Induction of endogenous IL-10 promotes resolution and tolerance of nitric oxide in microglia

Endogenous interleukin-10 (IL-10), a potent anti-inflammatory cytokine, is induced in a timely and coordinated manner to dampen microglia-mediated brain inflammation. However, it remains unclear how it alters the inflammatory process to shape the immune polarization of microglia. This study aimed to investigate the anti-inflammatory mechanisms of endogenous IL-10 in activated and tolerized microglia using in vitro multiple-reconstituted primary brain cell cultures and an in vivo IL-10 knockout (IL-10KO) animal model. Upon a single or repeated lipopolysaccharide (LPS) treatment regimen, the expression levels of the inflammatory factors during the neuroinflammatory/tolerance process were measured by quantitative real-time polymerase chain reaction, enzyme-linked immunosorbent assay (ELISA), and Griess reagent assay. ELISA data showed that cell-autonomous induction of endogenous IL-10 occurs in LPS-activated and LPS-tolerized microglia. Furthermore, comparing the LPS-elicited pro-inflammatory factor expressions at different neuroinflammatory stages between the wild-type and IL-10KO groups, our data revealed the failure of negative-feedback suppression of inducible nitric oxide synthesis (iNOS) during immune resolution in the IL-10KO brains. Moreover, LPS-treated IL-10KO microglia increase the supernatant level of nitrite and become overactive during late-stage inflammation, despite no changes in cell number; in contrast, LPS-tolerized IL-10KO microglia fail to program endotoxin tolerance of nitric oxide/inducible nitric oxide synthesis (iNOS). In summary, our data demonstrate that the cell-autonomous induction of endogenous IL-10 in microglia is crucial for mitigating brain immune responses, particularly in the resolution and tolerance of nitric oxide.