Difference between revisions of "Cytokines"

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==Introduction==
 
==Introduction==
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The term cytokine is a generic name for the soluble proteins that mediate reactions between cells, acting via specific receptors on those cells. They are particularly important during effector stages of the immune system, the development of haematopoietic cells, and the repair of damaged tissue. Cytokines such as the Growth Factors have even been linked to the spread of cancer.
+
The term cytokine is a generic name for the soluble molecules that mediate reactions between cells, acting via specific receptors on those cells. They are particularly important during effector stages of the immune system and the development of haematopoietic cells
  
 
==Nomenclature and classification==
 
==Nomenclature and classification==
 
Nomenclature of the cytokines was first based on their activity both ''in vivo'' and ''in vitro'', with the name being abbreviated to acronyms. For example, when a factor isolated from antigen-activated lymphocytes was added to non-immune peritoneal macrophages, their migration from capillary tubes was inhibited. This led to the factor being named migration inhibition factor, or MIF for short. It was then discovered that many biological functions were being produced by the same chemicals, which led to the classification of cytokines based on either the cell populations that secrete them or their function:
 
Nomenclature of the cytokines was first based on their activity both ''in vivo'' and ''in vitro'', with the name being abbreviated to acronyms. For example, when a factor isolated from antigen-activated lymphocytes was added to non-immune peritoneal macrophages, their migration from capillary tubes was inhibited. This led to the factor being named migration inhibition factor, or MIF for short. It was then discovered that many biological functions were being produced by the same chemicals, which led to the classification of cytokines based on either the cell populations that secrete them or their function:
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*'''Monokines'''- produced by myeloid cells (macrophages, [[Monocytes|monocytes]])
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*'''Monokines'''- produced by myeloid cells (macrophages, monocytes)
 
*'''Lymphokines'''- produced primarily by lymphocytes
 
*'''Lymphokines'''- produced primarily by lymphocytes
 
*'''Interleukins'''- produced by leukocytes in general
 
*'''Interleukins'''- produced by leukocytes in general
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|interleukins || IL || IL-1, IL-2
 
|interleukins || IL || IL-1, IL-2
 
|-  
 
|-  
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|interferons || IFN || IFN-γ
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|interferons || IFN || IFN-alpha
 
|-  
 
|-  
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|tumour necrosis factors || TNF || TNF-α
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|tumour necrosis factors || TNF || TNF-alpha
 
|-  
 
|-  
 
|growth factors || GF || NGF, EGF
 
|growth factors || GF || NGF, EGF
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|-  
 
|-  
 
|chemokines || - || RANTES, MCP-1
 
|chemokines || - || RANTES, MCP-1
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|}
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|}  
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==Functions of cytokines==
 
==Functions of cytokines==
 
'''Mediating and regulating innate immunity''': bacterial and viral products, such as LPS, stimulate macrophages and natural killer cells to secrete cytokines that primarily act on endothelial cells and leukocytes. They stimulate the early stages of the inflammatory reaction to microbes.
 
'''Mediating and regulating innate immunity''': bacterial and viral products, such as LPS, stimulate macrophages and natural killer cells to secrete cytokines that primarily act on endothelial cells and leukocytes. They stimulate the early stages of the inflammatory reaction to microbes.
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'''Mediating and regulating adaptive immunity''': in response to specific recognition by T lymphocytes, cytokines are produced that have a wide range of functions, including:
 
'''Mediating and regulating adaptive immunity''': in response to specific recognition by T lymphocytes, cytokines are produced that have a wide range of functions, including:
 
*regulating the growth and differentiation of some lymphocyte populations
 
*regulating the growth and differentiation of some lymphocyte populations
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*recruitment, activation and regulation of specialised effector cells, e.g. mononuclear phagocytes, [[Neutrophils|neutrophils]], eosinophils
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*recruitment, activation and regulation of specialised effector cells, e.g. mononuclear phagocytes, neutrophils, eosinophils
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'''Stimulating haematopoiesis''': produced primarily by bone marrow stromal cells and leukocytes, these cytokines stimulate the growth and differentiation of immature leukocytes.
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'''Stimulating haematopoiesis''': produced primarily by bone marrow stromal cells and leukocytes, these cytokines stimulate the growth and differentiation of immature leukocytes
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By binding onto specific receptors on cell membranes, cytokines are able to exert their actions by triggering signal-cascade mechanisms that ultimately result in altered gene expression. Cytokines have an incredibly high affnity for their receptors, and are therefore able to act at picomolar concentrations. Cytokines can exert antagonistic effects on each other.
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By binding onto specific receptors on cell membranes, cytokines are able to exert their actions by triggering signal-cascade mechanisms that ultimately result in altered gene expression. Cytokines have an incredibly high affnity for their receptors, and are therefore able to act at picomolar concentrations
 +
Cytokines can exert antagonistic effects on each other
  
 
==Important cytokines==
 
==Important cytokines==
 
Cytokines primarily produced by '''macrophages''':
 
