Researchers at TU Wien and the Vienna School of Medicine report that T-cell receptors act alone, rather than interact with one another, for effective immune signaling. Their study (“Monomeric TCRs Drive T Cell Antigen Recognition”) appears in Nature Immunology.

“T cell antigen recognition requires T cell antigen receptors (TCRs) engaging MHC [major histocompatibility complex]-embedded antigenic peptides (pMHCs) within the contact region of a T cell with its conjugated antigen-presenting cell. Despite micromolar TCR:pMHC affinities, T cells respond to even a single antigenic pMHC, and higher-order TCRs have been postulated to maintain high antigen sensitivity and trigger signaling. We interrogated the stoichiometry of TCRs and their associated CD3 subunits on the surface of living T cells through single-molecule brightness and single-molecule coincidence analysis, photon-antibunching-based fluorescence correlation spectroscopy and Förster resonance energy transfer measurements,” write the investigators.

“We found exclusively monomeric TCR–CD3 complexes driving the recognition of antigenic pMHCs, which underscores the exceptional capacity of single TCR–CD3 complexes to elicit robust intracellular signaling.”

Johannes Huppa, Ph.D., an Immunologist from the Medical University Vienna, and Mario Brameshuber, Ph.D., a biophysicist from the TU Wien, succeeded in visualizing T-cell receptors on the surface of living T cells at the molecular level.

“Even though the mechanisms underlying T-cell recognition are integral to the inner workings of the immune system, our understanding is still limited,” Dr. Huppa says. This is the case because an electron microscope is necessary to see such tiny structures. With such a microscope, however, only dead cells that are specifically prepared for this kind of analysis can be studied.

“A unique aspect of our joint venture is the use of specialized microscopy approaches that allow for quasi-biochemical studies on living T cells,” adds Dr. Brameshuber, due to a combined application of different techniques. Specially labeled molecules serve as highly accurate molecular probes for targeting the protein of interest and newly developed microscopy methods are used.

“As a biochemist, this has always been a big dream of mine, simply because this combined experimental approach allows us to study short-lived molecular processes within their native cellular context, and not in the test tube, removed from the all-defining context of life,” explains Dr. Huppa.

According to Dr. Brameshuber, the outer membrane of the T cell is not like a solid skin. “Molecules embedded in the membrane are relentlessly on the move. This is also true for the receptors that bind to antigens. They constantly change their location,” he points out.

As a result, the mechanism underlying the sensitivity of T cells toward antigens was regarded to depend on T-cell receptors forming pairs or even larger groups to collectively signal once a T-cell receptor binds to a single antigen. However, as was now shown by the Viennese researchers, this assumption is wrong. 

“Obviously the T-cell receptor is a fine-tuned molecular machine, which acts as an individual entity and translates antigen-binding events into intracellular signaling,” says Dr. Huppa, adding that one can learn important lessons from how these receptors work, not only in purely academic terms but also for medical applications. 

Only by understanding in detail what goes wrong in T cells when they cause disease can one develop means of precise intervention, emphasize the scientists. Fundamental knowledge of all involved molecular processes will allow for devising effective high-precision therapies against cancer and autoimmune diseases, and also for more success in protecting against the rejection of organ transplants.