Transcriptional switching in macrophages associated with the peritoneal foreign body response

We previously demonstrated that myeloid cells are the source of fibrotic tissue induced by foreign material implanted in the peritoneal cavity. This study utilised the MacGreen mouse, in which the Csf1r promoter directs myeloid-specific enhanced green fluorescent protein (EGFP) expression, to determine the temporal gene expression profile of myeloid subpopulations recruited to the peritoneal cavity to encapsulate implanted foreign material (cubes of boiled egg white). Cells with high EGFP expression (EGFP(hi)) were purified from exudate and encapsulating tissue at different times during the foreign body response, gene expression profiles determined using cDNA microarrays, and data clustered using the network analysis tool, Biolayout Express(3D). EGFP(hi) cells from all time points expressed high levels of Csf1r, Emr1 (encoding F4/80), Cd14 and Itgam (encoding Mac-1) providing internal validation of their myeloid nature. Exudate macrophages (days 4-7) expressed a large cluster of cell cycle genes; these were switched off in capsule cells. Early in capsule formation, Csf1r-EGFP(hi) cells expressed genes associated with tissue turnover, but later expressed both pro- and anti-inflammatory genes alongside a subset of mesenchyme-associated genes, a pattern of gene expression that adds weight to the concept of a continuum of macrophage phenotypes rather than distinct M1/M2 subsets. Moreover, rather than transdifferentiating to myofibroblasts, macrophages contributing to later stages of the peritoneal foreign body response warrant their own classification as 'fibroblastoid' macrophages.