KIR3DL3 allelic diversity: six new alleles exhibit both conservative and non-conservative substitutions

Conservation, Extensive Heterozygosity, and Convergence of Signaling Potential All Indicate a Critical Role for KIR3DL3 in Higher Primates

Natural killer (NK) cell functions are modulated by polymorphic killer cell immunoglobulin-like receptors (KIR). Among 13 human KIR genes, which vary by presence and copy number, KIR3DL3 is ubiquitously present in every individual across diverse populations. No ligand or function is known for KIR3DL3, but limited knowledge of expression suggests involvement in reproduction, likely during placentation. With 157 human alleles, KIR3DL3 is also highly polymorphic and we show heterozygosity exceeds that of HLA-B in many populations. The external domains of catarrhine primate KIR3DL3 evolved as a conserved lineage distinct from other KIR. Accordingly, and in contrast to other KIR, we show the focus of natural selection does not correspond exclusively to known ligand binding sites. Instead, a strong signal for diversifying selection occurs in the D1 Ig domain at a site involved in receptor aggregation, which we show is polymorphic in humans worldwide, suggesting differential ability for receptor aggregation. Meanwhile in the cytoplasmic tail, the first of two inhibitory tyrosine motifs (ITIM) is conserved, whereas independent genomic events have mutated the second ITIM of KIR3DL3 alleles in all great apes. Together, these findings suggest that KIR3DL3 binds a conserved ligand, and a function requiring both receptor aggregation and inhibitory signal attenuation. In this model KIR3DL3 resembles other NK cell inhibitory receptors having only one ITIM, which interact with bivalent downstream signaling proteins through dimerization. Due to the extensive conservation across species, selection, and other unusual properties, we consider elucidating the ligand and function of KIR3DL3 to be a pressing question.

A small, variable, and irregular killer cell Ig-like receptor locus accompanies the absence of MHC-C and MHC-G in gibbons

The killer cell Ig-like receptors (KIRs) of NK cells recognize MHC class I ligands and function in placental reproduction and immune defense against pathogens. During the evolution of monkeys, great apes, and humans, an ancestral KIR3DL gene expanded to become a diverse and rapidly evolving gene family of four KIR lineages. Characterizing the KIR locus are three framework regions, defining two intervals of variable gene content. By analysis of four KIR haplotypes from two species of gibbon, we find that the smaller apes do not conform to these rules. Although diverse and irregular in structure, the gibbon haplotypes are unusually small, containing only two to five functional genes. Comparison with the predicted ancestral hominoid KIR haplotype indicates that modern gibbon KIR haplotypes were formed by a series of deletion events, which created new hybrid genes as well as eliminating ancestral genes. Of the three framework regions, only KIR3DL3 (lineage V), defining the 5' end of the KIR locus, is present and intact on all gibbon KIR haplotypes. KIR2DL4 (lineage I) defining the central framework region has been a major target for elimination or inactivation, correlating with the absence of its putative ligand, MHC-G, in gibbons. Similarly, the MHC-C-driven expansion of lineage III KIR genes in great apes has not occurred in gibbons because they lack MHC-C. Our results indicate that the selective forces shaping the size and organization of the gibbon KIR locus differed from those acting upon the KIR of other hominoid species.

Seventeen novel alleles add to the already extensive KIR3DL3 diversity

Exons 2-9 of KIR3DL3 alleles were characterized by genomic DNA sequencing in two families and in 78 bone marrow transplant samples. Several strategies were used to isolate single alleles for characterization and to resolve alternative allele combinations. We describe 17 novel 3DL3 alleles carried by 30 individuals. Compared with the most closely matched alleles, the new alleles differ by from one to three nucleotides and from zero to three amino acids. The majority of the substitutions were shared with other 3DL3 alleles although three novel polymorphic codons, 151 (exon 4), 327 (exon 7) and 352 (exon 9), are described. Of the 36 different 3DL3 alleles detected in the transplant population, the three most common alleles accounting for 47% of the total were KIR3DL3*00101 (13.5%), KIR3DL3*003 (21.2%) and KIR3DL3*00901 (12.2%).

Comprehensive approach to high-resolution KIR typing

Multiple studies suggest that prospective KIR typing may be beneficial for the outcome of bone marrow transplants, but to date no practical high-resolution KIR typing system has been developed. Here we propose a comprehensive high-resolution typing approach that provides allele level KIR typing. Based on the low-resolution typing obtained by SSO, the 14 KIR loci are divided in groups according to the level of polymorphism in exons coding for extracellular Ig-like domains and cytoplasmic tail. The first group is typed by sequence-specific oligonucleotide only; the second is typed by sequence-based typing (SBT) based on the amplification of a fragment coding the Ig-like domains; and the third is typed by SBT based on amplification of a fragment coding the cytoplasmic tail. SBT for the fourth group includes both the Ig-like and cytoplasmic domains. Because of a considerable number of polymorphisms scattered throughout all nine KIR exons, SBT results may still produce a number of ambiguities, which can be resolved by sequence-specific primers. This combined high-resolution approach was applied to the complete KIR typing of 205 Caucasian hematopoietic stem cell donors in support of the National Marrow Donor Program High-resolution KIR Typing Pilot Project. High-resolution typing of several KIR loci produced numerous novel alleles, whereas some loci demonstrated very limited polymorphism. Several of the novel alleles appeared in more than four donors, suggesting that these alleles are not rare. Our results showed that the comprehensive KIR typing approach presented here provides the balance of high-resolution typing and cost effectiveness.

Diversity of the killer cell immunoglobulin-like receptor gene KIR2DS4 in the Chinese population

Human killer cell immunoglobulin-like receptors are a subfamily of the immunoglobulin superfamily, which map to the leukocyte receptor complex on chromosome 19. Here, we established polymerase chain reaction-sequence-based typing (PCR-SBT) procedures to identify alleles of the KIR2DS4 gene. The method was designed around the specific amplification of exons 4-5 of the KIR2DS4 gene. Genomic DNA from 105 healthy unrelated Chinese Han individuals were typed for the KIR2DS4 alleles. Each sample was assigned the KIR2DS4 alleles combination, consistent with the pairwise combinations of sequences of all the known KIR2DS4 alleles. We observed eleven different genotypes and four KIR2DS4 alleles in the population, with the KIR2DS4*00101 having the highest frequency, 0.576, and also confirmed the new KIR2DS4*007 allele. Our data demonstrated that the established PCR-SBT method for the KIR2DS4 allele typing was reliable.