Here, we show that bacteria induce de novo synthesis of both major histocompatability complex (MHC) class I and II molecules in a mouse dendritic cell culture system. The neo-biosynthesis of MHC class I molecules is delayed as compared with that of MHC class II. Furthermore, bacteria stabilize MHC class I molecules by a 3-fold increase of their half-life. This has important consequences for the capacity of dendritic cells to present bacterial antigens in the draining lymph nodes. In addition, a model antigen, ovalbumin, expressed on the surface of recombinant Streptococcus gordonii is processed and presented on MHC class I molecules. This presentation is 106 times more efficient than that of soluble OVA protein. This exogenous pathway of MHC class I presentation is transporter associated with antigen processing (TAP)-dependent, indicating that there is a transport from phagolysosome to cytosol in dendritic cells. Thus, bacteria are shown to be a potentially useful mean for the correct delivery of exogenous antigens to be presented efficiently on MHC class I molecules
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