The tyrosine kinase Syk is required for light chain isotype exclusion but dispensable for the negative selection of B cells.

In this study we set out to test whether Syk was required for negative selection of immature B cells. B cells expressing a B cell antigen receptor (BCR) transgene (3-83, anti-H-2K(k)) underwent negative selection independently of Syk in both fetal liver organ culture and radiation chimera models. Furthermore, Syk-independent negative selection was not reversed by transgenic overexpression of Bcl-2. Receptor editing was not apparent in Syk-deficient B cells, presumably as a consequence of the failure of mature edited B cells to develop in the absence of Syk. Interestingly, light chain isotype exclusion by the BCR transgene failed in the absence of Syk. We observed a dramatic reduction in the overall BCR-mediated tyrosine phosphorylation of cellular proteins in Syk-deficient immature B cells. However, the tyrosine phosphorylation of a number of substrates including phospholipase C gamma 2, although reduced, was not completely abrogated. BCR ligation triggered an increase in calcium flux in the absence of Syk. Thus signaling events that mediate negative selection can still occur in the absence of Syk. This may be due to redundancy with zeta-associated protein 70 (ZAP-70), which we demonstrate to be expressed in immature B cells.