Daëron M
Laboratoire d'Immunologie Cellulaire et Clinique, INSERM U.255, Institut Curie, Paris, France.
Immunol Res. 1992;11(3-4):191-202. doi: 10.1007/BF02919126.
Murine low-affinity receptors for the Fc portion of IgG are of two types: Fc gamma RII and Fc gamma RIII. Murine Fc gamma RII and III have 95% homologous extracellular (EC) domains and bind the same ligands, but different transmembrane (TM) and intracytoplasmic (IC) domains. They, however, have unrelated TM and IC domains. Murine Fc gamma RII are single-chain receptors, encoded by the beta-Fc gamma R gene. Murine Fc gamma RIII are composed of two subunits: the ligand-binding alpha-subunit, encoded by the alpha-Fc gamma R gene and the gamma-subunit, encoded by another gene which belongs to a family of genes encoding dimeric subunits of multichain receptors. The expression of murine Fc gamma RII and Fc gamma RIII depends on a number of mechanisms which do the following: (1) determine the tissue-specific expression of the alpha- and beta-Fc gamma R genes by selectively unmethylating DNA in specific 5' sequences in different cell types; (2) regulate the initiation of the transcription of the alpha- and beta-Fc gamma R genes via several transcription factors; (3) up- and downregulate the amount of alpha- and beta-Fc gamma R transcripts in response to cytokines; (4) decide the alternative splicing of IC exons of the beta-Fc gamma R gene and generate the different Fc gamma RII isoforms; (5) possibly regulate the translation of alpha- and beta-Fc gamma R transcripts in different cells; (6) control the assembly of the Fc gamma RIII subunits and their membrane insertion, and (7) determine the turnover of Fc gamma RII and III in the presence and absence of ligands by affecting the internalization, shedding and proteolytic cleavage of the receptors. These mechanisms altogether contribute to make a variety of cells capable of responding differently to antigen-antibody complexes, depending on environmental stimuli.
小鼠IgG Fc段低亲和力受体有两种类型：FcγRII和FcγRIII。小鼠FcγRII和III的胞外（EC）结构域有95%的同源性，且结合相同的配体，但跨膜（TM）和胞质（IC）结构域不同。然而，它们的TM和IC结构域没有关联。小鼠FcγRII是单链受体，由β-FcγR基因编码。小鼠FcγRIII由两个亚基组成：配体结合α亚基，由α-FcγR基因编码；γ亚基，由另一个基因编码，该基因属于编码多链受体二聚体亚基的基因家族。小鼠FcγRII和FcγRIII的表达取决于多种机制，这些机制如下：（1）通过在不同细胞类型的特定5'序列中选择性地使DNA去甲基化，来决定α-和β-FcγR基因的组织特异性表达；（2）通过几种转录因子调节α-和β-FcγR基因转录的起始；（3）响应细胞因子上调和下调α-和β-FcγR转录本的量；（4）决定β-FcγR基因IC外显子的可变剪接，并产生不同的FcγRII异构体；（5）可能调节不同细胞中α-和β-FcγR转录本的翻译；（6）控制FcγRIII亚基的组装及其膜插入，以及（7）通过影响受体的内化、脱落和蛋白水解切割，在有配体和无配体的情况下决定FcγRII和III的周转。这些机制共同作用，使多种细胞能够根据环境刺激，对抗抗原-抗体复合物做出不同反应。
