question: Background:Hemophagocytic lymphohistiocytosis (HLH) is a rare and often fatal immune disorder characterized by overwhelming immune activation, an autocrine loop of pro-inflammatory cytokines, resulting in unremitting fever, tissue damage and multiorgan failure. HLH patient survival is an area of high unmet medical need and the development of novel and effective therapeutic strategies remains of high importance. The phosphorylation-dependent activation of the Janus kinase-signal transducers and activators of transcription (JAK-STAT) pathway are hallmarks of the common pathway in HLH, and several clinical trials have been initiated to investigate the role of JAK inhibitors in this disease. Aims:To characterize the immunomodulatory profile and disease modifying potential of ruxolitinib, a JAK1/2 inhibitor, in mouse models of primary (genetic origin) and secondary (immune trigger) HLH. Materials and Methods:For the secondary HLH model, C57BL/6 mice were injected with 50 μg of CpG DNA on days 0, 2, 4, 6, and 8. Starting on day 5, corresponding mice were treated with vehicle, ruxolitinib (P.O.), anti-IL-1R or anti-IL-6R (I.P.). In the primary (genetic) model of HLH, perforin deficient mice were infected with lymphocytic choriomeningitis virus (LCMV) and were treated with either standard or ruxolitinib chow starting day 4 post-infection. In both models, mouse ruxolitinib doses were designed to mimic clinically relevant JAK target inhibition. Efficacy readouts included splenomegaly, serum cytokine levels and bi-lineage cytopenias (anemia and thrombocytopenia). NanoString RNA analysis from splenocytes was performed to characterize JAK-STAT pathway regulation and downstream immunomodulation. Results: Consistent with previous results, oral ruxolitinib dosing designed to mimic clinically relevant JAK-STAT target inhibition significantly reduced CpG-induced HLH splenomegaly, and diminished pro-inflammatory cytokines KC/GRO and TNFa in the plasma and spleen. In contrast, neither anti IL-1R nor anti IL-6R blockade were effective in the CpG-induced HLH model. NanostringTM analysis confirmed that ruxolitinib treatment ameliorated the pathogenic expression pattern profile observed in vehicle treated HLH mice. In the LCMV-induced HLH model, animals treated with ruxolitinib had reduced splenomegaly and lower serum levels of inflammatory cytokines when compared with animals fed standard chow. Additionally, ruxolitinib was also able to abrogate HLH-induced anemia and thrombocytopenia. Conclusion:Ruxolitinib is a potent and selective JAK1/2 inhibitor currently being evaluated in diseases associated with cytokine storm (i.e. COVID19 NCT04377620 and NCT04362137, and HLH). Oral ruxolitinib at doses that mimic clinically achievable human JAK-STAT target inhibition significantly reduced splenomegaly and cytokines levels. This data provides additional scientific support for targeting JAK1/2 during aberrant immune hyperreactivity. Disclosures: Huarte:Incyte corporation:Current Employment.Peel:Incyte corporation:Current Employment.Fay:Incyte corporation:Current Employment.Nichols:Incyte corporation:Research Funding.Smith:Incyte corporation:Current Employment.