Induction of Peripheral T Cell Tolerance by Antigen-Presenting B Cells. I. Relevance of Antigen Presentation Persistence

Epitope-based precision immunotherapy of Type 1 diabetes

Antigen-specific immunotherapies (ASITs) address important clinical needs in treating autoimmune diseases. However, Type 1 diabetes is a heterogeneous disease wherein patient characteristics influence responsiveness to ASITs. Targeting not only disease-relevant T cell populations, but also specific groups of patients using precision medicine is a new goal toward achieving effective treatment. HLA-restricted peptides provide advantages over protein as antigens, however, methods for profiling antigen-specific T cells need to improve in sensitivity, depth, and throughput to facilitate epitope selection. Delivery approaches are highly diverse, illustrating the many ways relevant antigen-presenting cell populations and anatomical locations can be targeted for tolerance induction. The role of persistence of antigen presentation in promoting durable antigen-specific tolerance requires further investigation. Based on the outcome of ASIT trials, the field is moving toward using patient-specific variations to improve efficacy, but challenges still lie on the path to delivering more effective and safer treatment to the T1D patient population.

Sticking to the Subject: Multifunctionality in Microbial Adhesins

Bacterial and fungal adhesins mediate microbial aggregation, biofilm formation, and adhesion to host. We divide these proteins into two major classes: professional adhesins and moonlighting adhesins that have a non-adhesive activity that is evolutionarily conserved. A fundamental difference between the two classes is the dissociation rate. Whereas moonlighters, including cytoplasmic enzymes and chaperones, can bind with high affinity, they usually dissociate quickly. Professional adhesins often have unusually long dissociation rates: minutes or hours. Each adhesin has at least three activities: cell surface association, binding to a ligand or adhesive partner protein, and as a microbial surface pattern for host recognition. We briefly discuss Bacillus subtilis TasA, pilin adhesins, gram positive MSCRAMMs, and yeast mating adhesins, lectins and flocculins, and Candida Awp and Als families. For these professional adhesins, multiple activities include binding to diverse ligands and binding partners, assembly into molecular complexes, maintenance of cell wall integrity, signaling for cellular differentiation in biofilms and in mating, surface amyloid formation, and anchorage of moonlighting adhesins. We summarize the structural features that lead to these diverse activities. We conclude that adhesins resemble other proteins with multiple activities, but they have unique structural features to facilitate multifunctionality.

Oncogene-specific T cells fail to eradicate lymphoma-initiating B cells in mice

To date, little is known about the interaction between (pre-)malignant B cells and T cells. We generated transgenic mice that allow B cell-specific induction of the oncogene SV40 large T-antigen (TAg) to analyze the role of oncogene-specific T cells during sporadic B-cell lymphoma development. Constitutive TAg expression in CD19-Cre × LoxP-Tag mice resulted in TAg-tolerant CD8⁺ T cells and development of B-cell lymphomas. In contrast, CD19-CreER^T2 × LoxP-Tag mice retained TAg-competent CD8⁺ T cells at time of oncogene induction and TAg expression in few B cells of adult mice resulted in exceptionally rare lymphoma formation late in life. Increased lymphoma incidence in the absence of TAg-specific T cells suggested T cell-mediated inhibition of lymphoma progression. However, TAg-initiated B cells were not eliminated by T cells and detected long term. Our results demonstrate a failure of the immune system to eradicate lymphoma-initiating B cells, retaining the risk of lymphoma development.

Regulatory T cells induced by B cells: a novel subpopulation of regulatory T cells

Regulatory T cells play a crucial role in the homeostasis of the immune response. In addition to CD4⁺Foxp3⁺ regulatory T cells, several subsets of Foxp3^- regulatory T cells, such as T helper 3 (Th3) cells and type 1 regulatory T (Tr1) cells, have been described in mice and human. Accumulating evidence shows that naïve B cells contribute to tolerance and are able to promote regulatory T cell differentiation. Naïve B cells can convert CD4⁺CD25^- T cells into CD25⁺Foxp3^- regulatory T cells, named Treg-of-B cells by our group. Treg-of-B cells express LAG3, ICOS, GITR, OX40, PD1, and CTLA4 and secrete IL-10. Intriguingly, B-T cell-cell contact but not IL-10 is essential for Treg-of-B cells induction. Moreover, Treg-of-B cells possess both IL-10-dependent and IL-10-independent inhibitory functions. Treg-of-B cells exert suppressive activities in antigen-specific and non-antigen-specific manners in vitro and in vivo. Here, we review the phenotype and function of Foxp3⁺ regulatory T cells, Th3 cells, Tr1 cells, and Treg-of-B cells.

