Staphylococcus aureus is the most common cause of skin and soft tissue infections, and rapidly emerging antibiotic-resistant strains are creating a serious public health concern. If immune-based therapies are to be an alternative to antibiotics, greater understanding is needed of the protective immune response against S. aureus infection in the skin. Although neutrophil recruitment is required for immunity against S. aureus, a role for T cells has been suggested. Here, we used a mouse model of S. aureus cutaneous infection to investigate the contribution of T cells to host defense. We found that mice deficient in γδ but not αβ T cells had substantially larger skin lesions with higher bacterial counts and impaired neutrophil recruitment compared with WT mice. This neutrophil recruitment was dependent upon epidermal Vγ5 + γδ T cell production of IL-17, but not IL-21 and IL-22. Furthermore, IL-17 induction required IL-1, TLR2, and IL-23 and was critical for host defense, since IL-17R-deficient mice had a phenotype similar to that of γδ T cell-deficient mice. Importantly, γδ T cell-deficient mice inoculated with S. aureus and treated with a single dose of recombinant IL-17 had lesion sizes and bacterial counts resembling those of WT mice, demonstrating that IL-17 could restore the impaired immunity in these mice. Our study defines what we believe to be a novel role for IL-17-producing epidermal γδ T cells in innate immunity against S. aureus cutaneous infection.

Complications arising from cutaneous and soft tissue infections with Staphylococcus aureus are a major clinical problem owing to the high incidence of these infections and the widespread emergence of antibiotic-resistant bacterial strains. If prophylactic vaccines or immunotherapy for certain patient populations are to be developed as an alternative to antibiotics, it will be essential to better understand the immune mechanisms that provide protection against S. aureus skin infections. Recent discoveries have identified a key role for interleukin-1 (IL-1)- and IL-17-mediated immune responses in promoting neutrophil recruitment to the site of infection in the skin, a process that is required for host defence and bacterial clearance. This Review describes these new insights and discusses their potential impact on immune-based therapies and vaccination strategies.

BackgroundPost-arthroplasty infections represent a devastating complication of total joint replacement surgery, resulting in multiple reoperations, prolonged antibiotic use, extended disability and worse clinical outcomes. As the number of arthroplasties in the U.S. will exceed 3.8 million surgeries per year by 2030, the number of post-arthroplasty infections is projected to increase to over 266,000 infections annually. The treatment of these infections will exhaust healthcare resources and dramatically increase medical costs.Methodology/Principal FindingsTo evaluate novel preventative therapeutic strategies against post-arthroplasty infections, a mouse model was developed in which a bioluminescent Staphylococcus aureus strain was inoculated into a knee joint containing an orthopaedic implant and advanced in vivo imaging was used to measure the bacterial burden in real-time. Mice inoculated with 5×103 and 5×104 CFUs developed increased bacterial counts with marked swelling of the affected leg, consistent with an acute joint infection. In contrast, mice inoculated with 5×102 CFUs developed a low-grade infection, resembling a more chronic infection. Ex vivo bacterial counts highly correlated with in vivo bioluminescence signals and EGFP-neutrophil fluorescence of LysEGFP mice was used to measure the infection-induced inflammation. Furthermore, biofilm formation on the implants was visualized at 7 and 14 postoperative days by variable-pressure scanning electron microscopy (VP-SEM). Using this model, a minocycline/rifampin-impregnated bioresorbable polymer implant coating was effective in reducing the infection, decreasing inflammation and preventing biofilm formation.Conclusions/SignificanceTaken together, this mouse model may represent an alternative pre-clinical screening tool to evaluate novel in vivo therapeutic strategies before studies in larger animals and in human subjects. Furthermore, the antibiotic-polymer implant coating evaluated in this study was clinically effective, suggesting the potential for this strategy as a therapeutic intervention to combat post-arthroplasty infections.

Neutrophil abscess formation is critical in innate immunity against many pathogens. Here, the mechanism of neutrophil abscess formation was investigated using a mouse model of Staphylococcus aureus cutaneous infection. Gene expression analysis and in vivo multispectral noninvasive imaging during the S. aureus infection revealed a strong functional and temporal association between neutrophil recruitment and IL-1β/IL-1R activation. Unexpectedly, neutrophils but not monocytes/macrophages or other MHCII-expressing antigen presenting cells were the predominant source of IL-1β at the site of infection. Furthermore, neutrophil-derived IL-1β was essential for host defense since adoptive transfer of IL-1β-expressing neutrophils was sufficient to restore the impaired neutrophil abscess formation in S. aureus-infected IL-1β-deficient mice. S. aureus-induced IL-1β production by neutrophils required TLR2, NOD2, FPR1 and the ASC/NLRP3 inflammasome in an α-toxin-dependent mechanism. Taken together, IL-1β and neutrophil abscess formation during an infection are functionally, temporally and spatially linked as a consequence of direct IL-1β production by neutrophils.

Post-arthroplasty infections are a devastating problem in orthopaedic surgery. While acute infections can be treated with a single stage washout and liner exchange, chronic infections lead to multiple reoperations, prolonged antibiotic courses, extended disability and worse clinical outcomes. Unlike previous mouse models that studied an acute infection, this work aimed to develop a model of a chronic post-arthroplasty infection. To achieve this, a stainless steel implant in the knee joints of mice was inoculated with a bioluminescent S. aureus strain (1×102–1×104 CFUs) and in vivo imaging was used to monitor the bacterial burden for 42 days. Four different S. aureus stains were compared in which the bioluminescent construct was integrated in an antibiotic selection plasmid (ALC2906), the bacterial chromosome (Xen29 and Xen40) or a stable plasmid (Xen36). ALC2906 had increased bioluminescent signals through day 10, after which the signals became undetectable. In contrast, Xen29, Xen40 and Xen36 had increased bioluminescent signals through 42 days with the highest signals observed with Xen36. ALC2906, Xen29 and Xen40 induced significantly more inflammation than Xen36 as measured by in vivo EGFP-neutrophil florescence of LysEGFP mice. All four strains induced comparable biofilm formation as determined by variable-pressure scanning electron microscopy. Using a titanium implant, Xen36 had higher in vivo bioluminescence signals than Xen40 but had similar biofilm formation and adherent bacteria. In conclusion, Xen29, Xen40 and especially Xen36, which had stable bioluminescence constructs, are feasible for long-term in vivo monitoring of bacterial burden and biofilm formation to study chronic post-arthroplasty infections and potential antimicrobial interventions.