Antibodies (also known as immunoglobulins) are used by the immune system to recognize foreign substances (so-called ‘antigens’), e.g. parts of bacteria and viruses. Antibodies are Y-shaped molecules, consisting of four polypeptide chains (two ‘light chains’ and two ‘heavy chains’), linked by a disulphide bridge. At the end of the two “arms” of the antibody, two antigen-binding fragments are positioned, the so-called ‘Fab’s. It is the variable regions located at the N-terminal ends of the Fab fragments, which are responsible for antigen binding. Antigens can be any kinds of substances that are recognized by the immune system. Usually, they are proteins or polysaccharides. These molecules trigger a cellular immune response, which leads to the destruction of the offending cells.
Structure of Fab 14F7 and its modeled complex with the NeuGc-GM3 trisaccharide.
About the project
The current research project is directed towards the study of the binding interactions of two monoclonal antibodies, called P3 and 14F7, with their carbohydrate ligands, in an effort to understand the structural basis of the specificity and affinity that characterize these interactions. Both P3 and 14F7 bind, with different specificity, to N-glycolyl sialic acid-containing molecules that are expressed on breast and melanoma cancer tumors.
They are capable of selectively recognizing tumor tissues and can thus distinguish between normal and tumor cells. Thus, these molecules constitute possible targets for both passive and active (vaccines) cancer therapies. P3 and 14F7 monoclonal antibodies motivate a special interest because of their unique binding properties (Vázquez et al., 1995; Moreno et al., 1998; Lindquist et al., unpublished) and in vivo activity (Carr et al., 2002; Oliva et al., 2006).
We plan to determine the structures of both antibodies and their complexes with various carbohydrate ligands by X-ray crystallography. Complementary studies will involve phage display, site directed mutagenesis and molecular modeling.
The main goal of these studies is to engineer the binding site of the antibodies in order to improve the antibody's binding properties.
To this end, we have been able to solve the structure of 14F7 Fab and generate a model of its ligand complex, which explains the unique binding characteristics of this antibody (Krengel et al., 2004).
More recently, we have solved the crystal structure of the P3 antibody and modeled its complexes with N-glycolyl GM3 as well as its anti-idiotypic antibody 1E10 (Talavera et al., 2009a). The results shed light on shedding light on the functional resemblance between the 1E10 and its sugar mimic: It appears as if the anti-idiotpypic antibody (Ab2) serves as an imprint of the primary antibody (Ab1), P3, providing a cast to generate Ab3 antibodies similar to Ab1, rather than providing strict structural mimicry.
A second research project: Nimotuzumab
A second, unrelated, research project on anti-tumor antibodies is concerned with the Nimotuzumab (also known as h-R3), an antibody targeting the EGF receptor. This antibody has been approved in several countries for the treatment of head and neck tumors and glioma, and is in clinical trials for various other tumor types. Intriguingly, Nimotuzumab shows only minor side effects (Allan, 2005), while still being effective.
Based on the crystal structure that we recently solved at 2.5 Å resolution and its modeled complex with the extracellular domain of the EGF receptor, we have proposed a model for the favorable mechanism of action of this antibody (Talavera et al., 2009b), which we now intend to probe, in collaboration with the group of Inger Helene Madshus at the Norwegian National Hospital.
This project involves the close collaboration with the Center for Molecular Immunology (CIM), Havana, Cuba.
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Talavera et al. (2009b), Cancer Res. 69, 5851-5859.
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Vázquez et al. (1995). Hybridoma 14, 551-556.