Tumor Localization and Quantitation of Adoptively Transfered T Lymphocytes in a Murine Model.

Tracking adoptively transferred antigen-specific T lymphocytes is an important prerequisite for devising better protocols for cellular therapy. To this end we have developed a highly sensitive method for “in situ” visualization of labelled lymphocytes in vivo by combined PET and magnetic resonance imaging (MRI) to monitor the distribution of adoptively transferred tumour-specific T cells in a mouse model system. Moreover, quantitation of the adoptively transferred cells in tumor was performed by flow cytometry.

C57BL/6J mice carrying subcutaneous tumours of the ovalbumin (OVA)-expressing malignant melanoma cell line B16-OVA were adoptively transferred with OVA-specific CD8+ T cells labelled with 124IdU. Five days after transfer of T cells, mice were killed and subjected to PET and MR imaging. Using a newly developed method for co-registration of the two image modalities, the anatomical localisation of the transferred cells was visualised and the amount of radioactivity in various anatomical locations very accurately determined.

For quantitation of tumor infiltrating non-labelled OVA-specific CD8+ T cells by flow cytometry (using AbsoluteCount Beads), tumors were removed from mice day 1 until day 8 following adoptive transfer (6 mice/group) and prepared for single cell suspension before labeled with anti-CD8-FITC and SIINFEKL-Tetramer-PE.

Results showed a clear tumor localization of the adoptively transferred OVA-specific T cells in the tumours. In two independent experiments comprising 12 and 13 evaluable mice, respectively, we found a mean value of 0.909 +/− 0.468 Bq and 0.926 +/− 0.553 Bq in the tumours, and only 0.182 +/− 0.479 Bq and 0.026 +/− 0.480 Bq in the corresponding contralateral control volumes. The difference in activity between the tumour regions and the control regions was statistically highly significant with 2p-values of 0.002 and 0.006 for the two experiments. Using flow cytometry it was shown that the number of OVA specific T lymphocytes accumulating in tumor gradually increased until day 5 after transfer when an average of 3.3 million SIINFEKL-specific cells per gram tumor tissue was found. From day 5 until day 8 the number of SIINFEKL-specific cells per gram tumor tissue fluctuated at a fairly constant level.

This method presented for tracking adoptively transfered tumor specific T lymphocytes represent a significant advancement for studies of adoptively transferred specific T cells, and could potentially be developed for diagnostic purposes. Moreover, since these studies show that tumor-specific T cells home to subcutaneous tumours in substantial numbers, we suggest that these migrating cells could be employed in a new form of therapy as carriers of toxic substances to tumors.