Body: Relapse following haploidentical haematopoietic stem cell transplantation (HSCT) can be driven by immune escape mechanisms, including HLA loss of heterozygosity (LOH). Malignant cells evade donor-derived cytotoxic T-cell recognition through HLA LOH causing donor lymphocyte infusions (DLI) ineffective when the lost HLA is targeted. Investigation of HLA LOH is crucial for appropriate donor selection for subsequent HSCT or DLI. We here present a case of HLA LOH-mediated relapse which was diagnosed using multiple approaches.
A patient diagnosed with T-cell acute lymphoblastic leukemia (T-ALL), underwent allogenic HSCT using a maternal haploidentical donor and a cord blood unit (CBU). Full engraftment of 100% CBU was observed by STR chimerism analysis on Day 30. However, a decline in CBU chimerism from 100% to 82% was detected after Day 46, concurrent with negative anti-HLA antibody results, suggesting early relapse. HLA LOH of the maternal haplotype was suspected which may lead to the immune escape of the blast cells from the graft vs leukemia (GvL) effect of the CBU. Table 1 showed the HLA typing of the patient and donors, in which the mismatched maternal alleles were the targets of investigation. NGS AllTypeTM Fastplex HLA typing assay (One Lambda, US) was used to estimate the proportion of HLA alleles by counting the read depth of the allele-specific variants. An in-house HLA-A*11 sequence-specific primers (SSP) assay was also employed to detect the presence of the maternal A*11:02 allele. Both NGS HLA typing and HLA-A*11 SSP results revealed that the maternal HLA alleles became nearly undetectable by Day 69 (Table 2). Reduced maternal HLA-B*46 and potential duplication of paternal HLA-DPB1*05 was further confirmed by DigitalTRACETM digital PCR chimerism assay (Jeta Molecular, the Netherlands). In addition, loss of HLA-A*11:02 protein expression was demonstrated by flow cytometry. Based on these findings, disease relapse with HLA LOH of the maternal haplotype was confirmed in the patient and the patient's mother was selected as the donor for the next allo-HSCT.
Conclusion: HLA LOH represents a significant immune escape mechanism leading to post-HSCT relapse. Effective identification of HLA LOH can be achieved through comprehensive assessment using NGS HLA typing, SSP assays, digital PCR, and flow cytometry. These can guide the selection of suitable donors for DLI or subsequent HSCT.
