Pan-caspase inhibition as a potential host-directed immunotherapy against MRSA and other bacterial skin infections

Abstract

A pan-caspase inhibitor, Q-VD-OPH, has therapeutic efficacy against preclinical methicillin-resistant Staphylococcus aureus skin infection. Anti-infection immunotherapy Antibiotic-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) are increasingly prevalent and difficult to treat. Alphonse et al. show that a pan-caspase inhibitor called Q-VD-OPH functioned as an effective immunotherapy in mouse models of community-acquired MRSA, Streptococcus pyogenes, and Pseudomonas aeruginosa skin infections. Q-VD-OPH reduced apoptosis in neutrophils and monocytes and increased necroptosis in macrophages, thereby increasing TNF production and infection clearance in mice. This work suggests a potential strategy to target bacterial infections without requiring the use of antibiotics. Staphylococcus aureus causes most skin infections in humans, and the emergence of methicillin-resistant S. aureus (MRSA) strains is a serious public health threat. There is an urgent clinical need for nonantibiotic immunotherapies to treat MRSA infections and prevent the spread of antibiotic resistance. Here, we investigated the pan-caspase inhibitor quinoline–valine–aspartic acid–difluorophenoxymethyl ketone (Q-VD-OPH) for efficacy against MRSA skin infection in mice. A single systemic dose of Q-VD-OPH decreased skin lesion sizes and reduced bacterial burden compared with vehicle-treated or untreated mice. Although Q-VD-OPH inhibited inflammasome-dependent apoptosis-associated speck-like protein containing caspase activation and recruitment domain (ASC) speck formation and caspase-1–mediated interleukin-1β (IL-1β) production, Q-VD-OPH maintained efficacy in mice deficient in IL-1β, ASC, caspase-1, caspase-11, or gasdermin D. Thus, Q-VD-OPH efficacy was independent of inflammasome-mediated pyroptosis. Rather, Q-VD-OPH reduced apoptosis of monocytes and neutrophils. Moreover, Q-VD-OPH enhanced necroptosis of macrophages with concomitant increases in serum TNF and TNF-producing neutrophils, monocytes/macrophages, and neutrophils in the infected skin. Consistent with this, Q-VD-OPH lacked efficacy in mice deficient in TNF (with associated reduced neutrophil influx and necroptosis), in mice deficient in TNF/IL-1R and anti-TNF antibody-treated WT mice. In vitro studies revealed that combined caspase-3, caspase-8, and caspase-9 inhibition reduced apoptosis, and combined caspase-1, caspase-8, and caspase-11 inhibition increased TNF, suggesting a mechanism for Q-VD-OPH efficacy in vivo. Last, Q-VD-OPH also had a therapeutic effect against Streptococcus pyogenes and Pseudomonas aeruginosa skin infections in mice. Collectively, pan-caspase inhibition represents a potential host-directed immunotherapy against MRSA and other bacterial skin infections.