Btk dosage determines sensitivity to B cell antigen receptor cross-linking

Mutations in Btk result in the B cell immunodeficiencies X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. Btk is a critical component of signaling pathways regulating B cell development and function. We used a genetic approach to determine whether Btk is also limiting for these processes. One allele of a murine Btk transgene expressed a dosage of Btk (25% of endogenous levels in splenic B cells) sufficient to restore normal numbers of phenotypically mature conventional B cells in xid mice. 2,4,6-trinitrophenyl–Ficoll response, anti-IgM-induced proliferation, B1 cell development, and serum IgM and IgG(3) levels remained significantly impaired in these animals. B cells from Btk −/− transgenic mice also responded poorly to anti-IgM, indicating that the xid mutation does not create a dominant negative form of Btk. Response to 2,4,6-trinitrophenyl–Ficoll and B cell receptor cross-linking were increased 3- to 4-fold in xid mice homozygous for the transgene. These results demonstrate that Btk is a limiting component of B cell antigen receptor signaling pathways and suggest that B cell development and response to antigen may require different levels of Btk activity.

布鲁顿酪氨酸激酶（Bruton's tyrosine kinase，Btk）的突变可导致人类罹患X连锁无丙种球蛋白血症（X-linked agammaglobulinemia，XLA），以及小鼠出现X连锁免疫缺陷症（X-linked immunodeficiency，xid），二者均属于B细胞免疫缺陷性疾病。Btk是调控B细胞发育与功能的信号通路关键组分。本研究采用遗传学方法，旨在明确Btk是否同样对上述过程起到限制性作用。本研究中，一个小鼠Btk转基因等位基因可表达特定水平的Btk，其表达量为脾脏B细胞内源性水平的25%，该水平足以恢复xid小鼠表型成熟的常规B细胞的正常数量。但此类小鼠的2,4,6-三硝基苯基-聚蔗糖（2,4,6-trinitrophenyl–Ficoll）应答、抗IgM诱导的细胞增殖、B1细胞发育，以及血清IgM与IgG3水平仍显著受损。从Btk基因敲除转基因小鼠中分离的B细胞，对抗IgM的应答同样较弱，这表明xid突变并未使Btk产生显性负突变形式。在携带该转基因的纯合xid小鼠中，对2,4,6-三硝基苯基-聚蔗糖的应答以及B细胞受体交联的响应提升了3至4倍。上述实验结果证实，Btk是B细胞抗原受体信号通路的限制性组分，同时提示B细胞发育与抗原应答可能需要不同水平的Btk活性。