Although there is no direct evidence on how Bacteroides fragilis improves immunotherapy through the PD-L1 expression and PD-1 checkpoint pathway thus far, the study by Vetizou et al. revealed that Bacteroides fragilis could improve the therapeutic effect of another immune checkpoint inhibitor CTLA-4 blockade on melanoma mice, which to some extent supports our inference [47]. This evidence concerns the gene CTLA4 and melanoma.