These differences might be the cause of the observed distinct cytokine expression patterns (Hackstadt, 1995; Stephens et al., 1998; Greub et al., 2005b, 2009; Corsaro & Greub, 2006). Here, it should be stressed that major differences exist

in the biology of the classical Chlamydiae and the so-called Chlamydia-related organisms including a threefold larger genome size of Parachlamydia (Stephens et al., 1998; Greub et al., 2009) and its R428 ability to resist to the microbicidal effectors of free-living amoebae (Greub et al., 2003b). Immune cells can also be infected by Chlamydiales although not all do so with the same efficiency. For example C. pneumoniae can infect freshly derived monocytes, but cannot replicate in them and is degraded (Airenne et al., 1999; Wolf et al., 2005).

Chlamydia pneumoniae replicated to a lower extent in macrophages derived from human peripheral blood mononuclear cells (PBMC) as compared with HeLa cells or not at all in freshly derived PBMCs (Kaukoranta-Tolvanen et al., 1996; Wolf et al., 2005). To some degree, growth inhibition is probably due to TNF-α, because interference with antibodies causes increased bacterial growth in alveolar macrophages, although the late gene omcB was still poorly transcribed (Haranaga et al., 2003). Thus, in vivo macrophages seem to be refractory to C. pneumoniae replication compared with other Chlamydiales. Chlamydia trachomatis’ ability to perform a productive replication in macrophages depends on the biovar. Only the LGV biovars were able to replicate within macrophages, while

selleck others generally form persistent forms when infecting these phagocytic cells (reviewed in Beagley et al., 2009). Nonetheless, the persistent C. trachomatis are still metabolically active and can induce apoptosis of other immune cells (Jendro et al., 2004). Indeed, C. trachomatis-infected macrophages release TNF-α that with other components induces apoptosis of T cells, but not of the infected macrophages. Moreover, the factors released during apoptosis of T cells induce an immunosuppressing environment (transforming growth factor-β), thus creating a favorable environment for chlamydial persistence (Jendro et al., 2004). Controlled apoptosis may not only be P-type ATPase a mechanism used by some Chlamydiales to prevent bacterial clearance but might also provide enough time to complete a replication cycle or induce persistence. Waddlia chondrophila has a direct cytopathic effect on macrophages, suggesting that they are not the primary host cells for replication (Goy et al., 2008). This characteristic could help the bacteria prevent early infection recognition, display of antigens and attraction of other immune cells. Several Chlamydiales differ in their ability to induce cytokines after exposure to detrimental conditions such as heat or UV light. Thus, P.