During B cell development, proliferative expansion takes place after expression of the pre-BCR. At this pre-BII cell stage, the IL-7Ra is also expressed. Some in vitro studies suggest that pre-BCR-dependent expansion relies on the IL-7Ra, and others that it does not. It has also been suggested that the pre-BCR mediates down-regulation of the IL7Ra. However, the in vivo relationship between the pre-BCR and the IL-7Ra has not been previously examined. Here, we have investigated this by establishing mice lacking both receptors. Our results show that in the absence of the IL-7Ra, the pre-BII population is reduced, as previously seen in mice lacking the pre-BCR, demonstrating that the IL-7Ra is important at this stage. A deficiency in both receptors results in a further reduction of the pre-BII cell population. We conclude that both the IL-7Ra and the pre-BCR are required for optimal pre-BII cell expansion. Furthermore, IL-7Ra expression levels are normal in pre-BCR-deficient mice, suggesting that the pre-BCR does not mediate its down-regulation. As a consequence of the absence of both receptors, the peripheral B cell pool is severely depleted, resulting in atypical splenic B cell structures and reduced serum Ig levels.
IntroductionB lymphopoiesis takes place in the fetal liver and, postnatally, in the BM [1][2][3]. During early BM B cell development, critical checkpoints occur firstly after Ig heavy (H)-chain DJ H rearrangement and secondly after successful VDJ H rearrangement. Due to the inherent imprecision of the rearrangement process, each of these is followed by proliferative expansion at the pre-BI (DJ H ) and large pre-BII (VDJ H ) cell stages, which results in an increase in cell numbers and, consequently, the Ig repertoire. Functional VDJ H -rearrangement leads to expression of the lH chain, which, together with the surrogate light chain (SLC, comprised of VpreB and k5) [4, 5], forms the pre-B cell receptor (pre-BCR) [6][7][8]. After subsequent Ig light (L)-chain recombination, although not followed by an expansion phase, the end result is the production of large numbers of immature B cells, each carrying a unique BCR (IgM) on the cell surface. Immature B cells migrate to the spleen, where they mature and co-express IgD [9].Expansion at the pre-BI cell stage has been shown, both in vitro [10][11][12][13] and in vivo [14][15][16][17][18][19][20][21], to depend on the IL-7Ra [22]. Mice deficient in the IL-7Ra chain show a block in B cell development at the pre-BI stage. Although this block is complete in adult IL-7Ra -/-mice,
Leukocyte signalingThe last two authors contributed equally to this work. [20,21]. The IL-7Ra chain heterodimerizes with the common gamma (c C ) chain [22,23] to form the receptor for IL-7, while together with the thymic stromal lymphopoietin (TSLP) receptor chain [24,25], it forms the receptor for TSLP. Expansion at the pre-BII stage is dependent on the expression of the pre-BCR. While a lack of the transmembrane region of the lH chain (lMT) leads to a complete block at the pre-BI stag...