Dissecting fingolimod’s action: a novel immunomodulatory effect of the unphosphorylated compound on cytotoxic T cell function (P5159)

Abstract

Abstract Fingolimod (FTY), a sphingosine-1-phosphate receptor modulator, is a multiple sclerosis (MS) therapeutic that upon phosphorylation traps CCR7+ T cells in lymph nodes (LN). CCR7- effector T cells are spared, thus allowing effective infection clearance. Nonetheless, FTY-treated patients are more susceptible to viral infections, indicating a cytotoxic T (Tc) cell defect. Yet, FTY’s effect on Tc cells remains unclear. To address this question, we utilized experimental autoimmune encephalomyelitis (EAE) and a murine influenza model. FTY ameliorated EAE by sequestering T cells, but also inhibited splenic Tc cell IFNγ and Granzyme B(GrB) expression. Influenza was exacerbated and mortality was increased, as FTY inhibited Tc cell activation and lung infiltration. To elucidate its mechanism of action, murine splenocyte cultures were treated with phosphorylated and unphosphorylated FTY, as both exist in vivo. Remarkably, only the latter reduced IFNγ and GrB levels in Tc cells and inhibited their function in an LDH cytotoxicity assay. The addition of arachidonic acid rescued Tc cell function, suggesting that FTY’s effect is due to inhibition of cytosolic phospholipase A2. Herein, we demonstrate that FTY suppresses Tc cell function independently of its cell trafficking effects. This provides a novel explanation not only for the increased rate of viral infections in FTY-treated patients, but also for its efficacy in MS, as Tc cells have emerged as crucial mediators of MS pathogenesis.