Gamma Delta T Cells Blur the Line Between Innate and Adaptive Immunity

For more than a century, immunology has rested on a tidy division of labor. The innate immune system, the body’s first line of defense, responds quickly and broadly to danger, while the adaptive immune system takes longer to mobilize but learns from every encounter, building highly specialized receptors that recognize specific threats with exquisite precision. That dichotomy has filled textbooks, shaped research agendas and guided the design of virtually every immunotherapy developed to date. Now, a study published in Nature by researchers at the Francis Crick Institute, King’s College London and Complutense University of Madrid is forcing scientists to reconsider how clean that boundary really is.

The focus of the new work is a population of immune cells known as gamma delta T cells, a specialist lineage that patrols tissues throughout the body, including the skin and the gut, on the lookout for damage, infection and cancer. Unlike the more familiar alpha beta T cells that dominate adaptive immunity, gamma delta T cells have long been regarded as immunological hybrids. They can detect signs of cellular stress and dysregulation without needing the lengthy priming that conventional adaptive responses require, and they can act rapidly against abnormal cells. That combination of speed and broad reactivity has made them one of the most promising candidates for next-generation cancer immunotherapies, and scores of clinical trials are currently underway to harness their potential.

Yet a fundamental question has lingered beneath all of that clinical enthusiasm. Like other adaptive immune cells, gamma delta T cells carry signature receptors on their surface, the products of the same genetic rearrangement machinery that generates the vast receptor repertoires of conventional T and B cells. During development, these so-called T cell receptors help shape how gamma delta T cells mature, where they settle in the body and how prepared they are to respond to particular kinds of stress. But many researchers had assumed that once the cells were mature and deployed into tissues, the receptor’s job was essentially done. If gamma delta T cells respond so rapidly and so innately, surely their adaptive receptor could not be involved in real time?

Adrian Hayday, Principal Group Leader at the Francis Crick Institute who has studied gamma delta T cells for more than four decades, teamed up with Miguel Munoz-Ruiz, a former postdoc in his laboratory who now leads his own research programme at Complutense University of Madrid, to answer that question definitively. “There is a lot of interest and progress in using gamma delta T cells in cancer immunotherapies, or even other types of immune treatment, because they can recognise any kind of dysregulation and act quickly,” Hayday explains. “But if we are going to use them clinically, we need to understand what underpins that rapid response.”

The two teams designed a series of experiments to ask what happens when the gamma delta T cell receptor is removed from mature cells that are already stationed in tissues. In mice engineered so that the receptor could be deleted from selected populations of resident gamma delta T cells, the researchers, working with Nicolas Veland in Hayday’s lab and Bethania Garcia-Cassani in Munoz-Ruiz’s team, observed that the receptor was essential to the cells’ continuing tissue surveillance function. This was a striking result. The cells did not simply carry on with their innate-like patrol work once the receptor was gone; instead, their core identity as sentinels began to unravel.

When the receptor was removed, the gamma delta T cells remained physically present in the tissues, but their behaviour changed markedly. The researchers documented alterations in the activity of genes involved in activation, tissue surveillance and communication with other immune cells. Perhaps most tellingly, they also saw disruption to receptors associated with innate immune responses, the very molecular machinery that was supposed to operate independently of adaptive recognition. In other words, pulling out the adaptive receptor did not leave the innate program intact; it compromised it.

To determine whether the same principle holds in human cells, Veland undertook key parts of the experimental work in laboratory-grown human gamma delta T cells, using CRISPR gene-editing technology to disrupt a gene encoding part of the gamma delta T cell receptor. The result echoed the mouse findings closely. Removing the receptor altered gene activity and reduced the expression of several receptors associated with innate responses, indicating that the dependence on receptor signalling is not a quirk of the mouse immune system but a conserved feature of gamma delta T cell biology across species.

The consequences for cancer surveillance were equally significant. Across mouse models of both skin cancer and colorectal cancer, the loss of the gamma delta T cell receptor impaired the immune system’s ability to control tumour development. In further laboratory experiments, receptor-deficient human gamma delta T cells were less able to restrict the growth of melanoma and colorectal cancer cells. Taken together, these results demonstrate that the rapid, innate-like anti-tumour activity that makes gamma delta T cells so attractive for immunotherapy is not a receptor-independent property of the cells themselves; it depends, in real time, on the continuous signalling of their adaptive T cell receptor.

“This work came from a shared question that Adrian and I kept coming back to,” says Munoz-Ruiz. “If gamma delta T cells are so rapid and so innate-like, what is their receptor doing?” The answer, the teams have now shown, is that the receptor is doing a great deal. “We’ve shown that without receptor signalling, gamma delta T cells are much less able to carry out rapid tissue surveillance, showing that responses previously thought to be independent of adaptive immune recognition still rely on the gamma delta T cell receptor in real time.” The finding reframes the receptor not as a developmental scaffold that can be discarded after maturation, but as an ongoing operational requirement for the cells’ frontline duties.

For the growing field of gamma delta T cell therapy, the implications are immediate and practical. The findings suggest that gamma delta T cell treatments may depend on preserving or activating the receptor signal that helps these cells recognise and respond to damage and disease. Therapeutic strategies that expand or transfer gamma delta T cells without safeguarding their receptor function could, according to this new evidence, deliver cells that are present but functionally blunted. “These findings are incredibly important if we want those cells to attack tumours effectively,” Hayday adds. Beyond the clinic, the study points to what the researchers describe as a fundamental shift in how immune responses are understood. These cells defy the traditional classifications of immunity: an adaptive receptor being used as part of the innate immune response. That may, Hayday suggests, explain why gamma delta T cells have been conserved across many different species, offering an essential and unique form of surveillance that does not fit neatly into either category of the classical immune dichotomy.

Subject of Research: The role of gamma delta T cell receptors in rapid innate-like tissue surveillance and cancer immunosurveillance

Article Title: Immune cells defy textbook classifications

Article References: Immune cells defy textbook classifications. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: gamma delta T cells, innate immunity, adaptive immunity, T cell receptor, cancer immunotherapy, tissue surveillance, Francis Crick Institute, Nature, CRISPR, tumour immunology, immune cell receptors, immunosurveillance