Distinct mechanisms of neonatal tolerance induced by dendritic cells and thymic B cells

European Society for Immunodeficiencies guidelines for the management of patients with congenital athymia

Congenital athymia is a life-limiting disorder due to rare inborn errors of immunity causing impaired thymus organogenesis or abnormal thymic stromal cell development and function. Athymic infants have a T-lymphocyte-negative, B-lymphocyte-positive, natural killer cell-positive immunophenotype with profound T-lymphocyte deficiency and are susceptible to severe infections and autoimmunity. Patients variably display syndromic features. Expanding access to newborn screening for severe combined immunodeficiency and T lymphocytopenia and broad genetic testing, including next-generation sequencing technologies, increasingly facilitate their timely identification. The recommended first-line treatment is allogeneic thymus transplantation, which is a specialized procedure available in Europe and the United States. Outcomes for athymic patients are best with early diagnosis and thymus transplantation before the development of infectious and inflammatory complications. These guidelines on behalf of the European Society for Immunodeficiencies provide a comprehensive review for clinicians who manage patients with inborn thymic stromal cell defects; they offer clinical practice recommendations focused on the diagnosis, investigation, risk stratification, and management of congenital athymia with the aim of improving patient outcomes.

Thymic B Cells as a New Player in the Type 1 Diabetes Response

Type 1 diabetes (T1d) results from a sustained autoreactive T and B cell response towards insulin-producing β cells in the islets of Langerhans. The autoreactive nature of the condition has led to many investigations addressing the genetic or cellular changes in primary lymphoid tissues that impairs central tolerance- a key process in the deletion of autoreactive T and B cells during their development. For T cells, these studies have largely focused on medullary thymic epithelial cells (mTECs) critical for the effective negative selection of autoreactive T cells in the thymus. Recently, a new cellular player that impacts positively or negatively on the deletion of autoreactive T cells during their development has come to light, thymic B cells. Normally a small population within the thymus of mouse and man, thymic B cells expand in T1d as well as other autoimmune conditions, reside in thymic ectopic germinal centres and secrete autoantibodies that bind selective mTECs precipitating mTEC death. In this review we will discuss the ontogeny, characteristics and functionality of thymic B cells in healthy and autoimmune settings. Furthermore, we explore how in silico approaches may help decipher the complex cellular interplay of thymic B cells with other cells within the thymic microenvironment leading to new avenues for therapeutic intervention.

The Multifaceted Roles of B Cells in the Thymus: From Immune Tolerance to Autoimmunity

The thymus is home to a significant number of resident B cells which possess several unique characteristics regarding their origin, phenotype and function. Evidence shows that they originate both from precursors that mature intrathymically and as the entry of recirculating mature B cells. Under steady-state conditions they exhibit hallmark signatures of activated B cells, undergo immunoglobulin class-switch, and express the Aire transcription factor. These features are imprinted within the thymus and enable B cells to act as specialized antigen-presenting cells in the thymic medulla that contribute negative selection of self-reactive T cells. Though, most studies have focused on B cells located in the medulla, a second contingent of B cells is also present in non-epithelial perivascular spaces of the thymus. This latter group of B cells, which includes memory B cells and plasma cells, is not readily detected in the thymus of infants or young mice but gradually accumulates during normal aging. Remarkably, in many autoimmune diseases the thymus suffers severe structural atrophy and infiltration of B cells in the perivascular spaces, which organize into follicles similar to those typically found in secondary lymphoid organs. This review provides an overview of the pathways involved in thymic B cell origin and presents an integrated view of both thymic medullary and perivascular B cells and their respective physiological and pathological roles in central tolerance and autoimmune diseases.

The development and function of thymic B cells

Thymic B cells are a unique population of B lymphocytes that reside at the cortico-medullary junction of the thymus, an organ that is specialized for the development and selection of T cells. These B cells are distinct from peripheral B cells both in terms of their origin and phenotype. Multiple lines of evidence suggest that they develop within the thymus from B lineage-committed progenitors and are not recirculating peripheral B cells. Furthermore, thymic B cells have a highly activated phenotype. Because of their location in the thymic medulla, they have been thought to play a role in T cell negative selection. Thymic B cells are capable of inducing negative selection in a number of model antigen systems, including viral super antigen, peptides from immunoglobulin, and cognate self antigen presented by B cell receptor-mediated uptake. These findings establish thymic B cells as a novel and important population to study; however, much work remains to be done to understand how all of these unique aspects of thymic B cell biology inform their function.

Dominant trait linked to chromosome 1 in DBA/2 mice for the resistance to autoimmune gastritis appears in bone marrow cells

Neonatal thymectomy (NTx) induces autoimmune gastritis (AIG) in BALB/c mice, a model for human type A chronic atrophic gastritis, but not in DBA/2 mice and rarely in CDF1 mice (a hybrid of BALB/c and DBA/2 mice). The aim of this study was to clarify the mechanisms of AIG-resistance in mice bearing the dominant trait of DBA/2. Linkage groups associated with, and cells related to AIG resistance were examined with CDF1-BALB/c backcrosses. Intracellular staining and flow-cytometric bead array for several cytokines were performed on NTx BALB/c mice and NTx DBA/2-chimeric BALB/c mice receiving DBA/2-bone marrow cells. In NTx BALB/c mice, IFN-γ-secreting CD4(+) T cells were increased, but not in NTx DBA/2 mice. Because Vβ6(+) T cell-bearing mice of half of their backcrosses developed AIG, but the other half of Vβ6(+) T cell-negative mice developed scarcely, resistance for AIG generation is associated with the presence of the Mls-1a locus on chromosome 1 in DBA/2 mice, which deletes Vβ6(+) T cells. NTx DBA/2-chimera BALB/c mice showed dominant production of IL-10 and resistance for AIG, although the deletion of Vβ6(+) T cells was found not to be a cause of AIG-resistance from Mls-1a locus segregation experiments. Although NTx DBA/2-chimeric BALB/c mice did not suffer from AIG, they brought immediate precursors of T cells for AIG. It is concluded that DBA/2 mice generate bone marrow-derived cells that produce anti-inflammatory cytokines to prevent the activation of AIG-T cells.

