Protection against Chlamydia Promoted by a Subunit Vaccine (CTH1) Compared with a Primary Intranasal Infection in a Mouse Genital Challenge Model ¤ Anja Weinreich Olsen*, Michael Theisen , Dennis Christensen, Frank Follmann, Peter Andersen* Chlamydia and Adjuvant Research, Department of Infectious Disease Immunology, Statens Serum Institut, Copenhagen, Denmark Abstract Background: The chlamydial proteins CT443 (OmcB) and CT521 (rl16) have previously been identified as human B and/or T cell targets during a chlamydial infection in humans. Here we compare the protective effector mechanism promoted by a fusion protein composed of CT521 and CT443 (CTH1) with a primary intranasal Chlamydia muridarum infection known to provide high levels of protection against a genital chlamydial challenge. Methodology/Principal Findings: The fusion protein CTH1, adjuvanted with a strong Th1 inducing cationic adjuvant (CAF01), significantly reduced the bacterial shedding compared to a control group in both a C. trachomatis Serovar D and C. muridarum challenge model. The CTH1/CAF01 vaccine was found to induce polyfunctional T cells consisting of TNFa/IL-2 and TNFa/IL-2/IFN-c positive cells and high titers of CTH1 specific IgG2a and IgG1. By depletion experiments the protection in the C. muridarum challenge model was demonstrated to be mediated solely by CD4+ T cells. In comparison, an intranasal infection with C. muridarum induced a T cell response that consisted predominantly of TNFa/IFN-c co-expressing effector CD4+ T cells and an antibody response consisting of C. muridarum specific IgG1, IgG2a but also IgA. This response was associated with a high level of protection against challenge—a protection that was only partially dependent on CD4+ T cells. Furthermore, whereas the antibody response induced by intranasal infection was strongly reactive against the native antigens displayed in the chlamydial elementary body, only low levels of antibodies against this preparation were found after CTH1/CAF01 immunization. Conclusions/Significance: Our data demonstrate that CTH1 vaccination promotes a CD4+ T cell dependent protective response but compared with intranasal C. muridarum infection lacks a CD4 independent protective mechanism for complete protection. Citation: Olsen AW, Theisen M, Christensen D, Follmann F, Andersen P (2010) Protection against Chlamydia Promoted by a Subunit Vaccine (CTH1) Compared with a Primary Intranasal Infection in a Mouse Genital Challenge Model. PLoS ONE 5(5): e10768. doi:10.1371/journal.pone.0010768 Editor: Joy Sturtevant, Louisiana State University, United States of America Received November 30, 2009; Accepted April 23, 2010; Published May 21, 2010 Copyright: 2010 Olsen et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The study was funded by Statens Serum Institut. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: aol@ssi.dk (AWO); pa@ssi.dk (PA) ¤ Current address: Department of Clinical Biochemistry & Immunology, Statens Serum Institut, Copenhagen, Denmark Introduction Despite the existence of effective antimicrobial therapy, Chlamydia trachomatis (Ct) continues to be the leading sexually transmitted bacteria worldwide, causing an estimated 92 million new cases annually [1]. Over 225,000 cases were recorded in 2006 in Europe, making it the most frequently reported infectious disease on the continent [2]. If left untreated, approximately 20% of women with a chlamydial lower genital tract infection will develop pelvic inflammatory disease, 4% will develop chronic pelvic pain, 3% infertility and 2% adverse pregnancy outcomes [3]. Furthermore it has been suggested that Ct is a major cofactor for HIV transmission [4] and in the development of cervical neoplasia [5,6]. Early vaccine trials in humans and non-human primates showed that it was possible to induce protection against Ct infection with vaccines based on whole organisms [7,8]. Nonetheless, as the protection was short lived, and in some trials associated with immunopathology, the approach was abandoned [9]. Thus, identification of potential vaccine antigens and exclusion of potential immunopathogenic components is today an active area of research, one which has been accelerated by the availability of the complete Ct serovar D genome sequence [10]. However, so far only relatively few candidates have proven successful in animal models (reviewed in [11]). Rational vaccine design is based on detailed understanding of pathogen biology and the mechanism leading to protection. Immunity against intracellular pathogens like Ct is generally thought to depend on the cellular arm of the immune system. In agreement with this hypothesis, studies in mice and humans have highlighted the importance of CD4+ T-helper type 1 (Th-1) cells in the clearance of a chlamydial infection [4,12,13]. A number of reports dealing with various pathogens in different animal models have indicated that not only the magnitude but also the quality of the T cell response appears to have significant impact on the establishment of protective memory and protection [14–18]. PLoS ONE | www.plosone.org 1 May 2010 | Volume 5 | Issue 5 | e10768