A Sensitive Method for Detecting Peptide-specific CD4+ T Cell Responses in Peripheral Blood from Patients with Myasthenia Gravis

Myasthenia gravis (MG) is an autoimmune neurological disorder typified by skeletal muscle fatigue and most often production of autoantibodies against the nicotinic acetylcholine receptor (AChR). The present study was undertaken to assess the extent of AChR-peptide recognition in MG patients using co-culturing (DC:TC) of autologous monocyte-derived dendritic cells (moDCs) and highly enriched CD4⁺ T cells from the blood as compared to the traditional whole peripheral blood mononuclear cell (PBMC) cultures. We found that the DC:TC cultures were highly superior to the PBMC cultures for detection of reactivity toward HLA-DQ/DR-restricted AChR-peptides. In fact, whereas DC:TC cultures identified recognition in all MG patients the PBMC cultures failed to detect responsiveness in around 40% of the patients. Furthermore, reactivity to multiple peptides was evident in DC:TC cultures, while PBMC cultures mostly exhibited reactivity to a single peptide. No healthy control (HC) CD4⁺ T cells responded to the peptides in either culture system. Interestingly, whereas spontaneous production of IFNγ and IL-17 was observed in the DC:TC cultures from MG patients, recall responses to peptides enhanced IL-10 production in 9/13 MG patients, while little increase in IFNγ and IL-17 was seen. HCs did not produce cytokines to peptide stimulations. We conclude that the DC: TC culture system is significantly more sensitive and better identifies the extent of responsiveness in MG patients to AChR-peptides than traditional PBMC cultures.

Characterization of c-Maf+Foxp3- Regulatory T Cells Induced by Repeated Stimulation of Antigen-Presenting B Cells

The role of B cells in the development of CD4⁺ regulatory T cells has been emphasized recently. Our previous studies have demonstrated that the antigen-presenting splenic B cells converted naïve CD4⁺CD25⁻ T cells into CD4⁺CD25⁺Foxp3⁻ T cells without additional cytokines or chemicals with regulatory activity and that referred to as Treg-of-B cells. The present study further showed that Treg-of-B cells increased the IL-10-producing population, and the expression of c-Maf, inducible T-cell co-stimulator (ICOS) as well as cytotoxic T-lymphocyte-associated protein 4 (CTLA4) after repeated stimulation of B cells in a cell-cell contact-dependent manner. Long-term cultured Treg-of-B cells exerted IL-10 and CTLA4-mediated antigen-specific suppressive activity; moreover, the single antigen-specific Treg-of-B cells inhibited in a non-antigen-specific fashion. In conclusion, these results suggest that repeated stimulation of B cells induced IL-10-producing CD4⁺Foxp3⁻ regulatory T cells in a contact-dependent manner and these Treg-of-B cells possess IL-10 and CTLA4-dependent suppressive function.

Bile salt export pump-reactive antibodies form a polyclonal, multi-inhibitory response in antibody-induced bile salt export pump deficiency

Progressive familial intrahepatic cholestasis type 2 (PFIC-2) is caused by mutations in ABCB11, encoding the bile salt export pump (BSEP). In 2009, we described a child with PFIC-2 who developed PFIC-like symptoms after orthotopic liver transplantation (OLT). BSEP-reactive antibodies were demonstrated to account for disease recurrence. Here, we characterize the nature of this antibody response in 7 more patients with antibody-induced BSEP deficiency (AIBD). Gene sequencing and immunostaining of native liver biopsies indicated absent or strongly reduced BSEP expression in all 7 PFIC-2 patients who suffered from phenotypic disease recurrence post-OLT. Immunofluorescence, western blotting analysis, and transepithelial transport assays demonstrated immunoglobulin (Ig) G-class BSEP-reactive antibodies in these patients. In all cases, the N-terminal half of BSEP was recognized, with reaction against its first extracellular loop (ECL1) in six sera. In five, antibodies reactive against the C-terminal half also were found. Only the sera recognizing ECL1 showed inhibition of transepithelial taurocholate transport. In a vesicle-based functional assay, transport inhibition by anti-BSEP antibodies binding from the cytosolic side was functionally proven as well. Within 2 hours of perfusion with antibodies purified from 1 patient, rat liver showed canalicular IgG staining that was absent after perfusion with control IgG.

CONCLUSIONS

PFIC-2 patients carrying severe BSEP mutations are at risk of developing BSEP antibodies post-OLT. The antibody response is polyclonal, targeting both extra- and intracellular BSEP domains. ECL1, a unique domain of BSEP, likely is a critical target involved in transport inhibition as demonstrated in several patients with AIBD manifest as cholestasis.

