Cutting Edge: The Link between Lymphocyte Deficiency and Autoimmunity: Roles of Endogenous T and B Lymphocytes in Tolerance

The Role of B Lymphocyte Subsets in Adipose Tissue Development, Metabolism, and Aging

Adipose tissue contains resident B lymphocytes (B cells) with varying immune functions and mechanisms, depending on the adipose depot type and location. The heterogeneity of B cells and their functions affect the immunometabolism of the adipose tissue in aging and age-associated metabolic disorders. B cells exist in categorizations of subsets that have developmental or phenotypic differences with varying functionalities. Subsets can be categorized as either protective or pathogenic depending on their secretion profile or involvement in metabolic maintenance. In this article, we summarized recent finding on the B cell heterogeneity and discuss how we can utilize our current knowledge of adipose resident B lymphocytes for potential treatment for age-associated metabolic disorders. © 2022 American Physiological Society. Compr Physiol 12: 1-13, 2022.

ZAP70, too little, too much can lead to autoimmunity*

Establishing both central and peripheral tolerance requires the appropriate TCR signaling strength to discriminate self‐ from agonist‐peptide bound to self MHC molecules. ZAP70, a cytoplasmic tyrosine kinase, directly interacts with the TCR complex and plays a central and requisite role in TCR signaling in both thymocytes and peripheral T cells. By studying ZAP70 hypomorphic mutations in mice and humans with a spectrum of hypoactive or hyperactive activities, we have gained insights into mechanisms of central and peripheral tolerance. Interestingly, both hypoactive and hyperactive ZAP70 can lead to the development of autoimmune diseases, albeit through distinct mechanisms. Immature thymocytes and mature T cells rely on normal ZAP70 function to complete their development in the thymus and to modulate T cell responses in the periphery. Hypoactive ZAP70 function compromises key developmental checkpoints required to establish central tolerance, allowing thymocytes with potentially self‐reactive TCRs a greater chance to escape negative selection. Such ‘forbidden clones’ may escape into the periphery and may pose a greater risk for autoimmune disease development since they may not engage negative regulatory mechanisms as effectively. Hyperactive ZAP70 enhances thymic negative selection but some thymocytes will, nonetheless, escape negative selection and have greater sensitivity to weak and self‐ligands. Such cells must be controlled by mechanisms involved in anergy, expansion of Tregs, and upregulation of inhibitory receptors or signaling molecules. However, such potentially autoreactive cells may still be able to escape control by peripheral negative regulatory constraints. Consistent with findings in Zap70 mutants, the signaling defects in at least one ZAP70 substrate, LAT, can also lead to autoimmune disease. By dissecting the similarities and differences among mouse models of patient disease or mutations in ZAP70 that affect TCR signaling strength, we have gained insights into how perturbed ZAP70 function can lead to autoimmunity. Because of our work and that of others on ZAP70, it is likely that perturbations in other molecules affecting TCR signaling strength will be identified that also overcome tolerance mechanisms and cause autoimmunity. Delineating these molecular pathways could lead to the development of much needed new therapeutic targets in these complex diseases.

Regulatory B Cells in Autoimmune Diabetes

Since they were discovered almost three decades ago, a subset of B cells denoted as regulatory B cells (Bregs) have elicited interest throughout the immunology community. Many investigators have sought to characterize their phenotype and to understand their function and immunosuppressive mechanisms. Indeed, studies in murine models have demonstrated that Bregs possess varied phenotypic markers and could be classified into different subsets whose action and pivotal role depend on the pathological condition or stimuli. Similar conclusions were drawn in clinical settings delineating an analogous Breg population phenotypically resembling the murine Bregs that ultimately may be associated with a state of tolerance. Recent studies suggested that Bregs may play a role in the onset of autoimmune diabetes. This review will focus on deciphering the different subclasses of Bregs, their emerging role in autoimmune diabetes, and their potential use as a cell-based therapeutic.

Reporters of TCR signaling identify arthritogenic T cells in murine and human autoimmune arthritis

How arthritis-causing T cells trigger rheumatoid arthritis (RA) is not understood since it is difficult to differentiate T cells activated by inflammation in arthritic joints from those activated through their T cell antigen receptor (TCR) by self-antigens. We developed a model to identify and study antigen-specific T cell responses in arthritis. Nur77—a specific marker of TCR signaling—was used to identify antigen-activated T cells in the SKG arthritis model and in patients with RA. Nur77 could distinguish highly arthritogenic and autoreactive T cells in SKG mice. The enhanced autoreactivity was associated with increased interleukin-6 (IL-6) receptor signaling, likely contributing to their arthritogenicity. These data highlight a functional correlate between Nur77 expression, arthritogenic T cell populations, and heightened IL-6 sensitivity in SKG mice with translatable implications for human RA.

How pathogenic cluster of differentiation 4 (CD4) T cells in rheumatoid arthritis (RA) develop remains poorly understood. We used Nur77—a marker of T cell antigen receptor (TCR) signaling—to identify antigen-activated CD4 T cells in the SKG mouse model of autoimmune arthritis and in patients with RA. Using a fluorescent reporter of Nur77 expression in SKG mice, we found that higher levels of Nur77-eGFP in SKG CD4 T cells marked their autoreactivity, arthritogenic potential, and ability to more readily differentiate into interleukin-17 (IL-17)–producing cells. The T cells with increased autoreactivity, nonetheless had diminished ex vivo inducible TCR signaling, perhaps reflective of adaptive inhibitory mechanisms induced by chronic autoantigen exposure in vivo. The enhanced autoreactivity was associated with up-regulation of IL-6 cytokine signaling machinery, which might be attributable, in part, to a reduced amount of expression of suppressor of cytokine signaling 3 (SOCS3)—a key negative regulator of IL-6 signaling. As a result, the more autoreactive GFP^hi CD4 T cells from SKGNur mice were hyperresponsive to IL-6 receptor signaling. Consistent with findings from SKGNur mice, SOCS3 expression was similarly down-regulated in RA synovium. This suggests that despite impaired TCR signaling, autoreactive T cells exposed to chronic antigen stimulation exhibit heightened sensitivity to IL-6, which contributes to the arthritogenicity in SKG mice, and perhaps in patients with RA.

Frequency of CD19+CD24hiCD38hi regulatory B cells is decreased in peripheral blood and synovial fluid of patients with juvenile idiopathic arthritis: a preliminary study

BACKGROUND

To understand the relationship between regulatory B cells (Bregs) and juvenile idiopathic arthritis (JIA), we analyzed the percentages of Bregs and their function in peripheral blood (PB) and synovial fluid (SF) of JIA patients.

