Interleukin

Interleukin-10 (IL-10) is a small protein known as a cytokine that functions as an important regulator of the immune system. Although IL-10 is known to have many different roles in the immune system, its two major activities include inhibition of cytokine production by macrophages and inhibition of the accessory functions of macrophages during T cell activation (Abbas et al., 1994). The effects of these actions cause IL-10 to play mainly an anti-inflammatory role in the immune system. IL-10 was originally known as the cytokine synthesis inhibiting factor (CSIF), and the discovery of this protein was based on its biological activity (Delves et al., 1998).

Human IL-10 (hIL-10) is a 18.5 kDa acid-sensitive protein that lacks detectable carbohydrate moieties It is a member of the four alpha-helical bundle cytokine superfamily and appears to function as a homodimer (Delves et al., 1998). Its subunits have a length of 160 amino acids (Ibelgauft et al., 1999). Mouse IL-10 (mIL-10) is a 35kDa homodimeric cytokine that differs from the human IL-10 due to glycosylation at the N-terminus. Both mouse and human IL-10 contain intrachain disulfide bonds that are essential to the biological function of IL-10 (Delves et al., 1998).

The gene for human and murine IL-10 is present as a single copy in the genome and maps to chromosome 1. The gene for hIL-10 contains four exons, while the gene for mIL-10 consists of five exons over a span of 5.1 kb. At the nucleotide level, human IL-10 shows 81% homology with mIL-10. Transcription of the IL-10 gene produces a single strand of mRNA of 1.4 kb in mouse and 2 kb in humans. Regulation of IL-10 transcription is not yet fully understood. However, it is known that its upstream regulatory sequence elements are similar to that of IL-6 (Delves et al., 1998). It is also known that the hIL-10 gene is closely related to the product of the BCRF-1 gene of Epstein-Barr virus (84% homology at the protein level). Interestingly, this protein and hIL-10 are more closely related than hIL-10 and mIL-10. For this reason, BCRF-1 is also called viral IL-10 (vIL-10) (Ibelgauft et al., 1999). Research shows that viral IL-10 shares in vitro activity with the T cell-derived cytokine, IL-10. This information suggests that the Epstein-Barr virus may have acquired the hIL-10 gene during evolution in an attempt to gain survival advantage by inhibiting anti-viral immunity (Abbas et al., 1994).

IL-10 is mainly produced by the T_H2 subset of CD4⁺ helper cells. However, it is also produced by some activated B cells, some T_H1 cells (in humans), activated macrophages, and some nonhematopoietic sources (e.g., keratinocytes, colon carcinoma, melanoma cells). Kinetics studies demonstrate that IL-10 is synthesized later than other immunoregulatory cytokines by activated T cells or monocytes. This data may reveal the regulatory role of IL-10 in later phases of the immune response (Delves et al., 1998).

Using radiolabeled IL-10, researchers have identified a receptor on murine and human cells. They have also demonstrated that mIL-10 is able to block binding of hIL-10 to mouse (but not human) cells. Cloning of the mIL-10 receptor reveals that this receptor is approximately 110 kDa and binds mIL-10 specifically. This IL-10 receptor demonstrates a structural likeness to receptors for IFN (Ibelgauft et al., 1999). This finding is interesting because IL-10 and IFN-gamma are antagonistic to one another (Delves et al., 1998). Characterization of the mIL-10 receptor demonstrates that this receptor activates STAT1, STAT2, and STAT5, which susequently form heterocomplexes and bind to the IFN-gamma response region of the Fc(gamma)R1 gene. Other signaling pathways have been defined for hIL-10/IL-10R complexes. One of these pathways involves phosphorylation of tyk2 and JAK1 with subsequent activation of STAT1alpha and STAT3. However, several other routes have also been defined. This data suggests that multiple and distinct signaling pathways may mediate the various functions of IL-10 (Delves et al., 1998). Other research has demonstrated that the immunosuppressive drugs cyclosporin A, rapamycin, and FK506 are able to inhibit IL-10 production by human T cells. This mechanism may involve the binding of IL-2 and may be under the control of the internal feedback mechanisms of IL-2 (Cohen et al., 1997).

