A Chlamydia trachomatis OmcB C-Terminal Fragment Is Released into the Host Cell Cytoplasm and Is Immunogenic in Humans

Abstract

The Chlamydia trachomatis outer membrane complex protein B (OmcB) is an antigen with diagnostic and vaccine relevance. To further characterize OmcB, we generated antibodies against OmcB C-terminal (OmcBc) and N-terminal (OmcBn) fragments. Surprisingly, the anti-OmcBc antibody detected dominant signals in the host cell cytosol, while the anti-OmcBn antibody exclusively labeled intrainclusion signals in C. trachomatisinfected cells permeabilized with saponin. Western blot analyses revealed that OmcB was partially processed into OmcBc and OmcBn fragments. The processed OmcBc was released into host cell cytosol, while the OmcBn and remaining full-length OmcB were retained within the chlamydial inclusions. The organism-associated OmcB epitopes became detectable only after the C. trachomatis-infected cells were permeabilized with strong detergents such as SDS. However, the harsh permeabilization conditions also led to the leakage of the already secreted OmcBc and chlamydia-secreted protease (CPAF) out of the host cells. The OmcBc processing and release occurred in all biovars of C. trachomatis. Moreover, the released OmcBc but not the retained OmcBn was highly immunogenic in C. trachomatis-infected women, which is consistent with the concept that exposure of chlamydial proteins to host cell cytosol is accompanied by increased immunogenicity. These observations have provided important information for further exploring/optimizing OmcB as a target for the development of diagnosis methods and vaccines.

Chlamydial Plasmid-Dependent Pathogenicity

“…However, it is unclear whether pGP3 can activate caspase-1 although bacterial lipopolysacchride (LPS) in the host cell cytosol is known to directly activate other inflammatory caspases [125, 126]. pGP3 [27] and various other chlamydial proteins [127–130] are both associated with pathologies in Chlamydia -infected humans [66, 106] and secreted into the cytosol of the infected cells. These proteins may have the opportunity to access the cytosolic inflammatory pathways.…”

A path forward for the chlamydial virulence factor CPAF

“…These approaches have led to the discovery of many putative effectors. However, chlamydial proteins that have been visualized in the cytosol of Chlamydia-infected cells are CPAF [1], cHtrA [25], CT621 [26], CT622 [27], CT311 [28, 29], CT795 [30], GlgA [31], the C-terminus of OmcB [32] & Pgp3 [17]. Nevertheless, despite the definitive evidence for their localization in the host cell cytosol, the precise roles of these factors in chlamydial pathogenesis remain largely unknown.…”

“…Only the cleaved forms of these proteins can be detected in cells lysed in 8 M urea, suggesting that observations of cleavage by CPAF in cell-free assays are indeed true. For OmcB, it has been known that its C-terminus (OmcBc) serves as an immunodominant CD8+ T cell antigen, but not until recently was it shown that this portion of the protein is released into the host cell cytosol [32]. The advent of another CPAF inhibitor, a peptide that mimics an inhibitory domain of the native protein and gains access to the host cell through a polyarginine cell-penetrating peptide tag, has allowed for the further characterization of CPAF-specific cleavage events.…”

Host-Pathogen Co-Evolution: Chlamydia trachomatis Modulates Surface Ligand Expression in Genital Epithelial Cells to Evade Immune Recognition

“…Interestingly, some of the chlamydial antigens that trafficked to the ER were seen to co-localize with the MHC-like molecule CD1d (see below) (Wyrick 2010). Others have shown that proteins such as the chlamydial outer membrane complex protein B (OmcB) (Qi, Gong et al 2011) and the chlamydia protease, CPAF (see below), are present in the host cell cytosol (Sharma, Bosnic et al 2004;Kawana, Quayle et al 2007), the latter being secreted through a Sec-dependent pathway (Chen, Lei et al 2010). C. trachomatis also uses the type III secretory system (T3SS) to move bacterial virulence proteins from EB into the host cytosol (Clifton, Fields et al 2004;Hower, Wolf et al 2009).…”