Table_2_The Splenic Marginal Zone in Children Is Characterized by a Subpopulation of CD27-Negative, Lowly IGHV-Mutated B Cells.xlsx

Young children and older adults suffer from enhanced susceptibility to infections with blood-borne pathogens. An essential step towards immunity is the establishment of a splenic marginal zone (sMZ), which is immature at below 2 years of age. At approximately 5 years of age, an adult level of protection is reached but wanes again in older adults. Although the infant sMZ is thought to contain mostly naïve B cells, memory B cells are recruited to and recirculate from the sMZ throughout life, and class-switched sMZ B cells dominate in older adults. For a better resolution of naïve versus memory B-cell subset accumulation in the sMZ, we performed a single cell-based gene expression analysis of (CD21highIgMhigh) sMZ B cells among five healthy donors (age 3 to 48 years) and validated the sMZ B-cell subset composition by flow cytometry of 147 spleen biopsies (age 0 to 82 years). We identified a major sMZ B-cell subpopulation, which is abundant at birth but decreases with age. These cells lack CD27 expression but carry a weak-to-intermediate memory B-cell signature. These CD27neg sMZ B cells are either IGHV-unmutated or carry only a few IGHV mutations early in life but show average memory B-cell IGHV mutation frequencies (>3%) in adults. The activation and proliferation potential of CD27neg sMZ B cells is significantly above that of non-sMZ B cells already in children. Our study suggests that the human sMZ B-cell pool changes with age, encompassing a major population of lowly Ig-mutated CD27neg but antigen-experienced B cells early in life.

低龄儿童与老年群体均对血源性病原体感染具有更高的易感性。机体获得免疫能力的关键环节是脾脏边缘区（splenic marginal zone, sMZ）的形成，2岁以下个体的脾脏边缘区尚未发育成熟。约5岁时，机体可达到成人水平的免疫保护能力，但该能力会在老年群体中再次衰退。尽管既往认为婴儿的脾脏边缘区以初始B细胞（naïve B cell）为主，但实际上记忆B细胞（memory B cell）会在一生中持续被招募至脾脏边缘区并从中再循环，而类别转换型脾脏边缘区B细胞在老年群体中占据主导地位。为更清晰地解析脾脏边缘区内初始B细胞与记忆B细胞亚群的积累特征，本研究对5名年龄介于3至48岁的健康供体的（CD21高表达、IgM高表达）脾脏边缘区B细胞开展了单细胞基因表达分析，并通过对147份年龄跨度为0至82岁的脾脏活检样本进行流式细胞术（flow cytometry），验证了脾脏边缘区B细胞的亚群构成。本研究鉴定出一类主要的脾脏边缘区B细胞亚群：该亚群在出生时含量丰富，但会随年龄增长而逐渐减少。此类细胞不表达CD27，但呈现弱至中等强度的记忆B细胞特征。这类CD27阴性的脾脏边缘区B细胞在生命早期要么未发生免疫球蛋白重链可变区（IGHV）突变，要么仅携带少量IGHV突变；而在成年个体中，其IGHV突变频率则达到记忆B细胞的平均水平（＞3%）。在儿童群体中，CD27阴性脾脏边缘区B细胞的活化与增殖能力已显著高于非脾脏边缘区B细胞。本研究结果表明，人类脾脏边缘区B细胞库会随年龄发生动态变化：在生命早期，该细胞库包含大量Ig突变率较低的CD27阴性但已接触过抗原的B细胞。