Large granular lymphocyte (LGL) leukemia is a disorder of mature cytotoxic cells. LGL leukemia is characterized by accumulation of cytotoxic cells in blood and infiltration in bone marrow and other tissues. Leukemic LGL could arise from expansion of either CD3⁺ CD8⁺ T-cells (T-cell LGL leukemia or T-LGL leukemia) or those arising from CD3^- natural killer (NK)-cells (NK-cell LGL leukemia or NK-LGL leukemia). LGL leukemia is a rare disorder consisting of less than 5% of non-B cell leukemia. Clinically, LGL leukemia can manifest along a spectrum of disorders ranging from slowly progressing indolent disorder to an aggressive leukemia that could be fatal within months. About fifty percent of LGL leukemia patients also present with variety of autoimmune conditions, rheumatoid arthritis being the most common one.

Normally, following antigen clearance, cytotoxic T-lymphocytes (CTL) become sensitive to Fas-mediated apoptosis resulting in activation-induced cell death (AICD). In contrast, the leukemic LGLs are resistance to Fas-mediated apoptosis. It is believed that constitutively active survival signals keep leukemic LGLs alive in the face of functional Fas-apoptotic machinery. Previously, constitutive activation of Jak-Stat signaling pathway, PI3k-Akt pathway, and Ras-Mek-Erk signaling pathways have been described in leukemic LGL.

In this study, we identified unique gene signature from PBMC of thirty LGL leukemia patients. Pathway-based analysis of gene signature identified that sphingolipid metabolism and signaling are differentially regulated in leukemic LGL. We also showed that unlike CD8⁺ cells from healthy donors, sphingosine-1-phosphate (S1P) receptor S1P5 (EDG8) is the predominant S1P-receptor (S1PR) leukemic LGL. Disruption of balance between pro- and anti-apoptotic sphingolipids (also known as sphingolipid rheostat) leads to selective induction of apoptosis in leukemic LGL. We further showed that sphingosine-1-phosphate (S1P)-mediated signaling induces resistance to Fasmediated apoptosis in normal activated PBMC. On the other hand, inhibition of S1P-mediated signaling by FTY720 restored Fas-sensitivity in leukemic LGL. Collectively these results suggest that S1P-S1P5 mediated signaling may contribute to abnormal survival of leukemic LGL that is mediated by Gα₁₂ signaling.

We further investigated how FTY720 induced cell death in leukemic LGL. FTY720 induced cell death in leukemic LGL that is insensitive to pertussis toxin (PTX) suggesting a role of Gα_12/13-mediated signaling in survival of leukemic LGL. FTY720-mediated cell death is independent of sodium orthovanadate or okadaic acid (OA) suggesting that FTY720 mediates cell death independent of phosphatases. Our results indicate that FTY720-mediated cell death in leukemic LGL was independent of action of extrinsic- or intrinsic caspase cascades suggesting a possibility that FTY720 mediated apoptosis-like programmed cell death rather than classical apoptosis. FTY720 damaged mitochondrial membrane prior to induction of cell death and downregulated MCL1 independent of STAT3 activity in one patient sample but not in others.

FTY720 is a novel immunomodulatory agent currently being evaluated for post renal transplant patients and in patients with autoimmune conditions such as multiple sclerosis. Recent work has identified its role as a novel antineoplastic drug as well. Due to significant association of LGL leukemia with autoimmune conditions, it is possible that FTY720 may be of therapeutic benefit to these patients.