肿瘤蛋白SND1阻挠肿瘤细胞中MHC-I类分子抗原呈递导致肿瘤免疫逃逸的分子机制

SND1, staphylococcal nuclease domain-containing protein 1, (also named p100 or Tudor-SN) is a multi-functional protein, which is recently identified as an oncoprotein promotes tumorigenesis by diverse mechanisms in different cancers. .Our group has been focusing on investigating the function of SND1 for many years. We discovered that SND1 forms a loop with SMAD family members and regulate reciprocally to promote breast cancer metastasis through TGF-β signaling pathway. Additionally, SND1 acts as an upstream regulator of SLUG, which is a key transcription factor in regulating the expression of E-CAD/N-CAD switch, resulting in the EMT in ovarian cancer cells. As a multi-functional protein, it is likely that SND1 regulates diversity signal pathways involved in proliferation or metastasis of different cancers..Defects in MHC-I antigen presentation represent a common feature of cancer and allow evasion from T cell recognition. Recently, we found that the expression level of MHC-I, on the membrane surface of tumor cells, was remarkably enhanced with knockout of oncoprotein SND1 (SND1-KO) in the tumor cells. Most importantly, after implanting the SND1-KO tumor cells in the homologous mice, the amount of infiltrated immune cells in the tumor tissue was significantly increased. Meanwhile, at the cellular level, we found that SND1 could associate with not only Cnx and nascent heavy chain (HC) of MHC-I molecule, but also the protein complexes (derlin-1/sec61/p97) involved in ERAD pathway. Based on above, we hypothesize that high expression of SND1 in tumor cells interrupted the MHC-I antigen presentation. In the ER of tumor cells, the association of SND1 and Cnx prevented it from stabilizing the nascent HC, as a result, HC cannot enter the normal maturation process, instead, it is hijacked into the ERAD pathway, which is degraded by the ubiquitin modification in the cytoplasm. With knockout of SND1 in tumor cells, HC can effectively combine with β2m to form mature MHC-I molecule and present antigen peptide to cell surface, consequently promote immune cell infiltration, especially CD8+T cells to effectively kill tumor cells.

肿瘤细胞MHC-I表达水平下调是肿瘤免疫逃逸的重要原因。我们最近发现在肿瘤细胞中敲除肿瘤蛋白SND1（已进行长期研究）后，细胞表面MHC-I分子明显增加，接种同源小鼠后瘤体内免疫细胞浸润数量显著增加；同时发现SND1不仅与Cnx和MHC-I分子重链(HC)结合，还与ER上参与ERAD途径的复合物结合（Derlin-1/Sec61/p97）。由此提出假设：肿瘤蛋白SND1阻挠了MHC-I分子在肿瘤细胞的抗原呈递。在肿瘤细胞ER中，SND1通过与Cnx和新生HC结合，阻止Cnx对HC的稳定作用，使HC不能进入正常成熟过程，而是被劫持进入ERAD途径，转运到胞浆中发生泛素化修饰而被降解。敲除细胞中SND1，HC能有效结合β2m形成成熟MHC-I分子呈递抗原肽到细胞表面，促进免疫细胞浸润（尤其是CD8+T），有效杀伤肿瘤细胞。该研究有助于深入探讨如何促进MHC-I/抗原呈递，改善肿瘤免疫微环境。

肿瘤细胞主要组织相容性复合物1类分子（MHC-1）表达水平下调或缺失，无法诱导CD8+T淋巴细胞活化和杀伤肿瘤细胞，是导致肿瘤逃逸最重要的原因之一。SND1是一种新的肿瘤蛋白，几乎在所有的肿瘤中都能检测到它的高表达，是增强免疫应答和抑制肿瘤生长的一个潜在治疗靶点。本项目深入探讨SND1蛋白抑制肿瘤细胞MHC-1类分子抗原呈递的分子机制。我们利用SND1敲除的小鼠肿瘤细胞和C57BL/6小鼠成瘤模型，发现SND1在促进肿瘤免疫逃逸中发挥重要作用，肿瘤细胞中过表达的SND1通过“劫持”MHC-1分子，并迫使它们进入降解程序，导致肿瘤细胞MHC-1表达降低，这使得CD8+T细胞识别肿瘤细胞的能力下降，从而逃离免疫系统的追杀。此外，我们首先明确了SND1与MHC-1 (HLA-A)的关键结合位点，筛选到一种能够靶向SND1与HLA-A结合位点的小分子药物，即表没食子儿茶素(EGC)；并利用HeLa和B16F10两个不同的细胞株，证明了EGC药物能够阻断SND1/HLA-A的结合，从而抑制SND1对HLA-A的降解，使肿瘤细胞表面HLA-A表达增加，促进肿瘤抗原呈递，促进CD8+T细胞对肿瘤细胞的杀伤作用。.黑色素瘤细胞分泌的外泌体通过重塑免疫微环境，成为黑色素瘤肺转移的重要调节因子。靶向黑色素瘤外泌体，抑制免疫逃逸成为阻断肺转移的有效手段。我们基于外泌体干预黑色素瘤肺转移模型系列实验发现肿瘤蛋白SND1通过肿瘤细胞外泌体重塑肿瘤微环境，促进肿瘤免疫逃逸和肿瘤转移，又通过ITRAQ分析筛选出CD47分子及复合物成员ITGAV/ITGB3，提示SND1可能影响细胞内膜蛋白运输在重塑免疫微环境中发挥重要作用。本项目从不同角度揭示肿瘤细胞过量表达SND1在肿瘤免疫逃逸中的分子机制。资助期间发表SCI论文6篇，已培养4名博士研究生毕业，2名硕士研究生转博。.