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On the basis of a long-standing experience in the immunological research field of antigen presentation, the Momburg group has recently shifted the focus towards aspects of T and NK cell responses in tumor and viral immunology. Major ongoing projects are outlined below.

Activation of tumor-specific T cells

In addition to vaccination with synthetic peptides, recombinant proteins, DNA/RNA, or dendritic cells (DC) loaded with these components, the adoptive transfer of genetically engineered T cells is becoming a promising strategy in tumor immunotherapy. Through retroviral transduction of recombinant T cell receptors, larger numbers of autologous cytotoxic T lymphocytes (CTL) can be endowed with the capacity to lyse tumor cells. In the case of adoptive T cell transfer the need of priming (or restimulation) of tumor-specific CTL is circumvented. This could be beneficial because T cells are often inactivated by tumor-related immunosuppressive mechanisms. We presently isolate T cell receptors (TCR) that recognize MHC-bound peptide epitopes encompassing mutations in proto-oncogenes/tumor suppressor genes. Such mutations that frequently occur in p53, K-ras, B-raf, APC etc. represent ideal tumor antigens as they are selectively expressed by tumor cells and causally linked to oncogenesis. In order to functionally characterize and clone such TCR more easily, we utilize the T cell hybridoma technology to immortalize in vitro preselected, peptide-specific, primary T cells.

In parallel, we pursue peptide- and protein-based vaccination strategies with focus on particular oncogene mutations, oncofetal tumor antigens and differentiation antigens overexpressed in tumors. In a cooperative effort with Prof. Beckhove and Dr. Cid (DKFZ), we utilize long synthetic peptides as well as recombinant tumor antigen fusion proteins with DC-stimulating components such as GM-CSF for the immunization of HLA-transgenic mice. For the selection and in vitro restimulation of T cells from immunized mice, antigen presenting cells expressing larger quantities of selected, peptide-loaded MHC class I molecules (recombinant MHC single chain trimers) are being used in addition to autologous, peptide-loaded DC.

Activation of NK cells for tumor targeting

In malignant tumors, expression of various components of the class I antigen presentation machinery is frequently downmodulated, leading to escape from CTL recognition due to the lack of MHC-I-presented peptides. Such tumor cells with low or absent MHC I expression may, however, become targets for NK cells since MHC class I molecules usually inhibit NK cell-mediated cytotoxicity. In order for NK cells to be activated and lyse target cells, not only the absence of inhibitory MHC class I molecules, but also the presence of NK activating ligands on the surface of virus-infected or tumor cell targets is mostly required. The investigation of the function of activating receptors on NK cells and their binding partners is of importance to employ the potential of NK cells to kill tumor targets. Together with C. Watzl (University of Heidelberg) and A. Cerwenka (DKFZ) we are in the process of identifying tumor cell-borne binding partners recognized by the Natural Cytotoxicity Receptors (NCR), NKp46, NKp44, and NKp30. While the B7 family member B7-H6 has been identified as ligand for NKp30, tumor cell ligands for NKp46 and NKp44 still remain elusive. Our work supports the contention that heparan sulfate proteoglycans, which are expressed on many kinds of tumor cells, function as co-ligands for the recognition of tumor cells by NCRs. Moreover, we have recently identified the hemagglutinin-neuraminidase of the paramyxovirus NDV (Newcastle Disease Virus) and the hemagglutinin of vaccinia virus as novel viral ligand structures for NKp44/NKp46 and NKp30/NKp46, respectively.

NK cells not only have the potential to attack tumor cells with low MHC expression but also to mediate antibody-dependent cellular cytotoxicity (ADCC) against tumor cells, provided the low-affinity IgG Fc receptor (CD16) is sufficiently triggered by tumor cell-binding antibodies. Together with Dr. G. Moldenhauer, DKFZ, and Dr. A. Salnikov, DKFZ, we are presently developing recombinant antibody-based reagents, which will, on the one hand, trigger ADCC through CD16 cross-linking, and on the other hand target NK cells to molecules enriched on tumor endothelial cells such as DLL4, or towards the epithelial cancer antigen EpCAM or the B cell lymphoma antigen CD19. In order to target NK cells to tumor-associated neo-vasculature, we moreover utilize molecules such as VEGF, endocan, or vitronectin as part of recombinant IgG Fc fusion proteins. Preactivated NK cells will be charged in vitro with CD16/endothelium-binding proteins and used in adoptive transfers. We hope that we will be able to efficiently harness NK cells for anti-angiogenetic tumor therapy.