Abstract Chlamydia pathogenesis Sexually transmitted infection with Chlamydia trachomatis (CT) is a leading infectious cause of tubal infertility due to the induction of excessive fibrosis observed as adhesion and hydrosalpinx in the female upper genital tract under laparoscopy. Both CT ascending infection and host immune responses likely contribute to the sequelae. Intravaginal inoculation with Chlamydia muridarum (CM) induces hydrosalpinx in mice, which is frequently used for investigating the pathogenic mechanisms of CT. The mouse model reveals a two-hit mechanism for chlamydial induction of hydrosalpinx: The lower genital Chlamydia ascends to the upper genital tract and infects oviduct epithelial cells, causing direct damages, as the 1st hit, and the Chlamydia-induced lymphocytes with a profibrotic phenotype, after recruiting to the oviduct, may provide a 2nd hit to convert the initial damage-triggered tissue repairing into pathological fibrosis. Both hits may be required for chlamydial induction of the sequelae. CM that spreads from the genital tract to the gastrointestinal (GI) tract can most efficiently induce pathogenic CD8+ T cells to provide a 2nd hit for promoting CM pathogenicity in the upper genital tract of mice. Although CT is also frequently detected in the GI tract of women, it is unclear how CT reaches the human GI tract and where CT induces 2nd hits for promoting CT pathogenicity in women. Regardless of how a 2nd hit is induced, we hypothesize that chlamydial antigen-specific T lymphocytes, particularly CD8+ T cells, are able to provide a 2nd hit for promoting chlamydial pathogenicity in the upper genital tract by delivering profibrotic effectors. Since T cells are known to contribute to both chlamydial pathogenesis and immunity, we further hypothesize that pathogenic and protective T cells may recognize both common and distinct CT antigens. We will test these hypotheses by identifying profibrotic effectors of pathogenic CD8+ T cells (Aim I), systematically mapping the antigen specificities of functionally defined pathogenic vs. nonpathogenic CD8+ T cells from mice (Aim II) and identifying T cell antigens in CT-exposed women with or without tubal infertility (Aim III). The information to be obtained will advance our understanding of chlamydial pathogenic mechanisms and improve differential diagnosis of tubal infertility as well as facilitate the development of Chlamydia subunit vaccines.