Extrafollicular B cell responses drive pathogenic antibodies production in Heparin-induced thrombocytopenia 2259334

Introduction Heparin-induced thrombocytopenia (HIT) is a severe complication of heparin therapy caused by IgG antibodies (Abs) against platelet factor 4—heparin (PF4/H) complexes. These Abs arise rapidly, mimicking a memory response even on first exposure, yet decline quickly and recur with similar kinetics. The mechanisms remain unclear. Methods CITE-seq combined with scVDJ-seq was performed on B cells from HIT patients, non-HIT patients (NHIT) who developed PF4/H Abs without developing HIT, and healthy donors (HD). Integrated transcriptomic, surface protein, and repertoire analyses were used to define B cell subsets and clonal features. Selected clones were expressed and functionally evaluated. Results Compared to HD, HIT and NHIT patients showed expansion of activated naïve and atypical memory B cells (AtyMBCs), strongest in HIT. B cell repertoires and cloned monoclonal Abs from HIT displayed lower mutation rates, longer HCDR3s enriched for basic and tyrosine residues, broad poly- and autoreactivity, indicative of an extrafollicular (EF) response. These features were present in naïve B cells but were most pronounced in AtyMBCs. Preferential VH3—JH6 recombination and enrichment of clones bearing structural motifs associated with pathogenicity were observed in HIT naïve and AtyMBCs, along with elevated IFN-γ, TLR, and BCR signaling. Conclusion Heparin exposure selectively expands pre-existing autoreactive B cells via an EF response, driving the rapid production of pathogenic PF4/H Abs characteristic of HIT. Funding Source R01 HL130724, R01HL161127, R01HL148120, AHA 25PRE1374068 Topic Categories Immune Mechanisms of Human Disease (HUM)