Targeting neutrophils in ischemic stroke: translational insights from experimental studies

Abstract

Neutrophils have key roles in ischemic brain injury, thrombosis, and atherosclerosis. As such, neutrophils are of great interest as targets to treat and prevent ischemic stroke. After stroke, neutrophils respond rapidly promoting blood-brain barrier disruption, cerebral edema, and brain injury. A surge of neutrophil-derived reactive oxygen species, proteases, and cytokines are released as neutrophils interact with cerebral endothelium. Neutrophils also are linked to the major processes that cause ischemic stroke, thrombosis, and atherosclerosis. Thrombosis is promoted through interactions with platelets, clotting factors, and release of prothrombotic molecules. In atherosclerosis, neutrophils promote plaque formation and rupture by generating oxidized-low density lipoprotein, enhancing monocyte infiltration, and degrading the fibrous cap. In experimental studies targeting neutrophils can improve stroke. However, early human studies have been met with challenges, and suggest that selective targeting of neutrophils may be required. Several properties of neutrophil are beneficial and thus may important to preserve in patients with stroke including antimicrobial, antiinflammatory, and neuroprotective functions.

Figure 1

Neutrophils activation and adhesion in acute ischemic stroke. After ischemic brain injury, a number of cytokines and DAMPs are released. These promote neutrophil recruitment and activation, including the release of reactive oxygen species, proteases, and cytokines. Neutrophils adhere to activated endothelium through adhesion molecules that promote neutrophil–endothelial interactions and neutrophil migration with resulting effects on the blood–brain barrier and brain parenchyma. CCL, CC-chemokine ligand; CXCL, CXC-chemokine ligand; CCR, CC-chemokine receptor; CXCR, CXC chemokine receptor; DAMPs, damage-associated molecular patterns; IL, interleukin; ILR, interleukin receptor; FPR, formyl peptide receptor; HMGB1, high mobility group box 1; HSP72, heat-shock protein 72; ICAM-1, intracellular adhesion molecule-1; MAC-1, macrophage 1 antigen; MMP-9, matrix metalloproteinase 9; PSGL-1, P-selectin glycoprotein ligand-1; TLR, toll-like receptor; TNFα, tumor necrosis factor alpha.

Figure 2

Change in neutrophil and lymphocyte count in blood over time after ischemic stroke.

Figure 3

Neutrophil recruitment, adhesion, and transmigration. Neutrophils express specific adhesion molecules that bind activated endothelium to promote adhesion and migration. In ischemic stroke, the adhesion molecules expressed on neutrophils and endothelium involved in neutrophil tethering, rolling, arrest, adhesion, crawling, and transmigration remain poorly defined and may differ from the peripheral vasculature. Further studies are required to delineate the molecules involved in the neutrophil recruitment cascade after ischemic stroke and to determine the extent of transmigration. ‘?' indicates unclear role in ischemic stroke; ICAM-1, intracellular adhesion molecule-1; LFA-1, lymphocyte function-associated antigen 1; MAC-1, macrophage 1 antigen; PSGL-1, P-selectin glycoprotein ligand-1.

Figure 4

Change in matrix metalloproteinase 9 (MMP-9) mRNA in peripheral leukocytes over the first 24 hours in patients with ischemic stroke. (*P=0.02, **P=9.9 × 10-5, ***P=8.6 × 10-8)

Figure 5

Role of neutrophils in thrombus formation. Neutrophils promote thrombosis through interactions with platelets, proteolytic cleavage of clotting factors (TFPI, coagulation factor X), and release of prothrombotic molecules (NETs and tissue factor). ICAM-1, intracellular adhesion molecule-1; MAC-1, macrophage 1 antigen; NET, neutrophil extracellular traps; PSGL-1, P-selectin glycoprotein ligand-1; TF, tissue factor; TFPI, tissue factor pathway inhibitor; vWF, von Willebrand factor.

Figure 6

Role of neutrophils in atherosclerosis. (A) Neutrophils promote the formation of atherosclerosis though interactions with platelets and by enhancing monocyte infiltration into damaged endothelium. (B) Neutrophils promote atherosclerotic plaque progression and rupture via the release of cytokines, reactive oxygen species that activated macrophage foam cells, producing oxidized-LDL, and proteolytic degradation of the fibrous cap. BM, basement membrane; CCL, CC-chemokine ligand; CRAMP, cathelicidin antimicrobial peptide; ECM, extracellular matrix; FPR, formyl peptic receptor; ICAM-1, intracellular adhesion molecule-1; IL, interleukin; INF-γ, interferon gamma; LL37, Cathelicidin; MAC-1, macrophage 1 antigen; MMP, matrix metalloproteinase; oxLDL, oxidized low density lipoprotein; PSGL-1, P-selectin glycoprotein ligand-1; TLR, toll-like receptor; TNFα, tumor necrosis factor alpha.