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Dual Requirement for the Ig Immunoreceptor Tyrosine-Based Activation Motif (ITAM) and a Conserved Non-Ig ITAM Tyrosine in Supporting Ig-Mediated B Cell Development¹

Kelly A. Pike^*, and Michael J. H. Ratcliffe^2,*,

^* Department of Immunology, University of Toronto, and Sunnybrook and Women’s College Health Sciences Center, Toronto, Ontario, Canada

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 CD8 or CD8 are fused to the cytoplasmic domains of chicken Ig (chIg) or Ig, respectively (murine CD8 (mCD8):chIg + mCD8:chIg), or an mCD8:chIg homodimer supported bursal B cell development as efficiently as endogenous sIg. In this study we demonstrate that B cell development, in the absence of chIg, requires both the Ig ITAM and a conserved non-ITAM Ig tyrosine (Y3) that has been associated with binding to B cell linker protein (BLNK). When associated with the cytoplasmic domain of Ig, the Ig 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 Ig Y3 severely compromised calcium mobilization when expressed as either a homodimer or a heterodimer with the cytoplasmic domain of Ig. However, coexpression of the cytoplasmic domain of Ig partially complemented the Ig Y3 mutation, rescuing higher levels of BLNK phosphorylation and, more strikingly, supporting B cell development.

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