Anders Brunmark

An αβ T cell receptor structure at 2.5 Å and its orientation in the TCR-MHC complex

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 αβ T cell receptor (TCR). The x-ray structure of the complete extracellular fragment of a glycosylated αβ 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 Vα and Vβ domains but deviates in the constant Cα domain and in the interdomain pairing of Cα with Cβ. Four of seven possible asparagine-linked glycosylation sites have ordered carbohydrate moieties, one of which lies in the Cα-Cβ interface. The TCR combining site is relatively flat except for a deep hydrophobic cavity between the hypervariable CDR3s (complementarity-determining regions) of the α and β chains. The 2C TCR covers the class I MHC H-2Kb binding groove so that the Vα CDRs 1 and 2 are positioned over the amino-terminal region of the bound dEV8 peptide, the Vβ chain CDRs 1 and 2 are over the carboxyl-terminal region of the peptide, and the Vα and Vβ CDR3s straddle the peptide between the helices around the central position of the peptide.

Characterization of Protease-activated Receptor-2 Immunoreactivity in Normal Human Tissues

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.

Kinetics and affinity of reactions between an antigen-specific T cell receptor and peptide-MHC complexes

We show here that the net rate of accumulation of complexes formed by the antigen-specific receptor of T cells (TCR) of a T cell clone with its natural ligand, an octapeptide in association with Ld, a class I protein of the major histocompatibility complex (MHC), approaches the maximal value determined by the affinity of the TCR for this peptide-MHC ligand in 1-2 min, which is well within the lifetime of transient T cell-target cell conjugates. Consistent with this finding, we also found that the widely divergent affinity values (equilibrium constants) of this TCR for six related peptide-MHC complexes correlate well with the extent of specific lysis of target cells bearing various level of these complexes.

Altered Antigen Presentation in Mice Lacking H2-O

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.

Structural Basis of 2C TCR Allorecognition of H-2Ld Peptide Complexes

MHC class I H-2Ld complexed with peptide QL9 (or p2Ca) is a high-affinity alloantigen for the 2C TCR. We used the crystal structure of H-2Ld with a mixture of bound peptides at 3.1 A to construct a model of the allogeneic 2C-Ld/QL9 complex for comparison with the syngeneic 2C-Kb/dEV8 structure. A prominent ridge on the floor of the Ld peptide-binding groove, not present in Kb, creates a C-terminal bulge in Ld peptides that greatly increases interactions with the 2C beta-chain. Furthermore, weak electrostatic complementarity between Asp77 on the alpha1 helix of Kb and 2C is enhanced in the allogeneic complex by closer proximity of QL9 peptide residue AspP8 to the 2C HV4 loop.

Peptide Antagonism and T Cell Receptor Interactions with Peptide-MHC Complexes

We describe antagonist peptides that specifically inhibit cytolytic activity of T cell clones and lines that express the antigen-specific receptor of CD8+ T lymphocyte clone 2C, which recognizes peptides in association with syngeneic (Kb) and allogeneic (Ld) MHC proteins. Addition of an antagonist peptide that can bind to Kb on 2C cells decreased the tyrosine phosphorylation of CD3 zeta chains elicited by prior exposure of the cells to an agonist peptide-Kb complex. Contrary to previous agonist-antagonist comparisons, the 2C T cell receptor had higher affinity for an antagonist peptide-Kb complex than for a weak agonist peptide-Kb complex. This difference is considered in light of evidence that antigen-specific receptor affinity values can be substantially higher when determined with the receptor on live cells than with the receptor in cell-free systems.

