A long-standing paradox in the study of T cell antigen recognition is that of the high specificity–low affinity T cell receptor (TCR)–major histocompatibility complex peptide (MHCp) interaction. The existence of multivalent TCRs could resolve this paradox because they can simultaneously improve the avidity observed for monovalent interactions and allow for cooperative effects. We have studied the stoichiometry of the TCR by Blue Native–polyacrylamide gel electrophoresis and found that the TCR exists as a mixture of monovalent (αβγεδεζζ) and multivalent complexes with two or more ligand-binding TCRα/β subunits. The coexistence of monovalent and multivalent complexes was confirmed by electron microscopy after label fracture of intact T cells, thus ruling out any possible artifact caused by detergent solubilization. We found that although only the multivalent complexes become phosphorylated at low antigen doses, both multivalent and monovalent TCRs are phosphorylated at higher doses. Thus, the multivalent TCRs could be responsible for sensing low concentrations of antigen, whereas the monovalent TCRs could be responsible for dose-response effects at high concentrations, conditions in which the multivalent TCRs are saturated. Thus, besides resolving TCR stoichiometry, these data can explain how T cells respond to a wide range of MHCp concentrations while maintaining high sensitivity.

For CD8⁺ T cells, a relatively short antigen pulse seems sufficient for antigen-presenting cells to drive clonal expansion and differentiation. It is unknown whether the requirement for antigen is similarly ephemeral for CD4⁺ T cells. To study the dependence of a CD4⁺ T cell response on antigen persistence in a quantitatively and temporally controlled manner in vivo, we engineered a mouse line expressing a major histocompatibility complex class II–restricted epitope in dendritic cells under the control of a tetracycline-inducible promoter. Experiments tracking the proliferation of CD4⁺ T cells exposed to their cognate antigen in various amounts for different time periods revealed that the division of such cells was contingent on the presence of antigen throughout their expansion phase, even in the presence of an inflammatory stimulus. This previously unrecognized feature of a CD4⁺ T cell response contrasts with the proliferative behavior of CD8⁺ T cells that has been documented, and it implies that the two T cell subsets might require different strategies for efficient vaccination.

It has been reported that the differentiation of CD4⁺CD25⁺ regulatory T cells (T reg cells) can be induced by agonist peptide/major histocompatibility complex ligands in the thymus. Exploiting a transgenic mouse line wherein expression of a particular T cell epitope can be controlled temporally and quantitatively, we found that diversion of differentiating thymocytes into the FoxP3 T reg cell pathway by this agonist ligand was essentially nonexistent. However, CD4⁺CD25⁺ thymocytes were much less sensitive than their CD4⁺CD25⁻ companions, by two to three orders of magnitude, to agonist-induced clonal deletion, such that their proportion increased, giving the false impression of induced differentiation. To account for these and prior observations, one can propose that differentiation along the CD4⁺CD25⁺ pathway is induced by cues other than recognition of self-agonist cues, which are poorly read by thymocytes, whose T cell receptors are conducive to selection toward the conventional CD4⁺CD25⁻ lineage. Thus, selective survival, rather than induced differentiation, may explain the apparent enrichment observed here and in previous studies.

Cell-mediated immune responses are essential for protection against many intracellular pathogens. For Mycobacterium tuberculosis (MTB), protection requires the activity of T cells that recognize antigens presented in the context of both major histocompatibility complex (MHC) class II and I molecules. Since MHC class I presentation generally requires antigen to be localized to the cytoplasmic compartment of antigen-presenting cells, it remains unclear how pathogens that reside primarily within endocytic vesicles of infected macrophages, such as MTB, can elicit specific MHC class I-restricted T cells. A mechanism is described for virulent MTB that allows soluble antigens ordinarily unable to enter the cytoplasm, such as ovalbumin, to be presented through the MHC class I pathway to T cells. The mechanism is selective for MHC class I presentation, since MTB infection inhibited MHC class II presentation of ovalbumin. The MHC class I presentation requires the tubercle bacilli to be viable, and it is dependent upon the transporter associated with antigen processing (TAP), which translocates antigenic peptides from the cytoplasm into the endoplasmic reticulum. The process is mimicked by Listeria monocytogenes and soluble listeriolysin, a pore-forming hemolysin derived from it, suggesting that virulent MTB may have evolved a comparable mechanism that allows molecules in a vacuolar compartment to enter the cytoplasmic presentation pathway for the generation of protective MHC class I-restricted T cells.

Misfolded membrane proteins are rapidly degraded, often shortly after their synthesis and insertion in the endoplasmic reticulum (ER), but the exact location and mechanisms of breakdown remain unclear. We have exploited the requirement of MHC class I molecules for peptide to achieve their correct conformation: peptide can be withheld by introducing a null mutation for the MHC-encoded peptide transporter, TAP. By withholding TAP-dependent peptides, the vast majority of newly synthesized class I molecules fails to leave the endoplasmic reticulum and is degraded. We used mice transgenic for HLA-B27 on a TAP1-deficient background to allow visualization by immunoelectron microscopy of misfolded HLA-B27 molecules in thymic epithelial cells. In such HLA transgenic animals, the TAP mutation can be considered a genetic switch that allows control over the extent of folding of the protein of interest, HLA-B27, while the rate of synthesis of the constituent subunits remains unaltered. In TAP1-deficient, HLA-B27 transgenic animals, HLA-B27 molecules fail to assemble correctly, and do not undergo carbohydrate modifications associated with the Golgi apparatus, such as conversion to Endoglycosidase H resistance, and acquisition of sialic acids. We show that such molecules accumulate in an expanded network of tubular and fenestrated membranes. This compartment has its counterpart in normal thymic epithelial cells, and is identified as an ER-Golgi intermediate. We detect the presence of ubiquitin and ubiquitin-conjugating enzymes in association with this compartment, suggesting a nonlysosomal mode of degradation of its contents.