Regulation of macrophage activation and polarization by interferon-gamma

Interferon-gamma (IFN-gamma) is a potent activator of macrophages capable of synergizing with pro-inflammatory pathways to enhance macrophage response to activating stimuli. Our laboratory has demonstrated that a major mechanism by which IFN-gamma primes macrophages for enhanced immune responses is through suppression of negative feedback pathways, and this dissertation further explored that concept. Using microarray analysis to identify novel IFN-gamma-suppressed genes in human monocytes, we discovered an M2 macrophage genetic program that was strikingly upregulated during early monocyte differentiation and potently suppressed by IFN-gamma. M2 gene expression was largely dependent on cell adhesion and PI3K activity, two pathways that are inhibited by IFN-gamma. We also identified a novel role for the IFN-gamma-suppressed transcription factor c-Maf in regulating a subset of the M2 genetic program. Diversion of monocyte polarization away from the anti-inflammatory and homeostatic M2 phenotype represents a mechanism by which IFN-gamma promotes more pro-inflammatory responses.;One of the most global manifestations of feedback inhibition is endotoxin tolerance, a phenomenon whereby previous exposure of cells or organisms to microbial products induces a transient period of hyporesponsiveness upon subsequent endotoxin challenge and is characterized by diminished release of pro-inflammatory cytokines. Previous reports have demonstrated that IFN-gamma can inhibit endotoxin tolerance, but the underlying mechanisms remained unclear. We found that IFN-gamma efficiently blocked the tolerization of IL-6 and TNFalpha induced by LPS pre-treatment. Unexpectedly, the ability of IFN-gamma to block tolerance was not associated with a reversal of upstream signaling defects in tolerized monocytes. Instead, regulation by IFN-gamma occurred at the level of gene expression, as IFN-gamma restored 1L6 and TNF transcription and binding of transcriptional machinery to the promoters of these genes in tolerized monocytes. Furthermore, IFN-gamma restored the recruitment of chromatin remodeling complexes to the 1L6 promoter, and restriction enzyme accessibility assays confirmed that IFN-gamma completely restored 1L6 promoter accessibility in tolerized monocytes. Our results suggest that IFN-gamma overcomes endotoxin tolerance by facilitating TLR-induced chromatin remodeling to allow continued expression of pro-inflammatory genes. Overall, this dissertation underscores the concept that suppression of negative feedback and anti-inflammatory pathways is a major mode by which IFN-gamma primes macrophages for enhanced innate immune responses.