Large granular lymphocytic leukemia: molecularpathogenesis, clinical manifestations, and treatment

Dan Zhang1 and Thomas P. Loughran Jr1

1Hershey Cancer Institute, Pennsylvania State University College of Medicine, Hershey, PA

Large granular lymphocyte (LGL) leukemia represents a spectrum of rare lymphoproliferative diseases defined byclonal amplification of either CD3� cytotoxic T-lymphocytes or CD3� natural killer cells. This chapter focuses on theT-cell form of LGL leukemia. Clinical features include neutropenia, anemia, and rheumatoid arthritis. LGL leukemia isthought to arise from chronic antigenic stimulation, with the long-term survival of LGL being promoted by constitutiveactivation of multiple survival signaling pathways, such as the JAK/STAT3, sphingolipid, and Ras/MEK/ERK pathways.Therefore, these lead to global deregulation of apoptosis and resistance to normal pathways of activation-induced celldeath. The majority of LGL leukemia patients eventually need treatment. Treatment of leukemic LGL is based onimmunosuppressive therapy, primarily using low doses of methotrexate or cyclophosphamide. However, no standardtherapy has been established because of the lack of large, prospective trials. In addition, because some patients arerefractory to currently available treatments and none of these therapeutic modalities can cure LGL leukemia, newtherapeutic options are needed. Understanding the current state of the pathogenesis of LGL leukemia may provideinsights into novel therapeutic options.

IntroductionLarge granular lymphocytes (LGLs) are large WBCs (15-18 �m)with reniform or round nuclei and abundant cytoplasm that containstypical azurophilic granules. In normal adults, LGLs account for10%-15% of peripheral blood mononuclear cells (PBMCs). Thispopulation of cells can be further classified into 2 different lineagesof lymphocytes based on their cell surface markers: CD3� naturalkiller (NK) cells and CD3� cytotoxic T lymphocytes (CTLs). LGLleukemia represents a rare chronic lymphoproliferative disorder ofCTLs, a malignancy that involves lymphocyte infiltration of mul-tiple organs, including the BM, liver, and spleen. Phenotypically,LGL leukemia can arise from either CD3� CTLs or CD3� NKcells.1 The World Health Organization (WHO) classification in-cludes T-cell LGL leukemia in the subgroup of mature peripheralT-cell neoplasms and distinguishes it from aggressive NK-cellleukemia. In addition, a new provisional entity of chronic lymphopro-liferative disorder of NK cells was created by the WHO in 2008.Many features of molecular pathogenesis and clinical presentationof T-LGL leukemia are similar to those of chronic lymphoprolifera-tive disorders of NK cells.

Clinical manifestations and diagnosisThis chapter focuses on T-LGL leukemia (referred to hereafter asLGL leukemia). Table 1 summarizes clinicopathologic features ofthis disease, as well as the NK disorders that should be considered inthe differential diagnosis. LGL leukemia occurs equally in men andwomen with typical age of onset of approximately 60 years.Approximately one-third of patients are asymptomatic at diagno-sis, whereas two-thirds of patients will become symptomaticduring the course of their disease. Predominant symptomsinclude neutropenia, anemia, and rheumatoid arthritis (RA).2

Eighty-five percent of patients with LGL leukemia experienceneutropenia, and 45% develop severe neutropenia (absoluteneutrophil count [ANC] � 500/�L).3 Recurrent bacterial infec-tions are a hallmark of LGL leukemia as a consequence ofneutropenia. Anemia is becoming more frequently recognized

and transfusion dependence may occur in 5%-35% of patients.3

There appear to be multiple mechanisms of anemia, includingautoimmune hemolysis and pure RBC aplasia. RA has beenreported in every series of LGL leukemia patients,2 and is usuallydiagnosed before the onset of LGL leukemia. Activating STAT3mutation is associated preferentially with the LGL/Felty syn-drome phenotype (ie, neutropenia and RA).4 Splenomegaly isseen in 20%-50% of patients; thrombocytopenia is less common,being detected in 20% of patients.3