Soluble CD40 ligand-activated B cells from patients with chronic hepatitis B virus infection as antigen presenting cells to induce hepatitis B virus specific cytotoxic T lymphocytes

Chronic hepatitis B virus (HBV) infection is the result of an inadequate antiviral immune response to the virus. In this study, we aimed to investigate whether the soluble CD40 ligand-activated B (CD40-B) cells could present antigen and induce specific cytotoxic T lymphocytes (CTLs) in patients with chronic HBV infection. We observed that after activated by sCD40L, the expression of CD80, CD86, major histocompatibility complex (MHC) I and II molecules on the CD40-B cells was significantly increased. Cytometry and fluorescence microscopy showed that more than 41.34% CD40-B cells were loaded by the HBcAg peptide. Furthermore, after been activated and HBcAg18-27 antigen peptide pulsed, B cells obtained from patients with chronic HBV infection could induce HBcAg18-27 specific CTLs in vitro. Taken together, our results show that B cells from patients with chronic HBV infection can be activated by sCD40L and may function as antigen presenting cells and induce HBV-specific CTLs.

Migratory, and not lymphoid-resident, dendritic cells maintain peripheral self-tolerance and prevent autoimmunity via induction of iTreg cells

There is evidence that dendritic cells (DCs) induce peripheral tolerance. Nevertheless, it is not known whether immature DCs in general are able to tolerize CD4(+) T cells or if this is a prerogative of specialized subtypes. Here we show that, when autoantigen presentation is extended to all conventional mouse DCs, immature lymphoid tissue resident DCs are unable to induce autoantigen-specific regulatory T (iTreg) cell conversion. In contrast, this is an exclusive prerogative of steady-state migratory DCs. Because only lymph nodes host migratory DCs, iTreg cells develop and are retained solely in lymph nodes, and not in the spleen. Mechanistically, in cutaneous lymph nodes, DC-derived CCL22 contributes to the retention of iTreg cells. The importance of the local generation of iTreg cells is emphasized by their essential role in preventing autoimmunity.

CD40-activated B cells can efficiently prime antigen-specific naïve CD8+ T cells to generate effector but not memory T cells

Background

The identification of the signals that should be provided by antigen-presenting cells (APCs) to induce a CD8⁺ T cell response in vivo is essential to improve vaccination strategies using antigen-loaded APCs. Although dendritic cells have been extensively studied, the ability of other APC types, such as B cells, to induce a CD8⁺ T cell response have not been thoroughly evaluated.

Methodology/Principal Findings

In this manuscript, we have characterized the ability of CD40-activated B cells, stimulated or not with Toll-like receptor (TLR) agonists (CpG or lipopolysaccharide) to induce the response of mouse naïve CD8⁺ T cells in vivo. Our results show that CD40-activated B cells can directly present antigen to naïve CD8⁺ T cells to induce the generation of potent effectors able to secrete cytokines, kill target cells and control a Listeria monocytogenes infection. However, CD40-activated B cell immunization did not lead to the proper formation of CD8⁺ memory T cells and further maturation of CD40-activated B cells with TLR agonists did not promote the development of CD8⁺ memory T cells. Our results also suggest that inefficient generation of CD8⁺ memory T cells with CD40-activated B cell immunization is a consequence of reduced Bcl-6 expression by effectors and enhanced contraction of the CD8⁺ T cell response.

Conclusions

Understanding why CD40-activated B cell immunization is defective for the generation of memory T cells and gaining new insights about signals that should be provided by APCs are key steps before translating the use of CD40-B cell for therapeutic vaccination.

Mammalian target of rapamycin inhibition and alloantigen-specific regulatory T cells synergize to promote long-term graft survival in immunocompetent recipients

Minimization of immunosuppression and donor-specific tolerance to MHC-mismatched organ grafts are important clinical goals. The therapeutic potential of regulatory T cells (Tregs) has been demonstrated, but conditions for optimizing their in vivo function posttransplant in nonlymphocyte-depleted hosts remain undefined. In this study, we address mechanisms through which inhibition of the mammalian target of rapamycin (Rapa) synergizes with alloantigen-specific Treg (AAsTreg) to permit long-term, donor-specific heart graft survival in immunocompetent hosts. Crucially, immature allogeneic dendritic cells allowed AAsTreg selection in vitro, with minimal expansion of unwanted (Th17) cells. The rendered Treg potently inhibited T cell proliferation in an Ag-specific manner. However, these AAsTreg remained unable to control T cells stimulated by allogeneic mature dendritic cells, a phenomenon dependent on the release of proinflammatory cytokines. In vivo, Rapa administration reduced danger-associated IL-6 production, T cell proliferation, and graft infiltration. Based on these observations, AAsTreg were administered posttransplant (day 7) in combination with a short course of Rapa and rendered >80% long-term (>150 d) graft survival, a result superior to that achieved with polyclonal Treg. Moreover, graft protection was alloantigen-specific. Significantly, long-term graft survival was associated with alloreactive T cell anergy. These findings delineate combination of transient mammalian target of Rapa inhibition with appropriate AAsTreg selection as an effective approach to promote long-term organ graft survival.

