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Cytokines

This module will help you

• learn the general properties of cytokines and naming conventions.
• understand how cytokines regulate immunity.
• test your knowledge and immunology problem solving skills.

Properties of Cytokines
Cytokine Activities
Cytokine Receptors

Properties of Cytokines

Cytokines are small secreted proteins which mediate and regulate immunity, inflammation, and hematopoiesis. They must be produced de novo in response to an immune stimulus. They generally (although not always) act over short distances and short time spans and at very low concentration. They act by binding to specific membrane receptors, which then signal the cell via second messengers, often tyrosine kinases, to alter its behavior (gene expression). Responses to cytokines include increasing or decreasing expression of membrane proteins (including cytokine receptors), proliferation, and secretion of effector molecules.

Cytokine is a general name; other names include lymphokine (cytokines made by lymphocytes), monokine (cytokines made by monocytes), chemokine (cytokines with chemotactic activities), and interleukin (cytokines made by one leukocyte and acting on other leukocytes). Cytokines may act on the cells that secrete them (autocrine action), on nearby cells (paracrine action), or in some instances on distant cells (endocrine action).

It is common for different cell types to secrete the same cytokine or for a single cytokine to act on several different cell types (pleiotropy; see the table below.) Cytokines are redundant in their activity, meaning similar functions can be stimulated by different cytokines. Cytokines are often produced in a cascade, as one cytokine stimulates its target cells to make additional cytokines. Cytokines can also act synergistically (two or more cytokines acting together) or antagonistically (cytokines causing opposing activities).

* CTL: cytotoxic T lymphocytes; DC: dendritic cells; GM-CSF: Granulocyte-Monocyte Colony Stimulating Factor; IL: interleukin; IFN: Interferon; TGF: Tumor Growth Factor; TNF: Tumor Necrosis Factor.
** Italicized activities are inhibited.

Additional information is available at
http://www.copewithcytokines.de/
http://cmbi.bjmu.edu.cn/cmbidata/cgf/CGF_Database/cytweb/
http://www.beckmancoulter.com/products/instrument/flowcytometry/ecatalog/cdchart.asp

Other groups of cytokines include interferons and chemokines. Interferons IFNa and IFNb inhibit virus replication in infected cells, while IFNg also stimulates antigen-presenting cell MHC expression. Chemokines attract leukocytes to infection sites. Chemokines have conserved cysteine residues that allow them to be assigned to four groups. The groups, with representative chemokines, are C-C chemokines (RANTES, MCP-1, MIP-1a, and MIP-1b), C-X-C chemokines (IL-8), C chemokines (Lymphotactin), and CXXXC chemokines (Fractalkine). Some cytokines are predominantly inhibitory. For example, IL-10 and IL-13 inhibit inflammatory cytokine production by macrophages.

Helper T cells have two important functions: to stimulate cellular immunity and inflammation, and to stimulate B cells to produce antibody. Two functionally distinct subsets of T cells secrete cytokines which promote these different activities. Th1 cells produce IL-2, IFNg, and TNFb, which activate Tc and macrophages to stimulate cellular immunity and inflammation. Th1 cells also secrete IL-3 and GM-CSF to stimulate the bone marrow to produce more leukocytes. Th2 cells secrete IL-4, IL-5, IL-6, and IL-10, which stimulate antibody production by B cells.

T cells are initially activated as Th0 cells, which produce IL-2, IL-4 and IFNg. The nearby cytokine environment then influences differentiation into Th1 or Th2 cells. IL-4 stimulates Th2 activity and suppresses Th1 activity, while IL-12 promotes Th1 activities. Th1 and Th2 cytokines are antagonistic in activity. Th1 cytokine IFNg inhibits proliferation of Th2 cells, while IFNg and IL-2 stimulate B cells to secrete IgG_2a and inhibit secretion of IgG₁ and IgE. Th2 cytokine IL-10 inhibits Th1 secretion of IFNg and IL-2; it also suppresses Class II MHC expression and production of bacterial killing molecules and inflammatory cytokines by macrophages. IL-4 stimulates B cells to secrete IgE and IgG₁. The balance between Th1 and Th2 activity may steer the immune response in the direction of cell-mediated or humoral immunity. (See The Big Picture: Immunity to Infection.)