Patients with unexplained cytopenias or RA should be tested forcirculating LGLs. Diagnosis of LGL leukemia is established bydocumentation of an increased number of clonal LGLs (Figure 1).5-7

LGL counts can be determined by peripheral blood smear evalua-tion and/or flow cytometry. The normal number of LGLs in theperipheral blood is � 0.3 � 109/L. Initially, a circulating LGLcount � 2 � 109/L was considered as mandatory,1 but now a lowercount (range, 0.4-2 � 109/L) can be compatible with the diagnosisin the proper clinical setting. Phenotypic analyses have revealed thatthe leukemic LGL cells are terminal effector memory T cells(CD45RA�CD62L�).8 The majority (80%-90%) of patients withT-LGL leukemia show a CD3�CD8�CD57�CD56�CD28�, TCR-��� phenotype.3 Uncommon variants include CD4� with orwithout coexpression of CD8. Less than 10% of patients haveTCR-, which is clinically similar to TCR-��, and have afavorable survival of 85% at 3 years.9 Clonality is readily ascer-tained by detecting TCR gene rearrangement using Southernblotting and/or PCR. More recently, flow cytometry has been usedto assess clonality by demonstrating a predominant expression of aTCR V� family, because mAbs now are available for approximately70% of the variable region families of the TCR� chain.10 In patientswith low LGL counts, we recommend BM aspirate and biopsy to aidin diagnosis. Pathologic findings of lymphoid interstitial infiltrationwith linear arrays of CD8�, TIA-1 (�), and granzyme B (�)supports the diagnosis of LGL leukemia.

UNTANGLING UNCOMMON LYMPHOPROLIFERATIVE DISORDERS

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Molecular pathogenesis of LGL leukemiaDuring infection exposure or Ag stimulation, LGLs undergovigorous proliferation by approximately 50 000-fold upon primingby target cells, and at a later time after Ag clearance, are selectivelyeliminated by a process called activation induced cell death (AICD).However, in LGL leukemia patients, the AICD process is dysfunc-tional and activated CTL cells do not undergo apoptosis efficiently,leading to elevated number of LGLs in the peripheral blood. Multiplecell survival pathways, including JAK2/STAT3,4,11 sphingolipid signal-ing,12-15 RAS/MEK/ERK,16 and SFK/PI3K/Akt,17,18 have been foundto be constitutively activated in LGL leukemia patients. A systemsbiology approach identified IL-15 and PDGF as master survival

signaling switches that may have a profound effect on all knownderegulations in T-LGL leukemia.13 Most recently, STAT3 mutationswere found to be highly associated with LGL leukemia, suggesting thataberrant STAT3 signaling underlines the pathogenesis of LGL leuke-mia.4 Global deregulation of cell proliferation and survival signalingpathways in LGL leukemia are summarized in Figure 2.

Deregulation of Fas-FasL–mediated apoptosisin LGL leukemiaFas, a member of the TNF receptor family, is involved intransducing death signals. The interaction of Fas ligand (FasL) withits receptor (Fas) results in the formation of death inducing signaling

Figure 1. How to establish the diagnosis of LGL leukemia. The diagnosis is based on a LGL peripheral expansion (� 0.5 � 109/L). Specific criteriafor T-LGL leukemia include expression of LGL surface markers compatible with an activated T-cell (commonly CD3�/CD8�/CD57� and/or CD16�)phenotype and clonal rearrangement of the TCR-� gene using PCR or specific and clonal V� expression using flow cytometry. Specific criteria forNK-LGL leukemia and NK-LGL lymphocytosis include expression of LGL surface markers compatible with an NK-cell (commonly CD3�/CD8�/CD16�

and/or CD16�/CD56�) phenotype. The term chronic NK-LGL lymphocytosis is used for patients with relatively few symptoms and chronic illness,whereas patients with massive tissue LGL infiltration of the spleen, liver, and BM and presenting aggressive clinical behavior are considered to haveNK-LGL leukemia. Reproduced with permission from Lamy and Loughran.6

