DataSheet_1_IRF4 Has a Unique Role in Early B Cell Development and Acts Prior to CD21 Expression to Control Marginal Zone B Cell Numbers.pdf

Strict control of B lymphocyte development is required for the ability to mount humoral immune responses to diverse foreign antigens while remaining self-tolerant. In the bone marrow, B lineage cells transit through several developmental stages in which they assemble a functional B cell receptor in a stepwise manner. The immunoglobulin heavy chain gene is rearranged at the pro-B stage. At the large pre-B stage, cells with a functional heavy chain expand in response to signals from IL-7 and the pre-BCR. Cells then cease proliferation at the small pre-B stage and rearrange the immunoglobulin light chain gene. The fully formed BCR is subsequently expressed on the surface of immature B cells and autoreactive cells are culled by central tolerance mechanisms. Once in the periphery, transitional B cells develop into mature B cell subsets such as marginal zone and follicular B cells. These developmental processes are controlled by transcription factor networks, central to which are IRF4 and IRF8. These were thought to act redundantly during B cell development in the bone marrow, with their functions diverging in the periphery where IRF4 limits the number of marginal zone B cells and is required for germinal center responses and plasma cell differentiation. Because of IRF4’s unique role in mature B cells, we hypothesized that it may also have functions earlier in B cell development that cannot be compensated for by IRF8. Indeed, we find that IRF4 has a unique role in upregulating the pre-B cell marker CD25, limiting IL-7 responsiveness, and promoting migration to CXCR4 such that IRF4-deficient mice have a partial block at the pre-B cell stage. We also find that IRF4 acts in early transitional B cells to restrict marginal zone B cell development, as deletion of IRF4 in mature B cells with CD21-cre impairs plasma cell differentiation but has no effect on marginal zone B cell numbers. These studies highlight IRF4 as the dominant IRF family member in early B lymphopoiesis.

要在对多种外来抗原产生体液免疫应答的同时维持自身耐受，严格调控B淋巴细胞发育过程至关重要。在骨髓中，B系细胞历经多个发育阶段，逐步组装功能性B细胞受体（B cell receptor, BCR）：免疫球蛋白重链基因在前B细胞（pro-B）阶段发生重排；于大前B细胞阶段，携带功能性重链的细胞会在白细胞介素7（IL-7）与前B细胞受体（pre-BCR）的信号刺激下增殖；随后细胞在小前B细胞阶段停止增殖，并重排免疫球蛋白轻链基因。完全组装完成的BCR随后会在未成熟B细胞表面表达，自身反应性细胞则通过中枢耐受机制被清除。迁移至外周后，过渡性B细胞可分化为边缘区B细胞、滤泡B细胞等成熟B细胞亚群。上述发育过程受转录因子网络调控，其中干扰素调节因子4（IRF4）与干扰素调节因子8（IRF8）处于核心地位。此前认为二者在骨髓B细胞发育过程中功能冗余，而在外周组织中功能出现分化：IRF4可限制边缘区B细胞的数量，且对于生发中心反应与浆细胞分化不可或缺。鉴于IRF4在成熟B细胞中的独特作用，我们推测其在B细胞发育早期或许也存在无法被IRF8代偿的功能。实验结果证实，IRF4可通过上调前B细胞标志物CD25的表达、限制细胞对IL-7的应答性以及促进细胞向C-X-C趋化因子受体4（CXCR4）介导的迁移发挥作用，IRF4缺陷小鼠的前B细胞阶段会出现部分发育阻滞。此外，我们还发现IRF4可在早期过渡性B细胞中发挥作用，以限制边缘区B细胞的发育：借助CD21-Cre重组酶系统在成熟B细胞中敲除IRF4后，虽会损害浆细胞分化，但对边缘区B细胞的数量无显著影响。本研究明确了IRF4是早期B细胞生成过程中占主导地位的干扰素调节因子家族成员。