Multi-modal single-cell and genetic integration defines cytotoxic T-cell regulatory states, HSPC suppression and inherited susceptibility in aplastic anaemia

Acquired aplastic anaemia is caused by immune-mediated loss of haematopoietic stem and progenitor cells (HSPCs), but the regulatory states that sustain cytotoxic immunity and their relationship to inherited susceptibility remain incompletely understood. We integrated two single-cell RNA-sequencing cohorts spanning healthy, non-severe and severe aplastic anaemia with single-cell chromatin accessibility profiling, genome-wide association meta-analysis, Bayesian fine-mapping and stratified LD-score regression. Single-cell transcriptomics revealed a coordinated shift across the immune and haematopoietic compartments. Cytotoxic CD8 and {gamma}{delta} T cells converged on a shared NKG7/CCL5/PRF1 effector program, indicating that cytotoxic differentiation extends across T-cell lineages. Effector-memory T cells combined inflammatory signalling with SOCS, DUSP, TNFAIP3, RGS1 and TOX, consistent with sustained stimulation accompanied by extensive feedback regulation. With increasing disease severity, these inflammatory states were further coupled to hypoxic, oxidative and unfolded-protein-response programmes, suggesting qualitative remodeling of the immune compartment rather than uniform amplification of perforin-granzyme expression. Single-cell chromatin accessibility provided a regulatory counterpart to these transcriptional states. Naive and memory-associated cells retained TCF7/LEF1/BACH2 accessibility, whereas cytotoxic cells acquired coordinated accessibility across CCL5, NKG7, PRF1, granzymes and killer-receptor loci. Pseudotime, motif activity and integrated RNA-chromatin profiles positioned AP-1, NFAT and TBX21 along this transition, linking loss of memory-associated regulation to acquisition of cytotoxic effector competence. Genetic meta-analysis independently recovered association at the HLA-B region, reinforcing antigen presentation as the principal inherited susceptibility axis. Fine-mapping additionally prioritized a non-HLA locus without resolving its effector gene, while stratified LD-score regression found no detectable preferential enrichment of common-variant heritability within effector-memory or cytotoxic regulatory elements. Integrated with the cellular data, these findings support a mechanistic hierarchy in which HLA-linked antigen presentation establishes the selective context, persistent cytotoxic T-cell state remodeling maintains pathogenic immune pressure, and IFN{gamma}-responsive HSPC suppression translates this pressure into haematopoietic failure.