METHODS

Twenty-one JIA patients and 11 children with growing pain but without known rheumatic diseases as controls were included. The B cell phenotype and intracellular production of IL-10 of Bregs were assessed by flow cytometry. Mononuclear cells from PB and SF were stimulated to produce IL-10 in vitro for the identification of IL-10- producing regulatory B cells.

RESULTS

The percentage of CD24^hiCD38^hi Bregs in the PB of JIA patients was significantly decreased compared to that in controls, and it was even lower in the SF of JIA patients compared to that in the PB. CD24^hiCD38^hi Bregs frequency was significantly lower in the PB of RF-positive patients than in RF-negative patients. Frequency of IL-10-producing regulatory B cells (B10 cells) was significantly lower in active JIA patients than that in inactive patients.

CONCLUSIONS

The inability of the host to produce enough regulatory B cells in PB and especially in SF of JIA patients may contribute to the disease, especially the local inflammation.

Correlates of Follicular Helper Bias in the CD4 T Cell Response to a Retroviral Antigen

CD4+ T cell differentiation is influenced by a plethora of intrinsic and extrinsic factors, providing the immune system with the ability to tailor its response according to specific stimuli. Indeed, different classes of pathogens may induce a distinct balance of CD4+ T cell differentiation programmes. Here, we report an uncommonly strong bias toward follicular helper (Tfh) differentiation of CD4+ T cells reactive with a retroviral envelope glycoprotein model antigen, presented in its natural context during retroviral infection. Conversely, the response to the same antigen, presented in different immunization regimens, elicited a response typically balanced between Tfh and T helper 1 cells. Comprehensive quantitation of variables known to influence Tfh differentiation revealed the closest correlation with the strength of T cell receptor (TCR) signaling, leading to PD-1 expression, but not with surface TCR downregulation, irrespective of TCR clonotypic avidity. In contrast, strong TCR signaling leading to TCR downregulation and induction of LAG3 expression in high TCR avidity clonotypes restrained CD4+ T cell commitment and further differentiation. Finally, stunted Th1 differentiation, correlating with limited IL-2 availability in retroviral infection, provided permissive conditions for Tfh development, suggesting that Tfh differentiation is the default program of envelope-reactive CD4+ T cells.

Relationship between CD4 Regulatory T Cells and Anergy In Vivo

Selective suppression of effector CD4+ T cell functions is necessary to prevent immune cell-mediated damage to healthy tissues. This appears especially true during pregnancy or in individuals predisposed to autoimmunity. Foxp3+ regulatory T (Treg) cells and induction of anergy, an acquired state of T cell functional unresponsiveness in Foxp3- cells, have both been implicated as mechanisms to suppress dangerous immune responses to tissue-restricted self-Ags. Anergic CD4+ T cells and Treg cells share a number of phenotypic and mechanistic traits-including the expression of CD73 and folate receptor 4, and the epigenetic modification of Treg cell signature genes-and an interesting relationship between these two subsets has recently emerged. In this review, we will compare and contrast these two subsets, as well as explore the role of anergy in the generation of peripheral Treg cells.

Graft-versus-host disease in paediatric solid organ transplantation: A review of the literature

GvHD is a rare and serious complication of organ transplantation. The literature is sparse following solid organ transplantation. The aim of this report was to review the literature of GvHD in paediatric solid organ transplantation. We searched PubMed for English-language full-text manuscripts between 1990 and 2015 for eligible studies. A total of 28 publications were found pertaining to paediatric GvHD following solid organ transplantation. GvHD had a mean incidence of 11% (range 8.3-13.4%) following SBTx and 1.5% following liver transplantation. Where described, the most common sites for presentation of GvHD were the skin (87%), the native GI tract (43%), the lungs (7%), the eyes (4%), HA (4%), and the kidneys (1%). Diagnosis was confirmed with biopsy (93%) and/or chimerism (41%). Treatments used include steroids (80%), of which 75% showed partial or complete resolution. Mortality was 33.3% (range 0-100%). Novel therapies include ECP and MSC therapy. GvHD is a rare but serious disease with high mortality. Novel therapies may offer hope in the future, but currently there is limited evidence for their efficacy in the context of intestinal or liver transplantation.

CD4(+) T cell anergy prevents autoimmunity and generates regulatory T cell precursors

The role of anergy, an acquired state of T cell functional unresponsiveness, in natural peripheral tolerance remains unclear. In this study, we found that anergy was selectively induced in fetal antigen-specific maternal CD4(+) T cells during pregnancy. A naturally occurring subpopulation of anergic polyclonal CD4(+) T cells, enriched for self antigen-specific T cell antigen receptors, was also present in healthy hosts. Neuropilin-1 expression in anergic conventional CD4(+) T cells was associated with hypomethylation of genes related to thymic regulatory T cells (Treg cells), and this correlated with their ability to differentiate into Foxp3(+) Treg cells that suppressed immunopathology. Thus, our data suggest that not only is anergy induction important in preventing autoimmunity but also it generates the precursors for peripheral Treg cell differentiation.

Interleukin 21 blockade modulates activated T- and B-cell homeostasis via B-cell activating factor pathway-mediated inhibition in a murine model of acute graft-versus-host disease

Interleukin (IL) 21 plays a key role in the development of acute graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation. Therapeutic manipulation of IL-21 activity may improve acute GVHD during the early-posttransplant period. We investigated the mechanisms regulating T- and B-cells during IL-21 blockade in acute GVHD. Interleukin 21 blockade enhanced regulatory T and T helper (Th) 2 cell differentiation and inhibited Th1- and Th17-derived transcription factors and cytokines as a modulator of activated T-cells. Interleukin 21(-/-) cell recipients showed increased mature B- and marginal-zone B-cells, but decreased memory B-cells, germinal center formation, and plasma cells that did not lead to immunoglobulin production. B-cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL) are involved in the induction and maintenance of T- and B-cell responses. We observed decreased levels of only BAFF during acute GVHD and confirmed that mammalian target of rapamycin complex 1 was reduced by the BAFF/BAFF-receptor pathway. Therefore, this study suggests that IL-21 blockade modulates activated T- and B-cell homeostasis via BAFF-pathway-mediated inhibition in acute GVHD following murine allogeneic bone marrow transplantation.

Acute graft-versus-host disease: a bench-to-bedside update

Over the past 5 years, many novel approaches to early diagnosis, prevention, and treatment of acute graft-versus-host disease (aGVHD) have been translated from the bench to the bedside. In this review, we highlight recent discoveries in the context of current aGVHD care. The most significant innovations that have already reached the clinic are prophylaxis strategies based upon a refinement of our understanding of key sensors, effectors, suppressors of the immune alloreactive response, and the resultant tissue damage from the aGVHD inflammatory cascade. In the near future, aGVHD prevention and treatment will likely involve multiple modalities, including small molecules regulating immunologic checkpoints, enhancement of suppressor cytokines and cellular subsets, modulation of the microbiota, graft manipulation, and other donor-based prophylaxis strategies. Despite long-term efforts, major challenges in treatment of established aGVHD still remain. Resolution of inflammation and facilitation of rapid immune reconstitution in those with only a limited response to corticosteroids is a research arena that remains rife with opportunity and urgent clinical need.