Clonal heterogeneity of thymic B cells from early-onset myasthenia gravis patients with antibodies against the acetylcholine receptor

Myasthenia gravis (MG) with antibodies against the acetylcholine receptor (AChR-MG) is considered as a prototypic autoimmune disease. The thymus is important in the pathophysiology of the disease since thymus hyperplasia is a characteristic of early-onset AChR-MG and patients often improve after thymectomy. We hypothesized that thymic B cell and antibody repertoires of AChR-MG patients differ intrinsically from those of control individuals. Using immortalization with Epstein-Barr Virus and Toll-like receptor 9 activation, we isolated and characterized monoclonal B cell lines from 5 MG patients and 8 controls. Only 2 of 570 immortalized B cell clones from MG patients produced antibodies against the AChR (both clones were from the same patient), suggesting that AChR-specific B cells are not enriched in the thymus. Surprisingly, many B cell lines from both AChR-MG and control thymus samples displayed reactivity against striated muscle proteins. Striational antibodies were produced by 15% of B cell clones from AChR-MG versus 6% in control thymus. The IgVH gene sequence analysis showed remarkable similarities, concerning VH family gene distribution, mutation frequency and CDR3 composition, between B cells of AChR-MG patients and controls. MG patients showed clear evidence of clonal B cell expansion in contrast to controls. In this latter aspect, MG resembles multiple sclerosis and clinically isolated syndrome, but differs from systemic lupus erythematosus. Our results support an antigen driven immune response in the MG thymus, but the paucity of AChR-specific B cells, in combination with the observed polyclonal expansions suggest a more diverse immune response than expected.

Gaucher disease gene GBA functions in immune regulation

Inherited deficiency of acid β-glucosidase (GCase) due to biallelic mutations in the GBA (glucosidase, β, acid) gene causes the classic manifestations of Gaucher disease (GD) involving the viscera, the skeleton, and the lungs. Clinical observations point to immune defects in GD beyond the accumulation of activated macrophages engorged with lysosomal glucosylceramide. Here, we show a plethora of immune cell aberrations in mice in which the GBA gene is deleted conditionally in hematopoietic stem cells (HSCs). The thymus exhibited the earliest and most striking alterations reminiscent of impaired T-cell maturation, aberrant B-cell recruitment, enhanced antigen presentation, and impaired egress of mature thymocytes. These changes correlated strongly with disease severity. In contrast to the profound defects in the thymus, there were only limited cellular defects in peripheral lymphoid organs, mainly restricted to mice with severe disease. The cellular changes in GCase deficiency were accompanied by elevated T-helper (Th)1 and Th2 cytokines that also tracked with disease severity. Finally, the proliferation of GCase-deficient HSCs was inhibited significantly by both GL1 and Lyso-GL1, suggesting that the "supply" of early thymic progenitors from bone marrow may, in fact, be reduced in GBA deficiency. The results not only point to a fundamental role for GBA in immune regulation but also suggest that GBA mutations in GD may cause widespread immune dysregulation through the accumulation of substrates.

MHC class II-dependent T-T interactions create a diverse, functional and immunoregulatory reaction circle

Unlike conventional T cells, innate-like T cells such as natural killer (NK) T cells are selected by homotypic T-cell interactions. Recently, a few reports have shown that T-T CD4(+) T cells can be generated in a similar manner to that for NKT cells. These two types of cells share common functional properties such as rapid response to antigenic encounters and the potential for a panoply of cytokine secretion. However, T-T CD4(+) T cells differ from NKT cells in that they are restricted by highly polymorphic major histocompatibility complex (MHC) II molecules and have a diverse T-cell receptor repertoire. Additional example of T-T interactions was recently reported in which peripheral T cells re-circulate to the thymus and participate in the thymocyte selection process. In this review, we dissect the cellular mechanisms underlying the production of T-T CD4(+) and NKT cells, with particular emphasis on the differences between these two T-cell prototypes. Finally, we propose that T-T CD4(+) T cells serve two major functions: one as an acute-phase reactant against viral infection and the other is the generation of anti-ergotypic CD4(+) T cells for regulatory purposes. All of these features make it possible to create a diverse set of functional cells through MHC class II-restricted T-T interactions.

Immune tolerance: mechanisms and application in clinical transplantation

The achievement of immune tolerance, a state of specific unresponsiveness to the donor graft, has the potential to overcome the current major limitations to progress in organ transplantation, namely late graft loss, organ shortage and the toxicities of chronic nonspecific immumnosuppressive therapy. Advances in our understanding of immunological processes, mechanisms of rejection and tolerance have led to encouraging developments in animal models, which are just beginning to be translated into clinical pilot studies. These advances are reviewed here and the appropriate timing for clinical trials is discussed.

Regulatory dendritic cell therapy in organ transplantation

Dendritic cells (DCs) are uniquely well equipped antigen (Ag)-presenting cells. Their classic function was thought to be that of potent initiators of innate and adaptive immunity to infectious organisms and other Ags (including transplanted organs). Evidence has emerged, however, that DCs have a central and crucial role in determining the fate of immune responses toward either immunity or tolerance. This dichotomous function of DCs, coupled with their remarkable plasticity, renders them attractive therapeutic targets for immune modulation. In transplantation, much recent work has focused on the ability of DCs to silence immune reactivity in an Ag-specific manner in the hope of preventing rejection and diminishing reliance on potentially harmful immunosuppressive agents. Experimental strategies have included in vivo targeting of DCs, as well as ex vivo generation of regulatory (or tolerogenic) DCs with subsequent reinfusion (i.e. cell therapy). Different approaches to 'program' DC toward tolerogenic properties include genetic (transgene insertion), biologic (differential culture conditions, anti-inflammatory cytokine exposure) and pharmacologic manipulation. Recent data suggest a promising role for pharmacologic treatment as a means of generating potent regulatory DCs and have further stimulated speculation regarding their potential clinical application. Herein, we discuss evidence that the potential of regulatory DC therapy is considerable and that there are compelling reasons to evaluate it in the setting of organ transplantation in the near future.

Role of regulatory dendritic cells in allergy and asthma

PURPOSE OF REVIEW

Dendritic cells are the most efficient inducers of all immune responses, and are capable of inducing either productive immunity or maintaining the state of tolerance to self-antigens and allergens. The present review summarizes the emerging literature on dendritic cells, with the emphasis on regulatory function of dendritic cells in allergy and asthma. In particular we summarize recent data regarding the relationship between dendritic cell subsets and Th1, Th2 and regulatory T (TReg) cells.

