Mef2c介導之Flt3表達調控共同淋巴前驅細胞發育成漿狀樹突細胞

Abstract

樹突細胞 (DC) 可分為傳統樹突細胞 (cDC) 與漿狀樹突細胞 (pDC)，其中pDC專門產生第一型干擾素 (type I interferon, IFN-I) 以抵禦病毒感染。在發育方面，樹突細胞可由淋巴或骨髓前驅細胞持續分化而來，此過程極度仰賴細胞激素訊號與轉錄因子網路 (transcription factor network) 的協同調控，然而其分子調控機制尚未完全釐清。我們的研究聚焦於一個高度表達於pDC的轉錄因子Mef2c。在先前實驗室的研究已證實Mef2c具有正向促進pDC發育及負向抑制pDC活化的功能，但Mef2c促進pDC發育的具體分子機制仍不清楚。利用MxCre所驅動的誘導型Mef2c剔除 (Inducible Mef2c knockout) 小鼠，我們發現pDC的淋巴前驅細胞群的組成發生明顯改變，且共同淋巴前驅細胞 (CLP) 分化為pDC的能力也顯著下降；進一步透過競爭性細胞移植 (competitive adoptive transfer) 實驗證實，Mef2c以細胞內源性 (cell intrinsic) 方式調控CLP的pDC分化潛能。機制分析顯示，Mef2c缺失導致CLP中pDC發育相關細胞激素受體 (cytokine receptor) Flt3及表面標記Siglec-H的表達下降。當給予高劑量Flt3配體 (Flt3 ligand, FL) 後，可部分挽救Mef2c缺失所導致的pDC發育缺陷，顯示維持Flt3在CLP的表達為Mef2c促進pDC發育的重要機制之一。此外，轉錄組定序 (RNA-seq) 分析發現，Mef2c缺失亦伴隨Tcf4、Irf8及Runx2等pDC關鍵轉錄因子在FL刺激後的表達顯著下降。此外，有一群IFN刺激所引發的基因 (IFN-stimulated genes, ISGs) 會隨著FL的處理24小時後大幅度增加表現。有意思的是IFN-I、IFN-II或IFN-III在FL刺激的早期 (6小時) 或晚期 (24小時) 都沒有被偵測到，顯示這群ISGs可能不是透過IFN刺激所引發的，即所謂的非傳統 (non-canonical) ISGs。以上結果表明，Mef2c作為一種調控pDC發育的新穎轉錄因子，早在前驅細胞階段即參與pDC譜系的決定，透過上調Flt3與Siglec-H等關鍵表面分子之表達，並促進pDC特異性轉錄因子網路的運作，共同驅動CLP朝pDC分化，最終維持體內pDC的正常生成與穩態。

Dendritic cells (DCs) can be classified into conventional dendritic cells (cDCs) and plasmacytoid dendritic cells (pDCs). Among them, pDCs specialize in producing type I interferons (type I interferon, IFN-I) to combat viral infection. The development of DCs from lymphoid or myeloid progenitors is tightly regulated by cytokine signaling and transcription factor (TF) networks. However, the molecular mechanisms governing pDC lineage commitment remain incompletely understood. In this study, we focused on myocyte enhancer factor 2C (Mef2c), a transcription factor highly expressed in pDCs. Our previous study demonstrated that Mef2c positively regulates pDC development while negatively modulating pDC activation, yet the molecular basis underlying its role in pDC development remains unclear. Using MxCre-driven inducible Mef2c-deficient mice, we found that Mef2c deficiency markedly altered the composition of pDC lymphoid precursor populations and impaired the ability of common lymphoid progenitors (CLPs) to differentiate into pDCs. Competitive adoptive transfer further demonstrated that Mef2c regulates the differentiation potential of CLPs in a cell-intrinsic manner. Mechanistically, Mef2c deficiency resulted in reduced expression of a key cytokine receptor, Flt3, for DC development, and the surface marker Siglec-H in CLPs. Supplementation with a high dose of Flt3 ligand (FL) partially rescued the developmental defect in Mef2c-deficient CLPs, suggesting that maintenance of Flt3 expression in CLPs is an important mechanism by which Mef2c promotes pDC development. Furthermore, transcriptomic analysis revealed that loss of Mef2c was accompanied by decreased expression of key pDC-associated transcription factors, including Tcf4, Irf8, and Runx2. Moreover, a subset of IFN-stimulated genes (ISGs) was upregulated in CLPs following FL stimulation for 24 h. Interestingly, the expression of IFN-I, IFN-II, or IFN-III was not detectable at either 6 h or 24 h post-treatment, suggesting that these ISGs are IFN-independent and therefore non-canonical ISGs. Collectively, these findings identify Mef2c as a novel regulator of pDC development that acts at the progenitor stage to promote pDC lineage commitment. Mef2c facilitates pDC differentiation by maintaining Flt3 and Siglec-H expression and supporting the pDC-specific transcription factor network, thereby directing CLPs toward the pDC lineage and maintaining pDC homeostasis under steady-state conditions.