Effect of Calcitriol on the Immunomodulatory Properties and Hyaluronic Acid Metabolic Pathways of Human Mesenchymal Stem Cells

Abstract

Mesenchymal stem cells (MSCs) are non-hematopoietic stem cells which have remarkable immunomodulatory and tissue reparative properties, sparking clinical interest in their use in regenerative medicine. MSC-mediated immunomodulation occurs primarily via production of soluble factors like indolamine-2,3-dioxygenase (IDO), programmed death ligand-1 (PDL-1), prostaglandin E2 (PGE2), among others which function to inhibit effector immune cell function while promoting regulatory subtypes. Our lab focuses on the role of hyaluronic acid (HA) metabolism and signaling through HA receptor, CD44, in MSC’s ability to modulate the immune system and promote tissue repair. Data from our lab has demonstrated alterations in HA metabolism, production, and signaling in MSCs that have been primed with an inflammatory signal. Emerging research has suggested that calcitriol, the biologically active form of Vitamin D, may modulate soluble factor production by MSCs in response to inflammation. Taken together, we then hypothesized that calcitriol may also augment HA metabolic pathways. We utilized secretomes from Staphylococcal enterotoxin B (SEB)-activated peripheral blood mononuclear cells (PBMCs) to simulate the inflammatory microenvironment. In PBMC supernatant-activated MSCs we saw a consistent upregulation in gene expression of CD44, HA synthase isoform HAS-3, IDO-1, and PDL-1. Preliminary data suggests possible alterations in the molecular weight of hyaluronic acid secreted by activated MSCs when treated with calcitriol and PBMC supernatant when visualized by gel electrophoresis, though further replicates are needed. xi On its own, calcitriol was not able to modulate the alterations seen in HA metabolism in activated MSCs, as there was no statistically significant change in HAS3 gene expression between calcitriol and vehicle at 24-hour time-points. Though, calcitriol was shown to decrease total HA production in activated MSCs, as well as partially restore high molecular weight HA in PBMC supernatant- activated MSCs. Calcitriol was not demonstrated to meaningfully alter gene expression of IDO-1 or PDL-1 compared to PBMC supernatant-treated cells, suggesting that calcitriol does not affect MSC activation by inflammatory stimuli. However, optimization of the calcitriol delivery process and exposure time is necessary. Preliminary results following optimization of calcitriol dose and timing demonstrate a significant alteration in expression of HAS3, though further replicates are needed to confirm this effect. Additional studies investigating the effects of secretomes from MSCs treated with calcitriol on immune cell activity, and co-culturing experiments between MSCs and immune cells in the presence of calcitriol may provide more insight into the role of calcitriol and its effect on the immunomodulatory properties of MSCs.