Shepherding regenerative medicine to new territory

More than 20 bruises mysteriously appear on her body. Her gums bleed when she brushes her teeth. Her legs ache and cause excruciating pain. This is what Emily Whitehead, a five-year-old at the time of her ordeal in 2010, was experiencing before her parents rushed her to a Pennsylvania emergency room.

Shortly after doctors assessed her, Emily was diagnosed with an aggressive cancer called acute lymphoblastic leukemia. She and her parents were told that 85% of people with the disease receive chemotherapy and can return to their normal lives. Following two relapses after intensive rounds of chemotherapy, however, it was clear Emily fell outside the overwhelming majority. Doctors were stumped, and Emily’s condition deteriorated. A glimmer of hope arrived, though, at the suggestion of CAR-T therapy, now known as the U.S. Food and Drug Administration-approved treatment Kymriah. It entails extracting T cells from one’s bloodstream, reprogramming them to target cancer cells, multiplying them and then introducing them to one’s system to combat destructive cells. Fast-forward to 2017 after Emily became the first person to be treated with the therapy, and she’s cancer free—and like many teens, YouTube-obsessed and prone to snapping photos of her poodle.

This is the power of regenerative medicine.

It is no secret that regenerative medicine is, and has been, revolutionizing health care. Defined as the reproduction, renewal or replacement of defective cells, organs and tissues, it is at the heart of some of science’s most significant breakthroughs, including the discovery of stem cells by University of Toronto scientists Drs. Ernest McCulloch and James Till in the early 1960s. It is continually being improved and advanced by scientists worldwide, and one researcher keen to deepen his imprint is Dr. Juan Carlos Zúñiga-Pflücker, senior scientist in Biological Sciences at Sunnybrook Research Institute (SRI). He is preparing to bring his pioneering work in the field of immune-regeneration to the threshold of clinical testing.

Zúñiga-Pflücker—who is also the chair of and a professor in the department of immunology at U of T, and the Canada Research Chair in developmental immunology—studies T cells, focusing on building them to bolster weakened immune systems. Essential for immunity, T cells are white blood cells that develop in the thymus after traveling there as progenitors from bone marrow. “Without T cells, you can’t have normal health. They’re an important part of the immune system; they’re critical effectors and regulators,” Zúñiga-Pflücker says.

The research he wishes to explore further and potentially translate to clinical practice entails manufacturing large quantities of T cells from blood stem cells and then delivering them to patients with leukemia. The project would be the first of its kind done in humans. These cancer patients will have undergone myeloablative chemotherapy, a high-dose treatment that kills cells—cancer cells included—in bone marrow. Following this form of chemotherapy, patients receive a stem cell transplant; however, a consequence of this is a depletion in T cells, which leaves patients susceptible to infection and relapse.

Zúñiga-Pflücker hopes to establish an immunotherapy that simultaneously regenerates the immune system and targets anti-tumour activity. He predicts the clinical trials will begin in two to three years, and says the work could be broadened to treat cancers other than leukemia. “The idea is to guide the development of T cells to recognize different types of tumour cells. To regulate T cells would also allow us to regulate the immune system and control autoimmunity,” he says.

The research would be performed with colleagues at Memorial Sloan Kettering Cancer Center in New York City, SickKids in Toronto, the University of British Columbia in Vancouver and the University of Minnesota in Minneapolis. It builds on the results Zúñiga-Pflücker has achieved through lab work and in preclinical studies.

Having fathered the OP9-DL cell system, which is the gold standard for generating T cells in the lab and which will play a key role in the proposed project, Zúñiga-Pflücker has been steadily working toward this goal. As an undergraduate student at the University of Maryland in the mid-1980s, he landed a position in the lab of Dr. Ada Kruisbeek at the National Institutes of Health. She explained to him that stem cells travel to the thymus and emerge as T cells, but what happens in the thymus to complete the transformation is a mystery. “She drew a picture for me, of a black box representing what happens in the thymus, and I was immediately smitten by that enigma. What happens to make these cells become T cells? It was a question of development, and I’ve been studying it for over 30 years,” he says.

Aside from the translational work he hopes to get underway, it was announced in August that Zúñiga-Pflücker would receive a seven-year fund worth nearly $3 million from the Canadian Institutes of Health Research to study an evolutionarily conserved pathway called Notch signalling. Notch is a gene that directs cell types to select a lineage—to become T cells, for example—and Zúñiga-Pflücker discovered that the pathway can be controlled to promote T cell development. Gaining a more thorough understanding of Notch signalling will allow him to enhance his knowledge of T cells, specifically their production and function in regulating the immune system.

Already this year, Zúñiga-Pflücker teamed with fellow SRI senior scientist Dr. Michele Anderson to break new ground in immunology. By using T cells developed from stem cells and an innovative gene-editing technique called CRISPR-Cas9, they unlocked the significance of a transcription factor known as HEB in the early developmental processes of humans. The landmark finding was outlined in the September issue of Stem Cell Reports.

Considering the strides he has made in T cell development research, which he calls his “first love,” it is unsurprising to learn that Zúñiga-Pflücker has no intention of turning off the trail he’s been treading. “It’s been a career-long path to understand how this organ—the thymus—works, what the molecular underpinnings are and how one can take this knowledge and translate it,” he says, before he pauses and adds, “while still digging for more knowledge, of course.” He recognizes that the value of antibiotics, drugs and devices is indisputable, but regenerative medicine is where he sees the most promise, and where he plans to continue expanding the scope of possibility. “Giving you a cell that can live in your body and give you proper function, that’s what I want to do. I want to do more. I want to do it bigger, better, faster, and with more impact. I’d like to push the frontiers of science, train the next generation and create new things.”

This work was funded by the Canadian Cancer Society Research Institute, Canadian Institutes of Health Research, Krembil Foundation, Medicine by Design, U.S. National Institutes of Health and Ontario Institute of Regenerative Medicine. Infrastructure support was provided by the Canada Foundation for Innovation.