DataSheet_1_Human IgA-Expressing Bone Marrow Plasma Cells Characteristically Upregulate Programmed Cell Death Protein-1 Upon B Cell Receptor Stimulation.pdf

The functions of bone marrow plasma cells (BMPC) beyond antibody production are not fully elucidated and distinct subsets of BMPC suggest potential different functions. Phenotypic differences were identified for human BMPC depending on CD19 expression. Since CD19 is a co-stimulatory molecule of the B-cell-receptor (BCR), and IgA+ and IgM+ BMPC express the BCR on their surface, we here studied whether CD19 expression affects cellular responses, such as BCR signaling and the expression of checkpoint molecules. We analyzed 132 BM samples from individuals undergoing routine total hip arthroplasty. We found that both CD19+ and CD19− BMPC expressed BCR signaling molecules. Notably, the BCR-associated kinase spleen tyrosine kinase (SYK) including pSYK was higher expressed in CD19+ BMPC compared to CD19− BMPC. BCR stimulation also resulted in increased kinase phosphorylation downstream of the BCR while expression of CD19 remained stable afterwards. Interestingly, the BCR response was restricted to IgA+ BMPC independently of CD19 expression. With regard to the expression of checkpoint molecules, CD19− BMPC expressed higher levels of co-inhibitory molecule programmed cell death protein-1 (PD-1) than CD19+ BMPC. IgA+ BMPC characteristically upregulated PD-1 upon BCR stimulation in contrast to other PC subsets and inhibition of the kinase SYK abrogated PD-1 upregulation. In contrast, expression of PD-1 ligand, B and T lymphocyte attenuator (BTLA) and CD28 did not change upon BCR activation of IgA+ BMPC. Here, we identify a distinct characteristic of IgA+ BMPC that is independent of the phenotypic heterogeneity of the subsets according to their CD19 expression. The data suggest that IgA+ BMPC underlie different regulatory principles and/or exert distinct regulatory functions.

骨髓浆细胞（bone marrow plasma cells, BMPC）除抗体产生之外的功能尚未完全阐明，且其不同亚群提示存在功能异质性。已有研究证实，人骨髓浆细胞的表型差异取决于CD19的表达水平。由于CD19是B细胞受体（B-cell-receptor, BCR）的共刺激分子，且IgA+与IgM+骨髓浆细胞表面均表达BCR，本研究旨在探讨CD19的表达是否会影响细胞应答，如BCR信号转导及检查点分子的表达。本研究分析了132份来自接受常规全髋关节置换术个体的骨髓样本。结果显示，CD19+与CD19-骨髓浆细胞均表达BCR信号分子。值得注意的是，与CD19-骨髓浆细胞相比，CD19+骨髓浆细胞中BCR相关激酶脾酪氨酸激酶（spleen tyrosine kinase, SYK）及其磷酸化形式pSYK的表达水平更高。BCR刺激还可导致BCR下游激酶磷酸化水平升高，且此后CD19的表达保持稳定。有趣的是，BCR应答仅局限于IgA+骨髓浆细胞，且这一现象与CD19的表达无关。就检查点分子的表达而言，CD19-骨髓浆细胞的共抑制分子程序性死亡蛋白-1（programmed cell death protein-1, PD-1）表达水平高于CD19+骨髓浆细胞。与其他浆细胞亚群不同，IgA+骨髓浆细胞在BCR刺激后会特异性上调PD-1的表达，且抑制脾酪氨酸激酶SYK可消除PD-1的上调效应。与之相反，在IgA+骨髓浆细胞的BCR激活过程中，PD-1配体、B和T淋巴细胞衰减蛋白（B and T lymphocyte attenuator, BTLA）以及CD28的表达并未发生改变。本研究揭示了IgA+骨髓浆细胞的独特特性，这一特性与其根据CD19表达所划分的亚群表型异质性无关。研究数据表明，IgA+骨髓浆细胞遵循不同的调控机制，并/或发挥独特的调控功能。