They published the results in the New England Journal of Medicine.
Dr Ewelina Mamcarz, from the St June Department of Bone Marrow Transplantation and Cellular Therapy, said: "These patients are toddlers now, who are responding to vaccinations and have immune systems to make all immune cells they need for protection from infections as they explore the world and lead normal lives".
"This is a first for patients with SCID-X1", she added, referring to the most common type of the disease.
Eight babies have survived so far - developing full and totally normal immune systems because of gene modification and the HIV virus. Babies born with the condition are confined to completely sterile conditions and, if left untreated, usually die within the first two years of life.
John Rasko, head of cell and molecular therapies at Royal Prince Alfred Hospital and who was not involved with the study, said that while long-term follow up studies are needed, the results so far are strong. "Through our collaborations with St. Jude and NIH, we are working to offer this life-saving therapy to these patients as well", said Mustang Bio CEO Manuel Litchman, MD. He is the senior author and died after the manuscript was submitted for publication.
All babies born in the United States are now screened for this condition, and the best treatment today - a bone marrow transplant - succeeds more than 90 percent of the time.
Their bone marrow was collected and processed at a central facility to enrich for stem cells that were then transfected with the lentivirus for gene therapy.
The disorder, which is estimated to affect about one in 100,000 newborns, is caused by a mutation in the gene that produces a protein essential for normal immune function.
Currently, the best treatment for SCID-X1 is bone marrow transplantation with a tissue-matched sibling donor. "They must rely on stem cells from other donors". Still, previous gene therapy techniques either did not fully restore immune function or resulted in side effects as serious as leukemia.
But doctors have seemingly found a way to cure the disease in babies (most often seen in males) via gene therapy: by inserting a corrective gene into a patient's cells via a tweaked version of HIV, in a way that they say it will not lead to development of AIDS or increase the risk of leukemia. The cells were then frozen and underwent quality testing.
Omarion Jordan, who turns 1 later this month, had the therapy in December to treat severe combined immunodeficiency syndrome, or SCID.
Prior gene therapy attempts in SCID-X1 have restored T cells but not B-cell and NK-cell immunity.
Gene therapy has been used successfully over the past decade. All are developing and growing normally. He was not involved in the new treatment, but helped develop an initial gene therapy for the condition 20 years ago.
This lentiviral gene therapy may find a role beyond immune disorders, too. Before reinfusion, patients got individualized doses of nonmyeloablative busulfan chemotherapy for a "cumulative AUC of 22 mg × hour per liter, which is approximately 25% of the typical cumulative AUC used for allogeneic hematopoietic stem-cell transplantation", the researchers noted. "The vector also has been designed with insulators, which protects the site where the vector lands in a cell from activating unintended genes that could cause side effects".
Without treatment, SCID-X1 causes the body to produce no T cells or NK cells and B cells that are normal in number but not functional. Before the cells are returned, the babies are given a drug to destroy some of their marrow, which grants the modified cells more space for growth. Both are important steps for expanding access to treatment and commercializing production.
"In previous trials, waning of the immune system was observed much, much earlier - within the first year", Mamcarz said.
Dr Mamcarz said the current therapy, which is also being trialled among older SCID-XI patients whose bone marrow transplants had been unsuccessful, was able to deliver gene-corrected cells to a patient in a way not possible before. He and Mamcarz are principal investigators of the trial.