Induction of the antigen receptor expression on B lymphocytes results in rapid competence for signaling of SLP-65 and Syk

The binding of antigen to the B cell antigen receptor (BCR) results in the activation of protein tyrosine kinases (PTKs) such as Lyn and Syk, and the phosphorylation of several substrate proteins including HS1 and SLP-65, How these signaling elements are connected to the BCR is not well understood. Using an expression vector for a tamoxifen-regulated Cre recombinase, we ha ve developed a method that allows the inducible expression of the BCR, Disr uption of the VH leader reading frame of the immunoglobulin heavy chain by two loxP sites is overcome by Cre-mediated DNA recombination and results in the cell surface expression of the BCR starting 4 h after exposure of tran sfected B cells to tamoxifen, This method can, in principle, be employed fo r the inducible expression of any secreted or type I transmembrane protein. By monitoring the activation of signaling elements in pervanadate-stimulat ed B cells expressing different levels of the BCR, we show here that phosph orylation of SLP-65 and Syk, but not of Lyn, is strictly dependent on the e xpression of the BCR on the cell surface. These data suggest that the BCR r eorganizes its signaling molecules as soon as it appears on the cell surfac e.