The Relationship Between CSF1R Signaling, Monocyte-Macrophage Differentiation, and Susceptibility to Inflammatory Bowel Disease

More than 300 genomic loci have been associated with increased susceptibility to inflammatory bowel disease (IBD) through genome-wide association studies. A major challenge in the translation of genome-wide association studies to mechanistic insights lies in connecting noncoding variants to function. For example, single-nucleotide variants (SNVs) in the vicinity of the gene encoding the transcription factor ETS2 on human chromosome 21 are associated with the risk of developing IBD in Europeans. The peak of SNV association lies within a distal enhancer that may regulate ETS2 transcription. The interpretation of this and many other SNV associations with IBD depends on a model linking variation in transcriptional regulation to the likelihood of developing chronic intestinal inflammation. One model for the ETS2 locus is that overexpression in monocytes is causally associated with the risk allele, which in turn leads to a hyperinflammatory state. Here we summarize evidence for an alternative mechanism focused on negative regulators of monocyte-macrophage activation. We argue that IBD susceptibility arises from dysregulation of monocyte adaptation in the intestinal milieu to form resident intestinal macrophages that are anergic to inflammatory stimuli. This process depends on signals initiated by macrophage colony–stimulating factor (CSF1) binding to its receptor (CSF1R). Within this framework, ETS2 is a myeloid-specific transcription factor, expressed in pluripotent and committed progenitors and monocytes, and is down-regulated by CSF1, in common with many genes associated with IBD susceptibility, including NOD2. ETS2 is also both a downstream target and a mediator of the CSF1/CSF1R signaling pathway. Therapeutic targeting of ETS2 and its upstream regulators has the potential to prevent CSF1-dependent monocyte differentiation toward a prorepair resident macrophage phenotype and consequently exacerbate intestinal inflammation.