Siglec-5和Siglec-14對於模式識別受體與抗病毒免疫反應的調節作用

Abstract

在感染的過程中，類鐸受體能夠藉由識別病原相關分子模式以啟動先天性免疫反應。Siglec受體為廣泛表現在免疫細胞上的免疫調節分子，其能透過與病原體表面的唾液酸化配體結合，並和類鐸受體訊號路徑產生交互作用來調控免疫反應。Siglec-5與Siglec-14是主要表現在髓系細胞上的成對受體，儘管兩者具有高度相似的細胞外結構域，能夠辨識相同的配體，但它們卻會傳遞相反的胞內訊號：Siglec-5含有ITIM基序，可傳遞抑制型訊號；而Siglec-14則藉由含有ITAM基序的銜接蛋白DAP12傳遞活化型訊號。本研究利用嚴重病毒性肺炎的小鼠模型探討此成對受體在免疫調控中的功能。體外實驗結果顯示，在小鼠骨髓源性巨噬細胞(BMDM)中，表現Siglec-14可以促進類鐸受體活化或病毒感染後TNF的表現並抑制IL-10的生成。對於IFN-β，Siglec-14則呈現刺激特異性的調節作用：在Poly(I:C)或仙台病毒刺激時，Siglec-14能夠促進IFN-β表現，但在LPS-EK或H5N1感染後則呈現抑制效果。相較之下，Siglec-5傾向抑制IL-10與IFN-β的表現，對於TNF與IL-6的影響則相對有限。體內實驗中，儘管SIGLEC14Myeloid TG小鼠與野生型小鼠感染後的病毒量相似，但感染之後第三天SIGLEC14Myeloid TG小鼠嗜中性球與自然殺手細胞的數量明顯下降，而感染之後第六天SIGLEC14Myeloid TG小鼠CD8+ T細胞的比例與數量以及細胞因子IL-6與IFN-γ的含量均顯著上升。這些結果顯示，Siglec-14可能透過調控病毒感染期間免疫細胞的分布與活化狀態以促進肺部的發炎反應。總結而言，我們的研究結果表明Siglec-5與 Siglec-14在調節巨噬細胞反應及肺部免疫環境方面具有獨特且依賴刺激條件的功能。

Upon infection, Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns (PAMPs) and initiate immune responses. Siglecs, which are abundantly expressed on immune cells, serve as key immunomodulatory receptors that regulate immune activation by recognizing sialylated ligands on pathogens and interacting with TLR pathways. Among them, Siglec-5 and Siglec-14 are paired receptors primarily found on myelomonocytic cells. Despite having nearly identical extracellular domains, they transmit opposing signals: Siglec-5 inhibits signaling via an ITIM motif, whereas Siglec-14 activates immune responses through the ITAM-containing adaptor DAP12 upon engaging the same ligand. We investigated the functional roles of this receptor pair using mouse models of severe viral pneumonia. In vitro, Siglec-14 expression in mouse bone-marrow-derived macrophage (BMDMs) enhanced TNF and suppressed IL-10 after TLR stimulation or viral infection. Siglec-14 also regulated IFN-β in a stimulus-specific manner-enhancing expression following Poly(I:C) or Sendai virus stimulation, but suppressing it after LPS-EK or H5N1 exposure. In contrast, Siglec-5 consistently downregulated IL-10 and IFN-β with minimal impact on TNF or IL-6. In vivo, despite similar viral loads, SIGLEC14Myeloid TG mice displayed reduced neutrophils and NK cells on day 3 post-infection, but increased CD8⁺ T cells and elevated IL-6 and IFN-γ by day 6 compared to wild-type (WT) mice. These findings suggest Siglec-14 skews immune cell dynamics and promotes an inflammatory lung environment during viral infection. Together, our results demonstrate distinct and context-dependent roles for Siglec-5 and Siglec-14 in regulating macrophage responses and shaping pulmonary immunity.