Fourteen KIR genes plus two pseudogenes are joined in the leukocyte receptor complex (LCR) on chromosome 19q13.4 and display a high degree of genetic diversity concerning gene content and allelic polymorphism [13]. This genomic structure drives to non-allelic homologous recombination events, which potentially generates considerable genetic diversity in KIR gene repertoire among individuals and populations [14].
HLA and KIR gene clusters are functionally linked but segregate independently creating a genetic diversity that could have a different impact on transplantation outcome. Hence, studies regarding KIR and HLA genes in different populations can provide valuable information to several scientific fields [15, 16].
In our study, we evaluated the repertoire of KIR genes in patients with hematopoietic disorders. Overall, the frequencies of the presence/absence of KIR genes were similar to the frequencies observed in European populations, which would be expected considering the predominance of White Euro-descendants in southern Brazil.
The framework genes KIR3DL3, KIR3DP1, KIR2DL4 and KIR3DL2 were observed in all samples, in accordance to their presence in all known KIR haplotypes. The remaining inhibitorys and activators KIR genes showed frequencies that varied between individuals. The distribution of KIR gene frequencies among patients showed low frequencies as follows: KIR2DL2 (23 %), KIR2DL5 (28 %), KIR2DS1 (15 %), KIR2DS3 (13 %), KIR2DS5 (28 %) and KIR3DS1 (26 %). Despite the small sample size, the results are in agreement with those observed for populations of Caucasian origin. The KIR genes that showed higher frequencies in the patient group were: KIR2DL1 (90 %), KIR2DL3 (92 %), KIR2DP1 (95 %), KIR2DS4 (92 %) and KIR3DL1 (93 %).
A recent study of the association between polymorphisms in KIR and HLA genes and pediatric ALL in Hispanic and non-Hispanic children provided additional evidence about the contribution of genetic variation in ALL incidence. When the incidence and survival were evaluated between the two ethnic groups, a high incidence of ALL and a significantly worse survival was found in Hispanic children compared to non-Hispanic Whites. The genotypes diversity related to KIR and HLA ligands are very suggestive that these two loci may determine a different susceptibility effect depending on the ethnic groups. Such observed differences are probably multifactorial due to an interaction between KIR and environmental factors, e.g. patterns of infection, rather than merely allele frequencies differences between ethnic groups [14].
In a study carried out in Italian population Bontadini and colleagues reported the same general KIR gene patterns distribution observed in other Caucasian and non-Caucasian populations. Australian Aborigine, Chinese Han, and Japanese showed the most markedly different patterns, with significant differences from Italian population and other Caucasian populations, in particular for inhibitory gene KIR2DL2 and non inhibitorys KIR2DS1, KIR2DS2, KIR2DS3, KIR3DS1 [17–20]. The findings with respect to KIR gene diversity in different populations could provide relevant genomic diversity data for further studies on viral infection, autoimmune diseases, and reproductive fitness.
Inhibitorys genes KIR2DL2 and KIR2DL5B were also found at lower frequencies in the Italian population, as well as the activating genes KIR2DS3 and KIR2DS4 [15]. Notably, as to KIR2DS4 alleles, Han Chinese showed an inverse pattern compared to the Italian population [19].
The activating KIR2DS4 gene is unique in the haplotype A, whereas haplotype B contains up to five activating KIRs. Haplotypes A and B have been preserved in the human population (about 25 and 75 % in Caucasian), thus suggesting the occurrence of a balancing selection [13, 15].
Linkage disequilibrium or the non-random associations between alleles at two loci are also present in KIR genes repertoire. A high positive linkage disequilibrium between KIR2DL1 and KIR2DL3 has been observed in Caucasian and non-Caucasian populations [21]. Our data are consistent with this hypothesis since the frequencies of these two genes were the highest observed in both control and patient groups.
KIR repertoire comparisons between patients and healthy family members (Fig. 1) showed that the inhibitory genes KIR2DL2 (p = 0.0005) and KIR2DL5 (p = 0.0067), as well as the activating genes KIR2DS1 (p = 0.0013), KIR2DS2 (p = 0.0038), KIR2DS3 (p = 0.0153) were more frequently found (p <0.05) in healthy individuals than in patients.
HLA-KIR genotypes have been associated with susceptibility to a variety of diseases such as psoriatic arthritis, type I diabetes, infectious diseases, cancer, and reproduction. Just a few of these studies revealed an influence of HLA-KIR gene interactions on disease outcome [9]. Others studies have investigated the frequency of KIR genes in patients with hematologic malignancies [22]. Most of these investigations have been performed in patients with different diseases such as AML, CML and MDS [23–27].
There are several studies that investigate the association between KIR haplotypes distribution and diseases; it is observed that Haplogroups A and B vary considerably between ethnic groups [15, 28–30]. The association of KIR gene/haplotypes has been investigated in patients with indication for hematopoietic stem cell transplantation, highlighting the role of KIR genes in the transplant outcome [7].
