Subcompartments of the macrophage recycling endosome direct the differential secretion of IL-6 and TNFα

Activated macrophages secrete an array of pro-inflammatory cytokines, including tumor necrosis factor-alpha (TNF alpha) and interleukin 6 (IL-6), that are temporally secreted for sequential roles in inflammation. We have previously characterized aspects of the intracellular trafficking of membrane-bound TNF alpha and its delivery to the cell surface at the site of phagocytic cups for secretion (Murray, R.Z., J.G. Kay, D.G. Sangermani, and J.L. Stow. 2005. Science. 310:1492-1495). The trafficking pathway and surface delivery of IL-6, a soluble cytokine, were studied here using approaches such as live cell imaging of fluorescently tagged IL-6 and immunoelectron microscopy. Newly synthesized IL-6 accumulates in the Golgi complex and exits in tubulovesicular carriers either as the sole labeled cargo or together with TNF alpha, utilizing specific soluble NSF attachment protein receptor (SNARE) proteins to fuse with the recycling endosome. Within recycling endosomes, we demonstrate the compartmentalization of cargo proteins, wherein IL-6 is dynamically segregated from TNF alpha and from surface recycling transferrin. Thereafter, these cytokines are independently secreted, with TNFa delivered to phagocytic cups but not IL-6. Therefore, the recycling endosome has a central role in orchestrating the differential secretion of cytokines during inflammation.