Department of Immunology, Erasmus MC, Rotterdam, The Netherlands.
Department of Periodontology, ACTA, University of Amsterdam and VU University, Amsterdam, The Netherlands.
Immunol Cell Biol. 2017 Oct;95(9):744-752. doi: 10.1038/icb.2017.43. Epub 2017 May 26.
The mechanisms involved in sequential immunoglobulin G (IgG) class switching are still largely unknown. Sequential IG class switching is linked to higher levels of somatic hypermutation (SHM) in vivo, but it remains unclear if these are generated temporally during an immune response or upon activation in a secondary response. We here aimed to uncouple these processes and to distinguish memory B cells from primary and secondary immune responses. SHM levels and IgG subclasses were studied with 454 pyrosequencing on blood mononuclear cells from young children and adults as models for primary and secondary immunological memory. Additional sequencing and detailed immunophenotyping with IgG subclass-specific antibodies was performed on purified IgG memory B-cell subsets. In both children and adults, SHM levels were higher in transcripts involving more downstream-located IGHG genes (esp. IGHG2 and IGHG4). In adults, SHM levels were significantly higher than in children, and downstream IGHG genes were more frequently utilized. This was associated with increased frequencies of CD27IgG memory B cells, which contained higher levels of SHM, more IGHG2 usage, and higher expression levels of activation markers than CD27IgG memory B cells. We conclude that secondary immunological memory accumulates with age and these memory B cells express CD27, high levels of activation markers, and carry high SHM levels and frequent usage of IGHG2. These new insights contribute to our understanding of sequential IgG subclass switching and show a potential relevance of using serum IgG2 levels or numbers of IgG2-expressing B cells as markers for efficient generation of memory responses.
顺序免疫球蛋白 G（IgG）类别转换涉及的机制在很大程度上仍然未知。顺序 IG 类别转换与体内更高水平的体细胞高频突变（SHM）相关，但尚不清楚这些突变是在免疫反应过程中还是在二次反应中激活时产生的。我们旨在将这些过程分离出来，并区分记忆 B 细胞与原发性和继发性免疫反应。我们使用 454 焦磷酸测序技术研究了来自幼儿和成年人的血液单核细胞中的 SHM 水平和 IgG 亚类，以作为原发性和继发性免疫记忆的模型。在纯化的 IgG 记忆 B 细胞亚群上进行了额外的测序和详细的免疫表型分析，并使用 IgG 亚类特异性抗体。在儿童和成人中，涉及更下游 IGHG 基因（特别是 IGHG2 和 IGHG4）的转录本中的 SHM 水平更高。在成年人中，SHM 水平明显高于儿童，并且更频繁地利用下游 IGHG 基因。这与 CD27IgG 记忆 B 细胞的频率增加有关，这些细胞含有更高水平的 SHM、更多的 IGHG2 使用和更高的激活标志物表达水平，而 CD27IgG 记忆 B 细胞则不同。我们得出结论，继发性免疫记忆随着年龄的增长而积累，这些记忆 B 细胞表达 CD27、高水平的激活标志物，并且携带高水平的 SHM 和频繁使用 IGHG2。这些新的见解有助于我们理解顺序 IgG 亚类转换，并显示出使用血清 IgG2 水平或表达 IgG2 的 B 细胞数量作为有效产生记忆反应的标志物的潜在相关性。
