Investigation of the molecular mechanisms of human mesenchymal stem cells and S. aureus interactions in osteomyelitis

Abstract

Osteomyelitis (OM) is bone inflammation that typically arises due to bacterial infection. Current treatment strategies for eradicating this condition include surgical debridement and systemic and local antibiotics. Both strategies, however, come with limitations that impact the disease outcomes. New biological approaches that can reduce the bacterial burden and help regenerate damaged bone are being sought. The aims of this study were to identify the potential of ameliorating bone marrow mesenchymal stem cells (bmMSC) anti-inflammatory and anti-bacterial function for treating OM through modulating the Toll-like receptors (TLRs), the impact of bmMSC conditioned medium (MSC-CM) on osteogenic properties of osteoblasts and testing the therapeutic potential of MSCs in a 3D model of OM using a human ex vivo bone model. Stimulation of bmMSCs with TLR agonists (Pam3CK4 and PGN-SA) resulted in significant growth inhibition of S. aureus compared to the unstimulated control cells. Infected bmMSCs overexpressed indoleamine 2, 3-dioxygenase enzyme (IDO) after Pam3CSK4 stimulation. The inhibition correlated with IDO expression in response to PGN-SA but not Pam3CSk4 stimulation. The inflammatory cytokines, IL-6 and CCL2, were upregulated in MSCs stimulated with Pam3CK4 and PGN-SA, respectively. Upon administration of MSC-CM with infected osteoblasts revealed antibacterial properties of human bmMSCs on infected osteoblasts. Moreover, MSC-CM inhibited the growth of S. aureus using CM harvested from different cell densities of MSCs. An ex vivo model of OM infection was established by infecting bone explants from human femoral head with S. aureus. Studies using the model revealed differential expression of bone resorption marker including cathepsin K (CTSK), tartrate-resistant acid phosphatase (ACP5), extracellular matrix-degrading enzymes matrix metalloproteinase 1 and 13 (MMP-1 and MMP-13), and the inflammatory cytokines IL-6. The expression profile suggested alignment of the ex vivo model with the pathological profile of OM. The addition of MSC-CM to the infection model dampened the inflammatory and resorption response. There was significant downregulation of genes responsible for osteoclastic and inflammatory activities upon culture with MSC-CM. The results may suggest that MSC- CM should be investigated further to reveal its trophic composition and derive a biological treatment for OM.