The Role of Neutrophil, Monocyte and Macrophage Calprotectin and S100A12 in the Fibrotic Process

Fibrosis is a progressive and irreversible mechanism affecting any organ. During tissue injury, fibroblast activation is necessary for wound healing but the uncontrolled accumulation of fibrotic tissue leads to local organ damage. The fibrotic process involves the excessive accumulation of extracellular matrix components and inflammatory mediators. Since sustained inflammation precedes fibrosis, the involvement of immune cells, like neutrophils, monocytes and macrophages, is crucial to elucidate its pathogenesis. These immune cells release proinflammatory cytokines and chemokines, and also proteins that act as fibroblast proliferation mediators, such as the S100/calgranulins subgroup, comprising S100A8, S100A9, and S100A12 proteins. Moreover, a homodimer of S100A8 binds to a homodimer of S100A9 forming the heterodimer S100A8/A9, called calprotectin, which is abundant in the cytosol of neutrophils during immune activation. Although calprotectin (S100A8/A9) is the most predominant form, calgranulins S100A8 and S100A9 have independent functions of calprotectin (S100A8/9) complex formation. These calcium-binding proteins have proinflammatory functions and are potential inflammation biomarkers. More evidence in different fibrosis disorders highlights their role as relevant fibroblast proliferation mediators and prognosis markers. Hence, this review focuses on the current understanding of the role of S100A8, S100A9, and S100A12 calgranulins and calprotectin (S100A8/A9) in the fibrotic process of different disorders, and their potential application as disease severity and prognosis biomarkers.