Lymphopenia-induced proliferation in aire-deficient mice helps to explain their autoimmunity and differences from human patients

Studies on autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) and its mouse model – both caused by mutant AIRE – have greatly advanced the understanding of thymic processes that generate a self-tolerant T-cell repertoire. Much is now known about the molecular mechanisms by which AIRE induces tissue-specific antigen expression in thymic epithelium, and how this leads to negative selection of auto-reactive thymocytes. However, we still do not understand the processes that lead to the activation of any infrequent naïve auto-reactive T-cells exported by AIRE-deficient thymi. Also, the striking phenotypic differences between APECED and its mouse models have puzzled researchers for years. The aim of this review is to suggest explanations for some of these unanswered questions, based on a fresh view of published experiments. We review evidence that auto-reactive T-cells can be activated by the prolonged neonatal lymphopenia that naturally develops in young Aire-deficient mice due to delayed export of mature thymocytes. Lymphopenia-induced proliferation (LIP) helps to fill the empty space; by favoring auto-reactive T-cells, it also leads to lymphocyte infiltration in the same tissues as in day 3 thymectomized animals. The LIP becomes uncontrolled when loss of Aire is combined with defects in genes responsible for anergy induction and Treg responsiveness, or in signaling from the T-cell receptor and homeostatic cytokines. In APECED patients, LIP is much less likely to be involved in activation of naïve auto-reactive T-cells, as humans are born with a more mature immune system than in neonatal mice. We suggest that human AIRE-deficiency presents with different phenotypes because of additional precipitating factors that compound the defective negative selection of potentially autoaggressive tissue-specific thymocytes.

Recent progress in managing graft-versus-host disease and viral infections following allogeneic stem cell transplantation

Despite recent reductions in transplant-related mortality, post-transplant complications such as graft-versus-host disease (GvHD) remain major obstacles to the successful application of allogeneic hematopoietic transplantation. Steroid-refractory GvHD has a poor outcome. Although there are a variety of new approaches to the treatment of refractory GvHD, many have limited evidence of efficacy. Other approaches appear to be unacceptably toxic. It would be preferable to improve GvHD prophylaxis. There is good evidence that rates of GvHD can be reduced without unacceptable reduction of the graft-versus-leukemia effect or compromising overall survival. However, prophylactic measures aimed at reducing T-cell numbers or functions are associated with high rates of reactivation of latent viruses. New technologies that allow rapid generation of virus-specific T-cells show promise to reduce the frequency and severity of such reactivations and have the potential to revolutionize the approach to post-transplant infectious complications.

Advances in graft-versus-host disease biology and therapy

Allogeneic haematopoietic stem cell transplantation is used to treat a variety of disorders, but its efficacy is limited by the occurrence of graft-versus-host disease (GVHD). The past decade has brought impressive advances in our understanding of the role of stimulatory and suppressive elements of the adaptive and innate immune systems from both the donor and the host in GVHD pathogenesis. New insights from basic immunology, preclinical models and clinical studies have led to novel approaches for prevention and treatment. This Review highlights the recent advances in understanding the pathophysiology of GVHD and its treatment, with a focus on manipulations of the immune system that are amenable to clinical application.

Arthritogenic self-reactive CD4+ T cells acquire an FR4hiCD73hi anergic state in the presence of Foxp3+ regulatory T cells

Rheumatoid arthritis develops in association with a defect in peripheral CD4(+) T cell homeostasis. T cell lymphopenia has also been shown to be a barrier to CD4(+) T cell clonal anergy induction. We therefore explored the relationship between clonal anergy induction and the avoidance of autoimmune arthritis by tracking the fate of glucose-6-phosphate isomerase (GPI)-reactive CD4(+) T cells in the setting of selective T cell lymphopenia. CD4(+) T cell recognition of self-GPI peptide/MHC class II complexes in normal murine hosts did not lead to arthritis and instead caused those T cells to develop a Folate receptor 4(hi)CD73(hi) anergic phenotype. In contrast, hosts selectively depleted of polyclonal Foxp3(+)CD4(+) regulatory T cells could not make GPI-specific CD4(+) T cells anergic and failed to control arthritis. This suggests that autoimmune arthritis develops in the setting of lymphopenia when Foxp3(+)CD4(+) regulatory T cells are insufficient to functionally inactivate all autoreactive CD4(+) T cells that encounter self-Ag.

B cells and TCR avidity determine distinct functions of CD4+ T cells in retroviral infection

The T cell-dependent B cell response relies on cognate interaction between B cells and CD4(+) Th cells. However, the consequences of this interaction for CD4(+) T cells are not entirely known. B cells generally promote CD4(+) T cell responses to pathogens, albeit to a variable degree. In contrast, CD4(+) T cell responses to self- or tumor Ags are often suppressed by B cells. In this study, we demonstrated that interaction with B cells dramatically inhibited the function of virus-specific CD4(+) T cells in retroviral infection. We have used Friend virus infection of mice as a model for retroviral infection, in which the behavior of virus-specific CD4(+) T cells was monitored according to their TCR avidity. We report that avidity for Ag and interaction with B cells determine distinct aspects of the primary CD4(+) T cell response to Friend virus infection. Virus-specific CD4(+) T cells followed exclusive Th1 and T follicular helper (Tfh) differentiation. High avidity for Ag facilitated expansion during priming and enhanced the capacity for IFN-γ and IL-21 production. In contrast, Tfh differentiation was not affected by avidity for Ag. By reducing or preventing B cell interaction, we found that B cells promoted Tfh differentiation, induced programmed death 1 expression, and inhibited IFN-γ production by virus-specific CD4(+) T cells. Ultimately, B cells protected hosts from CD4(+) T cell-mediated immune pathology, at the detriment of CD4(+) T cell-mediated protective immunity. Our results suggest that B cell presentation of vaccine Ags could be manipulated to direct the appropriate CD4(+) T cell response.

