Monocytes

Monocytes are a population of circulating white blood cells with the potential to differentiate into tissue macrophages and dendritic cells (DCs). Monocytes are derived from precursors in the bone marrow and can be subdivided into subsets that differ in size, trafficking and innate immune receptor expression. Monocytes mediate host antimicrobial defense and are also implicated in many inflammatory diseases, including atherosclerosis.

In addition, monocyte-like cells can be recruited to tumor sites and inhibit tumor-specific immune defense mechanisms. In both mice and humans, monocytes display some typical morphological features, such as irregular cell shape, oval- or kidney-shaped nuclei and cytoplasmic vesicles. However, they are still very heterogeneous and difficult to distinguish from other cells just by morphology or light scatter analysis.

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Development and Differentiation

Once in the blood stream, monocytes can enter peripheral tissues and further differentiate into macrophages or dendritic cells, depending on the environmental stimuli. Researchers have been also successful in differentiating macrophages and DCs in vitro from monocytes. Although these cells may differ from their naturally, in vivo monocyte-derived counterpart, they have been instrumental in the study of the immune system and their functions.

Click on a dropper to differentiate the monocyte in the test tube:

Monocyte

M1/M2
macrophages

Dendritic cells

In vitro
derived cells

GM-CSF
IFN-γ, TNF-α

IL-4, IL-13

GM-CSF
IL-4

Mouse and Human Subsets

In general, monocytes can be divided depending on their surface expression level of two important chemokine receptors. These are CCR2 and CX3CR1. The pattern of chemokine expression also correlates with the expression of another surface molecule, Ly6C. In mice, Ly6C^hi monocytes are considered proinflammatory with anti-microbial function, whereas Ly6Clo patrol in the blood stream and participate in tissue repair. Their recognized human equivalents are the CD14^hiCD16^neg and CD14^loCD16^hi, respectively.

Subset	Markers	Chemokine receptors	Functions
Mouse
Ly6C^hi	CD11b⁺CD115⁺Ly6C^hi	CCR2^hiCX3CR1^low	Pro-inflammatory and antimicrobial roles
Ly6C^low	CD11b⁺CD115⁺Ly6C^low	CCR2^lowCX3CR1^hi	Patrolling; early responses; tissue repair

Human
Classical	CD14⁺⁺CD16^-	CCR2^hiCX3CR1^low	Resemble Ly6C^hi monocytes based on gene-expression arrays
Intermediate	CD14⁺⁺CD16⁺	CCR2^lowCX3CR1^hi	Pro-inflammatory
Non-classical	CD14⁺CD16⁺⁺	CCR2^lowCX3CR1^hi	Patrolling; antiviral roles

Figure illustrating the pattern of Ly6C expression that can be seen on T/B/NK^- (CD3, CD19, NK1.1/CD49b) CD115⁺ CD11b⁺ cells in mouse whole blood.

For more information see references 5 and 6 (under Other Resources).

Equivalents of mouse monocytes in human blood.

A major feature of is to patrol the body and populate several tissues as needed, where they can further differentiate into other cell types. It happens during both steady state and inflammatory conditions. This is a very important physiological function and multiple factors are involved, most notably chemokines and their receptors, as well as adhesion molecules.

Click on a chemokine or adhesion molecules to see their function:

Emigration of Ly6C^hi Monocytes from bone marrow

Patrolling

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True-Stain Monocyte Blocker™

True-Stain Monocyte Blocker™ has been formulated to block non-specific binding of PE/Dazzle™ 594, PE/Cyanine5, PE/Cyanine7, APC/Cyanine7, APC/Fire™ 750, and PE/Fire™ 744 commonly seen on monocytes and macrophages. The blocker is most effective when doing surface staining on live cells prior to the fixation step in a staining protocol. Learn more.

Cell Markers and Other Resources

Just like the related macrophages and dendritic cells, monocyte subtype characterization is complex. In the pages below, you can find guidelines for working with surface molecules to study monocytes. We also recommend the following literature to learn more about monocyte development, differentiation, subsets, and function: