The avian B‐cell receptor complex: distinct roles of Igα and Igβ in B‐cell development

Summary: The bursa of Fabricius has evolved in birds as a gut‐associated site of B‐cell lymphopoiesis that is segregated from the development of other hematopoietic lineages. Despite differences in the developmental progression of chicken as compared to murine B‐cell lymphopoiesis, cell‐surface immunoglobulin (sIg) expression has been conserved in birds as an essential checkpoint in B‐cell development. B‐cell precursors that express an sIg complex that includes the evolutionarily conserved Igα/β heterodimer colonize lymphoid follicles in the bursa, whereas B‐cell precursors that fail to express sIg due to non‐productive V(D)J recombination are eliminated. Productive retroviral gene transfer has allowed us to introduce chimeric receptor constructs into developing B‐cell precursors in vivo . Chimeric proteins comprising the extracellular and transmembrane regions of murine CD8α fused to the cytoplasmic domain of chicken Igα efficiently supported B‐cell development in precursors that lacked endogenous sIg expression. By contrast, expression of an equivalent chimeric receptor containing the cytoplasmic domain of Igβ actively inhibited B‐cell development. Consequently, the cytoplasmic domains of Igα and Igβ play functionally distinct roles in chicken B‐cell development.