In vivo assessment of the relative contributions of deletion, anergy, and editing to B cell self-tolerance

Transcriptional basis of lymphocyte tolerance

Signaling through lymphocyte antigen receptors has the potential to initiate several distinct outcomes: proliferation, differentiation, apoptosis, or functional unresponsiveness. Expansion and differentiation of effector T cells is required for defense against foreign antigens, whereas functional unresponsiveness, termed anergy, is a cell-intrinsic mechanism that contributes to peripheral self-tolerance. Other mechanisms of peripheral tolerance include the 'dominant' tolerance imposed by regulatory T cells and immunosuppression mediated by interleukin-10 and transforming growth factor-beta. T- and B-cell antigen receptor ligation induces an increase in intracellular calcium levels as well as activating additional signaling pathways that are further potentiated by costimulatory receptors. In this review, we argue that cell-intrinsic programs of peripheral anergy and tolerance are imposed by sustained calcium signaling in lymphocytes. We address in particular the role of the calcium-dependent transcription factor nuclear factor for activation of T cells, which is activated by antigen receptor stimulation and, depending on the presence or absence of input from its transcriptional partner, activator protein-1, dictates two distinct transcriptional programs: activation or tolerance.

Identification of Anergic B Cells within a Wild-Type Repertoire

The contribution of anergy to silencing of autoreactive B cells in physiologic settings is unknown. By comparing anergic and nonanergic immunoglobulin-transgenic mouse strains, we defined a set of surface markers that were used for presumptive identification of an anergic B cell cohort within a normal repertoire. Like anergic transgenic B cells, these physiologic anergic cells exhibited high basal intracellular free calcium and did not mobilize calcium, initiate tyrosine phosphorylation, proliferate, upregulate activation markers, or mount an immune response upon antigen-receptor stimulation. Autoreactive B cells were overrepresented in this cohort. On the basis of the frequency and lifespan of these cells, it appears that as many as 50% of newly produced B cells are destined to become anergic. In conclusion, our findings indicate that anergy is probably the primary mechanism by which autoreactive B cells are silenced. Thus maintenance of the unresponsiveness of anergic cells is critical for prevention of autoimmunity.

Peripheral B cell receptor editing may promote the production of high-affinity autoantibodies in CD22-deficient mice

CD22-deficient mice are characterized by B cell hyperactivity and autoimmunity. We have constructed knock-in CD22-/- mice, expressing an anti-DNA heavy (H) chain (D42), alone or combined with Vkappa1-Jkappa1 or Vkappa8-Jkappa5 light (L) chains. The Ig-targeted mice produced a lupus-like serology that was age- and sex-dependent. High-affinity IgG autoantibodies were largely dependent on the selection of B cells with a particular H/L combination, in which a non-transgenic, endogenous L chain was assembled by secondary rearrangements through the mechanism of receptor editing. Moreover, we present evidence that these secondary rearrangements are very prominent in splenic peripheral B cells. Since CD22 is primarily expressed on the surface of peripheral B cells, we propose a model for the development of a lupus-like autoimmune disease by a combination of peripheral receptor editing and abnormal B cell activation.

Receptor editing in lymphocyte development and central tolerance

The specificities of lymphocytes for antigen are generated by a quasi-random process of gene rearrangement that often results in non-functional or autoreactive antigen receptors. Regulation of lymphocyte specificities involves not only the elimination of cells that display 'unsuitable' receptors for antigen but also the active genetic correction of these receptors by secondary recombination of the DNA. As I discuss here, an important mechanism for the genetic correction of antigen receptors is ongoing recombination, which leads to receptor editing. Receptor editing is probably an adaptation that is necessitated by the high probability of receptor autoreactivity. In both B cells and T cells, the genes that encode the two chains of the antigen receptor seem to be specialized to promote, on the one hand, the generation of diverse specificities and, on the other hand, the regulation of these specificities through efficient editing.

Silencing of autoreactive B cells by anergy: a fresh perspective

B-cell antigen receptor (BCR) signals are crucial for initiation of humoral immune responses and must be actively modulated and/or terminated in preparation for receipt of subsequent cues for progression. BCR signaling is also actively inhibited in autoreactive cells in which unresponsiveness is maintained by anergy. This serves to prevent cell activation and autoimmunity. Importantly, the feedback mechanisms that modulate and/or terminate signaling during normal antigen-induced B-cell activation appear to also be involved in maintaining B-cell anergy. In fact, it is suggested that anergy reflects nothing more than the normal inability of cells to respond to antigen following preceding stimulation of normal inhibitory feedback mechanisms. Thus, the time-honored two-signal hypothesis is almost certainly correct, with second signals being required to release the cell from inhibitory BCR-specific and trans-active feedback regulation.

B-cell tolerance

Autoreactive B cells are actively tolerized to more abundant self-antigens by a series of checkpoints involving receptor editing, deletion, anergy and competition for growth factors. In contrast, B cells reactive against rare, sequestered or tissue specific self-antigens remain functionally naïve. During an immune response, the autoimmune danger from these cells is countered by a variety of mechanisms comprising control of self-antigen presentation, limitation of immunogenic and tolerogenic costimuli including T cell help, homeostatic control of growth and strict regulation of germinal centre reactions. In this overview we consider how knowledge of these checkpoints may be used to gain a better understanding of transplant tolerance and the generation of alloantibodies.

Receptor editing for better or for worse

Receptor editing has emerged from its original identification as a minor secondary mechanism of B cell tolerance to be considered as a dominant mechanism by which autoreactive immature B cells are rendered tolerant. Clonal deletion, previously regarded as the major mechanism of central B cell tolerance, has been shown by recent studies to operate secondarily and only when receptor editing is unable to provide a non-autoreactive specificity. Receptor editing has also been shown to operate during the development of wild-type B lymphocytes, and ongoing investigations demonstrate the influence of particular signaling molecules in the induction and/or inhibition of receptor editing. Together, these studies begin to map the signaling pathways that regulate receptor editing in autoreactive and non-autoreactive immature B cells.

Induction of tolerance in arthritogenic B cells with receptors of differing affinity for self-antigen

Multiple mechanisms of tolerance induction limit autoimmunity, but their relative contribution for lymphocytes recognizing self-antigens of differing availability is incompletely understood. The mechanisms applied to arthritogenic B cells expressing antigen-specific B cell receptors (BCRs) with different affinities for glucose-6-phosphate isomerase (GPI) were examined in the corresponding Ig gene knock-in mice. This ubiquitously expressed and blood-borne enzyme is the target autoantigen in the K/BxN model of inflammatory arthritis and perhaps in some humans with arthritis. Negative selection of B cells expressing high-affinity anti-GPI specificities, whose surface receptors were occupied by GPI, operated mainly at the transitional B cell stages in the spleen, preventing their final differentiation and entry into follicular areas. Receptor editing contributed to the purging of cells displaying anti-GPI BCRs, and significant numbers of autoreactive cells escaped through expression of an additional Ig light (L) chain, accumulating gradually in lymphoid organs. In contrast, low-affinity anti-GPI B cells, whose surface receptors did not carry GPI, matured normally. The "escaped" dual-L-chain cells and the "ignored" low-affinity cells are the likely precursors of cells that produce pathogenic autoantibodies once T cell help is provided. These studies portray, in a single system, the range of tolerance mechanisms applied to potentially pathogenic B cells, and serve as a base for dissecting where T cell help intervenes and where therapeutic agents impinge.