In dieser Arbeit wurde die Affinitätsreifung des murinen anti-c-myc-Peptid-Antikörpers 9E10 analysiert. Hierfür wurden Fab-Fragmente mit Keimbahnrückmutationen gentechnisch hergestellt und in ihrem Bindungsverhalten zum humanen c-myc-Peptid charakterisiert. Das von 9E10 erkannte Epitop besitzt die Aminosäuresequenz EQKLISEEDLLRKR mit den darin sehr selektiv erkannten Schlüsselpositionen LISEXXL.Der 3300-fache Affinitätsgewinn während der 9E10-Reifung kommt sowohl durch eine Zunahme der Assoziations- als auch durch eine Abnahme der Dissoziationsgeschwindigkeit des Komplexes zustande. Der Affinitätsgewinn resultiert weniger aus zusätzlichen Kontakten des Antikörpers zum Peptid, sondern vor allem aus der Beeinflussung der Konformation und/oder der Flexibilität der an der Bindung beteiligten CDRs. Die außergewöhnlich lange CDR-H3 liefert einen wesentlichen Beitrag zur Affinitätsreifung. Die variable leichte Domäne dient dabei mit der langen CDR-L1 und -L3 als eine Bindungsplattform für die flexible CDR-H3. Änderungen in der Spezifität von 9E10 sind vorrangig auf die Reifung der variablen schweren Domäne zurückzuführen. Dabei ist die selektive Erkennung der Schlüsselpositionen im Peptid im Anfangsstadium der Affinitätsreifung von 9E10 stark ausgeprägt. / In this work the affinity maturation of the murine anti c-myc-peptide antibody 9E10 was analysed. Therefore Fab fragments with reversed mutations directed towards germline genes were genetically produced and characterised for their binding to the human c-myc peptide. The epitope recognized by 9E10 consists of the amino acid sequence EQKLISEEDLLRKR of which the key positions LISEXXL are very selectively recognized. The maturation of 9E10 leads to a 3300-fold higher affinity, which is achieved by a faster association as well as by a slower dissociation of the complex. For the gain in affinity formation of additional contacts to the peptide is less important than conformational and/or flexibility changes of the CDRs which are involved in binding. The exceptionally long CDR-H3 contributes essentially to the affinity maturation. The variable light domain serves thereby with its long CDR-L1 and -L3 as a binding platform for the flexible CDR-H3. Changes in specificity of 9E10 are primarily due to maturation of the variable heavy domain. Selective recognition of the key positions in the peptide is already highly pronounced in the initial stage of affinity maturation of 9E10.

Cell separation based on antibody-targeted magnetic beads has been widely used in a number of applications in immunology, microbiology, oncology and more recently, in the isolation of circulating tumour cells (CTCs) in cancer patients. Although other cell separation techniques such as size based cell filtration and Fluorescence Activated Cell Sorting have also been in popular use, immunomagnetic cell isolation possesses the advantages of high throughput, good specificity and reduced cell stress. However, certain fundamental features of the cell-bead interface are still unknown. In this study, some of the key features of the cell-bead synapse were investigated in an effort to improve the efficiency of immunomagnetic cell isolation and reduce its dependence on high expressing cell surface markers. A clinically relevant antibody fragment (Fab) against tyrosine kinase receptor HER2 was applied to study the immunomagnetic isolation of HER2 expressing cancer cells. First, the minimum number of target proteins required on a cell for it to be isolated was determined. Second, the importance of the primary antibody affinity was investigated, using a series of Fab mutants with known kinetics and it was shown that despite starting with sub-nanomolar affinity, improving Fab affinity increased cell isolation. Third, the influence of the connection between the primary antibody and the bead was studied by comparing Fab bridged to the magnetic bead via a secondary antibody, Protein L or streptavidin; the high affinity biotin-streptavidin linkage increased isolation sensitivity by an order of magnitude. Fourth, the effect of manipulating cytoskeletal polymerization and cell membrane fluidity using small molecules was tested; cholesterol depletion decreased isolation and cholesterol loading increased cell isolation. The insights from these observations were then applied to isolate a panel of cell lines expressing a wide range of surface HER2. While the standard approach isolated less than 10% of low HER2 expressing cancer cells from spiked rabbit and human blood, our enhanced approach with the optimized cholesterol level, antibody affinity and antibody-bead linkage could specifically isolate more than 80% of such cells. The final part of this work focussed on developing an antibody clamp that could physically restrict the antigen within its binding site on the Fab and prevent antigen dissociation, using the HER2-Fab complex and the anti-myc peptide antibody 9E10. Work from this thesis provides useful insights into the molecular and cellular parameters guiding immunomagnetic cell isolation and can be used to extend the range of target receptors and biomarkers for tumour cell isolation and other types of cell separation, thereby enhancing the power and capacity of this approach.