Staphylococcus aureus Infects Osteoclasts and Replicates Intracellularly

Abstract Osteomyelitis (OM), or inflammation of bone tissue, occurs most frequently as a result of bacterial infection and severely perturbs bone structure. The majority of OM is caused by Staphylococcus aureus , and even with proper treatment, OM has a high rate of recurrence and chronicity. While S. aureus has been shown to infect osteoblasts, persist intracellularly, and promote the release of pro-osteoclastogenic cytokines, it remains unclear whether osteoclasts (OCs) are also a target of intracellular infection. In this study, we examined the interaction between S. aureus and OCs, demonstrating internalization of GFP-labeled bacteria by confocal microscopy, both in vitro and in vivo . Utilizing an intracellular survival assay and flow cytometry during OC differentiation from bone marrow macrophages (BMMs), we found that the intracellular burden of S. aureus increases after initial infection in cells with at least 2 days of exposure to the osteoclastogenic cytokine receptor activator of nuclear factor kappa-B ligand (RANKL). Presence of dividing bacteria was confirmed via visualization by transmission electron microscopy. In contrast, undifferentiated BMMs, or those treated with interferon-γ or IL-4, had fewer internal bacteria, or no change, respectively, at 18 hours post infection, compared to 1.5 hours post infection. To further explore the signals downstream of RANKL, we manipulated NFATc1 and alternative NF-κB, which controls NFATc1 and other factors affecting OC function, finding that intracellular bacterial growth correlates with NFATc1 levels in RANKL-treated cells. Confocal microscopy in mature OCs showed a range of intracellular infection that correlated inversely with S. aureus and phagolysosome colocalization. The ability of OCs to become infected, paired with their diminished bactericidal capacity compared to BMMs, could promote OM progression by allowing S. aureus to evade initial immune regulation and proliferate at the periphery of lesions where OCs and bone remodeling are most abundant. Author Summary The inflammation of bone tissue is called osteomyelitis, and most cases are caused by an infection with the bacterium Staphylococcus aureus . To date, the bone building cells, osteoblasts, have been implicated in the progression of these infections, but not much is known about how the bone resorbing cells, osteoclasts, participate. In this study, we show that S. aureus can infect osteoclasts and proliferate inside these cells, whereas macrophages, immune cells related to osteoclasts, destroy the bacteria. These findings elucidate a unique role for osteoclasts to harbor bacteria during infection, providing a possible mechanism by which bacteria could evade destruction by the immune system. Therapeutic interventions that target osteoclasts specifically might reduce the severity of OM or improve antibiotic responses.