Tumor necrosis factor: a mechanistic link between angiotensin-II-induced cardiac inflammation and fibrosis

Continuous angiotensin-II infusion induced the uptake of monocytic fibroblast precursors that initiated the development of cardiac fibrosis; these cells and concurrent fibrosis were absent in mice lacking tumor necrosis factor receptor 1 (TNFR1). We now investigated their cellular origin and temporal uptake and the involvement of TNFR1 in monocyte-to-fibroblast differentiation.

Within a day, angiotensin-II induced a proinflammatory environment characterized by production of inflammatory chemokines, cytokines, and TH1-interleukins and uptake of bone marrow-derived M1 cells. After a week, the cardiac environment changed to profibrotic with growth factor and TH2-interleukin synthesis, uptake of bone marrow-derived M2 cells, and the presence of M2-related fibroblasts. TNFR1 signaling was not necessary for early M1 uptake, but its absence diminished the amount of M2 cells. TNFR1-knockout hearts also showed reduced levels of cytokine expression, but not of TH-related lymphokines. Reconstitution of wild-type bone marrow into TNFR1-knockout mice was sufficient to restore M2 uptake, upregulation of proinflammatory and profibrotic genes, and development of fibrosis in response to angiotensin-II. We also developed an in vitro mouse monocyte-to-fibroblast maturation assay that confirmed the essential role of TNFR1 in the sequential progression of monocyte activation and fibroblast formation.

Development of cardiac fibrosis in response to angiotensin-II was mediated by myeloid precursors and consisted of 2 stages. A primary M1 inflammatory response was followed by a subsequent M2 fibrotic response. Although the first phase seemed to be independent of TNFR1 signaling, the later phase (and development of fibrosis) was abrogated by deletion of TNFR1.