The absence of B lymphocytes reduces the number and function of T-regulatory cells and enhances the anti-tumor response in a murine tumor model

Increasing evidence suggests that B lymphocytes play a central role in inhibiting the immune response against certain tumors, but the underlying mechanisms by which B cells facilitate tumor growth are still poorly understood. In this study, we investigated how the presence or absence of B cells affects expansion and function of T-regulatory cells ('T-regs') in a murine breast tumor model (EMT-6). We compared tumor growth, and the number and function of T-reg cells in wild-type immune-competent mice (ICM) and B-cell-deficient mice (BCDM). Mice were either tumor-naive or implanted with EMT-6 mammary adenocarcinoma cells. Tumor growth was markedly inhibited in BCDM, compared to wild-type mice (ICM). Increased T-reg expansion as defined by CD4+/CD25+/FOXP3+ cells was evident following EMT-6 inoculation in ICM in comparison with non-tumor-bearing mice or compared to BCDM in which tumor had been implanted. The percentage and absolute number of T-regs in the spleen, tumor draining lymph nodes, and tumor bed were significantly reduced in BCDM compared to ICM. T-reg function, measured by suppression and proliferation assays, was also reduced in tumor inoculated BCDM compared to ICM. Our studies indicate that absence of B cells may play a role in augmenting the T-cell anti-tumor response, in part due to effects on T-regulatory cell expansion and function.

Differential requirements for Th1 and Th17 responses to a systemic self-antigen

T cell-APC interactions are essential for the initiation of effector responses against foreign and self-antigens, but the role of these interactions in generating different populations of effector T cells in vivo remains unclear. Using a model of CD4(+) T cell responses to a systemic self-antigen without adjuvants or infection, we demonstrate that activation of APCs augments Th17 responses much more than Th1 responses. Recognition of systemic Ag induces tolerance in self-reactive CD4(+) T cells, but induction of CD40 signaling, even under tolerogenic conditions, results in a strong, Ag-specific IL-17 response without large numbers of IFN-γ-producing cells. Transfer of the same CD4(+) T cells into lymphopenic recipients expressing the self-antigen results in uncontrolled production of IL-17, IFN-γ, and systemic inflammation. If the Ag-specific T cells lack CD40L, production of IL-17 but not IFN-γ is decreased, and the survival time of recipient mice is significantly increased. In addition, transient blockade of the initial MHC class II-dependent T cell-APC interaction results in a greater reduction of IL-17 than of IFN-γ production. These data suggest that Th17 differentiation is more sensitive to T cell interactions with APCs than is the Th1 response, and interrupting this interaction, specifically the CD40 pathway, may be key to controlling Th17-mediated autoimmunity.

B cells--masters of the immunoverse

The immune system involves the complex interplay between many different cell types. Over the last decade, T cells, dendritic cells (DC) and macrophages have all been implicated as the key regulator cells of the immunological response, linking innate and adaptive immunity. The forgotten cell in this discourse has been the B-cell. Long considered as simple antibody production units dictated to by T-cells, recent years have begun to shift this assumption. The discovery that numerous B-cell subsets exist, with specific regulatory functions capable of modulating T-cell and chronic inflammatory responses has revealed a hitherto unappreciated role of B-cells. In particular, these ideas have been developed in light of the surprisingly successful responses delivered in autoimmune settings following depletion of B-cells with the anti-CD20 antibody rituximab. Here we summarise the history of the humble B-cell and discuss some of the key recent findings that lead us to propose it as an important regulator of ongoing immune responses and as such, one of the masters of the immunoverse.

Decreased production of interleukin-10 and transforming growth factor-β in Toll-like receptor-activated intestinal B cells in SAMP1/Yit mice

A unique subset of B cells expressing interleukin-10 (IL-10) and transforming growth factor-β (TGF-β) plays an essential role in preventing inflammation and autoimmunity. We investigated the presence of this cell subset in intestines and its role in the pathogenesis of ileitis using SAMP1/Yit and age-matched control AKR/J mice. Mononuclear cells were isolated from mesenteric lymph nodes (MLNs) and the expressions of B220, CD1d, CD5, Toll-like receptor 4 (TLR4) and TLR9 in isolated cells were analysed. Purified B cells were stimulated with lipopolysaccharide (LPS) or CpG-DNA, then IL-10 and TGF-β(1) expressions were examined by enzyme immunoassay and flow cytometry. Production of IL-1β by TLR-mediated macrophages co-cultured with or without purified MLN B cells from SAMP1/Yit and AKR/J mice was evaluated. In addition, interferon-γ (IFN-γ) production in intestinal T cells co-cultured with MLN B cells were also assessed in SAMP1/Yit and AKR/J strains. The production levels of IL-10 and TGF-β(1) stimulated by LPS and CpG-DNA were significantly lower in B cells separated from MLNs from the SAMP1/Yit strain. B cells expressing IL-10 and TGF-β(1) were mainly located in a population characterized by the cell surface marker CD1d(+) . Interleukin-1β production by TLR-activated macrophages co-cultured with MLN B cells from SAMP1/Yit mice was significantly higher than that of those from AKR/J mice. Interestingly, IFN-γ production by T cells was noted only when they were co-cultured with SAMP1/Yit but not the AKR/J B cells. These results are the first to show that disorders of regulatory B-cell function under innate immune activation may cause disease pathogenesis in a murine model of Crohn's disease.

Regulatory B cells: evidence, developmental origin and population diversity

The adaptive immune system has developed several highly effective mechanisms in order to avoid excessive or unwanted reactions and promote resolution of immune activation. An emerging, significant body of evidence indicates that B cells can actively modulate immune responses by mechanisms that do not directly involve the production of antibodies. B cells appear to have the capacity to both induce and suppress immune effector mechanisms and they exert these functions both by contact-dependent interactions and through the secretion of cytokines. In this review we will focus on the regulatory suppressive function of several recently described B cell populations, functionally defined "regulatory B cells" or Breg cells. We will first outline the evidence that has led to their identification and then we will summarize current hypotheses on their ontogeny and possible lineage relationship.

Duration of antigen receptor signaling determines T-cell tolerance or activation

The early events that determine the decision between lymphocyte tolerance and activation are not well-understood. Using a model of systemic self-antigen recognition by CD4(+) T cells, we show, using single-cell biochemical analyses, that tolerance is characterized by transient signaling events downstream of T-cell receptor engagement in the mammalian target of rapamycin (mTOR) and NF-κB pathways. Parallel studies done by live cell imaging show that the key difference between tolerance and activation is the duration of the T cell-antigen presenting cell (APC) interaction, as revealed by stable T-cell immobilization on antigen encounter. Brief T cell-APC interactions result in tolerance, and prolonged interactions are associated with activation and the development of effector cells. These studies show that the duration of T cell-APC interactions and magnitude of associated TCR-mediated signaling are key determinants of lymphocyte tolerance vs. activation.