Table 1. Types and features of LGL leukemia

Types Clinical featuresAssociateddiseases Markers Treatment

T-LGL leukemia Asymptomatic or symptomaticwith cytopenias, recurrentinfection, splenomegaly,liver, BM,

RA other autoimmunedisease

CD3�CD8�CD16�CD57�;TCR-��

Observation or immunosuppressiveregimen (see Figure 3)

CLPD-NK �60% asymptomatic and40% symptomatic withcytopenias, neuropathy andsplenomegaly

Malignancy and RArare

CD3�CD16�CD56� Observation or immunosuppressiveregimen

Aggressive NK-LGLleukemia

Fulminant, B symptoms,organomegaly, cytopenias

EBV as etiology CD3�CD16�CD56� Intensive acute lymphoblasticleukemia–like induction

CLPD indicates chronic lymphoproliferative disorder.

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complex (DISC) and the activation of many apoptotic effectors.Under physiological conditions, after Ag clearance, activated CTLsare eliminated through the process of AICD in part by the Fas-FasLpathway. However, leukemic LGLs are resistant to this deathpathway despite high levels of surface expression of Fas, abundantconstitutive expression of FasL, and absence of mutations in the Fasreceptor gene.19 In addition, cells from T-LGL leukemia patientsexhibit impaired Fas-induced DISC formation after cross-linking ofthe Fas receptor.8 Further study found that the DISC inhibitoryprotein cellular Fas-associated protein with death domain (FADD)–like IL-1 converting enzyme-inhibitory protein (c-FLIP) is overex-pressed in leukemic LGLs, resulting in reduced DISC assembly andresistance to Fas-FasL–mediated apoptosis.8 Moreover, a solubleform of Fas (sFas) that is capable of blocking Fas-dependentapoptosis was detected in the sera of LGL leukemia patients but notin normal serum.20 It is believed that sFas competes with Fas byacting as soluble decoy receptor, leading to resistance to Fas-mediated apoptosis in leukemic LGLs. Impaired Fas-FasL–mediated apoptosis contributes to the pathogenesis of LGL leukemia.

IL-15IL-15 is a member of the IL-2 family that regulates T- and NK-cellactivation and proliferation by altering expression of Bcl-1 familymembers (eg, Bcl-2 and Bcl-XL). IL-15 is also required to providelong-term survival signals to maintain normal NK and CD8�

memory T cells13 and plays an important role in the survival ofleukemic LGL cells. Inhibition of IL-15 led to apoptosis in leukemicLGL cells,21 whereas interruption of IL-15 signaling using anIL-15–neutralizing Ab was able to prevent LGL leukemia inIL-15–transgenic mice.22 In NK cells, IL-15 specifically reduces thelevel of BH3 interacting domain death agonist (Bid) by up-

regulating the E3 ligase E3 ubiquitin-protein ligase double minute2 protein (HDM2) that targets Bid to proteasome for degradation.21

Interestingly, low levels of Bid could be reversed with bortezomib(a proteasome inhibitor) treatment, leading to apoptosis of leukemicLGLs.21 Recently, the IL-15 private soluble receptor subunitIL-15R� has been shown to be up-regulated in the PBMCs of someT-LGL leukemia patients.23 Moreover, PBMCs from some T-LGLpatients proliferated at higher levels in response to exogenouslyadded IL-15 compared with normal PBMCs. It has been suggestedthat the higher level expression of IL-15R� contributes to thepathogenesis of T-LGL leukemia by lowering the IL-15 responsethreshold in T-LGL leukemia.