The stratification of the patients according to AML vs ALL groups, revealed that the KIR2DS3 gene presented higher frequencies (p = 0.0031) in AML when compared to ALL patients (Fig. 2). Although the sample size was relatively small, recent publications suggest important roles for specific KIR genes that may influence allogeneic hematopoietic stem cell transplantation (HSCT) outcome in HLA-compatible siblings, GvHD, relapse and other complications related to transplantation [31] in small and heterogeneous samples [32].
A study in China investigated KIR genotypes in 54 patients with hematopoietic malignancies, classified into two risk groups: standard and high. The frequency of activating KIR genes in standard-risk group was higher when compared to the high-risk group, specifically for KIR2DS1, KIR2DS2 and KIR3DS1. A secondary analysis of this study, comparing standard-risk group vs high-risk group in AML patients, revealed higher frequencies of activating KIR genes in the standard-risk group, particularly for KIR2DS1, KIR2DS2, and KIR2DS3 genes, the latter one in agreement with our findings in AML patients group [22].
In the same line of investigation, Kim and colleagues reported the influence of KIR genes in AML patients and HLA compatible donor siblings after HSCT. All the activating KIR genes in the donors showed an important role in transplant outcome and in the occurrence of acute graft-versus-host disease (GvHD) in HSCT in AML patients. Particularly, the KIR2DS2 gene and the allele KIR2DS4*003 were correlated with acute GvHD. This evidence suggests an immunogenic specificity in the Korean population compared to Caucasians since the frequency of KIR2DL2 and KIR2DS2 genes are comparatively lower in Koreans than in other countries. Long-term survival was noted even if the KIR2DS1 gene was only present in the donor and not in the recipient. The presence of both genes KIR2DS3-KIR2DS5 was more frequently found in a variety of complications related to transplant [31].
Mancusi and colleagues reported that donors, possessing KIR2DS1, KIR3DS1 or both activating genes, showed reduced infection rates and mortality, and a better event-free survival (EFS) [33].
Donor cells that express KIR haplotype B have been reported to contribute to relapse protection and improved survival after myeloablative allogeneic transplantation. Haplotype B/x donor cells have also been associated with a higher incidence of chronic GvHD. In HLA-haploidentical transplant setting, adonor KIR B haplotype has been associated with lower risk of relapse for patients with hematologic malignancies [2].
NK cells receptors of the family KIR may confer specific protector effect for different diseases. In a Turkish study carried out in a heterogeneous group of leukemia patients and controls, a protective effect was observed associated with KIR2DL2 and/or KIR2DS2 against CML [32].
From an evolutionary perspective, activating KIRs arose more recently from inhibitorys homologous genes [34]. A wide variation in KIR activators gene frequencies have been reported for different populational groups [35]. Nevertheless, the allelic diversity related to inhibitory receptors genes is limited when compared to activators genes [36]. The strong negative correlation observed between certain activating KIR and its ligands across populations, in contrast to weak positive correlations between several KIR inhibitory genes and their ligands, put forward a hypothesis that a pressure selection mechanism involving autoimmune disease is acting on the maintenance of lower frequencies of activator KIR receptor and their ligands [35].
KIR phenotypes analysis in Belgian leukemia patients indicated significantly higher frequencies of inhibitory KIR2DL1, KIR2DL2, and KIR2DL3 genes, suggesting their contribution for the lack of antitumor responses of NK cells [10]. On another study, conducted in 35 patients with a lymphoproliferative NK cell disease, inhibitory genes KIR2DL5A and KIR2DL5B were more frequently found in patients compared to healthy controls [37].
Epstein-Barr virus has been associated to the development of Hodgkin’s disease in some pathological conditions. An important review summarises current knowledge of the pathogenesis of Hodgkin’s disease with particular emphasis on the association with EBV. Besson and colleagues identified in a family study a stronger protector effect related to KIR2DS1/KIR3DS1 in patients with Hodgkin’s lymphoma [38].
A total of 50 Han Chinese patients were studied to explore the correlation between KIR genes and susceptibility to leukemia. The comparison made between patient and control groups showed lower frequencies of KIR3DL1 and KIR2DL1 genes amongst patients. Additionally, the results highlighted a negative correlation between the pathogenesis of leukemia and KIR3DL1, KIR3DS1, KIR2DL1, and KIR2DL5 genes [39], a very suggestive finding that KIR polymorphisms are associated with susceptibility to leukemia in Hans.
In the present study, patients had lower frequencies of KIR3DS1 (26 % versus 39 %) and KIR2DL5 (28 % versus 53 %) as compared to healthy family members group.
McQueen and collaborators analyzed KIR genes repertoire in donors, and found that KIR2DS3 conferred a protective effect against chronic GvHD in transplantation with HLA-compatible unrelated donor [40] and with donors who have more than four activating KIR in haploidentical transplants.
NK cells were components previously not recognized in HSCT rejection process and GvHD. More recently, investigation of human health/disease effects associated with KIR receptors have been reported and the majority of the described associations have been with activators KIR genes. Our data suggest that susceptibility to leukemia can be influenced, at least, partly by KIR receptors and an increased sample can confirm these findings for further investigations.