A monoclonal antibody recognizes a subset of the H-2Dd mouse major class I antigens

Opportunities and Challenges for Antibodies against Intracellular Antigens

Therapeutic antibodies are one most significant advances in immunotherapy, the development of antibodies against disease-associated MHC-peptide complexes led to the introduction of TCR-like antibodies. TCR-like antibodies combine the recognition of intracellular proteins with the therapeutic potency and versatility of monoclonal antibodies (mAb), offering an unparalleled opportunity to expand the repertoire of therapeutic antibodies available to treat diseases like cancer. This review details the current state of TCR-like antibodies and describes their production, mechanisms as well as their applications. In addition, it presents an insight on the challenges that they must overcome in order to become commercially and clinically validated.

T cell receptor-like recognition of tumor in vivo by synthetic antibody fragment

A major difficulty in treating cancer is the inability to differentiate between normal and tumor cells. The immune system differentiates tumor from normal cells by T cell receptor (TCR) binding of tumor-associated peptides bound to Major Histocompatibility Complex (pMHC) molecules. The peptides, derived from the tumor-specific proteins, are presented by MHC proteins, which then serve as cancer markers. The TCR is a difficult protein to use as a recombinant protein because of production issues and has poor affinity for pMHC; therefore, it is not a good choice for use as a tumor identifier outside of the immune system. We constructed a synthetic antibody-fragment (Fab) library in the phage-display format and isolated antibody-fragments that bind pMHC with high affinity and specificity. One Fab, fE75, recognizes our model cancer marker, the Human Epidermal growth factor Receptor 2 (HER2/neu) peptide, E75, bound to the MHC called Human Leukocyte Antigen-A2 (HLA-A2), with nanomolar affinity. The fE75 bound selectively to E75/HLA-A2 positive cancer cell lines in vitro. The fE75 Fab conjugated with ⁶⁴Cu selectively accumulated in E75/HLA-A2 positive tumors and not in E75/HLA-A2 negative tumors in an HLA-A2 transgenic mouse as probed using positron emission tomography/computed tomography (PET/CT) imaging. Considering that hundreds to thousands of different peptides bound to HLA-A2 are present on the surface of each cell, the fact that fE75 arrives at the tumor at all shows extraordinary specificity. These antibody fragments have great potential for diagnosis and targeted drug delivery in cancer.

Conformational changes within the HLA-A1:MAGE-A1 complex induced by binding of a recombinant antibody fragment with TCR-like specificity

Although there is X-ray crystallographic evidence that the interaction between major histocompatibility complex (MHC, in humans HLA) class I molecules and T cell receptors (TCR) or killer cell Ig-like receptors (KIR) may be accompanied by considerable changes in the conformation of selected residues or even entire loops within TCR or KIR, conformational changes between receptor-bound and -unbound MHC class I molecules of comparable magnitude have not been observed so far. We have previously determined the structure of the MHC class I molecule HLA-A1 bound to a melanoma antigen-encoding gene (MAGE)-A1-derived peptide in complex with a recombinant antibody fragment with TCR-like specificity, Fab-Hyb3. Here, we compare the X-ray structure of HLA-A1:MAGE-A1 with that complexed with Fab-Hyb3 to gain insight into structural changes of the MHC molecule that might be induced by the interaction with the antibody fragment. Apart from the expulsion of several water molecules from the interface, Fab-Hyb3 binding results in major rearrangements (up to 5.5 A) of heavy chain residues Arg65, Gln72, Arg145, and Lys146. Residue 65 is frequently and residues 72 and 146 are occasionally involved in TCR binding-induced conformational changes, as revealed by a comparison with MHC class I structures in TCR-liganded and -unliganded forms. On the other hand, residue 145 is subject to a reorientation following engagement of HLA-Cw4 and KIR2DL1. Therefore, conformational changes within the HLA-A1:MAGE-A1:Fab-Hyb3 complex include MHC residues that are also involved in reorientations in complexes with natural ligands, pointing to their central importance for the peptide-dependent recognition of MHC molecules.

