Virtual memory T cells (T
VM) are a subtype of
T lymphocytes
T cells (also known as T lymphocytes) are an important part of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes
A lymphocyte is a type of white blood cell (leukoc ...
. These are cells that have a memory
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
but have not been exposed to a foreign
antigen
In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response.
...
. They are classified as memory cells but do not have an obvious memory function. They were first observed and described in 2009.
The name comes from a computerized "
virtual memory
In computing, virtual memory, or virtual storage, is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a ver ...
" that describes a working memory based on an alternative use of an existing space.
Origin
T
VM arise from autoreactive T cells during the
thymus
The thymus (: thymuses or thymi) is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. The thymus ...
section. Autoreactive lymphocytes are commonly removed in the thymus because they recognize the body's own structure and could induce
autoimmune disease
An autoimmune disease is a condition that results from an anomalous response of the adaptive immune system, wherein it mistakenly targets and attacks healthy, functioning parts of the body as if they were foreign organisms. It is estimated tha ...
. However, some autoreactive
CD8
CD8 (cluster of differentiation 8) is a transmembrane protein, transmembrane glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). Along with the TCR, the CD8 co-receptor plays a role in T cell Cell signaling, signaling and aid ...
+ T cells are intended for development into T
VM. This process is controlled by CD8-
Lck. The generation and maintenance of the TVM population depends on the transcription factors
Eomes
Eomesodermin also known as T-box brain protein 2 (Tbr2) is a protein that in humans is encoded by the ''EOMES'' gene.
The Eomesodermin/Tbr2 gene, ''EOMES'', encodes a member of a Conserved sequence, conserved protein family that shares a commo ...
and
IRF4
Interferon regulatory factor 4 (IRF4) also known as MUM1 is a protein that in humans is encoded by the ''IRF4'' gene. IRF4 functions as a key regulatory transcription factor in the development of human immune cells.Nam S, Lim J-S (2016). "Essentia ...
, type I interfering signaling. The presence of the
IL-15 cytokine is then essential.
Function
The existence of memory T cells is also known in non-immunized animals. T
VM are cells specific and reactive to foreign antigens that have never met. There are different phenotypic mismatches between naive, true memory and T
VM. We can find functional differences after activation. It is easy to distinguish naive cells from memory, but true memory from T
VM can only be distinguished by
CD49d
CD49d is an integrin
Integrins are transmembrane receptors that help cell–cell and cell–extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulat ...
and
CD122
Interleukin-2 receptor subunit beta is a protein that in humans is encoded by the ''IL2RB'' gene. Also known as CD122; IL15RB; P70-75.
Function
The interleukin 2 receptor, which is involved in T cell-mediated immune responses, is present in 3 ...
markers.
T
VM produce a stronger inflammatory response using
IL-12 and
IL-18 cytokines than naive T cells.
They are a significant producer of
IFN-γ
Interferon gamma (IFNG or IFN-γ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. ...
.
Compared to other naive phenotypes, T
VM represents only 10—30% of the population, but outperforms other types of subpopulation with its stronger proliferation. However, the reaction is slower than true memory cells. These properties suggest that virtual memory T cells may participate in both innate and adaptive immune responses during the immune response.
Another indispensable feature is the suppression of potential states. This occurs already in the development of virtual memory cells from autoreactive T cell clones. For this reason, some scientists thought that TVM could be used in the fight against autoimmunity, but so far no evidence has been found.
The physiological role of virtual memory T lymphocytes has yet to be investigated, but research suggests that they have a unique type of response to pathogens and contribute to the functional diversity of the T cell immune system, which is required for effective immune defense.
CD4 positive TVM
Previous paragraphs dealt with only CD8+ T lymphocytes, but
CD4
In molecular biology, CD4 (cluster of differentiation 4) is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). CD4 is found on the surface of immune cells such as helper T cells, monocytes, macrophages, and dendritic c ...
+ virtual memory T cells are also described.
The function of these cells is not known, but an even more likely relationship to autoimmune conditions is assumed, whether in their suppression or formation.
References
{{Lymphocytes
T cells