Calprotectin: The Link Between Acute Lung Injury and Gastrointestinal Injury in Covid-19: Ban or Boon

The pathogenesis of SARS-CoV-2 infection is related to the direct cytopathic effect and associated hyper-inflammation due to exaggerated immune response. Different experimental and clinical studies revealed that many biomarkers could be used to determine the Covid-19 severity, such as D-dimer, procalcitonin, C-reaction protein (CRP), IL-6, and ferritin. Calprotectin (CP) is associated with intestinal inflammation, intestinal injury, and different respiratory diseases such as cystic fibrosis. Thus, CP might be a possible biomarker linking intestinal injury and acute lung injury (ALI) in Covid-19. Therefore, this study aimed to find a potential role of CP regarding GITI and ALI in Covid-19. CP is a complex protein consisting of S100A8 and S100A9, belonging to the Ca+2-binding proteins S100 family abundant in the cytosol of neutrophils and expressed on the monocyte membranes, macrophages, and intestinal epithelial cells. CP is a proinflammatory protein that acts through activation of the receptor for the advanced glycation end product (RAGE) and toll-like receptor 4 (TLR4). CP is a biomarker of neutrophil activation and is released following the turnover of neutrophils. CP could be controversial; it increases airway inflammation or protects lung and airway epithelium from an exaggerated immune response. Therefore, a high level of CP in different respiratory disorders might be protective and compensate against abnormal immune responses. CP level is high in Covid-19 and correlated with Covid-19 severity and oxygen demand due to activation of proinflammatory cytokines and inflammatory signaling pathways. Therefore, CP level is elevated in both ALI and intestinal inflammation so that it could be a potential biomarker that links the respiratory and intestinal injury in Covid-19.