RECENT FINDINGS

The diverse functions of dendritic cells have been attributed to distinct lineages of dendritic cells, which arise from common immature precursor cells that differentiate in response to specific maturation-inducing or local microenvironment conditions. These subsets induce different lineages of T cells such as Th1, Th2 and TReg cells, including Th1Reg and Th2Reg cells, which regulate allergic diseases and asthma.

SUMMARY

Subsets of dendritic cells regulate the induction of a variety of T-cell subtypes, which suppress the development of allergy and asthma, thus providing antiinflammatory responses and protective immunity.

B cells and B cell products-helping to restore cellular immunity?

T cells that provide vital protection against tumors, viruses and intracellular bacteria are thought to develop independently of B cells. However, recent discoveries suggest that development of T cells depends on B cells. One way B cells promote T cell development is by providing diverse peptides that may promote positive selection of thymocytes. Diverse peptides and B cells help in diversification of the T cell receptor repertoire and may decrease cross-reactivity in the mature T cell compartment. These new insights may provide the basis for the design of novel therapeutics.

Natural killer T cells are required for the development of a superantigen-driven T helper type 2 immune response in mice

We show, here, that one single injection or weekly injections of staphylococcal enterotoxin B (SEB), starting in 1-day-old newborn mice, induced a powerful immune response with a T helper type 2 (Th2) pattern, as judged by the isotype and cytokine profile, with the production of large amounts of SEB-specific immunoglobulin G1 (IgG1), detectable levels of SEB-specific IgE and increased production of interleukin-4 by spleen cells. These protocols also induced an increase in the levels of total IgE in the serum. Memory of SEB was transferred to secondary recipients by using total spleen cells from primed animals. The secondary humoral response in transferred mice was diminished if spleen cells from SEB-treated mice were previously depleted of CD3+ or Vbeta8+ T cells or NK1.1+ cells. In vivo depletion of NK1.1+ cells in adult mice resulted in a marked reduction in the SEB-specific antibody response in both the primary and secondary immune responses. Additionally, purified NK1.1+ T cells were able to perform SEB-specific helper B-cell actions in vitro and in vivo. These results suggest that NK1.1+ T cells are required for the full development of humoral immunological memory, whilst making neonatal tolerance to SEB unachievable.

Role of regulatory dendritic cells in allergy and asthma

Dendritic cells (DCs) are the most efficient inducers of all immune responses, and are capable of either inducing productive immunity or maintaining the state of tolerance to self antigens and allergens. In this review, we summarize the emerging literature on DCs, with emphasis on the regulatory function of DCs in allergy and asthma. In particular, we summarize recent data regarding the relationship between DC subsets and TH1, TH2, and regulatory T (TReg) cells. The diverse functions of DCs have been attributed to distinct lineages of DCs, which arise from common immature precursor cells that differentiate in response to specific maturation-inducing or local microenvironment conditions. These subsets of DCs induce different lineages of T cells, such as TH1, TH2, and TReg cells, including Th1Reg and Th2Reg cells, which regulate allergic diseases and asthma. Subsets of DCs regulate the induction of a variety of T-cell subtypes, which suppress the development of allergy and asthma, thus providing anti-inflammatory responses and protective immunity.

Mls presentation by peritoneal cavity B cells

DBA/2J spleen and peritoneal cells were compared for their ability to present the minor lymphocyte stimulatory superantigen Mls-1a. Although capable of Mls presentation in vivo, peritoneal cells were less effective than spleen cells in vitro. This difference was not due to cell concentration or culture duration. Flow cytometric comparison of spleen and peritoneal B cells revealed no significant differences in cell surface markers needed for cognate interaction with T cells. Resolution of peritoneal B cell subsets by cell sorting revealed that even though B-1 cells were capable of Mls presentation, they were less effective than B-2 cells. Mixing experiments showed that B-1 cells did not inhibit B-2 cell presentation of Mls. In contrast, total peritoneal cells inhibited T cell responses to Mls presented by spleen cells. The peritoneal cavity harbors B cells that can present Mls as well as other cells that can suppress this response.

Antibody repertoire development in fetal and neonatal piglets. XI. The thymic B-cell repertoire develops independently from that in blood and mesenteric lymph nodes

The origin and function of thymic B cells is currently unresolved. In the present study we compared V(H) gene repertoire diversification in >3500 cloned VDJs (from 11 animals at three time-points, using three to five animals per time-point) that were expressed with immunoglobulin (Ig)M, IgD, IgG, IgA and IgE in thymus, mesenteric lymph nodes (MLN) and peripheral blood B cells (PBB) of newborn piglets and 5-week-old isolator piglets maintained germfree (GF) or colonized with Escherichia coli. The results showed that the repertoire expressed with IgM, IgD, IgG and IgA in PBB and MLN remained polyclonal, undiversified and unselected in piglets maintained GF for 5 weeks, that age and colonization resulted in significant repertoire diversification of IgG and IgA in the MLN and of IgG in blood, that the thymic B-cell repertoire was polyclonal, unaffected by colonization and showed no clonal selection in any isotype, and that the thymic IgA and IgE repertoires were more diverse at birth than the repertoire of any isotype in MLN or PBB. IgD was seldom recovered from the PBB of newborn piglets or at any time-point in thymus, but was recovered in the MLN of all 11 animals examined. The IgD and IgM repertoires in all tissues remained polyclonal and unselected, although V(H) usage by IgD transcripts did not always parallel that of IgM in the same tissue. Therefore, isotype-switched B cells in the thymic medulla cannot be accounted for by immigration of B cells diversified by colonization of the gut, and thymic B cells undergo switch recombination and repertoire diversification before birth without clonal selection.

B cell-dependent T cell development

T and B cells are thought to develop independently. While it is widely recognized that T cells help B cells in the production of antibodies to protein antigens, less well understood is whether or how B cells contribute to T cell development and function. Defects in cell-mediated immunity in individuals with B cell deficiency and in B cell-deficient mice suggest that B cells contribute to T cell function. The question of whether T cell development is B cell dependent was revisited using two novel mouse strains: mice with monoclonal T cells (MT) and mice with monoclonal compartments of both B and T cells (MBT). It was found that T cell development and thymocyte selection is modified by the presence of B cells. These results suggest that B cells, or B cell products, contribute to thymocyte selection and T cell development.