Cytokines primarily produced by '''macrophages''':
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*GM-CSF (granulocyte macrophage colony stimulating factor)- stimulates growth and differentiation of granulocytes, macrophages, [[Neutrophils|neutrophils]] and eosinophils
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*GM-CSF (granulocyte macrophage colony stimulating factor)- stimulates growth and differentiation of granulocytes, macrophages, neutrophils and eosinophils
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*IL-1 - stimulates T<sub>H</sub>2 cells and acute phase response
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*IL-1- stimulates Th2 cells and acute phase response
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*IL-6 - stimulates growth and differentiation of B and T cells and acute phase response
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*IL-6- stimulates growth and differentiation of B and T cells and acute phase response
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*IL-12 - stimulates T<sub>H</sub>1 cells
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*IL-12- stimulates Th1 cells
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*IL-18 - stimulates IFN-gamma production by T cells and NK cells, favours Th1 response
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*IL-18- stimulates IFN-gamma production by T cells and NK cells, favours Th1 response
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*TNF-α - stimulates local inflammation and endothelial activation
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*TNF-α- stimulates local inflammation and endothelial activation
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Cytokines primarily produced by '''T<sub>H</sub>1 cells''':
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Cytokines primarily produced by '''Th1 cells''':
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*IL-2 - stimulates proliferation and differentiation of T cells, activates NK cells and macrophages
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*IL-2- stimulates proliferation and differentiation of T cells, activates NK cells and macrophages
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*IFN-γ - activates macrophages, increases expression of MHC I and II molecules, increases antigen presentation
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*IFN-γ- activates macrophages, increases expression of MHC I and II molecules, increases antigen presentation
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*TNF-β - stimlulates killing mechanisms in T and B cells and endothelial activation
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*TNF-β- stimlulates killing mechanisms in T and B cells and endothelial activation
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Cytokines primarily produced by '''T<sub>H</sub>2 cells''':
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Cytokines primarily produced by '''Th2 cells''':
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*IL-4 - activates B cells and [[IgE]] switch, supresses T<sub>H</sub>1 cells
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*IL-4- activates B cells and IgE switch, supresses Th1 cells
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*IL-5 - stimulates eosinophil growth and differentiation
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*IL-5- stimulates eosinophil growth and differentiation
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*IL-10 - suppresses macrophage functions
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*IL-10- suppresses macrophage functions
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Although [[Neutrophils|neutrophils]] produce a lower amount of cytokines per cell than other immune cell types, they are often the first and most common cell type present at sites of infection. This makes them a physiologically important source of cytokines, such as IL-12. [[Eosinophils|Eosinophils]] and [[Mast Cells|Mast cells]] also produce a number of cytokines that are important in the immunology of [[Immunity to Parasites|parasites]] and the pathology of allergic reactions.
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Although neutrophils produce a lower amount of cytokines per cell than other immune cell types, they are often the first and most common cell type present at sites of infection. This makes them a physiologically important source of cytokines, such as IL-12.
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==Chemokines==
 
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The chemokines are a superfamily of cytokines, all related in terms of sequence and gene structure. The family is also known as the 'small cytokine' family (scy) or the intercrines. All have a relatively small molecular weight of ~5-10kDa and can be divided in one of two groups based on the position of the cystein residues (important for the tertiary structure):
 
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*C-C subgroup- cysteine residues are adjacent to each other. Important members include:
 
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**RANTES - Regulated upon Activation, Normal T cell Expressed and Secreted (CCL5)
 
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**MIP-1α in mice, LD-78 in humans - Macrophage Inflammatory Protein (CCL3)
 
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**MIP-1β in mice, ACT-2 in humans - Macrophage Inflammatory Protein (CCL4)
 
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**MCP - Monocyte Chemoattractant Protein (CCL2)
 
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*C-X-C subgroup- residues are separated by another amino acid. Important members include:
 
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**Interleukin 8 (CXCL8)
 
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**MGSA - melanoma growth stimulatory activity (CXCL2)
 
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**PF4 - platelet factor 4 (CXCL4)
 
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**βTG - β-thromboglobulin (CXCL7)
 
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Chemokines are released by many cell types, and are present in the earliest phase of infection, with actions including the following:
 
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*Lymphoid trafficking
 
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*Wound healing
 
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*T<sub>H</sub>1/T<sub>H</sub>2/T<sub>H</sub>17 development
 
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*Angiogenesis/angiostasis
 
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*Lymphoid organ development
 
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*Inflammation
 
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*Cell recruitment
 
−
 
 
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==Cytokines in pathology==
 
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===Bacterial septic shock===
 
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This is the overproduction of cytokines developing a few hours after infection by certain Gram-negative bacteria, including:
 
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*''E. coli''
 
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*''K. pneumoniae''
 
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*''P. aeruginosa''
 
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*''E. aerogenes''
 
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Bacterial cell wall endotoxins (Lipopolysaccharide, LPS) are the cause of septic shock, stimulating macrophages to release IL-1 and TNF-α at excessive, systemic levels. The condition is often fatal and symptoms include a sudden drop in blood pressure, fever, diarrhoea and blood-clotting in multiple organs.
 
