Several recent studies have demonstrated that T-helper cell-dependent events during the initial priming period are required for the generation of CD8 ؉ T cell-mediated protective immunity. The underlying mechanisms of this phenomenon have not yet been determined, mostly because of difficulties in studying memory T cells or their precursor populations at early stages during immune responses. We identified IL-7 receptor (CD127) surface expression as a marker for long-living memory T cells, most importantly allowing the distinction between memory and effector T cells early after in vivo priming. The combination of surface staining for CD127 and CD62L further separates between two functionally distinct memory cell subsets, which are similar (if not identical) to cell subsets recently described as central memory T cells (CD127 high and CD62L high ) and peripheral effector memory T cells (CD127 high and CD62L low ). Using this new tool of memory T cell analysis, we demonstrate that CD8 ؉ T cell priming in the absence of T cell help or CD40L specifically alters the generation of the effector memory T cell subset, which appears to be crucial for immediate memory responses and long-term maintenance of effective protective immunity. Our data reveal a unique strategy to obtain information about the quality of long-term protective immunity early during an immune response, a finding that may be applied in a variety of clinical settings, including the rapid monitoring of vaccination success.

Individuals with systemic lupus erythematosus and rheumatoid arthritis are characterized by the presence of high levels of circulating IgM and IgG autoantibodies. Although IgG autoantibodies often are pathogenic, the role of IgM autoantibodies in autoimmune disease is not clear. Using mice that are unable to secrete IgM but are able to express surface IgM and IgD and to secrete other classes of immunoglobulins, we examined the effect of the absence of secreted IgM in the development of IgG autoantibodies and autoimmune disease in lupus-prone lymphoproliferative (lpr) mice. Compared with regular lpr mice, lpr mice that lack secreted IgM developed elevated levels of IgG autoantibodies to doublestranded DNA and histones and had more abundant deposits of immune complexes in the glomeruli; they also suffered more severe glomerulonephritis and succumbed to the disease at an earlier age. Similarly, the absence of secreted IgM also resulted in an accelerated development of IgG autoantibodies in normal mice. These findings suggest that secreted IgM, including IgM autoantibodies produced naturally or as part of an autoimmune response, may lessen the severity of autoimmune pathology associated with IgG autoantibodies.

Our knowledge about the kinetics and dynamics of complex pathogen-specific CD8 + T cell responses and the in vivo development of CD8 + memory T cells has increased substantially over the past years; in comparison, relatively little is known about the CD4 + T cell compartment. We monitored and directly compared the phenotypical changes of pathogen (Listeria monocytogenes)-specific CD8 + and CD4 + T cell responses under conditions leading to effective and long-lasting protective immunity. We found that the general kinetics of bacteriaspecific CD8 + and CD4 + T cells during the effector and post-effector phases are synchronized. However, later during the memory phase, CD8 + and CD4 + T cell populations differ substantially. Whereas CD8 + memory T cell populations with immediate effector function are readily detectable in lymphoid and non-lymphoid tissues and remain remarkably stable in size, antigen-specific CD4 + effector-memory T cells decline continuously in frequency over time. These findings have important implications for the better understanding of the in vivo development of protective immunity towards intracellular pathogens.

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Differences in the cleavage specificities of constitutive proteasomes and immunoproteasomes significantly affect the generation of MHC class I ligands and therefore the activation of CD8-positive T cells. Based on these findings, we investigated whether proteasomal specificity also influences CD8-positive T cells during thymic selection by peptides derived from self proteins. We find that one of the self peptides responsible for positive selection of ovalbuminspecific OT-1 T cells, which is derived from the f-actin capping protein (Cp␣1), is efficiently generated only by immunoproteasomes. Furthermore, OT-1 mice backcrossed onto low molecular mass protein 7 (LMP7)-deficient mice show a 50% reduction of OT-1 cells. This deficiency is also observed after transfer of BM from OT-1 mice in LMP7-deficient mice and can be corrected by the injection of the Cp␣1 peptide. Interestingly, WT and LMP7-deficient mice mount comparable immune responses to the ovalbumin-derived epitope SIINFEKL. However, their cytotoxic T lymphocytes (CTL) differ in the use of T cell receptor V␤ genes. CTL derived from WT mice use V␤8 or V␤5 (the latter is also used by OT-1 cells), whereas SIINFEKL-specific CTL from LMP7-deficient mice are exclusively V␤8-positive. Taken together, our experiments provide strong evidence that proteasomal specificity shapes the repertoire of T cells participating in antigen-specific immune responses. selection ͉ T cell repertoire C ytotoxic T lymphocytes (CTL) recognize a complex of MHC class I molecules and peptides derived mainly from intracellular proteins. The generation of these peptides crucially depends on the activity of proteasomes, which represent the main proteolytic activity present in the cytoplasm and are responsible for the generation of the C termini of most peptides presented by MHC class I molecules (1-5). The 20S proteasome represents the proteolytic core complex, which is composed of seven different ␣-and seven different ␤-subunits organized in a complex of four stacked rings with ␣ 7 ␤ 7 ␤ 7 ␣ 7 stoichiometry (6-8). The specificity of this protease is influenced by the incorporation of three IFN-inducible ␤-subunits (␤1i, ␤2i, and ␤5i), which replace the three proteolytically active constitutive subunits (␤1, ␤2, and ␤5) during proteasome assembly, resulting in the formation of so-called immunoproteasomes (9, 10). This type of proteasome generates reduced numbers of peptides with acid C termini and increased numbers of peptides with hydrophobic C termini (11,12), which is in favor of a more efficient transporter associated with antigen-presentation transport and binding to MHC class I molecules (13). In addition, a change in proteolytic specificity is apparent from the numerous examples reporting that either constitutive proteasomes or immunoproteasomes are required for the efficient generation of certain MHC class I ligands and consequently for the activation of CTL (14-19). As a result, proteasomal specificities provide a major contribution to the hierarchies of epitopes recognized by pathog...