TLR5 and TLR7 amplify different stage of myeloid cells

Abstract

Abstract Balance between immune activation and immune suppression is important for immune homeostasis. Toll-like receptors (TLRs) play key roles in the innate immune system, initiating inflammatory responses against pathogen infections and internal danger signals. Different TLRs recognize different kinds of ligands, but not all the TLRs do have same effect: some TLRs are immune stimulatory, while the others are immune suppressive. For instance, TLR5 ligand has been known to be immune suppressive, while TLR7 ligand has been used for immune activation. To understand how these TLR ligands lead to opposite immune outcomes, we investigated the effects of the ligands on myeloid cell differentiation. Treatment of TLR5 ligand flagellin resulted in the increase of CD11b+ Ly6C+Ly6G+ myeloid-derived suppressor cells (MDSCs) on days 2 and 3 after culture initiation. However, TLR7 ligand gardiquimod led to enhancement of dendritic cell differentiation on day 1, and then increase of CD11c− MHCII− CD11b− Ly6C− Ly6G− progenitor population on day 2, which differentiate into CD11c− MHCIIow CD11b+ Ly6C+ cells with DC differentiation potential afterwards. Based on these results, we suggest that different immune outcomes of TLR ligands would depend on the stages of myeloid cells at which corresponding TLRs act, contributing to overall immune homeostasis.