Discovering the cancer-fighting potential of gamma delta T cells

It’s one of the most aggressive and dangerous types of cancer. In Canada, incidences of it have more than tripled in the last 30 years, making it one of the few cancers on the rise. The Canadian Cancer Society posits that in 2017 there were an estimated 7,200 diagnoses in this country, and that the disease was responsible for about 1,250 deaths. In North America, it kills one person every hour.

Its name is melanoma, the deadliest type of skin cancer.

These figures paint a bleak picture, but a recent discovery by Dr. Juan Carlos Zúñiga-Pflücker, a senior scientist in Biological Sciences at Sunnybrook Research Institute (SRI) and the chair of and a professor in the department of immunology at the University of Toronto, and Dr. Erin Adams, Joseph Regenstein professor of biochemistry at the University of Chicago, offers hope. These teams have shown that human gamma delta T cells can identify and destroy melanoma cells, and they have also visualized how the interaction transpires. Not only have the researchers unleashed a new weapon to combat this sinister disease, but they have also rewritten an element of fundamental biology.

At the centre of Zúñiga-Pflücker’s discovery, which was published in the Dec. 14 issue of Science Immunology, are T cells. Critical to healthy immunity, T cells are white blood cells that attack and destroy invaders in the body. Among these invaders are antigens, foreign substances that trigger an immune response. Until now, the belief has been that only one type of T cell, alpha beta T cells, could kill tumour-associated antigens.

For this to happen, alpha beta T cells create T cell receptors (TCRs) during development, which recognize antigens—cancer-associated antigens included—and alert the T cell to kill the intruder. The receptors on gamma delta T cells have never been thought capable of this function, but Zúñiga-Pflücker, who holds the Canada Research Chair in developmental immunology, discovered otherwise. “We didn’t know these T cells could do this. It’s like we’ve been walking around with one arm, and now we have two,” he says. “We have a new arm to fight disease. There’s a new weapon out there that we didn’t know existed to fight cancer.”

To achieve their result, Zúñiga-Pflücker and his team—including Dr. Patricia Benveniste of SRI, co-first author on the Science Immunology paper—generated T cells from stem cells in the lab, and asked whether these T cells were equipped to identify tumour antigens. Using umbilical cord blood stem cells extracted from consenting mothers post-delivery, the researchers guided the stem cells to become T cells using the OP9-DL system that Zúñiga-Pflücker created, and investigated whether they could recognize and kill tumour antigens. They chose melanoma-associated antigens because they are well understood, readily available for testing and easily analyzed.

Informed by their existing understanding of biology, the scientists predicted they’d find that alpha beta T cells could detect and destroy the tumour displaying these antigens. They were shocked to find that gamma delta T cells could, too. Zúñiga-Pflücker explains: “As we were pulling out these T cell receptors that were recognizing the tumour antigens, we came across a few that were not the T cells we were looking for. We said, ‘If these aren’t the T cells we were looking for, then what are they?’” Upon realizing that the mystery cells were gamma delta T cells, Zúñiga-Pflücker says he was floored. “I didn’t expect it,” he adds after a chuckle.

The group needed to validate their findings. “We isolated the receptors from these gamma delta T cells and then transferred them to other cells. We asked, ‘Can you now make the cell see that same [antigen]?’ It’s sort of the perfect experiment,” Zúñiga-Pflücker says. “These cells that didn’t have T cell receptors before, but now had them, they saw [the melanoma antigens]. We knew then that we were in business.” These newly created cells, empowered by gamma delta TCRs, pinpointed the tumour antigens and killed them.

To witness how this behaviour between cloned TCR and antigen unfolds, Zúñiga-Pflücker and his team partnered with Adams and Dr. Sobhan Roy, also co-first author on the Science Immunology paper, at the University of Chicago. Through crystallography, which enables scientists to see and understand how components of cells interact, they were able to follow the confrontation. “It’s one thing to think it happens, but it’s another to see it,” Zúñiga-Pflücker says. Owing to this tool, the collaborators could compare and contrast the ways in which alpha beta TCRs and gamma delta TCRs encounter tumour antigens.

Not only does their discovery bolster the arsenal against cancer, but it is also a new biological principle. As Zúñiga-Pflücker pulls an immunology textbook from a shelf in his office, he reads aloud that gamma delta TCRs do not identify the tumour antigens his group studied. “Every textbook will say this,” he says. With his work, he has demonstrated a need to rewrite those textbook statements.

Asked why the capabilities of gamma delta T cells have remained unknown until now, Zúñiga-Pflücker says they are in the minority and frequently overlooked. He refers to the “old lamppost problem” when discussing this further: “You lose your keys outside, it’s dark, you look for them by the lamppost because that’s where the light is. We knew alpha betas [had this potential], so we looked for alpha betas.” Through a laugh, he admits that a colleague even calls gamma delta T cells “the redheaded, strange stepchild of the T cell family.”

Turning back to his discovery, the clinical possibilities extend beyond melanoma to other cancers. “These T cell receptors can be used with conventional T cells to direct their recognition of tumours,” Zúñiga-Pflücker says. “There is a strong potential that other cancer antigens will be recognized by gamma delta T cells in a similar manner.”

He adds that designing the work in a human context made the results more directly applicable to translational approaches. In other words, the team bypassed experiments with mouse cells and used human stem cells from the outset, inching several steps closer to possible clinical practice. A future aim is to generate these T cells in the lab and then deliver them to people with cancer, so that tumour antigens can be killed. For this to happen, pharmaceutical companies would need to get involved, some of which have already reached out.

As he puts his feelings about his team’s work into words, Zúñiga-Pflücker beams. “It was really thrilling to see that we had uncovered a new reactivity for cells that were known to be there for a long time. T cell receptors were first described in 1984 by several groups, including by Dr. Tak Mak, who’s from Princess Margaret Cancer Centre and is a professor in the department of immunology at U of T, so over 30 years ago, and it’s been that long of not knowing what [gamma delta TCRs] can see,” he says.

Unearthing the potential of gamma delta TCRs may not have been the initial goal, but it is a welcome result. “One aspect of science is that you have to be ready to accept something different,” Zúñiga-Pflücker says. Thankfully, for the sake of biology and the fight against melanoma, he and his team were prepared to do just that.

This work was funded by the Canadian Institutes of Health Research, the Krembil Foundation and the National Institutes of Health. The Canada Foundation for Innovation provided infrastructure support through the Centre for Research in Image-Guided Therapeutics.