Conclusion
Our findings suggest that Ly6C+ monocytes and mtDNA-induced Ly6C+ monocyte PGE2 production may be part of a feedback mechanism that contributes to the resolution of cationic nanocarrier-induced inflammatory toxicity and may have important implications for understanding nanoparticle biocompatibility and designing better, safer drug delivery systems.
Methods
C57BL/6 mice were intravenously injected with a dosage of 1-100 mg/kg cationic DOTAP liposomes as well as other cationic materials. Cell necrosis was detected by flow cytometry. Release of mitochondrial DNA was quantified by qPCR via Taqman probes. Signal proteins were detected by Western blotting. PGE2 production in the supernatant was quantitated using an enzyme immunoassay (EIA). The infiltrated inflammatory cells were observed in WT mice, Ccr2-/- mice, Sting-/- mice and Tlr9-/- mice.
Results
The early stage (24-48 h) inflammatory neutrophil infiltration was followed by an increasing percentage of monocytes; and, compared with WT mice, Ccr2-/- mice presented with more severe pulmonary inflammation. A previously uncharacterized population of regulatory monocytes expressing both inflammatory and immunosuppressive cytokines was identified in this model. The alteration in monocyte phenotype was directly induced by mtDNA release from cationic nanocarrier-induced necrotic cells via a STING- or TLR9-dependent pathway. Neutrophil activation was specifically inhibited by PGE2 from Ly6C+ inflammatory monocytes, and intravenous injections of dual-phenotype monocytes beneficially modified the immune response; this inhibitory effect was abolished after treatment with indomethacin. Moreover, we provide clear evidence that mitochondrial DNA activated Ly6C+ monocytes and increased PGE2 production through TLR9- or STING-mediated MAPK-NF-κB-COX2 pathways.