2022-03-11 16:15:28
One of the most important questions in understanding the functioning of the immune system is finding out the origin of all the different types of cells that make it up and their fate in the body. This is not an easy task, considering that the immune system contains dozens of different cell types, which are located in various compartments and tissues and which carry out specific and sometimes highly specialized functions. How can we find out the origin of each of these cell types? In this article, we are going to describe the basis of one of the most advanced technologies that are being used to achieve this goal and some of the discoveries to which it has led.
As we know, the cells of an organism have an identical genome, that is, they all have the same genes. The differences between some cells and others are due to the different set of genes that each cell type has in operation. The set of genes that will function in an adult cell is determined by following a program during the development of organisms. Embryonic stem cells communicate with each other through molecular signals and literally decide how they should be organized and located in the embryo and which genes should activate the different types of daughter cells that will derive from them, following the development program.
To get going, genes have particular regions of “letters”, called promoters, to which so-called transcription factors must bind. These are proteins that bind to promoter regions and allow the generation of ARN messenger that, generally, will be used next to manufacture one or another protein. It is these, the proteins, that generate the molecular machinery necessary for the cells to locate themselves in the right place in the body and perform a specific function. Thus, during development, the cells multiply, and as they do so, they also become different and form the different organs of the adult animal. However, in the case of the immune system, the cell differentiation program does not end in the adult organism. In this case, the stem cells in the bone marrow continuously generate their different types of cells and the other cells in the blood.
Since each type of immune cell has a different set of functioning genes, with their corresponding promoters, it has been possible to identify some whose function occurs exclusively in a specific cell type or even at a specific moment in the development of one or another cell type. By ingeniously manipulating the promoter region of one of these genes, we can mark the cell and its descendants in a way that makes it easy to visualize and track.
Mapping the destination.
This is what is achieved with the cell fate mapping technique, which, as its name suggests, seeks to locate in the organism, as if it were on a map, the cells derived from a certain ancestor. Let’s see how it is achieved.
Once the promoter of a gene that functions exclusively in the original cell and its descendants has been identified, this promoter is used to make a synthetic genetic system. The system is then introduced into the genome of a laboratory mouse, which is to be used as a model organism to study cell fate.
The system works in a way that it will direct the generation of a protein called Cre. This is an enzyme capable of cutting the ADN between two identical sequences of 38 “letters”, called loxp, which are also part of this system. Once cut the ADNthe cut is repaired by the cells and the ADN is reattached, but in this repair process the area of ADN located between the two loxp sequences is removed. In summary, therefore, the genetic system generates a “scissors” of ADN which only cuts it between two specific points, the loxp sequences. Furthermore, it only cuts it when the scissor is produced. The scissors is the Cre enzyme, which can only be made when the chosen gene promoter begins to function in the ancestral cell from which immune cells of a particular type will be derived.
The properties of this recombination system, that is, the cutting and joining of the ADN, allow it to be used in a very ingenious way to mark cells, which is nothing more than identifying them with a mark. For this, the ADN that is placed before and after the two loxp sequences, which are going to be cut by the Cre enzyme, is a ADN of braking, that is, a ADN which stops the functioning of another gene that is also introduced into the mouse genome. This gene generates a fluorescent protein that will give cells a particular color when the gene that produces it starts working.
Initially, the genetic system that is to produce the Cre enzyme does not work in the stem cells because the promoter has not yet been put into operation early in development. When stem cells begin to develop into the particular type of daughter cell that kicks off the chosen promoter, the Cre enzyme begins to be produced. This now acts on the loxp sequences and cuts the region between them, thus eliminating the zone of ADN that prevented the fluorescent protein gene from working. This gene is thus put into operation and produces the fluorescent protein, which allows cells to color when illuminated with a certain type of light. In short, the system allows staining only cells in which the promoter of the chosen gene has been put to work, and which, therefore, are of a specific type. In this way, various imaging techniques, microscopy etc., can be used to study where the colored cells are located in the animal’s body and what they do there.
useful discoveries
This technology, and more sophisticated evolutions of it, has been used to generate laboratory mice in which different types of immune system cells have been marked. Labeling has allowed interesting discoveries to be made that would have been impossible without the use of this cell fate mapping technique. Among them, we can mention that heart macrophages are continuously renewed autonomously, while kidney macrophages have been generated during embryonic development and come from the yolk sac.
The technology has yet to be used to study perhaps the even more mysterious cells of the immune system: eosinophils and basophils. However, specific promoters have been identified in the ancestors of those cells. These promoters will be used in the immediate future to develop transgenic mice that will make it possible to mark these types of cells and make significant progress in the study of their distribution and function. The knowledge gained may be important for the treatment or prevention of allergies, in which these cells are heavily involved.
Cell fate mapping technology has also been used to study T cells from the moment they are generated in the thymus and also when they are activated to fight infection. The technique has made it possible to identify a new class of CD8 T cells involved in the fight against Listeria bacteria and against cancer. It has also made it possible to study the role of regulatory T cells in preventing rejection of heart transplants.
The future use of this technology and its derivatives promises not only to discover new and perhaps unsuspected properties or functions of the cells of the immune system, and even to identify new cell types, but also to acquire fundamental knowledge that may be of great importance for the development of , for example, novel cancer immunotherapy strategies, or better vaccines.
Reference:
Scarlett E. Lee, Brian D. Rudd, Norah L. Smith. Fate-mapping mice: new tools and technology for immune discovery. January 27, 2022 DOI: https://doi.org/10.1016/j.it.2022.01.004
(Jorge Laborda 03/11/2022)
Works by Jorge Laborda.
Your defenses against coronavirus
Your defenses against coronavirus
Deflamed immunology: An introduction to the immune system and its pathologies
Deflamed immunology: An introduction to the immune system and its pathologies
Deflamed immunology: An introduction to the immune system and its pathologies
Kilo of Science Volume XII eBook
Kilo of Science Volume XII Paper
Kilo of Science Volume I. Jorge Laborda
Kilo of Science Volume II. Jorge Laborda
Kilo of Science Volume III. Jorge Laborda
Kilo of Science Volume IV. Jorge Laborda
Kilo of Science Volume V. Jorge Laborda
Kilo of Science Volume VI. Jorge Laborda
Kilo of Science Volume VII. Jorge Laborda
Kilo of Science Volume VIII. Jorge Laborda
Kilo of Science Volume IX. Jorge Laborda
Kilo of Science Volume X. Jorge Laborda
Kilo of Science Volume XI. Jorge Laborda
Matrix of homeopathy
Chained circumstances. Ed.Lulu
Chained circumstances. Amazon
One moon, one civilization. Why the Moon tells us that we are alone in the Universe
One moon, one civilization. Why the Moon tells us that we are alone in the Universe
One Moon one civilization why the Moon tells us we are alone in the universe
The thousand and one bases of ADN and other science stories
Adenius Fidelius
The intelligence funnel and other essays
The gods have been cloned
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