Developmentally Distinct Tregs Decelerate Immune Response

La Jolla researchers have described how specialized T cells keep the immune system from overstepping and when these regulatory cells are deployed. The findings, set to be published in Proceedings of the National Academy of Sciences, help fill in knowledge gaps in the complex choreography that is triggered by the immune system in response to infection.

The study focused on Tregs, a subset of CD4+ T cells that keep infection-fighting CD8+ T cells from harming too many healthy cells. Uncovering how Tregs are generated—and how exactly they do their jobs—is critical for understanding how the body battles infections.

"This work shows how the immune system has devised this remarkable way to rapidly protect the host from acute inflammation," says La Jolla Institute for Immunology (LJI) Professor Stephen Schoenberger, Ph.D., who led the new study.

The work builds on findings in a 2020 Cell Reports study in which Schoenberger’s lab was the first to show an entirely new way that the body makes Tregs. For that study, the team showed that upon infection with bacterial pathogen Listeria monocytogenes, 20–40% of CD4+ T cells rapidly converted to Tregs. These converted Tregs serve as a rapid response to shape CD8+ T cell expansion and reduce autoimmune damage to the body's own tissues early in infection.

The new PNAS study is the first to show that there is a another, developmentally distinct Tregs involved in immune response. "The literature says Tregs basically sit around doing nothing early in an infection, and they're just there to shut down the immune response later on," says study first author Joseph Dolina, Ph.D., a senior scientist at Pfizer Inc., and former member of the Schoenberger Lab. "But we show there's an early wave of Tregs and then a distinct second wave of Tregs."

By day 7 of infection, a second Tregs takes over. This new wave of Tregs has distinct markers, meaning they are developed through a different pathway than the first wave of Tregs. These second-wave Tregs are a specialized to address the mass of CD8+ T cells that gathers in response to the ongoing infection. "These Tregs don't turn into each other. The Treg population is heterogeneic, meaning the first and second waves represent distinct populations, and there are separate subsets built to do certain jobs," says Dolina.

The researchers discovered that the second wave of Tregs could take their time arriving at the site of infection because they aren’t aiming to control inflammation. Instead, these Tregs show up when it is time to shut down the entire CD8+ T cell response, signaling that the infection has been cleared.

These findings were possible thanks to T cell receptor sequencing (TCR-Seq) by study collaborators at Repertoire Genesis Inc. in Osaka, Japan. The researchers were able to follow a single Treg clone over time to see if it expanded into more Tregs, disappeared or stopped developing.

As they compared the two Treg subsets, the researchers noticed that the two types of Tregs reacted differently to adenosine, a nucleotide that is found in DNA but becomes part of the wreckage when cells die. The first-responder Tregs see the loose adenosine as a danger signal and gather molecules to form adenosine clouds. The adenosine binds to receptors on CD8+ T cells, interrupting the CD8+ T cell response and slowing down the capacity of these CD8+ to expand and harm healthy tissues. By contrast, the later wave of Tregs arrives as the CD8+ T cell response is already fading. These Tregs instead produce and store a potent regulatory molecule derived from adenosine called cAMP, which they deliver directly to nearby CD8+ T cells.

One reason for distinct Treg populations is to maintain a healthy balance of killer cells and regulatory cells during the course of an infection. “The trade-off between Tregs and CD8+ T cells is not like an on-off switch," Schoenberger says. "Instead, our research shows distinct pathways for accelerating and decelerating immune responses."

Going forward, the researchers hope to investigate where in the body the different Tregs spend their time. "We need to look at where they are in relation to the CD8+ T cells," says Dolina. The team also wants to see how Treg activity is affected by antigen-presenting cells such as dendritic cells and macrophages.