Dendritic cells (DC) and monocytes, along with macrophages, constitute the Mononuclear Phagocyte System (MPS), which scans for and provides the first line of defence against invading pathogens. Some cells of the MPS internalise the invading pathogens. These cells are called Professional Antigen Presenting Cells (APC). Following the internalisation, these cells also play a key role in MHC class II-dependent B and CD4 T-cell responses and MHC class I-dependent CD8 T-cell toxicity. DCs are not only the most potent APCs, but can also induce immune tolerance under certain circumstances. Depletion of murine DCs leads to spontaneous development of autoimmune diseases, highlighting the role played by DCs in immune homeostasis. The immunostimulatory and immunoregulatory functions of DCs have led them to be studied extensively and made them attractive targets for cancer immunotherapy and autoimmunity interventions.
In humans, two main subsets of DCs occur within circulating HLA-DR+ cells and can be distinguished by their expression of CD11c and CD123. These are plasmacytoid DCs or pDCs (CD11c–CD123+) and conventional DCs or cDCs (CD11c+CD123-/dim). Other pDC specific markers that have been identified include CD303 and CD304. Conventional DCs can be divided into two main subsets within
CD14−HLA-DR+CD11c+ cells based on the expression of CD141 and CD1c: cDC1s and cDC2s, respectively. A more recent discovery of inflammatory DCs, called DC3, has been described as a distinct lineage within the DC family.
The classical definition of monocytes is based on three subsets: classical CD14+CD16− (cMo), intermediate CD14+CD16+ (iMo), and nonclassical CD14−CD16+ (ncMo) monocytes.
Reassessing the definitions of macrophages and DCs
In this article, Mair et.al have re-assessed the conventional definitions of DC and monocyte subsets provided by earlier studies. This has become necessary according to the authors, as the emergence of high-parameter multicolour flow cytometry and single-cell RNA sequencing have revealed a great complexity and heterogeneity of the human mononuclear phagocyte system than was known previously. Therefore, there was a need to re-visit and update the definition of human DCs and monocytes based on functional and transcriptional signatures.
Towards this end, the authors propose an updated gating strategy to delineate human blood DCs and monocyte subsets. They propose, based on a new panel and gating strategy, that CD88/CD89 should be included to define monocytes and CD5 and CD163 to subset cDC2s and DC3s.
Mair F, Liechti T. Comprehensive Phenotyping of Human Dendritic Cells and Monocytes. Cytometry. Part A: the Journal of the International Society for Analytical Cytology. 2020 Nov. DOI: 10.1002/cyto.a.24269.