[T cell development and selection in extrathymic environments]

Thymocytes express randomly diversified TCRs by TCR gene rearrangement and those reactive to self antigens are negatively selected while those capable of recognizing antigens in the context of self MHC antigens are positively selected. The selection process in the thymus for forming the TCR repertoire is a basic rule for the formation of T cell repertoire. Recently, however, certain T cells have been reported to be generated extra-thymically and exhibit the restricted TCR repertoire. Some of these T cells are suggested to recognize self antigens in a non-MHC-restricted fashion. Those self antigens, such as endogenous retrovirus, surface activation antigens, and bacterial antigens, are induced by exogenous stress, transformation and invading microorganisms. T cells which recognize such self antigens seem to be activated and expand clonally whenever the antigens are expressed in the periphery. Although the biological roles of the unique T cells remain obscure, it is suggested that they may participate in immune surveillance system to monitor cell integrity and to destroy cells altered by transformation and infection by microorganisms.