The Cytoplasmic Domain of Igα Is Necessary and Sufficient to Support Efficient Early B Cell Development

The B cell receptor complex (BcR) is essential for normal B lymphocyte function, and surface BcR expression is a crucial checkpoint in B cell development. However, functional requirements for chains of the BcR during development remain controversial. We have used retroviral gene transfer to introduce components of the BcR into chicken B cell precursors during embryonic development. A chimeric heterodimer, in which the cytoplasmic domains of chicken Igα and Igβ are expressed by fusion with the extracellular and transmembrane domains of murine CD8α and CD8β, respectively, targeted the cytoplasmic domains of the BcR to the cell surface in the absence of extracellular BcR domains. Expression of this chimeric heterodimer supported all early stages of embryo B cell development: bursal colonization, clonal expansion, and induction of repertoire diversification by gene conversion. Expression of the cytoplasmic domain of Igα, in the absence of the cytoplasmic domain of Igβ, was not only necessary, but sufficient to support B cell development as efficiently as the endogenous BcR. In contrast, expression of the cytoplasmic domain of Igβ in the absence of the cytoplasmic domain of Igα failed to support B cell development. The ability of the cytoplasmic domain of Igα to support early B cell development required a functional Igα immunoreceptor tyrosine-based activation motif. These results support a model in which expression of surface IgM following productive V(D)J recombination in developing B cell precursors serves to chaperone the cytoplasmic domain of Igα to the B cell surface, thereby initiating subsequent stages of development.