Pentraxin 3 plays an important role in defence against Klebsiella pneumoniae infection

Sepsis is a life-threatening organ dysfunction due to dysregulated host response to an infection. Sepsis is characterized by the combination of pro and anti-inflammatory responses in the patient. Klebsiella pneumoniae is the second cause of Gram-negative bacteremia and a common pathogen in human sepsis. The emergence of multi drug resistance strains makes the treatment of Klebsiella pneumoniae a big challenge. The long pentraxin PTX3 is a key component of humoral innate immunity, induced by pro-inflammatory cytokines and microbial moieties. It is involved in modulation of inflammation and complement activity, microbe recognition and opsonization, tissue remodeling and resistance to selected pathogens of fungal, bacterial and viral origin. PTX3 has been reported to play a dual role in Klebsiella pneumoniae infections in transgenic mice, depending on the bacterial load and on the amount of PTX3 expressed by the mice. In addition, PTX3 has been shown to contribute to innate immune responses against the outer membrane protein A from Klebsiella pneumoniae (KpOmpA). Nevertheless, the role of PTX3 and the mechanism of its action in innate immunity against K. pneumoniae infections are not clearly defined. The aim of this thesis was to define the actual role of PTX3 in defence against Klebsiella pneumonia, taking advantage of PTX3 deficient mice and well-established in vivo models of Klebsiella pneumonia severe infections. To assess the involvement of PTX3 in K. pneumoniae infection, we used models of pulmonary or systemic infections and investigated the expression of PTX3 during the infection and its relevance using ptx3-/- mice. We evaluated the mortality and local and systemic bacterial load at different time points post infection. The interaction between PTX3 and K. pneumoniae was investigated through in vitro experiments. The levels of pro- and anti-inflammatory cytokines and coagulation parameters were measured, and histopathological damage and fibrin deposition were assessed in the lung. Results showed that PTX3 was induced in the lung and blood of C57BL/6 mice following infection, which was associated with the up-regulation of PTX3 inducing cytokines, TNF and IL-1β. The bacterial burden was significantly higher in ptx3-/- mice compared to WT in the local and systemic compartments 44 hours after infection. Infected WT mice survived significantly longer than infected ptx3-/- mice. The levels of pro-inflammatory cytokines were significantly higher in ptx3-/- mice. PTX3 did not interact with K. pneumoniae directly, suggesting that it is involved in regulating K. pneumoniae infection through an opsonophagocytosis independent mechanism. Lung histopathological analysis showed more severe lesions in PTX3-deficient mice, where fibrinosuppurative, necrotizing and haemorrhagic bronchopneumonia were observed, in contrast with suppurative bronchopneumonia in WT mice. Increased fibrin deposition was observed in the lung of Ptx3-/- mice with decreased amount of circulating fibrinogen in their blood. These results indicate that PTX3 contributes to the control of K. pneumoniae infection by modulating inflammatory and tissue damage responses, whereas PTX3-dependent facilitated opsophagocytosis and complement regulation were irrelevant in this infection. This study contributed in understanding the relevance of PTX3 in the innate response to K. pneumoniae infection and in dissecting the multifaced roles of this molecule in conditions of infection and inflammation. In particular, the study demonstrates that regulation of inflammation and tissue remodelling are key PTX3-dependent mechanisms involved in K. pneumoniae severe infection.