• Overview
• So what do cytokines do?
• How do cytokines exert their effects?
• What are the limitations of cytokines for therapeutic use?
• In what ways can infectious agents manipulate the cytokine network to counteract rejection?
• What are some examples of how specific pathogens interfere with the cytokine network?
• What are some vaccine design strategies using cytokines?
• Some links
• References
• Back to Immune Evasion Strategies HOME
• Back to Bio 160 Homepage

Cytokines are small molecules, usually proteins or glycoproteins less than 30 kDa that play an essential role in the immune response to infectious agents. They are, according to Kuby (1997), "the messengers of the immune system." In this respect, cytokines are sometimes compared to hormones as a class of molecules. However, they seem to exert their effects much more locally than hormones. They are usually secreted by T helper cells or macrophages but are sometimes produced by other types of cells as well. There are at least over 30 different cytokines that are grouped together according to which class of molecules they belong.

The tertiary structure of most cytokines is alpha helical, although some consist of beta sheets.

Cytokines act to stimulate the various arms of the immune system. For example, some are responsible for the activation of the inflammatory response and delayed-type hypersensitivity reactions. Others stimulate B cells to produce antibody. Still others act to offset these effects by downregulation. In fact, the synergistic and antagonistic effects that cytokines often have on each other is the key to the network and how it regulates the immune system.

Back to Table of Contents

Cytokines control the immune response by influencing and changing the balance of T helper 1 (TH1) and T helper 2 (TH2) cells. This in turn has a direct effect on what type of immune response will occur. When the TH1 cells outnumber the TH2 cells, cell-mediated immunity is induced, including the activation of cytotoxic T cells (CTLs) and the delayed-type hypersensitivity reaction. When TH2 cells predominate, the humoral arm of the immune response is targeted (Romagnani, 1988).

Not surprisingly, there is significant cross-regulation between these two T helper cell populations. That is, when TH1 cells are stimulated, there is a simultaneous downregulation of TH2 cells and vice versa. Likewise, when antibody production is increased, the delayed-type hypersensitivity reaction is suppressed.

Cytokines act to change the T helper cell balance in a number of ways. Most are involved in cell activation, proliferation or differentiation. Sometimes the activity of cytokines can be to induce the expression of certain cell surface molecules, such as MHC and even its own receptor. Another activity is to promote class switching of antibodies.

Skip the following table

In summary, cytokines influence:

• Cell activation
• Cell growth and differentiation
• Induction of MHC
• Induction of cytokine receptors
• Promotion of antibody class-switching

The type of immune response generated is not at all random. TH1 responses are usually implicated in the control of viral infections and other intracellular pathogens. TH2 responses, which promote antibody production, are most often associated with larger microorganisms, such as free-living bacteria and helminthic parasites. This response is also important in generating an allergic reaction.

Back to Table of Contents

Though there is still much to learn about how cytokines perform their duties, the general mechanism of action seems to be signal transduction via cytokine receptors. When a cytokine binds to its receptor, the signal is transduced and the JAK-STAT pathway is activated. The end result of this pathway is to activate the transcription of certain genes, usually genes involved in cell activation or in cell growth and differentiation.

One intriguing question about the action of cytokines is specificity. Compared to the immune system as a whole, cytokines are highly nonspecific. How can these two observations be reconciled? The nonspecific nature of cytokines results in a specificity of the immune response in a number of ways. Usually, only antigen-activated lymphocytes will express appropriate cytokine receptors on their surface. In this way, the activity of nonspecific cytokines can be limited so that these molecules aren't "overactivating" the immune system. There are also a diverse array of cytokine receptors. For example, the Interleukin-2 (IL-2) receptor has three subunits which, in different combinations, act as either low-, intermediate-, or high-affinity receptors, thus further regulating the activity of cytokines. For a cytokine secreted by one cell to target another cell, there is often a requirement of cell-cell interaction. Another mechanism by which an out-of-control immune response is avoided is the short half-life of cytokines. Finally, to slow down or stop an immune response, cells can produce cytokine antagonists that can bind either to the receptor or the cytokine itself, thus blocking signal transduction. For example, the production of IL-2Receptor a (IL-2Ra) helps to regulate the intensity of the inflammatory response. Similarly, soluble IL-2 receptor is released after chronic T cell activation to sequester the IL-2 cytokine and keep it in check. The synergistic and antagonistic effects that cytokines have on each other contribute significantly to the specificity of the immune response.

