Currently learn more there are no studies that have evaluated the protective efficacy of a vaccine targeting urogenital infections (the closest simply measuring immune responses at multiple mucosal sites following immunization [78]). Nevertheless, recent studies have shown the NHP model to be a promising platform for the evaluation of trachoma vaccines [79] and [80], including one recent study showing promise with a live, plasmid-free, attenuated vaccine [81]. Although NHP models offer a biological system much more comparable to that of

the human they are not without limitations. Currently there is no known natural NHP strain of Chlamydia. High inoculum doses of C. trachomatis are required to establish an infection (and pathology) [81] and [82], as well as the fact that differences in immune responses and disease states have been found with different infecting serovars [82] and [83], as well as the NHP species used [78]. Therefore, for the successful use of NHPs in vaccine evaluation, it is essential to define the immunological IWR-1 molecular weight mechanisms behind clearance of the human strains,

and to compare that to the paradigm associated with clearance in humans. If this can be done, then NHP models will indeed be valuable in the development of C. trachomatis vaccines for humans. Given the global importance of C. trachomatis STIs, and the strong case for a vaccine to curb increasing infection rates, how are we progressing towards the goal of an effective vaccine? The critical questions to ask are, (i) why does not natural infection result in strong protection? and (ii) how successful have past vaccination attempts been, or at least, what can we learn from these trials? The answers to both of these questions are actually quite promising.

Natural infection does lead to a degree of protection. In the mouse model this is certainly the case, with animals given a live infection being very solidly protected against a second (challenge) infection in that they shed very low levels of organisms [64]. A similar effect was observed in the early trachoma vaccine trials in which inactivated C. trachomatis organisms offered some degree of protection [84]. Indeed, there are some Casein kinase 1 valuable lessons that can be learned from the early trachoma trials as well as more recent studies of ocular C. trachomatis natural infections (reviewed by Mabey et al., [85] The early trachoma vaccine trials in countries such as Saudi Arabia, Taiwan, The Gambia, India and Ethiopia, showed that it was possible to induce short term immunity to ocular infection, and also to reduce the incidence of inflammatory trachoma, by administering vaccines based on killed or live whole organisms. The problem though is that these whole organism vaccines, whether infectious chlamydial elementary bodies or whole inactivated organisms, contain both protective as well as deleterious antigens.