The central event in the cellular immune response to invading microorganisms is the specific recognition of foreign peptides bound to major histocompatibility complex (MHC) molecules by the alphabeta T cell receptor (TCR). The x-ray structure of the complete extracellular fragment of a glycosylated alphabeta TCR was determined at 2.5 angstroms, and its orientation bound to a class I MHC-peptide (pMHC) complex was elucidated from crystals of the TCR-pMHC complex. The TCR resembles an antibody in the variable Valpha and Vbeta domains but deviates in the constant Calpha domain and in the interdomain pairing of Calpha with Cbeta. Four of seven possible asparagine-linked glycosylation sites have ordered carbohydrate moieties, one of which lies in the Calpha-Cbeta interface. The TCR combining site is relatively flat except for a deep hydrophobic cavity between the hypervariable CDR3s (complementarity-determining regions) of the alpha and beta chains. The 2C TCR covers the class I MHC H-2Kb binding groove so that the Valpha CDRs 1 and 2 are positioned over the amino-terminal region of the bound dEV8 peptide, the Vbeta chain CDRs 1 and 2 are over the carboxyl-terminal region of the peptide, and the Valpha and Vbeta CDR3s straddle the peptide between the helices around the central position of the peptide.

At the site of contact between T cells and antigen-presenting cells (APCs), T cell receptor (TCR)–peptide–major histocompatibility complex (MHC) interaction is intensified by interactions between other molecules, notably by CD28 and lymphocyte function-associated antigen 1 (LFA-1) on T cells interacting with B7 (B7-1 and B7-2), and intracellular adhesion molecule 1 (ICAM-1), respectively, on APCs. Here, we show that during T cell–APC interaction, T cells rapidly absorb various molecules from APCs onto the cell membrane and then internalize these molecules. This process is dictated by at least two receptors on T cells, namely CD28 and TCR molecules. The biological significance of T cell uptake of molecules from APCs is unclear. One possibility is that this process may allow activated T cells to move freely from one APC to another and eventually gain entry into the circulation.

HLA-DM catalyzes the release of MHC class II-associated invariant chain-derived peptides (CLIP) from class II molecules. Recent evidence has suggested that HLA-DO is a negative regulator of HLA-DM in B cells, but the physiological function of HLA-DO remains unclear. Analysis of antigen presentation by B cells from mice lacking H2-O (the mouse equivalent of HLA-DO), together with biochemical analysis using purified HLA-DO and HLA-DM molecules, suggests that HLA-DO/H2-O influences the peptide loading of class II molecules by limiting the pH range in which HLA-DM is active. This effect may serve to decrease the presentation of antigens internalized by fluid-phase endocytosis, thus concentrating the B cell-mediated antigen presentation to antigens internalized by membrane immunoglobulin.

PAR-2 is a second member of a novel family of G-protein-coupled receptors characterized by a proteolytic cleavage of the amino terminus, thus exposing a tethered peptide ligand that autoactivates the receptor. The physiological and/or pathological role(s) of PAR-2 are still unknown. This study provides tissue-specific cellular localization of PAR-2 in normal human tissues by immunohistochemical techniques. A polyclonal antibody, PAR-2C, was raised against a peptide corresponding to the amino terminal sequence SLIGKVDGTSHVTGKGV of human PAR-2. Significant PAR-2 immunoreactivity was detected in smooth muscle of vascular and nonvascular origin and stromal cells from a variety of tissues. PAR-2 was also present in endothelial and epithelial cells independent of tissue type. Strong immunolabeling was observed throughout the gastrointestinal tract, indicating a possible function for PAR-2 in this system. In the CNS, PAR-2 was localized to many astrocytes and neurons, suggesting involvement of PAR-2 in neuronal function. A role for PAR-2 in the skin was further supported by its immunolocalization in the epidermis. PAR-2C antibody exemplifies an important tool to address the physiological role(s) of PAR-2.