Development and maturation of the immune system in zebrafish, Danio rerio: a gene expression profiling, in situ hybridization and immunological study

The development and maturation of the immune system in zebrafish was investigated using immune-related gene expression profiling by quantitative real-time polymerase chain reaction, in situ hybridization (ISH), immunoglobulin (Ig) detection by immuno-affinity purification and Western blotting as well as immersion immunization experiments. Ikaros expression was first detected at 1 day post-fertilization (dpf) and thereafter increased gradually to more than two-fold between 28 and 42dpf before decreasing to less than the initial 1dpf expression level in adult fish (aged 105dpf). Recombination activating gene-1 (Rag-1) expression levels increased rapidly (by 10-fold) between 3 and 17dpf, reaching a maximum between 21 and 28dpf before decreasing gradually. However, in adult fish aged 105dpf, the expression level of Rag-1 had dropped markedly, and was equivalent to the expression level at 3dpf. T-cell receptor alpha constant region and immunoglobulin light chain constant region (IgLC) isotype-1, 2 and 3 mRNAs were detected at low levels by 3dpf and their expression levels increased steadily to the adult range between 4 and 6 weeks post-fertilization (wpf). Using tissue-section ISH, Rag-1 expression was detected in head kidney by 2wpf while IgLC-1, 2 and 3 were detected in the head kidney and the thymus by 3wpf onwards. Secreted Ig was only detectable using immuno-affinity purification and Western blotting by 4wpf. Humoral response to T-independent antigen (formalin-killed Aeromonas hydrophila) and T-dependent antigen (human gamma globulin) was observed in zebrafish immunized at 4 and 6wpf, respectively, indicating that immunocompetence was achieved. The findings reveal that the zebrafish immune system is morphologically and functionally mature by 4-6wpf.

Functional and morphological development of lymphoid tissues and immune regulatory and effector function in rhesus monkeys: cytokine-secreting cells, immunoglobulin-secreting cells, and CD5(+) B-1 cells appear early in fetal development

Little is known regarding the timing of immune ontogeny and effector function in fetal humans and nonhuman primates. We studied the organization of lymphocyte and antigen-presenting cell populations in developing lymphoid tissues of rhesus monkey fetuses during the second and third trimesters (65 to 145 days of gestation; term = 165 days). Immunoglobulin-secreting and cytokine-secreting cells were detected at day 80. The thymus, spleen, lymph nodes, and intestinal mucosa were examined for cells expressing CD3, CD5, CD20, CD68, p55, and HLA-DR. In the spleens of 65-day-old fetuses (early second trimester), the overwhelming majority of total lymphocytes were CD5(+) CD20(+) B-1 cells. The remaining lymphocytes were CD3(+) T cells. By day 80, splenic B and T cells were equal in number. Intraepithelial CD3(+) CD5(-) T cells and lamina propria CD20(+) CD5(+) B cells were present in the intestines of 65-day-old fetuses. By day 80, numerous CD20(+) CD5(+) B cells were present in the jejunums and colons and early lymphocyte aggregate formation was evident. The spleens of 80- to 145-day-old fetuses contained immunoglobulin M (IgM)-secreting cells, while IgA-, IgG-, interleukin-6-, and gamma interferon-secreting cells were numerous in the spleens and colons. Thus, by the second trimester, the lymphoid tissues of the rhesus monkey fetus have a complete repertoire of properly organized antigen-presenting cells, T cells, and B cells.

B-cell development in the thymus is limited by inhibitory signals from the thymic microenvironment

B-cell precursors are present in the thymus, and the thymic microenvironment is the source of lymphopoietic factors that include interleukin-7 (IL-7). Despite the fact that intrathymic B-cell progenitors are bone marrow-derived cells, the data in this report demonstrate that these progenitors accumulate at an early pro-B-cell stage of development, cycle less than their bone marrow counterparts, and fail to differentiate efficiently. Additional studies presented herein indicate that these effects are mediated, at least in part, by soluble factors produced by the thymic microenvironment and suggest that they affect the ability of pro-B cells to respond optimally to IL-7. Taken together, these observations demonstrate a specific inhibition of intrathymic B lymphopoiesis, which in turn may explain why lymphoid cell production in the thymus is largely restricted to production of T-lineage cells despite the fact that B-cell precursors and B-lymphopoietic stimuli are present in that organ.

Acceptance of third-party cardiac but not skin allografts induced by neonatal exposure to semi-allogeneic lymphohematopoietic cells

Neonatal tolerance is exclusively donor-specific when assessed by skin allograft survival and in vitro alloreactivity assays. In contrast, we reported previously that acceptance of primarily vascularized cardiac allografts was not donor-specific in C3H/He (C3H, H-2(k)) mice treated as neonates with BALB/c-derived (BALB, H-2(d)) lymphohematopoietic cells, but included third-party C57BL/10 (B10, H-2(b)) allografts. The present study examined whether this unusual pattern is limited to heart grafts in this strain combination, and defined the relative importance of the donor cell H-2(d) haplotype for third-party cardiac allograft acceptance. C3H neonates were injected with (C3HxBALB)F1 bone marrow and spleen cells. Tolerance was assessed at age 8-10 weeks by transplantation of heart or skin allografts from several donor strains, and by in vitro assays of proliferation and cytotoxicity. Additionally, cells from H-2(d) and H-2(b)-expressing strains on BALB or non-BALB minor histocompatibility (miH) antigen backgrounds were tested as tolerizing inocula. Prolonged survival of cardiac grafts from all donor strains was observed in neonatally treated mice, whereas skin grafting and in vitro assays demonstrated donor-specific hyporesponsiveness. Both H-2(d) haplotype and non-H-2 miH background of graft donor and tolerizing cell donor were important to third-party cardiac allograft acceptance. These results suggest that the functional alteration in alloreactivity induced by neonatal alloantigen exposure depends partly on method of assessment.

