Activation-induced mobilization of secretory vesicles in bovine neutrophils

Objective-To characterize mobilization of secretory granules in bovine neut rophils. Sample Population-Neutrophils obtained from four 6- to 18-month-old Holstei n cattle. Procedure-Mobilization of secretory granules in bovine neutrophils was dete rmined by measuring changes in cell-surface alkaline phosphatase activity o n cells treated with various inflammatory mediators. Subcellular distributi on of the alkaline phosphatase activity was determined by analysis of bovin e neutrophil homogenates fractionated on density gradients. Results-Alkaline phosphatase-containing secretory granules of bovine neutro phils were readily mobilized by a number of inflammatory agents, including platelet-activating factor, interleukin-8, tumor necrosis factor-alpha, lip opolysaccharide, leukotriene B-4, and zymosan-activated plasma. In contrast , N-formyl-methionyl-leucyl-phenylalanine did not have a significant effect . Phorbol myristate acetate induced a biphasic response with up-regulation of cell-surface alkaline phosphatase at low doses and a return to baseline or even a reduction in cell-surface alkaline phosphatase at higher doses (g reater than or equal to 10 ng/ml). Subcellular fractionation of bovine neut rophil homogenates revealed that alkaline phosphatase activity resided in l ight-density membrane vesicles (ie, location of secretory granules), which were distinct from specific, azurophil, and large granules. Conclusions and Clinical Relevance Bovine neutrophils respond to various in flammatory mediators by mobilizing alkaline phosphatase-containing secretor y granules. This suggests that the process is an important early step in th e host-defense response of bovine neutrophils.