Abstract
Infectious osteomyelitis associated with periprosthetic joint infections is often recalcitrant to treatment and has a high rate of recurrence. In the case of Staphylococcus aureus, the most common pathogen in all forms of osteomyelitis, this may be attributed in part to residual intracellular infection of host cells, yet this is not generally considered in the treatment strategy. Osteocytes represent a unique cell type in this context due to their abundance, their formation of a syncytium throughout the bone that could facilitate bacterial spread and their relative inaccessibility to professional immune cells. As such, there is potential value in studying the host-pathogen interactions in the context of this cell type in a replicable and scalable in vitro model. Here, we examined the utility of the human osteosarcoma cell line SaOS2 differentiated to an osteocyte-like stage (SaOS2-OY) as an intracellular infection model for S. aureus. We demonstrate that S. aureus is capable of generating stable intracellular infections in SaOS2-OY cells but not in undifferentiated, osteoblast-like SaOS2 cells (SaOS2-OB). In SaOS2-OY cells, S. aureus transitioned towards a quasi-dormant small colony variant (SCV) growth phenotype over a 15-day post-infection period. The infected cells exhibited changes in the expression of key immunomodulatory mediators that are consistent with the infection response of primary osteocytes. Thus, SaOS2-OY is an appropriate cell line model that may be predictive of the interactions between S. aureus and human osteocytes, and this will be useful for studying mechanisms of persistence and for testing the efficacy of potential antimicrobial strategies.
Highlights
Infectious osteomyelitis, known as deep bone infection, is a serious clinical feature of periprosthetic joint infection (PJI) associated with chronic disease
That we have found PJI to be associated with increased osteocytemediated bone matrix degradation (Ormsby et al, 2021), which could facilitate the clearance of osteocytes harbouring dormant bacteria by both the destruction of the host cell and the facilitation of access to the cell-mediated immune system, these possibilities remain to be explored
This result was consistent with similar work using SaOS2 by Dusane et al, who found that removal of antibiotic pressure resulted in the proliferation and escape of intracellular S. aureus (Dusane et al, 2018)
Summary
Infectious osteomyelitis, known as deep bone infection, is a serious clinical feature of periprosthetic joint infection (PJI) associated with chronic disease. PJI occurs in around 1-4% of hip and knee arthroplasties, this incidence is likely underreported (Tande and Patel, 2014; Mitchell et al, 2017). The intra-osteocytic bacteria transitioned to the quasi-dormant small colony variant (SCV) phenotype and importantly for the longevity of these infections, induced minimal cytotoxic effects on the host cell. This scenario has significant implications as a discrete phenomenon that can be modelled to inform assessments of sustained infection
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