A Guide to Monocyte Markers

Monocytes, which comprise up to 10 percent of leukocytes in the blood, play essential roles in development, homeostasis, and immunity. As professional phagocytes, they recognize and engulf pathogens, infected cells, lipids, and cellular debris. They also contribute to the immune system by acquiring and presenting antigens, migrating to sites of inflammation, and producing cytokines. Finally, this myeloid cell acts as a reservoir to replenish subsets of macrophages and dendritic cells. Monocyte biology is fundamental in understanding not only infection response, but also in the longer term effects of inflammation.

Cell surface markers have become the standard method for identifying monocytes and their unique subtypes. While monocytes have a few morphological features, such as irregular shape, elongated nuclei, and the presence of cytoplasmic vesicles, these are insufficient to distinguish among other immune cell types. Two-color flow cytometry has set the stage for marker detection by establishing CD14 and CD16 as key markers in identifying the three main human monocyte subpopulations.

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Additional protein markers continue to be uncovered through high-level approaches, such as transcriptomics, mass cytometry, functional assays, and single-cell profiling. These new markers bring forth new insights, revealing implications in disease states, inflammation, cancer, and more. This article highlights common monocyte markers that have been mentioned in the literature.

Classical Monocytes

Comprising about 80 to 90 percent of circulating blood monocytes, the “classical” subset is associated with higher levels of phagocytic and antimicrobial activity, sensing and immune response, and cell migration. These monocytes exhibit high peroxidase activity and have been found to be more efficient in the production of reactive oxygen species (ROS). They also produce higher levels of chemokine receptors, indicating the potential of migrating to sites of injury and inflammation. This subset differentiates into macrophages and dendritic cells. 

In humans, classical monocytes are classified based on high expression of CD14 but lacking CD16 (designated as CD14⁺⁺ CD16⁻). CD14 is a co-receptor for bacterial lipopolysaccharides and specifically expressed in monocytes and macrophages. CD16 is an antibody (IgG) receptor that is expressed by two nearly identical genes, FCGR3A and FCGR3B. High levels of CCR2, CCR5, and CD62L have also been reported. The equivalent subset in mice, sometimes called "inflammatory monocytes", has been defined by the marker combination Ly6C^Hi CX3CR1^low CCR2⁺ CD62L⁺ CD43^Low. 

Highlighted classical monocyte marker antibodies: CD14 antibodies, Ly6C1 antibodies, CCR5 antibodies, CX3CR1 antibodies, CD62L antibodies

Non-classical Monocytes

In contrast to classical monocytes, the non-classical subset is associated with reduced phagocytosis and lower production of ROS and pro-inflammatory cytokines. Instead of performing more traditional monocyte duties, they are known to patrol the walls of blood vessels, promote wound healing, and respond to viral infections. This subset does not proliferate to the same extent as classical monocytes but can circulate in the blood longer. Non-classical monocytes originate from their classical counterparts. 

Non-classical monocytes in humans are identified by low levels of CD14 but high levels of CD16 (sometimes written as CD14⁺ CD16⁺⁺ or CD14^dim CD16⁺). Some additional proteins found to be differentially expressed in this subset include CX3CR1, and HLA-DR as well as secretion of IL-1 beta and TNF alpha. In mice, this subset has been referred to as "patrolling monocytes," with Ly6C^Low as the key distinguishing marker. Other markers include CX3CR1^Hi CCR2^Low CD62L^- CD43⁺ and TREML4⁺.

Highlighted non-classical monocyte marker antibodies: CD16 antibodies, CX3CR1 antibodies, CCR2 antibodies, TREML4 antibodies, 

Intermediate Monocytes

A third subset, intermediate monocytes, has been identified that expresses both CD14 and CD16 in humans (CD14⁺⁺ CD16⁺) and intermediate levels of Ly6C in mice. Like classical monocytes, this subpopulation also displays inflammatory properties and does not patrol blood vessels. However, like non-classical monocytes, they feature lower peroxidase activity and also secrete TNF and IL-1B. They are suspected to play an important role in antigen presentation and rapid pathogen defense, as they been found in increased levels in patients with systemic infections, such as HIV. Human intermediate monocytes have been reported to express the markers CCR2, CCR5, HLA-DR, CD86, and CD11c. Intermediate Ly6C^int mice can be recognized by CD209 and MHCII expression. 

