Molecular mechanisms of proliferation and differentiation of germinal center B cells

生发中心(GC)在次级淋巴组织中发育，以响应胸腺依赖(TD)抗原(Ag)。为探讨X-连锁免疫缺陷(XID)小鼠GC-B细胞分化的分子机制，阐明X-连锁免疫缺陷(XID)小鼠GC-B细胞的功能，我们从TD-Ag免疫的小鼠脾中扩增GC-B细胞，检测其功能分子的表达，并测定其在体外对抗CD40单抗和细胞因子诱导分化为抗原特异性Iggi抗体形成细胞(AFCs)的能力。GC B细胞表达IL-4受体(R)，约5%的GC B细胞表达IL-5R。在抗CD40单抗作用下，IL-4可诱导野生型小鼠GC B细胞分化为抗原特异性IgGI AFC，并增强野生型小鼠GC B细胞上IL-5R的表达。IL-5进一步增强IGGI AFC反应。来自XID小鼠的GC B细胞对抗CD40单抗和IL-4产生免疫应答，而IL-5不能增强这种应答。提示抗CD40单抗、IL-4和IL-5在小鼠GC B细胞分化中起重要作用。XID小鼠GCB细胞在IL-5介导的分化过程中存在功能缺陷，我们检测了CD40和CD38对GC和卵泡套(FM)B细胞分化的影响。结果表明，IL-5Rpha阳性细胞主要分布于GC，同时表达CD40，而CD38在GC B细胞表达下调。IL-5加抗CD38单抗刺激GC B细胞不能诱导IgGI反应。IL-5Rα、CD38、CD40在西德小鼠体内的表达与野生型小鼠相似。此外，IL-5Rpha缺陷小鼠对TD Ag的反应正常，表现为GC的发育和Ag特异性IgGI的产生。这些结果表明，IL-5在小鼠GC-B细胞分化过程中起关键作用，它是一种共刺激因子，而不是必需因子。

Germinal center (GC) develops in secondary lymphoid tissues in response to thymus-dependent (TD) antigens (Ag). To investigate the molecular mechanism of mouse GC B cell differentiation and elucidate the function of GC B cells from X-linked immunodeficient(Xid)mice, we enriched GC B cells from spleen of TD Ag-immunized mice, monitored the expression of functional molecules, and determined the differentiation into Ag-specific IgGI antibody forming cells (AFCs) in response to anti-CD40 mAb and cytokines in vitro. GC B cells expressed IL-4 receptor (R), and approximately 5% of GC B cells expressed IL-5R.IL-4 in the presence of anti-CD40 mAb induced the differentiation of GC B cells from wild-type mice into Ag-specific IgGI AFCs, and enhanced the IL-5R expression on GC B cells from wild-type mice. IL-5 enhanced further the IgGI AFC response. The GC B cells from Xid mice responded to anti-CD40 mAb and IL-4 resulting in the IgGI response, whereas IL-5 did not enhance the response. These findings suggest that anti-CD40 mAb, IL-4, and IL-5 play a critical role in differentiation of mouse GC B cells. The GCB cells from Xid mice show functional defect with respect to IL-5-mediated differentiation.We then examined effect of CD40 and CD38 on differentiation of GC and follicular mantle(FM)B cells. The results showed that IL-5Ralpha positive cells were mainly distributed in GC which also expressed CD40, whereas CD38 expression was down-regulated on GC B cells. Stimulation of GC B cells with IL-5 plus anti-CD38 mAb did not induce the IgGI response. The expression of IL-5Ralpha, CD38, and CD40 in Xidmice was similar to that in wild-type mice. Furthermore, IL-5Ralpha deficient mice showed normal response to TD Ag, regarding GC development and Ag-specific IgGI production. These results suggest that IL-5 plays a critical role in differentiation of mouse GC B cells as a costimulatory factor rather than an essential factor.