Bone marrow B lymphopoiesis is compromised, to variable extents, in aged mice of several inbred strains (1
). Multiple developmental stages show deficits, including pre-B cells, pro-B cells, and CLPs (1
). While the mechanisms responsible remain to be fully characterized, diminished responses of B cell precursors and progenitors to IL-7, increased apoptosis, and reduced capacity for V gene recombination have been described in aged mice (5
). We have shown that the transcriptional program underlying B lineage specification and commitment is altered in aged mice (10
). Aged B cell precursors are variably deficient in E2A expression; this results from increased turnover of E2A encoded proteins (13
). E2A, together with EBF, regulates transcription of the surrogate light chain genes λ5 and VpreB (15
). Given the reductions in E2A, it is not surprising that B cell precursors from aged mice are often deficient in surrogate light chains (3
). We hypothesize that reductions in surrogate light chain level may limit formation and function of the preBCR in nascent pre-B cells in aged mice (10
). This would be expected to affect the capacities for pre-B cell expansion and further maturation in senescence.
As shown by Kawano et. al., different μ heavy chain V regions show varying capacities for binding surrogate light chains and this correlates well with preBCR signaling and further differentiation (17
). Therefore, each pre-B cell, individually, undergoes further proliferation and differentiation dictated by its particular capacity for expressing the preBCR. In previous studies, lack of preBCR expression in surrogate light chain knockout mice does not preclude μ heavy chain rearrangements, but does dampen pre-B cell proliferation (18
). In λ5 gene knockout mice, where preBCR expression can not occur, the remaining pre-B cells have a unique phenotype and retain surface expression of the tyrosine kinase c-kit and fail to up-regulate CD25 and CD2 (19
). In studies by Kawano et. al., the extent of preBCR signaling coincided with down-regulation of c-kit in pre-B cells (17
). As shown by ten Boekel et. al. (22
pre-B cells, from either λ5 knockout or wild-type mice, differ from c-kit-
pre-B cells in their Vh repertoires, the former being enriched in rearranged 3' Vh gene families. Notably, the majority of μ chains cloned from c-kit+
pre-B cells failed to associate with surrogate light chains while μ chains derived from c-kit-
pre-B cells bound surrogate light chains and were competent to form the preBCR (22
). Therefore, the expression (or lack thereof) of c-kit serves as a phenotypic marker of two distinct pre-B subsets that likely differ in their capacities for preBCR expression and signaling, and consequently, expansion and differentiation to the B cell stage.
B cell development in the bone marrow is dominated by the B2 B cell lineage, the precursors of typical follicular B cells. In aged mice, we would anticipate that poor surrogate light chain expression, by compromising the preBCR checkpoint, would diminish the numbers of pre-B cells generated in the bone marrow, but would allow continued, albeit less effective, B cell differentiation along a more “preBCR deficient” path. Since preBCR mediated selection, which is dependent upon Vh sequence, would be minimized, newly generated B cells in aged mice would be expected to have altered specificity and possibly function.
Consistent with this hypothesis, we have previously reported that aged mice show an increased proportion of immature B cells within the bone marrow that appear to have undergone activation as evidenced by altered surface phenotype (e.g., expression of CD43, CD5, CD11b, PD-1 antigens), dependence on the BCR pathway kinase Btk, increased size, and an altered Vh repertoire as evidenced by increased usage of the VhS107 family (23
). In this report, we show that B cells with a comparable phenotype are preferentially derived from preBCR deficient c-kit+
pre-B cells within the B2 lineage. Consequently, we hypothesize that the quality and quantity of new B cells derived from senescent bone marrow is impacted by reduced preBCR dependent B2 B lymphopoiesis, subsequent to low surrogate light chain expression.