IgM Fc receptor, FCMR, promotes B cell development and modulates antigen-driven immune responses

FcR specific for pentameric IgM (FCMR) is expressed at high levels by B cells. Although circulating IgM has profound effects on responses to pathogens, autoimmunity, and B cell homeostasis, the biologic consequences of its binding to FCMR are poorly understood. We interrogated FCMR contributions to B cell function by studying mice that lack FCMR. FCMR transcripts are expressed at different levels by various B cell subsets. FCMR-deficient mice have reduced numbers of developing B cells, splenic follicular and peritoneal B-2 cells, but increased levels of peritoneal B-1a cells and autoantibodies. After immunization, germinal center B cell and plasma cell numbers are increased. FCMR-deficient B cells are sensitive to apoptosis induced by BCR ligation. Our studies demonstrate that FCMR is required for B cell differentiation and homeostasis, the prevention of autoreactive B cells, and responsiveness to antigenic challenge.

Dynamics of the splenic innate-like CD19⁺CD45Rlo cell population from adult mice in homeostatic and activated conditions

In the adult spleen, CD19⁺CD45R(-/lo) (19⁺45R(lo)) lymphocytes of embryonic origin exist as a distinct population to that of the conventional B cell lineage. These cells display a plasmablast phenotype, and they spontaneously secrete IgG1 and IgA, whereas the bone marrow population of 19⁺45R(lo) cells contains B1 progenitors. In this study, we show that 19⁺45R(lo) cells are also present in Peyer's patches and in the spleen throughout the life span of wild-type mice, beginning at postnatal day 7. Although this population is heterogeneous, the surface phenotype of most of these cells distinguishes them from follicular, transitional, marginal zone, and B1 cells. In CBA/CaHN mice, few 19⁺45R(lo) cells were detected at postnatal day 7, and none was observed in the adult spleen. Splenic 19⁺45R(lo) cells exhibited homeostatic BrdU uptake in vivo and actively transcribed cell cycle genes. When transferred to immunodeficient RAG2⁻/⁻γchain⁻/⁻ recipient mice, 19⁺45R(lo) cells survived and differentiated into IgG1- and IgA-plasma cells. Moreover, in vitro stimulation of splenic 19⁺45R(lo) cells with LPS, CpG, BAFF/IL4, and CD40/IL4 induced cell proliferation, IgG1/IgA secretion and the release of IL-10, suggesting a potential immunoregulatory role for this subset of innate-like B cells.

Transgenic overexpression of G5PR that is normally augmented in centrocytes impairs the enrichment of high-affinity antigen-specific B cells, increases peritoneal B-1a cells, and induces autoimmunity in aged female mice

To investigate signals that control B cell selection, we examined expression of G5PR, a regulatory subunit of the serine/threonine protein phosphatase 2A, which suppresses JNK phosphorylation. G5PR is upregulated in activated B cells, in Ki67-negative centrocytes at germinal centers (GCs), and in purified B220(+)Fas(+)GL7(+) mature GC B cells following Ag immunization. G5PR rescues transformed B cells from BCR-mediated activation-induced cell death by suppression of late-phase JNK activation. In G5PR-transgenic (G5PR(Tg)) mice, G5PR overexpression leads to an augmented generation of GC B cells via an increase in non-Ag-specific B cells and a consequent reduction in the proportion of Ag-specific B cells and high-affinity Ab production after immunization with nitrophenyl-conjugated chicken γ-globulin. G5PR overexpression impaired the affinity-maturation of Ag-specific B cells, presumably by diluting the numbers of high-affinity B cells. However, aged nonimmunized female G5PR(Tg) mice showed an increase in the numbers of peritoneal B-1a cells and the generation of autoantibodies. G5PR overexpression did not affect the proliferation of B-1a and B-2 cells but rescued B-1a cells from activation-induced cell death in vitro. G5PR might play a pivotal role in B cell selection not only for B-2 cells but also for B-1 cells in peripheral lymphoid organs.

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.

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.