Highlighted intermediate monocyte marker antibodies: CD14 antibodies, Ly6C1 antibodies, HLA-DR antibodies, CD86 antibodies, CD11c antibodies

Table of Monocyte Markers

The table provides a general list of characteristic proteins found in monocytes. These include cell markers as well as upregulated and secreted proteins. They are categorized by their human NCBI gene symbol (Gene), common names (Synonyms), use as a marker (Marker Type), protein type, cellular localization, and protein size. Accompanying each entry are listings of relevant antibodies and ELISA kits. These immunodetection products are widely used in the study of specific cell types and protein expression. The listings are sourced by an assortment of reagent suppliers, providing an extensive pool of products from which to compare and choose.

*Some proteins are multi-subunit protein complexes composed of several distinct genes. Information on Protein Type, Localization, and Size (kDa) obtained from UniProt.org.

References:

1. Auffray, Cedric, et al. “Blood Monocytes: Development, Heterogeneity, and Relationship with Dendritic Cells.” Annual Review of Immunology, vol. 27, 2009, pp. 669–92. PubMed, doi:10.1146/annurev.immunol.021908.132557.
2. Guilliams, Martin, et al. “Dendritic Cells, Monocytes and Macrophages: A Unified Nomenclature Based on Ontogeny.” Nature Reviews. Immunology, vol. 14, no. 8, Aug. 2014, pp. 571–78. PubMed Central, doi:10.1038/nri3712.
3. Yang, Jiyeon, et al. “Monocyte and Macrophage Differentiation: Circulation Inflammatory Monocyte as Biomarker for Inflammatory Diseases.” Biomarker Research, vol. 2, Jan. 2014, p. 1. PubMed Central, doi:10.1186/2050-7771-2-1.
4. Jakubzick, Claudia V., et al. “Monocyte Differentiation and Antigen-Presenting Functions.” Nature Reviews. Immunology, vol. 17, no. 6, June 2017, pp. 349–62. PubMed, doi:10.1038/nri.2017.28.
5. Guilliams, Martin, et al. “Developmental and Functional Heterogeneity of Monocytes.” Immunity, vol. 49, no. 4, 16 2018, pp. 595–613. PubMed, doi:10.1016/j.immuni.2018.10.005.
6. Wolf, Anja A., et al. “The Ontogeny of Monocyte Subsets.” Frontiers in Immunology, vol. 10, July 2019. PubMed Central, doi:10.3389/fimmu.2019.01642.
7. Günther, Patrick, and Joachim L. Schultze. “Mind the Map: Technology Shapes the Myeloid Cell Space.” Frontiers in Immunology, vol. 10, Oct. 2019. PubMed Central, doi:10.3389/fimmu.2019.02287.
8. Kapellos, Theodore S., et al. “Human Monocyte Subsets and Phenotypes in Major Chronic Inflammatory Diseases.” Frontiers in Immunology, vol. 10, Aug. 2019. PubMed Central, doi:10.3389/fimmu.2019.02035.
9.Tissue:Monocyte AND Organism:"Homo Sapiens (Human) [9606]" in UniProtKB. https://www.uniprot.org/uniprot/?query=tissue:monocyte&fil=organism%3A%22Homo+sapiens+%28Human%29+%5B9606%5D%22&sort=score. Accessed 1 Sept. 2020.
10. CellMarker. http://biocc.hrbmu.edu.cn/CellMarker/search.jsp?quickSearchInfo=monocyte. Accessed 1 Sept. 2020.