DENDRITIC CELL SUBPOPULATIONS
Dendritic cells (DCs) have an essential function in the immune system by participating in primitive defense responses that constitute the innate immunity, as well as in the induction and regulation of antigen-specific immune responses. This allows DCs to control infections caused by parasitic and microbial pathogens, to block tumour growth and to exert a precise regulation of T cell, B cell and NK cell immune responses. In addition, DCs also fulfill a pivotal role in the induction and maintenance of T cell tolerance. The functional diversity characterizing the DC system relies essentially on the remarkable plasticity of the DC differentiation process, which dictates the acquisition of DC functional specialization through the generation of a large collection of DC subpopulations (reviewed by Shortman and Liu, 2002). Dendritic cells are located both in the lymphoid organs (such as the spleen or the lymph nodes), and in non-lymphoid tissues (such as the skin or the liver), and can be classified in two major categories: conventional DCs (cDCs), and plasmacytoid DCs (pDCs). Whereas in turn cDCs comprise multiple DC subpopulations endowed with specific functions, little is known about the functional heterogeneity of pDCs. A summary of the most relevant phenotypic and functional characteristics of the main DC subpopulations present in mice is shown in Table 1.1.
A first group of cDCs includes those that are common, and largely restricted, to the majority of organized lymphoid organs of the immune system, and perform their specific functions, as immature or mature DCs, within these organs.