Figure 1.
The migration of effector and memory T cells to sites of localized infection. DC acquisition of antigen occurs at peripheral sites such as the lung, skin or gut after a breach in the epithelial layer or local infection. After capturing antigen, DCs access initial afferent lymphatics mediated by signals from integrins, chemokines and semaphorins to migrate to draining lymph nodes to activate naive or memory cells. For example, breach of the intestinal mucosa leads to T cell priming in the MLNs and/or Peyer’s patches. The spleen is poised to generate a substantial T cell response if pathogens are not compartmentally contained and gain access to the bloodstream. In either scenario, a robust T cell population expansion program follows with DC-mediated instruction for migration to the site of initial infection. Activated T cells and circulating memory cells exit the lymph node via the efferent lymphatics and return to circulation through the thoracic duct (colors indicate T cells instructed to home to specific tissues). As T cells migrate through the circulation, integrin and chemokine signals direct their emigration into tissues. In this manner, imprinted T cells have a specific key that allows access to restricted tissues (gates) under normal homeostatic conditions. IEL, intraepithelial lymphocyte; IEC, intestinal epithelial cell; VLA-4, integrin α4β1.