The direct major histocompatibility complex (MHC) class I antigen presentation pathway ensures intracellular peptides are displayed at the cellular surface for recognition of infected or transformed cells by CD8 ؉ cytotoxic T lymphocytes. Chlamydia spp. are obligate intracellular bacteria and, as such, should be targeted by CD8 ؉ T cells. It is likely that Chlamydia spp. have evolved mechanisms to avoid the CD8 ؉ killer T cell responses by interfering with MHC class I antigen presentation. Using a model system of self-peptide presentation which allows for posttranslational control of the model protein's stability, we tested the ability of various Chlamydia species to alter direct MHC class I antigen presentation. Infection of the JY lymphoblastoid cell line limited the accumulation of a model host protein and increased presentation of the model-protein-derived peptides. Enhanced selfpeptide presentation was detected only when presentation was restricted to defective ribosomal products, or DRiPs, and total MHC class I levels remained unaltered. Skewed antigen presentation was dependent on a bacterial synthesized component, as evidenced by reversal of the observed phenotype upon preventing bacterial transcription, translation, and the inhibition of bacterial lipooligosaccharide synthesis. These data suggest that Chlamydia spp. have evolved to alter the host antigen presentation machinery to favor presentation of defective and rapidly degraded forms of self-antigen, possibly as a mechanism to diminish the presentation of peptides derived from bacterial proteins.

Background Chlamydia species are obligate intracellular bacteria that infect a broad range of mammalian hosts. Members of related genera are pathogens of a variety of vertebrate and invertebrate species. Despite the diversity of Chlamydia, all species contain an outer membrane lipooligosaccharide (LOS) that is comprised of a genus-conserved, and genus-defining, trisaccharide 3-deoxy-D-manno-oct-2-ulosonic acid Kdo region. Recent studies with lipopolysaccharide inhibitors demonstrate that LOS is important for the C. trachomatis developmental cycle during RB- > EB differentiation. Here, we explore the effects of one of these inhibitors, LPC-011, on the developmental cycle of five chlamydial species. ResultsSensitivity to the drug varied in some of the species and was conserved between others. We observed that inhibition of LOS biosynthesis in some chlamydial species induced formation of aberrant reticulate bodies, while in other species, no change was observed to the reticulate body. However, loss of LOS production prevented completion of the chlamydial reproductive cycle in all species tested. In previous studies we found that C. trachomatis and C. caviae infection enhances MHC class I antigen presentation of a model self-peptide. We find that treatment with LPC-011 prevents enhanced host-peptide presentation induced by infection with all chlamydial-species tested.ConclusionsThe data demonstrate that LOS synthesis is necessary for production of infectious progeny and inhibition of LOS synthesis induces aberrancy in certain chlamydial species, which has important implications for the use of LOS synthesis inhibitors as potential antibiotics.

The direct MHC class I antigen presentation pathway presents cytosolic proteins for immune surveillance. While this pathway is an attractive target for therapeutics, we must first understand the efficiency of antigen presentation. Previous studies have shown the presentation efficiency from pathogen-derived antigens ranges from 0.03–25%. The intracellular lifestyle of Chlamydia makes them an attractive model to study class I presentation. It is likely that Chlamydia have developed mechanisms to evade CD8+ T cells through manipulation of this pathway. Here, we investigate the impact of Chlamydial infection on a recombinant fusion protein expressed in a human lymphoblastoid cell line (JY). The fusion protein contains a destabilization domain, an antigenic peptide, and a reporter. Chlamydial infection led to decreased accumulation of the model protein and increased presentation of model protein-derived antigens. This enhanced self-peptide presentation was only observed when antigen presentation was restricted to the DRiP form of the protein. We also investigated the antigen presentation efficiency of different model recombinant proteins in JY cells. We calculated the antigen presentation efficiency to be 0.35%, much lower than was previously observed in murine cells with a similar protein. The skewed antigen presentation phenotype observed following Chlamydial infection was not statistically different from the efficiency observed in the absence of infection. These data suggest that Chlamydia have evolved a mechanism to skew the host antigen presentation machinery toward presentation of DRiPs, and that the efficiency of direct class I presentation may be mechanistically unique among common immune signaling pathways.