an individual KIR gene, rather than a combination of several genes, and successful ageing could be identified, the frequencies of named KIR genes/pseudogenes were ana- lysed in both the aged and control cohorts. Through this analysis, a significant association was observed for the KIR2DS3 and KIR2DL5 genes in the healthy elderly before applying Bonferroni’s correction, suggesting that these KIR genes may play a role in successful ageing. When a second cohort of aged individuals and control subjects were analysed for the presence of KIR2DS3 and KIR2DL5, the significance of this finding was lost (Table 6). It is, however, interesting to note that both activating and inhibitory KIR genes encode receptors with immune functions that clearly have the potential to optimise NK cell activity in the elderly, thereby contributing to prolonged lifespan.

*Uncorrected p-value 0.02; **uncorrected p-value 0.03.

were combined, an increase in the frequencies of both of these KIR genes was observed in the complete aged group relative to the total control group (KIR2DS3, 31 vs 24% and KIR2DL5, 50 vs 44%) (Table 6). However, the difference in the frequencies of these two genes between the elderly and young control groups was not statistically significant, and their observed frequencies in both these sample groups were comparable to those previously obtained for a large healthy Irish control group ðn ¼ 512Þ (data not shown).

Certain activating KIR genes encode NK receptors that induce cytolysis of abnormal cells displaying reduced HLA class I expression. Hence, the increased prevalence of such a KIR gene in the healthy aged may reflect a role for the activating receptor it encodes in providing an improved level of innate immune protection against virus infection and cancer. The precise mechanism through which this may be fulfilled tempts some degree of speculation. It is conceivable that certain activating KIR genes may be readily up-regulated in their expression, in response to oncogenesis and infection. If this were the case, such an activating KIR may confer improved NK cell-mediated killing of abnormal cells and thus eliminate viral infections and tumour cells more efficiently.

4. Discussion

NK cells play a pivotal role in the innate immune response and mediate their immune function through the elimination of abnormal cells that display compromised HLA class I expression. The cytolytic activity of NK cells is regulated through the expression of a plethora of activating and inhibitory receptors with HLA class I ligand specificity, such as the KIRs. In the present study, the potential role of KIR diversity in immunosenescence and longevity was investigated in the Irish population.

Inhibitory KIRs also mediate an immune function capable of modulating NK cell activity, which again may provide beneficial effects that contribute to healthy ageing. Certain inhibitory KIR genes encode receptors that prevent NK cell-mediated lysis of healthy autologous cells with normal levels of HLA class I expression. Hence, optimal functional activity of inhibitory KIRs is crucial in prevent- ing self-reactivity and autoimmune disorders. If the mechanisms controlling the expression levels of inhibitory KIR genes became dysfunctional, the reduced cell surface expression of HLA class I restricted inhibitory KIRs may permit NK cell-mediated lysis of healthy autologous cells expressing normal levels of HLA class I. This scenario may arise via the interaction of HLA class I independent activating NK receptors with their ligands. A possible class of receptor that may mediate this effect are the natural cytotoxicity receptors, which possess ligands other than HLA class I that are expressed by normal autologous cells (Moretta et al., 2000). Thus, in the context of ageing, it is conceivable that certain inhibitory KIR genes may be more susceptible to aberrant expression and thus may provide an increased disposition to the development of autoimmune

A PCR–SSOP typing system that determines the presence or absence of the currently known KIR genes, was applied to a cohort of healthy aged individuals and young control subjects, in order to elucidate their KIR genotypes. Subsequent analysis of the data obtained, failed to identify an individual KIR genotype that was associated with successful ageing, thereby suggesting that the poly- genic nature of the KIR loci may not be a major influence in longevity. A considerably high level of genotypic diversity was encountered in the sample groups investigated. This phenomenon was not entirely unexpected as the polygenic nature of the KIR permits variation in KIR gene type and number between individuals, and has previously been observed to manifest itself in the existence of high levels of genotypic diversity within the human population (Uhrberg et al., 1997; Witt et al., 1999; Crum et al., 2000; Hsu et al., 2002b; Shilling et al., 2002), In the context of the present study, such an extreme level of genotype diversity makes the identification of a specific KIR gene combination

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study, the differences observed in the KIR2DS3 and KIR2DL5 frequencies between the first and second aged cohorts are unlikely to arise from variation in geographical location has both groups were part of the BELFAST study, and overall the small and homogeneous nature of the Northern Ireland Caucasian population dramatically lowers the potential for diversity in ancestral geographical origin and the bias that this may introduce.

disorders. Conversely, other inhibitory KIR genes may offer a protective effect against self-reactivity and autoimmunity.