Regulation of B cell self-tolerance by BAFF

To avoid the generation of pathogenic autoantibodies, self-reactive lymphocytes are deleted at several distinct checkpoints during B cell maturation. BAFF is required for mature B cell development and survival but causes B cell hyperplasia and autoimmunity when it is overexpressed. Self-reactive B cells have reduced responsiveness to BAFF and therefore die due to the limiting levels of BAFF available in vivo. Elevated BAFF expression subverts B cell self-tolerance by rescuing self-reactive B cells normally deleted relatively late during maturation. Strongly self-reactive B cells are deleted prior to expression of BAFF-R and are therefore resistant to rescue by BAFF.     

In vivo administration of interleukin-2 turns on anergic self-reactive T cells and leads to autoimmune disease.

One major mechanism of self tolerance involves the deletion of T cell clones in the thymus. In athymic mice, tolerance to self antigens must be generated extrathymically. T cells with self-reactive receptors undergo either peripheral clonal deletion or become unresponsive (i.e. anergic). The unresponsive state of human and mouse T cell clones in vitro can be reversed by the addition of exogenous interleukin (IL)-2, thus transforming anergic T cells to an activated state. Here it is shown that the in vivo delivery of IL-2 to athymic BALB/c nu/nu mice abrogates the anergic state of self-reactive V beta 3+ and V beta 11+ T cells [which are normally deleted in the minor lymphocyte stimulatory (Mls)-1b-, I-E(+)-expressing euthymic counterparts]. Thus, V beta 3+ and V beta 11+ T cells from IL-2-treated nude mice proliferate in response to T cell receptor cross-linking and acquire effector functions as measured by their ability to deliver aid to B cells upon specific stimulation. This activation correlates with the development of autoimmune manifestations (DNA autoantibodies, rheumatoid factors, erythroleukopenia and minimal change nephritis) in these IL-2-treated mice.     

Dendritic cells and interaction with other cell types. Immune tolerance

T cell tolerance to self antigen is mainly established in the thymus were self-reactive T cells are deleted. Interdigitating dendritic cells and medulary epithelial cells are directly involved in the deletion process. Some self-reactive T cells escape, however this thymic censorship and enter the peripheral pool of naive T cells. Multiple mechanisms are also at play in the periphery to control this potentially armfull T cells, this include deletion and immune deviation.     

Effect of cell:cell competition and BAFF expression on peripheral B cell tolerance and B-1 cell survival in transgenic mice expressing a low level of Igkappa-reactive macroself antigen

In mice carrying a synthetic Igkappa-reactive superantigen ("kappa macroself antigen"), low level expression induced split peripheral B cell tolerance in the sIgkappa+ compartment, with striking reductions in follicular and marginal zone (MZ) B cells and the retention of significant numbers of sIgkappa+ B-1a but not B-1b cells in the peritoneum. Here, we characterize the transgenic line pKkappa with this split tolerance phenotype and assess the effects of B cell competition and the survival cytokine BAFF (B cell activating factor belonging to the TNF family) on peripheral tolerance. In pKkappa mice the surviving peritoneal and splenic kappa+ B cells were largely lost in mice carrying one copy of the human Ckappa exon in place of the mouse version, a maneuver that generates additional antigen non-reactive competitor B cells in this model. Furthermore, overexpression of BAFF suppressed kappa-macroself antigen-induced deletion and promoted production of both IgM,kappa and IgA,kappa antibodies in mice with normal Igkappa alleles but not in mice carrying one copy of the human Ckappa allele. These findings suggest that BAFF overexpression has minimal effects on the survival of autoreactive B cells in a polyclonal immune system and that B cell:B cell competition plays a potent role in suppressing the survival of B-1 and splenic B cells with excessive autoreactivity.     

Self-reactive T cells are present in the peripheral lymphoid tissues of cyclosporin A-treated mice.

