Memory T cell

For other uses, see Memory cell (disambiguation).

A lymphocyte is shown in the center of this picture

1. After the naive T cell (N) encounters an antigen it becomes activated and begins to proliferate (divide) into many clones or daughter cells.
2. Some of the T cell clones will differentiate into effector T cells (E) that will perform the function of that cell (e.g. produce cytokines in the case of helper T cells or invoke cell killing in the case of cytotoxic T cells).
3. Some of the cells will form memory T cells (M) that will survive in an inactive state in the host for a long period of time until they re-encounter the same antigen and reactivate.

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 subpopulations have been described and can be recognised by the differential expression of chemokine receptor CCR7 and L-selectin (CD62L).

• Stem memory T_SCM cells, like naive cells, are CD45RO−, CCR7+, CD45RA+, CD62L+ (L-selectin), CD27+, CD28+ and IL-7Rα+, but they also express large amounts of CD95, IL-2Rβ, CXCR3, and LFA-1, and show numerous functional attributes distinctive of memory cells.

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

• Effector memory T_EM 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 T_CM and T_EM 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 (T_CM). The T_CM cells are thought to contain some properties associated with memory cells stem cells. T_CM display a capacity for self-renewal due to high levels of phosphorylation of an important transcription factor known as STAT5. In mice, T_CM cells have been shown to confer superior protection against viruses, bacteria, and cancer in several different model systems compared with T_EM 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 (T_EM)
  • effector memory RA (T_EMRA)

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

See also

• Naive T cell
• V(D)J recombination of DNA
• Memory B cell

External links

• T-cell Group - Cardiff University