Understanding the Role of Natural Killer Cell Receptors and Their Human

act in concert to mediate NK cell and T-lymphocyte Understanding the Role of activation.6 Cognate ligands for the inhibitory KIRs are Natural Killer Cell the ubiquitously expressed human leukocyte antigen (HLA) class I molecules, whereas ligands for the stim- Receptors and Their ulatory KIRs are for the most part unknown. Recent analysis suggests that certain KIR and HLA class I li- Human Leukocyte Antigen gand combinations may impact predisposition to several autoimmune diseases, including psoriatic arthritis, Ligands in Multiple ankylosing spondylitis, type I diabetes, and Crohn disease, among others.3,7–14 The report by Lorentzen et Sclerosis al15 in this issue of Annals of Neurology is the first study to address the question of the role of KIRs and their HLA ligands in the development of MS. The study by Lorentzen and colleagues is Multiple sclerosis (MS) is a complex, multifactorial au- population-based, using a large and clinically well- toimmune disease causing tissue injury of the central documented Norwegian cohort of 631 MS patients (fe- nervous system. Although the etiology of MS is un- male:male ratio, 2.6) and 555 healthy control (female: known, a genetic-environmental interaction is believed male ratio, 1.9) samples from a bone-marrow donor to negatively influence immunoregulation, which re- registry. They found a significant deficit of HLA Bw4 sults in destruction of myelin and/or oligodendrocytes. ϭ ϭ ϫ in patients (odds ratio 1.76; pcorrected 3.2 Ϫ Although the precise role of innate immunity in the 10 5), independent of known HLA DRB1 associa- pathogenesis of MS is unclear, several lines of evidence tions. Thus, the presence of Bw4 appears to confer suggest that natural killer (NK) cells might contribute protection from MS in this population. The Bw4 spec- to susceptibility and/or protection from disease or dis- ificity is defined by an isoleucine or threonine residue ease progression. NK cells are highly versatile, critical 1 at position 80 in the C-terminal region of the alpha- regulators of innate and adaptive immunity. The mul- helix of HLA-B (and some alleles of HLA-A); Bw4 is tifunctional NK cells act as cytotoxic effectors and/or part of a dimorphism in this region (Bw4/Bw6) that is cytokine producers and regulators, engaging with mac- present in all human populations and believed to be rophages, T cells, B cells, dendritic cells, and endothe- maintained by balancing selection. The authors have lial cells to control the immune response. NK cells can carefully adjusted for linkage disequilibrium to the respond to (eg, interleukin [IL]-2, IL-12, IL-15, IL-21) well-known DRB1 risk factors. They have not, how- ␥ or secrete (eg, interferon- , tumor necrosis factor) an ever, rigorously distinguished between an HLA-B allele 2 array of cytokines in the inflammatory response. NK effect and an NK receptor ligand (Bw4) effect. cells have long been associated with an immunoregula- HLA Bw4 is the cognate ligand for the inhibitory tory role in susceptibility to or protection from auto- receptor KIR3DL1, and in humans, recognition of self 3 immune disease. Studies in animal models and hu- HLA-Bw4 helps to confer functional competence, or mans suggest a role for NK cells, through cytotoxicity “licensing”, of NK cells with KIR3DL1 receptors.16,17 or immunoregulatory cytokine activity in self tissues, In this case, the NK cells with self-HLA Bw4 receptors resulting in downregulation of MS disease progression may be licensed, that is, more readily activated, as or increased susceptibility and disease severity in con- compared to NK cells without self-HLA–specific recep- 4,5 junction with particular cytokines. The NK cells ap- tors (unlicensed). A deficit in HLA Bw4 could thus pear to play a pivotal role in the maintenance of a decrease the functionality of NK, resulting in a dimin- tuned immune response versus one that is autoim- ished response to infectious disease, and increased mune. susceptibility to MS. For example, Bw4 homozygotes The diverse NK population activity is regulated by provide very strong protection against human immu- different receptor systems that facilitate recognition nodeficiency virus infection compared with Bw4/Bw6 of infectious disease pathogens and tumors. The heterozygotes or Bw6 homozygotes.18 evolutionarily younger and highly diverse killer In addition to these effects, differences in affinity be- immunoglobulin-like receptors (KIRs) work in con- tween the many Bw4 and KIR3DL1 allelic pairs could junction with the older and more conserved lectin-like also influence the functional outcome. In the Lorent- NKG2 cell surface receptor family to modulate the im- zen study, the KIR3DL1 locus was found at a slightly mune response. The KIR genes code for stimulatory or higher frequency in patients compared with controls. inhibitory receptors, which vary in number and allelic However, the many KIR3DL1 allelic variants have dif- composition among individuals (Fig 1). Likewise, the ferent expression patterns and abilities to recognize KIR gene products are present to varying degrees on HLA-Bw4 ligand,19–21 with significant potential to ef- the surface of NK and several subsets of T cells, and fect disease, as for example found in the KIR3DL1/

626 Annals of Neurology Vol 65 No 6 June 2009 Fig 1. Killer immunoglobulin-like receptor (KIR) gene organization on chromosome 19q13.4. KIR diversity exists at the structural level (inhibitory versus stimulatory receptors), at the genotypic level (with between 7 and 12 genes lying between the centromeric KIR3DL3 and KIR3DL2 anchor genes), at the level of gene allelic polymorphism (Ͼ300 alleles have already been identified pre- dominately in the inhibitory genes), and at the level of cell surface expression. This diagram illustrates the composition of the pre- dominately inhibitory group A haplotype and group B stimulatory haplotype. There are 5 major common variants, and many more minor variants, of the B haplotype in human populations (Geraghty, personal communication), each with different combinations of stimulatory and inhibitory genes. The stimulatory KIR receptors (colored pink in this diagram) have short cytoplasmic tails, designated with an S in the name, and the inhibitory receptors (colored green in this diagram) have long cytoplasmic tails, designated with an L in the name. The anchor receptors are colored gray in this diagram. There is a region of reciprocal recombination between the centromeric and telomeric ends of the haplotypes.

