| BackgroundAccording to the 2020 global cancer statistics,gastric cancer(GC)ranks fifth in the global incidence and fourth in mortality.In China,the incidence and mortality of gastric cancer ranks third.The main treatment methods for advanced gastric cancer include chemotherapy,targeted therapy and immunotherapy.Among them,Immune checkpoint blockade(ICB)is widely used in the field of tumor therapy,however,the effective rate of ICB therapy for gastric cancer is not satisfactory.Screening the benefit population of immunotherapy is an important means to improve the efficiency of ICB,and the screening of benefit population relies on the detection of biological markers.Currently,single tumor cell-based indicators such as PD-L1,TMB,and MSI are not ideal for predicting the effect of immunotherapy.The tumor microenvironment affects tumor growth,metastasis,treatment,and prognosis,and a scoring model based on the tumor microenvironment can better predict the response to ICB therapy.Among them,the TMEscore scoring model that quantifies the tumor microenvironment can better predict the treatment effect of gastric cancer ICB,and the overall accuracy is higher than that of PD-L1,TIM3 and other predictive biomarkers.However,most of the current evaluation models of the microenvironment rely on biopsy tissue sequencing data,and obtaining tissue samples in gastric cancer is difficult,high-risk,and difficult to achieve dynamic sampling.Therefore,exploring non-invasive evaluation methods based on the immune microenvironment of gastric cancer can help stratify gastric cancer patients and formulate treatment strategies.Cancer-associated fibroblasts(CAFs)are an important part of the tumor microenvironment(TME),and CAFs play an important role in tumor cell proliferation,metastasis,immune escape,chemotherapy resistance and energy metabolism.More importantly,CAFs also play a key role in the regulation of the microenvironment.CAFs can recruit and activate immunosuppressive cells by secreting cytokines,thereby creating an "immune expulsion microenvironment".Among them,Fibroblast activation protein(FAP)is selectively overexpressed on the fibroblast membrane of most solid tumor stroma,especially breast cancer,colorectal cancer,pancreatic cancer and gastric cancer.Tumors with a desmoplastic response,but low expression in normal tissues and benign tumors.Therefore,FAP can be specifically used to identify CAFs,and it is also one of the potential imaging targets of immunosuppressive microenvironment.68Ga-labeled fibroblast activation protein inhibitor-04(68Ga-FAPI-04)PET/CT imaging can accurately image liver cancer,colorectal cancer,poorly differentiated gastric cancer,head and neck tumors,and its specific targeting of FAP is helpful for quantitative analysis of tumors infiltration of CAFs in the microenvironment.However,whether 68Ga-FAPI-04 can be used to evaluate the tumor microenvironment to guide ICB therapy remains unknown.Therefore,this study aimed to explore the role of 68Ga-FAPI-04 PET/CT in imaging the immunosuppressive microenvironment and the predictive efficacy of immunotherapy for gastric cancer.Methods1.To analyze the relationship between FAP and gastric cancer immune response.The NanoString cohort included 48 ICB-treated gastric cancer patients.The gastric cancer patients in the NanoString cohort were grouped according to their response to immunotherapy,the efficacy of gastric cancer immunotherapy was evaluated according to RECIST 1.1,and the expression of FAP in different gastric cancer prognosis groups was detected.The predictive power of FAP for ICB treatment was assessed by ROC analysis in the NanoString cohort.The patients in the TCGA-STAD,ACRG,and IMvigor210 cohorts were grouped according to the expression of FAP,and it was analyzed whether the different expression levels of FAP were related to clinical prognosis.2.The correlation between FAP expression and immunosuppressive characteristics in gastric cancer microenvironment was analyzed.The correlation between FAP and CAFs in the TCGA-STAD and ACRG cohorts was analyzed,and the characteristic scores of immune infiltrating cells in the FAP high and low expression groups were calculated,including tumor-associated fibroblasts,MDSCs,exhausted CD8+T cells,M2 macrophages and Treg cells.And then we calculated the TMEscore of FAP high and low expression groups.3.To evaluate the