UPenn Team Uncovers Surprising Regulatory T Cell Mechanism

Research led by the University of Pennsylvania has identified a new immune system regulatory mechanism that could explain why checkpoint inhibitors—cancer-fighting drugs that block proteins that normally restrain immune response—don’t work in all patients.

The immune system has many layers of checks and balances to balance controlling pathogens while protecting healthy tissue. T cells may be best known for their roles in fighting infections and killing cancer cells, but the immune system also has several mechanisms in place to counterbalance those responses to prevent out-of-control inflammation that could damage healthy tissue. Regulatory T cells, called Tregs, are one such mechanism.

A new study in Nature Immunology led by researchers from UPenn’s School of Veterinary Medicine has identified a new layer of complexity to the immune systems’ regulatory responses  and suggests a possible reason why checkpoint inhibitors don’t always work.

Essentially, the researchers found that checkpoint inhibitors can indeed encourage the activity of cancer killing T cells, but in some cases, they can also activate a population of regulatory T cells that serve to rein in that attack. UPenn Vet professor Christopher Hunter and doctoral student Joseph Perry discovered that blocking the activity of the checkpoint protein PD-L1, which interacts with a T cell receptor PD-1, enhanced the activity of a subset of T cells known as effector regulatory T cells, or effector Tregs. This intervention unexpectedly reduced the ability of mice to control a parasite infection.

The findings reveal a complexity to how the body “regulates the regulators” of the immune system, says Hunter. “Once you have those Tregs to control your T cell response, you also need to control them,” he says. “It’s like with a car. You have the ignition, the accelerator, and you also need a brake. PD-1 is a brake not only on killer T cells but also on Tregs.” Essentially, Tregs are like the health and safety inspectors of the immune system. “They’re really important, but sometimes, when you need to mount an immune response against a pathogen, you need them to take a back seat,” he says. “When an infection is present, we see Treg levels crash so that an effector response can emerge. But we never understood what causes that crash.”

The team further discovered that, when the checkpoint protein PD-L1 was blocked, mice infected with Toxoplasma gondii were less effective at fighting off the parasite than mice with an uninhibited PD-L1. “That was the opposite of what we expected,” Hunter says, as the dogma would have suggested that blocking this checkpoint inhibitor would allow for a better effector T cell response against infection.

Digging into the surprising result, the team realized that it aligned with what some cancer researchers had recently reported: in certain cancers, blocking PD-L1 led to worse outcomes, seemingly because of an increase in a population of Tregs that restrained killing of cancer cells.

When the Penn-led team looked in the context of a T. gondii infection, they discovered the signaling molecule interferon gamma turned on PD-L1, which precipitated a rapid decline in Treg numbers. A PD-L1 inhibitor mitigated this effect and stopped the Treg crash. This treatment alleviated the harmful effects of inflammation on mice but also impaired the ability of T cells to fight infection. Similarly, Tregs altered to lack PD-1, the receptor which with PD-L1 interacts, also led to increases in Treg activity. “It seems that the effector T cell versus Treg ratio is really important,” says Hunter.

As the researchers began to learn more about how Tregs were activated and operated during an infection, they were curious about whether this pathway worked when animals were in a normal, healthy state. Just as there are different types of “regular” T cells, including CD8 and CD4 T cells and many more subdivisions besides, the new work underscores that there are subpopulations of Tregs as well, which have different roles in the body. In healthy, uninfected animals, the team found differences between the proteins expressed by different Treg populations, including some that expressed PD-1, which the researchers termed effector Tregs.

The findings suggest that there is a large population of activated PD-1-positive Treg cells present as a normal part of everyday life that help limit the immune system.  “We think these PD-1 high cells are the most active Tregs,” says Hunter. “It’s a complex landscape, and it’s possible that some checkpoint inhibitor treatments have been inadvertently targeting these Tregs and not others, leading to unexpected outcomes.”

The findings could have implications not only in refining cancer checkpoint inhibitor therapies but also in conceiving new strategies for treating autoimmune disease. “In that case you would want to increase the number of Tregs,” Hunter says. “Maybe we could think about ways to augment that arm of the immune system to treat inflammatory diseases.”