ARTHRITIS & RHEUMATISM Vol. 52, No. 3, March 2005, pp 984–986 © 2005, American College of Rheumatology CONCISE COMMUNICATION complex–associated glomerulonephritis and proteinuria, as well as inflammation of the salivary gland, liver, and lung—all comparable with, if not worse than, the findings in their WT counterparts (Figure 1B and results not shown). Expression of IFN␥ in the spleens and kidneys was comparable between WT and KO animals, as was their overall mortality (survival at 24 weeks 6 of 12 WT animals and 7 of 14 KO animals). Thus, IL-18R␣ is not required for the development of murine lupus, as judged by autoantibody production and endorgan inflammation and dysfunction, at least in this severe autoimmunity model. These results indicate that, although it may in some contexts play a pathogenic role (8), IL-18 is not an essential mediator of either autoantibody generation or endorgan inflammation in lupus, and suggest that previous findings with a partially backcrossed IL-18R␣ allele against the MRL/lpr background (9) were confounded by the incomplete genetics of the backcross, which likely allowed protective 129 genes, perhaps associated with chromosome 1, to segregate with IL-18R␣ deficiency. Alternatively, our findings may reflect differences in housing conditions, where different diets, bacterial or other microorganismal flora, etc. may alter ongoing Th1/Th2 cytokine or other inflammatory responses, as has been suggested to explain the differences in penetrance of the 16/6 antiidiotype murine lupus model in different environments (17) as well as in models of other autoimmune diseases, such as diabetes in NOD mice (18). As such, the potential influence of bacterial flora and other environmental factors in the production, activity, and effects of endogenous IL-18 in murine lupus may be of interest. Similarly, it is interesting to note that the 129 genome, including chromosome 1, has generally been considered to harbor enhancers of autoimmunity, particularly when studied against the C57BL/6 genome (19), while the present results suggest that some 129 genes may be protective, at least on the MRL/lpr background. These observations suggest differential interactions between such disease-modifying 129 loci and the different background strains, e.g., C57BL/6 versus MRL. Since such differences may offer insight into the context-specific roles of such autoimmunity genes, further studies addressing the potential impact of 129 loci upon autoimmunity in other inbred mouse strains, including MRL, will likely be of continued interest. Finally, another study has demonstrated that intramuscular vaccination of young MRL/lpr mice with a complementary DNA (cDNA) encoding murine IL-18 elicited significant reductions in lymphoproliferation, renal disease, and mortality (20). Since these effects were associated with the development of anti–IL-18 autoantibodies, the authors concluded that neutralization of IL-18 activity was responsible for the protective effect. However, autoantibodies to cytokines are common in MRL/lpr, as well as other autoimmune-prone, mice (21), and separate studies have indicated that IL-18 may have an immunosuppressive role in lupus, acting in synergy with IL-12 in the inhibition of autoantibody production (22). Thus, perhaps IL-18 cDNA vaccination resulted in disease protection due not to anti–IL-18 autoantibody generation, but to a direct inhibitory effect of the ectopically generated IL-18 upon antinuclear autoantibody production. In contrast, results of a previous study with intraperitoneal administration of Escherichia coli–derived recombinant IL-18, which appeared to exacerbate disease (8), may perhaps have reflected contaminating elements in the preparation of recombinant cytokine (e.g., bacterial lipopolysaccharides, etc.), which could initiate and/or promote autoimmune responses (23), and/or differences DOI 10.1002/art.20961 Interleukin-18 receptor signaling is not required for autoantibody production and end-organ disease in murine lupus In both human and murine lupus, interleukin-18 (IL18) has emerged as a prominent pathogenic and therapeutic target: its serum levels correlate with disease activity (1–6), and it is known to act in synergy with IL-12 for the induction, in T cells, of interferon-␥ (IFN␥), a pathogenic cytokine in this disease (7). Intraperitoneal administration of recombinant IL-18 or of IL-18 plus IL-12 has been demonstrated to exacerbate renal disease and production of inflammatory cytokines in the MRL/lpr murine lupus model (8), and Kinoshita et al have recently demonstrated the reduced development of autoantibodies, renal disease, and mortality in lupus-prone animals that are homozygous for a targeted IL-18 receptor ␣ (IL-18R␣) allele (9). However, it is notable that those authors generated IL-18R␣–deficient lupusprone animals via intercross-backcross between the original IL18R␣–deficient strain on the 129 background and the target MRL/lpr strain, and only for 3 generations. Use of this strategy theoretically might generate animals with ⬎94% MRL/lpr genes; however, it could have easily generated animals with significantly fewer. Indeed, the IL-18R␣ gene is located on chromosome 1, which