The complement system modulates the intensity of innate and specific immunity. While it protects against infections by extracellular bacteria its role in infection with obligate intracellular bacteria, such as the avian and human pathogen Chlamydia (C.) psittaci, is still unknown. In the present study, knockout mice lacking C3 and thus all main complement effector functions were intranasally infected with C. psittaci strain DC15. Clinical parameters, lung histology, and cytokine levels were determined. A subset of infections was additionally performed with mice lacking C5 or C5a receptors. Complement activation occurred before symptoms of pneumonia appeared. Mice lacking C3 were ∼100 times more susceptible to the intracellular bacteria compared to wild-type mice, with all C3−/− mice succumbing to infection after day 9. At a low infective dose, C3−/− mice became severely ill after an even longer delay, the kinetics suggesting a so far unknown link of complement to the adaptive, protective immune response against chlamydiae. The lethal phenotype of C3−/− mice is not based on differences in the anti-chlamydial IgG response (which is slightly delayed) as demonstrated by serum transfer experiments. In addition, during the first week of infection, the absence of C3 was associated with partial protection characterized by reduced weight loss, better clinical score and lower bacterial burden, which might be explained by a different mechanism. Lack of complement functions downstream of C5 had little effect. This study demonstrates for the first time a strong and complex influence of complement effector functions, downstream of C3 and upstream of C5, on the outcome of an infection with intracellular bacteria, such as C. psittaci.

C3a and its receptor are critical for defense against C. psittaci in mouse lung infection. In this model, C3a acts via its receptor as immune modulator. Enhancement of specific B and T cell responses upon infection with an intracellular bacterium were identified as hitherto unknown features of C3a/C3aR. These new functions might be of general immunological importance.

The obligate intracellular bacterium Chlamydia (C.) pneumoniae causes respiratory infections and is associated with vascular diseases. To elucidate how temperature and host cells used for propagation alter chlamydial virulence, C. pneumoniae CWL0129 (Cpn) was cultured at 35 or 37°C in two different cell lines and then applied to mice. These mice infected with differentially propagated chlamydiae showed differences in clinical score, body weight and inflammatory cytokines in the lung. Our study demonstrates that Cpn cultured at 37°C in hamster fibroblast BHK-21 are able to colonize the mouse lung faster and better, and induce stronger symptoms and cytokine induction than bacteria cultured at 35°C. The temperature-triggered virulence alteration could not be observed for Cpn propagated in HeLa cells and was independent of host cell protein synthesis. Transcriptome analysis did not reveal temperature-induced effects on chlamydial gene expression, suggesting that the observed virulence changes are regulated on a different, so far unknown level. Preculture close to the central body temperature of its warm-blooded human or murine host might 'prepare' Cpn for subsequent in vivo infection. Our identification of culture-dependent virulence alteration helps to establish an optimized mouse lung infection model for Cpn and provides the basis to further unravel the molecular mechanisms underlying chlamydial pathogenicity.