Bioluminescent tags can reveal the fate of human embryonic stem cells within living, intact mice, suggesting that less precise post-mortem examinations used to track the survival of transplanted cells could soon be a thing of the past.

Researchers used bioluminescent imaging to see how long human embryonic stem cells (hES cells) last if injected into mice with an intact immune system. Several previous studies suggested that mouse ES cells can persist in rats and sheep, but mice apparently don't extend the same courtesy to hES cells. Joseph Wu and colleagues at Stanford University School of Medicine in California found that when injected into mouse leg muscles, hES cells tagged with the luciferase reporter gene provoked an aggressive cellular and humoral immune response that killed them off within 7 to 10 days, throwing cold water on the notion that hES cells are immune-privileged1. “To us, it's not a surprising finding,” says Wu, an assistant professor of cardiovascular medicine and of radiology.

If the in vivo bioluminescent imaging system the Stanford team used hasn't entirely settled the controversy over hES cell-triggered immune responses, the technique itself, in which cells are tagged with the firefly luciferase reporter gene, is fast gaining favour with researchers keen on a relatively affordable way to track cell fates and detect gene expression.

Jeff Bulte, a professor of radiology and director of cellular imaging at the Institute for Cell Engineering at Johns Hopkins University in Baltimore, Maryland, says the real strength of bioluminescent imaging is in quantifying the kind of in vivo cell dynamics that Wu's group examined. “It's very hard in histology,” Bulte says. “You have a cohort of animals that you kill at different time points and each provides a snapshot.” But a snapshot of cell survival at day 14, he says, may say little about the larger picture, and variations within individual mice can also be misleading.

Wu's group did use histology — tagging the hES cells with enhanced green fluorescent protein — as a post-mortem check, but supplemented it with the luciferase-based method to permit whole-body imaging of anaesthetized mice. Because luciferase is expressed only by living cells, injections of D-luciferin allowed Wu and his colleagues to use a sensitive camera to measure the photons emitted by surviving stem cells in the same mouse at different time points.

Wu says the technique allowed his team to demonstrate the even faster immune rejection after a second injection of hES cells into immunocompetent mice, and to screen for drugs capable of mitigating that response.

Wu hopes to take the imaging approach further by using the herpes simplex virus thymidine kinase reporter gene and a fluorine-18-labelled reporter probe to image cell fates in larger animals and humans using positron emission tomography (PET). The 18F tracer emits the equivalent of more than 250 times the number of photons as the luciferase–luciferin system, although the set-up for the experiment will be considerably more complex. Bulte agrees that the PET approach will be valuable, although a reporter gene that allowed researchers to perform magnetic resonance-based quantitative imaging, he says “would be the best of both worlds”.