infiltration of immunosuppressive cells in gastric cancer tissue.Immunohistochemistry(IHC)was performed on 31 gastric cancer tissue samples,including PD-1,MDSC(CD11b+/CD33+),M2 macrophages(CD163+).Quantitative analysis was performed to evaluate the relationship between FAP expression level and immunosuppressive tumor microenvironment.4.To assess the correlation of FAPI uptake and immunosuppressive tumor microenvironment in gastric cancer patients.Twenty-one advanced gastric cancer patients(FAPI cohort)were recruited and underwent 18F-FDG and 68Ga-FAPI-04 PET/CT imaging before immunotherapy.The corresponding gastric cancer specimens were subjected to immunohistochemical staining to verify the infiltration of immunosuppressive cells in the tumor.Transcriptome sequencing of some gastric cancer specimens was performed to calculate the TMEscore,and the Pearson correlation coefficient was used to measure the correlation between FAPI uptake and TMEscore.5.FAPI cohort to verify the correlation between FAPI uptake and gastric cancer immunotherapy.18F-FDG and 68Ga-FAPI-04 imaging analysis and immunotherapy efficacy evaluation were performed on the FAPI cohort to evaluate whether the FAPI uptake level of gastric cancer is related to the immunotherapy efficacy.The relationship between the SUVmax value of FAPI and the prognosis of different immunotherapy was calculated.Results1.High FAP expression indicates poor prognosis of gastric cancer immunotherapy.In the NanoString cohort,the FAP mRNA level in the non-responder group(NonResponder)was significantly higher than that in the responder group(Responder)after gastric cancer immunotherapy.ROC curve analysis found that FAP had higher prediction accuracy than PD-1,PD-L1,TIM3 and PDCD1LG2,and its area under the curve reached 0.733.In the TCGA-STAD,ACRG,and IMvigor210 cohorts,high FAP expression was also confirmed to be associated with poor patient prognosis.2.FAP expression is closely related to the immunosuppressive characteristics of gastric cancer.In the TCGA-STAD and ACRG cohorts,FAP expression levels were positively correlated with CAFs expression,and FAP expression was positively correlated with the infiltrating abundance of immunosuppressive cells.Our team previously verified that TMEscore is a more accurate prognostic biomarker for gastric cancer immunetherapy.In the NanoString cohort,the level of FAP expression was negatively correlated with the TMEscore score,indicating that the tumor microenvironment of patients with high FAP expression was in an immunosuppressive state.3.Tissue specimens showed that FAP expression was positively correlated with immunosuppressive cell infiltration.Immunohistochemical staining of gastric cancer tissue microarrays showed that there were more immunosuppressive cell infiltration in FAP-expressing tissues,including M2 macrophages,MDSCs,and the expression level of PD-1 was also significantly up-regulated.4.High uptake of 68Ga-FAPI-04 suggests an immunosuppressive tumor microenvironment.Immunohistochemical staining was performed on tumor specimens from gastric cancer patients with 68Ga-FAPI-04 PET/CT imaging,and it was found that the uptake of 68Ga-FAPI-04 was positively correlated with FAP expression,and in gastric cancer lesions with high FAPI uptake,MDSCs,M2 macrophages infiltration was increased,and PD-1 expression levels were elevated.Pearson correlation analysis found that FAPI uptake was negatively correlated with TMEscore.Thus,increased FAPI uptake indicated an immunosuppressive tumor microenvironment.5.FAPI high uptake gastric cancer patients with poor immunotherapy response.The FAPI cohort was evaluated by 68Ga-FAPI-04 PET/CT imaging and immunotherapy efficacy.It was found that the immunotherapy efficacy of gastric cancer patients with high FAPI uptake was poor,suggesting that the FAPI uptake value is related to the prognosis of gastric cancer immunotherapy.68Ga-FAPI-04 PET/CT can be used as a non-invasive means to predict the efficacy of immunotherapy.Conclusions1.High FAP expression indicates poor prognosis in gastric cancer.2.High FAP expression indicates an immunosuppressive tumor microenvironment.3.Increased FAPI uptake results in poor immunotherapy in gastric cancer patients,suggesting that 68Ga-FAPI-04 PET/CT can be used for gastric cancer predictive marker of immunotherapy. |
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