Investigation of an enhancer-based regulation of LILRB1 gene and the differential functions of LILRB1 variants in natural killer cells

• Author / Creator

  Yu, Kang

• Human cytomegalovirus (HCMV) causes severe disease in immunocompromised people such as transplant patients. NK cells are crucial in controlling HCMV whereas HCMV developed multiple strategies to evade NK cell surveillance. HCMV encodes a human MHC-I homolog called UL18 to target an inhibitory receptor called leukocyte immunoglobulin-like receptor B1 (LILRB1) expressed on NK cells. LILRB1 is also broadly expressed on other immune cells and associated with viral infection, autoimmune diseases, and cancer. LILRB1 expression exhibits dramatic heterogeneity among different types of immune cells and LILRB1 gene transcription in lymphoid and myeloid cells arises from the distal promoter and the proximal promoter, respectively. LILRB1 is expressed on subsets of human NK cells and the frequency of LILRB1-positive NK cells differs among people. I verified in this thesis that NK clones have either single or double allelic expression. Notably, the frequency of LILRB1-positive NK cells has been shown to increase in the context of HCMV infection. Our group demonstrated that LILRB1 polymorphisms are associated with the frequency of LILBR1+ NK cells, and there are “high” and “low” haplotypes involving the SNPs in the regulatory regions that are correlated with relatively high and low frequency of LILRB1-positive NK cells, respectively. Intriguingly, our group found that the kidney transplant patients homozygous for the SNPs linked with the “low” haplotype were more susceptible to HCMV infection. This thesis aimed to explore the mechanism contributing to the LILRB1 heterogeneity on NK cells and investigate how LILRB1 polymorphisms influence the NK cell response controlling HCMV in transplant patients.
  To understand by what mechanism polymorphisms may influence the LILRB1 expression in NK cells, our group previously compared the distal promoter activity from the two haplotypes using a luciferase reporter assay in an NK cell line but did not detect any difference. We further found one CpG site in the distal promoter of the “high” haplotype had a higher methylation rate compared with the “low” haplotype. In this thesis, I characterized a 3.2 kb enhancer in the intron 1 of the LILRB1 gene in NK cells. This polymorphic region possesses multiple YY1 sites and the promoter/enhancer complexes can be isolated using the YY1 antibody in a ChIP-loop assay. CRISPR-mediated deletion of this region reduced LILRB1 expression in an NK cell line. Together, these results suggest that the intronic enhancer positively regulates LILRB1 transcription in NK cells through the scaffold function of YY1.
  There are four non-synonymous SNPs in the region coding the ligand-binding domains of LILRB1 and two of which are strongly linked with the expression-correlated SNPs. The second part of my thesis investigated whether those SNPs influence the function of LILRB1 on NK cells. I found the two naturally occurring LILRB1 variants expressed in a model NK cell line showed functional differences with target cells expressing UL18 and classical MHC-I, but not with HLA-G. The altered functional recognition was recapitulated in a binding assay with the purified binding domains of LILRB1. Interestingly, the stronger binder is linked with the “low” haplotype and worse control of HCMV in transplant patients. Each of the four substitutions contributes to the binding tested and one SNP controls the addition of an N-linked glycan which is also important to ligand binding. These findings indicate that specific LILRB1 alleles correlated with poor control of HCMV are restricted by limiting surface expression on NK cells.
  Additionally, I observed that UL18 transduction of a B cell line could induce cell aggregation. This phenotype was not influenced by the functional blocking of LILRB1. CRISPR-mediated knockout of the LILRB1 gene indicated that LILRB1 is important in the formation while dispensable for maintaining the phenotype. I further revealed that this phenotype was dependent on LFA-1. However, neither the LFA-1 expression nor the LFA-1 activating status was notably changed by the UL18 transduction. Nevertheless, these results uncovered a novel function of UL18 in cell adhesion that is potentially involved in HCMV pathogenesis.
  Collectively, this thesis extends our knowledge of LILRB1 transcriptional regulation and LILRB1 heterogeneity in NK cells. It also explains how the genetic variation of the LILRB1 gene influence the differential responses of NK cells to HCMV, ultimately may inform developing LILRB1 as a marker to predict the outcome of HCMV in the transplant patients.

• Subjects / Keywords

  • Polymorphism
  • UL18
  • NK
  • Transcription
  • LILRB1
  • Cell adhesion
  • Enhancer
  • HCMV

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-y4rc-4p78

• License

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