Introduction

Recent technological developments in single-cell analysis have significantly advanced our understanding of the subset heterogeneity of antigen-presenting cells. Mair et al. responded to the rising suggestions of an updated definition of human dendritic cells and monocyte cells based on functional and transcriptional signatures, as well as the expression of key transcription factors and surface markers.¹  In their report, they propose an updated panel and gating strategy to delineate subsets of human dendritic cells and monocyte cells.¹

The role of dendritic cells in the mononuclear phagocyte system

The mononuclear phagocyte system is comprised of innate immune cells, including dendritic cells, monocyte cells and macrophages that work together as the first line of defense against invading pathogens.¹ As professional antigen-presenting cells, or APCs, dendritic cells help internalize pathogens invading the mononuclear phagocyte system, ensuring efficient patrolling and scanning for pathogens as well as danger-associated molecular patterns (DAMPs) from damaged tissue.¹  Following the internalization, 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.¹
 

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Dendritic cells and immunology

The immunostimulatory and immunoregulatory functions of dendritic cells make them attractive targets for cancer immunotherapy and autoimmunity interventions, some of which show promising results in human clinical trials.¹  Dendritic cells are not only the most potent of antigen-presenting cells but can induce immune tolerance under certain circumstances, such as cancer.¹  Dendritic cells also play a key role in immune homeostasis, as depletion of murine dendritic cells leads to spontaneous development of autoimmune diseases.¹
 

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Phenotyping human dendritic cells

Two main subsets of human dendritic cells occur within circulating HLA-DR⁺ cells and can be distinguished by their expression of CD11c and CD123, namely plasmacytoid (pDCs; CD11c⁻CD123⁺) and conventional dendritic cells (cDCs; CD11c⁺CD123^−/dim).¹  Conventional dendritic cells 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.¹  A more recent discovery of inflammatory dendritic cells, called DC3, has been described as a distinct lineage within the dendritic cell family.¹ These cells expressed dendritic cell-specific transcription factor IRF4 and mRNA for FLT3 and ZBTB46, which are typically not expressed in monocyte cells and macrophages.¹
 

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Phenotyping monocyte cells

The classical definition of monocyte cells is based on three subsets: classical CD14⁺CD16⁻ (cMo), intermediate CD14⁺CD16⁺ (iMo) and nonclassical CD14⁻CD16⁺ (ncMo) monocyte cells.¹ Monocyte cell subsets show functional differences, with classical and nonclassical monocyte cells exerting pro- and anti-inflammatory functions, playing unique roles in human diseases such as multiple sclerosis or arthritis.¹

Previously, monocyte cells were defined as a separate lineage of cells giving rise to macrophages in tissues. Interestingly, recent reports challenge the single differentiation trajectory of monocyte cells and suggest that they can differentiate into dendritic cells under certain circumstances.¹

Reassessing the definitions of dendritic cells and monocyte cells

According to Mair et al., the emergence of high-parameter, single-cell technologies such as multicolor flow cytometry, mass cytometry and single-cell RNA sequencing has revealed unprecedented complexity and heterogeneity of the human mononuclear phagocyte system.¹  For example, these new technologies suggest a more complex monocyte cell compartment with at least eight distinct subpopulations.¹  

Based on their an updated panel and gating strategy, Mair et al. conclude that CD88/CD89 should be included to define monocyte cells and CD5 and CD163 to subset cDC2s and DC3s.¹  They state that this new strategy accommodates the more advanced delineation of these subsets and serves as a starting point for more specific and detailed analysis of human dendritic cells and monocyte cells in health and disease.¹

Read the full report from Mair et al.: Comprehensive phenotyping of human dendritic cells and monocytes