(Circulation. 1997;95:551-552.)
© 1997 American Heart Association, Inc.
Articles |
Cytokines Score a Knockout
Harnessing Gene Targeting to Gain Insight Into the Pathogenesis of Myocarditis
the Departments of Medicine and Pathology, Brigham and Women's Hospital and Harvard Medical School, 221 Longwood Ave, Boston, Mass.
Correspondence to Peter Libby, MD, Vascular Medicine and Atherosclerosis Unit, Brigham and Women's Hospital, 221 Longwood Ave, Boston, MA 02115. E-mail plibby@bustoff.bwh.harvard.edu.
Key Words: Editorials • interleukins • lymphocytes • molecular biology • myocarditis
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Introduction |
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 Top Introduction References |
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Circumstantial evidence implicates a number of cytokines, protein mediators of inflammation and immunity, in the pathogenesis of cardiovascular diseases ranging from heart failure to atherosclerosis. One such cytokine, tumor necrosis factor-
(TNF-), has attracted considerable interest in this regard. The original descriptions of TNF- focused on its antitumor actions in mice primed with endotoxin1 or as a mediator of cachexia in parasitically infected animals.2 Although it was originally considered a product of macrophages, we now recognize that many cells can produce TNF-. In the context of cardiovascular biology, cardiac myocytes3 and vascular smooth muscle cells4 constitute potentially important sources of this multipotent mediator.
TNF may contribute to the pathogenesis of cardiovascular diseases in several ways. Although its effects on specific cells differ, TNF- generally elicits a spectrum of proinflammatory functions on target cells that may account for aspects of the pathological findings (Table
). For example, in septic shock, augmented endothelial procoagulant activity and increased production of plasminogen activator inhibitor can promote disseminated intravascular coagulation and its consequences, including purpura fulminans and the Waterhouse-Friderichsen syndrome. TNF-induced augmentation in leukocyte adherence to microvascular endothelium can promote vascular plugging and may contribute to such clinical scenarios as reperfusion injury or adult respiratory distress syndrome in septic patients.
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In congestive heart failure, the development of cardiac cachexia correlates with elevations in the circulating levels of TNF-.5 This cytokine can augment expression of nitric oxide synthetase and hence vasodilatation and diminished myocardial contractility in septic shock.6 In atherosclerosis7 and restenosis,8 overexpression of TNF- by injured smooth muscle cells may contribute to leukocyte recruitment and local induction of growth factors and further inflammatory mediators, yielding a sustained proinflammatory response in the injured vessel. TNF- expression by macrophages and smooth muscle cells within advanced human atheroma may similarly contribute to lesion evolution and complication.
Work from several quarters has implicated cytokines in the pathogenesis of myocarditis, modeled in mice by immunization with cardiac myosin.9 10 Thus, in myocarditis, as in many other diseases, temporal and spatial associations of cytokine overexpression with lesion generation inculpates these mediators in pathogenesis. Yet the challenge remains to define specific roles for a particular cytokine by demonstrating causality rather than mere association. This challenge has more than intellectual implications, because various strategies exist for neutralizing the action of various cytokines.
Embarking on therapeutic trials with such agents that target cytokines requires a more certain knowledge of the precise cytokine and/or cytokine receptor involved. The multiplicity and redundancy of cytokines and their receptors highlights this need. For example, the cytokines interleukin-1 and lymphotoxin, also known as TNF-ß, share most of the actions of TNF-. Thus, even effective neutralization of TNF- action might fail to interrupt the disease process. The repeated failures of anticytokine treatment in septic shock illustrate this possibility.11 Moreover, most cytokines can interact with a number of different receptors. In the case of TNFs, the p55 and p75 receptors belong to a family of receptor molecules on the surface of cells that appear to signal critical aspects of cell function ranging from immune stimulation to cell death.12
Present on virtually all nucleated cells, TNF receptors are transmembrane homodimers; the affinities of the p55 and p75 forms for their ligands (TNF and lymphotoxin-) are comparable, as are many but not all of the downstream biological effects they signal. The members of the TNF receptor family, including CD40, Fas, and OX-40, all contain cysteine-rich repeated motifs in their extracellular domains, but each has distinct intracytoplasmic sequences, presumably conferring unique activation cascades and functions in vivo. TNF receptors p55 and Fas, for example, have in common cytoplasmic "death domains," which serve to bind proteins involved in signaling the program of apoptotic cell death. Conversely, the cytoplasmic tail of the TNF receptor p75 molecule has been implicated in inducing T-lymphocyte proliferation.12
How can one order this confusing complexity? Fortunately, contemporary genetic technology provides us an avenue of approach to this daunting problem. The article by Bachmaier et al10 published in this issue of Circulation illustrates an application of this approach. These investigators used mice with a targeted disruption in the TNF p55 receptor to define the role of this particular TNF signaling molecule in the pathogenesis of myosin-induced myocarditis. They documented a striking reduction in the myocardial lesion in the absence of TNF receptor p55. Moreover, the finding that adoptive transfer of activated T cells from mice also immunized with myosin caused disease in the wild-type but not TNF receptor p55–deficient recipient suggested that the interruption of the pathogenic pathway occurred at the level of the end organ, ie, the myocardium, rather than the effector lymphocytes. Their finding of reduced class II major histocompatibility complex (MHC) antigen expression by interstitial cells led these investigators to conclude that inability of TNF- to induce class II antigen expression in the p55-deficient mice underlies at least some of the striking reduction in pathological findings. Although TNF- probably transcriptionally augments the expression of class I MHC gene expression, it is unlikely that TNF- directly regulates class II MHC expression at the transcriptional level. Rather, we interpret the results reported in this article as suggesting that p55 engagement signals the production of other relevant intermediates, including the cytokine 
-interferon, a transcriptional regulator of class II expression produced by T lymphocytes and natural killer cells.
