TGF beta 1

Some T cells (e.g. regulatory T cells) release TGF-β1 to inhibit the actions of other T cells. Specifically, TGF-β1 prevents the interleukin(IL)-1- & interleukin-2-dependent proliferation in activated T cells,[12][13] as well as the activation of quiescent helper T cells and cytotoxic T cells.[14][15] Similarly, TGF-β1 can inhibit the secretion and activity of many other cytokines including interferon-γ, tumor necrosis factor-alpha (TNF-α), and various interleukins. It can also decrease the expression levels of cytokine receptors, such as the IL-2 receptor to down-regulate the activity of immune cells. However, TGF-β1 can also increase the expression of certain cytokines in T cells and promote their proliferation,[16] particularly if the cells are immature.[11]

TGF-β1 has similar effects on B cells that also vary according to the differentiation state of the cell. It inhibits proliferation, stimulates apoptosis of B cells,[17] and controls the expression of antibody, transferrin and MHC class II proteins on immature and mature B cells.[11][17]

The effects of TGF-β1 on macrophages and monocytes are predominantly suppressive; this cytokine can inhibit the proliferation of these cells and prevent their production of reactive oxygen (e.g. superoxide (O₂⁻)) and nitrogen (e.g. nitric oxide (NO)) intermediates. However, as with other cell types, TGF-β1 can also have the opposite effect on cells of myeloid origin. For example, TGF-β1 acts as a chemoattractant, directing an immune response to certain pathogens. Likewise, macrophages and monocytes respond to low levels of TGF-β1 in a chemotactic manner. Furthermore, the expression of monocytic cytokines (such as interleukin(IL)-1α, IL-1β, and TNF-α),[15] and macrophage's phagocytic can be increased by the action of TGF-β1.[11]

TGF-β1 reduces the efficacy of the MHC II in astrocytes and dendritic cells, which in turn decreases the activation of appropriate helper T cell populations.[18][19]