particularly obvious in Faslpr/lprBim-/- mice. It is thus not a big surprise that natural selection would call on both apoptotic pathways (and also other tricks such as anergy and negative regulation) to prevent a fatal outcome.

2.2.3. Memory T cells

At the end of an immune response, the majority of effector T cells die by apoptosis, leaving behind a longlived population of memory T cells. Upon re-infection by the same pathogen, these cells proliferate strongly and undergo a rapid transition into effector cells.117,118 Memory T cells have long been considered to be generated from the pool of effector T cells that have responded to foreign antigens, and there is indeed evidence that some memory T cells have previously displayed effector functions.119 But memory T cells display many characteristics that are typical of na¨ıve cells, and this has led to the hypothesis that they may be generated directly from naive T cells without going through the effector state.120 The importance of cytokines, in particular of IL-7 and IL-15, in the maintenance and homeostasis of memory T cells is well recognized.118 Expression of IL-7Rα on T cells at the peak of the effector response to acute infection has been first correlated with the ability to survive the downsizing of immune response and progress to memory,121 but recent reports indicate that IL-7 signal is not sufficient for this process.122,123,124 IL-2 and some inflammatory signals such as IL-12 or type I interferons play an important role in the differentiation of memory

cells.125,126,127

The relative role of death receptor and mitochondrial pathways downstream of these cytokines in the maintenance of memory T cells is not clearly defined yet. The balance between Bim and Bcl-2 has been described as critical for the homeostasis of the memory T-cell population,19 and accumulation of memory T cells has also been observed in Bak/Bax-deficient mice.68 Interestingly, memory T-cell numbers were largely increased in mice lacking both Fas and Bim compared with mice lacking either of them alone,113 suggesting that here again, cooperation between both pathways may regulate the homeostasis of this cell population.

2.3. Control of apoptosis in B-cell development

2.3.1. Early B-cell development

The development of B cells resembles that of T cells in many aspects. The purpose of B-cell maturation is to produce a membrane BCR that can recognize foreign

antigens and ignore self-antigens.128 As is the case for TCR, production of a functional BCR first entails the rearrangement of genes encoding the different subunits of the pre-BCR, composed of the immunoglobulin heavy chain (Ig HC) with the λ5 and Vpre-B surrogate light chains. This happens at the pro-B cell stage, and successful assembly of the pre-BCR allows cells to survive and progress to the pre-B cell stages, during which immunoglobulin light chain (Ig LC) gene is rearranged and combined with Ig HC to form a functional BCR. As for the TCR components, rearrangement of BCR HC and LC is ensured by the recombination activating gene products RAG-1 and RAG-2, and only some of the rearrangement events lead to complete HC or LC proteins. All nonproductive HC or LC rearrangements lead to incomplete BCRs, and the lack of signaling that ensues results in apoptosis. Indeed, B cells in Rag-1– or Rag-2–deficient mice do not progress past the pro-B cell stage.48,129 This cell death process involves the Bcl-2–regulated pathway because over-expression of Bcl-2 and Bcl-xL prevents the deletion of pro-B cells in Rag-deficient mice, unable to produce rearrangement of Ig gene segments.130,131 Like for T cells, signals from IL-7 are necessary for the development of early B- cell progenitors, and B-cell development is arrested at the pro–B-cell stage in mice lacking either IL-739 or its receptor.38 Surprisingly, although Bcl-2 over-expression restores T-lymphocyte numbers similar to those of wildtype mice, it fails to produce the same effect on the B- cell population.40,41 Loss of Bim in IL-7Rα-deficient animals also rescued T cells, but had a much lesser effect on B cells.132,133 The requirement for IL-7 signaling in pre–B-cell survival ceases at the immature B-cell stage, where a signal from the BCR and a second signal from a TNF family ligand called BAFF (or BLys), through one of its receptors, are necessary for further differentiation and maturation of B cells.134,135 BAFF deficiency was found to arrest B-cell maturation at the immature transitional type I stage, whereas transgenic expression of BAFF causes accumulation of immature and mature B cells and autoimmunity.136 Current evidence shows that all known BAFF receptors are expressed on B cells at different levels depending on their maturation and/or activation state.134 BAFF-R is the key receptor that triggers BAFF-mediated survival, as mice deficient in this receptor display a phenotype similar to that of BAFF-null mice.137

Maturational stages beyond the immature B-cell stage and survival of mature B cells require continuous signaling through the BCR, as inducible deletion of Ig genes was shown to cause a rapid disappearance of B cells.138

