Functional Characterization of Signaling Events Underlying Immunostimulatory and Immunosuppressive Activity of Macrophages.

Macrophages (Mo) play an important role in combating infectious pathogens and regulating adaptive immune responses. Their reactivity is, among others, guided by pattern recognition receptors including Toll like receptor 4 (TLR4), which recognizes bacterial lipopolysaccharide (LPS). After stimulation, TLR4 can associate with different adaptor proteins like e.g. MyD88 or TRIF leading to the activation of signaling molecules like MAPK or NF-κB. However, stimulation with LPS may result in equally impressive immunostimulatory and immunosuppressive Mo responses. The signaling events responsible for this diametrical behavior of different Mo subsets are yet not fully understood. To study the effects of LPS on Mo we differentiated monocytes of healthy donors with GM-CSF and M-CSF into pro-inflammatory type 1 (Mo1) or anti-inflammatory type 2 (Mo2) subsets, respectively. As expected, upon LPS stimulation the pro-inflammatory Mo1 secreted low levels of the immunosuppressive cytokine IL-10 and high levels of IL-12, while the anti-inflammatory Mo2 produced neglectable amounts of IL-12 but high levels of IL-10. To elucidate the underlying molecular mechanisms we performed gene array analyses with Mo1 compared to Mo2 from three independent donors. Interestingly, among over 50 genes involved in TLR4 signaling, only TLR4 itself and its adaptor protein MyD88 were found to be differentially regulated: Within the immunosuppressive Mo2 subset, MyD88 was significantly downregulated, while a significant upregulation of TLR4 was observed compared to the immunostimulatory Mo1 subset. The differential expression pattern were further confirmed on protein level by western blot. Since TLR4 stimulation leads to MyD88-dependent activation of the MAPK JNK, we investigated whether JNK was differentially involved in cytokine production of Mo1 and Mo2. Addition of a specific JNK inhibitor (SB600125) concentration dependently reduced TLR4-induced IL-10 but enhanced IL-12 production in Mo1, pointing to an immunoinhibitory role of JNK in Mo1. In contrast, inhibition of JNK did not alter the secreted levels of IL-12 or IL-10 in Mo2, indicating that rather MyD88-independent pathways are responsible for the anti-inflammatory behavior of Mo2. In conclusion, our data suggest that the opposite regulation of the adaptor molecule MyD88 in pro- and anti-inflammatory Mo subsets is responsible for the fact that a single, non-polymorphic receptor like TLR4 can mediate both pro- and anti-inflammatory immune responses in Mo.