Centromeric AA motif in KIR as an optimal surrogate marker for precision definition of alloimmune reproductive failure

Abstract

Throughout pregnancy, the decidua is predominantly populated by NK lymphocytes expressing Killer immunoglobulin-like receptors (KIR) that recognize human leukocyte antigen-C (HLA-C) ligands from trophoblast cells. This study aims to investigate the association of KIR-HLA-C phenotypes in couples facing infertility, particularly recurrent pregnancy loss (RPL) and recurrent implantation failure (RIF), in comparison to a reference population and fertile controls. This observational, non-interventional retrospective case-control study included patients consecutively referred to our Reproductive Immunology Unit from 2015 to 2019. We analyzed the frequencies of KIR and HLA-C genes. As control groups, we analyzed a reference Spanish population for KIR analysis and 29 fertile controls and their male partners for KIR and HLA-C combinations. We studied 397 consecutively referred women with infertility and their male partners. Among women with unexplained RPL (133 women) and RIF (176 women), the centromeric (cen)AA KIR genotype was significantly more prevalent compared to the reference Spanish population (p = 0.001 and 0.02, respectively). Furthermore, cenAA was associated with a 1.51-fold risk of RPL and a 1.2-fold risk of RIF. Conversely, the presence of BB KIR showed a lower risk of reproductive failure compared to non-BB KIR (OR: 0.12, p < 0.001). Women and their partners with HLA-C1C1/C1C1 were significantly less common in the RPL-Group (p < 0.001) and RIF-Group (p = 0.002) compared to the control group. Moreover, the combination of cenAA/C1C1 in women with C1C1 partners was significantly higher in the control group than in the RPL (p = 0.009) and RIF (p = 0.04) groups, associated with a 5-fold increase in successful pregnancy outcomes. In our cohort, the cenAA KIR haplotype proved to be a more accurate biomarker than the classic AA KIR haplotype for assessing the risk of RPL and RIF, and might be particularly useful to identify women at increased risk among the heterogeneous KIR AB or Bx population. The classification of centromeric KIR haplotypes outperforms classical KIR haplotypes, making it a better indicator of potential maternal-fetal KIR-HLA-C mismatch in patients.

Keywords: HLA-C Lymphocyte Antigen; Immunoglobulin-Like Receptor; Killer Cell; Natural Killer Cells; Recurrent Implantation Failure; Recurrent Pregnancy Loss; Unexplained Recurrent Pregnancy Loss.

Figure 1

The KIR gene cluster is characterized by its centromeric (Cen) and telomeric (Tel) gene content motifs. Within the KIR locus, the genes are organized in a distinct manner. The centromeric and telomeric regions are separated by a unique recombination site sequence, which allows for the exchange of genes between the centromeric and telomeric motifs. The common motifs contain a set of conserved framework genes, shaded in gray, while the B haplotype genes are represented in orange, and the A haplotype genes are shown in pink. The centromeric genes crucial for haplotype generation, namely 2DL2, 2DL3, and 2DS2, play a pivotal role. The co-occurrence of 2DL2 and 2DL3 results in the AB haplotype, while the presence of solely 2DL3 yields the AA haplotype, and solely 2DL2 leads to the BB haplotype. The 2DS2 allele exclusively manifests within the AB or BB haplotypes. Turning to the telomeric haplotype, key genes involved in its determination include 3DL1, 3DS1, 2DS4, and 2DS1. Specifically, the AB haplotype emerges when a concurrent presence of 3DL1 and 2DS4 accompanies 3DS1 and 2DS1. Conversely, the exclusive presence of 3DL1 and 2DS4 delineates the AA haplotype, while the co-presence of 3DS1 and 2DS1 characterizes the BB haplotype.