A recombinant single-chain HLA-A2.1 molecule, with a cis active beta-2-microglobulin domain, is biologically active in peptide binding and antigen presentation

In vivo immunogenicity of Tax(11-19) epitope in HLA-A2/DTR transgenic mice: implication for dendritic cell-based anti-HTLV-1 vaccine

Viral oncoprotein Tax plays key roles in transformation of human T-cell leukemia virus (HTLV-1)-infected T cells leading to adult T-cell leukemia (ATL), and is the key antigen recognized during HTLV-associated myelopathy (HAM). In HLA-A2+ asymptomatic carriers as well as ATL and HAM patients, Tax(11-19) epitope exhibits immunodominance. Here, we evaluate CD8 T-cell immune response against this epitope in the presence and absence of dendritic cells (DCs) given the recent encouraging observations made with Phase 1 DC-based vaccine trial for ATL. To facilitate these studies, we first generated an HLA-A2/DTR hybrid mouse strain carrying the HLA-A2.1 and CD11c-DTR genes. We then studied CD8 T-cell immune response against Tax(11-19) epitope delivered in the absence or presence of Freund's adjuvant and/or DCs. Overall results demonstrate that naturally presented Tax epitope could initiate an antigen-specific CD8T cell response in vivo but failed to do so upon DC depletion. Presence of adjuvant potentiated Tax(11-19)-specific response. Elevated serum IL-6 levels coincided with depletion of DCs whereas decreased TGF-β was associated with adjuvant use. Thus, Tax(11-19) epitope is a potential candidate for the DC-based anti-HTLV-1 vaccine and the newly hybrid mouse strain could be used for investigating DC involvement in human class-I-restricted immune responses.

Properties and applications of single-chain major histocompatibility complex class I molecules

Stable major histocompatibility complex (MHC) class I molecules at the cell surface consist of three separate, noncovalently associated components: the class I heavy chain, the β(2)-microglobulin light chain, and a presented peptide. These three components are assembled inside cells via complex pathways involving many other proteins that have been studied extensively. Correct formation of disulfide bonds in the endoplasmic reticulum is central to this process of MHC class I assembly. For a single specific peptide to be presented at the cell surface for possible immune recognition, between hundreds and thousands of peptide-containing precursor polypeptides are required, so the overall process is relatively inefficient. To increase the efficiency of antigen presentation by MHC class I molecules, and for possible therapeutic purposes, single-chain molecules have been developed in which the three, normally separate components have been joined together via flexible linker sequences in a single polypeptide chain. Remarkably, these single-chain MHC class I molecules fold up correctly, as judged by functional recognition by cells of the immune system, and more recently by X-ray crystallographic structural data. This review focuses on the interesting properties and potential of this new type of engineered MHC class I molecule.

Calreticulin Promotes Folding of Functional Human Leukocyte Antigen Class I Molecules in Vitro

The assembly of MHC class I molecules with beta(2)-microglobulin and peptides is assisted by the housekeeping chaperones calnexin, calreticulin, and Erp57 and the dedicated accessory protein, tapasin. Tapasin and calreticulin are essential for efficient MHC class I assembly, but their precise action during class I assembly remains to be elucidated. Previous in vitro studies have demonstrated that the lectin calreticulin interacts with monoglucosylated MHC class I heavy chains, whatever their state of assembly with light chains and peptide, and inhibits their aggregation above physiological temperature. We used a soluble single chain HLA-A2/beta(2)-microglobulin molecule, A2SC, to study the effect of calreticulin on the peptide binding capacity of HLA class I molecules. Calreticulin inhibited the formation of A2SC aggregates both when co-expressed in insect cells and during incubations at elevated temperature. Calreticulin dramatically enhanced acquisition of peptide binding capacity when added to denatured A2SC molecules during refolding at 4 degrees C. However, it had no effect on the rapid loss of A2SC peptide binding capacity at physiological temperature. We conclude that calreticulin promotes the folding of HLA class I molecules to a state in which, at low temperature, they spontaneously acquire peptide binding capacity. However, it does not induce or maintain a peptide-receptive state of the class I-binding site, which is likely to be promoted by one or several other components of the class I loading complexes. By being amenable to complementation with additional proteins, the described system should be useful for identification of these components.

Immunological Monitoring of Patients with Melanoma After Peptide Vaccination Using Soluble Peptide/HLA-A2 Dimer Complexes

To facilitate the immunologic monitoring of a peptide vaccine trial, a novel, empty dimeric HLA-A2 molecule (A2 dimer) that could be loaded with peptides was produced. The dimer comprises the extracellular domain of HLA-A2 noncovalently linked to a fusion protein consisting of human beta2-microglobulin joined to the human IgG1 Fc domain. Peptide-loaded dimer complexes were used to assess the function of peptide-specific T cells. HLA-A2 gp100 peptide dimers stimulated interferon (IFN)-gamma production by the gp100-specific TIL-1520 cell line. Gp100/A2 dimer stimulation in combination with intracellular cytokine staining was used to analyze peptide-specific T-cell responses in patients with melanoma after vaccination with the modified gp100: 209-2M peptide in adjuvant. Titration analysis of the amount of peptide-loaded dimer required to stimulate gp100-specific T cells was used to estimate the functional avidity of effector/memory CD8+ T lymphocytes. The number of peptide-specific T cells detected in the peripheral blood of vaccinated patients using this assay was comparable to the number determined by staining with fluoresceinated gp100: 209-2M HLA-A2 tetramers. IFN-gamma production by T cells was comparable after stimulation with peptide-pulsed dimers, T2 cells, or autologous dendritic cells. Peptide-loaded A2 dimers could also be used directly to stimulate T cells in the ELISPOT assay.

Peptide-beta2-microglobulin-MHC fusion molecules bind antigen-specific T cells and can be used for multivalent MHC-Ig complexes

Recombinant soluble MHC molecules are widely used for visualization, activation and inhibition of antigen-specific immune responses. Using a genetic approach, we have generated two novel peptide-beta2-microglobulin-MHC constructs. We have linked the MHC molecule with the peptide of interest, without limiting the recognition by the cognate TCR. This molecule can also be joined with the IgG heavy chain resulting in a dimeric MHC-Ig fusion protein. These molecules bind antigen-specific T cells with high specificity and sensitivity, therefore, providing a valuable tool for detection as well as enrichment of antigen-specific T cells.

NK and CTL Recognition of a Single Chain H-2Dd Molecule: Distinct Sites of H-2Dd Interact with NK and TCR

We generated transgenic mice expressing a single-chain β2-microglobulin (β2m)-H-2Dd. The cell-surface β2m-H-2Dd molecule was expressed on a β2m-deficient background and reacted with appropriate mAbs. It was of the expected m.w. and directed the normal development of CD8+ T cells in the thymus of a broad TCR repertoire. It also presented both exogenously provided and endogenous peptide Ags to effector CD8+ T cells. In tests of NK cell education and function, it failed to reveal any interaction with NK cells, suggesting that the site of the interaction of NK receptors with H-2Dd was disrupted. Thus, the sites of TCR and NK receptor interaction with H-2Dd are distinct, an observation consistent with independent modes of TCR and NK receptor evolution and function.