Identification of a conserved regulatory network underlying the differentiation of immunosuppressive human macrophage

Abstract There is considerable interest in generating human macrophage capable of down-modulating over-exuberant immune responses that characterize inflammatory and autoimmune diseases. Historically referred to as ‘M2-like’, macrophage with suppressive activity are conventionally generated by treating human monocytes with macrophage colony-stimulating factor (M-CSF). Unfortunately, the pleiotropic effects of M-CSF prevent it from being used therapeutically. This work identifies the TLR2/1 agonist Pam3CSK4 (hereafter PAM3) as being unique among TLR agonists in replicating the ability of M-CSF to induce monocytes to differentiate into immunosuppressive macrophage. The macrophage generated by PAM3 or M-CSF suppressed the proliferation of activated T cells and endocytosed particles with high efficiency. Consistent with their functional activity, these macrophage up-regulated expression of the surface markers DC-SIGN, SR-A and PD-L2, and secreted the anti-inflammatory mediators IL-1RA and IL-10. To clarify the pathway(s) by which macrophage with these properties arise, the regulatory networks activated by PAM3 and M-CSF were examined. NF-κB and Akt were central to the general process by which monocytes differentiate into macrophage, as their blockade prevented such maturation. By comparison, the Erk1/2 and p38 MAPK pathways were specifically involved in the differentiation of monocytes into macrophage with immunosuppressive activity. These findings identify targetable pathways to modulate the generation of immunosuppressive macrophage and demonstrate the potential utility of PAM3 for the treatment of chronic inflammation and autoimmunity.