A baby whom doctors thought almost certain to die has been cleared of a previously incurable leukemia in the first human use of an "off-the-shelf" cell therapy that creates designer immune cells.

One-year-old Layla had run out of all other treatment options when doctors at Britain's Great Ormond Street Hospital gave her the highly experimental, genetically edited cells in a tiny 1-milliliter intravenous infusion.

Two months later, she was cancer-free and she is now home from hospital. The gene-edited cell treatment was prepared by scientists at the hospital and University College London together with the French biotech firm Cellectis, which is now funding full clinical trials of the therapy due to start next year.

It is designed to work by adding new genes to healthy donated immune cells known as T-cells, which arm them against leukemia. Layla's mother told media that the only options the family would not accept was inaction.

"We didn't see it as a tough decision, we saw it as the only decision. There was no other option, we didn't agree with going home to do nothing. We didn't want to know what ifs, we wanted to know we've tried everything for her," Lisa Foley said.

Layla's mother said they saw the experimental treatment as the only decision. (Reuters)

Using a gene-editing technology called TALEN, which acts as "molecular scissors," specific genes are then cut to make the T-cells behave in two specific ways: Firstly, they are rendered invisible to a powerful leukemia drug that would usually kill them and secondly they are reprogrammed to only target and fight against leukemia cells.

Other drugmakers including Novartis, Juno Therapeutics and Kite Pharma have tested genetically modified T-cells extracted from an individual patient. However, this is the first time cells from a healthy donor have been used in a process that could lead to a ready off-the-shelf supply for use in multiple patients.

Some scientists have questioned Cellectis' approach because of potential problems with patients rejecting foreign cells.

But the French biotech, working with the U.S. giant Pfizer, as well as Novartis believes its method is faster and cheaper than creating single patient-specific gene therapies.

Results from Layla's case were due to be presented at the American Society of Hematology's annual meeting in Orlando on Wednesday.

"It does mean we have a new weapon and it does mean therefore that children like Layla who really are up against it now have something that we can do, we've learnt has a significant chance of working," said Paul Veys, a professor and director of bone marrow transplant at the hospital who led the team treating Layla.

Doctors are watching if the success in Layla's case is sustained.

"It's very exciting but it is early days," Matt Kaiser, head of research at U.K. charity Bloodwise that funds research on blood cancers, and who wasn't involved in Layla's treatment, cautioned in the Wall Street Journal.

Long-term clinical studies on more patients are also needed to validate the approach and to understand who responds to the treatment and who doesn't, Kaiser added.