Cyclosporin A (CsA) prevents most immature thymocytes from progressing to a mature phenotype and blocks the deletion of T cells that express self-specific TCR in the small population of cells that achieve maturity. The latter effect may explain the paradoxical observation that this immunosuppressive drug can induce autoimmunity in irradiated rodents and humans if administered while new T cells are developing in the thymus. This study shows that the repopulation of the periphery with mature T cells is delayed in irradiated CsA-treated mice, presumably because CsA blocks T cell development in the thymus. The peripheral repertoire of these mice contained self-reactive IL-2 producing T cells that could be detected in a sensitive limiting dilution assay. In addition, self-reactive T cell hybridomas were isolated from the IL-2 receptor+ population present in CsA-treated mice. One of these hybridomas expressed a TCR V beta chain that is normally expressed on thymocytes that are deleted via recognition of self-antigens. Despite the presence of self-reactive T cells that had escaped clonal deletion, CsA-treated mice rarely developed lethal autoimmune disease, implying that a peripheral mechanism of tolerance can prevent the onset of autoimmune disease.     

Peripheral CD8+ T cell tolerance to self-proteins is regulated proximally at the T cell receptor

CD8(+) T cell tolerance, although essential for preventing autoimmunity, poses substantial obstacles to eliciting immune responses to tumor antigens, which are generally overexpressed normal proteins. Development of effective strategies to overcome tolerance for clinical applications would benefit from elucidation of the immunologic mechanism(s) regulating T cell tolerance to self. To examine how tolerance is maintained in vivo, we engineered dual-T cell receptor (TCR) transgenic mice in which CD8(+) T cells recognize two distinct antigens: a foreign viral-protein and a tolerizing self-tumor protein. Encounter with peripheral self-antigen rendered dual-TCR T cells tolerant to self, but these cells responded normally through the virus-specific TCR. Moreover, proliferation induced by virus rescued function of tolerized self-tumor-reactive TCR, restoring anti-tumor activity. These studies demonstrate that peripheral CD8(+) T cell tolerance to self-proteins can be regulated at the level of the self-reactive TCR complex rather than by central cellular inactivation and suggest an alternate strategy to enhance adoptive T cell immunotherapy.     

B cell positive selection by self antigens and counter-selection of dual B cell receptor cells in the peripheral B cell pools

The presence of B cells expressing two B cell receptors (BCR), described in BCR-transgenic, gene-targeted and normal mice, may represent an autoimmune hazard. We generated RAG-2-deficient mice bearing two complete rearranged immunoglobulin transgenes. In these mice most mature resting B cells express chains from the two transgenes. We studied selection of these dual receptor B cells in the presence of self antigens. In spite of the reduced surface density of the anti-self receptor, self-reactive B cells are deleted in the presence of membrane-bound self antigens. In contrast, the presence of soluble self antigen positively selects single receptor B cells expressing the self-reactive receptor. At the periphery these positively selected B cells down-regulate surface IgM expression and become unresponsive. A few dual receptor cells, however, escape tolerance induction. We examined the peripheral fate of the dual receptor B cells and showed that they are poorly selected into the activated B cell compartment and show a poor competitive capacity when in presence of populations of single receptor B cells. These results indicate that peripheral selection contributes to the very low frequencies of dual receptor B cells in normal mice and that multiple safeguard mechanisms operate to minimize the autoimmune hazard that allelically included B cells could represent.     

Does the BAFF dysregulation play a major role in the pathogenesis of systemic lupus erythematosus?

Given the prominent role currently assigned to B lymphocytes in systemic lupus erythematosus, it is not surprising that the B cell activity factor belonging to the tumor necrosis factor family (BAFF) is involved in its pathogenesis. This cytokine is produced in excess, and inserted into its receptors on the surface of circulating B cells. Up-regulation of BAFF is most likely to lead to breach of tolerance by aberrant survival of B cells directed to the self. Trials aimed at blocking BAFF have thus been set out. Yet the results are awaited.     

Rescue of self-reactive B cells by provision of T cell help in vivo

We have previously demonstrated that antigen-specific T cell help can rescue mature Ig transgenic (Tg) hen egg lysozyme (HEL)-specific B cells from tolerance induction upon transfer into soluble HEL-expressing Tg hosts. Here we extend these findings by showing that T cell help could also rescue both immature and mature self-reactive B cells from rapid deletion in response to high-avidity membrane-bound HEL. Moreover, although short-lived anergic peripheral B cells that had matured in the presence of soluble self antigen could not be rescued by provision of T cell help, a proportion of immature anergic IgM⁺ IgD⁻ CD23⁻ B cells from the bone marrow of the same donors survived and proliferated when given help following transfer to a soluble or membrane HEL-expressing host. In other words, T cell help must be available relatively soon after the antigen signal to prevent induction of tolerance. Consistent with this interpretation, the stronger stimulus provided by membrane-bound antigen, which deletes immature B cells before they leave the bone marrow, did not afford an opportunity for T cell help to rescue tolerant immature bone marrow-derived B cells upon transfer in vivo. Nevertheless, these B cells were capable of responding to T cell help in vitro, which speaks against an immutable susceptibility of immature B cells to tolerance induction. Taken together, these data indicate that the strength of the antigen signal and availability of T cell help are the primary determinants of the fate of both immature and mature B cells, consistent with the model proposed by Bretscher and Cohn more than 25 years ago.     

