Chronic inflammation is associated with up-regulation of the transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-?B) and excessive inflammatory cytokine secretion by M1 macrophages. The anti-inflammatory cytokine interleukin (IL)-4 converts pro-inflammatory M1 macrophages into an anti-inflammatory and tissue-regenerative M2 phenotype, thus reducing inflammation and enhancing tissue regeneration. We have generated NF-?B responsive, or constitutively active IL-4 expression lentiviral vectors transduced into murine bone marrow-derived mesenchymal stromal cells (MSCs). MSCs with a constitutively active IL-4 expression vector produced large quantities of IL-4 continuously, whereas IL-4 secretion was significantly induced by lipopolysaccharide (LPS) in the NF-?B sensing MSCs. In contrast, LPS had no effect on MSCs with IL-4 secretion driven by a constitutively active promoter. We also found that intermittent and continuous LPS treatment displayed distinct NF-?B activation profiles, and this regulation was independent of IL-4 signaling. The supernatant containing IL-4 from the LPS-treated MSCs suppressed M1 marker (inducible nitric oxide synthase [iNOS] and tumor necrosis factor alpha [TNFa]) expression and enhanced M2 marker (Arginase 1, CD206 and IL1 receptor antagonist [IL1Ra]) expression in primary murine macrophages. The IL-4 secretion at the basal, non-LPS induced level was sufficient to suppress TNFa and enhance Arginase 1 at a lower level, but had no significant effects on iNOS, CD206 and IL1Ra expression. Finally, IL-4 secretion at basal or LPS-induced levels significantly suppressed osteogenic differentiation of MSCs. Our findings suggest that the IL-4 secreting MSCs driven by NF-?B sensing or constitutive active promoter have great potential for mitigating the effects of chronic inflammation and promoting earlier tissue regeneration.
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