Memory T cell

Memory T cells are a subset of infection- as well as potentially cancer-fighting T cells (also known as a T lymphocyte) that have previously encountered and responded to their cognate antigen; thus, the term antigen-experienced T cell is often applied. Such T cells can recognize foreign invaders, such as bacteria or viruses, as well as cancer cells. Memory T cells have become "experienced" by having encountered antigen during a prior infection, encounter with cancer, or previous vaccination. At a second encounter with the invader, memory T cells can reproduce to mount a faster and stronger immune response than the first time the immune system responded to the invader. This behaviour is utilized in T lymphocyte proliferation assays, which can reveal exposure to specific antigens.

Sub-populations
Within the overall memory T cell population, at least three distinct sub-populations have been described and can be recognised by the differential expression of chemokine receptor CCR7 and L-selectin (CD62L).


 * Central memory TCM cells express L-selectin and the CCR7, they secrete IL-2, but not IFNγ or IL-4.


 * Effector memory TEM cells, however, do not express L-selectin or CCR7 but produce effector cytokines like IFNγ and IL-4.

More recently, other sub-populations have been explored using co-stimulatory molecules CD27 and CD28 expression in addition to CCR7 and CD62L.

Function
Antigen-specific memory T cells against viruses or other microbial molecules can be found in both TCM and TEM subsets. Although most information is currently based on observations in the cytotoxic T cells (CD8-positive) subset, similar populations appear to exist for both the helper T cells (CD4-positive) and the cytotoxic T cells.


 * central memory (TCM). The TCM cells are thought to contain some properties associated with memory cells stem cells. TCM display a capacity for self-renewal due to high levels of phosphorylation of an important transcription factor known as STAT5. In mice, TCM cells have been shown to confer superior protection against viruses, bacteria, and cancer in several different model systems compared with TEM cells.
 * two highly related effector memory sub-types, which strongly express genes for molecules essential to the cytotoxic function of CD8 T cells:
 * effector memory (TEM)
 * effector memory RA (TEMRA)


 * More recently, antigen-experienced CD8+ T cells with apparent self-renewal capabilities have been described in mice. This population, now termed stem cell memory (TSCM), can be identified by the markers CD44(low)CD62L(high)CD122(high)sca-1(+) and are capable of generating TCM and TEM subsets while maintaing themselves. In preclinical studies, adoptively transferred TSCM confer superior immunity compared with other T memory subsets. Whether such a population is found in humans is the subject of active investigation.

A recent study in Science found that T-cells in the spleen can synthesized acetylcholine in response to vagus nerve stimulation. (Science Sep 15, 2011. DOI : 10.1126/science.1209985) It is thought that this mechanism may play a role in down-regulating inflamation via TNF-alpha via macrophages.