PDGFPDGF is a major growth factor that has been found to mediate thesurvival of leukemia LGL of both T- and NK-cell origin. Networkmodeling predicted a key role for IL-15 and PDGF in LGL leukemiapathogenesis.13 We demonstrated that leukemic LGL survival wasdependent on a PDGF autocrine loop. PDGF regulated the long-term survival of leukemic LGLs through the PI3K-Akt and MER/ERK pathways.24

Constitutive activation of STAT3 in LGL leukemiaThe JAK/STAT signaling pathway controls a variety of biologicalprocesses such as cell proliferation, apoptosis, angiogenesis, andimmune responses. STAT3, a latent transcription factor, has beenshown to play a central role in conferring cell survival. Persistentactivation of STAT3 has been observed in more than 22 types ofcancers. STATs can be activated by cytokine and growth factors viaengagement with their respective receptors (ie, cytokine receptorsand growth factor receptors), leading to dimerization and rapid

Figure 2. The signaling network underlying LGL leukemia pathogenesis. Up-regulated or constitutively active nodes are highlighted in pink;down-regulated or inhibited signals are in green; the states of white nodes are unknown or unchanged compared with normal. ASAH indicates acidceramidase; RTK, receptor tyrosine kinase.

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translocation to nucleus, where they exert their transcriptionalactivities. In 2001, constitutive activation of STAT3 was found inall cells of LGL leukemic patients (N � 19). STAT3 promoted thesurvival of leukemic LGL by regulating the expression of antiapop-totic proteins such as myeloid cell leukemia sequence 1 (Mcl-1).11

Recently, genomic studies demonstrated that the reason for constitu-tive STAT3 activation in 40% (31 of 77) of LGL leukemia patientswas activating somatic mutations in STAT3 Src homology 2 domain,a critical region that mediates STAT3 protein dimerization andactivation.4 Downstream target genes of STAT3 pathway (eg,IFNGR2, JAK2, and Bcl2L1) were up-regulated in patients withLGL leukemia.4 It is conceivable that STAT3 inhibitors might beideal targeted therapies for LGL leukemia.

Constitutive activation of NF-�BNF- B, a transcription factor, plays a critical role in the hematopoi-esis, proliferation, and survival of immune cells. In its inactivatedstate, NF- B stays in the cytoplasm with its inhibitor, I B. Once theNF- B pathway is activated, I B is phosphorylated by I B-kinasecomplex, leading to the degradation of I B and the activation ofNF- B. NF- B has been shown to promote the expression ofprosurvival Bcl-2 family member and inhibitor of apoptosis pro-teins. Interestingly, leukemic LGLs expressed high levels of c-Rel, amember of the NF- B family,13 and exhibited higher NF- B activitythan normal PBMCs. Pharmacological inhibition of Akt and NF- Bdemonstrated that NF- B acts downstream of the PI3K-Akt pathway toprevent apoptosis through Mcl-1 independently of STAT3.13

Imbalance of sphingolipid rheostat in LGL leukemiaSphingolipids and their metabolites play important but oppositeroles in diverse biological processes such as proliferation, apoptosis,and cell migration. The balance between the proapoptotic (ie,ceramide and sphingosine) and prosurvival (ie, sphingosine-1-phosphate [S1P]) sphingolipids, rather than the absolute amount ofthese molecules, determines the cell fate (sphingolipid rheostat).Molecular profiling analysis revealed that the sphingolipid rheostatis deregulated in a way that tilts the balance away from proapoptoticmolecules (eg, ceramide) and in favor of survival molecules (eg,S1P) in LGL leukemia.12 For example, acid ceramidase (ASAH1)was found to be constitutively up-regulated in LGL leukemia,leading to decreased levels of ceramide and survival of LGL cells.12

Inhibition of ASAH1 or delivery of C6-ceramide into a rat model ofNK-LGL led to cell death of leukemic LGLs both in vitro and invivo.14 In addition, sphingosine kinase 1 (SphK1), a kinase thatconverts sphingosine to S1P, is overexpressed in LGL leukemiapatients. Pharmacological inhibition of SphK1 with SKI-I or S1P usingFTY720 induced significant apoptosis in leukemic LGLs.15 Further, theexpression of S1P receptors (S1PRs) are elevated. In particular, S1PR5is found to be constitutively overexpressed in leukemic LGL.12 Thederegulated sphingolipid rheostat contributes to the development ofLGL leukemia; therefore, molecules targeting the sphingolipid path-way might prove to be successful therapies in LGL leukemia.

