Protective role of IL-1β against post-arthroplasty Staphylococcus aureus infection

Abstract

MyD88 is an adapter molecule that is used by both IL-1R and TLR family members to initiate downstream signaling and promote immune responses. Given that IL-1β is induced after Staphylococcus aureus infections and TLR2 is activated by S. aureus lipopeptides, we hypothesized that IL-1β and TLR2 contribute to MyD88-dependent protective immune responses against post-arthroplasty S. aureus infections. To test this hypothesis, we used a mouse model of a post-arthroplasty S. aureus infection to compare the bacterial burden, biofilm formation and neutrophil recruitment in IL-1β-deficient, TLR2-deficient and wild-type (wt) mice. By using in vivo bioluminescence imaging, we found that the bacterial burden in IL-1β-deficient mice was 26-fold higher at 1 day after infection and remained 3- to 10-fold greater than wt mice through day 42. In contrast, the bacterial burden in TLR2-deficient mice did not differ from wt mice. In addition, implants harvested from IL-1β-deficient mice had more biofilm formation and 14-fold higher adherent bacteria compared with those from wt mice. Finally, IL-1β-deficient mice had ∼50% decreased neutrophil recruitment to the infected postoperative joints than wt mice. Taken together, these findings suggest a mechanism by which IL-1β induces neutrophil recruitment to help control the bacterial burden and the ensuing biofilm formation in a post-surgical joint.

Figure 1. IL-1β-deficient mice had increased in vivo bacterial burden compared with TLR2-deficient mice or wt mice

The right knee joints of IL-1β^−/−, TLR2^−/− and wt mice were inoculated with 1×10³ CFUs of S. aureus (n=8 mice/group) in the presence of an orthopaedic-grade K-wire implant. (A) Bacterial counts as measured by in vivo S. aureus bioluminescence (mean maximum flux [p/s/cm2/sr] ± sem) (logarithmic scale). †p<0.01, *p<0.05 IL-1β^−/− versus wt mice. (B) Representative in vivo S. aureus bioluminescence on a color scale overlaid on top of a grayscale image of mice.

Figure 2. IL-1β-deficient mice had substantially more biofilm formation on the implants than wt mice

The right knee joints of IL-1β^−/− and wt mice were inoculated with 1×10³ CFUs of S. aureus (n=3 mice/group) in the presence of an orthopaedic-grade K-wire implant. (A) Representative VP-SEM images of the biofilms on the intra-articular ends of the implants harvested from infected joints on postoperative days 7 and 42 are shown (1 of 3, with similar results). Top panels represent a low magnification (120x) and the bottom panels show a higher magnification (600x) of the area boxed in red. (B) Representative plates and bioluminescent colonies (left panels) and numbers of CFUs (right panel) of bacteria released from the implants (n=5 mice/group) on day 42 after overnight culture. *p<0.05 IL-1β^−/− versus wt mice.

Figure 3. IL-1β-deficient mice had decreased neutrophil recruitment to the infected knee joints compared with wt mice

The right knee joints of IL-1β^−/− and wt mice were inoculated with 1×10³ CFUs of S. aureus in the presence of an orthopaedic-grade K-wire implant. The infected joint tissue was harvested on postoperative day 1. (A) Representative photomicrographs of histologic sagittal sections of wt mice (top panels) and IL-1β^−/− mice (bottom panels) are shown (1 of 3 mice per group, with similar results). Left large panels: low magnification (12.5x) of H&E-stained joint specimens with a line drawing of the location of the implant extending into the joint from the femoral canal. Upper right small panels: higher magnification (100x) of H&E-stained joint specimens of the boxed area in the left panel at the location of the intra-articular end of the implant. Lower right small panels: higher magnification (400x) of H&E-stained sections in the boxed areas in the upper right panels. (B) Mean myeloperoxidase activity of the infected joint tissue specimens (ng/mg of tissue) ± sem (n=5 per group). ‡p<0.001 IL-1β^−/− versus wt mice.