An overview of chronic lymphocytic leukaemia biology

Chronic lymphocytic leukaemia (CLL) is characterised by accumulation of CD5(+) monoclonal B cells in primary and secondary lymphoid tissues. Genetic defects and stimuli originating from the microenvironment concur to the selection and expansion of the malignant clone. Several lines of evidence, including molecular and functional analysis of the monoclonal immunoglobulin, support the hypothesis that stimulation through the B-cell receptor affects life and death of leukaemic cells. The microenvironment also has a critical role in the survival and accumulation of leukaemic cells within lymphoid organs where signals delivered from the surrounding cells are likely crucial in inducing proliferation. Nevertheless, several major biological issues still remain to be solved including regulation of the balance between proliferation and survival of leukaemic cells and the links between emerging gene abnormalities and microenvironment. In this context, mouse models are helpful tools in understanding disease mechanisms and in evaluating the efficacy of novel therapeutic agents.

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.

14-3-3sigma regulates B-cell homeostasis through stabilization of FOXO1

14-3-3σ regulates cytokinesis and cell cycle arrest induced by DNA damage but its role in the immune system is unknown. Using gene-targeted 14-3-3σ-deficient (i.e., KO) mice, we studied the role of 14-3-3σ in B-cell functions. Total numbers of B cells were reduced by spontaneous apoptosis of peripheral B cells. Upon B-cell antigen receptor engagement in vitro, KO B cells did not proliferate properly or up-regulate CD86. In response to T cell-independent antigens, KO B cells showed poor secretion of antigen-specific IgM. This deficit led to increased lethality of KO mice after vesicular stomatitis virus infection. KO B cells showed elevated total FOXO transcriptional activity but also increased FOXO1 degradation. Coimmunoprecipitation revealed that endogenous 14-3-3σ protein formed a complex with FOXO1 protein. Our results suggest that 14-3-3σ maintains FOXO1 at a consistent level critical for normal B-cell antigen receptor signaling and B-cell survival.

Mzb1 protein regulates calcium homeostasis, antibody secretion, and integrin activation in innate-like B cells

Marginal zone (MZ) B cells of the spleen and B1 cells, termed innate-like B cells, differ from follicular B cells by their attenuated Ca(2+) mobilization, fast antibody secretion, and increased cell adhesion. We identified and characterized Mzb1 as an endoplasmic reticulum-localized and B cell-specific protein that was most abundantly expressed in MZ B and B1 cells. Knockdown of Mzb1 in MZ B cells increased Ca(2+) mobilization and nuclear NFAT transcription factor localization, but reduced lipopolysaccharide-induced antibody secretion and integrin-mediated cell adhesion. Conversely, ectopic expression of an Lck-Mzb1 transgene in peripheral T cells resulted in attenuated Ca(2+) mobilization and augmented integrin-mediated cell adhesion. In addition to its interaction with the substrate-specific chaperone Grp94, Mzb1 augmented the function of the oxidoreductase ERp57 in favoring the expression of integrins in their activated conformation. Thus, Mzb1 helps to diversify peripheral B cell functions by regulating Ca(2+) stores, antibody secretion, and integrin activation.

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.

The importance of natural IgM: scavenger, protector and regulator

The existence of IgM has been known for more than a century, but its importance in immunity and autoimmunity continues to emerge. Studies of mice deficient in secreted IgM have provided unexpected insights into its role in several diverse processes, from B cell survival to atherosclerosis, as well as in autoimmunity and protection against infection. Among the various distinct properties that underlie the functions of IgM, two stand out: its polyreactivity and its ability to facilitate the removal of apoptotic cells. In addition, new B cell-targeted therapies for the treatment of autoimmunity have been shown to cause a reduction in serum IgM, potentially disrupting the functions of this immunoregulatory molecule and increasing susceptibility to infection.