CD8+ T Cell Exhaustion During Persistent Viral Infection is Regulated Independently of the Virus-Specific T Cell Receptor

During chronic viral infections, responses by virus-specific CD8⁺ T cells become marginalized by the acquisition of functional defects and reduced cell numbers in a process defined as T cell exhaustion. Similarly, T cell tolerance to self-antigen is also characterized by impaired effector function and eventual deletion of self-reactive T cells. Induction of both tolerance and exhaustion involve many shared inhibitory mechanisms, thus similar therapeutic approaches have proven effective in these distinct environments. We previously demonstrated that tolerant self-reactive CD8⁺ T cells expressing dual-T cell receptors (i.e., dual-TCR) could be rescued by immunization through a second TCR specific for a foreign antigen. These data revealed that T cell tolerance was regulated at the level of the self-reactive TCR. Here, dual-TCR CD8⁺ T cells were used to examine if exhaustion during persistent viral infection could be rescued by an analogous strategy of immunization through a second TCR not involved in recognition of virus. In direct contrast to the rescue achievable in tolerant CD8⁺ T cells, exhausted T cells were equally impaired through both TCR. These findings suggest that exhaustion is maintained by defects downstream of the virus-specific TCR, and establish that exhaustion and tolerance are distinctly regulated states of T cell dysfunction.     

The top ten clues to understand the origin of chronic lymphocytic leukemia (CLL)

The fundamental task of the immune system is to protect the individual from infectious organisms without serious injury to self. The essence of acquired immunity is molecular self/non self discrimination. Chronic lymphocytic leukemia is characterized by a global failure of immune system that begins with the failure of immunological tolerance mechanisms (autoimmunity) and finish with the incapacity to response to non-self antigens (immunodeficiency). Immunological tolerance mechanisms are involved in chronic lymphocytic leukemia (CLL) development. During B cell development some self-reactive B cells acquire a special BCR that recognize their own BCR. This self-autoantibody-self BCR interaction promotes survival, differentiation and proliferation of self-reactive B cells. Continuous self-autoantibody-self BCR interaction cross-linking induces an increased rate of surface BCR elimination, CD5+ expression, receptor editing and anergy. Unfortunately, some times this mechanisms increase genomic instability and promote additional genetic damage that immortalize self-reactive B cells and convert them into CLL like clones with the capability of clonal evolution and transformed CLL B cells. This review summarizes the immunological effects of continuous self-autoantibody-self BCR interaction cross-linking in the surface of self-reactive B cells and their role in CLL development.     

The NIK of time for B cells

NF‐κB‐inducing kinase (NIK) is a key mediator of the noncanonical NF‐κB signaling pathway, which is critical for B‐cell development and function. Although complete deletion of NIK in mice has been shown to result in defective B cells and impaired secondary lymphoid organogenesis, the consequences of deleting NIK exclusively in B cells have not been determined. In this issue of the European Journal of Immunology, Hahn et al. [Eur. J. Immunol. 2016. 46: 732–741] describe mice in which the NF‐κB2 pathway mediator, NIK, is deleted at different points in B‐cell lineage differentiation and activation. The results show that the survival of mature peripheral B cells, as well as appropriate kinetics of germinal center reactions, rely on noncanonical NF‐κB signaling. These findings confirm and extend prior observations implicating a nonredundant role for NF‐κB2 downstream of BAFF signaling via BAFF‐R, and prompt assessment of the growing literature regarding the relative roles of BCR and BAFF signals in B‐cell homeostasis, as well as the downstream pathways responsible.     