Beyond the considerable contribution to understanding the pathogenesis of myocarditis, the approach taken by Bachmaier and colleagues illustrates how genetically altered mice provide a powerful tool to probe mechanisms of disease. In principle, targeted disruption can delete the function of any gene. Gene disruption by homologous recombination is now routine in laboratories throughout the world. Other, increasingly refined strategies for targeted gene disruption have also recently emerged. Tissue-specific gene disruption using the cre recombinase can enable experiments using gene deletion at an even finer level of analysis than the "classic" homologous recombination approach. The ability to introduce by "knock-in" technology mutant genes in the precise position in the genome of the endogenous gene provides another potent tool for pathophysiological investigation.
However, some caveats apply to the use of targeted gene disruption in probing the pathophysiology of disease. Most strategies for targeted gene disruption interrupt function of the particular gene product during ontogeny. Thus, during fetal life and early development, compensatory mechanisms to counterbalance the lost gene may distort the normal contribution of this gene. Strategies that yield selective deletion of gene function postnatally could alleviate this potential problem. Moreover, one must bear in mind that mouse models, no matter how useful and attractive, may not apply directly to the human situation. In the present example, myosin hypersensitivity may not lie at the heart of the majority of cases of myocarditis in humans.
Despite these complexities, we are entering an era of incredible promise in understanding the mechanisms of disease. Although we have made remarkable progress in treating cardiovascular disease, we doubt that heart disease will be "gone with the century." For this reason, the use of genetically altered animals to unravel pathogenic mechanisms is of not only scientific but also clinical interest. The ability to assign the functions of specific gene products in disease pathogenesis should permit pinpointing potential therapeutic interventions. For example, the results of Bachmaier et al10 suggest consideration of trials of treatment of certain myocarditides with antibody fragments or soluble receptors that can neutralize TNF in vivo. The short-term use of these strategies for interrupting TNF action may prove particularly useful when myocarditis presents acutely.
It is tempting to congratulate ourselves on a job well done in view of the enormous inroads that we have recently made in reducing mortality in survivors of acute myocardial infarction with aspirin, ACE inhibitors, ß-blockers, and lipid-lowering agents. However, congestive heart failure now causes the majority of hospital admissions and results in enormous human and financial costs. Effective therapies for myocarditis and idiopathic dilated cardiomyopathy would help to stem this contemporary epidemic. However, chronic vascular diseases such as hypertension and coronary artery disease remain by far the leading causes of congestive heart failure in the developed world. Therefore, we should strive to apply the modern technologies such as those used by Bachmaier and colleagues to address issues related to the pathogenesis of these chronic cardiovascular diseases as well.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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TopIntroductionReferences |
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1. Old LJ. Tumor necrosis factor. Science. 1985;230:630-633.
2. Beutler B, Cerami A. Cachectin and tumour necrosis factor as two sides of the same biological coin. Nature. 1986;320:584-588.[Medline] [Order article via Infotrieve]
3.
Torre-Amione G, Kapadia S, Lee J, Durand JB, Bies RD, Young JB, Mann DL. Tumor necrosis factor- and tumor necrosis factor receptors in the failing human heart. Circulation. 1996;93:704-711.
4. Warner SJC, Libby P. Human vascular smooth muscle cells: target for and source of tumor necrosis factor. J Immunol. 1989;142:100-109.[Abstract]
5. Levine B, Kalman J, Mayer L, Fillit HM, Packer M. Elevated circulating levels of tumor necrosis factor in severe chronic heart failure. N Engl J Med. 1990;323:236-241.[Abstract]
6.
Kelly R, Balligand J-L, Smith T. Nitric oxide and cardiac function. Circ Res. 1996;79:363-380.
7. Barath P, Fishbein MC, Cao J, Berenson J, Helfant RH, Forrester JS. Detection and localization of tumor necrosis factor in human atheroma. Am J Cardiol. 1990;65:297-302.[Medline] [Order article via Infotrieve]
8. Tanaka H, Swanson S, Sukhova G, Schoen F, Libby P. Smooth muscle cells of the coronary arterial tunica media express tumor necrosis factor alpha and proliferate during acute rejection of rabbit cardiac allografts. Am J Pathol. 1995;147:617-626.[Abstract]
9.
Smith SC, Allen PM. Neutralization of endogenous tumor necrosis factor ameliorates the severity of myosin-induced myocarditis. Circ Res. 1992;70:856-863.
10.
Bachmaier K, Pummerer C, Kozieradzki I, Pfeffer K, Mak TW, Neu N, Penninger J. Low-molecular-weight tumor necrosis factor receptor p55 controls induction of autoimmune heart disease. Circulation. 1997;95:655-661.
11.
Bone RC. Why sepsis trials fail. JAMA. 1996;276:565-566.
12.
Bazzoni F, Beutler B. The tumor necrosis factor ligand and receptor families. N Engl J Med. 1996;334:1717-1725.
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