Sequential changes of Legionella antigens and bacterial load in the lungs and urines of a mouse model of pneumonia

Legionella pneumophila is an important cause of community- and hospital-acquired pneumonia. In spite of the introduction of the urinary antigen detection method, Legionella pneumonia may be still underdiagnosed. We performed kinetic and quantitative analysis of diagnostic markers, such as bacterial loads, DNA assays, and antigen titers, in a 28-day time course murine model of L. pneumophila pneumonia. L. pneumophila replicated approximately 100-fold in the lungs of A/J mice in the first 48 h, and then became undetectable on day 14. Unexpectedly, pathogens other than L. pneumophila were consistently recovered from the lungs and livers at the acute phases, although those numbers were far below Legionella loads in the lungs. The peaks of specific antigen titer were observed on 48 h in the lungs, bronchoalveolar lavage (BAL) fluids, and urines and sustained positive even at 28 days after the infection. Especially, the lung homogenates and BAL fluids demonstrated 16 to 64 times higher levels of antigen titer than the urines by the end of observation. Legionella-specific DNA in the lungs was detected by polymerase chain reaction and loop-mediated isothermal amplification methods until 7 and 14 days after the infection, respectively. The inflammatory cytokines, such as tumor necrosis factor (TNF)-α, interleukin 6, and MIP-2, exhibited a peak on the acute phase, whereas the maximal production of high mobility group box 1 in the serum was observed on day 7. These results characterized the kinetic nature of diagnostic markers in L. pneumophila pneumonia. The present data suggested prolonged and compartmentalized deposition of antigen in the lungs, which may have an impact on the diagnosis of L. pneumophila pneumonia, especially in missed cases even after recovery from disease.