Identification of a B cell signature associated with renal transplant tolerance in humans

Establishing long-term allograft acceptance without the requirement for continuous immunosuppression, a condition known as allograft tolerance, is a highly desirable therapeutic goal in solid organ transplantation. Determining which recipients would benefit from withdrawal or minimization of immunosuppression would be greatly facilitated by biomarkers predictive of tolerance. In this study, we identified the largest reported cohort to our knowledge of tolerant renal transplant recipients, as defined by stable graft function and receiving no immunosuppression for more than 1 year, and compared their gene expression profiles and peripheral blood lymphocyte subsets with those of subjects with stable graft function who are receiving immunosuppressive drugs as well as healthy controls. In addition to being associated with clinical and phenotypic parameters, renal allograft tolerance was strongly associated with a B cell signature using several assays. Tolerant subjects showed increased expression of multiple B cell differentiation genes, and a set of just 3 of these genes distinguished tolerant from nontolerant recipients in a unique test set of samples. This B cell signature was associated with upregulation of CD20 mRNA in urine sediment cells and elevated numbers of peripheral blood naive and transitional B cells in tolerant participants compared with those receiving immunosuppression. These results point to a critical role for B cells in regulating alloimmunity and provide a candidate set of genes for wider-scale screening of renal transplant recipients.

Mechanisms maintaining peripheral tolerance

The presentation of self-peptide-MHC complexes in the periphery to potentially autoreactive T cells that have escaped negative selection in the thymus poses an important problem to the immune system. In this review, I discuss data that reveal barriers preventing peripheral T cell recognition of self-peptide-MHC complexes, as well as the physiological mechanisms that ensure the elimination or functional inactivation (anergy) of T cells that do come to recognize self-peptide-MHC and threaten the health of the individual.

Interleukin-2 in the development and control of inflammatory disease

Interleukin-2 (IL-2) has multiple, sometimes opposing, functions during an inflammatory response. It is a potent inducer of T-cell proliferation and T-helper 1 (Th1) and Th2 effector T-cell differentiation and provides T cells with a long-lasting competitive advantage resulting in the optimal survival and function of memory cells. In a regulatory role, IL-2 is important for the development, survival, and function of regulatory T cells, it enhances Fas-mediated activation-induced cell death, and it inhibits the development of inflammatory Th17 cells. Thus, in its dual and contrasting functions, IL-2 contributes to both the induction and the termination of inflammatory immune responses.

Memory-like CD8+ and CD4+ T cells cooperate to break peripheral tolerance under lymphopenic conditions

The onset of autoimmunity in experimental rodent models and patients frequently correlates with a lymphopenic state. In this condition, the immune system has evolved compensatory homeostatic mechanisms that induce quiescent naive T cells to proliferate and differentiate into memory-like lymphocytes even in the apparent absence of antigenic stimulation. Because memory T cells have less stringent requirements for activation than naive cells, we hypothesized that autoreactive T cells that arrive to secondary lymphoid organs in a lymphopenic environment could differentiate and bypass the mechanisms of peripheral tolerance such as those mediated by self-antigen cross-presentation. Here, we show that lymphopenia-driven proliferation and differentiation of potentially autoreactive CD8(+) T cells into memory-like cells is not sufficient to induce self-reactivity against a pancreatic antigen. Induction of an organ-specific autoimmunity required antigen-specific CD4(+) T cell help. Notably, we found that this function could be accomplished by memory-like CD4(+) T cells generated in vivo through lymphopenia-induced proliferation. These helper cells promoted the further differentiation of memory-like CD8(+) T cells into effectors in response to antigen cross-presentation, resulting in their migration to the tissue of antigen expression where autoimmunity ensued. Thus, the cooperation of self-reactive memory-like CD4(+) and CD8(+) T cells under lymphopenic conditions overcomes cross-tolerance resulting in autoimmunity.

Helminth infections associated with multiple sclerosis induce regulatory B cells

OBJECTIVE

To assess the importance of B-cell control during parasite infections in multiple sclerosis (MS) patients.

METHODS

Peripheral blood CD19+ B cells from 12 helminth-infected MS patients, 12 MS patients without infection, 10 patients infected with Trypanosoma cruzi, 8 subjects infected with Paracoccidioides brasiliensis, and 12 healthy control subjects were purified using magnetic cell sorting. Interleukin (IL)-4, IL-6, IL-10, tumor necrosis factor-alpha, lymphotoxin, transforming growth factor-beta, brain-derived neurotrophic factor, and nerve growth factor secretion were evaluated after stimulation with CDw32 L cells and CD40 antibody using enzyme-linked immunosorbent assays. The production of anti-myelin oligodendrocyte glycoprotein IgG and IgM antibodies was evaluated by enzyme-linked immunosorbent spot assays. Cell phenotype was assessed by flow cytometry.

RESULTS

Helminth infections in MS patients created a B-cell population producing high levels of IL-10, dampening harmful immune responses through a mechanism mediated, at least in part, by the ICOS-B7RP-1 pathway. The IL-10-producing B-cell phenotype detected expressed high levels of CD1d and was similar to the one observed in mature naive B2 cells (namely, CD11b(-), CD5(-), CD27(-), and IgD+). Moreover, B cells isolated from helminth-infected MS patients also produced greater amounts of brain-derived neurotrophic factor and nerve growth factor compared with those of normal subjects, T. cruzi-infected subjects, P. brasiliensis-infected subjects, or uninfected MS patients, raising the possibility that these cells may exert a neuroprotective effect on the central nervous system.

INTERPRETATION

Increased production of B-cell-derived IL-10 and of neurotrophic factors are part of the parasite's regulation of host immunity and can alter the course of MS, potentially explaining environmental-related MS suppression observed in areas with low disease prevalence.

Induction of CD4+ T-cell anergy and apoptosis by activated human B cells

B cells are well-known mediators of humoral immunity and serve as costimulators in the generation of T cell-mediated responses. In several mouse models, however, it was observed that B cells can also down-regulate immune reactions, suggesting a dual role for B cells. Due to this discrepancy and so far limited data, we directly tested the effects of primary human B cells on activated CD4(+) T helper cells in vitro. We found that under optimal costimulation large, activated CD25(+) B cells but not small CD25(-) B cells induced temporary T-cell anergy, determined by cell division arrest and down-regulation of cytokine production. In addition, large CD25(+) B cells directly induced CD95-independent apoptosis in a subpopulation of activated T cells. Suppression required direct B-T-cell contact and was not transferable from T to T cell, excluding potential involvement of regulatory T cells. Moreover, inhibitory effects involved an IL-2-dependent mechanism, since decreasing concentrations of IL-2 led to a shift from inhibitory toward costimulatory effects triggered by B cells. We conclude that activated CD25(+) B cells are able to costimulate or down-regulate T-cell responses, depending on activation status and environmental conditions that might also influence their pathophysiological impact.