Like other cytokines interleukin-10 has many effects upon the functions of cells such as lymphocytes, monocytes, natural killer cells, and dendritic cells. Specifically, IL-10 is a cytokine that regulates immune-mediated inflammation. It appears to have two major functions: (1) to inhibit cytokine (i.e., TNF, IL-1, chemokine, and IL-12) production by macrophages and (2) to inhibit the accessory functions of macrophages in T cell activation. IL-10 accomplishes the latter function through the reduced expression of MHC class II molecules and certain co-stimulators (e.g., B7). The cumulative effect of these functions acts to inhibit T cell-mediated immune inflammation. IL-10 also has stimulatory actions on B cells and may function as a switching factor for the production of IgG4 in humans (homologous to IgG1 in mice) (Delves et al., 1998) . A comprehensive summary of the effects of IL-10 on the various components of the immune system is given in Table 1.

Cell Line Affected by IL-10	General Effect of IL-10
T-lymphocytes	Specifically inhibits T_H1 cell cytokine synthesis in the mouse (especially when macrophages acted as the antigen-presenting cells) (Delves et al., 1998). This function is antagonized by IL-4 (Ibelgauft et al., 1999).
	Inhibitory effects on proliferation, survival, and cytokine production of human T-cells. For example, direct interaction of IL-10 with the IL-10 receptor on T cells acts to suppress transcription of the gene for IL-2, which inhibits T cell proliferation (Delves et al., 1998).
	Indirect inhibition of T cells. This inhibition is caused by the alteration of the antigen- presenting function of monocytes. IL-10 downregulates the expression of MHC class II antigen on monocyte, thereby decreasing the peptide/MHC complexes available for interaction and proper stimulation of T cells (Delves et al., 1998).
	Plays a role in causing T cells to undergo anergy(Chabot et al., 1999)
	Inhibits IFN (gamma) synthesis by CD8⁺ T cells without affecting the cytotoxic function of these CD8⁺ T cells (Delves et al., 1998).
	Human IL-10 demonstrates the ability to stimulate chemotaxis, proliferation, differenciation and cytolytic activity of human CD8⁺ T cells (Groux et al., 1999)
	Inhibits apoptotic death of IL-2 dependent T cells by prolonging cell survival. This prolongation of survival may occur through upregulation of Bcl-2 by IL-10 (In Vivo 1996).
Macrophages/Monocytes	Effects on morphology, phenotype and cytokine production (Delves et al., 1998)
	Causes deadherence and rounding up of monocytes (Delves et al., 1998)
	Inhibits the constitutive and inducible expression of MHC class II on monocytes (Delves et al., 1998)
	Able to block B7 and ICAM-1 expression in monocytes in response to IFN(gamma). This act disrupts monocyte-T cell interaction (Delves et al., 1998)
	Prohibits human monocytes (at mRNA level) from producing IL-1 alpha, IL-1 beta, IL-6, IL-8, TNF alpha, granulocyte macrophage, and granulocyte colony-stimulating factor (G-CSF) following activation (Delves et al., 1998)
	Inhibits T cell production by preventing IL-12 expression from monocytes (Delves et al., 1998)
	Inhibits its own production (IL-10) by monocytes, thus demonstrating the workings of a self-regulatory negative feedback loop (Delves et al., 1998)
	Inhibits macrophage activation by IFN-gamma (Yue Ho et al., 1993).
Natural Killer Cells (NK)	Inhibits monocyte-induced production of IFN gamma by NK cells. Indirectly causes this inhibition by suppressing monocyte production of IL-12 (Delves et al., 1998)
	Enhances production of IFN gamma, TNF alpha, and GM-CSF by IL-2-activated NK cells (Delves et al., 1998)
	Induces NK cytotoxic activity against NK-resistant tumor cell targets (Delves et al., 1998)
B-lymphocytes	Induces the expression of MHC class II antigen on resting B cells (Yue Ho et al., 1993).
	Leads to enhanced viability of B cells in vitro (Delves et al., 1998)
	Induces apoptosis in germinal center B cells. Mechanism unknown (In Vivo 1996).
	Stimulates differenciation of B cells into antibody secreting cells (especially after activation by CD40 antibody). Differenciation occurs when IL-10 acts with IL-2 and effectively increases the affinity of the B cells' IL-2 receptor (Delves et al., 1998).

Table 1. General effects of IL-10 on various cell lineages. Data in table collected from various sources as cited internally in the table.

Researchers have demonstrated that mice in which the gene for IL-10 has been disrupted by gene knock-out show few immunologic abnormalities. These mice show normal development of lymphocytes and antibody responses. However, research suggests that most animals are growth retarded and anemic and suffer from chronic enterocolitis. The intestines of these mice exhibit extensive mucosal hyperplasia, inflammatory reactions, and abnormal expression of MHC class II molecules on epithelia (Kuhn et al., 1993). If these mutant mice are kept in specific pathogen-free conditions, the mice are shown to develop only a local inflammation limited to the proximal colon, thereby suggesting that IL-10 is an essential immunoregulator in the intestinal tract. This finding also implies that the generalized bowel inflammation in IL-10-deficient mice is a result of uncontrolled immune responses stimulated by enteric antigens (Delves et al., 1998). For further information regarding the effects of mutated or absent forms of IL-10 see the portion of this page entitled: The role of IL-10 in MS.

