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Myeloid cells that express arginase 1 are upregulated by different stimuli, including trauma, and are capable of depleting arginine from the surrounding environment. Through arginine depletion, myeloid cells are capable of regulating T-cell function. We have previously reported increased arginase 1 expression in the peripheral blood mononuclear cells (PBMCs) after injury. The nature of the cells expressing arginase in humans after trauma is unknown and is the focus of this article.PBMCs were isolated using a Ficoll-Hypaque gradient. Arginase activity was measured by conversion of arginine to ornithine, and arginase 1 protein expression was measured by Western blot. The percent CD16+ granulocytes and phenotypical analysis of the cells present in PBMCs were determined by flow cytometry. Magnetic microbeads were used for isolation and exclusion of specific cell subpopulations.Trauma patients exhibited a dramatic increase in arginase activity (p < 0.05) and an increased percentage of CD16+ granulocytes in the PBMC layer (p < 0.05) compared with control volunteers. Increased arginase activity in the PBMC layer was due to the contamination of this layer by granulocytes, as their exclusion decreased arginase activity back to baseline (p < 0.05). Granulocytes isolated from the PBMC layer expressed increased CD11b (p < 0.05) and CD66b (p < 0.05), markers of granulocyte activation. Furthermore, these granulocytes were significantly more swollen and degranulated compared with noncontaminating granulocytes.In humans, increased arginase 1 expression after trauma observed in the PBMC layer seems to be exclusively the result of an increased number of activated granulocytes.