CXCL9 is a predictive biomarker of survival with IFNγ-targeted therapy in adult HLH patients

Abstract INTRODUCTION Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory state triggered by diverse conditions including malignancy, infection, and autoimmunity. HLH is fueled by interferon-γ (IFNγ) hyperactivity, resulting in its hallmark clinical and biochemical manifestations. CXCL9, a sensitive and specific peripheral blood surrogate of systemic IFNγ activity, is an emerging biomarker for diagnosing and monitoring HLH. Adult HLH patients (pts) with elevated CXCL9 levels may be candidates for IFNγ-targeted therapeutics. METHODS This was a retrospective study of hospitalized adults with HLH treated with emapalumab (directly neutralizes IFNγ) or ruxolitinib (inhibitor of IFNγ downstream signaling) at Memorial Sloan Kettering Cancer Center or the National Institutes of Health between January 2022–May 2025. Recipients of CAR-T cells within 30-days were excluded. All pts met HLH-2004 criteria and/or had an HScore ≥168 on the day of initiating IFNγ-targeted therapy. Serum CXCL9 and soluble CD25 (sCD25) levels were quantified in CLIA-certified laboratories via Ella™ microfluidic platforms. Overall survival (OS) was calculated from the first dose of IFNγ-targeted therapy. Medians with interquartile ranges are reported. RESULTS Seventy-seven pts were identified. Median age was 49 years (39–60); 42% were female and 69% were White, non-Hispanic. HLH triggers included malignancy (52%), infection (12%), transplant-associated (8%), rheumatologic (5%), and other (23%). Therapies included emapalumab (n=54), ruxolitinib (n=18), or both (n=5). Sixteen pts received therapy without any CXCL9 testing. All pts received prior corticosteroids and 39% received prior etoposide. ICU admission was required in 62%. Ferritin, sCD25, and CXCL9 levels were measured within 72-hours of IFNγ-targeted therapy in 100%, 84%, and 88% of pts, with median values of 12283 ng/mL (4302–30425), 15636 pg/mL (8795–22888), and 23858 pg/mL (10637–72148), respectively. Median OS following IFNγ-targeted therapy was 37 days (9–180), with 30- and 60-day OS rates of 66% and 46%. Decision-tree analysis incorporating HScore, ferritin, sCD25, and CXCL9 identified a CXCL9 level ≥3500 pg/mL (n=54) as predictive of improved OS with emapalumab and/or ruxolitinib treatment (p=0.03), while all other variables were non-significant. Excluding those without CXCL9 testing, median OS was 79 days (26–180) in pts with high (≥3500) levels vs. 8 days (8–99) in pts with low (<3500) levels (HR 0.34, 95% CI: 0.13–0.90, p=0.01). Pts with malignancy-associated HLH (mHLH) had worse 60-day OS (30%) compared to pts with HLH not triggered by malignancy (64%, p<0.001). Only three mHLH pts received ruxolitinib monotherapy. Therefore, a sensitivity analysis excluding ruxolitinib monotherapy pts was performed. Results were consistent, confirming improved 60-day OS in pts with high CXCL9 levels (57% vs. 16%; HR 0.23, 95% CI: 0.08–0.65, p<0.001). Multivariate Cox regression adjusting for demographics, HLH etiology, ICU admission, and etoposide exposure confirmed improved OS in pts with high CXCL9 levels. Notably, the absence of assessing pre-treatment CXCL9 levels prior to initiating IFNγ-targeted therapy associated with markedly poor outcomes, with a median OS of 4 days (3–12) vs. 60 days (17–180) in pts with pre-treatment CXCL9 testing (p<0.001). Time from admission to initiation of IFNγ-targeted therapy was similar between pts with no CXCL9 testing (13 days), low CXCL9 (17 days), and high CXCL9 (10 days). CONCLUSION Adult HLH patients with high CXCL9 levels—indicative of IFNγ-driven hyperinflammation—demonstrated improved OS with emapalumab and/or ruxolitinib compared to those with low CXCL9 levels or without CXCL9 testing. These findings support CXCL9 as a clinically relevant biomarker in adult HLH when considering IFNγ-targeted therapy.