# Herpesvirus-Entry Mediator Inhibits the NF-κB Pathway Activated by IL-17 and Fosters the Osteogenic Differentiation of Allogeneic Mesenchymal Stem Cells

## TL;DR

This study shows that expressing HVEM in allogeneic MSCs can reduce inflammation and boost bone formation, offering a new approach for tissue-engineered bone treatments.

## Contribution

The study reveals a novel mechanism where HVEM inhibits the NF-κB pathway activated by IL-17, enhancing osteogenic differentiation of allogeneic MSCs.

## Key findings

- MSCs expressing HVEM suppress immune responses and maintain strong osteogenic potential in an inflammatory environment.
- HVEM inhibits the IKK-NF-κB pathway, preventing β-catenin degradation and promoting bone formation.
- The study links the Wnt/β-catenin and IKK-NF-κB pathways during allogeneic MSC transplantation.

## Abstract

The challenge in developing tissue-engineered bones (TEBs) for clinical applications lies in the constraints associated with the source and availability of autologous mesenchymal stem cells (MSCs) derived from the bone marrow, which creates a bottleneck. While allogeneic MSCs have shown promise in TEB applications, their ability to promote bone growth is notably diminished because of the inflammatory reaction at the transplant site and the inherent immune response triggered by allogeneic MSCs. Hence, there is a pressing need to develop methods that enhance the osteogenic differentiation of allogeneic MSCs during transplantation. Previous studies have found that IL-17 is a key proinflammatory factor in initiating inflammation and cascade amplification in the early stages of an inflammatory response, and proinflammatory cytokines such as TNF-α and IL-17 can inhibit the osteogenic differentiation of MSCs in an immune environment. In this study, MSCs expressing HVEM were successfully constructed by viral transfection and further reconfirmed that IL-17 can inhibit the in vivo and in vitro osteogenesis of allogeneic MSCs through in vitro experiments and mouse calvarial bone defect (diameter about 3 mm) model, while MSCs that express herpesvirus-entry mediator (HVEM) exhibit the capacity to suppress immune responses and sustain strong osteogenic potential. We further pointed out that the mechanism by which HVEM promotes the osteogenesis of allogeneic MSCs is related to its inhibition of the IκB kinase (IKK)-NF-κB signaling pathway activated by IL-17 in the immune environment, which can significantly inhibit the ubiquitination and degradation of β-catenin in MSCs induced by the IKK-NF-κB pathway, upregulate the expression of β-catenin, and promote bone formation. Hence, this research provides an initial connection between the Wnt/β-catenin signaling pathway and the IKK-NF-κB pathway during allogeneic MSC transplantation, offering new avenues for investigation and establishing a theoretical foundation for the potential use of HVEM-expressing MSCs in clinical treatments for bone defects.