Human CD8+ T Cells Recognize the 60-kDa Cysteine-Rich Outer Membrane Protein fromChlamydia trachomatis

Abstract

The intracellular bacterial pathogen Chlamydia is sequestered from the host cell cytoplasm by remaining within an inclusion body during its replication cycle. Nevertheless, CD8+ T cells recognizing Chlamydia Ags in the context of MHC class I molecules are primed during infection. We have recently described derivation of Chlamydia-specific human CD8+ T cells by using infected dendritic cells as a surrogate system to reflect Chlamydia-specific CD8+ T cell responses in vivo. These CD8+ T cell clones recognize chlamydial Ags processed via the conventional class Ia processing pathway, as assessed by treatment of infected APC with lactacystin and brefeldin A, suggesting that the Ags are translocated from the chlamydial inclusion into the host cell cytosol. In this study, outer membrane protein 2 (OmcB) was identified as the Ag recognized by one of these Chlamydia-specific human CD8+ T cells, and we defined the HLA*A0101-restricted epitope from this Ag. CD8+ T cell responses to this epitope were present at high frequencies in the peripheral blood of both of two HLA*A0101 donors tested. In vitro chlamydial growth was completely inhibited by the OmcB-specific CD8+ T cell clone independently of lytic mechanisms. OmcB is a 60-kDa protein that has been postulated to be associated with the Chlamydia outer membrane complex. The subcellular localization of OmcB to the cytosol of infected cells, as determined by conventional MHC class I Ag processing and presentation, suggests the possibility of an additional, cytosolic-associated function for this protein.

Chlamydia trachomatis Outer Membrane Complex Protein B (OmcB) Is Processed by the Protease CPAF

“…The localization of OmcBc in the host cell cytosol correlates well with its immunodominance in C. trachomatis-infected women (29), which is consistent with the concept that exposure of chlamydial proteins to the host cell cytosol is accompanied by increased immunogenicity (19,52,53). OmcBc is dominantly recognized by not only human antibodies (19) but also human CD4 ϩ (54) and CD8 ϩ T cells (21), indicating that OmcBc can access multiple immune processing compartments (55). These observations suggest that OmcB is a highly dynamic molecule and that release of OmcBc into the host cell cytosol may allow OmcB to participate in both the chlamydial intracellular interaction with host cells and chlamydial modulation of host immune responses.…”

“…We have confirmed that OmcB, a chlamydial outer membraneassociated, highly conserved, abundant and immunogenic protein (12,(18)(19)(20)(21)(25)(26)(27)(28), is processed during chlamydial infection since the processing was still detectable after the infected culture samples were harvested in 8 M urea, which has been shown to prevent proteolysis during cell lysis (30). Although we have previously reported that OmcB is partially cleaved (29), careful confirmation of OmcB processing in the current study is still necessary since a recent study has revealed that almost all previously reported protein cleavages in Chlamydia-infected cell samples are due to proteolysis during cell lysis (30).…”

“…OmcB was detected at the inner surface of the outer membrane and only became surface accessible after treatment with reducing reagents and proteases (17). The identification of CD8 T-cell epitopes in the OmcB C terminus (21) suggests that the C-terminal region can enter the host cell cytosol. We have recently reported that OmcB is partially processed into C-terminal (OmcBc) and N-terminal (OmcBn) fragments, and the processed OmcBc is released into the host cell cytosol while the processed OmcBn and remaining fulllength OmcB are retained within the chlamydial inclusions (29).…”

“…OmcB may also be involved in the conversion of RBs to EBs (11,17) and contribute to cell wall rigidity and osmotic stability of the EBs (11). OmcB is an immunodominant antigen with both B-cell and T-cell epitopes (18)(19)(20)(21)(22)(23)(24) and has been considered a candidate for developing both serodiagnostics (12, 25) and subunit vaccines (26-28). Nevertheless, there has been considerable debate regarding the precise location and role of OmcB during C. trachomatis infection.…”

The Frequency ofChlamydia trachomatisMajor Outer Membrane Protein-Specific CD8+T Lymphocytes in Active Trachoma Is Associated with Current Ocular Infection

“…CTL have been used to identify antigens, such as major histocompatibility complex (MHC) class I-accessible protein 1, which were not detected by other techniques (11). In humans, C. trachomatis-specific CTL have been identified in subjects exposed to chlamydial infection (12,13,17,19,20). In these and other studies, CTL generated to a number of C. trachomatis epitopes that were restricted by common and disease-associated HLA types were used to identify peptide-specific responses (12,15,24).…”