Background: We previously showed in several models of cardiac hypertrophy and failure that, in the absence of cell death, cardiac interstitial fibrosis was mediated by the uptake of monocyte-derived fibroblast precursor cells; fibrosis and concurrent cardiac remodeling were blocked by deletion of monocyte chemoattractant protein-1 (MCP-1), as well as of tumor necrosis factor receptor-1 (TNFR1). We now investigated the cellular origin, kinetics of uptake, and subpopulation of fibroblast precursors in the angiotensin-II (Ang-II)-challenged heart.
Methods: Mice with genetic deletion of TNFR1 (TNFR1-KO mice) were irradiated and rescued with bone marrow from wild-type (WT) mice, and were subjected to continuous infusion of Ang-II for 7 days. Flow cytometry was performed on isolated cells, immunostaining on perfusion-fixed tissue, quantitative PCR on whole heart mRNA isolations.
Results: In WT mice, Ang-II induced the early uptake of monocytes which became M1 macrophages (CD86+CD45+) in an M1/Th1 environment. Monocytes entering the heart after 2-3 days polarized to M2 macrophages (CD301+CD45+) in an M2/Th2 milieu with concurrent appearance of collagen-producing CD301+ fibroblasts. M1 cells produced TNF, whereas M2 cells did not; both expressed TNFR1. Ang-II-exposed TNFR1-KO hearts showed similar cardiac infiltration of M1 cells, but the amount of M2 cells was significantly lower. They also had reduced expression of M1 and M2 cytokines, but not of Th1 and Th2 interleukins. Transplantation of WT bone marrow to TNFR1-KO mice before Ang-II exposure restored cardiac fibrosis, uptake of M2 cells, and expression of cytokines to levels observed in Ang-II-exposed WT animals. Many TNFR1+ cells in chimeric TNFR1-KO/WT were committed to the fibroblast lineage.
Conclusion: Our data show that monocytic fibroblast precursor cells originated in the bone marrow, and that signaling through TNFR1 was required for their uptake and maturation into collagen-producing M2/fibroblasts that mediated the development of cardiac fibrosis in response to Ang-II. They also suggest a mechanistic link between inflammation and fibrosis, i.e. pro-inflammatory, M1-produced TNF initiates pro-fibrotic M2/fibroblast maturation via TNFR1 signaling.