SIX1 Inhibits Anti-tumor Immune Response Through TGF-?-mediated Regulation Of Collagens

As a global public health problem,cancer has been seriously affecting human health.Cancer treatment mainly include surgery,radiotherapy,chemotherapy,and immunotherapy,which has brought new hope for cancer patients because of its high efficacy and safety.Although immunotherapy has obtained satisfactory results in some tumor patients,it is not effective for most solid tumors.The efficacy of immunotherapy is mainly affected by the tumor microenvironment(TME).The TME includes cancer cells,non cancer cells and extracellular matrix(ECM).Cancer cells are the main component of the TME.Non cancer cells in the TME are mainly composed of stromal cells(such as fibroblasts,pericytes and mesenchymal stromal cells)and immune cells(including T and B lymphocytes,tumor associated macrophages and NK cells).Tumor infiltrating lymphocytes(TILs)are immune cells that play a key role in the occurrence and development of tumors.According to the infiltration and activity of T cells,the immune function phenotype in the TME can be roughly divided into "cold tumor" and "hot tumor".On the other hand,the tumor matrix in the TME is mainly composed of the ECM,which is an important regulator of cell growth.Traditionally,the ECM is mainly regarded as a physical scaffold of tumor tissue,which connects cells and tissues.However,recent studies have shown that the ECM can also induce biochemical and biophysical signals of cancer cell adhesion,migration and metastasis.Collagen is the most abundant component of the ECM.Its increase or decrease can have a substantial impact on the pathogenesis and progression of tumor by changing the infiltration of immune cells.As an important regulator of organogenesis,transcription factor sine oculis homeobox 1(SIX1)is expressed at low levels in normal adult tissues,although highly expressed during embryonic development.Previous studies have clarified the role of SIX1 in tumor metabolism,growth and poor prognosis,et al.Its potential impact on the TME and its potential mechanism are still unknown.Therefore,we first downloaded a large number of clinical sample information in the database to analyze the expression level of Sixl in tumor tissues and its impact on the survival of tumor patients.Then we used CRISPR cas9 technology to knock out Six1 in tumor cells and established mouse tumor model.Through RNA-seq,flow cytometry,immunofluorescence and other technologies,we observed the effect of Six1 deletion on remodeling TME and anti-tumor immune responses,and deeply explored its mechanism in vitro and in vivo.In this study,we found that the transcription factor SIX1 plays a regulatory role in antitumor immune response.Firstly,through the analysis of TCGA database,it was found that SIX1 in tumor tissues of cancer patients was significantly up-regulated and negatively correlated with the survival rate and the number of immune cells infiltration.In animal models,Six1 deficiency in cancer cells inhibited tumor growth in an immune dependent manner.In addition,Six1 deletion enhanced the infiltration and activation of CD8+T cells,activated the host adaptive immune response,and maintained the host's long-term tumor immune protection from the re-challenge of tumor cells.Mechanismly,SIX1,as a transcription factor,can directly target tgfbr2 and activate TGF-? signal transduction through Smad2/3 phosphorylation to induce collagen gene expression.Collagen deposition in the TME hindered the infiltration and activation of CD8+T cells.Therefore,the deletion of Six1 in cancer cells improves the immunogenicity of cancer cells and enhances the anti-tumor immune response.Our study revealed the regulatory role of SIX 1,which are usually up-regulated in tumor tissues of most cancer patients,on host anti-tumor immune response,and suggested that Six1 may become a new target in tumor immunotherapy.