Monocytes are a type of white blood cells (leukocytes). They are the largest of all leukocytes. They are part of the innate immune system of vertebrates including all mammals (humans included), birds, reptiles, and fish. They are amoeboid in shape, having agranulated cytoplasm. Monocytes have unilobar nuclei, which makes them one of the types of mononuclear leukocytes (containing azurophil granules). The archetypal idea of the nucleus is that it is bean-shaped or kidney-shaped, although the most important distinction is that it is not deeply furcated into lobes, as occurs in polymorphonuclear leukocytes. Monocytes constitute 2% to 10% of all leukocytes in the human body. They play multiple roles in immune function. Such roles include: (1) replenishing resident macrophages under normal states, and (2) in response to inflammation signals, monocytes can move quickly (approx. 8–12 hours) to sites of infection in the tissues and divide/differentiate into macrophages and dendritic cells to elicit an immune response. Half of them are stored in the spleen. Monocytes are usually identified in stained smears by their large kidney shaped or notched nucleus. These change into macrophages after entering into the tissue spaces, and in endothelium can transform into foam cells.
- Physiology 1
- Monocyte subpopulations 2
Diagnostic use 3
- Monocytosis 3.1
- Monocytopenia 3.2
- Dendritic cells 4
- Blood content 5
- See also 6
- References 7
- External links 8
Monocytes are produced by the bone marrow from precursors called monoblasts, bipotent cells that differentiated from hematopoietic stem cells. Monocytes circulate in the bloodstream for about one to three days and then typically move into tissues throughout the body. They constitute between three to eight percent of the leukocytes in the blood. Half of them are stored as a reserve in the spleen in clusters in the red pulp's Cords of Billroth. In the tissues, monocytes mature into different types of macrophages at different anatomical locations. Monocytes are the largest corpuscles in the blood.
Monocytes which migrate from the bloodstream to other tissues will then differentiate into tissue resident vesicles for processing foreign material.
Monocytes and their macrophage and dendritic-cell progeny serve three main functions in the immune system. These are phagocytosis, antigen presentation, and cytokine production. Phagocytosis is the process of uptake of microbes and particles followed by digestion and destruction of this material. Monocytes can perform phagocytosis using intermediary (opsonising) proteins such as antibodies or complement that coat the pathogen, as well as by binding to the microbe directly via pattern-recognition receptors that recognize pathogens. Monocytes are also capable of killing infected host cells via antibody-dependent cell-mediated cytotoxicity. Vacuolization may be present in a cell that has recently phagocytized foreign matter.
Many factors produced by other cells can regulate the chemotaxis and other functions of monocytes. These factors include most particularly chemokines such as monocyte chemotactic protein-1 and monocyte chemotactic protein-3; certain arachidonic acid metabolites such as Leukotriene B4 and members of the 5-Hydroxyicosatetraenoic acid and 5-oxo-eicosatetraenoic acid family of OXE1 receptor agonists (e.g., 5-HETE and 5-oxo-ETE); and N-Formylmethionine leucyl-phenylalanine and other N-formylated oligopeptides which are made by bacteria and activate the formyl peptide receptor 1.
Microbial fragments that remain after such digestion can serve as antigens. The fragments can be incorporated into MHC molecules and then trafficked to the cell surface of monocytes (and macrophages and dendritic cells). This process is called antigen presentation and it leads to activation of T lymphocytes, which then mount a specific immune response against the antigen.
Other microbial products can directly activate monocytes and this leads to production of pro-inflammatory and, with some delay, of anti-inflammatory cytokines. Typical cytokines produced by monocytes are TNF, IL-1, and IL-12.
There are at least three types of monocytes in human blood:
- The classical monocyte is characterized by high level expression of the CD14 cell surface receptor (CD14++ CD16- monocyte)
- The non-classical monocyte shows low level expression of CD14 and additional co-expression of the CD16 receptor (CD14+CD16++ monocyte).
- The intermediate monocyte with high level expression of CD14 and low level expression of CD16 (CD14++CD16+ monocytes).
