Allergic airway inflammation and susceptibility to pneumococcal pneumonia in a murine model with real-time in vivo evaluation

The relationship between allergic airway inflammation and pneumococcal pneumonia is not well understood. We assessed susceptibility to experimental pneumococcal pneumonia in mice with and without allergic airway inflammation. Susceptibility to pneumococcal pneumonia was evaluated by challenging mice with a bioluminescent Streptococcus pneumoniae strain after sensitization with ovalbumin (OVA), with subsequent monitoring of pneumococcal infection using real-time photonic imaging. Of 46 OVA-sensitized mice challenged with pneumococci, 13 (28%) developed imaging findings consistent with pneumococcal pneumonia. In comparison, 28 (57%) of 49 non-sensitized control mice developed pneumococcal pneumonia (P = 0·005). While none of the control group developed meningitis (0%, none of 28), two mice in the OVA-sensitized group developed meningitis (15·4%, two of 13) (P = 0·09). The mean bacterial count in the lung was significantly lower in the OVA-sensitized than the non-sensitized group (8·26 ± 0·69 versus 9·21 ± 0·67 log₁₀ colony-forming units (CFU)/g, P = 0·002). There was a trend towards the mean bacterial count in the spleen being higher in the OVA-sensitized versus the non-sensitized group (8·14 ± 0·89 versus 7·45 ± 1·07 log₁₀ CFU/g, P = 0·071). A high level of interleukin (IL)-4 in lung homogenates was associated with risk of pneumococcal infection independent of sensitization with OVA (odds ratio: 49·7, 95% confidence interval 2·92-846·5, per increment of 1·0 pg/ml). In the murine model studied, acute allergic airway inflammation reduced susceptibility to pneumococcal pneumonia. IL-4 may increase the risk of pneumococcal pneumonia independently of allergic airway inflammation.