Our results showed that PPARδ inhibitor is efficient in inhibiting tumor- and CD40 agonist-induced IL-10+ Bregs, and preventing transition of CD19+CD24hiIgDlo/−CD38lo Bregs to CD19+CD24hiIgDlo/−CD38hi Bregs with increased expression of PPARδ and IL-10, leading to markedly improved antitumor responses. Here, PPARD is linked to neoplasm.