Inhibition of Nuclear Transport of NF-ĸB p65 by the Salmonella Type III Secretion System Effector SpvD.

Salmonella enterica replicates in macrophages through the action of effector proteins translocated across the vacuolar membrane by a type III secretion system (T3SS). Here we show that the SPI-2 T3SS effector SpvD suppresses proinflammatory immune responses. SpvD prevented activation of an NF-ĸB-dependent promoter and caused nuclear accumulation of importin-α, which is required for nuclear import of p65. SpvD interacted specifically with the exportin Xpo2, which mediates nuclear-cytoplasmic recycling of importins. We propose that interaction between SpvD and Xpo2 disrupts the normal recycling of importin-α from the nucleus, leading to a defect in nuclear translocation of p65 and inhibition of activation of NF-ĸB regulated promoters. SpvD down-regulated pro-inflammatory responses and contributed to systemic growth of bacteria in mice. This work shows that a bacterial pathogen can manipulate host cell immune responses by interfering with the nuclear transport machinery.

Expression and characterization of a novel recombinant version of the secreted human mucin MUC5AC in airway cell lines.

Molecular manipulation and expression of mucins, large glycoproteins that provide the structural framework of mucus, are challenging due to mucins' size and numerous domains, including variable number tandem repeat (VNTRs) regions that are sites of O-glycosylation. Only individual human mucin domains have been expressed in mammalian cells. We produced recombinant versions of MUC5AC, a major secreted mucin in the respiratory tract, encoding the N-terminus, C-terminus, N- and C-termini together, and N- and C-termini interspersed with two native tandem repeat sequences (N+2TR+C) in both tracheal and bronchial cell lines. The latter protein contains all of the functional domains required for the biosynthesis and secretion of glycosylated mucin. The N-terminus protein was found in monomeric and higher molecular mass forms suggesting that secreted MUC5AC may form a branched netlike structure analogous to that described for MUC2. At the C-terminus, proteins underwent cleavage, polymerization, and glycosylation. Thus, they appear to undergo pivotal processing steps as predicted for native MUC5AC, which is analogous to that for other individual recombinant mucin domains. Secretion occurred when cells were grown on transwell filter inserts but not on plastic, indicating that the extracellular environment likely plays a role in mucin processing. The secreted N+2TR+C protein differed in molecular mass from the intracellular form, indicating that additional processing occurred. These recombinant proteins, expressed in different backgrounds, can potentially address the role of different mucin domains on MUC5AC processing and function as well as the role of MUC5AC in health and disease.

Use of Recombinant Mucin Glycoprotein to Assess the Interaction of the Gastric Pathogen Helicobacter pylori with the Secreted Human Mucin MUC5AC.

There is intense interest in how bacteria interact with mucin glycoproteins in order to colonise mucosal surfaces. In this study, we have assessed the feasibility of using recombinant mucin glycoproteins to study the interaction of the gastric pathogen Helicobacter pylori with MUC5AC, a mucin which the organism exhibits a distinct tropism for. Stable clonal populations of cells expressing a construct encoding for a truncated version of MUC5AC containing N- and C-termini interspersed with two native tandem repeat sequences (N + 2TR + C) were generated. Binding of H. pylori to protein immunoprecipitated from cell lysates and supernatants was assessed. High molecular weight mucin could be detected in both cell lysates and supernatants of transfected cells. Recombinant protein formed high molecular weight oligomers, was both N and O glycosylated, underwent cleavage similar to native MUC5AC and was secreted from the cell. H. pylori bound better to secreted mucin than intracellular mucin suggesting that modifications on extracellular MUC5AC promoted binding. Lectin analysis demonstrated that secreted mucin was differentially glycosylated compared to intracellular mucin. H. pylori also bound to a recombinant C-terminus MUC5AC protein, but binding to this protein did not inhibit binding to the N + 2TR + C protein. This study demonstrates the feasibility of using recombinant mucins containing tandem repeat sequences to assess microbial mucin interactions.

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

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.