Immune Responses to Anti-Chlamydial Peptide Vaccine Candidates in a Murine Infection Model

Abstract

Chlamydia trachomatis is a pathogenic bacterium that is a major cause of sexually transmitted disease in humans. Other chlamydial species also cause human or animal diseases. While antibiotics may cure some infections, a large number of human and animal chlamydial infections cause disease without symptoms until late stage disease is detected. A vaccine capable of preventing these infections would be of great public health and veterinary value. The goals of this project were to test T and B cell-mediated immune responses induced by several peptides fulfilling the need for a vaccine that protects against chlamydial infections. Several novel peptides representing mimotopes of the carbohydrate moiety of the chlamydial glycolipid exoantigen were identified by phage display1. Enzyme-Linked Immunosorbent Assays (ELISA), which measure specific antibody responses, had shown that peptide-specific serum antibodies were induced by vaccination before exposure to whole Chlamydia (infection). Those antibody responses correlated with reduced infectious load and tissue pathology when mice were infected. We used lymphocyte proliferative assays (LPA) to measure T lymphocyte responses to peptides and chlamydial organism. The LPA showed dose-dependent peptide-specific proliferation only with lymphocytes from immunized mice. Spleen and lymph node cells from mice vaccinated with our peptide candidates demonstrated immunity both pre- and post-challenge when cells were stimulated with the peptides in vitro. Additional experiments included Direct Fluorescent Antibody (DFA) staining of vaginal smears to test whether vaccination reduced in vivo infectious load. DFA and isolation cultures performed with duplicate vaginal swabs showed reduction of infectious loads in mice vaccinated with the peptides compared to unimmunized control mice. ELISA indicated increased levels of peptide- specific IgG antibodies, particularly after 3 immunizations, in serum from immunized mice. Importantly, the same peptide-specific antibodies induced by four of our initial peptide candidates recognized whole chlamydial organism in infected HEp-2 cells though mice had not been previously exposed to Chlamydia. These results indicated that antibody responses were peptide- and whole organism-specific before challenge and confirmed the results of ELISA. These combined findings provide proof of principle that the vaccine candidates are at least partially protective against infectious chlamydial challenge, and that they stimulate T and B cell immune responses beyond those that occur due to infection alone. These results support the potential of these vaccine candidates for protection against Chlamydia trachomatis infections. Because the peptides represent genus-wide Chlamydial antigens, they theoretically could serve as vaccine candidates against other chlamydial species infecting humans and animals.