CD4+ T cells
CD4+T cells recognise peptides presented on MHC class II molecules, which are found on antigen presenting cells (APCs). As a whole, they play a major role in instigating and shaping adaptive immune responses. This is in contrast to the cytotoxic CD8+ T cells.
Th1/Th2 cells Th1-polarised cells are responsible for control of intracellular pathogens such as viruses and some bacteria. IL-12 and IFN-γ are important cytokines involved in Th1 responses, and the intracellular transcription factors T-bet and STAT-4 are essential for Th1 cell differentiation and function. Th2 polarised cells are important in the defence against large extracellular organisms such as helminths, utilising cytokines such as IL-4, IL-5 and IL-13, promoting eosinophilia, mastocytosis and goblet cell hyperplasia. Gata-3 and STAT-6 are essential for Th2 cell differentiation and function.
Allergy/Autoimmunity
If the Th1/Th2 balance is disturbed there can be severe consequences. Asthma and allergy are Th2-driven and some autoimmune diseases, such as type 1 diabetes and multiple sclerosis are Th1-driven.
Th17 cells
This is a recently discovered T helper cell subset, characterised by its production of IL-17. IL-23 promotes the expansion of these cells and Th17 cells have been linked to several inflammatory conditions such as arthritis and IBD.
CD4 T cells play a key role in the functioning of a healthy immune system. They assist B cells to make antibodies, activate the microbe killing capacity of macrophages and recruit other immune cells to infected or inflamed areas of the body. These activities are orchestrated through their production of various cytokines and chemokines. It has been known for some time that uncommitted CD4+ T-cells can differentiate into Th1 or Th2 cells, based on the prevailing pro-inflammatory/anti-inflammatory environment, and that these activated Th1 and Th2 cells had distinct cytokine production patterns and functions. Generally, Th1 cells were associated with the eradication of intracellular pathogens whereas Th2 cells were heavily involved in responses against extracellular pathogens and parasites. Uncontrolled Th1 responses were implicated in autoimmunity and aberrant Th2 responses were associated with allergy and asthma development. However, this model did not explain the observation that a deficiency in Th1 signalling and/or cytokines still allowed the development of autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. More recently (2006) a third subset of CD4 T cells, Th17 cells, which have a pro-inflammatory bias was identified. Subsequent research using animal models and human studies has demonstrated a key role for Th17 cells in the immune system’s defence against extracellular bacteria and fungi as well as the development of autoimmune diseases, mediated by the secretion of IL-17 by these cells. The secretion of IL-23 from antigen-presenting cells such as dendritic cells, which have been activated by the uptake and processing of pathogens, in turn activates Th17 cells.
Treg cells
Regulatory T cells are a subpopulation of cells that maintain homeostasis and tolerance within the immune system. Subsets include inducible Tregs, CD25+CD45RBlo Tregs etc.