Memory T cell

T memory cells or memory T - cells are a subgroup ( subpopulation ) of the T - lymphocytes. They have the function of an immunological memory and improve the protection of an individual when re- infection with the same pathogen ( reinfection ). As a former T- helper cells (TH1 or TH2), the memory cells are stored once learned specific immune response. If the organism is confronted with the same antigen, the memory T cells triggers a rapid and effective immune response. Some of them are changing again in T- helper cells to.

History

The existence of memory cells could be adopted by epidemiological observations of a measles epidemic in the 19th century in the Faroe Islands. At that time it was found after a single virus exposure and apparent lack of re-exposure in the intervening time, even after 30 years and more protection against new disease in the original case of measles residents.

Properties

The adaptive immune system after stimulation by an antigen, there is a 10 - to 100-fold increase of the specific T cells. Of these, some can provide a faster and more efficient secondary response upon re-exposure to the same antigen. This memory function can be provided by CD4 - positive and CD8 - positive T- cell memory.

The from naive T cells caused by differentiation of effector cells usually have a short lifetime and usually die after an immune response by an activation -induced cell death (AICD, Eng. Activation-induced cell death ), a process of programmed cell death. This program different mechanisms are involved, including the provision of Fas -mediated signals ( CD95), intracellular pro-apoptotic molecules and loss of growth factors.

Of this programmed cell death of the specific immune response is formed during memory T cells less than 5% of the effector cells are excluded. The cellular and molecular factors that determine the fact that differentiate as a result of activation of naive T cells into short-lived effector cells and the one to the other in long-lived memory cells, are known only in its infancy. The following models for the differentiation of memory T cells are discussed:

• One model assumes the existence of two different naive T - cell subsets, one of which has the potential to mature into effector T cells, while the other can only develop into memory T cells. Convincing experimental data that could support this model, lacking at present.
• In another model, different signals determine in the activation of naive T cells whether an effector or memory T cell is formed. Costimulator -mediated signals convey whether a short-lived effector cell to be created as a result of strong activation or long-lived memory cell for weak signals. This model is supported by animal experimental observations.
• In the third model survived a subset of activated effector memory T cells as because they will either receive signals that hold this differentiation, or because they escape the signals, otherwise initiate programmed cell death. It remains open whether these additional stimuli at random ( stochastic) act on the effector T cells or act by selectively providing only certain cells. Animal experimental results are consistent with this model.

Types

As naive T - cells to different CD4 or CD8 positive cells in the memory T cells. In addition, distinctions at least two functionally different groups within these two types. These are for an effector - memory T cells ( engl. effector memory T cells, TEM) and other central memory T cells ( engl. central memory T cells, TCM). Both groups may be present in both CD4 and CD8 positive in memory T cells. The effector memory T cells are reactivated when a new antigen contact and provide by several divisions many functional effector cells ready. The central memory T cells, however, rest in the secondary lymphatic organs and will not initially have effector function at the site of infection. However, they can easily be activated and in order to develop into effector cells, as soon as the antigen persists. The division of labor between effector memory T cells and central memory T cells has similar features as that between plasma cells and memory B cells.

• Effector memory T - cells of humans are phenotypically characterized by the fact that they no CCR7 and do not express CD62L as a rule, two markers that are typically expressed on naive T cells. Effector memory T cells can be activated very quickly through their antigen and secrete IFN- γ within hours, IL -4, and IL- fifth

• Central memory T cells possess not only the memory T cell marker CD45R0 (naive T cells express CD45RA ) and CCR7 and CD62L. These cells are readily activated as a naive T cell costimulation and require less in order to be activated. They regulate the expression of CD40L on their cell surface high, enabling them to be more easily activated by the dendritic cells or B cells. After activation of the T cell receptor, these cells produce mainly interleukin -2, after an initial growth phase they differentiate into effector cells, which produce IFN- γ also ( in TH1) or IL -4 ( in TH2).