Bw4 relationship in delayed progression to acquired The complex regulation of NK cells by receptors immunodeficiency syndrome.22 These observations evolved to protect against pathogens and inhibit or make future analysis of KIR3DL1 and Bw4 allelic limit autoimmune processes. Does the NK-HLA recep- combinations in the Norwegian and other cohorts crit- tor interplay have a role in subsequent cellular response ical to a fuller understanding of this receptor and Bw4 leading to deleterious autoimmune disease, and/or is function in MS. the adverse cellular response regulated by NK without Lorentzen and colleagues also found a trend for a prior NK-receptor coupling? In vitro studies show that lower frequency of the inhibitory KIR2DL1 with cog- NK cells can directly lyse neural tissue, and may there- nate HLA ligand C2, and a higher frequency of acti- fore directly contribute to the tissue injury in MS. 27,28 vating KIR2DS4 in patients, leading overall to higher Given the numerous examples of KIRs affecting pre- activating NK potential in MS patients. KIR2DL1 al- disposition to autoimmune diseases, a plausible role lotypic recognition of its ligand C2 (HLA-Cw2, 4, 5, can be envisioned for these genes and their HLA class 6, and 15 allotypes defined by a Lys at amino acid I ligands in MS pathogenesis, and the work by Lorent- position 80) is a stronger inhibitory combination than zen et al has shown us that there is indeed a relation- KIR2DL2 and KIR2DL3 with their cognate ligand C1 ship that needs further analysis. Examination of allelic (Cw1, 3, 7, and 8 allotypes, defined by Asn at position variation in HLA has been historically integral to a 80).23,24 A common variant of KIR2DS4 is not expressed on the NK cell surface, but its role as a soluble intracellular receptor has not yet been identified.25,26 In addition, the activating KIR2DS2 and inhibitory KIR2DL2 genes (which are in near complete linkage disequilibrium in most populations) tended to be found in patients with more severe disease, although missing cognate ligand (C1) for KIR2DL2 was not associated with disease. The KIR2DS2/2DL2 combination has been associated with many other autoimmune diseases.3 Future allelic analysis of KIR2DL1, KIR2DL2, KIR2DS2, and KIR2DL3 and their ligands, and of KIR2DS4 should be performed among patients and controls to tease apart potential significant associations in MS. Figure 2 illustrates a possible hier- archy of KIR/HLA association with MS found in the Fig 2. Modeling potential killer immunoglobulin-like receptor Lorentzen study. These results need to be validated in (KIR)–human leukocyte antigen interactions in protection from additional cohorts, as there was no independent repli- or predisposition to severe disease in multiple sclerosis (MS) cation presented in the Lorentzen et al study. (based on the work by Lorentzen and colleages15).

Trachtenberg: NK Cell Receptors and HLA Ligands in MS 627 fuller understanding of HLA associations with disease, 13. Lopez-Larrea C, Blanco-Gelaz MA, Torre-Alonso JC, et al. and it is clear that a very detailed analysis of the highly Contribution of KIR3DL1/3DS1 to ankylosing spondylitis in polymorphic KIRs in relationship with their HLA li- human leukocyte antigen-B27 Caucasian populations. Arthritis Res Ther 2006;8:R101. gands will be integral to elucidating the role of NK, 14. Hollenbach JA, Ladner MB, Saeturn K, et al. Susceptibility to their receptors, and HLA in disease. Further investiga- Crohn’s disease is mediated by KIR2DL2/KIR2DL3 heterozy- tion into the relationship between KIR and HLA poly- gosity and the HLAC ligand. Immunogenetics (submitted). morphism and susceptibility to MS is critical to further 15. Lorentzen AR, Karlesen TH, Olsson M, et al. Killer our understanding of the innate and adaptive immune immunoglobulin-like receptor ligand HLA-Bw4 protects against multiple sclerosis. Ann Neurol 2009;65:658–666. response in this disease. 16. Kim S, Sunwoo JB, Yang L, et al. HLA alleles determine differences in human natural killer cell responsiveness and po- Elizabeth A. Trachtenberg, MS, PhD, D(ABHI) tency. Proc Natl Acad SciUSA2008;105:3053–3058. 17. Kim S, Poursine-Laurent J, Truscott SM, et al. Licensing of Children’s Hospital & Research Center Oakland natural killer cells by host major histocompatibility complex (CHRCO) class I molecules. Nature 2005;436:709–713. 18. Flores-Villanueva PO, Yunis EJ, Delgado JC, et al. Control of Potential conflict of interest: Nothing to report. HIV-1 viremia and protection from AIDS are associated with HLA-Bw4 homozygosity. Proc Natl Acad SciUSA2001;98: References 5140–5145. 1. Vivier E, Tomasello E, Baratin M, et al. Functions of natural 19. Carr WH, Pando MJ, Parham P. 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