Integrin function in T-cell homing to lymphoid and nonlymphoid sites: getting there and staying there

The continuous recirculation of naive T cells and their subsequent migration to tissue following activation is crucial for maintaining protective immunity against invading pathogens. The preferential targeting of effector and memory T cells to tissue is instructed during priming and mediated by cell surface expressed adhesion receptors such as integrins. Integrins arc involved in nearly all aspects of T-cell life, including naive T-cell circulation, activation, and finally effector T-cell trafficking and localization. Recent research has revealed that microenvironmental factors present during T-cell priming result in the specific regulation of adhesion/integrin and chemokine receptor expression. Once antigen-experienced T cells enter tissue, further changes in integrin expression may occur that arc critical for T-cell localization, retention, effector function, and survival. This review discusses the function of integrin expression on T cells and the multiple roles integrins play on naive T cells and in directing effector T-cell trafficking to nonlymphoid sites in order to maintain protective adaptive immunity at body barriers.