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Research

Identification of Endogenously Presented Peptides from Chlamydia trachomatis with High Homology to Human Proteins and to a Natural Self-ligand of HLA-B27^*,S

Juan J. Cragnolini and José A. López de Castro

From the Centro de Biología Molecular Severo Ochoa (Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid), Universidad Autónoma, 28049 Madrid, Spain

A strategy for the stable expression of proteins, or large protein fragments, from Chlamydia trachomatis into human cells was designed to identify bacterial epitopes endogenously processed and presented by HLA-B27. Fusion protein constructs in which the green fluorescent protein gene was placed at the 5'-end of the bacterial DNA primase gene or some of its fragments were transfected into B*2705-C1R cells. One of these constructs, including residues 90–450 of the bacterial protein, was stably and efficiently expressed. Mass spectrometry-based comparative analysis of HLA-B27-bound peptide pools led to identification of three HLA-B27 ligands differentially presented in the transfectant cells. Sequencing of these peptides confirmed that they were derived from the bacterial DNA primase. One of them, spanning residues 211–221, showed 55% sequence identity with a known self-ligand of HLA-B27 derived from its own molecule. The other two bacterial ligands, P-(112–121) and P-(112–122), were derived from the same region and differed in length by one residue at the C terminus. Both peptides showed >50% identity with multiple human protein sequences that possessed the optimal peptide motifs for HLA-B27 binding. Thus, expression of proteins from arthritogenic bacteria in HLA-B27-positive human cells allows identifying bacterial peptides that are endogenously processed and presented by HLA-B27 and show molecular mimicry with known self-ligands of this molecule and human proteins.