Figure 1.

Common and differential steps of the mammalian immune response: comparison of the mouse host (left side) with the human host (right side). Common steps are listed as (A–D). Progression from step (B) to step (C) is mediated by different mechanisms. The human host deploys a strategy of Trp depletion via the action of an IFN-γ induced indoleamine dioxygenase (IDO). In the mouse host p47 GTPase production is the anti-chlamydial strategy that, in turn, is met with the chlamydial counter-strategy of cytotoxin production and its subsequent export from the chlamydial inclusion body. The cytotoxin molecules can be viewed as a Trp sink, with the exportation step effectively accomplishing Trp depletion. Free Trp molecules are depicted as blue “w”s, whereas proteinaceous Trp molecules are shown against bright orange backgrounds.