Constitutive activation of Ras/Raf/MEK/ERKsignalingThe Ras/Raf/MEK/ERK pathway controls many fundamental cellu-lar processes, including cell proliferation, survival, differentiation,apoptosis, motility, and metabolism. The Ras/Raf/MEK/ERK path-way transfers various signals from growth factors (eg, PDGF),G-protein coupled receptors (eg, S1PR5), and NF- B to eitheractivate transcription factors (eg, Fos and Jun of Ets family) orpromotes the transcription of FLIP and Mcl-1 directly. Mutations in

critical components (eg, Ras and Raf) of this pathway occur inapproximately 30% of all human cancers.

Ras/Raf/MEK/ERK signaling has been reported to be constitutivelyactivated in NK LGL leukemia patients. Inhibition of Ras resultedin the inhibition of ERK and apoptosis of patient cells. Further,inhibition of ERK by inhibitors (eg, PD98059 and U0126) or adominant-negative form of MEK reduced the survival of patientcells.16 Therefore, these findings demonstrated that the constitu-tively active Ras/Raf/MEK/ERK pathway contributes to the sur-vival of LGL cells.

Deregulation of the PI3K/Akt pathwayThe PI3K/Akt signaling pathway plays a pivotal role in promotingcell proliferation and survival, and therefore cancer progression.PI3K/Akt signaling is activated by growth factors through either Srcfamily kinase (SFK) or the Ras signaling cascade. The main targetsdownstream of PI3K/Akt include the prosurvival transcriptionfactors NF- B, Bcl-2 antagonist of cell death, and pro-caspase 9. InLGL leukemia, SFK maintains PI3K in its constitutively activeform, which results in the increased phosphorylation of Akt andglycogen synthase kinase-3 (GSK3).18 After activation, PI3K/Aktsignaling enhances the survival of T cells by inhibition of Fasclustering and DISC formation. Inhibition of the PI3K/Akt pathwayalone leads to spontaneous apoptosis of T-LGL.18 These resultssuggest that PI3K/Akt pathway activation protects leukemic LGLsfrom undergoing homeostatic apoptosis.

In summary, various signaling pathways, including the Fas/FasL,IL-15, PDGF, JAK/STAT3, NF- B, sphingolipid, and Ras/MEK/ERK, PI3K/Akt pathways, are deregulated in LGL leukemia. Avariety of therapeutics targeting these deregulated signalingpathways are under clinical study for the treatment of differentcancers, raising the possibility of future clinical trials in LGLleukemia (Table 2).

Therapeutic options for LGL leukemiaCurrently, there is no standard treatment for patients with LGLleukemia. For asymptomatic LGL leukemia patients with anindolent course, one can consider a wait-and-see approach with asingle G-CSF injection to test the potential myeloid progenitormobilization, which then could be administered urgently at time ofneutropenic fever if the patient is G-CSF responsive. For symptom-atic patients, indications for immunosuppressive treatment includesevere or moderate anemia (ie, transfusion dependent or symptom-atic anemia) and severe or moderate neutropenia (ie, an ANC � 500or recurrent infection associated with higher ANC � 500). Finally,symptomatic patients with RA warrant therapy.7

The therapeutic approach is summarized in Figure 3.7 The mainstayof treatment is immunosuppressive therapy, with single-agentmethotrexate (MTX), oral cyclophosphamide, or cyclosporine Areported to have activity in LGL leukemia. We recommendlow-dose MTX initially for LGL leukemia patients with neutropeniaand/or RA. Currently, MTX (10 mg/m2/wk orally) or cyclophospha-mide (100 mg/d orally) is our first choice for LGL patients withanemia. Response to treatment is evaluated by blood count 4 monthsafter the beginning of the treatment. Hematologic complete re-sponse (CR) is considered achieved when blood counts return tonormal ranges (eg, platelets � 150 � 109/L, ANC � 500/�L, hemo-globin � 12 g/dL, and lymphocytes � 4 � 109/L) and circulatingLGL in the normal range. In addition, complete molecular remission

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is considered achieved when T-cell clone is nondetectable usingPCR. Partial response (PR) is defined as an improvement in bloodcounts in the absence of CR. Treatment failure is defined as failureto achieve either CR or PR within 4 months after starting therapy. Ifat least a PR is not attained after 4 months of treat-ment, then therapy is changed to the other agent or cyclosporine(2.5-5 mg/kg twice daily). The following section briefly reviews theavailable treatments in LGL leukemia.