On the role of dendritic cells in peripheral T cell tolerance and modulation of autoimmunity

Recently, it has become clear that dendritic cells (DCs) are essential for the priming of T cell responses. However, their role in the maintenance of peripheral T cell tolerance remains largely undefined. Herein, an antigen-presenting cell (APC) transfer system was devised and applied to experimental allergic encephalomyelitis (EAE), to evaluate the contribution that DCs play in peripheral T cell tolerance. The CD8alpha(-)CD4(+) subset, a minor population among splenic DCs, was found to mediate both tolerance and bystander suppression against diverse T cell specificities. Aggregated (agg) Ig-myelin oligodendrocyte glycoprotein (MOG), an Ig chimera carrying the MOG 35-55 peptide, binds and cross-links FcgammaR on APC leading to efficient peptide presentation and interleukin (IL)-10 production. Furthermore, administration of agg Ig-MOG into diseased mice induces relief from clinical EAE involving multiple epitopes. Such recovery could not occur in FcgammaR-deficient mice where both uptake of Ig-MOG and IL-10 production are compromised. However, reconstitution of these mice with DC populations incorporating the CD8alpha(-)CD4(+) subset restored Ig-MOG-mediated reversal of EAE. Transfer of CD8alpha(+) or even CD8alpha(-)CD4(-) DCs had no effect on the disease. These findings strongly implicate DCs in peripheral tolerance and emphasize their functional potency, as a small population of DCs was able to support effective suppression of autoimmunity.

Preferential infection of immature dendritic cells and B cells by mouse mammary tumor virus

Until now it was thought that the retrovirus mouse mammary tumor virus preferentially infects B cells, which thereafter proliferate and differentiate due to superantigen-mediated T cell help. We describe in this study that dendritic cells are infectable at levels comparable to B cells in the first days after virus injection. Moreover, IgM knockout mice have chronically deleted superantigen-reactive T cells after MMTV injection, indicating that superantigen presentation by dendritic cells is sufficient for T cell deletion. In both subsets initially only few cells were infected, but there was an exponential increase in numbers of infected B cells due to superantigen-mediated T cell help, explaining that at the peak of the response infection is almost exclusively found in B cells. The level of infection in vivo was below 1 in 1000 dendritic cells or B cells. Infection levels in freshly isolated dendritic cells from spleen, Langerhans cells from skin, or bone marrow-derived dendritic cells were compared in an in vitro infection assay. Immature dendritic cells such as Langerhans cells or bone marrow-derived dendritic cells were infected 10- to 30-fold more efficiently than mature splenic dendritic cells. Bone marrow-derived dendritic cells carrying an endogenous mouse mammary tumor virus superantigen were highly efficient at inducing a superantigen response in vivo. These results highlight the importance of professional APC and efficient T cell priming for the establishment of a persistent infection by mouse mammary tumor virus.

Genetic control of T and B lymphocyte activation in nonobese diabetic mice

Type 1 diabetes in nonobese diabetic (NOD) mice is characterized by the infiltration of T and B cells into pancreatic islets. T cells bearing the TCR Vbeta3 chain are disproportionately represented in the earliest stages of islet infiltration (insulitis) despite clonal deletion of most Vbeta3(+) immature thymocytes by the mammary tumor virus-3 (Mtv-3) superantigen (SAg). In this report we showed that a high frequency of NOD Vbeta3(+) T cells that escape deletion are activated in vivo and that this phenotype is linked to the Mtv-3 locus. One potential mechanism of SAg presentation to peripheral T cells is by activated B cells. Consistent with this idea, we found that NOD mice harbor a significantly higher frequency of activated B cells than nondiabetes-prone strains. These activated NOD B cells expressed cell surface molecules consistent with APC function. At the molecular level, the IgH repertoire of activated B cells in NOD mice was equivalent to resting B cells, suggesting a polyclonal response in vivo. Genetic analysis of the activated B cell phenotype showed linkage to Idd1, the NOD MHC haplotype (H-2(g7)). Finally, Vbeta3(+) thymocyte deletion and peripheral T cell activation did not require B cells, suggesting that other APC populations are sufficient to generate both Mtv-3-linked phenotypes. These data provide insight into the genetic regulation of NOD autoreactive lymphocyte activation that may contribute to failure of peripheral tolerance and the pathogenesis of type I diabetes.

Increase of CD5(+) B cells during adolescence in female mice

We examined physiological changes in CD5(+) B lymphocytes in mice associated with aging (from 1 or 5 to 50 weeks of age). The most predominant populations of lymphocytes among these mice were mainly CD5(-) B cells in the spleen and peritoneal cavity, and T cells in the thymus. However, in the spleen, CD5(+) B cells increased from 1 to 15 weeks, and then decreased with aging. In the thymus, CD5(+) B cells increased from 3 to 9 weeks of age, and subsequently became more predominant than CD5(-) B cells. In the peritoneal cavity, CD5(+) B cells increased from 5 to 9 weeks of age, became the most predominant population in lymphocytes at 7 to 9 weeks of age, and decreased with aging. The proportion of CD5(+) B cells in total B cells increased from 5 to 7 or 9 weeks of age, and then decreased with aging, with the highest proportion at 9 weeks of age in the spleen (15%), thymus (94%), and peritoneal cavity (54%). These findings indicate that CD5(+) B cells increase physiologically during mouse adolescence, and subsequently decrease with aging.

Mixed chimerism

Induction of mixed chimerism has the potential to overcome the current limitations of transplantation, namely chronic rejection, complications of immunosuppressive therapy and the need for xenografts to overcome the current shortage of allogeneic organs. Successful achievement of mixed chimerism had been shown to tolerize T cells, B cells and possibly natural killer cells, the lymphocyte subsets that pose major barriers to allogeneic and xenogeneic transplants. Current understanding of the mechanisms involved in tolerization of each cell type is reviewed. Considerable advances have been made in reducing the potential toxicity of conditioning regimens required for the induction of mixed chimerism in rodent models, and translation of these strategies to large animal models and in a patient are important advances toward more widespread clinical application of the mixed chimerism approach for tolerance induction.

Transplantation tolerance induced by mixed chimerism

Although short- and long-term results after organ transplantation have improved considerably in recent years, morbidity and mortality rates in graft recipients remain high. The induction of lifelong donor-specific tolerance would dramatically improve outcome after organ transplantation. Although many tolerance protocols have been successful in rodent studies, most of these approaches have failed when attempted in large animals or humans. Robust tolerance, in contrast, has been demonstrated with mixed chimerism regimens not only in rodents but also in large animal models, including non-human primates. Furthermore, mixed chimerism protocols have been developed that would be feasible in cadaveric, and thus in thoracic, transplantation. The induction of mixed hematopoietic chimerism is therefore an attractive experimental approach for development of clinical tolerance protocols. One of the obstacles to widespread clinical application of this concept is the remaining toxicity of the host conditioning. Recent advances, however, have led to substantially milder protocols that could become clinically acceptable in the foreseeable future. This article provides a short overview of the basic mechanisms by which immunologic tolerance may be induced, describes the concept of mixed chimerism as a promising approach for clinical tolerance induction, and reviews recent progress in developing clinically feasible mixed chimerism protocols.

