, 2003). For C. pneumoniae, there was a reduction in chemokine expression only in the absence of TLR2 and TLR4 (Da Costa et al., 2004). Moreover, C. pneumoniae survival was significantly reduced upon double knock out of TLR2 and TLR4 (Rodriguez et al., 2006). Different combinations of antibodies or knock outs against TLRs may thus be useful to dissect the PAMP recognition network. Another very useful approach is to transfect TLRs into HEK cells

(that lack most of these receptors) and to use a reporter system such as luciferin to detect TLR activation (Brightbill et al., 1999). Activation of TLR4 or TLR2 also influences their own expression levels (Wissel et al., 2005), as well as those of cytokine receptors. This allows a more rapid and amplified response

to PAMPs by neighboring cells. Besides TLRs, other PRRs are triggered by C. pneumoniae and C. trachomatis infection. Nod1 not only controls cytokine activation Dabrafenib in vivo but also induces the production of the bactericidal NO by inducible nitric oxide synthase (iNOS) (Shimada et al., 2009). Failure to activate iNOS allows uncontrolled bacterial growth. CD14 recognizes chlamydial lipopolysaccharide, which is a much weaker inducer than other lipopolysaccharides see more (Heine et al., 2003). Thus, PPRs should be seen as a network that can lead to the activation of the same downstream components. Furthermore, PRRs have very specific effectors and their activation is cell and pathogen dependent. Chlamydiales seem to have effector proteins that counteract TLR-induced immune response (reviewed in Betts et al., 2009). For example, C. psittaci elicits IFN-γ receptor (IFN-γR) expression Guanylate cyclase 2C through TLR4 and TLR2, but at the same time its function is impaired (Shirey et al., 2006). How this inhibition is performed is unknown. Other interferons are also induced by C. pneumoniae infection, leading to an IFN-γ response. The interferons were activated by a TLR4/MyD88 signaling pathway (Rothfuchs et al., 2004). IFN-γ induces

tryptophan breakdown by increasing host cell indolamine 2,3 dioxygenase expression. This is detrimental for Chlamydiales because most cannot synthesize tryptophan. Chlamydia trachomatis genital strains can use indole produced by other bacteria of the vaginal flora to synthesize tryptophan. Ocular strains of C. trachomatis have a mutation that prevents correct enzyme activity (Bavoil, 2006). Parachlamydia acanthamoebae also does not encode the tryptophan synthase enzyme and can therefore not circumvent tryptophan depletion. Induction of IFN-γ by chlamydial PAMPs is thus a potent bacterial growth inhibitor, at least for some C. trachomatis strains and P. acanthamoebae. Moreover, recent studies highlighted new IFN-γ-inducible effectors, so-called p47 GTPases. The absence of any of the two members of the p47 GTPases (Igtp[Irgm3] and Irgb10) was linked to an increase in susceptibility to C. trachomatis infection (Bernstein-Hanley et al., 2006).