Mesenchymal stem cells (MSCs; also referred to as mesenchymal stromal cells) have attracted much attention for their ability to regulate inflammatory processes. Their therapeutic potential is currently being investigated in various degenerative and inflammatory disorders such as Crohn's disease, graft-versus-host disease, diabetic nephropathy and organ fibrosis. The mechanisms by which MSCs exert their therapeutic effects are multifaceted, but in general, these cells are thought to enable damaged tissues to form a balanced inflammatory and regenerative microenvironment in the presence of vigorous inflammation. Studies over the past few years have demonstrated that when exposed to an inflammatory environment, MSCs can orchestrate local and systemic innate and adaptive immune responses through the release of various mediators, including immunosuppressive molecules, growth factors, exosomes, chemokines, complement components and various metabolites. Interestingly, even nonviable MSCs can exert beneficial effects, with apoptotic MSCs showing immunosuppressive functions in vivo. Because the immunomodulatory capabilities of MSCs are not constitutive but rather are licensed by inflammatory cytokines, the net outcomes of MSC activation might vary depending on the levels and the types of inflammation within the residing tissues. Here, we review current understanding of the immunomodulatory mechanisms of MSCs and the issues related to their therapeutic applications.

Noninvasive blood tests that provide information about fetal development and gestational age could potentially improve prenatal care. Ultrasound, the current gold standard, is not always affordable in low-resource settings and does not predict spontaneous preterm birth, a leading cause of infant death. In a pilot study of 31 healthy pregnant women, we found that measurement of nine cell-free RNA (cfRNA) transcripts in maternal blood predicted gestational age with comparable accuracy to ultrasound but at substantially lower cost. In a related study of 38 women (23 full-term and 15 preterm deliveries), all at elevated risk of delivering preterm, we identified seven cfRNA transcripts that accurately classified women who delivered preterm up to 2 months in advance of labor. These tests hold promise for prenatal care in both the developed and developing worlds, although they require validation in larger, blinded clinical trials.

Mesenchymal stem cells (MSCs) have been demonstrated to be anti-inflammatory against various immune disorders through several factors, including indoleamine 2,3-dioxygenase (IDO) and TNF-stimulated gene 6 (TSG-6). However, little is known about the necessity for both of these key immunosuppressive factors. Here we employed the mouse lipopolysaccharide (LPS)-induced acute lung injury (ALI) model, and found that IDO is necessary to achieve the effect of human umbilical cord-derived MSC (hUC-MSC)-based treatment on ALI. Notably, when IDO was deleted or inhibited, the expression of TSG-6 was decreased. This specific IDO-mediated regulation of TSG-6 expression was found to be exerted through its metabolite, kynurenic acid (KYNA), as inhibition of KYNA production led to decreased TSG-6 expression. Importantly, KYNA pretreatment of human MSCs enhanced their therapeutic effect on ALI. Mechanistically, KYNA activates aryl hydrocarbon receptor (AhR), which directly binds to the TSG-6 promoter to enhance TSG-6 expression. Therefore, our study has uncovered a novel link between IDO and TSG-6, and demonstrates that a metabolite of IDO controls the TSG-6-mediated anti-inflammatory therapeutic effects of human MSCs.