Dual Function of Drosophila Cells as APCs for Naive CD8+ T Cells: Implications for Tumor Immunotherapy

With unseparated mouse spleen cells as responders, Drosophila cells expressing MHC class I (L(d)) molecules alone lead to peptide-specific responses of CD8+ cells in the absence of exogenous cytokines. Under these conditions, DNA released from dying cells stimulates the B cells in spleen to up-regulate costimulatory molecules; these activated B cells then provide bystander costimulation for CD8+ cells responding to class I-peptide complexes on the Drosophila APCs. By stimulating B cells and presenting antigen to T cells, Drosophila cells thus serve two different functions in promoting primary responses of CD8+ cells in vitro. With this system, we show that Ld-transfected Drosophila cells are able to induce autologous spleen cells to respond to a tumor-specific peptide in vitro and, after transfer, cause tumor rejection in vivo.

The Crystal Structures of Kbm1 and Kbm8 Reveal that Subtle Changes in the Peptide Environment Impact Thermostability and Alloreactivity

The K(bm1) and K(bm8) natural mutants of the murine MHC class I molecule H-2K(b) were originally identified by allograft rejection. They also bind viral peptides VSV8 and SEV9 with high affinity, but their peptide complexes have substantially decreased thermostability, and the K(bm1) complexes do not elicit alloreactive T cell responses. Crystal structures of the four mutant complexes at 1.7-1.9 A resolution are similar to the corresponding wild-type K(b) structures, except in the vicinity of the mutated residues, which alter the electrostatic potential, topology, hydrogen bonding, and local water structure of the peptide binding groove. Thus, these natural K(b) mutations define the minimal perturbations in the peptide environment that alter antigen presentation to T cells and abolish alloreactivity.

A novel B-RAF inhibitor blocks interleukin-8 (IL-8) synthesis in human melanoma xenografts, revealing IL-8 as a potential pharmacodynamic biomarker.

B-RAF mutations have been identified in the majority of melanoma and a large fraction of colorectal and papillary thyroid carcinoma. Drug discovery efforts targeting mutated B-RAF have yielded several interesting molecules, and currently, three compounds are undergoing clinical evaluation. Inhibition of B-RAF in animal models leads to a slowing of tumor growth and, in some cases, tumor reduction. Described within is a novel series of diaryl imidazoles with potent, single-digit nanomolar, anti-B-RAF activity. One compound from this series has been detailed here and has been shown to block B-RAFV600E-dependent extracellular signal-regulated kinase 1/2 phosphorylation in SK-MEL-28 melanoma cells as well as soft agar colony formation and proliferation. Importantly, interleukin-8 (IL-8) was identified by quantitative real-time PCR and ELISA as a product of the elevated mitogen-activated protein kinase signaling in these cells. Plasma concentrations of IL-8 in mice bearing melanoma xenografts were significantly reduced following exposure to B-RAF inhibitors. Taken together, these data suggest that IL-8 could serve as a tractable clinical biomarker. [Mol Cancer Ther 2008;7(3):492–9]

Early Antigen-Specific Response by Naive CD8 T Cells Is Not Altered with Aging

Both a dramatic decline in CD8 responses and a switch to memory T cell predominance occur with aging. The extent to which the loss of responsiveness is the consequence of the accumulation of more differentiated vs intrinsically defective T cells (or both) has been unclear. Using similar conditions of Ag stimulation, we have examined the responses generated by CD8+ cells isolated from aged TCR transgenic mice. We found that the naive transgene+ CD8+ cells from aged 2C mice expressed activation markers, produced IL-2, proliferated, and differentiated into cytotoxic T cells as efficiently as their young counterparts. The extent of responsiveness and the level of the responses were comparable in both age groups regardless of the stimulatory conditions used, i.e., partial costimulation/adhesion molecule expression on APCs, or presentation of lower affinity peptide or diminished peptide concentrations. By day 4 after Ag stimulation, no significant age-related differences were observed in the number of effector cells generated nor in the levels of secreted IL-2 or IFN-γ. Upon restimulation of effector cells, IL-2 secretion and to a lesser extent TNF-α expression, but not IFN-γ secretion, were diminished with age. These findings suggest that age-associated alterations in naive CD8 cell function are not found after primary stimulation, but may become apparent upon restimulation.