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Chimeric antigen receptor T cell therapy (CAR-T) has been rolled out as a new treatment for hematological malignancies. For solid tumor treatment, CAR-T has been disappointing so far. Challenges include the quantification of CAR-T trafficking, expansion and retention in tumors, activity at target sites, toxicities, and long-term CAR-T survival. Non-invasive serial in vivo imaging of CAR-T using reporter genes can address several of these challenges. For clinical use, a non-immunogenic reporter that is detectable with exquisite sensitivity by positron emission tomography (PET) using a clinically available non-toxic radiotracer would be beneficial. Here, we employed the human sodium iodide symporter to non-invasively quantify tumor retention of pan-ErbB family targeted CAR-T by PET. We generated and characterized traceable CAR T cells and examined potential negative effects of radionuclide reporter use. We applied our platform to two different triple-negative breast cancer (TNBC) models and unexpectedly observed pronounced differences in CAR-T tumor retention by PET/CT (computed tomography) and confirmed data ex vivo. CAR-T tumor retention inversely correlated with immune checkpoint expression in the TNBC models. Our platform enables highly sensitive non-invasive PET tracking of CAR-T thereby addressing a fundamental unmet need in CAR-T development and offering to provide missing information needed for future clinical CAR-T imaging., Graphical Abstract, CAR-T immunotherapy remains challenging in solid tumors, not least as quantitation of CAR-T in vivo distribution, activity, and fate remains elusive. Fruhwirth and colleagues developed and validated a platform for non-invasive in vivo CAR-T tracking by highly sensitive PET imaging and demonstrated its application in triple-negative breast cancer models. Beyond supporting preclinical CAR-T development, they provide a clinically compatible CAR-T monitoring tool.

Gammadelta T (γδ-T) cells are strong candidates for adoptive immunotherapy in oncology due to their cytotoxicity, ease of expansion, and favorable safety profile. The development of γδ-T cell therapies would benefit from non-invasive cell-tracking methods and increased targeting to tumor sites. Here we report the use of [89Zr]Zr(oxinate)4 to track Vγ9Vδ2 T cells in vivo by positron emission tomography (PET). In vitro, we showed that 89Zr-labeled Vγ9Vδ2 T cells retained their viability, proliferative capacity, and anti-cancer cytotoxicity with minimal DNA damage for amounts of 89Zr ≤20 mBq/cell. Using a mouse xenograft model of human breast cancer, 89Zr-labeled γδ-T cells were tracked by PET imaging over 1 week. To increase tumor antigen expression, the mice were pre-treated with PEGylated liposomal alendronate. Liposomal alendronate, but not placebo liposomes or non-liposomal alendronate, significantly increased the 89Zr signal in the tumors, suggesting increased homing of γδ-T cells to the tumors. γδ-T cell trafficking to tumors occurred within 48 hr of administration. The presence of γδ-T cells in tumors, liver, and spleen was confirmed by histology. Our results demonstrate the suitability of [89Zr]Zr(oxinate)4 as a cell-labeling agent for therapeutic T cells and the potential benefits of liposomal bisphosphonate treatment before γδ-T cell administration., Graphical Abstract, Man et al. demonstrate that [89Zr]Zr(oxinate)4 allows in vivo PET imaging of therapeutic γδ-T cells over 1 week. Furthermore, the trafficking of γδ-T cells to the tumor is increased by treatment with a liposomal aminobisphosphonate drug. This radiotracer is potentially translatable for clinical trials of therapeutic T cells.

The exit of metastasizing tumor cells from the vasculature, extravasation, is regulated by their dynamic interactions with the endothelial cells that line the internal surface of vessels. To elucidate signals controlling tumor cell adhesion to the endothelium and subsequent transendothelial migration, we performed phosphoproteomic analysis to map cell-specific changes in protein phosphorylation that were triggered by contact between metastatic MDA-MB-231 breast cancer cells and endothelial cells. From the 2669 unique phosphorylation sites identified, 77 and 43 were differentially phosphorylated in the tumor cells and endothelial cells, respectively. The receptor tyrosine kinase ephrin type A receptor 2 (EPHA2) exhibited decreased Tyr(772) phosphorylation in the cancer cells upon endothelial contact. Knockdown of EPHA2 increased adhesion of the breast cancer cells to human umbilical vein endothelial cells (HUVECs) and their transendothelial migration in coculture cell assays, as well as early-stage lung colonization in vivo. EPHA2-mediated inhibition of transendothelial migration of breast cancer cells depended on interaction with the ligand ephrinA1 on HUVECs and phosphorylation of EPHA2-Tyr(772). When EPHA2 phosphorylation dynamics were compared between cell lines of different metastatic ability, EPHA2-Tyr(772) was rapidly dephosphorylated after ephrinA1 stimulation specifically in cells targeting the lung. Knockdown of the phosphatase LMW-PTP reduced adhesion and transendothelial migration of the breast cancer cells. Overall, cell-specific phosphoproteomic analysis provides a bidirectional map of contact-initiated signaling between tumor and endothelial cells that can be further investigated to identify mechanisms controlling the transendothelial cell migration of cancer cells.

We report the orthologous screening, engineering, and optimization of amino acid conversion enzymes for cell-specific proteomic labeling. Intracellular endoplasmic-reticulum-anchored Mycobacterium tuberculosis diaminopimelate decarboxylase (DDC(M.tub-KDEL)) confers cell-specific meso-2,6-diaminopimelate-dependent proliferation to multiple eukaryotic cell types. Optimized lysine racemase (Lyr(M37-KDEL)) supports D-lysine specific proliferation and efficient cell-specific isotopic labeling. When ectopically expressed in discrete cell types, these enzymes confer 90% cell-specific isotopic labeling efficiency after 10 days of co-culture. Moreover, DDC(M.tub-KDEL) and Lyr(M37-KDEL) facilitate equally high cell-specific labeling fidelity without daily media exchange. Consequently, the reported novel enzyme pairing can be used to study cell-specific signaling in uninterrupted, continuous co-cultures. Demonstrating the importance of increased labeling stability for addressing novel biological questions, we compare the cell-specific phosphoproteome of fibroblasts in direct co-culture with epithelial tumor cells in both interrupted (daily media exchange) and continuous (no media exchange) co-cultures. This analysis identified multiple cell-specific phosphorylation sites specifically regulated in the continuous co-culture. Given their applicability to multiple cell types, continuous co-culture labeling fidelity, and suitability for long-term cell-cell phospho-signaling experiments, we propose DDC(M.tub-KDEL) and Lyr(M37-KDEL) as excellent enzymes for cell-specific labeling with amino acid precursors.