Cutting edge: contributions of apoptosis and anergy to systemic T cell tolerance

Multiple pathways can induce and maintain peripheral T cell tolerance. The goal of this study was to define the contributions of apoptosis and anergy to the maintenance of self-tolerance to a systemic Ag. Upon transfer into mice expressing OVA systemically, OVA-specific DO11 CD4+ T cells are activated transiently, cease responding, and die. Bim is the essential apoptosis-inducing trigger and apoptosis proceeds despite increased expression of Bcl-2 and Bcl-x. However, preventing apoptosis by eliminating Bim does not restore proliferation or cytokine production by DO11 cells. While Foxp3 is transiently induced, anergy is not associated with the stable development of regulatory T cells. Thus, apoptosis is dispensable for tolerance to a systemic self-Ag and cell-intrinsic anergy is sufficient to tolerize T cells.

Integration of B cells and CD8+ T in the protective regulation of systemic epithelial inflammation

Mechanisms that control abnormal CD4(+) T cell-mediated tissue damage are a significant factor in averting and resolving chronic inflammatory epithelial diseases. B cells can promote such immunoregulation, and this is thought to involve interaction with MHC II- or CD1-restricted regulatory T cells. The purpose of this study is to genetically define the interacting cells targeted by protective B cells, and to elucidate their regulatory mechanisms in CD4(+) T cell inflammation. Transfer of G alpha i2-/- CD3(+) T cells into lymphopenic mice causes a dose-dependent multi-organ inflammatory disease including the skin, intestine, and lungs. Disease activity is associated with elevated levels of serum TNF-alpha and IFN-gamma, and an activated IL-17 producing CD4(+) T cell population. Mesenteric node B cells from wild type mice suppress disease activity, serum cytokine expression, and levels of CD4(+) T cells producing TNF-alpha IFN-gamma, and IL-17. The protective function of B cells requires genetic sufficiency of IL-10, MHC I and TAP1. Regulatory B cells induce the expansion and activation of CD8(+) T cells, which is correlated with disease protection. These results demonstrate that CD8(+) T cells can ameliorate lymphopenic systemic inflammatory disease, through peptide/MHC I-dependent B cell interaction.

Inhibition of ICAM-1/LFA-1 interactions prevents B-cell-dependent anti-CD45RB-induced transplantation tolerance

BACKGROUND

Allogeneic tolerance can be reliably obtained with monoclonal antibody therapy targeting CD45RB. Although regulatory T cells play an important role in the mechanism, we have recently demonstrated the active participation of host B lymphocytes. After anti-CD45RB therapy, B lymphocytes demonstrate phenotypic alterations that include up-regulation of CD54 (intercellular adhesion molecule [ICAM]-1). We have investigated the hypothesis that alteration in ICAM-1 expression is required for tolerance induction.

MATERIALS AND METHODS

Recipients of heterotopic allogeneic cardiac grafts (C3H donors into B6 recipients) were treated with anti-CD45RB, anti-ICAM, anti-lymphocyte function-associated antigen-1 (LFA), or the combination of these agents. These data were extended by performing allogeneic cardiac transplants into ICAM or LFA recipients treated with a 5-day course of anti-CD45RB. Finally, B-cell-deficient animals were reconstituted with ICAM splenocytes to create a recipient with a selective deficiency of ICAM-1 restricted to the B-cell compartment.

RESULTS

Anti-CD45RB alone or the combination of anti-LFA/anti-ICAM reliably induced transplantation tolerance. However, the triple combination was routinely unsuccessful and induced long-term graft survival in no recipients. ICAM-deficient or LFA-deficient recipients were also resistant to tolerance induced by anti-CD45RB. Finally, transfer of control splenocytes to B-cell-deficient recipients permitted anti-CD45RB-induced tolerance, whereas transfer of ICAM cells was unable to support tolerance induction.

CONCLUSIONS

Expression of ICAM-1 by B lymphocytes and interaction with LFA-1 form a central aspect of transplantation tolerance induced by anti-CD45RB therapy. These data further elucidate the cellular mechanisms used by B lymphocytes in the induction of transplantation tolerance.

Migration, cell-cell interaction and adhesion in the immune system

Migration is an essential function of immune cells. It is necessary to lead immune cell precursors from their site of generation to the places of maturation or function. Cells of the adaptive immune system also need to interact physically with each other or with specialized antigen presenting cells in lymphatic tissues in order to become activated. Thereby a complex series of controlled migration events, adhesive interactions and signalling responses is induced. Finally cells must be able to leave the activating tissues and re-enter the bloodstream from which they extravasate into inflamed tissue sites. Cells of the innate immune system can function directly without the need for previous activation. However, these cells have to adapt their function to a panoply of pathogens and environmental niches which can be invaded. The current review highlights the central aspects of cellular dynamics underlying adaptive and innate cellular immunity. Thereby a focus will be put on recent results obtained by microscopic observation of live cells in vitro or by intravital 2-photon microscopy in live animals.

B cells as a cellular adjuvant: induction or control of T-cell immunity--or both?

Evaluation of: Reichhardt P, Dornbach B, Rong S et al. Naive B cells generate regulatory T cells in the presence of a mature immunologic synapse. Blood 110 (5), 1519-1529 (2007). Cellular adjuvants, such as dendritic cells (DCs) are in the focus of tumor immunology and immunotherapy as tools to enhance immune responses in order to control cancer. Recent observations suggest that so-called tolerogenic DCs can be employed to achieve the opposite--the control of adverse immune reactions in transplantation or autoimmunity. However, the use of DCs is limited due to their low frequency, lack of purity and expansion in cell culture. Activated B cells have previously been shown to efficiently present antigen and be expandable to almost unlimited amounts, rendering them an interesting alternative to induce antitumor immune responses. The paper under evaluation extends this concept to B cells as tolerogenic cells by providing insight in a novel mechanism--the induction of regulatory T cells.

p300 protein acetyltransferase activity suppresses systemic lupus erythematosus-like autoimmune disease in mice

Conditional knock-in mice expressing a histone acetyltransferase-deficient version of the transcriptional coregulator p300 exclusively in B lymphocytes die prematurely with full penetrance. The mice develop an autoimmune disease similar to systemic lupus erythematosus in its pathological manifestations, such as splenomegaly, glomerulonephritis, vasculitis, deposition of immune complexes, and production of autoantibodies against dsDNA. Aged mice show a severe reduction of transitional and marginal zone B cells and generate aberrant mature B cells. These B cells show diminished proliferation in response to stimulation of the BCR, but respond normally to other stimuli. Yet, the mice mount a normal primary immune response against a T-dependent Ag. In contrast, the memory response is impaired. In addition, serum Ig levels, in particular IgG2b, are increased. We conclude that p300 acetyltransferase activity is essential for maintaining self-tolerance of B lymphocytes. These findings support the concept of treating lupus with inhibitors of protein deacetylases and point to B cells as a critical target of these drugs.

