The role of gamma delta T cells in the normal and disordered immune system.

A small population of T cells does not express the conventional T cell receptor characterized by the alpha and beta polypeptide chains (TCR alpha beta) but instead, two polypeptides termed gamma and delta (TCR gamma delta). This alternative receptor is able to recognize antigen. It appears early in T cell ontogeny, but its role in the thymus prior to the availability of TCR alpha beta remains unclear. In selected sites such as skin or gut TCR gamma delta predominates in mice which might suggest a role of gamma delta T cells in the first line of defense against infection. gamma delta T cells secrete lymphokines and display cytotoxic activity. However, their activation requirements may differ from what is known for alpha beta T cells since MHC-nonrestricted and also CD4 and CD8 negative gamma delta T cells have been described. Preferential activation by mycobacterial antigens possibly indicates a special repertoire of the gamma delta T cells. In various diseases slightly increased numbers of gamma delta T cells were found, but these preliminary studies have not yet provided evidence for a major pathogenetic role of gamma delta T cells.