Dendritic cells: functional aspects of glycosylation and lectins.

Dendritic cells (DC) direct immune responses either toward tolerance to a presented antigen or toward inflammatory reactions of effector cells. Many crucial cytokines and cell surface proteins have been identified in this process using gene expression profiling. However, it is becoming evident that important steps involve carbohydrate-protein interactions, which cannot be anticipated by gene expression profiling in most cases. These contacts are crucial for the uptake of certain antigens, migration, and homing, but also for infection by viruses. On one hand, DC use numerous C-type lectins, such as DC-SIGN, dectin-1, langerin, and DEC-205, for antigen uptake. Other lectins, such as CD83, siglecs, and galectins, may be involved in regulation of the immune response to a given antigen. On the other hand, cell surface glycosylation of DC themselves changes significantly depending on the environment and the functional state, generating different signals by altered glycans. Because DC occur at the interface between innate and acquired immunity, it may not be surprising that glycans and lectins play an important role in many biological functions of DC. In this review, we focus on glycobiological aspects of antigen uptake and processing, immune modulation, and viral infections in the context of DC biology.