Travellers in many guises: the origins and destinations of dendritic cells.

The migratory behaviour of dendritic cells (DC) is tightly linked to their differentiation state. Precursor DC constitutively repopulate normal tissues from the bloodstream, and are recruited in elevated numbers to sites of inflammation. Whilst maturing in response to antigenic stimulation, DC acquire the capability to enter lymph nodes via afferent lymphatic vessels, thus facilitating their presentation of antigen to naïve T cells. Peripheral blood monocytes constitute a second DC precursor population, which during an inflammatory response are recruited to the affected site where some differentiate into functional DC. The availability of separate DC precursor populations is thought to be significant for the character, amplification and perpetuation of the resultant immune response. In addition, the balance between steady-state trafficking of incompletely activated DC bearing self-antigens from the periphery, and the migration of fully mature DC from inflammatory sites into lymph nodes might have profound effects upon tolerance induction and activation of T cells, respectively.