Molecular analysis of inhibitory signals through FcgammaRIIB

B细胞对抗原的反应能力依赖于通过B细胞抗原受体(BCR)复合体传递的信号。胞质蛋白酪氨酸激酶(PTKs)的激活是对BCR交联的最早可测量的生化反应。这一初始事件导致二次信号的产生，包括RAS激活、磷脂酰肌醇3-激酶(P1-3K)激活、磷脂酰肌醇的周转。和钙动员，BCR信号的强度和持续时间在指导B细胞的生物学反应如增殖、分化方面都是重要的。和细胞凋亡。因此，这些激活信号的衰减和终止也是B细胞反应的关键成分。通过将FcGammaRIIB与BCR交联来抑制B细胞的激活，FcGammaRIIB的胞浆结构域包含一个免疫受体酪氨酸抑制基序(ITIM)，这是受体抑制功能所必需的。激活的PTK(S)使ITIM中的酪氨酸磷酸化是其抑制机制的关键。虽然磷酸化的FcGammaRIIB ITIM与含有SH2的蛋白酪氨酸磷酸酶SHP-1和含有SH2的肌醇多聚磷酸5‘-磷酸酶SHIP有关，但我们的数据表明FcGammaRIIB的抑制主要涉及SHIP。FcGammaRIIB的膜募集是FcGammaRIIB结合BCR产生抑制信号的原因。SHIP通过PH结构域-磷脂醇脂质相互作用调节BTK与膜的结合。BTK作为膜相关嵌合体的表达抑制了FcyRIIB连接对BCR信号的抑制。因此，我们的结果表明，FcGammaRIIB招募的SHIP降低了PIP3的水平，导致BTK下调，从而导致钙流动。

The ability of B cells to respond to antigen relies on signals transmitted through the B cell antigen receptor (BCR) complex. Activation of cytoplamic protein tyrosine kinases (PTKs) is the earliest measurable biochemical response to BCR cross-linking. This initial event leads to the generation of secondary signals including Ras activation, phosphatidylinositol 3-kinase (P1-3K) activation, turnover of phosphoinositides. and calcium mobilization, Both the strength and duration of the BCR-elicited signal are important in directing biological responses of B cells such as proliferation, differentiation. and apoptosis. Thus, attenuation and termination of these activation signals are also critical components for B cell response.B cell activation is inhibited by crosslinking FcgammaRIIB with the BCR, The cytoplasmic domain of FcgammaRIIB contains an immunoreceptor tyrosine-based inhibitory motif (ITIM), which is necessary for the inhibitory function of the receptor. Phosphorylation of the tyrosine in the ITIM by the activated PTK(s) is critical to its inhibitory mechanism. Although the phosphorylated FcgammaRIIB ITIM associates with the SH2-containing protein tyrosine phosphatase SHP-1 and the SH2-containing inositol polyphosphate 5'-phosphatase SHIP, our data have shown that inhibition by FcgammaRIIB involves primarily SHIP.Membrane recruitment of SHIP was responsible for the inhibitory signal generated by FcgammaRIIB coligation to the BCR.By reducing the level of PIP3. SHIP regulated the association of the Btk with the mebrane through PH domain-phospholinositol lipid interactions. Inhibition of BCR signaling by FcyRIIB coligation was suppressed by the expression of Btk as a membrane-associated chimera. Thus, our results suggest that recruited SHIP by FcgammaRIIB reduces the level of PIP3, leading to Btk downregulation and thereby calcium fluxes.