Lymphocyte T

The lymphocytes T , also called thymocytes or cells T , are a category of Lymphocyte S which plays a great part in the secondary Immunizing response. “T” is the Abréviation of thymus, the body in which them development is completed.
Ils is responsible for cellular immunity: the cells (bacteria, cancer cells) recognized as foreign (i.e. others that those which the cells T learned how to tolerate during their maturation) are destroyed by a complex mechanism.

The lymphocytes all T are CD3+.

Types of cells T

There are several types of cells T:

the cytotoxic cells T (CD₈+) destroy the infected cells. These cells function like tueuses cells (“English killer”) or cytotoxic because they are capable to destroy target cells which express specific antigens that they recognize.
cells CD4+ (in English T-Helper) are intermediaries of the immunizing response and proliferate to activate quantity of other types of cells which will act in a more direct way on the answer. The CD₄+ cells control or “help” with the realization of other functions lymphocytaires. It is known that they are the target of the infection with HIV; the AIDS involves the fall of their population.
T regulating help to prevent the activation of the autoimmune lymphocytes which destroy the cells of their own organization. Previously called “T suppressors”, they are very important for the maintenance of homeostasis. The main role is to repress the activity of the cells of immunity, either auto-immune, or at the end of the immune reaction. They are distinguished easily from the other lymphocytes T: they carry on their surface the markers CD4 and CD25 in their basal state, and express the molecule FOXP3 in their Cytosol.
the lymphocytes NKT are a type of lymphocytes presenting of the markers of cell T (CD3) and of the markers of cells NK. They are thus a bond between the innate Immune system and the adaptive Immune system. Contrary to the lymphocytes T conventional, whose TCR recognizes a peptide presented in a molecule of CMH, the NKT are able to recognize a glyclipide presented in a molecule called CD1d, structurally near to the CMH of class one. Once activated, the NKT are able to lyse the targets and to secrete cytokines.
the cells γδ T concealment S represent a population of cells T having a particular TCR. The majority of T have a TCR made up of two Glycoprotéines, the chains α and β. However, the cells γδ have a made TCR of a chain γ and a chain δ. These lymphocytes are less abundant than the αβ (they account for 5% of the total of the LT), but are found in greater quantity in the intestinal mucous membrane, among the population lymphocytaires named intraepithelial Lymphocytes. The antigenic determinant which these lymphocytes answer is unknown at the present time. Their TCR does not seem restricted with the recognition of a peptide, but would be able to react to the presence of a whole protein, without requiring the presentation via the molecules of CMH.

cellular immunity (the Pathogenic immunizing response with respect to organizations S inside the cells) implies the activation of the cells T.

CD₄ and CD₈ refer to the Antigène S characteristics on the surface of the various sub-types of lymphocytes T. These molecules CD are useful diagnostic markers to identify and quantify these cells by cytometry by means of Anticorps directed against them.

In the past, instead of CD₄ and CD₈, etc, one spoke about OKT ₄ and OKT₈,… and even of T₄ and T₈

Development of the cells T

All the lymphocytes T derive from hematopoietic original cells pluripotentes coming from osseous marrow. The progéniteurs hematopoietic colonize the thymus, where they undergo an intense multiplication, allowing the constitution of a great number of Thymocytes immature. Those express neither the CD4 nor the CD8, and are called for this reason of the negative “double” thymocytes. During their development, they will become “double positive” (CD4+CD8+) then will become naive lymphocytes mature simple positive (CD4+CD8- or CD4-CD8+), stage to which they leave the thymus to gain peripheral fabrics.

Roughly 98% of the thymocytes will die during their development, because they undergo two selections, the positive selection and the negative selection.

Positive selection

The positive double thymocytes migrate in the thymique cortex, where they are put in contact with auto- Antigène S peptide presented in the molecules of CMH of the epithelial cells of the thymique cortex. Only the thymocytes which are able to bind to a CMH/peptide complex with sufficient affinity receive a signal of survival. The others will die by Apoptose and their remains will be eliminated by Macrophage S. This phenomenon is called “positive selection” because the surviving cells are those which bound an interaction.
Selon the nature of the CMH which them TCR could bind, the positive double thymocytes lose one of the two markers. The cells whose TCR can bind molecules of the CMH of class I keep the CD8 and lose the CD4; those which bind a molecule of class II lose the CD8 and keep the CD4.

Negative selection

The cells having survived the positive selection will migrate in thymique marrow (medulla). Once in the medulla, the thymocytes are put again in the presence of peptides resulting from self, complexed with the molecules of the CMH carried by epithelial cells. This time, these are the cells whose TCR strongly interacts with the antigens which will die by apoptose secondary with a hyperactivation. As this time in fact the cells do not bind an interaction which survives, one speaks about negative selection. It is this phenomenon which allows the early elimination of lymphocytes car-reagents which are the cause of autoimmune Maladies.

At the time when the naive lymphocytes leave the thymus, they are unable to react to the presence of " leur" peptide.

Receiver TCR

TCR is the membrane receiver which makes it possible the lymphocytes T to locate its targets. It is resulting from a Recombinaison VDJ. In fact the TCR allows the specificity of the recognition of the complex peptide/CMH.

Activation of the cells T

Cellular interactions

The naive LT circulate in blood and the lymph and forward in the lymphatic ganglia.
Là, they meet Cellules presenters of antigens (CPA), among which the dendritic cells, the Macrophage S and the lymphocytes B. The lymphocytes will screen the complexes CMH/peptides present on these CPA, and will react only to the presence of " leur" complex CMH/peptide. The reaction will be influenced by the context cytokinic (i.e. different the Cytokine S present and the state from activation of the CPA.

Molecular interactions

The first molecular meeting is that of the molecules of adhesion. Once this established contact, the TCR will meet the CMH/peptides complexes. In the event of meeting épitope/Paratope, the connection of high affinity between the TCR and the CMH cause the Transduction of signals in the lymphocyte. It is what is called the " first signal". This first activation will involve the synthesis of molecules CD28 and CD40L with the membrane of the lymphocyte. These molecules will interact with membrane proteins of the CPA: CD80 and CD86 for CD28, CD40 for the CD40L. In the absence of these molecules on the CPA, the activation of the lymphocyte will be fallen through. On the other hand, if link CD80/86- CD28 is made, the lymphocyte will be activated. Connection CD40/CD40L allows as for it the final activaton CPA which will secrete cytokines which will direct the induced immunizing response.

Constitution of the memory

Extinction of the answer

See too

Leucocyte

Sources

Janeway, and Al , Immunobiology. 6th ED., Garland Science, 2005. ISBN 0815341016. NCBI makes the 5th edition availiable electronically At.
Michael H. Ross, and Al Histology: In Text and Atlas , 4th ED., Lippincott Williams & Wilkins, 2003, ISBN 0683302426
Marjan Vozelj, Temelji imunologije , 1th ED., DZS, 2005, Ljubljana, Slovenia, ISBN 8634128636

External bonds

PLOS To precede: Antigen-Specific T Concealments: Analyzes off the Needles in the Haystack
T Concealment Workshop, Research by the Immunology off Diabetes Society

Simple: T concealment

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