Regulation of human monocyte differentiation into m1- and m2-like macrophages
Myeloid-derived suppressor cells (MDSC) play a key role in down-regulating activated T and NK cells. MDSC are emerging as targets for cancer immunotherapy since they protect tumor cells from immune elimination. We previously showed that the TLR7/8 agonist R848 and the TLR2/1 dual agonist PAM3 had opposite effect on the maturation of human monocytic MDSC (mMDSC). While the former triggered them to differentiation in M1-like macrophages with pro-inflammatory/anti-tumoricidal capacity, the latter generated immunosuppressive M2-like macrophages. This work seeks to identify the soluble factors that regulate the differentiation of mMDSC into macrophages. Our studies reveal that TNFα and M-CSF are essential for mMDSC to mature into functional M1- and M2-like macrophages, respectively. IL-6 and IL-10 play secondary roles but when used in combination with TNFα or M-CSF exceed the effects of TLR agonists. Understanding the response of mMDSC to cytokines should help efforts to direct the mMDSC maturation to therapeutic benefit. The finding that PAM3 could induce human mMDSC to mature into M2-like macrophage triggered us to study the effect of this TLR agonist on other monocyte populations. Our findings reveal that PAM3 was unique among TLR agonists in generating M2-like macrophages. We compared the polarizing activity of PAM3 to that of M-CSF. PAM3 was slightly less efficient than M-CSF in driving maturation of HLA-DR+ monocytes based on phenotypic characterization and phagocytic ability. Yet macrophages generated by PAM3 or M-CSF were equally capable of suppressing T cell proliferation. Analysis of gene regulatory networks by microarray and subsequent validation of the pathways identified by using specific inhibitors defined the NF-κB – COX-2 axis as playing a primary role. However, PAM3 also induced monocyte differentiation via an IL-6-dependent pathway that was largely absent from M-CSF driven cultures. Our findings clarified the pathways by which immunosuppressive M2-like macrophage arise from human monocytes and identify PAM3 as a potential therapeutic modulator of monocyte differentiation in patients with autoimmune disease. Extracellular vesicles (EV) are a heterogeneous population of biological nanoscaled particles that serve as vectors to enhance intercellular communication. In addition to this physiological role evidence indicates that EV can be harnessed as therapeutic agents for cancer. The major limitation to EV-based therapeutics is their rapid clearance by the reticuloendothelial system (RES). To overcome this problem, we sought to reduce macrophage uptake of EV by blocking scavenger receptors. In vitro results using human and murine cells suggests that inhibiting class A scavenger receptors selectively impairs EV uptake by monocytes and macrophages. In vivo studies document reduced liver accumulation and enhanced plasma circulation of i.v. injected EV after such blockade. These findings provide a strategy for reducing EV uptake by the RES thereby increasing their targeting and activity.