Scientists have appreciated for many decades that T cells have a sophisticated alarm system that can tell viruses, bacteria, tumors, and other dangers apart from healthy cells in the body. This alarm system helps us stay healthy by allowing T cells to mount potent but precise responses to eliminate any detected threats.
While this alarm system works is generally well understood, a more thorough understanding could help scientists engineer immune cells in the lab.
Each T cell has a receptor on its surface that it uses as a surveillance tool. When a string of amino acids (or peptides) from another cell binds to a T cell receptor in a structure called a major histocompatibility complex (pMHC), the T cell measures the strength of the binding (the affinity) to determine whether these peptides come from foreign agents or from healthy tissues. They also sense the amount, or dose, of the pMHC ligand, which indicates the size of the potential threat (the amount of peptides present).
The ability of a T cell to precisely and selectively assess danger is crucial. Failing to adequately respond to a pathogen could lead to unchecked infection, while overreacting or attacking healthy tissue could trigger an autoimmune disease. For these reasons, researchers are eager to better grasp how exactly T cells perceive pMHC affinity and dose to regulate immune responses.
Solving a Molecular Puzzle
Dr. Hao Yuan Kueh, an associate professor of bioengineering and a member of the Institute for Stem Cell and Regenerative Medicine (ISCRM), leads a lab that investigates open questions about the molecular circuitry in immune cells.
“One mystery for the field has been how T cells can distinguish between peptides from pathogens and signals from healthy tissue,” says Kueh. “This is challenging because these signals are very similar molecularly. They can sometimes differ only by a few amino acids, which results in a small difference in binding affinity. There are very small molecular differences that need to be discerned. That’s the basis of the molecular puzzle we are trying to solve.”
Now, a paper from the Kueh Lab, published in the journal PNAS, reveals new insights about the signaling circuitry that allows T cells to elicit tailored responses to a diverse range of threats and proposes a model for future study. The first author of the paper is Matthew Wither, a PhD student in the Kueh Lab.
Mounting Calibrated Responses
In the investigation, the researchers focused their attention on two signaling pathways (Erk and NFAT) that are activated within minutes of foreign pMHC binding to a T cell receptor, and that are responsible for triggering downstream responses. Previous studies have shown that the initial signaling response appears to be the same regardless of pMHC stimulus, at least at first. However, Kueh and his team wanted to gauge how the intensity of the signaling varied over a longer period of time, up to a day.
Kueh explains the significance of timescale. “In the initial moments after pMHC recognition, T cells appear to either activate signaling or not. The prevailing theory has been that there is a threshold affinity that allows the T cell to respond only when triggered by a high affinity pHMC from a foreign source. One of the main findings here is that it’s not all-or-none. It’s much more subtle. When we look at longer times, we see that T cells mount signaling responses to low-affinity pMHCs that are distinct from those coming from invasive antigens, and that can lead to qualitatively different functional responses.”
In other words, T cells have a remarkable ability to discriminate between foreign threats and self-antigens and to make informed decisions about how to respond.
Engineering Optimal Immune Responses
To test their ideas, the investigators developed a dual fluorescent reporter mouse strain and a quantitative imaging assay that allowed them to simultaneously monitor signaling activity of the Erk and NFAT pathways in the same live cell over the course of a full day. RNA sequencing further showed that T cells can independently encode pMHC affinity and dose through the Erk and NFAT pathways, thereby giving the immune system appropriate instructions for dealing with the perceived threat.
Kueh says this is vital information for a lab that is attempting to engineer immune cells to fight cancer and other diseases more effectively. “We know that T cells are capable of sensing pathogen-derived peptides with incredible selectivity. What we have learned about the dynamics at work in these signaling pathways can now tell us, as engineers, how to achieve the optimal response at the right time for advanced immunotherapies.”
Acknowledgements:
This study
was funded by an NIH/NIBIB Trailblazer Award (R21EB027327, H.Y.K.), an NIH/NHGRI Program Project Grant (RM1HG010461, D.M.F.), a NSF Graduate Research Fellowship (M.J.W.), and startup funds from the Bioengineering Department at the University of Washington (H.Y.K).