Interleukin-10 mediated autoregulation of murine B-1 B-cells and its role in Borrelia hermsii infection

B cells are typically characterized as positive regulators of the immune response, primarily by producing antibodies. However, recent studies indicate that various subsets of B cells can perform regulatory functions mainly through IL-10 secretion. Here we discovered that peritoneal B-1 (B-1P) cells produce high levels of IL-10 upon stimulation with several Toll-like receptor (TLR) ligands. High levels of IL-10 suppressed B-1P cell proliferation and differentiation response to all TLR ligands studied in an autocrine manner in vitro and in vivo. IL-10 that accumulated in cultures inhibited B-1P cells at second and subsequent cell divisions mainly at the G1/S interphase. IL-10 inhibits TLR induced B-1P cell activation by blocking the classical NF-κB pathway. Co-stimulation with CD40 or BAFF abrogated the IL-10 inhibitory effect on B-1P cells during TLR stimulation. Finally, B-1P cells adoptively transferred from the peritoneal cavity of IL-10^−/− mice showed better clearance of Borrelia hermsii than wild-type B-1P cells. This study described a novel autoregulatory property of B-1P cells mediated by B-1P cell derived IL-10, which may affect the function of B-1P cells in infection and autoimmunity.

Circulating CD21low B cells in common variable immunodeficiency resemble tissue homing, innate-like B cells

The homeostasis of circulating B cell subsets in the peripheral blood of healthy adults is well regulated, but in disease it can be severely disturbed. Thus, a subgroup of patients with common variable immunodeficiency (CVID) presents with an extraordinary expansion of an unusual B cell population characterized by the low expression of CD21. CD21(low) B cells are polyclonal, unmutated IgM(+)IgD(+) B cells but carry a highly distinct gene expression profile which differs from conventional naïve B cells. Interestingly, while clearly not representing a memory population, they do share several features with the recently defined memory-like tissue, Fc receptor-like 4 positive B cell population in the tonsils of healthy donors. CD21(low) B cells show signs of previous activation and proliferation in vivo, while exhibiting defective calcium signaling and poor proliferation in response to B cell receptor stimulation. CD21(low) B cells express decreased amounts of homeostatic but increased levels of inflammatory chemokine receptors. This might explain their preferential homing to peripheral tissues like the bronchoalveolar space of CVID or the synovium of rheumatoid arthritis patients. Therefore, as a result of the close resemblance to the gene expression profile, phenotype, function and preferential tissue homing of murine B1 B cells, we suggest that CD21(low) B cells represent a human innate-like B cell population.

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

B-1a cells constitutively express phosphorylated, activated ERK, but the origin of pERK in B-1 cells has not been determined. To address this issue, we examined specific mediators of intracellular signaling in unmanipulated B-1a cells. We found that constitutive pERK was rapidly lost from B-1a cells following addition of metabolic inhibitors that block src kinase, Syk, PI-3K, and PLC function. We examined Syk and PLC in more detail and found rapid accumulation of phosphorylated forms of these molecules in B-1a cells, but not B-2 cells, when phosphatase activity was inhibited, and this change occurred in the majority of B-1a cells. Further, we showed that inhibition of src kinase activity eliminated "downstream" pSyk and pPLC accumulation in phosphatase-inhibited B-1a cells, indicating a pathway connection. CD86 expression is greater on B-1 than B-2 cells and plays a role in antigen presentation by B-1 cells to T cells. We found that when Syk or PI-3K was inhibited, CD86 expression was diminished in a reversible fashion. All together, these results indicate that continual activation of intracellular signaling leads to constitutive activation of ERK in B-1 cells, with attendant consequences for co-stimulatory molecule expression.