Monoclonal and recombinant antibodies with T cell receptor-like reactivity

We will explain why major histocompatibility complex (MHC) molecules presenting peptides derived from tumour-associated antigens can be recognized not only by T cell receptors (TCR), but also by soluble proteins endowed with TCR-like reactivity. To understand how an antibody can display high affinity and specificity for a particular MHC:peptide complex, we have employed X-ray crystallography to determine the structure of a recombinant antibody, Hyb3, bound to human HLA-A1 molecules presenting the peptide EADPTGHSY that is derived from the tumour-associated antigen MAGE-Al. The results indicate that although Hyb3 recgonizes its target in a TCR-like diagonal binding mode, important differences between the two types of proteins exist that are probably due to the fact that TCR are part of a molecular assembly on the surface of effector cells, while antibodies such as Hyb3 have to carry out their function as individual molecules.

Novel antibodies as anticancer agents

In recent years antibodies, whether generated by traditional hybridoma technology or by recombinant DNA strategies, have evolved from Paul Ehrlich's 'magic bullets' to a modern age 'guided missile'. In the recent years of immunologic research, we are witnessing development in the fields of antigen screening and protein engineering in order to create specific anticancer remedies. The developments in the field of recombinant DNA, protein engineering and cancer biology have let us gain insight into many cancer-related mechanisms. Moreover, novel techniques have facilitated tools allowing unique distinction between malignantly transformed cells, and regular ones. This understanding has paved the way for the rational design of a new age of pharmaceuticals: monoclonal antibodies and their fragments. Antibodies can select antigens on both a specific and a high-affinity account, and further implementation of these qualities is used to target cancer cells by specifically identifying exogenous antigens of cancer cell populations. The structure of the antibody provides plasticity resonating from its functional sites. This review will screen some of the many novel antibodies and antibody-based approaches that are being currently developed for clinical applications as the new generation of anticancer agents.

T-cell receptor-like antibodies: novel reagents for clinical cancer immunology and immunotherapy

Major histocompatibility complex class I molecules play a central role in the immune response against a variety of cells that have undergone malignant transformation by shaping the T-cell repertoire and presenting peptide antigens from endogeneous antigens to CD8+ cytotoxic T-cells. Diseased tumor or virus-infected cells are present on class I major histocompatibility complex molecule peptides that are derived from tumor-associated antigens or viral-derived proteins. Due to their unique specificity, such major histocompatibility complex-peptide complexes are a desirable target for novel approaches in immunotherapy. Targeted delivery of toxins or other cytotoxic drugs to cells which express specific major histocompatibility complex-peptide complexes that are involved in the immune response against cancer or viral infections would allow for a specific immunotherapeutic treatment of these diseases. It has recently been demonstrated that antibodies with the antigen-specific, major histocompatibility complex-restricted specificity of T-cells can be generated by taking advantage of the selection power of phage display technology. In addition to their tumor targeting capabilities, antibodies that mimic the fine specificity of T-cell receptors can serve as valuable research reagents that enable study of human class I peptide-major histocompatibility complex ligand presentation, as well as T-cell receptor peptide-major histocompatibility complex interactions. T-cell receptor-like antibody molecules may prove to be useful tools for studying major histocompatibility complex class I antigen presentation in health and disease as well as for therapeutic purposes in cancer, infectious diseases and autoimmune disorders.

A major histocompatibility complex-peptide-restricted antibody and t cell receptor molecules recognize their target by distinct binding modes: crystal structure of human leukocyte antigen (HLA)-A1-MAGE-A1 in complex with FAB-HYB3

Antibodies with T cell receptor-like specificity possess a considerable diagnostic and therapeutic potential, but the structural basis of the interaction between an antibody and an histocompatibility antigen has so far not been determined. We present here the crystal structure (at 2.15 A resolution) of the recombinant, affinity-matured human antibody fragment Fab-Hyb3 bound to the tumor-associated human leukocyte antigen (HLA)/peptide complex HLA-A1.MAGE-A1. Fab-Hyb3 employs a diagonal docking mode resembling that of T cell receptors. However, other than these natural ligands, the antibody uses only four of its six complementarity-determining regions for direct interactions with the target. It recognizes the C-terminal half of the MAGE-A1 peptide, the HLA-A1 alpha1-helix, and N-terminal residues of the alpha2-helix, accompanied by a large tilting angle between the two types of molecules within the complex. Interestingly, only a single hydrogen bond between a peptide side chain and Fab-Hyb3 contributes to the interaction, but large buried surface areas with pronounced shape complementarity assure high affinity and specificity for MAGE-A1. The HLA-A1.MAGE-A1.antibody structure is discussed in comparison with those of natural ligands recognizing HLA.peptide complexes.

