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New information about phagocytic cells of the immune system may lead to treatments for incurable autoimmune diseases

The origin of the mysterious macrophages, known in scientific language as Tingible Body Macrophages (TBM), is in progenitor cells of the blood system that emerge from the bone marrow and settle in the lymph nodes, where they quickly and efficiently digest the remains of antibody-producing cells of the immune system

A segment of the lymph node. Illustration:
A segment of the lymph node. Illustration:

Many parents instruct their children to finish eating everything on the plate until the last crumb. Swallowing cells (macrophages), located in certain areas of the lymph nodes, religiously follow this instruction. Although they were first described in 1884 by the German biologist Walter Fleming, their origin and mode of action remain a mystery. in the laboratory of Prof. Ziv Shulman, from the department of systemic immunology at the institute, recently studied these cells andThe findings are revealing Because the origin of the mysterious macrophages, known in scientific language as Tingible Body Macrophages (TBM), is in progenitor cells of the blood system that emerge from the bone marrow and settle in the lymph nodes, where they quickly and efficiently digest the remains of antibody-producing cells of the immune system; Cells that, if "crumbs" are left, could damage the body's tissues.

The immune system's antibody producers known as B cells wait in the lymph nodes for an order. Only when disease-causing agents, such as a virus or bacteria, invade our body do they come into action, divide rapidly and are put into "training camps" - germinal centers located in dedicated follicles in the lymph nodes. In the first stage of their activation, B cells produce antibodies that require improvement, that is, those whose ability to intercept the pathogen is not optimal. However, through a "training program" known as affinity maturation, they enter a process of accelerated evolution during which they undergo mutations at a rate a million times higher than the rest of the body's cells - random changes in the genetic code that may improve the antibodies produced, but at the same time may lead to the creation of antibodies that damage the body's tissues instead of the disease generator. At the end of the training program, only the mutated B cells that serve the war against the invader survive and they are the ones that will give the body protection against it for years. On the other hand, cells that have undergone ineffective and even harmful mutations will "kill themselves" by activating a mechanism of programmed cell death. But how do you prevent dead B cells from accumulating in the lymph nodes and causing damage?  

This is where the buccal cells come into play. In the new study, led by Prof. Shulman and research student Neta Gurevich, the researchers focused on TBM cells that trap dying B cells in the germinal centers and help break them down. In order to trace the mode and rate of their action, the researchers used genetically modified mice in which the splenic cells were colored green and the B cells in the training camps were colored red, using fluorescent proteins. As expected, about a week after the mice were injected with a vaccine, training camps opened in their lymph nodes, and B cells began to divide rapidly and train to improve antibodies. At this stage, through a delicate and focused surgical procedure, the scientists exposed the lymph nodes found in the knee of the mice and attached a microscope to it that allows viewing biological processes inside a living body with a resolution of one millionth of a meter.

""If we know how to get the phagocytic cells to clean the training camps more efficiently, we may have the key to new treatments for patients with diseases that are currently defined as incurable"

The live broadcast, in the full sense of the word, allowed the scientists to notice that while the B cells moved from place to place in the training camp, the phagocytic cells remained in place but sent out extensions to locate and surround dying B cells, similar to the arms of an octopus. They measured and saw that every 10 minutes a TBM cell captures a B cell and engulfs it, a fairly fast rate. Using a computer model, the scientists predicted that given this rate, 30 phagocytic cells are needed to clear all the dying cells in the training camps. In practice, an average of 25 phagocytic cells were observed in each germinal center, a finding that indicates that the cleaning mechanism is indeed effective and thorough.

The recycling bin of the lymph nodes

Later, the scientists tested whether the TBM cells would digest any B-cell that was caught in their environment, or if their action was specific to the B-cells found in the training camps. To this end, they injected the mice with mature, inactive B cells and discovered that the TBMs did not swallow the "bait" and continued to focus exclusively on active B cells that had already begun to mutate.

To solve the mystery of the TBM's origin, the scientists suppressed the mice's immune systems with radiation, then injected into their bone marrow hematopoietic stem cells labeled with a green fluorescent protein. They followed the development of the training camp and noticed that 75% of the inflammatory cells formed in the lymph follicles were colored green, meaning that they originated from the parent cells injected into the bone marrow of the mice. However, the green cells appeared in the training camps only at the end of a few weeks and after the vaccination, an observation that raised among the team of scientists the hypothesis that there is an intermediate station where the progenitor cells wait after leaving the bone skeleton and before turning into phagocytic cells during the training camps.

To locate the intermediate station, the researchers used radiation to damage all the cells of the immune system in the mice, except for the cells located in the lymph nodes, and gave the vaccine one more time. They saw that following the irradiation a small percentage of the TBMs that developed were green, meaning that they originated from the stem cells in the bone marrow, while most of the cells developed from stem cells that were already in the lymph follicles before the vaccine was given. They concluded from this that the origin of TBM is in progenitor cells that emerge from the bone marrow, enter the lymph follicles, accumulate and stay there for a long time.

By giving the vaccine to mice, the researchers were also able to discover what causes the progenitor cells to emerge from the bone marrow and enter the lymph follicles. About five days after the injection, when small concentrations of B cells began to appear in the training camps, a significant amount of TBM type cells were also observed in them for the first time. Therefore, the researchers concluded that the entry of the progenitor cells into the lymph follicles depends on the development of the germinal centers and is intended to serve them.

Finally, the researchers checked at which stage of B cell death the engulfment takes place. They used antibodies to detect proteins that appear at the beginning of the "cellular suicide" process and special staining of DNA segments that appear at the final stage of this process. To their surprise, they discovered that B cells are engulfed alive and undergo the final stage of death in the TBM cells. This discovery indicates that the engulfing cells are not only the "cleaning workers" of the cellular environment, as they have sometimes been described in the past, but constitute a biological recycling basket that groups dying cells within it that are about to die and disintegrate, thus preventing waste from being dispersed in the first place.

"It is known that when the TBM does not remove dying B cells in an optimal way, damage may occur to the body's tissues as a result of the production of unwanted antibodies that target the dead cells," says Prof. Shulman. "This process may be one of the causes of the autoimmune disease 'lupus'. A basic understanding of the origin of the TBM and their modes of action may help us in paving new ways to treat autoimmune diseases. If we know how to make TBM more effectively clean the training camps of cells and harmful proteins, we may have the key to new treatments for patients with diseases that are currently defined as incurable."

Dr. Liat Stoler-Barak, from the Department of Systemic Immunology of the Institute, also participated in the study; Niklas Schwan, Dr. Arnav Bandyopadhyay and Prof. Michael Mayer-Hermann from the Integrated Center for Systems Biology in Braunschweig, Germany.

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