The Salmonella Effector SteD Mediates MARCH8-Dependent Ubiquitination of MHC II Molecules and Inhibits T Cell Activation

Summary The SPI-2 type III secretion system (T3SS) of intracellular Salmonella enterica translocates effector proteins into mammalian cells. Infection of antigen-presenting cells results in SPI-2 T3SS-dependent ubiquitination and reduction of surface-localized mature MHC class II (mMHCII). We identify the effector SteD as required and sufficient for this process. In Mel Juso cells, SteD localized to the Golgi network and vesicles containing the E3 ubiquitin ligase MARCH8 and mMHCII. SteD caused MARCH8-dependent ubiquitination and depletion of surface mMHCII. One of two transmembrane domains and the C-terminal cytoplasmic region of SteD mediated binding to MARCH8 and mMHCII, respectively. Infection of dendritic cells resulted in SteD-dependent depletion of surface MHCII, the co-stimulatory molecule B7.2, and suppression of T cell activation. SteD also accounted for suppression of T cell activation during Salmonella infection of mice. We propose that SteD is an adaptor, forcing inappropriate ubiquitination of mMHCII by MARCH8 and thereby suppressing T cell activation. Graphical Abstract Highlights • Salmonella effector SteD promotes ubiquitination and surface depletion of mature MHCII • SteD binds both MHCII and the host E3 ubiquitin ligase MARCH8 • SteD uses MARCH8 to ubiquitinate and surface deplete MHCII • SteD suppresses T cell activation during Salmonella infection in vitro and in mice Dendritic cell infection by Salmonella depletes mature MHC class II (mMHCII) from the cell surface. Bayer-Santos et al. reveal that the Salmonella effector SteD binds the E3 ligase MARCH8 and mMHCII to promote mMHCII ubiquitination and surface depletion. SteD suppressed dendritic cell-mediated T cell activation in vitro and in mice.