Antibodies directed against the MHC-I molecule H-2Dd complexed with an antigenic peptide: similarities to a T cell receptor with the same specificity

alphabeta TCRs, which use an Ab-like structure to form a combining site, recognize molecular complexes consisting of peptides bound to MHC class I (MHC-I) or class II (MHC-II) molecules. To explore the similarities and differences between Ab and T cell recognition of similar structures, we have isolated two mAbs, KP14 and KP15, that specifically bind H-2D(d) complexed with an HIV envelope gp160-derived peptide, P18-I10. These Abs are MHC and peptide specific. Fine specificity of mAb binding was analyzed using a panel of synthetic peptides, revealing similarities between the mAb and a cloned TCR with the same specificity. These two mAbs used the same V(H) and J(H) gene segments, but different D, Vkappa, and Jkappa genes. Administered in vivo, mAb KP15 blocked the induction of CTL specific for recombinant vaccinia virus-encoded gp160, indicating its ability to bind endogenously generated MHC/peptide complexes. Analysis of the fine specificity of these mAbs in the context of their encoded amino acid sequences and the known three-dimensional structure of the H-2D(d)/P18-I10 complex suggests that they bind in an orientation similar to that of the TCR. Thus, the plasticity of the B cell receptor repertoire and the structural similarities among BCR and TCR allow Abs to effectively mimic alphabeta TCRs. Such mAbs may be useful in the therapeutic modulation of immune responses against infectious agents or harmful self Ags as well as in tracing steps in Ag processing.

The natural killer cell receptor Ly-49A recognizes a peptide-induced conformational determinant on its major histocompatibility complex class I ligand

Natural killer (NK) cells are inhibited from killing cellular targets by major histocompatibility complex (MHC) class I molecules. In the mouse, this can be mediated by the Ly-49A NK cell receptor that specifically binds the H-2Dd MHC class I molecule, then inhibits NK cell activity. Previous experiments have indicated that Ly-49A recognizes the alpha 1/alpha 2 domains of MHC class I and that no specific MHC-bound peptide appeared to be involved. We demonstrate here that alanine-substituted peptides, having only the minimal anchor motifs, stabilized H-2Dd expression and provided resistance to H-2Dd-transfected, transporter associated with processing (TAP)-deficient cells from lysis by Ly-49A+ NK cells. Peptide-induced resistance was blocked only by an mAb that binds a conformational determinant on H-2Dd. Moreover, stabilization of "empty" H-2Dd heavy chains by exogenous beta 2-microglobulin did not confer resistance. In contrast to data for MHC class I-restricted T cells that are specific for peptides displayed MHC molecules, these data indicate that NK cells are specific for a peptide-induced conformational determinant, independent of specific peptide. This fundamental distinction between NK cells and T cells further implies that NK cells are sensitive only to global changes in MHC class I conformation or expression, rather than to specific pathogen-encoded peptides. This is consistent with the "missing self" hypothesis, which postulates that NK cells survey tissues for normal expression of MHC class I.

H-2Dd exploits a four residue peptide binding motif

We have characterized the amino acid sequences of over 20 endogenous peptides bound by a soluble analog of H-2Dd, H-2Dds. Synthetic analogs corresponding to self, viral, tumor, or motif peptides were then tested for their ability to bind to H-2Dd by serologic epitope induction assays using both purified soluble protein and cell surface H-2Dd. The dominant primary sequence motif included glycine at position 2, proline at position 3, and a hydrophobic COOH terminus: leucine, isoleucine, or phenylalanine at position 9 or 10. Ancillary support for high affinity binding was contributed by a positively charged residue at position 5. Three-dimensional computer models of H-2Dds/peptide complexes, based on the crystallographic structure of the human HLA-B27/peptide complex, showed that the basic residue at position 5 was in position to form a salt bridge with aspartic acid at position 156, a polymorphic residue of the H-2Dd heavy (H) chain. Analysis of 28 such models, including 17 based on nonamer self-peptides, revealed considerable variation in the structure of the major histocompatibility complex (MHC) surrounding peptide residue 1, depending on the size and charge of the side chain. Interactions between the side chains of peptide residues 5 and 7, and 6 and 8 commonly occurred. Those peptide positions with limited sequence variability and least solvent accessibility may satisfy structural requirements for high affinity binding of the peptide to the MHC class I H chain, whereas the highly variable positions of the peptide (such as positions 4, 6, and 8) may contribute more to the T cell epitopes.

