Figure 2.

Neutrophil recruitment in the lung. Unlike most organs, in the lung neutrophils are sequestered in the capillaries, instead of post-venules. (1a) In the capillaries, neutrophils are activated by platelets releasing chemokines and the recruitment is promoted by endothelial stress. Due to the diameter of the capillaries, neutrophils are subjected to mechanical entrapment and the involvement of selectins for the recruitment process is not always occurring. The involvement of selectins and integrins is dependent on the inflammatory stimulus. (2a) For LPS-treated mice neutrophil recruitment is selectin and integrin dependent. Integrin activation occurs as described in the classical recruitment cascade. (1b) However, in mice treated with S. pneumoniae neutrophil recruitment was shown to be selectin independent. And recruitment was described as integrin-independent in mice administered with E. coli. In any case, L-selectin and LFA-1 can keep neutrophils within the capillary for several minutes, supporting the cell transmigration. (3) Neutrophil recruitment proceeds with transmigration to the interstitium or to the alveolar space. (4) In the alveolar space, alveolar macrophages and EpiCs are essential for guiding the neutrophil by the secretion of inflammatory mediators (e.g., cytokines and chemokine's). E. coli, Escherichia coli; EC, Endothelial cell; EpiC, Epithelial cell; GPCR, G protein-coupled receptor; ICAM, Intracellular adhesion molecule; LFA-1, Lymphocyte function-associated antigen 1; LPS, Lipopolysaccharide; Mac-1, Macrophage-1 antigen; PSGL-1, P-selectin glycoprotein ligand-1; S pneumoniae, Streptococcus pneumoniae.