Monocyte

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Two monocytes surrounded by red blood cells

Monocytes , also monocytes (from ancient Greek μόνος monos , German 'alone, unique' and ancient Greek κύτος cýtos , German 'cavity, vessel, shell' ), are cells of the immune system that circulate in the blood and the precursors of the u. a. macrophages localized in the tissues as well as part of the dendritic cells . Their task is the destruction of exogenous structures through phagocytosis and the activation of the acquired immune defense by means of antigen presentation . The main storage location for monocytes is the spleen . The monocytes belong to both the specific and the unspecific defense system. Circulating monocytes have a lifespan of 1 to 3 days before they differentiate and migrate into the tissues. There they live on as macrophages for several weeks to months.

construction

With a diameter of 5–20 µm, monocytes are among the largest of the white blood cells ( leukocytes ) and make up approx. 3–8% of the leukocyte population. They have a characteristic (from the Greek monos , "only") large nucleus of mostly bean-like shape and relatively little cytoplasm . Monocytes are not a homogeneous cell population, but different subpopulations are differentiated. In addition to the surface marker CD14 , which is typical for monocytes, there are subpopulations that also carry the marker CD16, for example. CD14 ++ CD16 + monocytes are u. a. described predictors of future cardiovascular events in dialysis patients, in patients with chronic kidney disease not requiring dialysis, and in patients with coronary heart disease. Since hardly any surface markers are known that are exclusively expressed on monocytes, combinations of surface markers are used to clearly identify the monocytes in the flow cytometer .

At least three subpopulations of monocytes have been distinguished since 2010:

  • Classical monocytes (CD14 ++ CD16 - )
  • Intermediate monocytes (CD14 ++ CD16 + )
  • Non-classical monocytes (CD14 + CD16 ++ )

The surface markers used for identification on the monocytes of other mammals differ in part from those of humans, which is due, for example, to the species-associated pH values.

Development and function

In bone marrow contained hematopoietic (blood-forming) stem cells specific to eg. Under the influence of growth factors such as GM-CSF (granulocyte macrophage colony stimulating factor) and M-CSF (macrophage colony stimulating factor) in so-called Monoblasts develop. From them, in turn, monocytes develop (“ differentiate ”), which leave the bone marrow and pass into the bloodstream. There they circulate for 1–3 days. During this period, they migrate into the affected tissue when they come into contact with infections. Monocytes have this ability in common with neutrophils , but they arrive at the site of infection at different times (up to a few days) (for a more detailed description, see neutrophil granulocytes ). The most important storage location of the monocytes is the red spleen pulp , from which they can be released in large numbers in acute inflammation. During the migration into the tissue, monocytes differentiate into macrophages. Even before differentiation into macrophages, monocytes are able to take up and destroy foreign structures such as microorganisms intracellularly. As a result, monocytes are able to present antigens and in this way initiate the acquired immune defense.

Monocytes can also be stimulated in vitro with certain cytokines and differentiated into dendritic cells or macrophages.

Vesicles of membrane fragments of monocytes in the blood, so-called circulating microparticles form, inter alia, tissue factor (engl. Tissue factor included). This is primarily involved in the process of blood clotting .

Cellular Signals

Components of bacterial cell walls, bacterial lipoproteins or antigen-bound antibodies activate certain Toll-like receptors (TLRs) , which are found on the plasma membrane of monocytes, macrophages and B cells . The activation of TLR2 and TLR4 is transmitted to the inside of the cell via signal transmission. As a result, z. B. inflammation-promoting substances ( mediators such as interleukin-1β , interleukin-6 or the tumor necrosis factor ) are produced or the phagocytosis process is initiated.

Monocyte Associated Diseases

Monocytes and macrophages naturally process LDL , an important part of lipid metabolism. However, if this is modified by acetylation of the protein component, this oxidized LDL can no longer be processed and accumulates inside the cell. The resulting macrophages are called foam cells . They are considered to be the main cause of arteriosclerosis in connection with injured blood vessels .

An increase in the number of monocytes as a result of chronic inflammation or necrosis is called monocytosis .

Immunophenotype

Cell type CD11b CD11c F4 / 80 Gr-1 Ly6C Ly6G iNOS TNFa
Monocyte +++ - + + +++ - +++ +++
Macrophage ++ + +++ + +/- - ++ ++
monocytoid MDSC ++ + ++ ++ +++ - +++ +
granulocyte. MDSC ++ + - ++ + ++ + +
plasmocyte. DC - + +/- + + - - -
Tip DC + + - + - + ++ ++
CD4-CD8a- DC ++ +++ + - +/- - - +/-
CD4-CD8a + DC - +++ - - +/- - - +/-
CD4 + CD8a- DC ++ +++ + + + / i - - +/-
Bone marrow of. DC + + +/- + - +/- + +
Skin DC + + +/- ? ? ? + +
Langerhans cell + + +/- - - - +/- +

Original table with references for all articles in PMID 22416241 (OA).

Web links

Wiktionary: Monocyte  - explanations of meanings, word origins, synonyms, translations
Commons : Monocytes  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b Filip K. Swirski et al: Identification of Splenic Reservoir Monocytes and Their Deployment to Inflammatory Sites. In: Science . 325, 2009, pp. 612-616.
  2. ↑ L. Jonas, C. Schütt, P. Neels, H. Walzel, E. Siegl: Electron microscopic study of receptor mediated endocytosis of a monoclonal antibody (RoMo-1) against the surface marker CD14 of human monocytes. In: Acta Histochem Suppl. 39, 1990, pp. 339-344. PMID 1706877
  3. ↑ AW Nockher, JE Scherberich: Expanded CD14 + CD16 + monocyte subpopulation in patients with acute and chronic infections undergoing hemodialysis. In: Infection and Immunity 66, 1998, pp. 2782-2790.
  4. ↑ M. Stec, K. Weglarczyk, J. Baran, E. Zuba, B. Mytar, J. Pryjma, M. Zembala: Expansion and differentiation of CD14 + CD16 (-) and CD14 ++ CD16 + human monocyte subsets from cord blood CD34 + hematopoietic progenitors. In: J Leukoc Biol . 82 (3), Sep 2007, pp. 594-602. PMID 17595380
  5. ↑ GH Heine et al. In: Kidney International 2008.
  6. ↑ Rogacev et al. Eur HJ 2011
  7. ↑ KS Roagecev among other things, Journal of the American College of Cardiology . 2012.
  8. ↑ L. Ziegler-Heitbrock et al: Nomenclature of monocytes and dendritic cells in blood. In: Blood . 2010.
  9. ^ A. Reddy, M. Sapp, M. Feldman, M. Subklewe, N. Bhardwaj: A monocyte conditioned medium is more effective than defined cytokines in mediating the terminal maturation of human dendritic cells. In: Blood. 90 (9), Nov 1, 1997, pp. 3640-3646. PMID 9345048
  10. ↑ N. Mackman: Role of tissue factor in hemostasis and thrombosis. In: Blood Cells Mol Dis. 36 (2), Mar-Apr 2006, pp. 104-107. Review.
  11. ^ M. Schmid, AK ways, U. Ritter: Characteristics of "Tip-DCs and MDSCs" and Their Potential Role in Leishmaniasis. In: Frontiers in Microbiology. Volume 3, 2012, p. 74, doi : 10.3389 / fmicb.2012.00074 , PMID 22416241 , PMC 3298847 (free full text).