Immunomodulatory properties of mesenchymal stem cells (MSCs). TGFβ: transforming growth factor beta; PGE2: prostaglandin E2; IDO: indoleamine 2,3-deoxygenase; HGF: hepatocyte growth factor; IFNγ: interferon gamma; TNF⍺: tumour necrosis factor alpha; M-CSF: macrophage colony-stimulating factor; IL-6: interleukin-6; IL-12: interleukin-12.