Chlamydia vaccines: recent developments and the role of adjuvants in future formulations

Bacteria of the genus Chlamydia cause a plethora of ocular, genital and respiratory diseases that continue to pose a considerable public health challenge worldwide. The major diseases are conjunctivitis and blinding trachoma, non-gonococcal urethritis, cervicitis, pelvic inflammatory disease, ectopic pregnancy, tubal factor infertility and interstitial pneumonia. The rampart asymptomatic infections prevent timely and effective antibiotic treatments, and quite often clinical presentation of sequelae is the first evidence of an infection. Besides, significant broad coverage in population screening and treatment is economically and logistically impractical, and mass education for public awareness has been ineffective. The current medical opinion is that an efficacious prophylactic vaccine is the best approach to protect humans from chlamydial infections. Unfortunately, a human vaccine has yet to be realized despite successful veterinary vaccines. Fortunately, recent advances in chlamydial immunobiology, cell biology, molecular pathogenesis, genomics, antigen discovery and animal models of infections are hastening progress toward an efficacious vaccine. Thus, it is established that Chlamydia immunity is mediated by T cells and a complementary antibody response, and several potential vaccine candidates have been identified. However, further advances are needed in effective vaccine delivery systems and safe potent adjuvants to boost and sustain immune responses for long-lasting protective immunity. This article focuses on the current status of human chlamydial vaccine research, specifically how application of new delivery systems and human compatible adjuvants could lead to a timely achievement of efficacious Chlamydia vaccines. The ranking of the candidate vaccine antigens for human vaccine development will await the availability of results from studies in which the antigens are tested by comparable experimental standards, such as antigen-adjuvant combination, route of delivery and possible toxicity.