In summary, these factors influence the specificity of the cytokine network:

• Only antigen-activated lymphocytes express receptors
• Different combinations of receptor subunits result in low-, intermediate-, or high-affinity receptors
• Requirement of cell-cell interaction
• Short half-life of cytokines
• Cytokine antagonists
• Synergistic and antagonistic interactions among cytokines

Back to Table of Contents

With all that cytokines do to induce an immune response appropriate to the infection and with the observation that a significant number of people lack this regulation (i.e., immunosupressed individuals such as those with AIDS, cancer or those on immunosupressive therapy after organ transplantation, etc.), it might seem that simply administering cytokines to affected patients would help the body to develop the reaction necessary to kick out these infectious agents. However, there are a number of limitations of cytokines for therapeutic use. These factors are directly related to cytokines conferring specificity of the immune response. For example, the high local concentrations of cytokines is difficult to achieve by injection. Also, cytokines have a relatively short half-life. Cells reacting to antigen in the body can serve as an undepleted source of short-lived cytokines but one shot can't. Finally, the pleiotropic effects of cytokines in the body that sometimes lead to control of the immune response can cause unpredictable and unwanted side-effects.

In summary, these factors limit cytokines for therapeutic use:

• High local concentrations of cytokines during immune response cannot be mimicked clinically
• Short half-life of cytokines
• Pleiotropic effects can cause unpredictable side-effects

Back to Table of Contents

Pathogens avoid an immune response or exploit the immune system to their advantage in a number of ways (see Introduction) One mechanism, seen in a variety of infectious agents is the modulation of the cytokine network, an essential component of the host immune system. Microbes can affect the balance of T helper cells (see above) in the immune system and thus determine which type of response is generated. They do this by secreting proteins that mimic cytokines in structure and function or proteins that mimic cytokine receptors that act to sequester host cytokines and prevent action. Some specialists believe that host cytokine mimicry in infectious organisms "may have evolved as a ploy to prevent destructive inflammatory responses (Marrack and Kappler, 1994). Infectious agents can also directly stimulate certain cytokines or interfere with specific cytokines, most often those that cause an inflammatory response.

Key mechanisms pathogens use to exploit the cytokine network include:

• Changes in balance of TH1 and TH2 cells
• cytokine or receptor mimicry
• stimulating or blocking the activation of certain cytokines
• Inhibition of local immune response by direct interference with cytokines that cause inflammation, such as IL-2 and interferon-gamma (IFN-g)

Back to Table of Contents

• Epstein -Barr virus (EBV) BCRF1 protein is an analog of human IL-10 and stimulates the production of TH2 cells. The simultaneous inhibition of TH1 cells allows EBV to avoid the inflammatory response. See detailed example of cytokine mimicry by EBV below.
• The poxviruses cowpox and vaccinia produce soluble IL-1-binding proteins which sequester secreted IL-1 and thus slow down the inflammatory response.
• Another poxvirus, Shope fibroma virus, codes for a TNF-receptor mimic and sequesters TNF, another cytokine involved in the inflammatory response.

• Certain bacterial endotoxins stimulate the production of IL-1 and TNF-a by macrophages consequently causing bacterial septic shock.
• The protozoan Trypanosoma cruzi somehow reduces the expression of the a subunit of the IL-2 receptor, which results in a lower-affinity receptor and severe immune suppression.
• Adenoviruses produce VA RNA transcripts that block the activation of an enzyme that phosphorylates a growth factor necessary for the inhibition of viral protein synthesis.
• Some parasites and nematodes induce high levels of IgE.

