trachomatis infection of an immortalized primary endocervical epithelial cell (A2EN). Our data suggest that NK cells lyse C. trachomatis-infected cells more efficiently at 34 hpi, when secondary differentiation to infectious EB is at an early stage, compared with a later stage (42 hpi). The increased activity of NK cells toward early stage C. trachomatis-infected cells may be beneficial to the host by reducing the levels of infectious EBs that can be released. We also investigated the effect of NK-mediated lysis of C. trachomatis-infected cells on the level of recoverable IFUs. Curiously,

although we observed that the recoverable IFUs decreased in the presence of NK cells, the magnitude LY2606368 of this decrease

was smaller than effects on cytolysis efficiency. NK cytolytic activity is primarily mediated by perforin, a pore-forming protein that acts as a channel for entry of granzymes (Reviewed in Lieberman, 2003), both of which are expressed in the NK cell line used here. Granzymes induce apoptosis VX-765 chemical structure in target cells, consistent with the membrane blebbing and cytolysis we observed when C. trachomatis-infected A2EN cells were exposed to the NK cell line (NK92MI). Therefore, while NK lysis may deprive C. trachomatis of its intracellular niche, we hypothesize that C. trachomatis may be equipped with a mechanism to survive or escape NK cell-mediated host cell lysis. Thus, we believe that our data warrants further

investigation on the Urease impact of NK cell activity on C. trachomatis, as this may reveal novel survival mechanisms used by this bacterium against host innate immune response. This capacity of Chlamydia is reminiscent of recent observations made with the sexually transmitted pathogen Neisseria gonorrheae, which is able to escape/suppress the effects of neutrophil-associated oxidative bursts (Johnson & Criss, 2011). Interestingly, while our data and that of Hook et al. (2004) demonstrate increased susceptibility of C. trachomatis-infected cells to NK cell lysis, Mavoungou et al. (1999) have demonstrated that NK cells purified from the peripheral blood of C. trachomatis-infected patients have reduced IFNγ release and lytic capacity. These patients included those with genital and nongenital C. trachomatis serovars. Discrepancies among existing human studies on the role of NK cells in clearing C. trachomatis may reflect heterogeneity among NK cell receptors and their host-expressed ligands. Gene polymorphism in the site encoding the human activating NK cell receptor, NKG2D, has been shown to influence NK cell activity and susceptibility to some infectious diseases (Ma et al., 2010). Polymorphisms in human MICA have also been reported and may alter susceptibility to NK cell lysis (Ahmad et al., 2002; Karacki et al., 2004; Tosh et al., 2006). In light of the recent findings by Mei et al. (2009) that C.