IL-18 serum concentrations increase with age (12) and are higher in healthy centenarians compared to younger controls (13). However, an association of higher IL-18 serum concentrations with aging phenotypes may not be causal. IL-18 concentrations may be raised as a by-product of disease or other processes producing physical decline. The role of common variation in the IL-to those who remained in the study for at least 6 years, and the associations remained similar. In conclusion, our study provides evidence that IL-18 may be a causal factor in inflammation and age-related poor physical function. The IL-18 rs5744256 SNP was associated with lower IL-18 serum concentrations and better physical function, based on walk tests in 65- to 80-year-olds. Further studies are needed to confirm this association.To those participants who reported being free of functional impairments at baseline, the regression coefficient for the SNP walking time association was in the same direction but reduced, and not nominally significant (coefficient 0.015; p-value .505). The details of measurement of the SPPB and walking times differed between the InCHIANTI and Iowa-EPESE participants. To deal with this in pooled analyses, we have ranked all scores within the study before combining them in ordinal regression models. This process should have removed distributional differences between the measures. The minor allele frequency for rs5744256 was higher in the Iowa-EPESE study than in the InCHIANTI study (44% vs 33%, respectively), and we, therefore, corrected for study in the pooled models. In the Iowa-EPESE study, all participants included in our analyses reported being white to a question on race, and the majority identified maternal and paternal family origins in Europe, although a limited group did not know these origins. Although the entire included sample is likely to have been of Caucasian origin, we have adjusted for this possible source of admixture. The finding of broadly consistent associations within two quite separate population studies also makes it unlikely that admixture could have biased the results.

Mendelian Randomization Using rs5744256: IL-18 serum concentrations (in the InCHIANTI data only) and walking time due to the rs5744256 SNP were tested, thus excluding reverse causation and confounding. Using instrumental variables analyses (Table 4), we found that there was a significant association between IL-18 serum concentrations and walk time due to the three rs5744256 genotypes (linear regression coefficient 0.324; 95% CI, 0.050-0.598; p-value .021). This is analogous to showing that the observed association between rs5744256 and functioning is consistent with the expected effect given the association between the SNP and IL-18 concentrations and the association between IL-18 concentrations and functioning. The estimate of the regression coefficient between IL-18 concentrations and walking speed using genotypes was larger than the observed regression coefficient (0.082; 95% CI, 0.029-0.135; p-value for difference .048). This difference may be due to chance or could reflect possible regression dilution bias caused by the error in the measurement of IL-18 concentrations.

Associations between serum markers and disease or functioning in observational studies always need to be viewed cautiously, as confounding and reverse causation are difficult to control in conventional analyses. Genetic variation is randomly inherited and is fixed at conception, and when such variation alters risk factor levels, these properties can be exploited to reduce confounding and reverse causation. The instrumental variable model presented has been used before for this purpose (14). In our study, the model clearly shows an unconfounded association of IL-18 concentrations with walk time when using IL-18 SNPs. If confirmed, this association indicates that IL-18 may play an active role in age-related physical decline.

DISCUSSION: In this study, we have tested three hypotheses on the relationships between IL-18 concentrations, IL-18 gene tagging SNPs, and functioning in younger old age. We have shown positive associations between higher IL-18 serum concentrations and tested physical functioning. We showed that the minor C allele of the rs5744256 SNP in IL-18 is associated with both IL-18 levels and altered functioning in old age; it is in intron 3 of the IL-18 gene and has no obvious function. Based on HapMapII data in Caucasians, the association could be due to nine other SNPs, including several outside of the IL-18 gene. It is likely that one or more of these SNPs alter IL-18 gene expression, and direct measurement of expression is needed. Previous in vitro experiments have shown IL-18 gene expression to be altered by two promoter polymorphisms, 607 (rs1946518), and 137 (rs187238) (28,29). The data from HapMap indicates that we have captured the 607 SNP with an r2 of 0.56 using rs1293344. This SNP was not associated with IL-18 levels (p-value .11), and 607 is not in strong linkage disequilibrium (LD) with the SNP we found is most strongly associated with levels (correlation, r2 0.14 with rs5744256). The 137 SNP is not in HapMap, and we have not analyzed it further. It, therefore, remains possible that there is further variation in the IL-18 gene that alters serum levels of IL-18.

Conclusion: There is mounting evidence that inflammation is a core process contributing to aging. IL-18 is a potent proinflammatory cytokine and is a good candidate for contributing to inflammation-mediated aging. We