T-cells

The effector T cell describes a group of cells that comprises several T cell types that actively respond to a stimulus, such as co-stimulation. It includes CD4+, CD8+, Treg cells.

Gamma delta T cells (γδ T cells) represent a very small subset of T cells that have a distinct T cell receptor (TCR) on their surfaces. The majority of T cells have a TCR consisting of two glycoprotein chains called α- and β- TCR chains. In γδ T cells, however, the TCR consists of a γ chain and a γ chain. This group of T cells is much less common in humans and mice (about 2% of total T cells) and is mainly found in the intestinal mucosa, within a population of lymphocytes known as intraepithelial lymphocytes. In rabbits, sheep and chickens, the number of γδ T cells can be up to 60% of the total T cells. The antigenic molecules that activate γδ T cells are still largely unknown. However, γδ T cells are not MHC-restricted and appear to be able to recognize whole proteins rather than requiring peptides to be presented by MHC molecules on APCs.  

Natural killer T cells (NKT cells - do not mix with the natural killer cells (CD56+3-) of the innate immune system) connect the adaptive immune system with the innate immune system. Unlike conventional T cells, which recognize peptide antigens presented by major histocompatibility complex molecules (MHCI and II), NKT cells recognize antigens presented by a surface receptor called CD1d. Activated cells can perform functions attributed to both helper T cells and cytotoxic T cells. They are also capable of recognizing and eliminating some tumor cells and cells infected with herpes viruses.

Regulatory T (Treg) cells (suppressor T cells) are essential for maintaining immune tolerance. Their main task is to stop the T cell-mediated immune response at the end of an immune reaction. A second main task is the suppression of autoreactive T cells that have escaped the process of negative selection in the thymus.

Two main classes of CD4+ Treg cells have been described - FoxP3+ Treg cells and FoxP3- Treg cells. FoxP3 (forkhead box protein P3) is an essential DNA-binding transcription factor for regulatory T cells.)

FoxP3+ Treg cells and FoxP3- Treg cells.

Regulatory T cells can either develop in the thymus during normal development and are then referred to as thymic Treg cells, or they can be induced peripherally and are referred to as peripherally derived Treg cells. Both subsets require the expression of the transcription factor FoxP3, which can be used to identify the cells. Mutations of the FoxP3 gene can prevent the development of regulatory T cells and cause the fatal autoimmune disease IPEX.

Many other types of T cells have suppressive activity but do not express FoxP3. These include Tr1 cells and Th3 cells, which are thought to arise during an immune response and act by producing suppressive molecules. Tr1 cells are associated with IL-10, and Th3 cells are associated with TGF-beta. Recently, Treg17 cells have been added to this list.

Memory T cells are a subgroup of antigen-specific T cells that persist in the long term after an infection has subsided. They rapidly expand into a large number of effector T cells after they come into contact with their specific antigen again, thus providing the immune system with a "memory" against previous infections.

Memory T cells can be CD4+ or CD8+. Memory T cells typically express the cell surface protein CD45RO.

T helper cells (Th cells) help other leukocytes in immunological processes, including the maturation of B cells into plasma cells and memory B cells. They also activate cytotoxic T cells and macrophages. The cells are also known as CD4+ T cells because they express the CD4 glycoprotein on their surfaces. T helper cells are activated when their T cell receptor specifically recognizes a peptide antigen presented by MHC class II molecules expressed on the surface of antigen-presenting cells (APCs). Once activated, they rapidly divide and secrete small proteins (cytokines) that regulate or support the active immune response. These cells can differentiate into one of several subtypes, including TH1, TH2, TH3, TH17, TH9 or TFH, which secrete different cytokine patterns to facilitate different types of immune responses. APC signaling directs T cells into specific subtypes.

Cytotoxic T cells (Tc cells, CTLs, T killer cells, killer T cells) destroy virus-infected cells and tumor cells and are also involved in transplant rejection. These cells are also known as CD8+ T cells because they express the CD8 glycoprotein on their surface. Cytotoxic T cells use their specific T cell receptor to recognize their targets by binding to antigens associated with MHC class I molecules present on the surface of all nucleated cells. Cytokines (e.g. IL-10) and other molecules secreted by regulatory T cells can induce the CD8+ cells into an anergic state that prevents autoimmune diseases.