SMAD4缺失通过上调THIK2介导MSI-H结直肠癌免疫逃逸及免疫耐药的机制

只有微卫星高度不稳定(MSI-H)的晚期结直肠癌（mCRC）才能从PD-1抗体中获益。即使是MSI-H患者，PD-1抗体有效率也仅40%，而原发性耐药的机制尚不清楚。mCRC中SMAD4缺失约占20%。我们发现对PD-1抗体敏感的CT26-MLH1 KO和MC38细胞，在敲除SMAD4后均对PD-1抗体耐药；SMAD4缺失与CD8+T细胞浸润减少密切相关。这高度提示SMAD4缺失通过抑制CD8+T细胞招募而导致PD-1抗体耐药。此外，肿瘤细胞钾离子通道THIK2的表达在SMAD4敲除后明显上调；进一步敲除THIK2能促进CD8+T细胞浸润至小鼠皮下肿瘤。我们提出科学假说：SMAD4缺失通过促进THIK2的表达，阻碍肿瘤微环境中CD8+T细胞的浸润，导致PD-1抗体耐药。本项目将利用基因敲除细胞及动物模型，明确SMAD4缺失上调THIK2，最终导致PD-1抗体耐药的机制，并提供克服耐药策略

For patients with metastatic colorectal cancer (mCRC), only those with microsatellite instability -high (MSI-H) tumor can benefit from PD-1 antibody. Even in patients with MSI-H, the response rate of PD-1 antibody is only 40%. The proportion of SMAD4 loss in mCRC is about 20%. It is unclear whether SMAD4 loss is involved in mCRC immune escape and immune resistance. We found that CT26-MLH1 KO cells and MC38 cells, which are sensitive to PD-1 antibody, are resistant to PD-1 antibody after SMAD4 knock out; SMAD4 loss is closely related to less CD8+ T cell infiltration. These results highly suggest SMAD4 loss hinders the infiltration of CD8+ T cells into the tumor, leading to PD-1 antibody resistance. We found that the expression of THIK2 was significantly up-regulated after SMAD4 was knocked out; further knocking out THIK2 could promote the infiltration of CD8+ T cells into mouse subcutaneous tumors. We put forward a scientific hypothesis: SMAD4 loss promotes the expression of THIK2 and hinders the infiltration of CD8+ T cells in the tumor microenvironment, leading to PD-1 antibody resistance. This project will further clarify the mechanism by which SMAD4 loss up-regulates THIK2 and cause PD-1 antibody resistance, and provide strategies to overcome resistance.