StemCell Therapy

April 30, 2019

Children born with a rare genetic disorder called X-linked severe combined immunodeficiency (X-SCID) dont have a functioning immune system. As a result, they cant fight off infections. Without treatment, an infant with X-SCID will usually die within the first year or two of life.

The best option for treatment of newly diagnosed infants with X-SCID has been stem-cell transplantation from a genetically matched sibling. But less than a quarter of children with X-SCID have a matched donor available. For those without a matched donor, standard treatment has been a half-matched bone marrow transplant from a parent. But most infants receiving this type of transplant only have part of their immune system, called T lymphocytes, restored. These infants will need lifelong injections of protective antibodies. In addition, as they grow into young adulthood, they may have chronic medical problems that affect growth, nutrition, and quality of life.

To develop a better approach to fix the immune systems of children with X-SCID, researchers have used gene therapy to alter patients own blood stem cells. An engineered virus brings a healthy copy of the gene into the stem cells to replace the mutated gene that causes the disease.

Early results from trials of gene therapy for X-SCID resulted in life-saving correction of T lymphocytes. But similar to bone marrow transplant from a parent, the immune restoration was incomplete. In addition, in those first gene therapy studies, almosta third of the children developed leukemia. The approach accidentally stimulated cells to grow uncontrollably. In later studies, improved design of the engineered virus didnt cause cancer, but also didnt fully restore a healthy immune system.

In 2010, Dr. Harry Malech of NIHs National Institute of Allergy and Infectious Diseases (NIAID) and Dr. Brian Sorrentino of St. Jude Childrens Research Hospital reported a new and safer version of gene therapy for X-SCID. They designed a harmless engineered virus (called a lentivector) that could deliver genes into cells without activating other genes that can cause cancer. Before the altered stem cells were returned to their bodies, patients were given low doses of the chemotherapy drug busulfan. This made it easier for the new stem cells to grow in the bone marrow. In young adults and children treated at the NIH Clinical Center, the new therapy proved to be both safe and effective at restoring the full range of immune functions.

Based on this work, a team led by Dr. Ewelina Mamcarz of St. Jude Childrens Research Hospital began treatment in 2015 of newly diagnosed infants with X-SCID using the lentivector and busulfan. The work was funded in part by NHLBI. The team described the treatment of eight infants with the disorder on April 18, 2019, in the New England Journal of Medicine.

By 3 to 4 months after infusion of the repaired stem cells, 7 of the 8 infants had normal levels of multiple types of immune cells in their blood. The last infant required a second stem-cell infusion, after which his immune-cell levels rose to a normal range.

The infants new immune systems were able to fight off infections that the researchers had detected before the gene therapy. Four of the eight discontinued immune-system boosting medications that theyd previously needed. Of those four, three developed antibodies in response to vaccination, indicating a fully functional immune system.

A year and a half after gene therapy, all children were healthy and growing normally.

The broad scope of immune function that our gene therapy approach has restored to infants with X-SCID as well as to older children and young adults in our continuing study at NIH is unprecedented, Malech says.

The researchers will continue to follow the participants over time. They plan to track how the childrens immune systems develop and look for any late side effects.

References:Lentiviral Gene Therapy Combined with Low-Dose Busulfan in Infants with SCID-X1. Mamcarz E, Zhou S, Lockey T, Abdelsamed H, Cross SJ, Kang G, Ma Z, Condori J, Dowdy J, Triplett B, Li C, Maron G, Aldave Becerra JC, Church JA, Dokmeci E, Love JT, da Matta Ain AC, van der Watt H, Tang X, Janssen W, Ryu BY, De Ravin SS, Weiss MJ, Youngblood B, Long-Boyle JR, Gottschalk S, Meagher MM, Malech HL, Puck JM, Cowan MJ, Sorrentino BP. N Engl J Med. 2019 Apr 18;380(16):1525-1534. doi: 10.1056/NEJMoa1815408. PMID: 30995372.

Funding:NIHs National Institute of Allergy and Infectious Diseases (NIAID); National Heart, Lung, and Blood Institute (NHLBI); and National Cancer Institute (NCI); American Lebanese Syrian Associated Charities; California Institute of Regenerative Medicine; and Assisi Foundation of Memphis.

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