Reactions of the subunits of the class II major histocompatibility complex molecule IAd

Functional analysis of the Salmonella pathogenicity island 2-mediated inhibition of antigen presentation in dendritic cells

Salmonella enterica is a facultative intracellular pathogen that is able to modify host cell functions by means of effector proteins translocated by the type III secretion system (T3SS) encoded by Salmonella Pathogenicity Island 2 (SPI2). The SPI2-T3SS is also active in Salmonella after uptake by murine bone marrow-derived dendritic cells (BM-DC). We have previously shown that intracellular Salmonella interfere with the ability of BM-DC to stimulate antigen-dependent T-cell proliferation in an SPI2-T3SS-dependent manner. We observed that Salmonella-mediated inhibition of antigen presentation could be restored by external addition of peptides on major histocompatibility complex class II (MHC-II). The processing of antigens in Salmonella-infected cells was not altered; however, the intracellular loading of peptides on MHC-II was reduced as a function of the SPI2-T3SS. We set out to identify the effector proteins of the SPI2-T3SS involved in inhibition of antigen presentation and demonstrated that effector proteins SifA, SspH2, SlrP, PipB2, and SopD2 were equally important for the interference with antigen presentation, whereas SseF and SseG contributed to a lesser extent to this phenotype. These observations indicate the presence of a host cell-specific virulence function of a novel subset of SPI2-effector proteins.

Stimulation of T cells by antigenic peptide complexed with isolated chains of major histocompatibility complex class II molecules

Major histocompatibility complex (MHC) class II molecules are heterodimeric glycoproteins with one alpha and one beta polypeptide chain of similar molecular size. In this report, we describe the binding of an acetylated N-terminal peptide of myelin basic protein, [Ala4]MBP-(1-14), to purified individual alpha and beta chains of murine I-Ak molecules. Purified complexes of isolated single chains and antigenic peptide bind to cloned T cells restricted by I-Ak and [Ala4]MBP-(1-14) tetradecapeptide. The binding is blocked by alpha/beta anti-T-cell receptor (TCR) monoclonal antibody. Cell triggering as measured by an increase in extracellular acidification rate is observed when cloned T cells are exposed to purified complexes of isolated chains and antigenic peptide. This increase in the extracellular acidification rate is antigen specific and MHC-restricted, as chains alone or irrelevant chain-peptide complexes do not trigger an increase in the metabolic acidification rate. These results together demonstrate that in vitro cloned T cells are triggered by complexes of specific antigenic peptides and isolated individual chains of their cognate MHC proteins.