Cutting edge: transplant tolerance induced by anti-CD45RB requires B lymphocytes

Selective interference with the CD45RB isoform by mAb (anti-CD45RB) reliably induces donor-specific tolerance. Although previous studies suggest participation of regulatory T cells, a mechanistic understanding of anti-CD45RB-induced tolerance is lacking. We report herein the unexpected finding that tolerance induced by this agent is not established in B cell-deficient mice but can be recovered by preemptive B lymphocyte transfer to B cell-deficient hosts. Using B cells from genetically modified donors to reconstitute B cell-deficient recipients, we evaluate the role of B lymphocyte-expressed CD45RB, T cell costimulatory molecules, and the production of Abs in this novel tolerance mechanism. Our data document an Ab-induced tolerance regimen that is uniquely B lymphocyte-dependent and suggest mechanistic contributions to tolerance development from the B cell compartment through interactions with T cells.

Tolerization of an established alphaB-crystallin-reactive T-cell response by intravenous antigen

Tolerance induction to prevent activation of a naïve T-cell repertoire has been well documented in rodents and can be readily achieved by intravenous, oral or intranasal administration of antigen in the absence of adjuvants. In autoimmune diseases such as multiple sclerosis (MS) the presence of an established memory/effector T-cell repertoire against self-antigens is likely to be more relevant than the potential reactivity of naive T cells. Methods to eliminate such an established T-cell response are less well understood. In this study, we explored the effectiveness of intravenous soluble antigen to eliminate a pre-existing T-cell response against alphaB-crystallin, a candidate autoantigen in MS. We used mice that are deficient for the target antigen. This condition allowed for a vigourous T-cell and antibody response to develop upon immunization, and eliminated all possible endogenous mechanisms of tolerance for alphaB-crystallin that are found in normal rodents. When applied 3 weeks after priming with alphaB-crystallin, intravenous administration of soluble antigen almost completely abrogated the established T-cell response in a dose-dependent manner as evidenced by T-cell non-responsiveness in tolerized animals to a re-challenge with antigen in complete Freund's adjuvant. Evaluating delayed-type hypersensitivity responses after tolerance induction revealed that the tolerizing effect was achieved within 24 hr. Furthermore, the tolerizing effect was found to be antigen-specific and long lasting. In contrast, serum antibody levels were markedly increased. Our data clarify that in the absence of any natural form of immune regulation, antigen-specific memory/effector T cells can be effectively silenced by intravenous antigen.

Resting B Cells Suppress Tumor Immunity via an MHC Class-II Dependent Mechanism

Resting B cells have been variously shown to induce direct tolerance of antigen specific CD8+ T cells, induce T-cell anergy via transforming growth factor-beta production, down-regulate interleukin-12 production by dendritic cells (DC) and influence TH1/TH2 differentiation via the production of regulatory cytokines. Through these mechanisms, B cells can exert a regulatory function in in vivo models of T-cell immunity including, experimental autoimmune encephalitis and rheumatoid arthritis. Here, we show that the resting B cells inhibit the ability of DC vaccination to provide protection from tumor growth. Inhibition of DC induced immunity by B cells was independent of presentation of major histocompatibility molecule (MHC) class-I bound tumor antigen but dependent on B-cell expression of MHC class-II. Administration of B cells did not alter the ability of DC to migrate from the injection site or impair DC-T cell interactions within the draining lymph node. The inhibitory effect of B cells was lost when they were activated by CD40L and partially reversed by the depletion of CD4+/CD25+ regulatory T cells. Together our findings indicate that the resting B cells are capable of limiting CD8+ T-cell effector function induced by DC vaccination via a mechanism that is dependent on the expression of MHC class-II molecules.

Naive B cells generate regulatory T cells in the presence of a mature immunologic synapse

Naive B cells are ineffective antigen-presenting cells and are considered unable to activate naive T cells. However, antigen-specific contact of these cells leads to stable cell pairs that remain associated over hours in vivo. The physiologic role of such pairs has not been evaluated. We show here that antigen-specific conjugates between naive B cells and naive T cells display a mature immunologic synapse in the contact zone that is absent in T-cell-dendritic-cell (DC) pairs. B cells induce substantial proliferation but, contrary to DCs, no loss of L-selectin in T cells. Surprisingly, while DC-triggered T cells develop into normal effector cells, B-cell stimulation over 72 hours induces regulatory T cells inhibiting priming of fresh T cells in a contact-dependent manner in vitro. In vivo, the regulatory T cells home to lymph nodes where they potently suppress immune responses such as in cutaneous hypersensitivity and ectopic allogeneic heart transplant rejection. Our finding might help to explain old observations on tolerance induction by B cells, identify the mature immunologic synapse as a central functional module of this process, and suggest the use of naive B-cell-primed regulatory T cells, "bTregs," as a useful approach for therapeutic intervention in adverse adaptive immune responses.

Balancing autoaggressive and protective T cell responses

A finely orchestrated balance between activating and inhibitory signals is fundamental for the ability of the immune system to effectively attack and eliminate pathogenic microbes but to not react against self-antigens. Derangements of this balance underlie the pathogenesis of autoimmune diseases. Conversely, elucidating the mechanisms of this balance may provide rational strategies for manipulating it in order to enhance the efficacy of vaccines and tumor immunotherapy. One of the clearest illustrations of precise regulation is in the generation of effector and regulatory T cells. In order to analyze the mechanisms of this regulation, we have developed a transgenic mouse model in which a single population of T cells reacts against its known cognate antigen in vivo. Here we summarize our studies with this experimental model, illustrating the sequence of T cell responses that develop and attempting to dissect the stimuli that control these responses.

Abrogation of antibody-mediated allograft rejection by regulatory CD4 T cells with indirect allospecificity

Alloantibody is an important effector mechanism for allograft rejection. In this study, we tested the hypothesis that regulatory T cells with indirect allospecificity can prevent humoral rejection by using a rat transplant model in which acute rejection of MHC class I-disparate PVG.R8 heart grafts by PVG.RT1(u) recipients is mediated by alloantibody and is dependent upon help from CD4 T cells that can recognize the disparate MHC alloantigen only via the indirect pathway. Pretransplant treatment of PVG.RT1(u) recipients with anti-CD4 mAb plus donor-specific transfusion abrogated alloantibody production and prolonged PVG.R8 graft survival indefinitely. Naive syngeneic splenocytes injected into tolerant animals did not effect heart graft rejection, suggesting the presence of regulatory mechanisms. Adoptive transfer experiments into CD4 T cell-reconstituted, congenitally athymic recipients confirmed that regulation was mediated by CD4 T cells and was alloantigen-specific. CD4 T cell regulation could be broken in tolerant animals either by immunizing with an immunodominant linear allopeptide or by depleting tolerant CD4 T cells, but surprisingly this resulted in neither alloantibody generation nor graft rejection. These findings demonstrate that anti-CD4 plus donor-specific transfusion treatment results in the development of CD4 regulatory T cells that recognize alloantigens via the indirect pathway and act in an Ag-specific manner to prevent alloantibody-mediated rejection. Their development is associated with intrinsic tolerance within the alloantigen-specific B cell compartment that persists after T cell help is made available.

