CD8+ T Cell Response in Experimental Chlamydia pneumoniae Infection

Chlamydia pneumoniae (C. pneumoniae) is a common human pathogen causing a range of respiratory diseases from pharyngitis to pneumonia. Even though the majority of diseases are mild or even asymptomatic, persistent C. pneumoniae infections have been associated with chronic diseases such as asthma and atherosclerosis which represent significant health problems in many countries. Consequently, the development of a safe and effective vaccine is an attractive approach that would have a great impact on the prevalence of chlamydial infections and the prevention of severe consequences. Studies on the pathogenesis of C. pneumoniae infection and the host immune response to infection have been greatly aided by well-established mouse models. Intranasal C. pneumoniae challenge of mice leads to a self-restricted pulmonary infection, and a partially protective immunity in which IFN-γ and T lymphocytes, especially CD8+ T cells, play a predominant role. Research objectives of this thesis included characterization of mechanisms for the targeted induction of cytotoxic CD8+ T lymphocyte (CTL) response against C. pneumoniae infection in vivo. Using a computer-based method for predicting the binding of chlamydial peptides to major histocompatibility complex (MHC) class I molecules, candidate CD8 epitopes were identified from C. pneumoniae proteins, and used for peptide immunization of mice. Nineteen C. pneumoniae-derived peptides were identified as CD8 epitopes by their ability to induce cytotoxic response after peptide immunization. Seven of the identified epitopes were able to induce long-term peptide-specific CTL lines, and three were natural epitopes presented by C. pneumoniae infected cells. Identification of CD8 epitopes served as a tool for the characterization of CD8+ T lymphocyte function after various anti-chlamydia immunization approaches. Similarly, human CD8 epitopes were identified in transgenic HHD mice expressing only human class I molecule (HLA-A2.1). Immunization of HHD mice with the whole chlamydial protein induced an HLA-A2.1-restricted peptidespecific CTL line, indicating that the classical human class I molecule can support the development of murine CD8+ T cell response against a chlamydial protein. In spite of a lower number of CD8+ T cells in HHD mice, the outcome of C. pneumoniae infection in the transgenic mice resembled that observed in the wild type mice. These findings encourage the use of HHD mouse model for the further characterization of C. pneumoniae infection and the host immunity against it, as well as for identification of human CD8 epitopes from chlamydial antigens. DNA-based vaccines coding for desired chlamydial protein are effective vehicles in delivering antigen into the eukaryotic cells and inducing both humoral and cellular antigen-specific immune response. However, DNA immunizations have induced only partial protection against C. pneumoniae infection. In an attempt to find a more potent immunization strategy, we cloned C. pneumoniae MOMP and Omp2 genes into the Semliki Forest virus vector and immunized mice with these recombinant viral particles (rSFV). rSFV vaccination, with or without DNA priming, induced antigen-specific humoral and cellular immune responses as well as partial protection against C. pneumoniae pulmonary infection. In addition, immunization with rSFV induced specific CTL responses against the very same chlamydial epitopes that were identified after DNA immunization, indicating that both antigen delivery systems can result in similar MHC class I-dependent antigen processing and presentation. Although the rSFV vaccination using chlamydial MOMP and Omp2 antigen is not sufficient alone to achieve complete protection, this immunization strategy mobilized a wide range of antigen-specific effector mechanisms and speaks in favor of genetic constructs for evaluating vaccine antigens against C. pneumoniae infection. Chlamydial proteins that are secreted out of the inclusion or associated with the inclusion membrane are potential targets for the host’s MHC class I-dependent antigen presentation, thereby representing ideal antigens for CD8+ T cell-based vaccination. Chlamydiae interact actively with the host cell, presumably by exploiting a type three secretion system (TTSS). We demonstrated that a TTSSsecreted chlamydial outer protein N (CopN) as a vaccine induced a strong humoral and cellular immune response, detected as antigen-specific antibodies, proliferation and IFN-γ production. Most importantly, the CopN immunization induced a significant level of protection against subsequent C. pneumoniae infection which certainly recommends this antigen as a candidate vaccine for further investigation.