Introduction: Macrophages are heterogeneous and thus can be correlated with distinct tissue outcomes after injury. Conflicting data have indicated that the M2-related phenotype directly triggers fibrosis. Conversely, we hypothesize here that the inflammatory milieu provided by early infiltration of pro-inflammatory macrophages dictates tissue scarring after injury.
Methods and results: We first determined that tissue-localized macrophages exhibit a pro-inflammatory phenotype (p40IL12(+)CCR7(+)CD11b(+)) during the early phase of a chronic injury model, in contrast to a pro-resolving phenotype (Arg1(+)IL10(+)CD206(+)CD11b(+)) at a later stage. Then, we evaluated the effects of injecting macrophages differentiated in vitro in the presence of IFNγ + LPS or IL4 + IL13 or non-differentiated macrophages (hereafter, M0) on promoting inflammation and progression of chronic injury in macrophage-depleted mice. In addition to enhancing the expression of pro-inflammatory cytokines, the injection of M (IFNγ + LPS), but not M (IL4 + IL13) or M0, accentuated fibrosis while augmenting levels of anti-inflammatory molecules, increasing collagen deposition and impairing organ function. We observed a similar profile after injection of sorted CCR7(+)CD11b(+) cells and a more pronounced effect of M (IFNγ + LPS) cells originated from Stat6(-/-) mice. The injection of M (IFNγ + LPS) cells was associated with the up-regulation of inflammation- and fibrosis-related proteins (Thbs1, Mmp7, Mmp8, and Mmp13).
Conclusions: Our results suggest that pro-inflammatory macrophages promote microenvironmental changes that may lead to fibrogenesis by inducing an inflammatory milieu that alters a network of extracellular-related genes, culminating in tissue fibrosis.
Keywords: Collagen; pro‐inflammatory macrophages; renal fibrosis.