Increased expression of mXBP-1 (TREB-5) in thymic B cells in New Zealand mice

New Zealand Black mice as well as several other murine models of murine lupus are well known for premature degeneration of thymus and development of autoimmunity. To focus on molecular events unique to murine lupus, we performed differential display using arbitrary primer pairs to distinguish NZB versus BALB/c thymus at 5 weeks of age. Following an extensive analysis of DNA bands that were either consistently up or downregulated and from studies of expression pattern of thymic genes by in situ nucleic acid hybridization, we focused on one clone that was consistently differentially expressed between NZB and BALB/c thymus. This clone was isolated, sequenced, and identified as the murine homologue of the human X box binding protein (hXBP-1), also known as TREB 5. mXBP-1 was found to be consistently upregulated in B cells in the thymic cortex of NZB and (NZBxNZW)F1, but not BALB/c, C3H/HeJ or C57BL/6 mice. In addition, it was dramatically elevated in MRL/ lpr but not MRL/++ mice; similarly, it was increased in BXSB/ Yaa male but not BXSB female thymic cortex. Of particular interest was an absence of mXBP-1 expression in the thymus of NZB/ Bln- Igh6(null)homozygotes. mXBP-1 has several putative functions, including the regulation of MHC class II expression and by virtue of its ability to recognize CRE-like elements shown to be involved in HTLV-1 transcription.

B cells are selectively associated with thymic cortical but not medullary pathology in NZB mice

Abnormal expansion of autoantibody-synthesizing B cells and self-reactive T cells, which most likely escape negative selection within the thymus, have both been characterized and reasoned to play a role in the pathogenesis of autoimmunity in NZB mice. Support for this thesis has been our observation that NZB mice have severe cortical and medullary thymic microarchitectural defects. As a means to dissect the roles of T and B cells in the induction of such abnormalities, B cell-deficient NZB mice were bred by backcrossing the Igh6(null)allele on to the NZB background (NZB-muMT mice). Such mice showed undetectable levels of autoantibodies. NZB-muMT mice, as compared to wild-type NZB mice, had lower absolute numbers of CD4(+)T cells. Furthermore, thymic abnormalities in NZB-muMT mice were restricted to the medulla. These data suggest that, while B cells may play a role in thymic cortical abnormalities, the medullary abnormalities are induced by other mechanisms.

Mixed chimerism and transplantation tolerance

Achieving transplantation tolerance is an important goal in the effort to reduce long-term morbidity and mortality in organ transplant recipients. Robust, lifelong, donor-specific tolerance can be reliably achieved by induction of mixed chimerism in various animal models. To date, the clinical application of these proto-cols has been impeded partly by the potential toxicity of the required host conditioning regimens and the lack of successful studies in large animals. This article reviews the progress achieved in recent years in developing considerably milder conditioning protocols in rodents, and in extending some of these models to achieve permanent mixed chimerism and tolerance in large animals. Advances in the induction of xenogeneic tolerance through mixed chimerism are also discussed.

Central tolerance induction in natural immunoglobulin-allotype-specific T cells

While self toleance is induced to IgG(b)(2a) in Igh(b / b) mice, an anti-IgG(b)(2a) T cell activity emerges in their Igh(a / a) congenic counterparts. This activity is revealed by postnatal transfer of Igh(a / a) T splenocytes into Igh(a / b) F(1), in which total suppression of IgG(2a)(b) expression is established. Here, we sought to determine whether the natural T cell unresponsiveness to IgG(2a)(b) in Igh(b / b) mice involved a central tolerance. Based on the kinetics of postnatal thymic C(gamma2a)(b) gene expression in Igh(b / b) mice, we transplanted thymi from Igh(b / b) donors of diverse ages into tolerogen-free Igh(a / a) nu / nu recipients. The state of T cell tolerance or responsiveness to IgG(2a)(b) in these reconstituted nu / nu hosts was determined by monitoring the capacity of their splenocytes to induce suppression in Igh(a / b) F(1). These experiments demonstrated that: (i) in the Igh(a / a) nu / nu recipients of adult Igh(b / b) thymi, 33 to 65 % T splenocytes were from nu / nu recipient origin, but these peripheral Igh(a / a) T cells were rendered tolerant to IgG(2a)(b) during their differentiation through the adult Igh(b / b) thymi, (ii) circulating IgG(2a)(b) was not a prerequisite for this tolerance induction, (iii) Igh(b / b) thymic epithelium was unable to induce tolerance to IgG(2a)(b) and (iv) IgG(2a)(b)-producing / presenting cells, colonizing the Igh(b / b) thymi, were certainly responsible of full tolerance induction to IgG(2a)(b).

The thymus in the mouse changes its activity during pregnancy: a study of the microenvironment

The mouse thymus changes dramatically during pregnancy. It shrinks in size, and the cortex is extensively reduced from midpregnancy onwards. Despite this, there is surprisingly little evidence for any increase in apoptosis, and considerable evidence that mitosis of thymocytes continues throughout pregnancy. In spite of overall involution the thymic medulla actually expands in midpregnancy due to a combination of mitosis of epithelial cells and an accumulation of lymphocytes. The extent and nature of these changes are examined in this study at the ultrastructural level. The epithelial cells of the subcapsular cortex (type 1 cells) become wrinkled and exhibit powers of phagocytosis, whilst the other cortical epithelial cells are relatively unchanged, although the formation of epithelial/thymocyte rosettes and thymic nurse cells is more clearly seen in midpregnancy than usual. Other changes associated with pregnancy involve the medullary epithelial cells that undergo an increased level of mitosis. Their greater numbers surround accumulations of lymphocytes to form the characteristic medullary epithelial rings. Cell movement through blood vessel walls was clearly observed in midpregnancy, but not at other times. Interdigitating cells in the medulla become more conspicuous as pregnancy proceeds and the cells become phagocytic. The endoplasmic reticulum in plasma cells becomes expanded, indicating increased secretory activity. These results highlight the active nature of the thymus in pregnancy in spite of its involution. This picture contradicts the conventional notion that an involuted thymus is inactive.

