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In immune-mediated acquired aplastic anemia (AA), the presence of an HLA allele, which is highly overrepresented or lost due to somatic mutations, may represent a specific immune pathophysiology and a clinical manifestation. HLA-B*14:02 is one of the most overrepresented class I alleles in AA and is also frequently affected by a somatic loss of expression; the inherited B*14:02 genotype was correlated with high-risk clonal evolution in two independent cohorts in the U.S. (Babushok DV et al. Blood Adv 2017; Zaimoku Y et al. manuscript in preparation). In contrast, HLA-B*14:02 is virtually absent in Japanese, in whom somatic mutations of AA have frequently been detected in HLA-B*40:02, B*54:01, and A*02:06, and occasionally in A*02:01, A*02:07, A*31:01, B*13:01, B*40:01, B*40:03, B*44:03, B*55:02, and B*56:01 (Mizumaki H et al. Haematologica 2021). A class II allele HLA-DRB1*15 is highly overrepresented in AA across various ethnic groups, including those in the U.S. and Japanese. This retrospective study in the Japanese population aimed to explore the clinical significance of disease-associated non-B*14:02 HLA class I and II alleles in AA. A total of 423 enrolled patients with AA (very severe [n = 81], severe [n = 266], transfusion dependent non-severe [n = 76]; median age 60 [range, 1-86] years) had undergone genotyping for HLA-A, HLA-B, HLA-C, and HLA-DRB1 at 2-field resolution. The HLA allele frequencies in these patients were compared to those in a Japanese HLA haplotype dataset (n = 19183; Ikeda N et al. Tissue Antigens 2016). The most overrepresented allele in AA was HLA-DRB1*15:02, followed by DRB1*15:01, B*40:02, and A*02:06 (Table); DRB1*13:02 and B*44:03, which are in linkage disequilibrium, were markedly underrepresented, consistent with a well-known protective role of DRB1*13 against autoimmune diseases. HLA-DRB1*15:02 was also significantly correlated with age and its frequency among patients aged <50 years was below the level of the control group (Figure A). Frequencies of HLA-DRB1*15:01, B*40:02, and A*02:06 were increased in both older and younger groups. HLA-A*31:01 and A*02:01 found to be enriched in young patients instead of DRB1*15:02 (Figure B); B*13:01, B*55:02, and B*56:01 showed a similar tendency for an early onset. HLA-B*40:02 was correlated with disease severity and was especially overrepresented in very severe AA (Figure C). The overall response rate to anti-thymocyte globulin-based immunosuppressive therapy at 6 months was 63% (139 of 220 treated and evaluable patients). A trend for a higher response was observed in patients harboring mutation-related HLA-B alleles (except for minor alleles B*13:01, B*40:03, and B*55:02) and the highly overrepresented or protective HLA-DRB1 alleles, but not in the HLA-A alleles (Figure D). A multivariate logistic regression revealed that the combination of the presence of any favorable alleles in HLA-B (odds ratio 3.6, P < 0.0001) or in HLA-DRB1 (odds ratio 2.3, P = 0.00085) was significantly and independently associated with a hematologic response; the tendencies for a lower or higher response in very severe disease and the presence of paroxysmal nocturnal hemoglobinuria clone did not reach statistical significance. Further, there was likely an additive effect when two favorable alleles coexisted in HLA-B or HLA-DRB1 (Figure E); the copy number of the favorable HLA-B and HLA-DRB1 alleles stratified the response rate to four groups: three or four copies, 95% (19 of 20); two copies, 72% (61 of 85); one copy, 59% (50 of 85); and zero copy, 30% (9 of 30). Only eight patients displayed clonal evolution to monosomy 7, myelodysplastic syndrome, or acute myeloid leukemia after immunosuppression without significant overrepresentation or underrepresentation of the pathogenic HLA alleles. Using a large dataset of homogeneous Japanese population with high-resolution HLA typing, we revealed, for the first time, a strong relationship between disease-associated (overrepresented, inactivated, or protecting) HLA alleles and the responsiveness to immunosuppressive therapy. Figure 1 Figure 1. Disclosures Takamatsu: Bristol-Myers Squibb: Honoraria, Research Funding; SRL: Consultancy; Adaptive Biotechnologies, Eisai: Honoraria; Janssen: Consultancy, Honoraria, Research Funding. Yamazaki: Novartis Pharma: Honoraria; Kyowa Kirin: Honoraria; Kyowa Kirin: Research Funding. Nakao: Symbio: Consultancy; Kyowa Kirin: Honoraria; Novartis Pharma: Honoraria; Alexion Pharma: Research Funding.

The prognostic value of minimal/measurable residual disease (MRD) detection in autografts of patients with multiple myeloma (MM) in an autologous stem‐cell transplantation setting has been reported. Next‐generation flow (NGF) cytometry has lower sensitivity (2 × 10−6) to detect MRD than next‐generation sequencing (NGS) (<10−6). We compared the clinical value of high‐sensitivity NGF (cutoff: <10−6) and NGS (cutoff: 10−6) for the detection of MRD in the cryopreserved autografts of 49 patients with newly diagnosed MM. The sensitivity test using frozen/thawed autografts revealed a strong correlation among MRD levels of 5 × 10−7 and 1 × 10−4 (r = 0.9997, p < 0.0001) when an adequate number of cells were analyzed. Autograft MRD levels determined using NGF and NGS were highly correlated (r = 0.811, p < 0.0001). MRD‐negative patients identified with NGF (cutoff: <10−6) showed significantly longer progression‐free survival (PFS) than MRD‐positive patients (p = 0.026). The PFS of MRD‐negative patients determined by NGS (cutoff: 10−6) was similar to that determined by NGF. These results show that the high‐sensitivity NGF method can assess MRD in frozen/thawed autografts, and its prognostic value is comparable to that of NGS.