Analysis of selection mechanisms of germinal center B cells by using a novel follicular dendritic cell line

在生发中心（GCs）中，B细胞通过与携带免疫复合物的滤泡树突状细胞和分泌Tfh细胞因子（包括IL-21）的滤泡辅助性T （Tfh）细胞相互作用而被选择。为了分析这些细胞相互作用，我们利用小鼠滤泡树突细胞系FL-YB，探索了可以模拟GC - B细胞体外选择的培养条件。FL-YB细胞以baff依赖的方式有效提高共培养小鼠B细胞的活力。有趣的是，我们发现添加IL-21（一种主要的Tfh细胞因子）很容易诱导与FL-YB细胞共培养的B细胞死亡，而单独IL-21在没有FL-YB细胞的情况下维持B细胞的活力。il -21诱导的死亡依赖于FL-YB细胞释放的低分子量可溶性因子，最终确定为PGE2。用IL-21和PGE2处理B细胞，而不是单独处理，导致促凋亡蛋白Bim和上游转录因子Foxo1的表达增强。pge2受体亚型EP4与IL-21/ pge2诱导的B细胞死亡有关。因此，pge2是一种内源性化学介质，可以改变IL-21对B细胞的多效性作用。通过CD40共刺激B细胞，IL-21/ pge2诱导的B细胞死亡得以挽救。在免疫小鼠中，B细胞中IL-21R的缺乏导致活化的caspase-3阳性GC B细胞频率显著降低，同时伴有抗体亲和力成熟受损。综上所述，结果提示IL-21/ pge2诱导的B细胞死亡在GC B细胞选择中的生理作用。

In germinal centers (GCs), B cells are selected through interaction with follicular dendritic cells bearing immune complexes and follicular helper T (Tfh) cells secreting Tfh cytokines, including IL-21. To analyze these cellular interactions, we have explored culture conditions that can simulate GC B cell selection in vitro using a mouse follicular dendriticcell line, FL-YB. FL-YB cells efficiently enhanced viability of cocultured mouse B cells in a BAFF-dependent fashion. Interestingly, we found that addition of IL-21, a major Tfh cytokine, readily induced death of B cells that were cocultured with FL-YB cells, whereas IL-21 alone sustained viability of B cells in the absence of FL-YB cells. The IL-21-induced death was dependent ona low m.w. soluble factor that was released from FL-YB cells, which was finally identified as PGE2. Treatment of B cells with IL-21 plus PGE2, but not either alone, resulted in enhanced expression of a proapoptotic protein Bim and the upstream transcription factor Foxo1. A PGE2receptor isoform, EP4, was responsible for IL-21/PGE2-induced B cell death. Thus, PGE2is an endogenous chemical mediator that can switch pleiotropic actions of IL-21 on B cells. IL-21/PGE2-induced B cell death was rescued if B cells were costimulated via CD40. In immunized mice, deficiency of IL-21R in B cells led to a significant decrease in the frequency of activated caspase-3-positive GC B cells concomitant with impaired affinity maturation of Abs. Taken together, results implicate a physiological role of IL-21/PGE2-induced B cell death in GC B cell selection.