MTX has been widely used in RA and was first reported to haveefficacy of 50% in hematologic CR in the treatment of a small seriesof LGL leukemia in 1994.25 Other series have demonstrated aresponse rate of 39%-67%.7 Our recent large, prospective EasternCooperative Oncology Group (ECOG) trial showed 39% responserate to MTX as initial therapy.26 In general, MTX is well tolerated.Side effects associated with low-dose MTX include hepatic injuryand, rarely, lung dysfunction. MTX is temporarily held or doseadjusted for transaminase values increased above 2-fold. Respond-ing patients are continued on MTX indefinitely.

Cyclophosphamide, an alkylating agent, is a prodrug used to treatvarious types of cancers and some autoimmune disorders. In Frenchseries of LGL leukemia patients, cyclophosphamide showed afavorable overall response rate compared with MTX, at 66% and55%, respectively.6 These data suggest that cyclophosphamidecould be also considered as a first-line therapy.6 It takes up to1-4 months to achieve any response, so the same 4-month timeframe is used to assess response. In patients achieving at least a PR,

cyclophosphamide is continued for a total of 9-12 months to avoidrare complications of myelodysplastic syndrome/acute myeloidleukemia, which appears to be dose dependent.

Cyclosporine is another alternative for first-line therapy. We preferMTX or cyclophosphamide because of the more extensive toxicityprofile of cyclosporine.7 Renal function and blood pressure need tobe monitored and treatment may be discontinued due to side effects.It is clear that cyclosporine must be continued indefinitely becauserelapse happens almost invariably just after stopping treatment.Treatment experience with purine analogs is limited to less than40 reported patients, but the overall response rate is impressive(79% or 30 of 38 patients).27 Considering the advantages of a shortperiod of treatment (from 1 to 4-6 monthly courses), high responserate, mild toxicity, and the potential of inducing durable remission, aclinical trial using purine analogs could be considered.

There have been a limited number of small clinical trials for LGLleukemia. R115077 (tipifarnib; trade name Zarnestra) is a farnesyl-transferase inhibitor designed to inhibit Ras-mediated signaling. Aphase 2 clinical study of 8 LGL leukemia patients achieved noclinical hematologic responses.28 However, promising biologicalresponses, including decreased LGL in the blood and BM, improvedBM hematopoiesis, and increased hematopoietic colony growth invitro were observed in most of these patients. In addition, R1150777treatment improved the pulmonary hypertension symptoms in aLGL leukemia patient.29

Table 2. Current and potential future therapeutics for LGL leukemia

Signaling pathway Deregulation in LGL leukemia Therapeutic intervention

Fas-FasL–mediated apoptosis ofactivated CTL

Fas-FasL machinery is blocked by sFas that iselevated in patient sera and is capableblocking Fas-mediated cell death ofleukemia LGLs8,20

Recombinant Ab targeting sFasL tumor cells toinduce apoptosis of LGL leukemic cells36; MMPinhibitors (marimastat) to inhibit the cleavage ofmembrane bound FasL into sFasL37

IL-15 promotes the survival of T and NKcells

IL-15 prevents apoptosis of leukemic LGLs byenhancing degradation of Bid, a criticalapoptotic factor; higher level of sIL-15R�may lower the IL-15 response threshold inleukemic LGL cells21,23

Ab (Mik-�1) against IL-15 receptor showed nosignificant toxicity in phase 1 clinical trial of LGLleukemia33,34

PDGF regulates cell growth and division Elevated PDGF regulates the long-termsurvival of leukemic LGL cells throughPI3K-Akt and MER/ERK pathways12,24

Tyrosine kinase (PDGFR) inhibitor, imatinibmesylate, exhibits effects in chronic myeloidleukemia and autoimmune disorders (such asRA)38

