Identification of sequences in the human peptide transporter subunit TAP1 required for transporter associated with antigen processing (TAP) function

The heterodimeric peptide transporter associated with antigen processing (TAP) consisting of the subunits TAP1 and TAP2 mediates the transport of cytosolic peptides into the lumen of the endoplasmic reticulum (ER). In order to accurately define domains required for peptide transporter function, a molecular approach based on the construction of a panel of human TAP1 mutants and their expression in TAP1(-/-) cells was employed. The characteristics and biological activity of the various TAP1 mutants were determined, and compared to that of wild-type TAP1 and TAP1(-/-) control cells. All mutant TAP1 proteins were localized in the ER and were capable of forming complexes with the TAP2 subunit. However, the TAP1 mutants analyzed transported peptides with different efficiencies and displayed a heterogeneous MHC class I surface expression pattern which was directly associated with their susceptibility to cytotoxic T lymphocyte-mediated lysis. Based on this study, the TAP1 mutants can be divided into three categories: those expressing a similar phenotype compared to TAP1(-/-) or wild-type TAP1 cells respectively, and those representing an intermediate phenotype in terms of peptide transport rate, MHC class I surface expression and immune recognition. Thus, the results provide evidence that specific regions in the TAP1 subunit are crucial for the proper processing and presentation of cytosolic antigens to MHC class I-restricted T cells, whereas others may play a minor role in this process.

Evidence for a differential avidity model of T cell selection in the thymus

Positive and negative selection of a lymphocytic choriomeningitis virus (LCMV) peptide-specific, H-2Db-restricted T cell clone (P14) was studied using TAP1- and TAP1+ mice transgenic for P14 T cell receptor (TCR) alpha and beta genes. Positive selection of transgenic CD8+ P14 cells was impaired in TAP1- mice. Addition of the LCMV peptide to TAP1- fetal thymic organ cultures (FTOCs) at low and high concentrations induced positive and negative selection of CD8+ P14 cells, respectively, while addition of the same peptide to TAP1+ FTOCs induced negative selection even at low concentrations. Both types of selection were peptide specific. Thus, a critical parameter that controls the fate of a thymocyte seems to be the number of TCRs engaged with complexes of peptide and major histocompatibility complex. When this number is low, positive selection occurs, and when it is high, negative selection takes place. These findings support a differential avidity model of T cell selection.

Separation of thymic education from antigen presenting functions of major histocompatibility complex class I molecules

Participation of transmembrane (TM) and glycosyl-phosphatidylinositol (GPI) anchored H-2Db molecules in antigen presentation and thymic selection events was investigated using transgenic mice. Both GPI-Db and TM-Db can efficiently present H-Y antigen, influenza and lymphocytic choriomeningitis virus (LCMV) peptides to primed cytotoxic, H-2Db-restricted T cells. Transgenic mice expressing GPI-Db, although unable to reject TM-Db skin grafts, nevertheless generate secondary CTL responses which can lyse TM-Db-bearing targets, indicating that GPI-Db mice fail to delete all TM-Db-reactive T cells. Furthermore, double-transgenic mice bearing GPI-Db and a T-cell receptor (TcR) for H-2Db+LCMV do not positively select receptor positive, CD8+CD4- T cells. This paradoxical behaviour of GPI-Db molecules suggests that the structural requirements for antigen presentation and thymic selection by class I molecules are different and may explain why GPI-linked class I molecules, such as Qa-2, do not appear to function as restriction elements in vivo.

T and B cell tolerance and responses to viral antigens in transgenic mice: implications for the pathogenesis of autoimmune versus immunopathological disease

Experiments with transgenic mice illustrate clonal elimination of T cells specific for antigens expressed appropriately in the thymus, but presence of inducible T cells when the antigen presented on class I MHC antigens is expressed exclusively on non-lymphohemopoietic cells in the periphery (pancreatic beta islet cells). TCR-transgenic LCMV-carrier mice expressing LCMV in the thymus exhibit clonal elimination at the early CD4+8+ thymocyte stage, causing CTL unresponsiveness in these mice. In contrast, studies with RIP LCMV-GP-transgenic mice (expressing GP in pancreatic beta cells) and with TCR-RIP LCMV-GP double-transgenic mice show that CTL reactivity is normal. These experiments argue against so-called peripheral anergy of class I MHC antigen-restricted cytotoxic T cells as a general mechanism of peripheral immunological tolerance to self. They reveal that self epitopes that are genetically self and presented by class I antigens may not be considered immunologically self if expressed solely extrathymically, despite the fact that they are antigenic and can be recognized by induced effector T cells. Genetic self that is presented on cells which can induce neither tolerance nor an immune response is immunologically dealt with as foreign and therefore may be called nonimmunological self. Appropriate presentation of the same epitope on antigen-presenting cells promptly induces effector T cells and causes disease; such disease should not be called autoimmune because it is an immunopathological T-cell mediated disease, comparable to an unfavorably balanced immunopathological T-cell response to a virus. Mechanisms that control autoantibody responses were studied in mice expressing a viral transgene. Such mice generate neutralizing antiviral autoantibody responses only when the transgenic viral antigen is linked to a foreign T-helper determinant. These findings, therefore, document differences in levels of T- vs B-cell tolerance (so-called split tolerance) under a given expression level of a "self" antigen. They illustrate how unresponsiveness of B cells to produce T-independent IgM is dose-dependent and that IgG autoantibodies are triggered by introducing foreign T-helper determinants that can be recognised in a linked fashion. This model suggests that, while T-cell tolerance to tolerogenic self in the thymus is solid, B-cell tolerance in general is not. From the point of view of autoantibody responses these T-helper cells may also be called immunopathological; i.e., these T-helper cells are specific for foreign epitopes that, via linked recognition, trigger truly autoimmune B cells.(ABSTRACT TRUNCATED AT 400 WORDS)

An example of enzyme hysteresis. The slow and tight interaction of some fully competitive inhibitors with small intestinal sucrase

Of the fully competitive inhibitors of small intestinal sucrase investigated in this or in other papers, acarbose, nojirimycin, and deoxynojirimycin (Fig. 2) have the highest affinity for the enzyme, their Ki values being in the 10(-7)-10(-8) M range. Furthermore, thier interaction with the enzyme is slow, the steady state being reached in their presence in a matter of minutes. Their overall "on" and "off" constants are small, which indicates that a conformational change accompanies the interaction of these substances with the active site of intestinal sucrase. The structure of these inhibitors, as well as the pH dependence of their Ki values, agrees with and allows additions to be made to the catalytic mechanism earlier suggested for this enzyme (Cogoli, A., and Semenza, G. (1975) J. Biol. Chem. 250, 7802-7809). None of these inhibitors of sucrase has any sizeable effect on the small intestinal Na+-dependent D-glucose transport system.