Ghattas et al. state that the "intermediate" monocyte population is likely to be a unique subpopulation of monocytes, as opposed to a developmental step, due to their comparatively high expression of surface receptors involved in reparative processes(including vascular endothelial growth factors type 1 and 2, CXCR4, and Tie-2) as well as evidence that the "intermediate" subset is specifically enriched in the bone marrow. After stimulation with microbial products the CD14+CD16++ monocytes produce high amounts of pro-inflammatory cytokines like tumor necrosis factor and interleukin-12.
Said et al. showed that activated monocytes express high levels of PD-1 which might explain the higher expression of PD-1 in CD14+CD16++ monocytes as compared to CD14++CD16- monocytes. Triggering monocytes-expressed PD-1 by its ligand PD-L1 induces IL-10 production which activates CD4 Th2 cells and inhibits CD4 Th1 cell function.
A monocyte count is part of a complete blood count and is expressed either as a ratio of monocytes to the total number of white blood cells counted, or by absolute numbers. Both may be useful in determining or refuting a possible diagnosis.
Monocytosis is the state of excess monocytes in the peripheral blood. It may be indicative of various disease states. Examples of processes that can increase a monocyte count include:
- chronic inflammation
- stress response
- Cushing's syndrome (hyperadrenocorticism)
- immune-mediated disease
- pyogranulomatous disease
- red blood cell regeneration
- Viral fever
A high count of CD14+CD16++ monocytes is found in severe infection (sepsis) and a very low count of these cells is found after therapy with immuno-suppressive glucocorticoids.
Monocytes can be used to generate dendritic cells in vitro by adding cytokines like Granulocyte Monocyte Colony Stimulating Factor (GM-CSF) and IL-4.
- Complete blood count
- Neutrophil granulocyte
- White blood cell
- Swirski, FK; Nahrendorf, M; Etzrodt, M; Wildgruber, M; Cortez-Retamozo, V; Panizzi, P; Figueiredo, J-L; Kohler, RH; Chudnovskiy, A; Waterman, P; Aikawa, E; Mempel, TR; Libby, P; Weissleder, R; Pittet, MJ (2009). "Identification of Splenic Reservoir Monocytes and Their Deployment to Inflammatory Sites". Science 325 (5940): 612–616.
- Sozzani, S; Zhou, D; Locati, M; Bernasconi, S; Luini, W; Mantovani, A; O'Flaherty, J. T. (1996). "Stimulating properties of 5-oxo-eicosanoids for human monocytes: Synergism with monocyte chemotactic protein-1 and -3". Journal of immunology (Baltimore, Md. : 1950) 157 (10): 4664–71.
- Ziegler-Heitbrock, L et al (2010) Nomenclature of monocytes and dendritic cells in blood" Blood116 (16): e74-e80. doi:10.1182/blood-2010-02-258558 PMID 20628149
- Ziegler-Heitbrock, L (2007). "The CD14+ CD16+ Blood Monocytes: their Role in Infection and Inflammation, Review". J Leukocyte Biology 81 (3): 584–92.
- Ghattas; et al. (2013). "Monocytes in Coronary Artery Disease and Atherosclerosis. Where are we now?". Journal of the American College of Cardiology 62: 1541–1551.
- Said, Elias A.; et al. (2010). "PD-1 Induced IL10 Production by Monocytes Impairs T-cell Activation in a Reversible Fashion". Nature Medicine: 452–9.
- "The novel subset of CD14+/CD16+ blood monocytes is expanded in sepsis patients". Blood 82 (10): 3170–6. November 1993.
- Fingerle-Rowson, G.; Angstwurm, M.; Andreesen, R.; Ziegler-Heitbrock, H.W.L. (1998). "Selective depletion of CD14+ CD16+ monocytes by glucocorticoid therapy". Clin. Exp. Immunol 112 (3): 501–506.
- Sallusto F, Cella M, Danieli C, Lanzavecchia A (1995). "Dendritic cells use macropinocytosis and the mannose receptor to concentrate macromolecules in the major histocompatibility complex class II compartment: downregulation by cytokines and bacterial products" (PDF). J. Exp. Med. 182 (2): 389–400.
- Histology image: 01702ooa – Histology Learning System at Boston University
- Human Monocytes — Prof. Dr. Ziegler-Heitbrock
- Circulation of Body Fluids