No abstract

T-cell antigen receptors (TCR) generally interact with moderate affinity with the complex formed by major histocompatibility complex (MHC) molecules and foreign peptides. MHC/TCR recognition is followed by the generation of a signal to the T cell through a monomorphic multicomponent system that includes the CD3 complex and accessory molecules such as CD4 and CD8. The interaction between the extracellular domains of MHC and TCR molecules, and the interaction of MHC and CD4/CD8 molecules, have been considered to occur independently of one another. We report here that the affinity of CD8 dimers for MHC class I molecules is independent of haplotype and peptide content, and that the affinity of the TCR for its specific ligand is enhanced through a reduced 'off' rate in the presence of either CD8alpha alpha homo- or CD8alpha beta heterodimers. Moreover, CD8 seems to help recognition of the specific MHC-peptide complex either by guiding an energetically favourable docking of TCR onto MHC, or by inducing conformational changes in the MHC complex that can augment the TCR/MHC-peptide interaction. CD8 should therefore be considered as an active participant in the T-cell recognition complex, rather than simply as an accessory molecule.

No abstract

We report here that the Intrinsic affinities of the antigen-specific T-cell receptors (TCR) of two unrelated CD8+ T-cell clones for their respective peptide-major histocompatibility complex (MHC) ligands are higher than the values generally thought to prevail for TCR. The TCR of one clone (2C) binds an aflogeneic class I MHC protein (Ld) in association with an a-ketoglutarate dehydrogenase nonapeptide (QLSPFPFDL, termed QL9) with an intrinsic anity (intrinsic equilibrium association constant) of 1-2 x 107 M-1. The TCR of the other clone (4G3) binds a syngeneic class I MHC protein (Kb) in association with an ovalbumin octapeptide (SIINFEKL, termed pOV8) with an inins affinity of 1.5x 106 M-l. A compain of the two clones, combined with current views of T-cell repertoire selection in the thymus, leads us to propose that TCR afities are generally likely to be higher for algeneic MHC-peptide complexes than for syngeneic MHC-peptide complexes.Because the somatic hypermutation that underlies the generation of high-affinity antibodies is not evident in T cells, it has long been surmised that the affinities of antigen-specific T-cell receptors (TCR) for their natural ligands, now known to be peptide-major histocompatibility complex (MHC) complexes, are likely to be low and in the narrow range (104 to 105 M-1) exhibited by antibodies made early in immune responses, before the onset of somatic hypermutation. Previously, however, we showed (1) that one of the two murine T-cell clones studied here (clone 2C) has a relatively high intrinsic affinity (2 x 106 M-1) for its ligand: Ld, a class I MHC protein, in association with an octapeptide (LSPF-PFDL, termed p2Ca) that derives from murine a-ketoglutarate dehydrogenase (a-KGDH) (2, 3). This clone also recognizes Ld in association with several other peptides from a-KGDH. One of them, the nonapeptide QLSPFPFDL (termed QL9), proved to be exceptionally active in sensitizing Ld+ target cells (T2-Ld) for lysis by 2C cells. We show here that the affinity of the TCR on 2C cells for the complex formed by this peptide with Ld (QL9-Ld) is about 10 times greater than for p2Ca-Ld and, to our knowledge, is the highest affinity value found so far for any TCR.Since the 2C clone arose in a mouse of the H-2b haplotype, its high-affinity binding of the peptide-Ld complex is an allogeneic reaction (alloreaction). To compare it with a CD8+ cytotoxic T lymphocyte (CTL) clone that is also highly reactive cytolytically but recognizes a syngeneic MHCpeptide complex, we took advantage of clone 4G3. This clone, which also arose in an H-2b mouse, reacts specifically with the ovalbumin octapeptide pOV8 (SIINFEKL) in association with Kb. In this syngeneic reaction, the TCR affinity for its peptide-MHC complex was about 10-fold lower than in the alloreaction. Though the comparison involves only two clones, when the findings are considered in light of recent studies ofT-cell repertoire selection in the thymus (4-6), they lead us to propose that the affinity ofa TCR for peptide-MHC complexes will ge...