Dendritic Cell Lectin Receptors (Dectin-2 Receptors Family)

Abstract

Dendritic cells (DCs) are special subsets of antigen presenting cells characterized by their potent capacity to activate immunologically naive T cells to induce initial immune responses. DCs are far more potent than macrophages and B cells in their capacity to activate immunologically naive T cells. Infact, DCs are responsible for initiating T cell-mediated immune responses to a vari-ety of antigens. Members of the DC family are distributed to virtually all the organs (except the brain), where they serve as tissue resident APCs, playing critical roles in presenting environmental, microbial, and tumor-associated antigens to the immune system. DCs operate at the inter-face of innate and acquired immunity by recognizing pathogens and presenting pathogen-derived peptides to T lymphocytes. As a component of the innate immune sys-tem, DCs organize and transfer information from the out-side world to the cells of the adaptive immune system. DCs can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biol-ogy and pathophysiology that shows the complexity of this cell system. Presentation of an antigen by immature (non-activated) DC leads to tolerance, whereas mature, antigen-loaded DCs are geared towards the launching of antigen-specific immunity. DCs Form a Major Resident Leukocyte Population in Human Skin Two main types of DCs are found in noninflamed skin: epidermal Langerhans cells (LCs) and dermal DCs. LCs express Langerin/CD207 that localizes to and forms Birbeck granules, as well as the CD1a class I–like molecule that presents glycolipids. Plasmacytoid dendritic cells (PDCs), which are also known as plasmacytoid T cells, plasmacytoid monocytes, natural IFN-a/ß–producing cells (natural IPCs), and type 2 predendritic cells (pDC2), consti-tute a subset of immature DCs, which is capable of differentiating in vitro into mature DCs with typical den-dritic cell morphology and potent T cell stimulatory function when exposed to IL-3 alone, IL-3 and CD40L, viruses, and bacterial DNA containing unmethylated CpG motifs (CpG-DNA) (Dzionek et al. 2001; Ebner et al. 2004). Dzionek et al. (2000) identified two novel markers of PDCs, blood dendritic cell antigen-2 (BDCA-2) and BDCA-4, which enable direct identification of PDCs in human blood. BDCA-2 is presum-ably involved in ligand internalization, processing and pre-sentation, as well as in inhibition of IFN-a/ß synthesis in PDCs.