Massively recruited sTLR9+ neutrophils in rapidly formed nodules at the site of tumor cell inoculation and their contribution to a pro-tumor microenvironment

Neutrophils exert either pro- or anti-tumor activities. However, few studies have focused on neutrophils at the tumor initiation stage. In this study, we unexpectedly found a subcutaneous nodule in the groin areas of mice inoculated with tumor cells. The nodule was developed 24 h after the inoculation, filled with tumor cells and massively recruited neutrophils, being designated as neutrophil-infiltrated nodules (NINs). 22% of the neutrophils in NINs are surface TLR9 (sTLR9) expressing neutrophils (sTLR9 + neutrophils). With tumor progression, sTLR9 + neutrophils were sustainably increased in NINs/tumor tissues, reaching to 90.8% on day 13 after inoculation, with increased expression of IL-10 and decreased or no expression of TNFα. In vivo administration of CpG 5805 significantly reduced sTLR9 expression of the sTLR9 + neutrophils. The reduction of sTLR9 on neutrophils in NINs contributed to the induction of an anti-tumor microenvironment conductive to the inhibition of tumor growth. Overall, the study provides insights for understanding the role of sTLR9 + neutrophils in the tumor development, especially in the early stages.