Resting B cells expand a CD4+CD25+Foxp3+ Treg population via TGF-beta3

Regulatory T cells (Treg) play critical roles in maintaining tolerance and preventing autoimmunity. It is not fully clear how these cells are generated and maintained. Here, we show that resting B cells are able to expand Treg. This expansion requires TGF-beta3 and signaling through the TCR and CD28. Upon activation, B cells express less TGF-beta3, which reduces their capacity to expand Treg and which also results in increased Treg death. This may ensure that B cells can function as potent professional antigen presenting cells during infections. However, in the absence of any infection, we find that B-cell-deficient microMT mice have decreased percentages of Treg in the periphery. Our data suggest that resting B cells, which may be presenting self-antigens to T cells, can expand and maintain specific Treg and thus might be involved in the prevention of autoimmunity.

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.

Antigen presenting cells treated in vitro by macrophage colony-stimulating factor and autoantigen protect mice from autoimmunity

Macrophage colony-stimulating factor (M-CSF) is a critical cytokine in the development of monocytic lineage and may have immunoregulatory properties. Here we show that peritoneal antigen presenting cells (APCs) treated with M-CSF produced decreased levels of proinflammatory cytokines IFN-gamma, TNF-alpha and IL-12. These APCs treated with M-CSF+autoantigen peptide significantly suppressed antigen-specific T cell proliferation, induced regulatory CD4(+) and CD8(+) T cells in vitro and in vivo, and significantly suppressed experimental autoimmune encephalomyelitis (EAE). Thus, in vitro treatment of APCs with M-CSF+autoantigen can be a novel therapeutic option for autoimmune diseases.

Cutting edge: primary B lymphocytes preferentially expand allogeneic FoxP3+ CD4 T cells

Despite the unequivocal role of B lymphocytes as effecter cells in humoral immunity, studies have reported that B cells are tolerogenic. The impact of B cell-mediated tolerance and its underlying mechanisms are incompletely understood. Using primary B cells as APCs and allogeneic CD4 T cells as responder cells in mixed leukocyte reactions, we find that B cells preferentially expand FoxP3(+) over FoxP3(-) CD4 T cells in the absence of exogenous cytokines. The preferential expansion of Foxp3(+) T cells is further enhanced by a partial blockade of class II MHC-TCR interaction but diminished by stimulatory anti-CD28 Ab or at high B to T cell ratios. Gamma irradiation of B cells selectively abrogates their ability to expand isolated CD25(+) but not CD25(-) CD4 T cells; exogenous IL-2 supplement can partially restore this function. B cell-expanded CD25(+) T cells express high levels of FoxP3 and are highly inhibitory in an Ag-specific manner.

Reversion of immune tolerance in advanced malignancy: modulation of myeloid-derived suppressor cell development by blockade of stem-cell factor function

Tumor growth induced a significant increase of myeloid-derived suppressor cells (MDSCs) in the tumor-bearing host. In our previous study, we showed that MDSCs induced tumor-specific T-cell tolerance and the development of T regulatory cells (Tregs). Tumor-derived factors have been implicated in the accumulation of MDSCs. We hypothesize that reduction of MDSC accumulation in tumor-bearing hosts, through the blockade of tumor factors, can prevent T-cell anergy and Treg development and thereby improve immune therapy for the treatment of advanced tumors. Several tumor-derived factors were identified by gene array analysis. Among the candidate factors, stem- cell factor (SCF) is expressed by various human and murine carcinomas and was selected for further study. Mice bearing tumor cells with SCF siRNA knockdown exhibited significantly reduced MDSC expansion and restored proliferative responses of tumor-infiltrating T cells. More importantly, blockade of SCF receptor (ckit)-SCF interaction by anti-ckit prevented tumor-specific T-cell anergy, Treg development, and tumor angiogenesis. Furthermore, the prevention of MDSC accumulation in conjunction with immune activation therapy showed synergistic therapeutic effect when treating mice bearing large tumors. This information supports the notion that modulation of MDSC development may be required to achieve effective immune-enhancing therapy for the treatment of advanced tumors.

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.

Frontiers of immunological tolerance

Herein, we succinctly review mechanisms underlying self-tolerance and the roles of dendritic leukocytes (DCs) in T-cell tolerance to self and foreign antigens. We also consider the properties of naturally arising and other populations of regulatory T cells (Treg), together with growing evidence that interplay between DCs and Treg cells can sustain antigen-specific tolerance. B-cell tolerance and the role of hematopoietic cell chimerism in the induction and maintenance of tolerance are also discussed, as is the impact of cosignaling pathway manipulation on tolerance induction. This overview also surveys prospects for technological advances in the monitoring and prediction of tolerance and the application of genomic and proteomic analysis. In addition, we consider potential novel therapeutic targets for promotion of tolerance induction.