Summary
Macrophage migration inhibitory factor (MIF) is an inflammatory cytokine with chemokine-like
functions and a role in atherogenesis. MIF is secreted by various cells including
endothelial cells and macrophages. Platelets are another prominent cell type with
a role in atherogenesis and are a rich source of atherogenic chemokines. We asked
whether platelets express and secrete MIF. In comparison, CXCL12 release was determined.
We examined the subcellular localisation of MIF in platelets/ megakaryocytes, studied
its co-localisation with other plateletderived mediators and asked whether platelets
contain MIF mRNA. Moreover, we probed the functional role of platelet-derived MIF
in inflammatory cell recruitment. Using Western blot and ELISA, we demonstrated and
quantitated MIF protein in human and mouse platelets. Applying confocal-microscopy,
MIF was found to localise in granularlike structures, but did not co-localise with
known platelet cytokines. qPCR indicated that platelets contain low levels of MIF
mRNA. ELISA measurements from human platelet supernatants showed that, whereas thrombin
and collagen triggered the release of MIF and CXCL12, ADP and oxidised LDL promoted
CXCL12 but not MIF secretion. Using Transwell assays, we demonstrated that platelet
supernatants promoted monocyte chemotaxis and that this was blocked by neutralising
MIF antibodies. This is the first report demonstrating MIF secretion from activated
platelets, suggesting that platelets are a previously unrecognised source of MIF in
inflammatory processes. There are distinct activating stimuli for MIF and CXCL12 secretion.
A substantial portion of the chemotactic capacity of stimulated platelet supernatants
is contributed by MIF, suggesting a role for platelet-derived MIF in atherogenic cell
recruitment.
Keywords
Atherosclerosis - chemokines - cytokines - inflammatory mediators - macrophage