Baricitinib reveals LGALS3BP as a novel pathogenic factor and therapeutic target in immune thrombocytopenia

Abstract Introduction Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder characterized by immune-mediated platelet destruction and impaired platelet production due to megakaryocyte (MK) dysfunction. However, the factors driving MK dysfunction in ITP remain poorly understood. Baricitinib, an oral JAK1/2 inhibitor, can alleviate innate and adaptive immune dysregulation without inducing thrombocytopenia and has shown efficacy in patients with relapsed ITP. In a recent open-label pilot study (n=35), baricitinib induced durable platelet responses in ~57% of steroid-resistant ITP patients, supporting the notion that cytokine-driven pathways contribute to the pathogenesis of ITP. Galectin-3-binding protein (LGALS3BP) is a secreted glycoprotein involved in immune regulation and fibrosis. Recent studies have implicated LGALS3BP in autoimmunity and inflammation—for example, platelet-derived LGALS3BP induces proinflammatory cytokine release by myeloid cells in systemic lupus erythematosus (SLE)—and in fibrosis, where LGALS3BP has emerged as a promising therapeutic target in TGF-β1–driven disease. Notably, LGALS3BP expression can be induced by inflammatory cytokines such as interferon-α, suggesting that it might link immune dysregulation to MK pathology in ITP. We hypothesized that LGALS3BP is a previously unrecognized contributor to ITP pathogenesis and evaluated its potential as a translational therapeutic target via pharmacologic inhibition in vitro. Methods Plasma LGALS3BP levels were measured by ELISA in adults with primary ITP (n=10) and healthy controls (n=10). Bone marrow aspirates (ITP, n=12; controls, n=12) were analyzed by flow cytometry to assess megakaryocyte LGALS3BP expression (intracellular vs. surface). Single-cell RNA sequencing (scRNA-seq) of bone marrow mononuclear cells (ITP, n=3; healthy, n=2) was performed to evaluate LGALS3BP mRNA expression in the MK clusters. Gene set variation analysis (GSVA) with AUCell scoring was used to quantify JAK/STAT pathway activity in the MK clusters. PMA-differentiated MEG-01 cells (an in vitro MK model) were treated with baricitinib (1 μM) for 72 hours. A P value < 0.05 was considered statistically significant. Results Compared with that in healthy controls, plasma LGALS3BP levels were significantly elevated in ITP patients (median, 14.2 vs. 5.8 μg/mL, P = 0.004; ~2.4-fold increase). Bone marrow flow cytometry revealed ~2.1-fold greater intracellular LGALS3BP levels (mean fluorescence intensity) in ITP MKs than in control MKs (P < 0.01), with no difference in surface expression, indicating that excess LGALS3BP is retained intracellularly (rather than secreted) in ITP MKs. Consistently, single-cell RNA-seq revealed upregulated LGALS3BP transcription levels in the MK clusters from ITP patients (3.5-fold higher than those from healthy controls, P < 0.001). Thus, to our knowledge, this is the first demonstration that LGALS3BP is markedly overexpressed in ITP patient samples, implicating this factor in the impaired platelet production in this disease. In vitro, baricitinib treatment of PMA-differentiated MEG-01 cells reduced LGALS3BP protein expression by ~60% relative to that in untreated cells (immunoblot densitometry ratio, 0.42 vs. 1.00, P < 0.01). Unexpectedly, GSVA/AUCell analysis indicated that JAK/STAT signaling activity in ITP MK clusters was significantly lower than that in healthy controls (normalized enrichment score, –1.35 vs. 0.47, P < 0.01). This paradox indicates that LGALS3BP upregulation in ITP is not driven by hyperactive JAK/STAT signaling and that baricitinib may reduce LGALS3BP via JAK-independent mechanisms or off-target effects. Notably, the ability of baricitinib to suppress MK LGALS3BP may represent a novel mechanism of action underlying its therapeutic efficacy in ITP. Conclusions Our findings establish LGALS3BP as a novel pathogenic factor in ITP and provide new insight into disease pathophysiology; patients exhibit abnormally high LGALS3BP levels in plasma and MKs, likely contributing to impaired platelet production. Notably, baricitinib, which has demonstrated clinical efficacy in ITP, downregulated MK LGALS3BP expression, suggesting a therapeutic mechanism beyond JAK/STAT inhibition. Together, these results highlight LGALS3BP as a promising therapeutic target in ITP.