Virgin alpha beta and gamma delta T cells recirculate extensively through peripheral tissues and skin during normal development of the fetal immune system.

Current models of T cell migration place severe restrictions on the recirculation of virgin T cells, condemning them to migrate exclusively via high endothelial venules in lymph nodes until they either die or acquire the capacity to migrate to skin and peripheral tissues as memory cells following stimulation with antigen. We have demonstrated in the sheep fetus (which is immunologically virgin until after birth) that virgin T cells and dendritic cells circulate through skin and peripheral tissues during fetal life in the same non-random manner as adult T cells but in much larger numbers than they do in adult animals. Our data also showed that T cells do not discriminate between peripheral tissues and skin or lymph nodes on the basis of virgin or memory CD45R phenotype, or CD2, CD58 or CD44 phenotype, and with the possible exception of CD11a/CD18, that it is not mandatory for lymphocytes to be activated to adhesion moleculehi status in order to home to fetal skin. Our results indicate that unique tissue-homing specificities for extra-lymphoid tissues can be imprinted on virgin T cells independent of foreign antigen. Virgin T cells have previously been thought to be denied access to peripheral tissues; however, the large-scale traffic of virgin T cells through extra-lymphoid tissues in the fetus reported here provides a mechanism whereby direct virgin T cell interactions with self-antigens expressed only on tissues outside the thymus can occur repeatedly during development of the fetal immune system.