This is the first case-control study of AD to explore KIR allelic variation. As previously reported, the presence of three KIR genes is associated with AD; KIR2DL5, KIR2DS1, and KIR2DS5.22 However, only the KIR2DL5*001:01 allele is independently associated with an increased risk of AD. The presence of KIR2DL5*001:01 as a homozygote, as compared to all other variants, is associated with an increased risk of having AD. Of those with the KIR2DL5 gene, only individuals with the KIR2DL5*001:01 allele have an increased risk of AD as compared to those without KIR2DL5 or who carry non- KIR2DL5*001:01 variants. Specifically, those with KIR2DL5*002:01 (the expression of the other common KIR2DL5 allele is usually silenced) do not appear to be at increased risk of AD.41,42 An association with other KIR2DLB alleles that are expressed on the cell surface such as KIR2DL5B*00602 could not be assessed because of the low frequency in our study. The increased risk associated with KIR2DL5*001:01 is augmented by an interaction with HLA-B*-21TT leader sequence, which is associated with less educated or less active NK cells, and has previously been associated with an increased risk of AD.22,24 It may also be augmented by C2 and the weaker Bw4 epitope B80T. Previous studies with HIV have shown that within a KIR gene, KIR allelic variation can have important influences on the functioning of NK cells as well as how the KIR receptor interacts with HLA ligand.43 Overall, our findings refine our previous investigation, which was based on genotyping the presence or absence of the KIR gene.22 The risk of AD due to KIR2DL5*001 likely does not vary significantly by race; although the effect of KIR2DL5*001:01 may be greater in Blacks with AD than Whites. The increased risk of AD appears to be associated with KIR2DL5 alleles when the KIR2DL5 gene is in the telomeric region rather than the centromeric region.
Two recent studies described the potential physiology of an association between AD and NK cell function and number.11,16 In a cohort of individuals with moderate to severe AD by Mack et al, individuals with AD had fewer circulating NK cells that are more homogenous than expected.11 The number of circulating NK cells could be returned to control subject levels after treatment with anti-IL-4 blockade and IL-15 super-agonism.11 Furthermore, Mack et al showed a low frequency of KIR2DL5 cell markers on the circulating NK cells but did not compare the frequency of these cell markers between groups with and without AD. Mobus et al evaluated NK cells using skin transcriptomics in a likely White European group with moderate to severe AD.16 These investigations noted an increased number of NK cells in AD lesional skin as compared to AD non-lesional skin or healthy control skin.16 Mobus et al further showed that surface markers on the NK cells from AD lesional skin consistently had an increase in inhibitory receptors and were poorly educated.16 Their findings could be reversed after treatment with anti-IL-4 blockade and calcineurin inhibition.16 The inhibitory receptors evaluated by these investigators were not KIR receptors but natural cytotoxicity receptors (NCR). 16,44,45 While Mobus et al implied that their results contradicted those of Mack et al, it is possible that our results help to unify and explain the conclusions from these two studies.11,16. Focusing on the human subject results from the three studies, NK cell numbers differ in circulation and lesional skin in those with AD as compared to those without AD. Those with AD are more likely to have poorly educated NK cells (less active), and are more likely to express inhibitory receptors from NCR or KIR families.11,12,16 There is a subset of NK cells called NK2 cells that are strongly influenced by TH2 cytokines and therefore more likely to respond to respond to anti-IL-4 blockade and calcineurin inhibition.46 Although the full functional consequences are unknown, it is possible that allelic polymorphisms such as KIR2DL5*001:01 impact KIR protein expression and function in a group of mature NK cells found in AD lesional skin. A previous study showed that specific KIR2DL5A alleles such as KIR2DL5*001:01 are expressed on the NK cell surface, whereas by a variety of mechanisms such as silenced transcription (e.g., KIR2DL5B*006:01) or intracellular retention (e.g., KIR2DL5A*005), some KIR2DL5 alleles are not expressed.41 Finally, Mack et al studied mouse lesional skin and found larger numbers of activated NK cells in mouse lesional skin, which does directly contradict the Mobus et al study; however, mouse models of AD may not reflect the natural human disease.11
The KIR2DL5 gene, as compared to most other KIR genes, is highly conserved among primate species.40 The frequency of the gene varies greatly among populations.37,39,40 It has been hypothesized that, unlike other KIR genes, this conservation resulted in an association with an invariant HLA domain required for its activation.40 It is also found in two different locations on chromosome 19, a telomeric location (KIR2DL5A, which includes KIR2DL5*001:01) and a centromeric location (KIR2DL5B, which includes KIR2DL5*002:01).39,40 Overall, the allelic variants form only 4 known extracellular segments based on amino acid sequences variation in D0-domain, D2-domain, and stem region.40 The KIR2DL5*001:01 and KIR2DL5*002:01 alleles form two distinct extracellular segments.40 In many settings, KIR2DL5B alleles, like KIR2DL5*002:01, are epigenetically silenced.41 While we confirm the association of KIR2DL5*001:01, it is possible that it might have been assumed because KIR2DL5*002:01 is silenced and we know that KIR2DL5 is associated with AD. Finally, KIR2DL5A, (e.g., KIR2DL5*001:01) forms a haplotype with KIR2DS1 and KIR2DS5 and this is not true for KIR2DL5B.40 As a result, the differing association of the KIR2DL5 alleles, KIR2DL5*001:01 and KIR2DL5*002:01, on the risk of AD are important and likely explained based on their genomic function.
