CCL20/CCR6 signaling modulates disease severity during the establishment of Staphylococcus aureus osteomyelitis

Chemokines are essential mediators of immune responses, and the CCL20/CCR6 chemokine signaling axis is known to be involved in inflammation, infectious diseases, and cancer progression. However, the role of the CCL20/CCR6 axis in host defense against Staphylococcus aureus osteomyelitis remains unknown. We hypothesized that the CCL20/CCR6 axis is critical for the recruitment and activation of immune cells against S. aureus, and the lack of CCL20 or its monogamous receptor CCR6 leads to exacerbation of S. aureus osteomyelitis. In vitro studies confirmed that osteoblasts and macrophages (M0 and M2 subtypes) secrete CCL20 following S. aureus exposure. Implant-associated osteomyelitis in C57BL/6, CCL20^-/-, and CCR6^-/- mice revealed an early increase in planktonic bacterial growth on day 1 and increased bacterial loads in soft tissue and bone on day 14 post-infection in both CCL20^-/- and CCR6^-/- mice. Immunohistochemistry and flow cytometry revealed that CCL20^-/- and CCR6^-/- mice have impaired recruitment of T cells, especially CCR6⁺ T cells, to the site of infection. Interestingly, CCR6^-/- mice exhibited increases in osteoclast numbers, reactive bone formation, and reduced bone mineral density. In a clinical pilot study, we observed a fivefold increase in serum CCL20 levels (P < 0.05) in S. aureus osteomyelitis patients (n = 23) vs uninfected controls (n = 10). Remarkably, serum CCL20 levels immediately following septic death were 100-fold higher vs uninfected patients (P < 0.05). Collectively, these results highlight the critical role of CCL20/CCR6-mediated host immunity during the establishment of S. aureus osteomyelitis and the potential of CCL20 as a biomarker of osteomyelitis-induced sepsis.

Staphylococcus aureus is the most common pathogen in orthopedic infections, and hard-to-treat strains (methicillin-resistant S. aureus) cause >50% of these infections. Thus, there is an urgent need to develop immunotherapies to treat these life-threatening infections. The role of the CCL20/CCR6 chemokine signaling axis on S. aureus osteomyelitis is unknown. In our efforts to uncover its role, we reveal that osteoblasts and macrophages secrete CCL20 in response to infection, and mice lacking CCL20 or its monogamous receptor CCR6 are more susceptible to S. aureus osteomyelitis. Mechanistically, we observed that increased infection severity in the knockout mice is associated with decreased T cell recruitment and increased osteoclastogenesis at the bone infection site. Importantly, in a clinical pilot study, we observed that CCL20 can be a useful biomarker of osteomyelitis-induced septic death. Overall, our study highlights the crucial immunomodulatory role that the CCL20/CCR6 axis plays during osteomyelitis.