Cytokine Receptors

Cytokines act on their target cells by binding specific membrane receptors. The receptors and their corresponding cytokines have been divided into several families based on their structure and activities. Hematopoietin family receptors are dimers or trimers with conserved cysteines in their extracellular domains and a conserved Trp-Ser-X-Trp-Ser sequence. Examples are receptors for IL-2 through IL-7 and GM-CSF. Interferon family receptors have the conserved cysteine residues but not the Trp-Ser-X-Trp-Ser sequence, and include the receptors for IFNa, IFNb, and IFNg. Tumor Necrosis Factor family receptors have four extracellular domains; they include receptors for soluble TNFa and TNFb as well as membrane-bound CD40 (important for B cell and macrophage activation) and Fas (which signals the cell to undergo apoptosis). Chemokine family receptors have seven transmembrane helices and interact with G protein. This family includes receptors for IL-8, MIP-1 and RANTES. Chemokine receptors CCR5 and CXCR4 are used by HIV to preferentially enter either macrophages or T cells.

Hematopoietin cytokine receptors are the best characterized. They generally have two subunits, one cytokine-specific and one signal transducing. An example is the GM-CSF subfamily, where a unique a subunit specifically binds either GM-CSF, IL-3, or IL-5 with low affinity and a shared b subunit signal transducer also increases cytokine-binding affinity. Cytokine binding promotes dimerization of the a and b subunits, which then associate with cytoplasmic tyrosine kinases to phosphorylate proteins which activate mRNA transcription. GM-CSF and IL-3 act on hematopoietic stem cells and progenitor cells and activate monocytes. With IL-5, they also stimulate eosinophil proliferation and basophil degranulation. All three receptors phosphorylate the same cytoplasmic protein. Antagonistic GM-CSF and IL-3 activities can be explained by their competition for limited amounts of b subunit.

The IL-2R subfamily of receptors for IL-2, IL-4, IL-7 , IL-9, and IL-15 have a common signal-transducing g chain. Each has a unique cytokine-specific a chain. IL-2 and IL-15 are trimers, and share an IL-2R b chain. Monomeric IL-2R a has low affinity for IL-2, dimeric IL-2R bg has intermediate affinity, and trimeric IL-2R abg binds IL-2 with high affinity. IL-2R a chain (Tac) is expressed by activated but not resting T cells. Resting T cells and NK cells constitutively express low numbers of IL-2R bg. Antigen activation stimulates T cell expression of high affinity IL-2R trimers as well as secretion of IL-2, allowing autocrine stimulation of T cell proliferation in an antigen-specific manner. Antigen specificity of the immune response is also maintained by the close proximity of antigen-presenting B cells and macrophages with their helper T cells, so that cytokines are secreted in the direction of and close to the membrane of the target cell. X-linked severe combined immunodeficiency (X-scid) is caused by a defect in IL-2R family g chain, which results in loss of activity from this family of cytokines.

Cytokine activity can be blocked by antagonists, molecules which bind cytokines or their receptors. IL-1 has a specific antagonist that blocks binding of IL-1a and IL-1b to their receptor. During immune responses, fragments of membrane receptors may be shed and then compete for cytokine binding. Microbes also influence cytokine activities. For example, Vaccinia virus (Smallpox and Cowpox) encodes soluble molecules which bind IFNg, while Epstein-Barr virus (Infectious Mononucleosis) encodes a molecule homologous to IL-10 that suppresses immune function in the host.