2.3.2. Deletion of autoreactive B cells

Like autoreactive T cells, B cells that express a BCR that recognizes self-antigen must be deleted (negative selection) to prevent autoimmunity. Such B cells may also undergo further receptor editing to reduce the affinity of the BCR for self139 or become anergic.140 Deletion of autoreactive B cells can occur in the bone marrow as well as in the spleen and can happen at several developmental stages, from immature pre-BCR bearing cells to mature B cells in germinal centers.141,142 BCR ligationinduced deletion of autoreactive B lymphocytes in vivo is independent of Fas143,144 and not inhibited by overexpression of FADD-DN or caspase-8 inhibitor CrmA in immature B cells lines.145 However, this process can be inhibited by Bcl-2 or Bcl-xL over-expression.131,140,146 Of all BH3-only proteins, Bim again appears as the best candidate to signal death downstream of BCR engagement. Indeed, mice lacking Bim were shown to develop autoimmunity and express autoantibodies.27 In the antiHEL Ig/HEL model of autoreactive B-cell deletion, loss of Bim protected B cells against BCR-induced ligation, demonstrating its role in the process.147

It is interesting to note that genetic background has a strong influence on the consequences of Bim deficiency. The original Bim-/- mice were generated by a targeted mutation in 129Sv ES cells and backcrossed onto C57BL/6 background. Mice of the few first generations developed fatal autoimmune disease at high frequency, whereas homozygous mice produced after >20 generations of backcross have less severe symptoms.27,64 Similarly, BCR-induced apoptosis was shown to be deficient in mice of the MRL background.145 It would be interesting to identify the genes responsible for these differences, as they may be modulators of the Bcl-2–regulated pathway.

2.3.3. Survival and death of activated B cells

Mature B cells critically depend on the BCR signal as well as on an auxiliary signal from the TNF family member BAFF for their survival.134 Both signals are not independent, because BCR ligation upregulates BAFF receptor (BAFF-R) expression on B cells.148 Although BAFF has three receptors (BAFF-R, TACI, and BCMA), only BAFF-R seems to be the key receptor that signals BAFF-mediated survival, as mice deficient for this receptor display the same phenotype (absence of peripheral B cells) as mice deficient for BAFF.149,150 By contrast, TACI seems to be a negative regulator of B-cell survival, as B-cell numbers are increased in TACI-deficient mice, and these animals eventually develop autoimmune disease.151 Survival

signaling through BAFF and BAFF-R involves the activation of the NF-κB pathway, but the details of the signaling cascade have not been elucidated yet. In any case, BAFF has become a major focus of research since the confirmation of its involvement in rheumatoid arthritis and systemic lupus erythematosus–like autoimmune diseases. Several clinical trials using BAFF-neutralizing agents are currently under way.134

The accumulation of antibody-forming cells (AFCs) and consequently abnormally high serum Ig levels in Bim-deficient mice demonstrated the role of Bim in the termination of B-cell immune responses.27

Because Bcl-2 transgenic mice present an increased number of memory T cells,152 it has been postulated that Bim could be involved in the shaping of the memory compartment. A recent study showed that Bim-deficient mice accumulated large numbers of low-affinity Igexpressing memory B cells, in addition to elevated numbers of AFCs.153 Therefore, Bim does not interfere with the affinity maturation process, but seems to be critical for the removal of low-affinity antibody-bearing memory B cells. However, because Bcl-2 transgenic mice accumulated even more antigen-specific B cells than Bim−/− mice, we cannot exclude that another BH3 only protein, such as Puma, could play a part in memory B-cell homeostasis.

The genetic studies mentioned previously involving deletion or over-expression of members of death receptor or Bcl-2–regulated pathways have highlighted that deregulation of cell death mechanisms in T and B lymphocytes have dramatic consequences, ranging from degenerative disorders to autoimmune pathologies. Excessive cell death due to the absence of survival signals (e.g., IL-7) or the lack of a prosurvival molecule (i.e., Bcl-2 or Mcl-1) can lead to a very fragile, if not completely absent, immune system. By contrast, insufficient cell death due to the absence of a proapoptotic mediator (FasL, Fas, Bim, etc.) can lead to an accumulation of immune cells that should normally die and culminate in a fatal autoimmune disease, such as glomerulonephritis. In the Bcl-2 family, Bcl-2 and Mcl-1 seem to be the most important prosurvival regulators. If we consider the expression of Bcl-2 family members in T and B cells,154 it appears that the role of A1 might have been underestimated so far, probably because the genetic deletion of all three A1 genes at once presents a major challenge. Among the BH3-only proteins, Bim really stands out for its multiple roles in so many critical steps of T- and B-cell maturation. Bim plays a major role as a Bcl-2 inhibitor, as demonstrated by the fact that lack of Bim completely rescued the degenerative diseases caused by the lack of Bcl-2.30 Puma also plays