Pike Kelly A, Iacampo Sandra, Friedmann Jennifer E, Ratcliffe Michael J H
Department of Immunology, University of Toronto, Toronto, Ontario, Canada.
J Immunol. 2004 Feb 15;172(4):2210-8. doi: 10.4049/jimmunol.172.4.2210.
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 Igalpha and Igbeta are expressed by fusion with the extracellular and transmembrane domains of murine CD8alpha and CD8beta, 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 Igalpha, in the absence of the cytoplasmic domain of Igbeta, 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 Igbeta in the absence of the cytoplasmic domain of Igalpha failed to support B cell development. The ability of the cytoplasmic domain of Igalpha to support early B cell development required a functional Igalpha 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 Igalpha to the B cell surface, thereby initiating subsequent stages of development.
B细胞受体复合物（BcR）对于正常B淋巴细胞功能至关重要，而表面BcR表达是B细胞发育中的关键检查点。然而，BcR各链在发育过程中的功能需求仍存在争议。我们利用逆转录病毒基因转移在胚胎发育期间将BcR的组分引入鸡B细胞前体中。一种嵌合异二聚体，其中鸡Igalpha和Igbeta的胞质结构域分别通过与鼠CD8alpha和CD8beta的胞外和跨膜结构域融合来表达，在没有胞外BcR结构域的情况下将BcR的胞质结构域靶向到细胞表面。这种嵌合异二聚体的表达支持胚胎B细胞发育的所有早期阶段：法氏囊定植、克隆扩增以及通过基因转换诱导受体库多样化。在没有Igbeta胞质结构域的情况下，Igalpha胞质结构域的表达不仅是必要的，而且足以像内源性BcR一样有效地支持B细胞发育。相反，在没有Igalpha胞质结构域的情况下Igbeta胞质结构域的表达未能支持B细胞发育。Igalpha胞质结构域支持早期B细胞发育的能力需要一个功能性的基于免疫受体酪氨酸的Igalpha激活基序。这些结果支持一种模型，即发育中的B细胞前体中发生有 productive V(D)J重组后表面IgM的表达有助于将Igalpha的胞质结构域陪伴到B细胞表面，从而启动后续发育阶段。