Hopefully, this section, while included as a matter of interest, will also aid in elucidating the role played by IL-10 in the function of the immune system.

Dr. Drew Weissman, MD is currently working to protect still uninfected cells in HIV⁺ persons by muzzling cytokines that promote viral infection. One focus of his work involves IL-10 (In Vivo 1996). In a recent study, the research team gave three HIV⁺ individuals one (1 micro:g/kg) dose injection of IL-10 each. The team found that this small dose suppressed the induction of TNF-a production for 4, 6, and 24 hours and caused rapid declines in viral load. A placebo injection failed to prevent the production of TNF-a. Repeating this experiment using higher doses of IL-10, the researchers found that results were similar - although possibly not as profound. This finding suggests that IL-10 may be producing unwanted side effects at higher doses. Thus IL-10 may be used in the future as a way to impede viral replication in HIV⁺ individuals (Nary 1996). For more information on this very promising topic of research go to: http://www.iapac.org/clinmgt/conferences/assisihost.html#cytokine.

Due to its anti-inflammatory properties, IL-10 has a favorable impact on inflammatory diseases of the CNS, such as MS. Many MS patients are treated with IFN-beta, and recent research indicates that the elevated levels of IL-10 in serum and cerebrospinal fluid associated with administration of IFN-beta may be partly accountable for the effectiveness of this treatment. Animal research indicates that during the recovery phase of MS, expression of IL-10 in the brains of mice afflicted with MS is elevated significantly. Some studies have even indicated that IL-10 is able to prevent MS in rats. However, this data is not confirmed. Also, mice deficient for IL-10 were shown to be more susceptible to MS and developed a more severe form of MS than did wild-type mice. Additionally, IL-10 transgenics were resistant to the development of MS (Chabot et al., 1999).

Because IL-10 is found to have an anti-inflammatory role in the immune system, it follows that IL-10 levels are generally very low in rheumatoid arthritis. Clinical trials at UCLA are currently being conducted in this area. Researchers hypothesize that abnormal joint inflammation associated with rheumatoid arthritis can be controlled by administering an injection of IL-10 to patients suffering from this disorder. However, previous studies suggest that administration of IL-10 in association with methotrexatate may provide a more effective treatment for rheumatoid arthritis than the administration of IL-10 alone (Bulpitt et al., 1999). To get more information on this study see: Summary of a Study of Interleukin-10 plus Methotrexatate to Treat Rheumatoid Arthritis.

Crohn's disease is an inflammatory bowel disease (IBD), which causes inflammation in the small intestine (NIDDK 1999). Because IL-10 is able to reduce the levels of various proinflammatory cytokines, it has been useful in the treatment of Crohn's disease. In recent study, over a period of seven days, researchers gave a series of IV infusions of IL-10 to a small group of patients with active Crohn's disease. A clinical response occurred in 81% of these patients at three weeks compared to 45% for placebo. Researchers also observed 50% clinical remission at three weeks for the active agent compared to 23% for placebo (MAC 1999). This research and other current studies around the world demonstrate the possibilities of using IL-10 to prevent recurrence of Crohn's disease or to maintain remissions (van Deventer et al., 1999).

The act of giving birth is associated with an increase in the body's production of proinflammatory mediators by gestational tissues. Researchers at the University of Auckland School of Medicine have shown that IL-10 may play a role in modulating or promoting the termination of inflammation during labor at term and in intrauterine infection-associated preterm labor (Simpson et al., 1998).

Additionally, tumors such as melanoma, ovarian carcinoma, and B-cell lymphoma have been shown to produce IL-10. It is possible that treatment with IL-10, therefore, could be used to treat such cancers (Janeway et al., 1999). Indeed, data from the Cytokines Web indicates that potential clinical roles of IL-10 may include: non-Hodgkin's lymphoma, multiple myeloma, melanoma, and ovarian cancer. It also finds that IL-10 may be used in transplantation, immunodeficiencies, and parasitic infections (Cytokines 1997).

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Simpson, KL, Keelan J, Mitchell M. 1998. Labor-Associated Changes in Interleukin-10 Production and Its Regulation by Immunomodulators in Human Choriodecidua. Journal of Endocrinology and Metabolism. 83(12): 4332-4337.

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