Highly sensitive and specific Alu-based quantification of human cells among rodent cells

Alu elements are primate-specific short interspersed elements (SINEs), over 1 million copies of which are present in the human genome; thus, Alu elements are useful targets for detecting human cells. However, previous Alu-based techniques for detecting human genomic DNA do not reach the theoretical limits of sensitivity and specificity. In this study, we developed a highly sensitive and specific Alu-based real-time PCR method for discriminating human cells from rodent cells, using a primer and probe set carefully designed to avoid possible cross-reactions with rodent genomes. From 100 ng of mixed human and rodent genomes, 1 fg of human genome, equivalent to 1 human cell in 100 million rodent cells, was detectable. Furthermore, in vivo mouse subrenal capsule xenotransplantation assays revealed that 10 human cells per mouse organ were detectable. In addition, after intravenous injection of human mesenchymal stem cells into NOD/SCID mice via tail vein, the biodistribution of human cells was trackable in the mouse lungs and kidneys for at least 1 week. Our findings indicate that our primer and probe set is applicable for the quantitative detection of tiny amounts of human cells, such as xenotransplanted human cancer or stem cells, in rodents.