Boosting Toll-like receptor 4 signaling enhances the therapeutic outcome of antibiotic therapy in pneumococcal pneumonia

The emergence and spread of antibiotic resistance emphasize the need for alternative treatment strategies against bacterial infections. Boosting the host innate immunity is not only readily deployable in most individuals but can also mobilize many different antibacterial defenses. This study tested the hypothesis whereby stimulation of the innate immune receptor Toll-like receptor 4 (TLR4) can be combined with antibiotics in the treatment of invasive pneumonia. In a mouse model of Streptococcus pneumoniae infection, a single oral administration of low-dose amoxicillin (AMX) or the systemic delivery of monophosphoryl lipid A (MPLA, a clinically-approved TLR4 activator) decreased the bacterial load in lung and spleen, although this was not sufficient for long-term survival. In contrast, a single treatment with a combination of MPLA and AMX induced significant bacterial clearance with little to no regrowth over time, and was associated with longer survival. Upregulation of genes related to granulocyte infiltration in lung tissue and elevation of blood levels of pro-inflammatory cytokines was immediate and transient in MPLA-treated mice; this indicates activation of the innate immune system in a context of infection. Combination treatment was associated with a well-preserved lung tissue architecture and more rapid recovery from inflammation - suggesting that immune activation by MPLA does not exacerbate pneumonia-induced damage. After AMX administration, plasma AMX concentrations rapidly reached the maximum and declined, whereas the downstream effects of MPLA extended beyond AMX elimination; these findings suggested a two-step effect. Our results demonstrated that leveraging host innate immunity increases the efficacy of antibiotic therapy in bacterial pneumonia.