Regulatory T cells (also known as suppressor T cells) belong to CD4+ subpopulations and express all or some of the following markers: high levels of CD25 (interleukin-2 IL-2 alpha chain), transcription factor FoxP3 (forkhead box p3), CTLA-4 (cytotoxic T-lymphocyte associated molecule-4) and GITR (glucocorticoid-induced TNF receptor). They mature in the thymus and comprise ~5-10% of the peripheral blood CD4+ helper T cell subpopulation in mice and about 1-2% of human CD4+ T cells. Absence of T regs or their dysfunction are associated with severe autoimmunity and tumors. Evidence is now accumulating that T regulatory activity is amplified in a variety of infectious contexts, mediating suppression during retroviral and bacterial infections. Moreover, CD4+CD25+ T cells curtail immunity to Leishmania and malaria parasites in mouse models. There is also some suggestion that the pathogens themselves can manipulate the regulatory cells to immunosuppress the host and so potentiate their own survival. T regs exhibit suppressor-regulatory activity through cell-to-cell contact with other T cells that are stimulated through their T cell receptor. The mechanism by which T regs display their suppressive function is currently uknown. In addition to FoxP3+ T regs, there are other suppressor T cells called Tr1 and Th3 cells which suppress T cell activation through secretion of cytokines IL-10 and TGF-beta.
