A detective will sometimes follow a hunch and investigate a suspect even if they have a cast-iron alibi. In a similar way, although early studies showed that ζ-chain-associated protein 70 (ZAP70) is expressed by T cells and natural killer cells, but not by B-cell lines, Schweighoffer and colleagues decided to test formally the role of this Syk tyrosine kinase in B-cell development. Their study, published in Immunity, shows surprisingly that not only is Zap70 expressed throughout B-cell development, but also it has a role in the pro-B- to pre-B-cell transition.

In the bone marrow, B cells develop through a series of developmental checkpoints: pro-B cells differentiate into pre-B cells after expression of and signalling through the pre-B-cell receptor (pre-BCR). Knockout studies have shown a role for Syk at this checkpoint, as B-cell development is partially blocked at the pro-B- to pre-B-cell transition in Syk−/− mice. Could the partial nature of this block result from functional redundancy between Syk and Zap70, the only other known Syk-family kinase?

To investigate this, Schweighoffer et al. generated chimaeras that were reconstituted with Zap70−/−Syk−/− fetal liver cells. In these mice, pro-B cells were present at higher numbers than in chimaeras that were reconstituted with wild-type or Zap70−/− cells, and no pre-B cells could be detected. Therefore, in the absence of Syk and Zap70, B-cell development is completely blocked at the pre-BCR checkpoint.

Although expression of Zap70 by B cells had not been detected previously, deficiency of Zap70 exacerbated the B-cell developmental blockade that was seen in Syk−/− mice. So, the authors re-examined the presence of Zap70 at different stages of B-cell development, and found it to be expressed by pro-B, pre-B and splenic B cells. By radiation chimaera experiments, they also showed that the requirement for Zap70 in B-cell development is intrinsic to cells of the B-cell lineage.

Further experiments showed that the arrested B-cell development that was seen in the Zap70−/−Syk−/− mice was due to defective pre-BCR signalling in the pro-B cells, and not due to defective synthesis or assembly of the pre-BCR. Heavy-chain allelic exclusion — a process known to require a pre-BCR signal — also failed in these cells, the first time that this has been seen in a signalling mutant.

This work shows that in B cells, functional redundancy does exist between Zap70 and Syk, as has been shown previously in T cells, and it establishes an important role for Zap70 in early B-cell development.