p38 mitogen-activated kinase (MAPK) is essential for equine neutrophil migration

Equine laminar tissues do not contain resident neutrophils and have less superoxide dismutase (SOD) activity than other equine tissues, which makes them inherently more vulnerable to damage induced by reactive oxygen species (ROS) produced by neutrophils that enter the tissues. In the advanced clinical stages of acute laminitis, pathologic events in affected feet include a breakdown in the basement membrane, neutrophil infiltration, and platelet-neutrophil aggregates in laminar dermal veins, highlighting the contribution of neutrophils to the pathophysiology of the disease. The aim of this study was to determine the role of p38 MAPK in the mechanism underlying equine neutrophil migration to potentially reveal therapeutic targets that may limit lamellar damage from the neutrophil influx that occurs in acute laminitis. We determined that the endogenous chemoattractant LTB4 transiently activated p38 MAPK and induced chemotaxis of equine primary neutrophils. Inhibition with the p38 MAPK specific inhibitor SB203580 reduced LTB4-induced migration in a dose-dependent manner with an IC50 of 2.8 μM. We then examined the potential mechanisms underlying the ability of SB203580 to abolish migration. We determined that inhibition of p38 MAPK with 10 μM SB203580 disrupted the ability of neutrophils to polarize in response to LTB4 and PAF. In contrast, p38 MAPK did not appear to be required for chemoattractant- or PKC-induced β2 integrin-dependent adhesion or chemoattractant-induced upregulation of surface β2 integrins, but was required for TNFα-induced adhesion. These findings support a function for p38 MAPK in equine neutrophil migration and suggest the potential for the ability of p38 MAPK inhibition to limit neutrophilic inflammation in the laminae during acute laminitis.