Defining the ability of cyclophosphamide preconditioning to enhance the antigen-specific CD8+ T-cell response to peptide vaccination: creation of a beneficial host microenvironment involving type I IFNs and myeloid cells

Although cyclophosphamide (CTX) has been clearly shown to enhance active specific and adoptive immunotherapies, the mechanism(s) underlying these beneficial effects have not been clearly defined. To define the impact of CTX preconditioning on the antigen-specific CD8 T-cell response to peptide vaccination, we used an adoptive transfer model based on the OT-1 T-cell receptor transgenic mouse. CTX preconditioning dramatically enhanced the antigen-specific CD8 T-cell response to peptide vaccination. Specifically, CTX significantly enhanced the expansion and function of responding CD8 T cells as demonstrated by flow cytometry and cytokine production. In parallel experiments, we attempted to define the mechanism(s) underlying these beneficial effects of CTX therapy. CTX therapy increased the relative number and activation status of myeloid dendritic cells, and was associated with the induction of significant levels of the inflammatory cytokines interferon-alpha, monocyte chemoattractant protein-1, and IL-6. Adoptive transfer experiments into type I IFNR-/- and CR3-/- mice confirmed that the beneficial effects of CTX were at least partially dependent on type I interferons and myeloid cells. Adoptive transfer of up to 150x10(6) naive spleen cells at the time of antigen-specific CD8 T-cell transfer did not abrogate the effects of CTX therapy, suggesting that the creation of a niche in the immune system may not be required. CTX decreased the absolute, but not relative number of CD4+CD25+ Treg cells, consistent with the possibility that regulatory T cells may be targeted by CTX therapy. Of note, combination therapy with CTX and a synthetic TLR3 agonist further enhanced the antigen-specific CD8+ T-cell response. Taken together, our data suggest that CTX modulates specific components of the innate immune system resulting in a beneficial host microenvironment. Specific targeting of these components may enhance the effectiveness of CTX preconditioning for adoptive immunotherapy.

CD25+CD4+ regulatory T cells and memory T cells prevent lymphopenia-induced proliferation of naive T cells in transient states of lymphopenia

Lymphopenia has been associated with autoimmune pathology and it has been suggested that lymphopenia-induced proliferation of naive T cells may be responsible for the development of immune pathology. In this study we demonstrate that lymphopenia-induced proliferation is restricted to conditions of extreme lymphopenia, because neither naive nor memory T cells transferred into T cell-depleted hosts proliferate unless the depletion exceeds 90% of the peripheral repertoire. Memory CD4 T cells as well as regulatory CD4 T cells proved to be relatively resistant to depletion regimes, and both subsets restrict the expansion and phenotypic conversion of naive T cells by an IL-7R-dependent mechanism. It therefore seems unlikely that lymphopenia-induced proliferation of peripheral T cells causes deleterious side effects that result in immune pathology in states of partial and transient lymphopenia.

Regulatory T cells in the periphery

Recognition of a systemic antigen by CD4+ T cells in a lymphopenic host leads to the sequential generation of pathogenic effector cells and protective CD25+ forkhead box protein (Foxp3+) regulatory T cells (Tregs) in the periphery. Such an experimental model is potentially valuable for defining the stimuli that determine the balance of effector and regulatory T cells. Our studies have shown that interleukin-2 (IL-2) enhances the development of effector cells and is essential for the peripheral generation of regulatory cells. Other models of peripheral Treg generation suggest that the concentration of antigen, the nature of the antigen-presenting cells, and cytokines such as transforming growth factor-beta and IL-10 may all influence the peripheral generation of Tregs.

Inhibitory effects of B cells on antitumor immunity

B-cell functions in antitumor immunity are not well understood. In this study, we evaluated the role of B cells in the development of antitumor immunity using Friend murine leukemia virus gag-expressing mouse EL-4 (EL-4 gag), D5 mouse melanoma, or MCA304 mouse sarcoma cells. To screen tumors for susceptibility to B-cell-deficient immune environments, spleen cells from naive C57BL/6 [wild-type (WT)] and B-cell knockout (BKO) mice were cultured with irradiated tumor cells in vitro. When cells were stimulated with EL-4 gag or D5 (but not MCA304 tumors), IFN-gamma production from CD8 T cells and natural killer cells was markedly decreased in WT compared with BKO cultures. IFN-gamma production was correlated with CD40 ligand expression on the tumor and inversely with interleukin-10 (IL-10) production by B cells. Sorted WT B cells produced more IL-10 than CD40 knockout (CD40KO) B cells when cocultured with EL-4 gag or D5 (but not MCA304). IFN-gamma production by BKO cells was reduced by the addition of sorted naive WT B cells (partially by CD40KO B cells) or recombinant mouse IL-10. In vivo tumor progression mirrored in vitro studies in that WT mice were unable to control tumor growth whereas EL-4 gag and D5 tumors (but not MCA304) were eliminated in BKO mice. Robust in vivo antitumor CTLs developed only in BKO tumor-challenged mice. Our studies provide the first mechanistic basis for the concept that B-cell depletion could therapeutically enhance antitumor immune responses to certain tumors by decreasing IL-10 production from B cells.

A Case for Regulatory B Cells

B cells are typically characterized by their ability to produce Abs, including autoantibodies. However, B cells possess additional immune functions, including the production of cytokines and the ability to function as a secondary APC. As with T cells, the B cell population contains functionally distinct subsets capable of performing both pathogenic and regulatory functions. Recent studies indicate that regulatory B cells develop in several murine models of chronic inflammation, including inflammatory bowel disease, rheumatoid arthritis, and experimental autoimmune encephalomyelitis. The regulatory function may be directly accomplished by the production of regulatory cytokines IL-10 and TGF-beta and/or by the ability of B cells to interact with pathogenic T cells to dampen harmful immune responses. In this review, we make a case for the existence of regulatory B cells and discuss the possible developmental pathways and functional mechanisms of these B cells.