A soluble, single-chain Kd molecule produced by yeast selects a peptide repertoire indistinguishable from that of cell-surface-associated Kd

Peptide binding to a soluble, single-chain Kd protein produced by the yeast strain Kluyveromyces lactis, and to Kd molecules on Kd-expressing cells (P815) was studied using radiolabeled Kd-restricted peptides. The stability of the peptide-Kd complexes formed was monitored in the absence and presence of unlabeled competitor peptides. Radioiodination of the Tyr anchor residue in position 2 of the peptide interferes with binding. A Kd-biased peptide library and a modified antigenic peptide in which a second Tyr was added in positions 6 and 8, respectively, were therefore used to assay binding. Recombinant and cell-associated Kd molecules are very similar in the following respects: the ease with which the proteins can be loaded with labeled peptide; the spectrum of peptides selected from a peptide library; the stability of the labeled peptide-Kd complex formed; and the ability to partially dissociate the class I-peptide complex with exogenous, unlabeled peptides. These results imply that measurements of peptide binding to soluble Kd molecules are a reliable indicator of the peptide-binding properties of Kd proteins on living cells. The large quantities of soluble recombinant Kd protein currently available represent an invaluable tool not only for dissecting the molecular mechanisms of antigen presentation but also for vaccinations and the design of T cell-specific toxins.

MHC class I/peptide interactions: binding specificity and kinetics

Recent developments in the preparation of soluble analogues of the major histocompatibility complex (MHC) class I molecules as well as in the application of real time biosensor technology have permitted the direct analysis of the binding of MHC class I molecules to antigenic peptides. Using synthetic peptide analogues with cysteine substitutions at appropriate positions, peptides can be immobilized on a dextran-modified gold biosensor surface with a specific spatial orientation. A full set of such substituted peptides (known as 'pepsicles', as they are peptides on a stick) representing antigenic or self peptides can be used in the functional mapping of the MHC class I peptide binding site. Scans of sets of peptide analogues reveal that some amino acid side chains of the peptide are critical to stable binding to the MHC molecule, while others are not. This is consistent with functional experiments using substituted peptides and three-dimensional molecular models of MHC/peptide complexes. Detailed analysis of the kinetic dissociation rates (kd) of the MHC molecules from the specifically coupled solid phase peptides reveals that the stability of the complex is a function of the particular peptide, its coupling position, and the MHC molecule. Measured kd values for antigenic peptide/class I interactions at 25 degrees C are in the range of ca 10(-4)-10(-6)/s. Biosensor methodology for the analysis of the binding of MHC class I molecules to solid-phase peptides using real time surface plasmon resonance offers a rational approach to the general analysis of protein/peptide interactions.

Soluble T cell receptor-like properties of an HLA-B35-specific monoclonal antibody (TU165)

A mouse monoclonal antibody of IgM class (TU165) was produced using Epstein-Barr virus (EBV)-infected mutant cells derived from the human BJAB-B95.8.6 cell line as immunogen. Binding studies with several HLA deletion mutant cell lines indicated that TU165 recognized the HLA-B35 molecule. In a panel of 89 EBV-transformed lymphoblastoid cell lines, all HLA-B35+ cells (n = 24) reacted with TU165 while all but two HLA-B35- lines (n = 65) were unreactive (r = 0.95). Surprisingly, peripheral blood lymphocytes of HLA-B35+ donors were unreactive; however, strong enhancement of TU165 recognition was observed with B cells of one of these individuals after transformation with EBV (B95.8 strain). Transfection of both HLA-B35 and human beta 2-microglobulin genomic DNA into mouse P815 cells led to high expression of HLA-B molecules; yet, expression of the TU165 epitope was not observed. Furthermore, the EBV-negative cell line BJAB as well as the EBV-infected (P3HR1 strain) line BJAB-HR1K were only weakly reactive, whereas the BJAB-B95.8 cell line was strongly positive. These results indicate that EBV-encoded or -controlled peptide(s) must be bound by HLA-B35 antigens to create the epitope which allows efficient binding of TU165.