Role of Regulatory T Lymphocytes in Health and Disease

T cells are conventionally categorized into two basic types, viz., CD4+ helper and CD8+ cytotoxic T cells. CD4+ T cells were known to “help” in the activation and differentiation of various immune cells such as NK cells, macrophages, and dendritic cells, whereas CD8+ T cells were known to kill foreign antigens. In 1970s, it was reported that functions exhibited by T cells were not merely restricted to augmenting an immune response but also to dampen it [1]. These T supressor cells were famously named as regulatory T cells or Tregs. Suppression caused by Tregs on various T cells was believed to mediate immunological tolerance by discriminating between self- and non-self-antigen [2, 3]. Tregs are believed to play an important role in maintaining homeostasis of the immune system by restricting the enormity of effector responses and permitting the initiation of immunological tolerance [4–6]. Treg populations are majorly divided into two major types: nTregs (natural Tregs) originating from the thymus and iTregs (induced Tregs) arising extrathymically, i.e., from secondary lymphoid organs or inflamed tissues [7]. Tregs are further differentiated into five subtypes based upon their origin, phenotypes, and expression of markers.