Dual Requirement for the Igα Immunoreceptor Tyrosine-Based Activation Motif (ITAM) and a Conserved Non-Igα ITAM Tyrosine in Supporting Igαβ-Mediated B Cell Development

Surface Ig (sIg) expression is a critical checkpoint during avian B cell development. Only cells that express sIg colonize bursal follicles, clonally expand, and undergo Ig diversification by gene conversion. Expression of a heterodimer, in which the extracellular and transmembrane domains of murine CD8α or CD8β are fused to the cytoplasmic domains of chicken Igα (chIgα) or Igβ, respectively (murine CD8α (mCD8α):chIgα + mCD8β:chIgβ), or an mCD8α:chIgα homodimer supported bursal B cell development as efficiently as endogenous sIg. In this study we demonstrate that B cell development, in the absence of chIgβ, requires both the Igα ITAM and a conserved non-ITAM Igα tyrosine (Y3) that has been associated with binding to B cell linker protein (BLNK). When associated with the cytoplasmic domain of Igβ, the Igα ITAM is not required for the induction of strong calcium mobilization or BLNK phosphorylation, but is still necessary to support B cell development. In contrast, mutation of the Igα Y3 severely compromised calcium mobilization when expressed as either a homodimer or a heterodimer with the cytoplasmic domain of Igβ. However, coexpression of the cytoplasmic domain of Igβ partially complemented the Igα Y3 mutation, rescuing higher levels of BLNK phosphorylation and, more strikingly, supporting B cell development.