Body: HLA mismatch in allogeneic hematopoietic stem cell transplantation (allo-HSCT) serves as a marker for chimerism monitoring. A panel of anti-HLA antibodies covering the prevalent HLA class I alleles in Hong Kong Chinese allows lineage-specific analysis by flow cytometric chimerism assay. With a limit of detection (LOD) of ~1%, it detects low-level chimerism that may precede relapse or graft failure. Three clinical cases were investigated for lineage-specific chimerism and the HLA loss of heterozygosity (LOH).
Case 1: A patient with β-thalassemia major underwent allo-HSCT from a haploidentical donor. Lineage-specific chimerism was performed after seven months by short tandem repeat (STR) chimerism for T-cells and granulocytes lineages. Flow cytometric chimerism assay used an anti-Bw6 antibody to target the mismatched HLA-B*55:02 allele. Both results indicated that T-cells and granulocytes chimerism were >99% donor, with cell surface staining, further demonstrating 100% donor chimerism in both B-cell and NK-cell.
Case 2: A patient with T cell post-transplantation lymphoproliferative disorder underwent allo-HSCT from a haploidentical donor. STR results showed 95% donor chimerism 3 months after HSCT. Increased γδ T cells were observed by immunophenotyping, but the origin remains indeterminate without a cell isolation kit. Flow cytometric chimerism assay was employed with an anti-HLA-A26 to target the mismatched A*26:01 allele in the αβ+ and γδ+ T cells labelled with specific antibodies. In the 45% donor chimerism of the CD3+ T cells detected, 28% of the αβ+ T cells and 87% of γδ+ T cells were from the donor.
Case 3: A patient with T-cell acute lymphoblastic leukaemia underwent sequential allo-HSCT from a haploidentical donor and cord blood unit (CBU). The STR result showed 100% engraftment of the CBU on D30 post-HSCT and a decline of donor chimerism from 100% to 82% on D75. Flow cytometric chimerism assay using anti-HLA-A11.2 serum failed to express the mismatch of the A*11:02 allele. The expression of HLA-B75/B76 and HLA-Cw8 shared by both the patient and the CBU was observed. It suggested there was HLA LOH in the blast cells.
Conclusion: Flow cytometric chimerism assays monitor engraftment across multiple cell lineages using HLA mismatches. This sensitive tool enables early relapse detection and improves understanding of immune reconstitution, facilitating timely intervention and personalized treatment.
