Widespread bone destruction is also characteristic of an infection in bone tissue and is typically mediated by activated multinucleated osteoclasts. During the bone formation process another cytokine, RANKL, is produced by osteoblasts which serves to modulate the activity or formation of osteoclasts. RANKL is a 317 amino acid polypeptide that is expressed either on the surface of osteoblasts or released into the local environment, and binds to and activates the RANK receptor expressed on osteoclasts. Once activated, osteoclasts begin the resorption of bone. However, the process of bone resorption needs to be tightly controlled, as uncontrolled signals can lead to excessive bone destruction and cause severe weakening of the skeletal system. BEZ235 clinical trial Consistent with previous results we found that S. aureus infection of osteoblasts led to an increase in RANKL expression in their membrane and also in a soluble form released into the surrounding environment. Previously it has been demonstrated that expression of RANKL leads to the migration and formation of osteoclasts, which is suggestive that S. aureus induced bone infection initiates localised bone resorption. Here we demonstrate that osteoclasts migrate toward S. aureus infected osteoblasts releasing RANKL. More importantly, deletion of SpA from S. aureus completely abolished RANKL expression and ablated migration and proliferation of the osteoclasts. This is most likely because S. aureus SpA is unable to bind to osteoblast TNFR-1 to induce release or expression of RANKL. These results are consistent with the finding that TNFa binding to TNFR-1 on osteoblasts results in increased RANKL expression which triggers subsequent bone destruction. RANKL is most likely released with other pro-osteoclastogenic cytokines that induce bone resorption e.g. Interleukin 6. Whether S. aureus SpA regulates these other pro-osteoclastogenic or not requires further investigation. A picture of the mechanisms that leads to bone infection is slowly starting to develop in the literature. Early reports demonstrated that as a defence mechanism, S. aureus can become internalised by osteoblasts. Uptake is promoted by fibronectin binding proteins that capture fibronectin and use it as a bridge between bacteria and the a5b1 integrin expressed on osteoblasts. Integrin clustering results in signalling that lead to bacterial uptake thus rendering the bacteria safe from both immune and antibiotic attack. In the current study we used a strain of S. aureus that does not express Fnbp’s on its surface. In addition, our group previously demonstrated that S. aureus strain Newman can bind to osteoblasts even in the absence of fibronectin or FnbpA and FnbpB. These results suggest that a second interaction between S. aureus and osteoblasts exists. Previously we demonstrated that an early step in S. aureus infection of the bone occurs when the major cell wall surface protein, SpA binds to TNFR1 on osteoblasts.