| Positron emission tomography (PET) gene expression imaging is a new technology permitting noninvasive, longitudinal observation of cell populations in living animals. The ability to apply PET technology to image the immune system noninvasively will greatly improve the dimension of the current knowledge for understanding the dynamics of immunology. In this dissertation, we focused on the development, characterization, and application of the PET detection system to repetitively monitor lymphocytes in vivo. We start with the demonstration of utilizing PET technology to image components of the immune system. We used immune T cells retrovirally transduced with the PET reporter gene, thymidine kinase (sr39TK), and imaged with the TK substrate [18F]FHBG (9-[4-[18F]-fluoro-3-(hydroxymethyl)butyl]guanine). We demonstrated, for the first time, the ability to image specific migration patterns of T cells in vivo. Next, we characterized the correlation of PET signal to cell number, and determined the limit of detection for PET imaging. Cells after in vitro uptake of [18F]FHBG were quantitated by PET either in vitro or in vivo. We also developed an in vivo model system to quantitate PET signals from in situ labeled-cells, and demonstrated that there were 72% retention and viability of the transferred cells over an one-hour period. Parenteral uptake of [18F]FHBG was then assessed in cells transferred to the in vivo model region, and showed a cell number-dependent signal, and a limit of detection of 106 cells in 0.1 mL volume. Lastly, we report the potential of utilizing PET imaging as a method to monitor tumor immunotherapy. We demonstrate the capacity to detect and quantitate small numbers of antigen-specific T cells early in the immune response by PET imaging. Naive or memory OT-1 CD8+ T cells, retrovirally marked with the sr39TK gene, were adoptively transferred into tumor bearing RAG I-/-. The animals then were microPET scanned periodically over 2 weeks. Memory cells demonstrated early homing and proliferation, with a 20-fold expansion of cell numbers at the antigen-positive tumor as early as day 1 after T cell transfer. Naive cells did not localize to the tumor until day 8, and had expanded only 5-fold. Only memory cells persisted in the local lymph nodes and lungs after clearance of the antigen. We observed a high correlation between PET detection of T cells at the tumor site to tumor rejection. PET may be useful to directly assess effector activity in anti-tumor immunotherapy and other cell-mediated immune responses. |
