TLR7/8 signaling balances cytokine responses in neonatal monocytes

Newborns are particularly vulnerable to nosocomial infection, but the underlying mechanisms are incompletely understood. Age-dependent differences in the expression of innate immune receptors and thus a reduced or delayed antimicrobial host response might contribute. Here we discovered that neonatal cord blood monocytes (CBMo) express lower levels of Toll-like receptor TLR8 but similar levels of TLR7 as compared to adult peripheral blood monocytes (PBMo). We investigated the TLR7/8 signal transduction by stimulating monocytes of adults (PBMo) and neonates (CBMo) with imiquimod (R₈₃₇) and resiquimod (R₈₄₈). Toll-like receptors (TLRs) bind various constituents of invading pathogens and trigger defense reactions. We found differential expressions of TLR7 and TLR8 which both bind to ssRNAs. We could show, that inflammatory cytokine response represented by TNF-α is comparable in PBMo and CBMo. This effect was stronger in resiiquimod (R₈₄₈) treated PBMo and CBMo. Whereas IL-6 secretion was found similar induced in PBMo and CBMo after stimulation with TLR7/8 ligands, IL-10 was only induced in PBMo, biasing the cytokine answer to the pro-inflammatory response in CBMo. Cytokine response was more dependent to NFκB signaling although MAP kinase activation was required for the IL-6 response in CBMo. The type 1 interferon response was found comparable in PBMo and CBMo and was correlated to IRF-7 activation. We utilized an in vitro model of gram-negative bacterial (E. coli) infection to study whether TLR7/8 pre-stimulation influence peri-phagocytic reactions. TLR7/8 pre-stimulation modified the E. coli PAMP receptor CD14 and CD32 expression. IL-6 and IL-10 secretion was not altered by TLR7/8 stimulation prior to infection. TLR7 pre-stimulation reduced E. coli survival in CBMo, but was not accompanied by higher TNF-α levels. Administration of imiquimod (R₈₃₇) displays therefore beneficial aspects for opportunistic bacterial infections in-vitro.