TCR signaling for initiation and completion of thymocyte positive selection has distinct requirements for ligand quality and presenting cell type

Thymocyte selection involves signaling by TCR engaging diverse self-peptide:MHC molecule ligands on various cell types in the cortex and medulla. Here we separately analyze early and late stages of selection to better understand how presenting cell type, ligand quality, and the timing of TCR signaling contribute to intrathymic differentiation. TCR transgenic CD4+CD8+ thymocytes (double positive (DP)) from MHC-deficient mice were stimulated using various presenting cells and ligands. The resulting CD69high cells were isolated and evaluated for maturation in reaggregate cultures with wild-type or MHC molecule-deficient thymic stroma with or without added hemopoietic dendritic cells (DC). Production of CD4+ T cells required TCR signaling in the reaggregates, indicating that transient recognition of self-ligands by DP is inadequate for full differentiation. DC bearing a potent agonist ligand could initiate positive selection, producing activated thymocytes that matured into agonist-responsive T cells in reaggregates lacking the same ligand. DC could also support the TCR signaling necessary for late maturation. These results argue that despite the negative role assigned to DC in past studies, neither the peptide:MHC molecule complexes present on DC nor any other signals provided by these cells stimulate only thymocyte death. These findings also indicate that unique epithelial ligands are not necessary for positive selection. They provide additional insight into the role of ligand quality in selection events and support the concept that following initiation of maturation from the DP state, persistent TCR signaling is characteristic of and perhaps required by T cells.

Pathology of ALY mice: congenital immunodeficiency with lymph node and Peyer's patch defects

The ALY-alyl/aly mouse is a new and unique animal model of primary immunodeficiency with autosomal recessive inheritance. The ALY mouse is devoid of superficial and profound lymph nodes and Peyer's patches. Furthermore, the lymphoid follicles and marginal zones are not clearly identified in the spleen. In addition to these structural defects, in the present study, we show that some B subpopulations are defective. Firstly, the thymic B lymphocytes are very rare. Secondly, the B220hi sIghi B subpopulation in the bone marrow is not detected as a clear cluster on FACS analyses. Thirdly, the B220 slg+ cells in the bone marrow are very rare in both ALY-aly/aly and ALY-alyl+ mice. By contrast, the NK activity is normal. Taken together, the ALY mouse is an invaluable model to elucidate the immunological networks between the lymphoid structures (lymph nodes, Peyer's patches, lymphoid follicles, etc.) and functions.

Evidence for migration of donor bone marrow stromal cells into recipient thymus after bone marrow transplantation plus bone grafts: A role of stromal cells in positive selection

Intrathymic T-cell differentiation is characterized by two selection events: positive and negative selection. It has been shown that thymic epithelial cells in the cortex are involved in the positive selection, while macrophages and dendritic cells, derived from hemopoietic stem cells, are involved in the negative selection. Here we investigate whether donor-derived bone marrow stromal cells can migrate into the thymus and participate there in positive selection after bone marrow transplantation plus bone grafts (to recruit bone marrow stromal cells). Allogeneic bone marrow transplantation with or without bone grafts was carried out in the [C57BL/6-->C3H] combination. Fluorescence-activated cell sorter analyses of recipient thymic adherent cells showed that donor-type bone marrow stromal cells exist in the thymus of mice that received bone marrow plus bone grafts but not in the mice that received bone marrow cells alone. Histological examination using confocal microscopy also confirmed the existence of donor-type stromal cells in the thymus of mice that received bone marrow cells plus bones. Both T-cell proliferation and plaque-forming cell assays indicated that the T cells of such mice show donor-type major histocompatibility complex-restriction. These findings strongly suggest that stromal cells can migrate from the bone marrow to the thymus, where they participate in the positive selection of thymocytes.

Failure to remove autoreactive Vbeta6+ T cells in Mls-1 newborn mice attributed to the delayed development of B cells in the thymus

Clonal deletion of autoreactive T cells in the thymus is one of the major mechanisms for establishing tolerance to self-antigens, and self-reactive T cells bearing Vbeta6 T-cell receptors are usually deleted before their maturation in Mls-1a mice. However, these T cells develop transiently in the neonatal thymus, and migrate to the periphery. In order to understand the mechanisms which permit these potentially auto-toxic T cells to generate, we investigated in vivo the physiological or functional properties of the elements involved, such as neonatal T cells, antigens and antigen-presenting cells (APC). Confirming the previous findings that each of these elements per se is already completed in function in neonates, we investigated the possibility of the absence or immaturity of particular APC with Mls antigens of their own products in the neonatal thymus. In the search for the cellular and histological changes occurring in the newborn thymus, we found that the elimination of Vbeta6+ T cells progressed in parallel with the development of thymic B cells. Involvement of B cells in purging the autoreactive T cells from the newborn thymus was shown by prevention of the deletion of Vbeta6+ T cells after the removal of B cells by the treatment of neonates with anti-immunoglobulin M antibodies. The restricted and stable expression of CD5 on the thymic B cells, but not on the splenic cells, suggests that these B cells are not postnatal immigrants from the periphery. Finally, it is concluded that the deficiency in the deletion of self-reactive T cells in the thymus of Mls-1a neonates is due to the delayed development of B cells.

Immunobiology of dendritic cells

Dendritic cells (DCs) are antigen-presenting cells with a unique ability to induce primary immune responses. DCs capture and transfer information from the outside world to the cells of the adaptive immune system. DCs are not only critical for the induction of primary immune responses, but may also be important for the induction of immunological tolerance, as well as for the regulation of the type of T cell-mediated immune response. Although our understanding of DC biology is still in its infancy, we are now beginning to use DC-based immunotherapy protocols to elicit immunity against cancer and infectious diseases.

DNA vaccination and the immune responsiveness of neonates

Neonates often respond poorly to conventional vaccines or microbial infections. Immaturity of the immune system has been considered to play a role in this regard. However, accumulating evidence shows that in certain conditions, neonatal inoculation of antigens leads to protective immunity. In the particular case of DNA vaccines administered to neonates, the rule is immunity rather than tolerance. Exceptions to the rule give opportunities to further understand the neonatal responsiveness and the mechanism of DNA vaccination. Due to the very nature of the vaccine vector, inhibition of neonatal DNA vaccination by maternal antibodies may be limited to the humoral immunity.

