Chlamydia-host cell interaction not only from a bird's eye view: some lessons from Chlamydia psittaci.

Chlamydia psittaci causes psittacosis/ornithosis in birds and is an economically important pathogen for poultry farming. It also infects nonavian domestic animals as well as rodents, and is a zoonotic human pathogen responsible for atypical pneumonia. The bacterium efficiently disseminates in host organisms causing pulmonary and systemic disease. Its rapid entry, fast replication cycle, and tight control of intracellular transport routes contribute to the host-to-host transmission and efficient growth observed with C. psittaci. Recent studies have revealed that the pathogen copes better than other chlamydial strains with proinflammatory effectors produced during the early immune reaction of infected hosts. These features likely contribute to successful infections and might explain the potent adaptation and evasion characteristics of the agent. Current findings on cell-autonomous, innate, and adaptive defenses against C. psittaci provide novel insights into the concerted immune mechanisms involved in the clearance of the pathogen. Further in-depth studies on C. psittaci and other related agents in cellular as well as animal models are needed to develop more efficient antichlamydial therapies and vaccination strategies.