Macrophage migration-dependent retraction fibers and migrasomes - a structural basis for spatiotemporal cytokine secretion.

Migrating cells leave behind trails of matrix-bound retraction fibers and migrasomes, but their cell-specific physiological functions are largely unknown. We comprehensively characterize retraction fibers and migrasomes of migrating macrophages in mouse and zebrafish in vitro and in vivo models. These structures share common and macrophage-centric components of tetraspanin enriched microdomains (TEMs) including tetraspanin 4 (TSPAN4), the integrin CD11b and the pTRAP, SCIMP; like TSPAN4, we show that SCIMP expression modulates retraction fiber and migrasome formation. In immune-activated migrating macrophages, rear-positioned recycling endosomes regulate the secretory trafficking of newly synthesized inflammatory cytokines into retraction fibers and migrasomes. Transmembrane TNF is delivered via SNARE-mediated carriers to the cell surface and to the surfaces of both retraction fibers and migrasomes, from where it can be released. Retraction fibers are thus newly identified here as cytokine secretion sites, and along with migrasomes they allow migrating macrophages to secrete more cytokine compared to stationary cells. Whereas cytokine secretion has been viewed as a process of ad hoc diffusion, instead, we reveal a new mechanism of migration-dependent, localized deposition and spatiotemporal release of bioactive cytokines from RFs and migrasomes. Cytokine deployment is thus organized in a constrained manner facilitating direct influence on local inflammatory responses.