B lymphopoiesis in the thymus

The thymus has been regarded as the major site of T cell differentiation. We find that in addition to alphabeta and gammadelta T cells, a significant number (approximately 3 x 104 per day) of B220+IgM+ mature B cells are exported from the thymus of C57BL/6 mice. Of these emigrating B cells, we estimate that at least approximately 2 x 104 per day are cells which developed intrathymically, whereas a maximum of approximately 0.8 x 104 per day are cells which circulated through the thymus from the periphery. The thymus possesses a significant number of pro-B and pre-B cells that express CD19, VpreB, lambda5, and pax-5. These B cell progenitors were found in the thymic cortex, whereas increasingly mature B cells were found in the corticomedullar and medullary regions. Other lymphoid cells, including NK cells and lymphoid dendritic cells, are not exported from the thymus at detectable levels. Thus, the thymus contributes to the formation of peripheral pools of B cells as well as of alphabeta and gammadelta T cells.

Functional comparison of thymic B cells and dendritic cells in vivo

In this report we present a transgenic mouse model in which we targeted gene expression specifically to B-lymphocytes. Using the human CD19 promoter, we expressed major histocompatibility complex class II I-E molecules specifically on B cells of all tissues, but not on other cell types. If only B cells expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. A comparison of the frequencies of I-E reactive Vβ5+ and Vβ11+ T cells shows that I-E expression on thymic B cells is sufficient to negatively select I-E reactive CD4+ T cells partially, but not CD8+ T cells. Thus partial negative but no positive selection events can be induced by B-lymphocytes in vivo.

Functional comparison of thymic B cells and dendritic cells in vivo

In this report we present a transgenic mouse model in which we targeted gene expression specifically to B-lymphocytes. Using the human CD19 promoter, we expressed major histocompatibility complex class II I-E molecules specifically on B cells of all tissues, but not on other cell types. If only B cells expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. A comparison of the frequencies of I-E reactive Vβ5+ and Vβ11+ T cells shows that I-E expression on thymic B cells is sufficient to negatively select I-E reactive CD4+ T cells partially, but not CD8+ T cells. Thus partial negative but no positive selection events can be induced by B-lymphocytes in vivo.

Notch1 deficiency dissociates the intrathymic development of dendritic cells and T cells

Thymic dendritic cells (DCs) form a discrete subset of bone marrow (BM)-derived cells, the function of which is to mediate negative selection of autoreactive thymocytes. The developmental origin of thymic DCs remains controversial. Although cell transfer studies support a model in which T cells and thymic DCs develop from the same intrathymic pluripotential precursor, it remains possible that these two types of cells develop from independent intrathymic precursors. Notch proteins are cell surface receptors involved in the regulation of cell fate specification. We have recently reported that T cell development in inducible Notch1-deficient mice is severely impaired at an early stage, before the expression of T cell lineage markers. To investigate whether development of thymic DCs also depends on Notch1, we have constructed mixed BM chimeric mice. We report here that thymic DC development from Notch1(-/)- BM precursors is absolutely normal (in terms of absolute number and phenotype) in this competitive situation, despite the absence of Notch1(-/)- T cells. Furthermore, we find that peripheral DCs and Langerhans cells are also not affected by Notch1 deficiency. Our results demonstrate that the development of DCs is totally independent of Notch1 function, and strongly suggest a dissociation between intrathymic T cell and DC precursors.

Differentiation from thymic B cell progenitors to mature B cells in vitro

The role of the thymic microenvironment in the development of murine thymic B cells has yet to be fully clarified. We therefore investigate the microenvironment that supports the development of mature thymic B cells (sIg+/B220+/CD43-B cells) from thymic B cell progenitors with immunophenotypes of sIg-/B220med/CD43+ cells. As we have previously reported, thymic B cells generated from these progenitors in the thymus are CD5+ B cells. We next study the in vitro condition that supports the differentiation of thymic B cell progenitors. Stromal cells (from the bone marrow or thymus), thymus-derived cell lines with the character of thymic nurse cells (TNCs) or thymic epithelial cells (TECs), or the bone marrow-derived cell line (MS-5) are tested for their ability to support B-lymphopoiesis from thymic B cell progenitors. Interestingly, thymic stromal cells (but neither stromal cells from the bone marrow nor stromal cell lines) support the differentiation of thymic B cell progenitors into thymic B cells in the presence of IL-7. Cortical epithelia (but not medullary epithelia, thymic macrophages or dendritic cells) are found to contribute to thymic B cell differentiation. Surface phenotype and Ig rearrangement analyses reveal that mature B cells generated in this condition are primarily CD5+ B cells, indicating that the thymic microenvironment (particularly cortical epithelia) determines the differentiation of thymic B cells.

The duration of antigen receptor signalling determines CD4+ versus CD8+ T-cell lineage fate

Signals elicited by binding of the T-cell antigen receptor and the CD4/CD8 co-receptor to major histocompatibility complex (MHC) molecules control the generation of CD4+ (helper) or CD8+ (cytotoxic) T cells from thymic precursors that initially express both co-receptor proteins. These precursors have unique, clonally distributed T-cell receptors with unpredictable specificity for the self-MHC molecules involved in this differentiation process. However, the mature T cells that emerge express only the CD4 (MHC class II-binding) or CD8 (MHC class I-binding) co-receptor that complements the MHC class-specificity of the T-cell receptor. How this matching of co-receptor-defined lineage and T-cell-receptor specificity is achieved remains unknown, as does whether signalling by the T-cell receptors, co-receptors and/or general cell-fate regulators such as Notch-1 contributes to initial lineage choice, to subsequent differentiation processes or to both. Here we show that the CD4 versus CD8 lineage fate of immature thymocytes is controlled by the co-receptor-influenced duration of initial T-cell receptor-dependent signalling. Notch-1 does not appear to be essential for this fate determination, but it is selectively required for CD8+ T-cell maturation after commitment directed by T-cell receptors. This indicates that the signals constraining CD4 versus CD8 lineage decisions are distinct from those that support subsequent differentiation events such as silencing of co-receptor loci.