Disease inspectors
T cells are white blood cells that patrol the bloodstream and body's organs for signs of disease, a process termed immune surveillance. When they encounter another cell, they "frisk" it to determine if it is normal or infected, cancerous, or foreign to the body. This inspection takes the form of the T cell brushing against the surface of the other cell. The T cell's surface bristles with receptors -- intricate webs of proteins designed to snag specific antigens, much as a lock accepts only certain keys. Each T cell displays a distinct TCR capable of binding to a specific antigen. The millions of T cells within the bloodstream protect people from a wide variety of invading germs or cells altered by cancerous changes.
TCRs are built of eight individual molecules. Investigators have sought to uncover the basic mechanics of the coupling between TCR and antigen by exploring the role of these eight molecules in recognizing foreign antigens and activating T cells' disease-fighting abilities.
First, immunologists identified monoclonal antibodies (mAbs) that target a portion of the TCR – known as CD3 subunits – involved in T cell activation. They determined which anti-CD3 mAbs activate T cells and which others are non-stimulatory. Using recombinant molecular biology, they generated pMHCs specific for, or irrelevant to, a particular TCR.
Next, structural biologists led by Harvard Medical School's Gerhard Wagner, PhD, used Nuclear Magnetic Resonance techniques to determine the shape of the TCR and the arrangement of its component molecules. Biomechanics scientists led by Matthew Lang, PhD, of MIT then devised a set of experiments involving mAbs and pMHC molecules.
The experiments sought to mimic, under controlled conditions, what normally happens when the TCR encounters an antigen from a diseased cell. The mAbs or pMHCs were mounted on tiny beads called microspheres that can be guided into place by laser beams. The mAbs and pMHCs were brought into contact with TCRs on T cells. By adjusting the angle of the laser beams, researchers could subtly alter the strength and direction with which the TCR and mAb or TCR and pMHC were brought together.
They found that although certain mAbs may bind quite well to the TCR, they were unable to activate the T cells if they bound in a perpendicular fashion -- that is, in a mode similar to pMHC binding to the TCR. The activation occurred only after the mAb or pMHC bound to the TCR was dragged along the T cell surface with optical tweezers. Application of force to other surface molecules including the co-receptor molecule CD8, failed to activate T cells.
The authors also observed that when certain anti-CD3 mAbs attached diagonally beneath a lever-like portion of the TCR, the T cell was signaled to activate without any additional force application. These mAbs bind to the most sensitive part of the TCR, suggesting how the relay of TCR signals operates via its various component parts.
"Our findings with mAbs demonstrate that TCR activation function depends on the angle at which anti-CD3 mAb binding takes place," says the study's lead author, Sun Taek Kim, PhD, of Dana-Farber and Harvard. "The mechanical energy generated by diagonal binding is converted into a signal for activating the T cell."
Kim explains that as a T cell scans the surface of antigen-displaying cells in the body looking for foreign intruders such as viruses or dangerous cancerous mutations, the binding of the TCR by pMHC pulls on the TCR. This dual "ligation plus scanning" operation converts a pull to a push, much like opening a flip lid on a can of soda. This diagonal force on the lever is equivalent to that given spontaneously by the stimulatory anti-CD3 mAb. Once the T cell recognizes its target antigen, T cell movement ceases and the cell transitions from its search mode into destroy mode.
"Immune system-based therapies such as cancer vaccines work by increasing the strength of the immune response to disease through expanding the number of T cells that see a particular tumor antigen," Reinherz explains. "Our findings concerning the mechanosensor function of the TCR imply that specific target antigens can be expressed at very low levels on tumor cells and still be recognized efficiently by these T cells. With this insight, the number of tumor target antigens for cancer-based vaccine therapies can be increased."
An animation illustrates how, when a T cell's receptors lock onto the targeted antigen, parts of the receptors bend in a way that signals the T cell to change from disease-scanning to disease-fighting mode.
(Photo Credit: Dana-Farber Cancer Institute)
This is a time-lapse video of T cells scanning epilthelial cells for antigens.
(Photo Credit: Dana-Farber Cancer Institute)
A highly focused laser beam (at right) is used to apply mechanical force (shown as a double headed arrow) to a microsphere (white) coated with histocompatibility protein. The microsphere abuts the surface of a single T cell, shown in gray (top). Activation of the T cell is measured by a change in calcium levels within the cell, which are shown by green colorization (left, prior to force application; bottom, after force application). The direction of force must be tangential, rather than perpendicular, to the T cell surface in order to trigger a rise in calcium levels. Without an application of force, the binding of the histocompatibility protein produces no such rise.
(Photo Credit: Dana-Farber Cancer Institute)
Source: Dana-Farber Cancer Institute