• Some adenoviruses express proteins that interfere with TNF action after it has bound to its receptor on the surface of the cell.
• A cowpox protein inhibits an enzyme necessary for proper cleavage of the IL-1b precursor thus preventing inflammation.

Epstein-Barr virus causes mononucleosis in about half of the people infected with it. It has recently been shown to be oncogenic in some cases, especially in immunosuppressed individuals. For example, EBV has been implicated in a variety of lymphoproliferative diseases such as Burkitt's lymphoma, Hodgkin's disease and nasopharyngeal carcinoma (Kieff, 1996).

Interestingly, most adults have in fact been infected with EBV (Kieff, 1996). The universal prevalence of this virus indicates that it must have developed an evolutionary survival advantage. One of the ways in which EBV exploits the human immune system to counteract rejection is by the modulation of the host cytokine network.

The EBV BCRF1 protein is an analog of human IL-10 (Moore et al., 1990), a key component of the cytokine pathway. It is suspected that EBV or its predecessor acquired the host gene because the amino acid sequence is so similar (about 78% sequence similarity) (Swaminathan and Kieff, 1994). Both IL-10 an viral BCRF1 play a role in the conversion of T cells into TH2 cells. As TH2 cells are upregulated, TH1 cells are simultaneously downregulated. By shifting the host T cell balance from TH1 to TH2 cells, EBV is able to successfully avoid the TH1 inflammatory response that would normally suppress infection. In this way, BCRF1 confers an evolutionary advantage for EBV, allowing it to establish latent infection and prevent eradication (Swaminathan and Kieff, 1994).

See also the modulation of the host cytokine network by cytomegalovirus.

Since many pathogens exploit the human cytokine network to confer a survival advantage, perhaps we can too through novel vaccine designs. For example, pathogens often express human cytokine mimics in an attempt to shift the T helper cell balance in their favor (see examples above). Current and future vaccine strategies should focus on selecting the appropriate branch of the immune system (humoral or cell mediated) to effectively target infectious diseases.

Shearer and Clerici (1997) suggest the following as factors in vaccine design to target cell mediated vs. humoral immune responses:

Recent approaches to vaccine design have attempted to tap into the cytokine network the same way some infectious agents do. A new idea is to use cytokines as adjuvants, administering them either alongside a vaccine or as a gene within a DNA vaccine itself (Shearer and Clerici, 1997). (For more on DNA vaccines and adjuvants, click here.) Phase clinical trials are currently underway to test the efficacy of cytokine-secreting tumor cell vaccines (Jaffee and Pardoll, 1997).

Chow et al. (1998) recently demonstrated that the nature of the immune response (i.e., humoral vs. cell-mediated) to a DNA vaccine could be determined by codelivery of cytokine genes. These researchers were able to show that codelivery of specific cytokine vectors with a Hepatitis B Virus (HBV) DNA vaccine could induce either TH1 or TH2 cells depending on which cytokine the vector coded for. For example, immunization of mice with the HBV DNA vaccine and IL-12 or IFN-g gene stimulated the production of TH1 and cell-mediated activity and suppressed the production of TH2 cells. Mice immunized with the DNA vaccine and the IL-4 gene increased activity of TH2 cells and suppressed the production of TH1 cells. They also showed that the efficacy of the HBV DNA vaccine was substantially enhanced by codelivery of the cytokine gene. In particular, the most significant immunity occurred when the HBV DNA vaccine was coadministered with the IL-12 gene, which induced the cell-mediated branch of the immune system.

The improved efficacy of DNA vaccines by the coadministration of cytokine adjuvants is a milestone in vaccine design. It seems that we can learn a lot about using the cytokine network to our advantage by observing the experts - the infectious agents we hope to prevent.

For questions and/or comments on this section of the website, please email Kimberly_Hemond@Brown.edu.

Back to Table of Contents

Back to Immune Evasion Strategies HOME

Back to Bio 160 Homepage

Interesting Cytokine Links

References