Restoring vision

November 26, 2019

A stem cell therapy for cornea regeneration reaches the clinical-trial stage


By Jessica Lau

As the eye’s outer layer of protection, the cornea needs to constantly regenerate to maintain a clear surface. Losing the cornea-regenerating stem cells leads to cloudiness — like trying to see through a perpetually dirty window. Now, Harvard Stem Cell Institute (HSCI) researchers have developed a therapy to replace cornea-generating stem cells and restore vision, and tested it in a patient for the first time.

“The idea that you can regenerate an organ with stem cells has been around for a long time,” said Natasha Frank, M.D. “So far it’s worked very well for blood stem cells, but the cornea is the first solid organ where this happens to be possible.”

Isolating the stem cells

Around the cornea sits a dark rim of tissue called the limbus, which contains the cornea-regenerating stem cells. Healthy limbus tissue from this rim could be transplanted to an eye that lacks stem cells, but the approach has serious limitations.

“If only one eye is damaged — due to an infection or burn, for example — you can take part of the limbus from the healthy eye and transplant it to the damaged eye,” said Natasha Frank. “But you’re limited in how much tissue you can take before you jeopardize the vision in the healthy eye, so very few stem cells actually get transplanted.”

In cases such as genetic disease, both eyes can be affected by stem cell deficiency. As a result, the limbus transplant has to be taken from a separate donor, and with that comes the risk of immune rejection.

“The limbus has many types of cells in addition to stem cells: pigment-producing cells, blood vessel cells, and nervous system cells. Altogether, they create an immune response,” said Natasha Frank. “Our idea is to purify limbal stem cells and expand them without any other mix of cells.”

To purify limbal stem cells, the researchers needed to identify a molecule that was expressed uniquely on the surface of those cells. That way, they could use the molecule as a “hook” to fish out the cells from the mixture.

In 2014, they succeeded. In work funded by an HSCI seed grant, Natasha Frank, Markus Frank, M.D., and collaborator Bruce Ksander, Ph.D. found precisely such a molecule: the protein ABCB5.

The researchers showed that in mice without ABCB5, corneas did not develop properly. When mice received limbal tissue transplants, the animals that got purified ABCB5-positive cells regenerated clear corneas.

“This was the first time that the limbal stem cell was identified at the molecular level,” said Markus Frank. “We were able to prospectively isolate the cells and demonstrate that they possess a stem cell functionality.”

An off-the-shelf therapy

To bring the therapy to patients, the HSCI researchers are collaborating with the company Rheacell in a clinical trial. The treatment process involves taking limbus tissue from a donor, isolating ABCB5-positive cells, culturing the cells to increase their numbers, and transplanting the purified cells to patients with stem cell deficiency.

The researchers hypothesize that because the limbal stem cells are purified, patients will not have an immune reaction to the transplant.

“This could be an enormous opportunity to create an off-the-shelf cell product. If you have stem cells ready in a tube that you could transplant without having to match the donor and patient’s immune systems, the treatment would be quicker and more universally accessible,” said Natasha Frank.

An alternate source of stem cells

In addition to isolating stem cells from the limbus, the research team is investigating a second, more accessible source of ABCB5-positive cells: the skin.

“Skin cells with ABCB5 normally contribute to skin development. But if we isolate ABCB5-positive cells from skin and put them in the context of an eye without limbal stem cells, they appear to help repair corneal wounds — just like those from the limbus,” said Markus Frank.

The researchers are developing ABCB5-positive skin cells into a cellular therapy for cornea regeneration, a process that could be expedited because the cells are already being evaluated in clinical trials to improve wound healing.

Early support

HSCI supported the team’s work in its early, high-risk stage. At that point, the biological and clinical significance of ABCB5 was still unclear.

“HSCI was one of the earliest funders of this work, when we were starting to characterize ABCB5 in mouse models. As the project evolved further and became more translationally oriented, we received funding from the National Institutes of Health,” said Markus Frank. “It didn’t take long for us to reach the stage of clinical translation.”