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.

Cellular plasticity of inflammatory myeloid cells in the peritoneal foreign body response.

Implantation of sterile foreign objects in the peritoneal cavity of an animal initiates an inflammatory response and results in encapsulation of the objects by bone marrow-derived cells. Over time, a multilayered tissue capsule develops with abundant myofibroblasts embedded in extracellular matrix. The present study used the transgenic MacGreen mouse to characterize the time-dependent accumulation of monocyte subsets and neutrophilic granulocytes in the inflammatory infiltrate and within the tissue capsule by their differential expression of the csf1r-EGFP transgene, F4/80, and Ly6C. As the tissue capsule developed, enhanced green fluorescent protein-positive cells changed from rounded to spindle-shaped morphology and began to co-express the myofibroblast marker alpha-smooth muscle actin. Expression increased with time: at day 14, 11.13 +/- 0.67% of tissue capsule cells co-expressed these markers, compared with 50.77 +/- 12.85% of cells at day 28. The importance of monocyte/macrophages in tissue capsule development was confirmed by clodronate-encapsulated liposome removal, which resulted in almost complete abrogation of capsule development. These results confirm the importance of monocyte/macrophages in the tissue response to sterile foreign objects implanted in the peritoneal cavity. In addition, the in vivo plasticity of peritoneal macrophages and their ability to transdifferentiate from a myeloid to mesenchymal phenotype is demonstrated.