CD19 signaling is impaired in murine peritoneal and splenic B-1 B lymphocytes

B-1 cells reside predominantly within the coelomic cavities, tonsils, Peyer's patches, spleen (a minor fraction - approximately 5%) and are absent in the lymph nodes. They are the primary sources of natural IgM in the body. B-1 cells express polyreactive B cell receptors (BCRs) that cross react with self-antigens and are thus implicated in auto-immune disorders. Previously, we reported that peritoneal B-1 cells are deficient in CD19-mediated intracellular signals leading to Ca(2+) mobilization. Here, we find that splenic B-1 cells, like peritoneal B-1 cells, are defective in Ca(2+) release upon B cell activation by co-cross-linking BCR and CD19. In the absence of extracellular sources of Ca(2+), intracellular Ca(2+) flux is similar between B-1 and B-2 cells. Moreover, the intracellular component of Ca(2+) release in both subsets of B cells is mostly PI3K dependent. BCR and CD19 co-cross-linking activates Akt, a key mediator of survival and proliferation signals downstream of PI3K in splenic B-2 cells. Splenic B-1 cells, on the other hand, do not phosphorylate Akt (S473) upon similar treatment. Furthermore, BCR+CD19 cross-linking induced phosphorylation of JNK is much reduced in splenic B-1 cells. In contrast, B-1 cells exhibited increased levels of constitutively active pLyn which appears to have an inhibitory role. The CD19 induced Ca(2+) response and BCR induced proliferation response were restored by a partial inhibition of pLyn with Src kinase specific inhibitors. These findings suggest a defect in CD19-mediated signals in both peritoneal and splenic B-1 B lymphocytes, which is in part, due to higher levels of constitutively active Lyn.

Constitutive activation of distinct BCR-signaling pathways in a subset of CLL patients: a molecular signature of anergy

Stimulation through the B-cell antigen receptor (BCR) is believed to be involved in the natural history of chronic lymphocytic leukemia (CLL). Some cases respond to the in vitro cross-linking of surface immunoglobulin (sIg) with effective activation. In contrast, the remaining cases do not respond to such stimulation, thereby resembling B cells anergized after antigen encounter in vivo. However the biochemical differences between the 2 groups are ill defined, and in humans the term B-cell anergy lacks a molecular definition. We examined the expression and activation of key molecules involved in signaling pathways originating from the BCR, and we report that a proportion of CLL patients (a) expresses constitutively phosphorylated extracellular signal-regulated kinase (ERK)1/2 in the absence of AKT activation; (b) displays constitutive phosphorylation of MEK1/2 and increased nuclear factor of activated T cells (NF-AT) transactivation; and (c) is characterized by cellular unresponsiveness to sIg ligation. This molecular profile recapitulates the signaling pattern of anergic murine B cells. Our data indicate that constitutive activation of mitogen activated protein (MAP) kinase signaling pathway along with NF-AT transactivation in the absence of AKT activation may also represent the molecular signature of anergic human B lymphocytes. CLL cases with this signature may be taken as a human model of anergic B cells aberrantly expanded.

B-1 lymphocytes increase metastatic behavior of melanoma cells through the extracellular signal-regulated kinase pathway

Increasing evidence indicates that tumors require a constant influx of myelomonocytic cells to support their malignant behavior. This is caused by tumor-derived factors, which recruit and induce functional differentiation of myelomonocytic cells, most of which are macrophages. Although myeloid lineages are the classical precursors of macrophages, B-lymphoid lineages such as B-1 cells, a subset of B-lymphocytes found predominantly in pleural and peritoneal cavities, are also able to migrate to inflammatory sites and differentiate into mononuclear phagocytes exhibiting macrophage-like phenotypes. Here we examined the interplay of B-1 cells and tumor cells, and checked whether this interaction provides signals to influence melanoma cells metastases. Using in vitro coculture experiments we showed that B16, a murine melanoma cell line, and B-1 cells physically interact. Moreover, interaction of B16 with B-1 cells leads to up-regulation of metastasis-related gene expression (MMP-9 and CXCR-4), increasing its metastatic potential, as revealed by experimental metastases assays in vivo. We also provide evidence that B16 cells exhibit markedly up-regulated phosphorylation of the extracellular signal-regulated kinase (ERK) when cocultured with B-1 cells. Inhibition of ERK phosphorylation induced by B-1 cells with inhibitors of MEK1/2 strongly suppressed the induction of MMP-9 and CXCR-4 mRNA expression and impaired the increased metastatic behavior of B16. In addition, constitutive levels of ERK1/2 phosphorylation in B-1 cells are necessary for their commitment to affect the metastatic potential of B16 cells. Our findings show for the first time that B-1 lymphocytes can contribute to tumor cell properties required for invasiveness during metastatic spread.