Lymphocytes from Autoimmune MRL lpr/lpr Mice Are Hyperresponsive to IL-18 and Overexpress the IL-18 Receptor Accessory Chain 1 Detlef Neumann, ‡ Elda Del Giudice,* Antonio Ciaramella,* Diana Boraschi, ² and Paola Bossu ` 2 * MRL lpr/lpr mice spontaneously develop a severe autoimmune lupus syndrome characterized by strong autoantibody production and massive lymphoproliferation, in which IFN- plays a major pathogenic effect. The role of the IFN--inducing cytokine IL-18 in the autoimmune syndrome of lpr/lpr mice has been investigated. In response to IL-18, lymph node cells of lpr/lpr mice produce significant amounts of IFN- and proliferate more potently as compared with cells from / mice. Cells likely responsible for such hyperrespon- siveness to IL-18 include NK cells and the CD4  /CD8  self-reactive T lymphocytes characteristically present in lymph nodes of lpr/lpr mice. Analysis of the expression of IL-18R complex revealed that mRNA for the IL-18R-chain is constitutively expressed at similar level both in / and lpr/lpr lymphocytes. In contrast, the expression of the accessory receptor chain IL-18R is low in unstimulated / cells but significantly high in lpr/lpr cells. Thus, the abnormally high expression of the IL-18R chain IL-18R could be one of the causes of the hyperresponsiveness of lpr/lpr cells to IL-18 at the basis of consequent enhancement of IFN- production and development of IFN--dependent autoimmune pathology. The Journal of Immunology, 2001, 166: 3757–3762. T he MRL lpr/lpr mouse spontaneously develops a severe autoimmune syndrome closely resembling human sys- temic lupus erythematosus, characterized by progressive lymphadenopathy, hypergammaglobulinemia, autoantibody pro- duction, and immunocomplex formation eventually leading to en- dorgan disease such as vasculitis, arthritis, and fatal renal failure (1, 2). The lpr mutation of the fas gene impairs Fas molecule functions (3), thus mutant mice show a defect in lymphocyte ap- optosis responsible for impaired deletion of autoreactive T lym- phocytes (4 – 6). Defective Fas is also at the basis of lymphade- nopathy, which is mainly attributable to the accumulation of a peculiar subset of T cells, defined as CD3 + B220 + CD4 - CD8 - double-negative (DN) 3 cells, likely deriving from self-reactive T lymphocytes (7, 8). Because of the deficiency of Fas-dependent apoptosis, autoreactive B cells also persist in lpr/lpr mice and are responsible for autoantibody-dependent pathological features (9) as well as for the activation of autoreactive T cells (10, 11). In consideration of the key role played by autoreactive T lymphocyte activation in this lupus model, several studies have been addressed to clarify the involvement of T cell-related cytokines in the pa- thology. IL-12, IL-4, and IFN- have been found to be involved in the pathogenesis of the lpr syndrome (12–17). Although deletion of either gene for IFN- or IL-4 in lpr/lpr mice results in the reduction of lymphadenopathy, endorgan disease, and early mor- tality (14), the important role of Th1-type cytokines in this pathol- ogy is indicated by the observation that the ratio of IFN-- to IL-4-secreting cells increases with disease progression (15), that DN T cells and autoantibodies are absent only in IFN--deficient mice (14), and that in lpr/lpr mice lacking the IFN-R the kidneys are significantly protected from glomerulonephritis damage (16). IL-18, originally named IFN--inducing factor (IGIF), is a cy- tokine capable of inducing IFN- production in primed T cells (18) and augmenting the NK activity and proliferation of spleen cells (18, 19). IL-18 does not induce development of Th1 cells or ex- pression of IFN- by itself, but it synergizes with IL-12 (20 –22), which induces IL-18R expression (23–25). Structurally, IL-18 is closely related to the IL-1 family (26) and shares with IL-1 the maturation mechanism through caspase-1 (IL-1-converting en- zyme) (27). IL-18 binding to its target cells is mediated by specific plasma membrane receptors, which strictly resemble the IL-1R complex. The previously orphan receptor IL-1R-related protein was identified as a low-affinity receptor for IL-18 (28) and re- named IL-18R (29). Recently, a second receptor subunit, the ac- cessory protein-like molecule (AcPL, or IL-18R), has been cloned (30). Like the IL-1R accessory protein, IL-18R does not bind IL-18 directly but forms the active signaling receptor com- plex with IL-18R bound to IL-18. The investigation on the role of IL-18 in autoimmune patholo- gies has just begun. In the development of autoimmune Th1-de- pendent insulitis in nonobese diabetic mice an association between the active stage of the disease and the expression of IL-18 was found (31, 32). Neutralizing Abs to IL-18 prevent the development of experimental autoimmune encephalomyelitis (33). More re- cently, enhanced expression of IL-18 has been observed in the gut mucosal tissues of Crohn’s disease patients (34, 35) and in syno- vial tissues of rheumatoid arthritis patients (36). *Dompe ´ Research Center, L’Aquila, Italy; ² Immunobiology and Cell Differentiation, Institute of Mutagenesis and Differentiation, National Research Council, Pisa, Italy; and ‡ Pharmacology, Hannover Medical School, Hannover, Germany Received for publication March 6, 2000. Accepted for publication January 2, 2001. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 This work has been conducted under a research contract with Consorzio Autoim- munita ` Tardiva, Pomezia, Italy, within the Programma Nazionale Farmaci seconda fase of the Ministero dell’Universita ` e della Ricerca Scientifica e Tecnologica. D.N. was supported in part by a grant of the Hannover Medical School (HILF). 2 Address correspondence and reprint requests to Dr. Paola Bossu `, Dompe ´ Research Center, Dompe ´ S.p.A., Via Campo di Pile, I-67100 L’Aquila, Italy. E-mail address: bossu.paola@dompe.it 3 Abbreviations used in this paper: DN, CD4 - CD8 - double negative; IGIF, IFN- -inducing factor; AcPL, accessory protein-like; HPRT, hypoxanthine phosphoribo- syltransferase; LN, lymph node. Copyright © 2001 by The American Association of Immunologists 0022-1767/01/$02.00