Characterization of the single-cell adaptive immune landscape and its clinical implications in high-grade serous ovarian cancer

Abstract Background High-grade serous ovarian cancer (HGSOC) remains the most lethal gynaecological malignancy, and its limited response to immunotherapy is largely attributed to poor T cell infiltration, functional exhaustion, and an incompletely characterised immune microenvironment. Although single-cell studies have begun to resolve the cellular ecosystems of HGSOC, the functional states, clonal architecture, and clinical relevance of tumour-infiltrating T and B lymphocytes—particularly the interplay between immunosuppressive T cell subsets and protective humoral responses—remain poorly defined. Methods We prospectively collected paired tumour and peripheral blood samples from five treatment-naïve HGSOC patients and performed integrated single-cell RNA sequencing, TCR/BCR repertoire sequencing, and flow cytometry. T and B cells were isolated by FACS and profiled using the 10x Genomics platform. Clonotype diversity, somatic hypermutation (SHM), and class-switch recombination (CSR) were analysed using scRepertoire, SHazaM, and Alakazam. Cell–cell communication was inferred with CellChat. Findings were validated by flow cytometry in an independent cohort ( n = 33), by IHC and mIHC in platinum-sensitive and platinum-resistant tumours, and by CIBERSORT deconvolution of the TCGA and ICGC OV cohorts with survival analysis. Results We obtained 35,103 high-quality single cells comprising 17,667 T cells and 17,436 B cells. The immune landscape was highly compartmentalised: CD4 Tregs and CD8 exhausted T (Tex) cells were enriched in tumours, whereas naïve/memory lymphocytes predominated in blood. TCR analysis revealed clonal expansion of CD8 Tex and CD4 Tregs within the tumour microenvironment (TME), with CD8 Tex showing the greatest expansion and a differentiation trajectory from Teff/Tm precursors. We identified a tumour-enriched HLADR-high Treg subset characterised by elevated MHC class II expression, oxidative phosphorylation, and TNFR2 upregulation. This subset was associated with platinum resistance and poor survival, and cell–cell communication analysis revealed dense TNFR2- and MIF/IL16-mediated crosstalk with CD8 Tex cells, which was corroborated by IHC and mIHC in platinum-resistant tumours. On the B cell side, tumour infiltration of plasmablasts and memory B cells was accompanied by active SHM and CSR to IgG/IgA, indicating antigen-driven activation. A high plasmablast signature was an independent favourable prognostic factor in both TCGA and ICGC cohorts, associated with an immunologically active TME, and mIHC confirmed fewer plasmablasts in platinum-resistant tumours Conclusion This study provides a comprehensive single-cell atlas of the adaptive immune landscape in HGSOC, revealing an HLADR-high Treg–TNFR2 axis that drives CD8 T cell exhaustion and platinum resistance, alongside a protective plasmablast signature linked to favourable prognosis. These findings highlight key immunosuppressive drivers and protective humoral responses, with implications for patient stratification and the development of combination immunotherapies in HGSOC.