The innate immune response to staphylococcus aureus biofilms on human sinonasal explants

Chronic Rhinosinusitis (CRS) is the persistent and symptomatic inflammation of the mucosa of the nose and paranasal cavities. It is a prevalent condition severely affecting the quality of life of around 10% of the population in Western countries. Its pathogenesis involves environmental factors such as viruses or bacteria on predisposed hosts triggering local mucosal inflammation. Staphylococcus aureus (S. aureus) is the most common isolated bacterium in CRS and, when forming biofilms, increases its resistance to antibiotics, being correlated with recalcitrant cases and higher rates of mucosal inflammation. The local inflammation can be explained by virulence factors from S. aureus, and also by innate and adaptive immune mechanisms of the host immune response. Although some researchers have explored the late or adaptive immune response associated with S. aureus biofilms, less is known about the initial or innate immune response that S. aureus biofilms trigger in the mucosa. This thesis aimed to study part of the mucosal innate immune response to S. aureus biofilms. We have challenged human sinonasal tissues—from normal donors undergoing transnasal pituitary surgery—with S. aureus biofilms ex vivo using an explant model. This model mimics in vivo conditions because it allows biofilms to grow at the air--‐liquid interface. Also, the biofilm--‐mucosa interaction is more physiological than primary cell cultures, because the communication between different host cells is preserved in the explant model. After the interaction with S. aureus biofilms, explant tissues produced IL--‐6 and other cytokines polarised to a Th1/Th17 type of immune response. The observed Th1/Th17 immune response differs from previous reports in eosinophilic CRS patients with nasal polyps (CRSwNP) showing a predominant Th2 response. Apparently there is an evolution from early Th1/Th17 immune responses to late Th2 in S. aureus biofilm associated infections. The turning point between these two types of immune responses seems critical in CRS because it could explain the origin of the Th2 inflammation. In The future, the use of long--‐term animal models could help to illustrate the progression from an initial Th1/Th17 to a late Th2 type of immune response in the sinonasal mucosa. S. aureus biofilms also induced apoptosis in the mucosa as demonstrated by the up--‐ regulation of cleaved caspase--‐3 in our settings. We also demonstrated the induction and activation of the Nod2 receptor and downstream pathway secondary to S. aureus biofilms. The Nod2 receptor recognises a small portion of peptidoglycan that is available during early phases of S. aureus biofilm formation. The role of Nod2 in CRS and biofilm infections should be evaluated in future studies. In conclusion, we demonstrated that early S. aureus biofilms induce a proinflammatory response in the sinonasal mucosa. This proinflammatory response seems to be crucial for biofilm attachment and persistence, and its modulation could represent an alternative to prevent S. aureus biofilm infections. Secreted staphylococcal products such as alpha (α) toxin and staphylococcal protein A (SpA) are two virulence factors critical during early biofilm growth. These proteins are also able to generate immune responses and represent targets for potential therapeutic intervention during biofilm infections in the sinonasal mucosa.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Medicine, 2014