Differential sensitivity of leukocyte populations to <i>Staphylococcus aureus</i> biofilm

<i>Staphylococcus aureus</i> is a leading cause of prosthetic joint infection (PJI) typified by biofilm formation. Anti-inflammatory granulocytic myeloid-derived suppressor cells (G-MDSCs) represent the main leukocyte population in a mouse model of <i>S. aureus</i> PJI, followed by neutrophils (PMNs), and macrophages (Mφs), which is also seen during human PJI. Defining how each leukocyte population responds to <i>S. aureus</i> biofilm vs planktonic bacteria could have important implications for how <i>S. aureus</i> evades immune detection to facilitate biofilm persistence. This study compared the kinetics of leukocyte death and relationship to mitochondrial ROS (mtROS) production following exposure to planktonic <i>S. aureus</i> or biofilm. Mφs were exquisitely sensitive to <i>S. aureus</i> biofilm with toxicity observed within 15 min following biofilm co-culture, whereas G-MDSCs and PMNs were more resilient, with appreciable survival out to 6 h. In contrast, G-MDSC viability was significantly decreased after extended exposure to planktonic <i>S. aureus</i> compared to PMNs and Mφs. Although leukocyte death coincided with increased mtROS production across all leukocyte populations, inhibiting mtROS had no impact on leukocyte survival following biofilm co-culture, suggesting alternative cell death triggers. Caspase-1-dependent pyroptosis was observed in PMNs, whereas Mφs and G-MDSCs were targeted by necrosis since an inhibitor of H₂O₂-induced necrosis improved cell survival of both populations, whereas programmed cell death inhibitors had no effect. These findings may account, in part, for the abundance of G-MDSCs and PMNs, but not Mφs, during PJI based on differential susceptibility to biofilm-induced cytotoxicity.