Method: Expression of Mer and other surface markers in normal human monocytes and monocyte-derived macrophages was assessed by flow cytometry and Western blot. Gas6 and soluble Mer levels were measured by ELISA.
Result: Among fresh circulating monocytes, Mer was detectable in a small population of CD16+CD14dimHLA-DR+SR-A1+ cells. Macrophages differentiated in the presence of M-CSF (M2 conditions) showed enhanced Mer expression, yet GM-CSF (M1 conditions) was inhibitory. Other M1 stimuli, such as IFNγ, also caused Mer down-regulation, and LPS exposure led to generation of soluble Mer in supernatants. Among M2 macrophages, Mer was overexpressed in CD14brightCD163+SR-A1+CD206brightCD209-CD16+ “M2c” cells, and was driven by dexamethasone and by M-CSF (in the presence of serum) or M-CSF+IL-10 (in serum-free conditions). TGFβ had a negative effect on Mer and CD163 expression, although it up-regulated CD16 and CD206. Except for rare cells, prototypical M2 macrophages driven by IL-4 (“M2a”), defined as CD14dim/nullCD209+CD206+CD163-CD16-, did not express Mer. Mer’s ligand Gas6 was released by IL-10- and glucocorticoid-differentiated “M2c” macrophages, though the highest levels were found in supernatants of IL-4-treated cells. In contrast, TGFβ inhibited Gas6 release. Down-regulation of Mer was dependent on PPARγ activation: in fact, GW9662 (PPARγ-antagonist) up-regulated Mer and CD163 in M1 and “M2a” cells; conversely, Rosiglitazone (PPARγ-agonist) suppressed Mer and CD163 expression in otherwise untreated macrophages.
Conclusion: The expression pattern of Mer and Gas6 in macrophages, sustained by anti-inflammatory cytokines and associated with other receptors involved in phagocytosis of apoptotic cells (SR-A1, CD14) and IL-10 production (CD163, FcγRs), reinforces the importance of Mer as a key regulator of inflammation and localizes its role mostly to “M2c” macrophages. In particular, the induction of Mer by glucocorticoids and cytokines may be of importance in the control of inflammation and has therapeutic implications. The role of PPARγ in Mer regulation may also be of clinical significance.
Disclosure: G. Zizzo, None; B. A. Hilliard, None; M. Monestier, None; P. L. Cohen, None.