We have previously demonstrated familial aggregation of high serum IFNα
activity in SLE families, suggesting that high serum IFNα
is a heritable SLE risk factor (7
). In this study, we demonstrated that the SLE risk haplotype of IRF5 is associated with higher serum IFNα
activity in SLE patients than the protective haplotypes. These data suggest that IRF5 haplotypes contribute to the heritability of serum IFNα
activity and play a role in the variance in IFNα
activity observed between different SLE patients. Thus, it seems likely that IRF5 mediates SLE risk, at least in part, by modulating serum IFNα
activity, which provides biologic relevance at the protein level for the SLE risk haplotype of IRF5. The protective/protective and protective/neutral genotype groups showed similar levels of serum IFNα
activity, as did the risk/risk and risk/neutral genotype groups, and the risk/protective genotype group had similar serum IFNα
activity as the neutral/neutral genotype group. These data suggest that risk and protective IRF5 haplotypes may have a dominant influence on serum IFNα
activity when combined with a neutral haplotype and that the risk/protective haplotype combination may have a balancing effect on serum IFNα
We demonstrated that separation of patients by autoantibody status is important, since the differential effect of IRF5 genotype on serum IFNα activity was detectable only in patients who were positive for either anti-RBP or anti-dsDNA and may have been obscured by higher background IFNα activity in patients who were positive for both categories of autoantibodies. It is interesting that the risk/risk and risk/neutral genotypes did not show significantly higher serum IFNα activity than the protective genotypes in the double-negative autoantibody group, despite having similar numbers of patients as the single-positive group. An effect of IRF5 genotype on IFNα activity in the double-negative patients is not ruled out by this study, however, since IFNα levels were much lower in this group in general than in the single-positive or double-positive patients, and greater numbers of patients may be required in order to detect a significant difference. The number of patients in the double-positive autoantibody group was small, and we were not able to exclude an association of IRF5 genotype with serum IFNα in this group.
The increased ratio of median serum IFNα
levels between the single-positive protective/protective and neutral/protective genotypes and the risk/risk and risk/neutral genotypes as compared with the double-negative patients suggests the possibility of a gene–autoantibody interaction. In vitro models have shown that the addition of sera containing anti-RBP or anti-dsDNA antibodies to dendritic cells in culture results in brisk production of IFNα
). This may result from the nucleic acid contained within these autoantibody–immune complexes triggering endosomal Toll-like receptors (TLRs) after uptake into cells via Fc receptors. If autoantibodies in SLE patient sera are required to see a differential effect of IRF5 genotype on serum IFNα
activity, then IRF5 may be operative in SLE downstream of the activation of endosomal TLRs by nucleic acid–containing autoantibody–immune complexes. If IFNα
is the protein mediator of SLE risk due to IRF5 genotype, then we would expect that if data from existing case–control genetic association studies are reanalyzed according to the autoantibody strata used in the present study, the IRF5 SLE risk haplotype would show a higher odds ratio for disease in single-positive patients than in double-negative patients. This may or may not be the case in the double-positive autoantibody group, but either result in this group would also be of high interest.
Emerging data suggest that an insertion/deletion polymorphism in the promoter region of IRF5 may play an important role in SLE susceptibility (15
). When we reanalyzed our serum IFNα
activity data using the model proposed in the present study and using rs3807306 as a proxy for the promoter insertion/deletion, we again saw significant differences in serum IFNα
activity by IRF5 genotype in the single-positive autoantibody group. Linkage is strong between the promoter insertion/deletion and previously described SLE risk variants in the 5′ region of the gene, including both rs3807306 and rs2004640, and therefore, the findings of this reanalysis share many similarities with the findings of our initial analysis. However, the 2-marker analysis by rs3807306/rs10488631 genotype categorized the 6% frequency TCTA haplotype as a relatively lower SLE risk haplotype than did our previous analysis, and this new analysis allowed for an association of genotype in the 5′ region of IRF5 with high levels of IFNα
, which is independent of 3′ risk alleles. This result could suggest that the promoter insertion/deletion plays a larger role in serum IFNα
activity than does the 5′ splice-site variation; however, future in vitro and mechanistic studies are required to more definitively address this question.
High levels of serum IFNα
activity show complex inheritance as a trait, and modeling the number of factors involved using relative recurrence-to-risk ratios suggests that 3–4 independent factors will be operative in a given SLE patient (7
). Therefore, while the effect of IRF5 genotype on serum IFNα
activity appears modest and limited to certain autoantibody groups in this study, we expect that a number of other genetic factors will also be important. It is likely that combinations of high IFNα
–predisposing genetic variants will be required to fully manifest the high serum IFNα
trait, and the other genetic factors that underlie high levels of serum IFNα
as an SLE risk factor are as yet unknown.