Multiple diseases including blood disorders, autoimmunity, infections, and cancer result from defects in proper hematopoiesis or immune cell function. Hematopoietic stem cell (HSC) transplants or genetic modification of these cells can be used as a therapy to correct the observed deficiencies. Engineering antigen specific T cell receptors (TCR) in T cells, or in HSCs, has provided new targeted therapies against cancer by arming the host's immune system to specifically recognize the cancer of interest. The standard method to monitor immune cells is peripheral blood samples that cannot provide whole-body measurements of cell locations. Positron emission tomography (PET) allows non-invasive and repetitive in vivo visualization of positron-emitting probes. Here we use two PET imaging techniques to monitor the immune system. PET reporter genes are applied to detect only the specific, engineered immune cell populations, and immunoPET is used for the detection of total endogenous CD8 cells by radiolabeling an antibody fragment.
We demonstrated that HSC transplants expressing PET reporter genes could be tracked for total cell engraftment, location, expansion, and elimination. The dual PET reporter/suicide gene sr39TK was expressed in a humanized mouse model of HSC transplant. Cells were detected through PET reporter imaging and then selectively eliminated through sr39TK suicide gene function validating its use for cell tracking and, if needed, ablation.
We then developed a new PET reporter with human deoxycytidine kinase containing three point mutations within the active site (hdCK3mut) in combination with the PET probe [18F]-L-FMAU (1-(2-deoxy-2-18fluoro-β-L-arabinofuranosyl)-5-methyluracil). Comparison of hdCK3mut and sr39TK established that hdCK3mut was approximately two fold more sensitive for in vivo detection. hdCK3mut was capable of tracking HSC engraftment, expansion, and the intratumor trafficking of engineered T cells. Expression of hdCK3mut was inert with no effect on cell longevity or function.
For detection of endogenous CD8 cells, a minibody (2.43 Mb) was engineered and radiolabeled with 64Cu. 2.43 Mb retained the ability to detect CD8 cells within the spleen and lymph nodes but did not cause antibody dependent cell depletion.
PET is a powerful imaging modality that can be adapted to track lymphoid cells in vivo. Our work demonstrates three applications of how PET imaging can be applied for whole-body, selective detection of hematopoietic cells.