The TNF receptor family molecules CD40 and Fas bind cell surface ligands on effector T cells: CD40L and FasL. CD40 is expressed on B cell and macrophage plasma membranes. T cell CD40L binding to B cell CD40 stimulates B cell proliferation and isotype switching. T cell CD40L binding to macrophage CD40 stimulates macrophages to secrete TNFa and become much more sensitive to IFNg. T cell FasL binding to Fas leads to the activation of caspase proteases that initiate apoptosis of the cell expressing membrane Fas. Activated lymphocytes express Fas, so that FasL-positive Tc cells can regulate the immune response by eliminating activated cells. An immune deficiency disease linked to expression of a mutant Fas is characterized by over-proliferation of lymphocytes.

Practice Quiz

Pick the one BEST answer for each question by clicking on the letter of the correct choice.

1. Cytokines may exhibit __________ action, signaling the cells that produce them.

a. antagonistic
b. autocrine
c. endocrine
d. paracrine.
e. synergistic

2. Cytokines are NOT

a. antigen specific.
b. capable of activating more than one cell type.
c. made by lymphocytes.
d. small protein molecules.
e. synthesized de novo in response to antigen or other cytokines.

3. Several cytokines may have the same effect on the cells they bind. This is an example of

a. a cascade.
b. antagonism.
c. pleiotropism.
d. redundancy.
e. synergy.

4. Characterization of cytokine activities is NOT made more difficult by their

a. gene structure.
b. pleiotropism.
c. redundancy.
d. secretion close to target cell membranes.
e. short half-lives.

5. Interferons

a. activate B cells to make virus-specific antibodies.
b. are Th2 cytokines.
c. are virus proteins that interfere with activation of cytotoxic T cells.
d. block virus infection of host cells.
e. inhibit virus replication by infected cells.

6. A cytokine can do all of the following EXCEPT

a. bind to receptors which do not share cytokine-binding subunits.
b. bind to its specific receptor on the same cell that produced it.
c. bind to receptor antagonists produced by pathogenic viruses.
d. compete with other cytokines whose receptors share signal-transducing subunits
e. upregulate (increase) synthesis of high affinity subunits for its receptor.

7. Members of a cytokine receptor family

a. all bind the same cytokines.
b. are grouped together because they share antigen specificity
c. are often found on the same cells
d. are similar in protein structure and sometimes in regions of amino acid sequence.
e. are specific for cytokines produced by a single cell type

8. The ability of a cytokine to change gene expression in the target cell is influenced by all of the following EXCEPT

a. presence of high-affinity receptors on the target cell.
b. presence of soluble cytokine receptors.
c. proximity of the producing and target cells.
d. rate of transport of cytokine-receptor complexes into the cytoplasm.
e. simultaneous production of another cytokine whose receptor uses the same signal transducing subunit.

9. Cytokines are NOT

a. able to inhibit the function of other cytokines.
b. able to stimulate the synthesis of other cytokines.
c. produced by more than one cell type.
d. small protein molecules.
e. stored in the cell for quick release.

10. The IL-2R subfamily consists of receptors for IL-2, IL-4, IL-7, IL-9, and IL-15. This group of cytokine receptors

a. bind all five cytokines to promote synergistic action on target cells.
b. bind cytokines which are produced by the same cell.
c. each has a unique high affinity cytokine-specific a chain.
d. shift the immune response towards cellular immunity.
e. each has a unique signal-transducing g chain.

11. An antagonist for cytokine X may NOT be

a. cytokine A competing for a shared receptor subunit.
b. cytokine B which acts synergistically with cytokine X.
c. cytokine C which inhibits the activation of the cell that produces cytokine X.
d. made by microorganisms.
e. soluble cytokine X receptors.

12. A knock-out mouse for a particular cytokine allows immunologists to characterize cytokine function

a. by doing a dose-response study with competing cytokines.
b. in the absence of all other cytokines.
c. on all cell types simultaneously.
d. under controlled conditions of local cytokine concentrations.
e. with defined cell populations.

13. Activated Tc can regulate immune responses by signaling activated lymphocytes to undergo

a. apoptosis.
b. clonal deletion.
c. clonal proliferation.
d. cytotoxicity.
e. somatic hypermutation.

Problem

Compare and contrast the consequences of a deletion in the gene encoding IL-3 and in the gene encoding the b subunit of IL-3 receptor.

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