AD is known to be associated with immune dysregulation. Currently, we do not have a full understanding of how NK cell number and function are associated with the pattern of immune dysfunction associated with AD.12,18,47 Cytokines associated with TH2 inflammation like thymic stromal lymphopoietin (TSLP), IL-4, and IL-13 are all part of the immune mechanisms associated with acute aspects of AD.48 Increased expression and circulation of TSLP has been associated with allergic illnesses like asthma, allergic rhinitis, food allergy, and AD.49 Mobus et al noted that the NK cells seen in lesional skin of individuals with AD treated with cyclosporine or dupliumab became more similar in function and number to NK cells found in non-lesional AD skin and healthy skin.16 Genetic variation in TSLP has been shown to increase the risk of eczema herpeticum (a form of disseminated cutaneious HSV) in those with AD as well as eosinophilic esophagitis.50,51 NK cells are known to play an important role in immune response to viral infections.18,47 TSLP genetic variation has also been associated with a less severe course of AD.52–54 The effect of these genetic variants on NK cells is not known, but at least one study noted that TSLP potentiated the cytotoxic activity of NK cells to malignant cells.55 However, TSLP effect may be further complicated by the fact that TSLP can be synthesized as two isoforms that each may have very different function.56 KIR mismatch between an organ donor and recipient is an important source of organ rejection and graft versus host disease.57 Furthermore, older studies indicated that cyclosporine may decrease NK apoptosis, making it a potent drug to maintain NK cell anti-tumor effects, that cyclosporine may influence the subpopulation of NK cells to proliferate, and that cyclosporine has minimal effect on the graft rejection mediated by NK cells.58–60 It is possible that the known effects of cyclosporine on NK cells contradicts Mobus et al findings or indicate that the effect that cyclosporine has on NK cell function is not directly associated with calcineurin inhibition.16 Assuming the latter and assuming that KIR allelic variation is functionally associated with KIR expression on NK cells and associated with the effect of cyclosporine on NK cell function, the potential exists that the success or failure of current therapies may be influenced by interactions associated with NK cell surface receptors.
This is an epidemiologic study and inherently has limitations. We focused on common allelic variants, not rare variants. This was primarily due to sample size and power considerations. It is likely that less common alleles in the genes KIR2DS1 and KIR2DS5 are associated with AD. However, our focus was from a population perspective, alleles likely to have an impact on the population at large. In addition, while rare variants might help explain disease mechanisms, common variants are critical for conducting translational studies on humans to further our understanding of the immunologic mechanisms associated with NK cell function and AD. Larger studies will be able to confirm the associations of less common variants. We evaluated genetic variation and did not evaluate the expression of NK surface receptors and how these receptors interact with keratinocytes, antigen presenting cells or other cells. We were also not able to evaluate NK cell numbers in circulation or in tissue. These studies are essential to further our understanding of how KIR alleles effect NK cell function with respect to AD. Finally, it is possible that our study does not generalize to other populations with AD. However, strengths of our study include that our participants were recruited from multiple centers and, unlike previous studies, we were able to assess associations in a relatively large number of African Americans.
In summary we conducted a case-control study of White and Black individuals from the United States with physician confirmed AD. We demonstrated that the common allele KIR2DL5*001:01 is associated with AD and that this effect is augmented in the presence of HLA-B*-21TT. This finding is consistent with previous tissue based reports.11 Our study adds to the growing literature on the genetic basis of the immune dysregulation of AD and, specifically, that KIR allelic variation is associated with an increased risk of AD.20,27,29,61,62 Further a paradigm shift has occurred in understanding of the immunology of AD suggesting that NK deficiency may be real, may interplay with barrier dysfunction and that treatment studies specifically examining NK induction and modulation are now warranted.11 Specifically, studies of NK cell immunologic function in those with AD may help explain disease variability, different disease immunologic endotypes, differences in genetic predilection for AD by race, as well as treatment failure.10,63−66