harbors strong disease-modfying loci in both human and murine lupus (10), and it is unclear from the findings of Kinoshita et al whether the reduction in disease parameters truly reflects IL-18R␣ deficiency as opposed to the presence of diseaseprotective 129 loci, some potentially on chromosome 1. To address this issue, we have generated congenic MRL/lpr animals deficient for IL-18R␣ by backcrossing mice with the same IL-18R␣–deficient allele (11) against mice of the MRL/lpr background (both from The Jackson Laboratory, Bar Harbor, ME) over 8 generations, using a speed-congenic strategy which ensures MRL homozygosity at all 24 proposed MRL disease susceptibility loci, as well as IgH, H-2, CD95, and importantly, chromosome 1 (12–16). IL-18R␣⫹/⫺ MRL/lpr animals generated in this manner were homozygotic for MRL chromosome 1 at markers D1Mit169 (15.0 cM) and D1Mit380 (36.9 cM), indicating the development of animals with an approximately ⱕ22-cM 129 congenic interval containing IL-18R␣ (at ⬃20.6 cM). These mice were intercrossed to generate IL18R␣⫹/⫹ (wild type [WT]) and IL-18R␣⫺/⫺ (knockout [KO]) MRL/lpr animals. As expected, RNA transcript for IL-18R␣ was detectable by reverse transcriptase–polymerase chain reaction in the spleens of WT, but not KO, animals (results not shown). Animals were observed and assessed over time, by established protocols (16), for the development of lupus. Surprisingly, KO animals developed full-blown lupus, essentially indistinguishable from that in their WT counterparts: as assessed at 12 weeks and 18 weeks of age, they had developed comparable, and sometimes increased, levels of anti–doublestranded DNA and rheumatoid factor (Figure 1A) as well as hypergammaglobulinemia of all isotypes, and comparable or increased degrees of lymphadenopathy due to the accumulation of CD3⫹,CD4⫺,CD8⫺,B220⫹ T cells (mean ⫾ SD weight of spleens and lymph nodes of WT versus KO animals 231 ⫾ 74 versus 259 ⫾ 82 mg and 593 ⫾ 116 versus 688 ⫾ 415 mg, respectively, at 12 weeks of age). Furthermore, KO animals developed full-blown end-organ disease, including immune 984 CONCISE COMMUNICATION 985 Figure 1. Serologic and histopathologic development of lupus in interleukin-18 receptor ␣ (IL-18␣)–deficient MRL/lpr mice. A, Sera from 12-week-old IL-18R␣–wild-type (WT; n ⫽ 8) or -deficient (knockout [KO]; n ⫽ 12) animals were assessed by enzyme-linked immunosorbent assay for anti–double-stranded DNA (anti-dsDNA) and chain–specific rheumatoid factor autoantibody specificities at 1:100 dilution, as well as for total serum IgG isotypes as previously described (16). Dashed lines indicate thresholds for positivity (3 SD above the mean optical density [OD] in sera from non-autoimmune BALB/c mice). All sera were also positive for anti-dsDNA by Crithidia luciliae immunofluorescence. B, Representative end-organ inflammation in the salivary glands and kidneys of 18-week-old WT versus KO mice. Note the development, in both WT and KO animals, of significant mononuclear infiltrates in the periacinar regions of the salivary glands, occupying the majority of the micrographs except for the vascular and ductal regions, as well as intact acinar complexes along the upper right edge. Note also moderate-to-severe hypercellularity and mesangial thickening of the renal glomeruli. Perivasculitis is prominent in both organs. Renal IgG deposits were assessed by direct immunofluorescence; note the presence of IgG deposition in both the renal glomeruli and the tubules of both WT and KO animals. Non-autoimmune C57BL/6 mice did not develop such inflammation of any organ, or renal IgG deposition (results not shown). Each panel reflects a different animal, representative of 10–12 animals examined per genotype (original magnification ⫻ 200). 986 CONCISE COMMUNICATION in the specific level of systemic IL-18 achieved, which may account for a pro- versus antiinflammatory effect (22). As such, the role of IL-18 may be varied and context-dependent in lupus, but given our present results, it nonetheless appears largely dispensable for disease pathogenesis in the MRL/lpr model. Therefore, as is the case for type I IFN, caution should be exercised in the interpretation of the results of studies that equate cytokine overexpression with pathogenicity and that utilize nonideal genetic intercrosses to assess the role of specific mutant loci in polygenic diseases (16). Supported by the Siteman Cancer, Rheumatic Diseases, Diabetes Research and Training, the Digestive Research Core Centers of the Washington University School of Medicine (grant DK52574), the NIH (grants AI-01803 and AI-057471), and the Lupus Research Institute. Dr. Peng’s work was supported by an Arthritis Investigator award from the Arthritis Foundation. Ling Lin, MS Stanford L. Peng, MD, PhD Washington University School of Medicine St. Louis, MO 1. Wong CK, Li EK, Ho CY, Lam CW. Elevation of plasma interleukin-18 concentration is correlated with disease activity in systemic lupus erythematosus. Rheumatology (Oxford) 2000;39: 1078–81. 2. Neumann D, del Giudice E, Ciaramella A, Boraschi D, Bossu P. 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Left histogram shows control staining.” We regret the error.