CXCR4 and acute respiratory distress syndrome: Although these data are consistent with the hypothesis that the CXCR4-CXCL12 axis mediates recruitment of fibrocytes to the lungs during ARDS, several other chemokine (Moore et al. 2005; Moore et al. 2006; Ishida et al. 2017; Ishida et al. 2007) and non-chemokine (Wang et al. 2015; García-de-Alba et al. 2010; Katebi et al. 2008; Aono et al. 2014) systems can mediate fibrocyte migration, and these mechanisms may operate in parallel or in series.