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===Bacterial toxic shock===
 
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This condition is caused by bacterial toxins known as superantigens (antigens that bind simultaneously to MHC II and the beta-V domain of the T cell receptor) that activate large numbers of T cells despite specificity. A number of bacteria have been implicated in the production of superantigens, including:
 
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*''S. aureus''- produces enterotoxins and toxic-shock syndrome toxin
 
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*''M. arthritidis''
 
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*''S. pyogenes''
 
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The large number of T cells activated by such toxins (between 5-25% of all T cells, compared to less than 0.01% activated towards conventional antigens) means an excessive amount of cytokines produced, such as IL-1 and TNF. These elevated amounts cause the same systemic reactions as seen in bacterial septic shock.
 
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===Lymphoid and myeloid cancers===
 
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The excessive production of cytokines has been linked to some types of cancer, e.g. IL-6 has been shown to be secreted by myeloma cells, plasmacytoma cells and cervical and bladder cancer cells. IL-6 is known to act in an autocrine manner to stimulate cell proliferation.
 
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{{Template:Learning
 
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|flashcards= [[Cytokines Flashcards|Cytokines]]
 
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}}
 
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{{review}}
 
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<br><br>
 
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{{Jim Bee 2007}}
 
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[[Category:Immunology]]
 

Revision as of 11:09, 9 September 2008

Introduction

The term cytokine is a generic name for the soluble molecules that mediate reactions between cells, acting via specific receptors on those cells. They are particularly important during effector stages of the immune system and the development of haematopoietic cells

Nomenclature and classification

Nomenclature of the cytokines was first based on their activity both in vivo and in vitro, with the name being abbreviated to acronyms. For example, when a factor isolated from antigen-activated lymphocytes was added to non-immune peritoneal macrophages, their migration from capillary tubes was inhibited. This led to the factor being named migration inhibition factor, or MIF for short. It was then discovered that many biological functions were being produced by the same chemicals, which led to the classification of cytokines based on either the cell populations that secrete them or their function:

  • Monokines- produced by myeloid cells (macrophages, monocytes)
  • Lymphokines- produced primarily by lymphocytes
  • Interleukins- produced by leukocytes in general
  • Chemokines- directing cell migration, activating cells in response to infectious agents/tissue damage
  • Interferons- produced by many different cells in response to viral infection

Common cytokines and nomenclature

Name Abbreviation Examples
interleukins IL IL-1, IL-2
interferons IFN IFN-alpha
tumour necrosis factors TNF TNF-alpha
growth factors GF NGF, EGF
colony stimulating factors CSF M-CSF, G-CSF
chemokines - RANTES, MCP-1

Functions of cytokines

Mediating and regulating innate immunity: bacterial and viral products, such as LPS, stimulate macrophages and natural killer cells to secrete cytokines that primarily act on endothelial cells and leukocytes. They stimulate the early stages of the inflammatory reaction to microbes.

Mediating and regulating adaptive immunity: in response to specific recognition by T lymphocytes, cytokines are produced that have a wide range of functions, including:

  • regulating the growth and differentiation of some lymphocyte populations
  • recruitment, activation and regulation of specialised effector cells, e.g. mononuclear phagocytes, neutrophils, eosinophils

Stimulating haematopoiesis: produced primarily by bone marrow stromal cells and leukocytes, these cytokines stimulate the growth and differentiation of immature leukocytes By binding onto specific receptors on cell membranes, cytokines are able to exert their actions by triggering signal-cascade mechanisms that ultimately result in altered gene expression. Cytokines have an incredibly high affnity for their receptors, and are therefore able to act at picomolar concentrations Cytokines can exert antagonistic effects on each other

Important cytokines

Cytokines primarily produced by macrophages:

  • GM-CSF (granulocyte macrophage colony stimulating factor)- stimulates growth and differentiation of granulocytes, macrophages, neutrophils and eosinophils
  • IL-1- stimulates Th2 cells and acute phase response
  • IL-6- stimulates growth and differentiation of B and T cells and acute phase response
  • IL-12- stimulates Th1 cells
  • IL-18- stimulates IFN-gamma production by T cells and NK cells, favours Th1 response
  • TNF-α- stimulates local inflammation and endothelial activation

Cytokines primarily produced by Th1 cells:

  • IL-2- stimulates proliferation and differentiation of T cells, activates NK cells and macrophages
  • IFN-γ- activates macrophages, increases expression of MHC I and II molecules, increases antigen presentation
  • TNF-β- stimlulates killing mechanisms in T and B cells and endothelial activation

Cytokines primarily produced by Th2 cells:

  • IL-4- activates B cells and IgE switch, supresses Th1 cells
  • IL-5- stimulates eosinophil growth and differentiation
  • IL-10- suppresses macrophage functions

Although neutrophils produce a lower amount of cytokines per cell than other immune cell types, they are often the first and most common cell type present at sites of infection. This makes them a physiologically important source of cytokines, such as IL-12.

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