Mechanisms of tolerance to self

Several mechanisms exist to prevent lymphocytes from reacting against self-antigens. As T cells develop in the thymus and express antigen-specific receptors, those with high-affinity to self-antigens existing within the thymus are deleted. Low-affinity self-reactive T cells and T cells with receptors against antigens not represented intrathymically will mature and join the peripheral T cell pool. They may either ignore self-antigens expressed by tissues unable to activate T cells through a lack of the appropriate costimulator signals, or they may, under certain conditions, be deleted or rendered anergic and unable to respond. Likewise, B cells that express surface Ig receptors with high binding affinity to membrane-bound self-antigens present in the bone marrow will be rescued by receptor editing or will be deleted, whereas those of lower affinity will migrate to the periphery in either an anergic or indifferent state depending on the degree of receptor engagement by antigen. Once there, their ultimate fate is determined by the availability of T cell help.     

Self-tolerance in B lymphocytes

Normally the immune system does not produce pathogenic antibodies to autologous antigens, due to induction of self-tolerance in both the T and B lymphocyte repertoires. The aim of this paper is to review the evidence for self-tolerance within the B cell repertoire, and the range of possible mechanisms responsible for it. In practice, the mechanism of B cell tolerance to autologous antigens in vivo remains controversial, and may in fact vary (depending on the nature of the self antigen and the properties of the self-reactive B cell. Recent work in transgenic mouse models of B cell tolerance has helped to assimilate the numerous and sometimes disparate findings from other models, firstly by allowing direct visualization of the fate of self-reactive B cells in vivo, and secondly, by enabling systematic genetic changes to be made either in the self antigen or in the self-reactive B cell.     

Expression and function of TNF family member B cell-activating factor in the development of autoimmune arthritis

B cell-activating factor (BAFF), a member of tumor necrosis factor family cytokines, has been shown to enhance the maturation and survival of peripheral B cells. While BAFF is implicated in regulating B cell function and autoimmunity, its role in the development of autoimmune arthritis has not been fully clarified. Using a collagen-induced arthritis (CIA) mouse model, we detected dysregulated expression of BAFF and its receptors in the peripheral lymphoid organs during arthritis induction. Elevated serum levels of BAFF were closely correlated with increased levels of anti-collagen antibodies during the CIA progression. Moreover, dendritic cells (DCs) and macrophages were found to express high amount of BAFF proteins at the acute and chronic stages of CIA, respectively. In cultures, recombinant BAFF suppressed apoptosis of splenic B cells from arthritic mice, and DC-induced B cell proliferation was specifically blocked by soluble decoy receptor B cell maturation antigen-Fc. These findings suggest that overproduction of BAFF by DCs and macrophages may play a crucial role in the pathogenesis of experimental arthritis.     

The role of BAFF in immune function and implications for autoimmunity

Summary: BAFF [B‐cell activating factor belonging to the tumor necrosis factor (TNF) family] is a ligand that is required for peripheral B‐cell survival and homeostasis. In addition to mediating B‐cell survival, BAFF also regulates expression of certain B‐cell‐surface proteins, such as CD21/35. BAFF deficiency results in a reduced number of peripheral B cells and a diminished ability to mount robust humoral immune responses. Overexpression of BAFF has been linked to murine and human autoimmunity, and recent data provide clues as to how excess BAFF may allow the emergence of autoreactive B cells. In vivo animal testing with BAFF inhibitors has generated exciting data that support the pathway as a target for modulating B cells. The role of BAFF in B‐cell biology, T‐cell biology, and autoimmunity is discussed, as well as current efforts to develop BAFF inhibitors for clinical testing in autoimmune disorders.     

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.     

Evidence for peripheral mechanisms inducing tissue tolerance during ontogeny

Tissue grafts from a histoincompatible donor of the same developmental stage were introduced into an early chick embryo host in order to probe the immune response to the graft after birth, when the host has reached immune maturity. Limb buds from B4 or B12 chicken strains were grafted in situ on (B15 x B21)F1 recipients that were allowed to hatch. The grafted wing grew normally and was tolerated in a nearly perfect way during the host's lifetime, although reversible rejection crises severely affected the fundamentally healthy state of the grafted tissues. Skin grafts of the same major histocompatibility complex haplotype as the wing were performed on the adult wing-chimera and were permanently tolerated. In contrast, host peripheral blood lymphocytes maintained their capacity to proliferate against donor cells in the mixed lymphocyte reaction. These results, while showing that in vitro and in vivo tolerance are separable phenomena, suggest the existence of a peripheral mechanism inducing tolerance to self that complements the elimination of self-reactive clones by the thymus.