The overall aim of this proposal is to develop novel approaches for repetitive in vivo imaging of T cell trafficking and activation that could be used to study the mechanisms of T-cell recruitment into immune response against brain tumors or infections. The invasive nature of biopsy does not allow for monitoring of the localization and activity of different cellular mediators of immune response in the brain in humans. Non-invasive CNS and whole body positron emission tomography (PET imaging of genetically enhanced immune-competent "tracer cells" expressing constitutive PET reporter genes (to visualize localization) and/or inducible PET reporter genes (to assess their activation status) would significantly aid in the development and clinical implementation of various vaccination and T-cell -based therapeutic approaches. These genetically enhanced immune-competent "tracer cells" (e.g., T-cells) will allow for non-invasive monitoring of their location and activity over a long period of observation using repeat administrations of specific radiolabeled PET reporter probes (e.g. 124I-FIAU).
The paradigm for non-invasive PET imaging of T-cells requires: 1) harvesting T-cells from a patient; 2) introduction of so-called PET reporter genes (e.g., HSV1-tk) into T-cells to generate "tracer cells"; 3) administration of these "tracer cells" back to the patient along or after therapeutic vaccination; 4) administration of a radiolabeled PET reporter probe (e.g., 124I-FIAU, 18F-FHBG), which will concentrate in "tracer cells"; and 5) visualization of in vivo radiolabeled "tracer T-cells" by PET imaging. In the paradigm, "tracer T-cell" injection will be performed once, while injections of a radiolabeled probe and PET imaging can be performed repeatedly over a period of several months.
The feasibility of this approach was previously demonstrated in a murine model of subcutaneously growing human EBV-lymphoma; genetically enhanced human autologous "tracer T-cells" localized to the sites of EBV-lymphoma growth in an HLA-restricted manner (Koehne, et al. 2003, Nat Biotech, in press). An earlier study demonstrated that T-cell receptor-mediated activation of cytolysis can be imaged using TCR-specific inducible reporter system (Ponomarev, et al. 2001). In the current proposal, the feasibility of "tracer T-cell" imaging paradigm in CNS malignances will be tested in a murine model of human EBV-lymphoma growing intracerebrally using both types of reporter systems and PET.
Non-invasive imaging of these "tracer cells" should help to address the several biological questions related to mechanisms of cellular immune reponses against CNS tumors and infections, including targeting, activation, and durability of immune responses. In summary, non-invasive imaging would aide in pre-clinical development and clinical implementation of new therapeutic strategies to control and direct cellular immune responses against CNS tumors or infections.