Regulatory macrophages (Mregs) represent a subset of anti-inflammatorymacrophages. In general,macrophages are a very dynamic and plastic cell type and can be divided into two main groups: classically activated macrophages (M1) and alternatively activated macrophages (M2).^[1] M2 group can further be divided into sub-groups M2a, M2b, M2c, and M2d.^[2] Typically the M2 cells have anti-inflammatory and regulatory properties and produce many different anti-inflammatorycytokines such asIL-4,IL-33,IL-10,IL-1RA, andTGF-β.^[3][4] M2 cells can also secreteangiogenic and chemotactic factors.^[5] These cells can be distinguished based on the different expression levels of various surface proteins and the secretion of different effector molecules.^[4]

M2a, mainly known as alternatively activated macrophages, aremacrophages associated withtissue healing due to the production of components ofextracellular matrix. M2a cells are induced byIL-4 andIL-13.^[2] M2b, generally referred to as regulatory macrophages (Mregs), are characterized by secreting large amounts ofIL-10 and small amounts ofIL-12.^[6][7] M2c, also known as deactivated macrophages, secrete large amounts ofIL-10 andTGF-β. M2c are induced byglucocorticoids andTGF-β.^[8] M2d arepro-angiogenic cells that secreteIL-10,TGF-β, andvascular endothelial growth factor and are induced byIL-6 and A2 adenosine receptor agonist (A2R).^[4][9]

Mregs can arise followinginnate oradaptive immune responses. Mregs were first described afterFcγR ligation byIgG complexes in the occurrence ofpathogen-associated molecular patterns (e. g. lipopolysaccharide or lipoteichoic acid) acting throughToll-like receptors.^[10] Coculture ofmacrophages withregulatory T cells (Tregs) caused differentiation ofmacrophages toward Mreg phenotype.^[11] Similar effect provoked interaction ofmacrophages andB1 B cells.^[12] Mregs can even arise following stress responses. Activation of thehypothalamic-pituitary-adrenal axis leads to production ofglucocorticoids that cause decreased production ofIL-12 bymacrophages.^[13]

Many cell types includingmonocytes, M1, and M2 can in a specific microenvironment differentiate to Mregs.^[7] Induction of Mregs is strongly linked with the interaction ofFc receptors located on the surface of Mregs withFc fragments ofantibodies.^[14] It has been shown that anti-TNF monoclonal antibodies interacting withFcγ receptor of Mregs induce differentiation of Mregs through activation ofSTAT3 signaling pathway.^[15][16] Some pathogens can promote the transformation ofcells into Mregs as animmune evasion mechanism.^[7][17] Two signals are needed for Mregs inducement. The first signal is stimulation byM-CSF,GM-CSF,PGE2,adenosine,glucocorticoid, orapoptotic cells.^[9][18] The second signal can be stimulation withcytokines ortoll-like receptor ligands. The first signal promotes the differentiation ofmonocytes tomacrophages and the second signal promotesimmunosuppressive functions.^[8] In vitro,M-CSF,IFNγ, andLPS are used for the inducement of Mregs.^[7]

Other cells such aseosinophils andinnate lymphoid cells type 2 (ILC2) can promote M2 polarization bycytokine secretion.IL-9 can function as agrowth factor for ILC-2 and thereby assist in the induction of Mregs. Anothercytokine that helps the induction of Mregs isIL-35 which is produced byTregs.^[7]

Surprisingly, Mregs resemble classically activated macrophages more than alternatively activated macrophages, due to higher biochemical similarity.^[19] The difference between M1 macrophages and Mregs is, inter alia, that Mregs secrete high levels ofIL-10 and simultaneously low levels ofIL-12. Out of allmacrophages, Mregs show the highest expression ofMHC II molecules andco-stimulatory molecules (CD80/CD86), which differentiates them from the alternatively activated macrophages, which show a very low expression of these molecules. Mregs also differ from alternatively activated macrophages by producing high levels ofnitric oxide and lowarginase activity.^[7][16][19] Lastly, they differ in the expression of FIIZ1 (Resistin-like molecule alpha1) and YM1 which are differentiation markers present on alternatively activated macrophages.^[4] Mregs are recognized by the expression ofPD-L1,CD206,CD80/CD86,HLA-DR, andDHRS9 (dehydrogenase/reductase 9).^[4][20]DHRS9 has been recognized as a stable marker for Mregs in humans.^[20]

The physiological role of Mregs is to dampen theimmune response andimmunopathology. Unlike classically activated macrophages, Mregs produce low levels ofIL-12, which is important becauseIL-12 induces differentiation ofnaïve helper T cells to Th1 cells which produce high levels ofIFNγ. Mregs do not contribute to the production ofextracellular matrix because they express low levels of arginase.^[12][4]

Mregs show up-regulation ofIL-10,TGFβ,PGE2,iNOS,IDO, and down-regulation ofIL-1β,IL-6,IL-12, andTNF-α.^[21] By secretingTGF-β they help with the induction ofTregs and by producingIL-10 they contribute to the induction oftolerance and regulatory cell types. Mregs can directly inhibit theproliferation ofactivated T cells. It has been shown that Mregs co-cultured withT cells have a negative effect on the T-cellular ability to secreteIL-2 andIFN-γ.^[22] Mregs can also inhibit thearginase activity of alternatively activated macrophages, theproliferation offibroblasts, and can promoteangiogenesis.^[23] The use of Mregs is widely studied as a potential cell-based immunosuppressive therapy afterorgan transplantation. Mregs could potentially solve the problems (susceptibility toinfectious diseases andcancer diseases) associated with the current post-transplant therapy. Since Mregs are still producingnitric oxide they may be more suitable than current treatments, when appropriately stimulated.^[22]

• Immune system
• Macrophages