Human monocytes store and secrete preformed CCL5, independent of de novo protein synthesis

Monocytes are a subset of circulating peripheral blood mononuclear cells with diverse roles in immunity, including sentinel roles in cytokine secretion. Conventionally, cytokines require an inductive stimulus for their expression and secretion, resulting in a time lag from the time of stimulation to when the proteins are packaged and secreted. Because cytokines are the main communicators in the immune system, their temporal expression is a key factor in coordinating responses to efficiently resolve infection. Herein, we identify that circulating human monocytes contain preformed cytokines that are stored intracellularly, in both resting and activated states. Having preformed cytokines bypasses the time lag associated with de novo synthesis, allowing monocytes to secrete immune mediators immediately upon activation or sensing of microbe-associated molecular patterns. We demonstrate here that, out of several cytokines evaluated, human monocytes contain a previously undescribed reservoir of the preformed chemokine CCL5. Furthermore, we showed that CCL5 could be secreted from monocytes treated with the protein synthesis inhibitor (cycloheximide) and Golgi blocker (brefeldin A). We examined the possibility for uptake of extracellular CCL5 from platelet aggregates and observed no significant levels of platelet binding to our enriched monocyte preparations, indicating that the source of preformed CCL5 was not from platelets. Preformed CCL5 was observed to be distributed throughout the cytoplasm and partially colocalized with CD63+ and Rab11A+ membranes, implicating endosomal compartments in the intracellular storage and trafficking of CCL5.