The New England Journal of Medicine -- September 7, 2000 -- Vol. 343, No. 10
Approaches involving the blockade of inflammatory cytokines such as interleukin-1 and tumor necrosis factor (alpha) (TNF-(alpha)) are entering mainstream clinical medicine: antibodies against TNF-(alpha) and soluble receptors for TNF-(alpha) are being used to treat rheumatoid arthritis and Crohn's disease. A large number of preclinical studies form the scientific basis for these anticytokine-based therapies. In experiments in animals in which a disease involving several cytokines has been induced, administration of neutralizing antibodies or the use of other means to block the biologic activity of a specific cytokine attenuates the effects of the disease or halts its progression.
A novel approach to unraveling the role of a particular cytokine in a disease comes from studies of genetically altered mice in which the production of a specific cytokine is prevented by the targeted deletion of its gene. These so-called knockout mice lack the ability to synthesize a specific cytokine and can be phenotypically normal or unusually susceptible to an experimentally induced disease. Of considerable interest is the fact that certain diseases develop spontaneously in some cytokine-knockout mice. For example, inflammatory bowel disease develops spontaneously in interleukin-10-knockout mice, perhaps because this cytokine has an essential role in regulating the immune responses to the microbial flora of the gastrointestinal tract.
In two recent publications, researchers reported that inflammatory autoimmune diseases developed spontaneously in mice that were genetically manipulated to prevent the production of the interleukin-1-receptor antagonist. (1,2) Interleukin-1-receptor antagonist is a naturally occurring cytokine and a member of the interleukin-1 family whose only function is to prevent a biologic response to interleukin-1. As shown in Figure 1, when interleukin-1 occupies its receptor, various proinflammatory events are initiated, but when interleukin-1-receptor antagonist occupies the receptor, no such events are initiated, because interleukin-1 cannot bind to the cells.
Receptors for interleukin-1 are found on cells with various functions, and when activated by this cytokine, they release secondary substances that mediate inflammation and tissue remodeling. Interleukin-1 is a very active cytokine that stimulates the production of prostaglandins and nitric oxide, both of which are highly inflammatory. In addition, interleukin-1 induces the synthesis of chemokines, small proteins that facilitate the entry of neutrophils, macrophages, and lymphocytes into tissues.
Interleukin-1 is a potential target of therapeutic intervention in a variety of inflammatory and autoimmune conditions. (3) In double-blind, placebo-controlled trials, administration of interleukin-1-receptor antagonist significantly reduced local inflammation of joints and bone erosions in patients with moderately severe rheumatoid arthritis. (4,5) The cytokine is being evaluated in clinical trials as a treatment for graft-versus-host disease and multiple sclerosis. Therefore, the finding that in mice that lacked the ability to produce interleukin-1-receptor antagonist an inflammatory disease developed spontaneously was of considerable importance. The implication of these studies is that there is a natural balance between the proinflammatory activities of interleukin-1 and the ability of interleukin-1-receptor antagonist to keep those activities in check by occupying interleukin-1 receptors.
In mice in which the gene for interleukin-1-receptor antagonist had been knocked out, two distinct diseases developed spontaneously that were similar to rheumatoid arthritis and arteritis in humans. One group of investigators reported that inflammatory arthropathy began to appear in these mice at 5 weeks of age and that by 13 weeks, all the mice had deformed joints with bone erosions, infiltrating neutrophils, and other histologic changes similar to those in the joints of patients with rheumatoid arthritis. (1) No such disease developed in the control mice, which had normal amounts of interleukin-1-receptor antagonist. The knockout mice also produced antibodies against double-stranded DNA, antibodies to type II collagen, and excessive amounts of immunoglobulins. Levels of interleukin-1 messenger RNA in the joints of these mice were 10 times as high as the levels in control mice.
Another group observed that a lethal arterial inflammation characterized by transmural infiltration of neutrophils, macrophages, and T lymphocytes developed in mice in which the gene for interleukin-1-receptor antagonist had been knocked out. (2) The lesions were commonly seen at branch points and flexures, which in humans are sites of high turbulence and of atherosclerotic-plaque formation. The arteritis in these mice caused arterial stenosis that was accompanied by infarction of organs and hemorrhage from aneurysmal rupture, which was the probable cause of death. The inflammatory nature of the lesions is consistent with the biologic effects of interleukin-1 on endothelial cells and smooth-muscle cells (Figure 1). In heterozygous mice -- the offspring of one parent with normal amounts of interleukin-1-receptor antagonist and one parent incapable of producing interleukin-1-receptor antagonist -- arterial lesions also developed, but these were less inflammatory than those in homozygous mice and were not lethal. The knockout mice with spontaneous arteritis did not have the rheumatoid-arthritis-like disease.
The two distinct patterns of disease observed in the two studies may be explained by differences in the normal flora in Japan and the United Kingdom, where the studies were conducted, or by differences in the genetic backgrounds of the two colonies of knockout mice. The knockout mice, like their normal littermates, lived in relatively clean but not sterile facilities and were free of obvious and detectable infections. Neither arthritis nor arteritis developed in the control mice.
One conclusion that can be drawn from these studies is that the interleukin-1-receptor
antagonist produced in healthy mice protects them from the effects of interleukin-1,
which is also produced in healthy animals under normal conditions. These
results are also consistent with the concept that the increased production
of interleukin-1 during an inflammatory disease in humans contributes to
the pathologic process by binding to and triggering its receptor (Figure
1). Normally, enough interleukin-1-receptor antagonist is produced to hold
interleukin-1-mediated inflammation at bay. In cases of runaway inflammation,
there is an insufficient amount of interleukin-1-receptor antagonist to
control the activity of interleukin-1. Hence, the administration of exogenous
interleukin-1-receptor antagonist or other agents that reduce the effects
of interleukin-1 should ameliorate inflammatory diseases.
Charles A. Dinarello, M.D.
University of Colorado Health Sciences Center
Denver, CO 80262
Supported by a grant (AI-15614) from the National Institutes of Health.