The mechanisms responsible for the differentiation of immature B cells emerging from the BM into mature B cells found in the periphery are poorly understood. It is, for instance, still unclear why only 10% of newly formed IgM+
B cells reach the spleen, and why only a very small proportion of these achieve maturation 32
. Many reports indicate that selection of specific transitional B cells takes place during B cell maturation in the periphery, and signaling through the BCR is a critical event in this process 111233
. Paradoxically, it is also clear that most immature transitional B cells die after stimulation of their BCR, a mechanism thought to eliminate autoreactive B cells 5689
. The exact nature of the positive signal driving maturation of B cells, although unknown, is either highly specific and/or requires concomitant signals apart from those through the BCR to allow transitional B cells to survive and differentiate into mature B cells.
We tested in vitro whether BAFF could be this extra factor needed to push immature B cells towards survival and differentiation. In our in vitro assays, we have been able to demonstrate that BAFF promotes the survival of immature T2 B cells and the differentiation of some of them into mature B cells when anti-μ antibodies were added to trigger signals through the BCR. Therefore, in vitro, BAFF synergizes with signals through the BCR to promote maturation of T2 B cells and as such is a critical new element for our understanding of this process.
The general expansion of the B cell compartment in the spleen of BAFF Tg mice, which is characterized by increased B cell numbers in all subsets, suggested that BAFF might act on all peripheral B cells. The discrepancy of this result with our in vitro results may be explained by environmental/tissue-specific signals acting in association with BAFF in vivo which the in vitro system could not mimic. However, the vastly unequal expansion of these subsets, favoring strongly the T2 and MZ B cell compartments, indicated that overexpression of BAFF directly or indirectly modified the dynamics of the entire B cell maturation and differentiation pathway in BAFF Tg mice. These results tend to indicate that BAFF might be a general survival factor for all B cells in vivo, with the specific potential to alter the process of B cell maturation.
Experiments in which BCMA-Ig was used to block BAFF in normal mice led to a marked and general reduction in B cell numbers in the periphery 22
. Similarly, previous studies have shown that shutting off the expression of the BCR on the surface of B cells led rapidly to a reduced number of mature B cells in the periphery 34
. The exact nature of the BCR-mediated survival signal in mature B cells is unknown. It is therefore possible that, in vivo,
constant signals from BAFF and through the BCR are needed to keep the mature B cells alive. The dynamics of B cell homeostasis are very complex, and other studies have suggested that immature B cells may also compete with recirculating B cells for survival signals 35
. We believe that normal recirculating B cells are probably not self-renewing and can only be replaced through the differentiation of immature B cells recruited in the periphery. Therefore, inhibition of BAFF, like the suppression of BCR signaling, could also affect B cell maturation and prevent the renewal of the mature B cell pool. 11-d treatment with BCMA-Ig markedly (although still incompletely) reduced B cell numbers in the periphery 22
. A longer treatment with BCMA-Ig, reflecting the life span of recirculating B cells, might allow most mature B cells to naturally disappear from the periphery without being replaced. Supporting this model, BAFF does not promote survival of naive mature B cells, in vitro, despite expression of both BAFF receptors on these cells 182223
. BAFF might only be an important cofactor for proliferation once mature B cells have been activated 1415161718
. Nevertheless, we cannot formally exclude the possibility that, in vivo,
BAFF directly supports survival of mature B cells. Adoptive transfer of B cell subsets in mice deficient for BAFF is the next strategy to obtain a final confirmation on whether BAFF is a general survival factor for all B cells or a specific survival and maturation factor for peripheral immature B cells. IL-4 and CD40 ligand are also factors involved in B cell survival; however, gene targeting experiments have shown that both factors are not required for B cell maturation 3637
. TACI and BCMA are also the receptors for another member of the TNF family, a proliferation-inducing ligand (APRIL), and it is at this point unclear whether or not this factor may also play a role in B cell survival 38
Other intriguing features of BAFF Tg mice are the dramatic expansion of the MZ B cell compartment, its possible relation with the parallel expansion of the T2 B cells compartment, and the autoimmune phenotype displayed in these mice. The MZ B cell population is an obscure subset, which contains memory as well as virgin B cells 3940
. Previous studies have shown that both CBA/N and CD45−/
− mice have their B cell development blocked at the T2 B cell stage, yet these mice have a perfectly normal population of MZ B cells 114142
. This observation led to the hypothesis that some T2 B cells, under specific conditions, may directly differentiate into MZ B cells without going through a mature B cell stage. It is therefore conceivable that in BAFF Tg mice, B cell maturation is aberrantly skewed toward the MZ compartment. Recently, studies using various Ig heavy chain Tg mice have shown that, depending on their BCR composition, some newly formed mature B cells could be positively selected into the MZ compartment 40
. Moreover, some B cells selected to migrate to the MZ are suspected to be potentially autoreactive 43
. In addition, recent work showed that CD1hi
B cells in the spleen of NZB × NZW mice, a model of SLE, are the major source of autoreactive B cells 31
. We showed that in normal mice, T2 and MZ B cells express high levels of CD1 compared with mature and T1 B cells, confirming previous reports 44
. We also showed that the population of CD1hi
B cells is dramatically enlarged in BAFF Tg mice. Our findings led us to a model described in . BAFF promotes the survival of T2 B cells, which in normal conditions will lead to the maturation of highly selected cells into the mature follicular B cell compartment. In contrast, in BAFF Tg mice, overexpression of BAFF may trigger excessive survival signals in T2 B cells, including autoreactive cells, which may fail to respond to censoring death signals. The autoreactive nature of their receptors might also force their massive assignment to the MZ compartment and subsequent expansion of this compartment, as observed in BAFF Tg mice. These B cells may become activated by autoantigens at this site, contribute to the formation of numerous germinal centers observed in these mice 26
, and differentiate into memory B cells, some of which may also localize in the MZ (). BAFF may also be important for the survival of germinal center B cells 28
, and we do not exclude the possibility that autoreactive B cells may also emerge at this stage, due to aberrant survival of self-reactive B cells created after errors during affinity maturation.
Figure 7 Proposed mechanism for one origin of autoimmune B cells in the spleen of BAFF Tg mice. BAFF induces excessive survival of all B cells in the spleen with a preferential role on immature T2 B cells. Autoreactive T2 B cells fail to respond to censoring death (more ...)
Recent work described the selective absence of MZ B cells in PyK-2–deficient mice and a possible defect in MZ-specific chemotaxis 45
. It is also possible that BAFF stimulates the production of an MZ-specific chemokine or a chemokine receptor on B cells driving most of these cells into the MZ of BAFF Tg mice. It is unclear whether the presence of T2 B cells in LNs of BAFF Tg mice also reflects an aberrant homing process or possible BAFF-enhanced B cell maturation in LNs. L-selectin expression was normal on freshly isolated splenocytes from BAFF Tg mice and cannot be evoked to explain this result. Previous studies indicated that B cell maturation takes place in the spleen 611
, yet immature B cells have also been detected in LNs 46
. B cell maturation outside of the spleen has not been investigated, but the relatively well-sustained numbers of B lymphocytes in splenectomized individuals raises the question of potential alternative sites for B cell maturation.
BAFF is a pivotal survival factor during B cell maturation, a process which is as yet poorly understood. Our results suggest that excessive BAFF-induced survival might induce the breakdown of immune tolerance at a critical stage of peripheral B cell maturation. Therefore, the highly selective recruitment of immature B cells into the mature B cell pool might be the result of active mechanisms of self-tolerance, possibly controlled by BAFF.