Examining the tolerance effect to non-inherited maternal antigen in allogeneic cord blood mononuclear cells in response to allogeneic stimulator cells

Abstract

Abstract of thesis entitled “Examining the tolerance effect to non-inherited maternal antigen in allogeneic cord blood mononuclear cells in response to allogeneic stimulator cells” Submitted by Dr Janette Kwok for the degree of Doctor of Philosophy at The University of Hong Kong in November 2018

Cord blood (CB) is an alternative stem cell source for allogeneic haematopoietic stem cell transplantation (HSCT). Its advantages lie in higher permissibility for human leukocyte antigen (HLA) mismatch, rapid availability and relatively risk-free cell collection procedures. The success of HSCT relies on best matching between donor and recipient at HLA-A, -B, -C, and -DRB1 loci. Since HLA is highly polymorphic and population-specific, optimally HLA-matched unrelated donors or cord blood units (CBUs) may not be available. In this thesis, we aimed to: 1) calculate the HLA haplotype frequencies and estimate the probability of identifying HLA matched donors in Hong Kong; 2) estimate the optimal donor pool size in the Hong Kong Bone Marrow Donor Registry (HKBMDR); 3) calculate the chance of increasing matched CBUs based on the inclusion of non-inherited maternal antigens (NIMA) as acceptable HLA mismatches; and 4) determine the allo-reactivities of CB mononuclear cells upon stimulation by maternal, paternal and unrelated stimulator cells.

In scientific paper I, gene and haplotype frequencies of HLA typing for volunteer donors from HKBMDR were determined and the results compared with other registries for population genetics study. Similar population genetics study was done on the Hong Kong Catherine Chow Cord Blood Bank in scientific paper II. Top 20 haplotype frequencies were comparable in the Hong Kong population structure despite significant population increase from 5 to 7 million over the past decade.

In scientific paper III, the study explored the likelihood of identifying HLA-matched donors in Hong Kong. The matching probabilities were 45% at 8/8 match and 65% at 7/8 match. By increasing the size of registry to 200,000, these probabilities become 54% and 73% at 8/8 and 7/8 matches respectively. With better donor recruitment targets and planning, a cost-effective registry with bigger donor pool size could increase matching chance and hence HSCT success.

In scientific paper IV, the chance of matched CBUs in Hong Kong by including NIMA acceptable HLA mismatches in donor selection was determined. The donor-recipient mismatch, identical to the NIMA CBU designated as 6/6 “virtual” HLA match, showed similar outcomes to 6/6 inherited HLA matched CBU. Such virtual HLA match of CBUs could be generated by replacing the inherited alleles with one or more NIMAs. Results showed that 6/6 HLA virtual matched CBU increased the effectiveness of NIMA matching by 50%, in line with the Center for International Blood and Marrow Transplant Research study. The elucidation of donors’ maternal HLA genotypes can offer significant numbers of 6/6 virtually matched CBUs and is potentially cost-effective.

In scientific paper V, cytotoxicity, cytokine profiling, cell proliferation assay, and enzyme linked immunosorbent assay was utilized to determine the allo-reactivities of CB mononuclear cells upon stimulation by maternal, paternal and unrelated third party cells with or without the presence of Regulatory T cells. It was found that NIMA had effect on CB tolerance to maternal antigens but not paternal antigens and Regulatory T cells might play a significant role in immune response.