Abstract
The processing and MHC class I-restricted presentation of antigenic peptides derived from the p60 protein of the facultative intracellular bacterium Listeria monocytogenes is tightly linked to bacterial protein synthesis. We used non-linear regression analysis to fit a mathematical model of bacterial antigen processing to a published experimental data set showing the accumulation and decay of p60-derived antigenic peptides in L. monocytogenes-infected cells. Two alternative models equally describe the experimental data. The simulation accounting for a stable and a hypothetical rapidly degraded form of antigen predicts that the antigenic peptides p60 217–225 and p60 449–457 are derived from a putative instable form of p60 with an average intracellular half-life of approximately 3 minutes accounting for approximately 31% of all p60 molecules synthesized. The alternative model predicts that both antigenic peptides are processed from p60 degraded intracellularly with a half-life of 109 min and that antigen processing only occurs as long as bacterial protein synthesis is not inhibited. In order to decide between both models the intracellular accumulation of p60 in infected cells was studied experimentally and compared with model predictions. Inhibition of p60 degradation by the proteasome inhibitor epoxomicin revealed that during the first 3 h post infection approximately 30% of synthesized p60 molecules were degraded. This value is significantly lower than the approximately 50% degradation of p60 that would be expected in the presence of the predicted putative short-lived state of p60 and also fits precisely with the predictions of the alternative model, indicating that the tight connection of bacterial protein biosynthesis and antigen processing and presentation of L. monocyctogenes-derived antigenic peptides is not caused by the presence of a highly instable antigenic substrate.
Highlights
The immune surveillance by cytotoxic CD8 T cells of cells infected by intracellular pathogens depends on the rapid presentation of pathogen-derived antigenic peptides in the context of MHC class I molecules on the surface of infected cells
The data set used was published by Sijts and Pamer [11] and shows the accumulation of antigenic peptides in L. monocytogenes-infected cells as well as the subsequent decay of accumulated peptide/MHC complexes after inhibition of bacterial protein synthesis by tetracycline added at 4h after infection of cells
The current analysis suggests that the experimentally observed tight linkage of bacterial protein synthesis and the presentation of L. monocytogenes-derived CD8 T cell epitopes is not caused by the presence of a highly instable antigenic substrate
Summary
The immune surveillance by cytotoxic CD8 T cells of cells infected by intracellular pathogens depends on the rapid presentation of pathogen-derived antigenic peptides in the context of MHC class I molecules on the surface of infected cells. The proteasome degrades pathogen-derived proteins and generates antigenic peptides that subsequently are transported into the endoplasmatic reticulum were binding to nascent MHC class I molecules occurs [1]. Individual proteins are degraded intracellularly with different half-lifes. 30% of a newly synthesized protein enters the pool of rapidly degraded proteins (RDP) which are degraded in less than 10 min. The remainder protein enters the pool of slowly degraded proteins which exhibit a much longer half-life [2,3,4,5]
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