The B cell antigen receptor (BCR) signaling cascade is initiated by phosphorylation of tyrosine residues in the Immunoreceptor Tyrosine based Activation Motif (ITAM) of Ig-α and Ig-β. Signals initiated by the pre-BCR in pre-B cells and the BCR in immature and mature cells can lead to cell maturation, selection, terminal differentiation or cell death. It is still unclear how a single receptor can mediate such a variety of cellular responses. Our aim is to develop a system for the structural and functional analysis of Ig-α in vivo and to understand its role in B cell development, selection and response. We use retroviral gene transfer to introduce different mb-1 (Ig-α encoding gene) mutants into hematopoietic stem cells of Ig-α-deficient mice. Transduced cells are reinjected into irradiated recipient mice where they can differentiate into B cells. Here we have focused on the serine and threonine residues of the cytoplasmic region of Ig-α. These residues are phosphorylated upon BCR engagement and studies in cell lines indicated their potential negative regulatory role in BCR signaling. We found that Ig-α deficient hematopoietic stem cells transduced with retroviral vectors containing either the wild-type or a serine/threonine mutated mb-1 sequence were able to differentiate into IgM expressing cells in vivo. Thus, our data indicate that the serine and threonine residues do not play a dominant role in pre-BCR signaling as pre-B cells and immature B cells expressing serine and treonine mutant Ig-α develop. We are in the process of testing the effect of these mutations in B cell negative selection and B cell response
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