Identification and characterization of human dendritic cell subsets in the steady state: a review of our current knowledge

Abstract

Dendritic cells (DC) are generally categorized as a group of rare antigen presenting cells that are to the crucial development of immune responses to pathogens and also of tolerance to self-antigens. Therefore, having the ability to identify DC in specific tissues and to test their functional abilities in the steady state are scientific gaps needing attention. Research on primary human DC is lacking due to their rarity and the difficulty of obtaining tissue samples. However, recent findings have shown that several different DC subsets exist, and that these subsets vary both by markers expressed and functions depending on their specific microenvironment. After discriminating from other cell types, DC can be split into myeloid and plasmacytoid fractions. While plasmacytoid DC express definite markers, CD123 and BDCA-2, myeloid DC encompass several different subsets with overlapping markers expressed. Such markers include the blood DC antigens BDCA-1 and BDCA-3, along with Langerin, CD1a and CD14. Marker specificity is further reduced when accounting for microenvironmental differences, as observed in the blood, primary lymphoid tissues, skin and lungs. The mixed leukocyte reaction (MLR) has been used to measure the strength of antigen presentation by specific DC subsets. Surface markers and MLR require standardization to enable consistent identification of and comparisons between DC subsets. To alleviate these issues, researchers have begun comparing DC subsets at the transcriptional level. This has allowed degrees of relatedness to be determined between DC in different microenvironments, and should be a continued area of focus in years to come.