Wang Jingwei, Li Tianbao, Zan Hong, Rivera Carlos E, Yan Hui, Xu Zhenming
Department of Microbiology, Immunology and Molecular Genetics, Joe R. and Teresa Lozano Long School of Medicine, University of Texas Health Science Center at San Antonio, San Antonio, TX, United States.
Division of Neonatology, Department of Pediatrics, The Second Xiangya Hospital, Central South University, Changsha, China.
Front Immunol. 2021 Apr 19;12:658048. doi: 10.3389/fimmu.2021.658048. eCollection 2021.
B cell activation by Tfh cells, i.e., through CD154 engagement of CD40 and IL-21, and survival within GCs are crucial for the T-dependent Ab response. LUBAC, composed of HOIP, SHARPIN, and HOIL-1, catalyzes linear ubiquitination (Linear M1-Ub) to mediate NF-κB activation and cell survival induced by TNF receptor superfamily members, which include CD40. As shown in this study, B cells expressing the null mutation ( ) could undergo proliferation, CSR, and SHM in response to immunization by a T-dependent Ag, but were defective in survival within GCs, enrichment of a mutation enhancing the BCR affinity, and production of specific Abs. B cells stimulated with CD154 displayed normal proliferation and differentiation, marginally impaired NF-κB activation and survival, but markedly exacerbated death triggered by IL-21. While activating the mitochondria-dependent apoptosis pathway in both and B cells, IL-21 induced B cells to undergo sustained activation of caspase 9 and caspase 8 of the mitochondria-dependent and independent pathway, respectively, and ultimately caspase 3 in effecting apoptosis. These were associated with loss of the caspase 8 inhibitor cFLIP and reduction in cFLIP Linear M1-Ub, which interferes with cFLIP poly-ubiquitination at Lys48 and degradation. Finally, the viability of B cells was rescued by caspase inhibitors but virtually abrogated - together with Linear M1-Ub and cFLIP levels - by a small molecule HOIP inhibitor. Thus, LUBAC controls the cFLIP expression and inhibits the effects of caspase 8 and IL-21-activated caspase 9, thereby suppressing apoptosis of CD40 and IL-21-activated B cells and promoting GC B cell survival.
滤泡辅助性T细胞（Tfh）通过CD154与CD40结合以及IL-21激活B细胞，且B细胞在生发中心（GC）内的存活对于T细胞依赖性抗体反应至关重要。线性泛素链组装复合体（LUBAC）由HOIP、SHARPIN和HOIL-1组成，催化线性泛素化（线性M1-Ub）以介导由肿瘤坏死因子受体超家族成员（包括CD40）诱导的核因子κB（NF-κB）激活和细胞存活。如本研究所示，表达无效突变（ ）的B细胞在受到T细胞依赖性抗原免疫时可发生增殖、类别转换重排（CSR）和体细胞高频突变（SHM），但在GC内的存活、富集增强B细胞受体（BCR）亲和力的突变以及产生特异性抗体方面存在缺陷。用CD154刺激的B细胞显示出正常的增殖和分化，NF-κB激活和存活略有受损，但由IL-21触发的死亡明显加剧。IL-21在野生型和突变型B细胞中激活线粒体依赖性凋亡途径时，分别诱导突变型B细胞使线粒体依赖性途径的半胱天冬酶9和线粒体非依赖性途径的半胱天冬酶8持续激活，最终激活半胱天冬酶3以实现凋亡。这些与半胱天冬酶8抑制剂cFLIP的缺失以及cFLIP线性M1-Ub的减少有关，cFLIP线性M1-Ub会干扰cFLIP在赖氨酸48处的多聚泛素化和降解。最后，半胱天冬酶抑制剂可挽救突变型B细胞的活力，但小分子HOIP抑制剂几乎消除了其活力——同时还有线性M1-Ub和cFLIP水平。因此，LUBAC控制cFLIP表达并抑制半胱天冬酶8和IL-21激活的半胱天冬酶9的作用，从而抑制CD40和IL-21激活的B细胞凋亡并促进GC B细胞存活。
