Dendritic cell biology and regulation of dendritic cell trafficking by chemokines

DC (dendritic cells) represent an heterogeneous family of cells which function as sentinels of the immune system. They traffic from the blood to the tissues where, while immature, they capture antigens. Then, following inflammatory stimuli, they leave the tissues and move to the draining lymphoid organs where, converted into mature DC, they prime naive T cells. The key role of DC migration in their sentinel function led to the investigation of the chemokine responsiveness of DC populations during their development and maturation. These studies have shown that immature DC respond to many CC and CXC chemokines (MIP-1 alpha, MIP-1 beta, MIP-3 alpha, MIP-5, MCP-3, MCP-4, RANTES, TECK and SDF-1) which are inducible upon inflammatory stimuli. Importantly, each immature DC population displays a unique spectrum of chemokine responsiveness. For examples, Langerhans cells migrate selectively to MIP-3 alpha (via CCR6), blood CD11c+ DC to MCP chemokines (via CCR2), monocytes derived-DC respond to MIP-1 alpha/beta (via CCR1 and CCR5), while blood CD11c- DC precursors do not respond to any of these chemokines. All these chemokines are inducible upon inflammatory stimuli, in particular MIP-3 alpha, which is only detected within inflamed epithelium, a site of antigen entry known to be infiltrated by immature DC. In contrast to immature DC, mature DC lose their responsiveness to most of these inflammatory chemokines through receptor down-regulation or desensitization, but acquire responsiveness to ELC/MIP-3 beta and SLC/6Ckine as a consequence of CCR7 up-regulation. ELC/MIP-3 beta and SLC/6Ckine are specifically expressed in the T-cell-rich areas where mature DC home to become interdigitating DC. Altogether, these observations suggest that the inflammatory chemokines secreted at the site of pathogen invasion will determine the DC subset recruited and will influence the class of the immune response initiated. In contrast, MIP-3 beta/6Ckine have a determinant role in the accumulation of antigenloaded mature DC in T cell-rich areas of the draining lymph node, as illustrated by recent observations in mice deficient for CCR7 or SLC/6Ckine. A better understanding of the regulation of DC trafficking might offer new opportunities of therapeutic interventions to suppress, stimulate or deviate the immune response.