Targeting PRMT5 in Primary Hemophagocytic Lymphohistiocytosis: A Novel Therapeutic Approach Using a Perforin-Deficient Mouse Model

Introduction. Primary hemophagocytic lymphohistiocytosis (HLH) is a rare but frequently fatal hyperinflammatory syndrome that occurs mostly in children due to predisposing genetic mutations in the perforin/granzyme secretory pathway. The pathophysiologic mechanisms of primary HLH are not fully understood but appear to implicate an initial ineffective cytotoxic T cell-based immune response that fails to eradicate an immunogenic trigger, such as infection. Continued antigen stimulation results in the activation and expansion of cytotoxic T cells, increased secretion of IFNγ and other pro-inflammatory cytokines and activation of myeloid immune cells such as macrophages, which infiltrate and damage tissues. Primary HLH is a potentially deadly disease. Development of novel therapies targeting the pathophysiology of primary HLH is urgently needed. The protein arginine methyltransferase 5 (PRMT5) enzyme catalyzes symmetric di-methylation of arginine and regulates numerous basic cellular processes, including gene transcription and protein signaling. PRMT5 expression is tightly regulated in normal tissues but high in inflammatory disease states such as acute graft versus host disease and autoimmune encephalomyelitis. We have previously identified a pro-inflammatory role of PRMT5 activity in the murine model of secondary HLH. Here we report that PRMT5 supports hyperinflammation in a perforin-deficient mouse model of primary HLH.

Materials and methods. HLH was induced in C57BL/6, prf^−/− mice by infection with the lymphocytic choriomeningitis virus. Mice were treated with a highly selective small molecule inhibitor of PRMT5, PRT808, dosed 5 mg/kg (po, in chow) once daily starting on Day 1, or vehicle chow. On Day 12, the mice were injected with Brefeldin A and sacrificed 6 hours later. Plasma IFNγ levels were analyzed by ELISA assay. Spleens were processed into a single-cell suspension. Splenocytes were analyzed by flow cytometry. Data were analyzed using one-way or two-way ANOVA with multiple comparisons as applicable.

Results. HLH led to the rise of plasma IFNγ (p=0.0004), and to expansion of splenic lymphocytes and myeloid cells, including CD8 T cells (CD8+, p=0.0441), total myeloid cells (CD11b+, p<0.0001), monocytes (CD11b+/Ly6C+/Ly6G-, p=0.0064), neutrophils (CD11b+/Ly6C+/Ly6G+, p=0.0003), and dendritic cells (DCs, CD11c+/IAIE+, p=0.0008) in mice with fulminant, untreated HLH compared to healthy controls. PRMT5 protein levels increased in CD8 T cells (p=0.0166), monocytes (p=0.0099) and DCs (p=0.0448) in HLH mice compared to healthy controls. Treatment with PRT808 significantly counteracted the HLH-driven rise of the pro-inflammatory cytokine, IFNγ, in plasma of the mice that received the drug, compared to animals treated with vehicle control (p=0.0027). Treatment with PRT808 also reduced the expansion of CD8 T cells (p=0.0384), total myeloid cells (p<0.0001) and monocytes (p=0.0495) in drug-treated HLH compared to vehicle-treated mice. Intracellular IFNγ increased in CD8 T cells upon induction of HLH (p=0.0007) compared to healthy controls and reduced significantly (p=0.0459) with PRT808 treatment. Furthermore, PRMT5 inhibition suppressed the number of activated CD8 T cells expressing the co-stimulatory molecule CD137 (P<0.05) and reduced the fraction of CD8 T cells expressing the immune checkpoint molecule PD-1 (p<0.05) in mice with drug-treated HLH, compared to the vehicle-treated cohort. Similar pattern of decreased expression of the markers of CD8 T cell proliferation (Ki-67, pS6) was observed in mice with HLH that received the PRMT5 inhibitor compared to the untreated HLH cohort, with results approaching statistical significance. Extended experiments examining the effect of PRMT5 inhibition on survival in the murine model of primary HLH are ongoing.

Discussion. Our findings identify PRMT5 as an actionable target in the murine model of primary HLH. Together with our previous findings on the pro-inflammatory role of PRMT5 in the murine model of secondary HLH, these results support further investigation of the role of PRMT5 in driving the inflammation of HLH in humans. Further studies are needed to investigate the mechanism of the anti-inflammatory impact of PRMT5 inhibition in murine models of HLH and in human disease.