The experimental chamber

used for T-cell force research,

with 3 glass micropipettes

shown under green light.

Georgia Tech/Rob Felt

Investigators say they’ve discovered how T-cell receptors (TCRs) use mechanical contact to decide if the cells they encounter

pose a threat to the body.

The team made their discovery using a sensor based on a red blood cell and a technique for detecting calcium ions emitted by T cells as part of the signaling process.

The researchers studied the binding of antigens to more than a hundred T cells, measuring the forces involved in the binding and the lifetimes of the bonds.

Results revealed that force prolongs TCR bonds for agonists but shortens them for antagonists. And the signaling outcome of an interaction between an antigen and a TCR depends on the magnitude, duration, frequency, and timing of the force application.

“This is the first systematic study of how T-cell recognition is affected by mechanical force, and it shows that forces play an important role in the functions of T cells,” said study author Cheng Zhu, PhD, of Georgia Tech and Emory University in Atlanta. “We think that mechanical force plays a role in almost every step of T-cell biology.”

Dr Zhu and his colleagues described this research in Cell.

The team used a biomembrane force probe to measure the strength and longevity of bonds between T cells and antigens. The probe consists, in part, of a red blood cell aspirated to a micropipette.

Attached to the red blood cell is a bead on which the investigators placed the antigen under study. Using a delicate mechanism that precisely controls motion, they moved the bead into contact with a TCR, allowing binding to take place.

To test the strength of the bond formed between an antigen and the TCR, the researchers applied piconewton forces to separate the bead holding the antigen from the TCR. The red blood cell then acted as a spring, stretching and allowing a measurement of the forces needed to separate the TCR and antigen.

To assess the impact of the binding on intracellular signaling, the investigators injected a dye into the cells that fluoresces when exposed to calcium signaling ions. Detecting the fluorescence allowed the team to determine when the mechanical force triggered T-cell signaling.

In this way, the researchers learned that interactions between the TCRs and agonist peptide-major histocompatibility complexes (MHCs) form catch bonds that become stronger with the application of additional force to initiate intracellular signaling.

And less active MHC complexes form slip bonds that weaken with force and don’t initiate signaling.

Overall, the investigators found that the signaling outcome of an interaction between an antigen and a TCR depends on the magnitude, duration, frequency, and timing of the force application.

“Force adds another dimension to interactions with T cells,” Dr Zhu explained. “Antigens that have a bond lifetime that is prolonged by force would have a higher likelihood of triggering signaling. Repeat engagements and lifetime accumulations play a role, and the decision to signal is usually made based on the accumulation of actions, not a single action.”

Researchers already have examples of how mechanical force can affect the operation of cellular systems. For instance, mechanical stress created by blood flow acting on the endothelial cells that line blood vessel walls plays a role in atherosclerosis.

So it isn’t surprising that mechanical forces play a role in the immune system, according to Dr Zhu.

“We now have a broader recognition that the physical environment and mechanical environment regulate many of the biological phenomena in the body,” he said. “When you exert a force on the TCR bonds, some of them dissociate faster, while others come off more slowly. This has an effect on the response of the T-cell receptor.”

As a next step, Dr Zhu’s team would like to explore the effects of force on T-cell development using the new experimental techniques. Evidence suggests the forces to which the cells are exposed while in a juvenile stage may affect the fates of their development.

How mechanical forces affect T cells