Perturbation of gut microbiota modulated the abscopal effects of immunoradiotherapy for colon cancer in mice
Objective To investigate whether the gut microbiota of mice treated with antibiotics has an effect on the distant effect of MC38 colon cancer radiotherapy combined with anti-PD-1 immunotherapy in mice.Methods Bilateral MC38 subcutaneous grafted tumors[in situ(radiotherapy site)tumors and distant tumors]were established with or without oral antibiotic therapy in 109 SPF grade female C57BL/6J mice.C57BL/6J mice were divided into 8 groups in a completely random way:control group(n=15),radiotherapy group(n=17),anti-PD-1 immunotherapy group(n=15),radiothera-py combined with anti-PD-1 immunotherapy group(n=17),antibiotics alone group(n=11),antibiotics combined with radiotherapy group(n=11),antibiotics combined with anti-PD-1 immunotherapy group(n=10),and antibiotics combined with radiotherapy anti-PD-1 immunotherapy group(n=13).The eight groups could be classified into two categories,the first 4 groups did not use antibiotics,which were normal intestinal flora(Normal),and the last 4 groups used antibiotics which were eliminated intestinal flora(ATB).16S rRNA sequencing of mouse feces was used to assess changes in the gut microbiome after ATB.A variety of bioinformatics methods were used to investigate the effects of spe-cific gut flora on specific flora associated with distant effects.Results In situ tumor after radiotherapy combined with an-ti-PD-1 immunotherapy,both Normal and ATB mice showed tumor shrinkage(P＜0.001).In the Normal group,the size of distant tumors in the radiotherapy combined with anti-PD-1 immunotherapy group decreased compared with the other 3 groups,with statistical significance,all P＜0.05.In the ATB group,there was no significant difference in the size of distant tumors in the antibiotic combined with radiation plus anti-PD-1 immunotherapy group compared with the other 3 groups,all P＞0.05.The a-diversity of gut microbiota decreased significantly after ATB,and the β-diversity be-tween Normal and ATB microbiota was significantly different.The results of linear discriminant analysis effect size(LEfSe)showed that the species with significant difference in Normal group was p Firmicutes and g_Ligilactobacillus,while the species with significant difference in ATB group was p_Bacteroidota and g_Bacteroides.At the phylum level,the relative abundance of Bacteroidota increased significantly(P＜0.001)and the relative abundance of Firmicutes is sig-nificantly reduced(P＜0.001)after ATB.At the genus level,after ATB,the relative abundance of Bacteroides increased significantly(P＜0.001),and the relative abundance of Lactobacillus salivary significantly decreased(P＜0.001)after ATB.Conclusion The changes of gut microbiota after ATB significantly affect the abscopal effects of iRT for colon cancer.
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