Identification and Imaging of 15N Labeled Cells with ToF-SIMS.

Stable isotope labeling may provide a novel method for tracking stem cells once they have been injected into a human or animal host. Here we present a simple pilot study to determine the potential for using ToF-SIMS to detect and localize 15N labeled cells in tissue biopsies for use in cell therapy studies. For this pilot study, 3T3 fibroblasts were grown in normal media and in two different media containing 15N labeled amino acids. Samples containing a mixture of 15N labeled and unlabeled cells were prepared, fixed and dried for analysis and were then imaged using a bunched Bi3+ primary ion source. The cells containing 15N labeled amino acids could be readily distinguished using nitrogen containing peaks which have been previously associated with the labeled amino acids. Contrast was sufficient to allow easy identification of labeled cells in both sparsely and densely plated cultures. Multivariate analysis showed that the image contrast could be improved by including peaks originating from characteristic fragments of the labeled amino acids as well as lower mass NH4+ and CH4N+ peaks. Additional work is being pursued to determine and improve the longevity of the label.