Title: The T cell response to Chlamydia trachomatis
Description: C. trachomatis is an important human pathogen for which T cells play a critical role in protective immunity. CD8 + T cells can efficiently lyse C. trachomatis -infected cells in vitro, but do not aid in the clearance of infected genital epithelia in mice. The goal of this work was to determine whether C. trachomatis -specific CD8 + T cells are unable to recognize infected epithelial cells in vivo because the T cells primed during infection are predominantly specific for antigens that can only be cross-presented by professional antigen presenting cells (APC). Therefore, the majority of CD8 + T cell antigens cannot be presented by the endogenous MHC class I pathway during C. trachomatis infection of epithelial cells. To this end, a novel approach was developed to identify the dominant T cell antigens recognized by the immune response generated during C. trachomatis infection. The method employs recombinant E. coli to express and specifically deliver every open reading frame (ORF) product encoded in the genome of C. trachomatis to either the MHC class I or class II presentation pathway of APC. C. trachomatis -specific T cells are then used to screen the APC to identify the antigen specificities. Twelve previously unknown CD8 + T cell antigens and two CD4 + T cell antigens of C. trachomatis were identified. Ten of the CD8 + T cell antigens were identified in screens using nonspecifically expanded CD8 + T cells isolated from the site of infection in vivo and therefore should represent many of the dominant specificities primed during the infection.

All ten of these antigens were homologs of well characterized proteins known to be localized to the cytoplasm or periplasm in other bacterial species. Consistent with these antigens only being presented by professional APC, infected epithelial and fibroblasts were unable to activate T cells specific for several cytoplasmic antigens, indicating that these antigens are not presented by infected host cells. These data strongly suggest that cross-presentation is the major presentation pathway used to prime CD8 + T cells during infection by C. trachomatis and can present antigen from all compartments of a bacterial cell. This work supports the hypothesis that CD8 + T cells do not aid in the protection against C. trachomatis genital infection because the majority of the T cells are specific for antigens that cannot be presented by infected epithelial cells in vivo. These results have significant implications for the rational design of a C. trachomatis vaccine.