How a lymphocyte feels for antigens
Biologists filmed T-lymphocytes in close-up
Denis Strigun, Naked Science
In mammals, T-lymphocytes provide acquired immunity. The identification of new pathogens occurs as follows. At the first stage, antigen-presenting cells, such as macrophages, capture a foreign substance and, together with the molecules of the main histocompatibility complex, present it to T cells. With the help of T-cell receptors (TCRs) and coreceptors (CD4, CD8), the latter "feel" the antigen and, upon recognition, begin to actively divide. Part of the clones at the same time turns into T-killers and T-helpers, destroying damaged cells of the body and stimulating innate immunity, respectively. The remaining copies, in the case of naive T-lymphocytes, are transformed into memory T-cells.
Despite various approaches to the description of this process, it has not yet been possible to obtain a three-dimensional visualization of the molecular mechanism of antigen recognition in real time. To fill the gap, specialists from the University of California at San Francisco, the Research Center for Applied Ecology and other institutions used the technique of planar illumination microscopy (LLS). Article Cai et al. Visualizing dynamic microvillar search and stabilization during ligand detection by T cells is published in the journal Science.
Scientists have derived a line of mouse cells whose T-lymphocytes carried TCRs specific to ovalbumin (a protein capable of provoking immunity). In an in vitro experiment, antibodies to CD45 phosphotase, necessary for signal transmission with TCR, or a fluorescent protein were applied to the membrane of T cells, after which the lymphocytes were treated with ovalbumin.
Dynamics of microvilli of T-lymphocyte (B) and antigen recognition scheme (A, C).
The arrows indicate the microvilli stabilized after recognition.
A drawing from an article in Science.
The diffraction limit of resolution when visualizing the reaction was 0.22–0.44 hertz. It turned out that the movements of lymphocyte microvilli obey fractal geometry: in the background, they make wave-like movements along the transverse plane with an average speed of about 5.2 ± 0.4 micrometers per minute. With an interval of 10-15 seconds, the activity of microvilli decreases and increases again, while for a long period of time they perform uniform interpenetration by the type of subdiffusion. Calculations have shown that, despite the apparent randomness, the "feeling" of the antigen (when it is recognized, the microvilli "freeze") it happens sequentially. So, in a minute, the lymphocyte managed to "inspect" 98 percent of the membrane of the antigen-presenting cell.
In order to repeat the process in real time and consider individual TCRs, the authors marked a bilayer simulating the membrane of an antigen-presenting cell with quantum dots based on the fluorescent pigment rhodamine. Since the diameter of the latter (about 16 nanometers) exceeded the size of the contact points, scientists could trace the sequential activation of microvilli through "holes" in the bilayer. In this case, a fluorescent microscope of total internal reflection (TIRF) was used for observations, and the new technique was called "synaptic mapping microscopy with quantum dots" (quantum dot–enabled synaptic contact mapping, SCM).
According to the group, the features of antigen recognition, which the described approach allowed to study, meet the needs of the body. In particular, fast fractal-like movements of microvilli are necessary for survival. At any given time, the body contains only about 100 T-lymphocytes, specialized for detecting each antigen familiar to the immune system. "At the first signs of a foreign element, the immune system really needs to be proactive. If one T cell fails to recognize the virus, the pathogen will have hours before another lymphocyte does, and it will have time to create tens of thousands of copies of itself," co-author Matthew Krummel said in a UCSF Video Imaging press release Reveals How Immune Cells Sense Danger.
Portal "Eternal youth" http://vechnayamolodost.ru 15.05.2017