Innate immune memory and homeostasis may be conferred through crosstalk between the TLR3 and TLR7 pathways

Mathematical modeling uncovers how macrophages use the JAK-STAT pathway to fine-tune their responses to different Toll-like receptor stimuli. Modeling immune responses During the course of an infection, the host is often exposed to numerous pathogen-derived stimuli at different times. Macrophages are activated through Toll-like receptor 3 (TLR3) by double-stranded viral RNA, whereas single-stranded viral RNA stimulates TLR7. Stimulation of TLR3 before TLR7 leads to the synergistically increased production of cytokines as part of the immune response. Liu et al. analyzed cytokine mRNA abundances in macrophages exposed to TLR3 and TLR7 stimuli in different orders and after different time intervals. Mathematical modeling of these data predicted and experiments validated that crosstalk occurred between both pathways and showed how the JAK-STAT pathway was required for TLR3 signaling to induce synergistic responses to subsequent TLR7 stimulation. This computational modeling approach may be applied to study other immune signaling pathways to uncover potential crosstalk mechanisms. Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns (PAMPs) and stimulate the innate immune response through the production of cytokines. The innate immune response depends on the timing of encountering PAMPs, suggesting a short-term “memory.” In particular, activation of TLR3 appears to prime macrophages for the subsequent activation of TLR7, which leads to synergistically increased production of cytokines. By developing a calibrated mathematical model for the kinetics of TLR3 and TLR7 pathway crosstalk and providing experimental validation, we demonstrated the involvement of the Janus-activated kinase (JAK)–signal transducer and activator of transcription (STAT) pathway in controlling the synergistic production of cytokines. Signaling through this pathway played a dual role: It mediated the synergistic production of cytokines, thus boosting the immune response, and it also maintained homeostasis to avoid an excessive inflammatory response. Thus, we propose that the JAK-STAT pathway provides a cytokine rheostat mechanism, which enables macrophages to fine-tune their responses to multiple, temporally separated infection events involving the TLR3 and TLR7 pathways.