JAK/STAT signaling regulates cellproliferation, survival, immuneresponse, etc

Persistent activation of STAT3 in LGLleukemia; somatic mutation of STAT3 inLGL leukemia4,11

STAT3 inhibitor (OPB-31121) is undergoingclinical trial39

NF-�B plays critical role in hematopoiesisand survival of immune cells

NF-�B is constitutively active in T-LGLs13 Bortezomib prevents the degradation of I�B;preclinical studies of bortezomib in T-LGLleukemia are promising21

Sphingolipid rheostat determines the cellfate

Proapoptotic signals (ceramide) are down-regulated and prosurvival signals (S1PR5)are up-regulated12,15

FTY720, a S1PR agonist, induced apoptosis ofLGL cells15; C6-ceramide integrated intonanoliposomes led to apoptosis in an NK-LGLrat leukemia model by targeting survivin14

Ras/MEK/ERK signaling regulates cellproliferation and differentiation inresponse to various growth factors

Ras and ERK are constitutively activated inNK-LGL leukemic cells16

Phase 2 study of R115077, a farnesyltransferaseinhibitor, in LGL leukemia patients achieved noclearly hematologic responses, but promisingbiological responses were observed in vitrofrom patient samples28,29

PI3K/Akt signaling promotes cellproliferation, survival and cancerprogression

Constitutive activation of PI3K/Akt signalingpromotes survival of T cells18

PI3K inhibitors (GDC-0941) show promisingresults in solid tumors40

RTK indicates receptor tyrosine kinase; sFas, soluble Fas; and MMP, matrix metalloproteinase.

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Alemtuzumab, the humanized anti-CD52 mAb, is capable of selec-tively killing CD52-expressing cells. Interestingly, CD52 is expressedon the leukemic T-LGLs. In one study, the overall response rate ofalemtuzumab was 50% (4 of 8 T-LGL leukemic patients).30 Currently,alemtuzumab is being studied in a phase 2 clinical trial for treatment ofT-LGL leukemia.31 It was approved by US Food and Drug Administra-

tion (FDA) as second-line therapy for CLL patients who have beentreated with alkylating agents and who have failed fludarabine therapy.32

Humanized MiK-�1 mAb is directed toward CD122, a commonsubunit of IL-2R and IL-15R. Recently, a phase 1 clinical study hasbeen completed for humanized MiK-�1 mAb in patients with

Figure 3. Algorithm of treatment of LGL leukemia. Adapted with permission from Figure 5 in Lamy and Loughran.6

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T-LGL leukemia.33 Although down-regulation of the IL-15� sub-unit was observed in leukemic LGL, no reduction in LGL countswere observed.34

Recently, the efficacy of extracorporeal photopheresis was evalu-ated in 5 refractory/relapsed patients with LGL leukemia, with 2 of5 patients achieving a CR.35 Because limited options are availablefor these refractory/relapsed patients, extracorporeal photopheresisshould be considered. Other drugs targeting the signaling pathwaysthat are deregulated in LGL leukemia are listed in Table 2. Suchtherapeutics could be considered as candidates for clinical trials.

SummaryLGL leukemia is a clonal disorder of CTLs. The mainstay oftreatment for LGL leukemia is immunosuppression; however, thereare no curative therapeutic modalities for this disease and newtherapeutic options are needed. Recognition of many importantderegulated signaling pathways has greatly stimulated the identifica-tion of potential new therapeutic targets in LGL leukemia. It ishoped that clinical trials using new drugs targeting the deregulatedsignaling pathways will soon be available for patients with LGLleukemia.

DisclosureConflict-of-interest disclosure: The authors declare no competingfinancial interests. Off-label drug use: immunosuppressive therapiesfor LGL leukemia that could include MTX, cyclophosphamide, andcyclosporine.

CorrespondenceThomas P. Loughran Jr, Penn State Hershey Cancer Institute, Rm4427, 500 University Dr, PO Box 850, Hershey, PA 17033-0850;Phone: 717-531-4034; Fax:717-531-0490; e-mail: tloughran@hmc.psu.edu.

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