Vaccination and Host-Targeted Interventions for Staphylococcus aureus Exotoxin-Mediated Diseases

Abstract

Staphylococcus aureus is a major human pathogen capable of infecting nearly every tissue and causing a wide range of disease including skin and soft tissue infections, pneumonia, osteomyelitis, endocarditis, and toxic shock syndrome. S. aureus produces an array of exotoxins including superantigens and cytolysins, which contribute directly to disease by causing inflammation, immune evasion, and tissue damage. Recent evidence also suggests that superantigens and cytolysins modulate host epithelial cell signaling at mucosal surfaces and contribute directly to disease progression. Antibiotics can eliminate S. aureus, but have no effect on exotoxins already present within tissues. In addition, antibiotic resistance continues to increase in strains of S. aureus. Therefore, I hypothesize that anti-staphylococcal therapies, which target exotoxins or the down-stream host responses to exotoxins, should be explored to treat or prevent S. aureus infections. In this study, I investigated the mechanism of action of the superantigen, Toxic Shock Syndrome Toxin (TSST)-1, and the cytolysins, alpha-toxin and gamma-toxin, at the vaginal mucosal surface. Historically, TSST-1 was determined to promote disease (toxic shock syndrome) by widespread activation of CD4+ T-cells and antigen-presenting cells. However, recently, TSST-1 was determined to have direct proinflammatory effects on vaginal epithelial cells through an epidermal growth factor receptor-dependent pathway. Alpha-toxin’s mechanism of action and synergy with TSST-1 at the vaginal mucosa was characterized using in vitro, ex vivo, and in vivo models. Furthermore, novel actions of gamma-toxin at the vaginal mucosa were identified and the mechanism of action characterized. From this work, I proposed a model of cell-signaling by TSST-1, alpha-toxin, and gamma-toxin at the vaginal mucosa, which demonstrates a conserved proinflammatory and immune-modulating effect by S. aureus exotoxins. Proof of concept was established for two novel therapies for prevention and treatment of S. aureus infections. The tyrosine kinase inhibitor, AG1478, which inhibits the epidermal growth factor receptor signaling pathway, inhibited cytokine production induced by TSST-1, alpha-toxin, and gamma-toxin at the vaginal mucosa and prevented lethal menstrual toxic shock syndrome in rabbits challenged with live S. aureus. In addition, a vaccine targeting multiple exotoxins of S. aureus was highly effective in preventing lethal intrapulmonary challenge with a wide range of clinical strains and for treatment of the lethal effects of exotoxins in rabbits. In summary, these studies describe the mechanism of action of multiple S. aureus exotoxins and provide proof of concept for targeting exotoxins for treatment or prevention of toxin-mediated diseases. These studies demonstrate the importance of local proinflammatory effects during S. aureus infections, which contribute directly to the initiation of clinical disease. The data further highlight the importance of exotoxins during disease and add both insight and complexity to the field of S. aureus pathogenesis.