Cytokines, Genetic Lesions and Signaling Pathways in Anaplastic Large Cell Lymphomas

Simple Summary This review summarizes the main features of anaplastic large cell lymphoma (ALCL) subtypes focusing on activated biological pathways that may have clinical significance for diagnosis, prognosis and personalized therapy. Abstract ALCL is a tumor of activated T cells and possibly innate lymphoid cells with several subtypes according to clinical presentation and genetic lesions. On one hand, the expression of transcription factors and cytokine receptors triggers signaling pathways. On the other hand, ALCL tumor cells also produce many proteins including chemokines, cytokines and growth factors that affect patient symptoms. Examples are accumulation of granulocytes stimulated by IL-8, IL-17, IL-9 and IL-13; epidermal hyperplasia and psoriasis-like skin lesions due to IL-22; and fever and weight loss in response to IL-6 and IFN-γ. In this review, we focus on the biology of the main ALCL subtypes as the identification of signaling pathways and ALCL-derived cytokines offers opportunities for targeted therapies.


Main ALCL Subtypes
The different ALCLs represent about 16% of peripheral T cell lymphomas [1]. As summarized in Table 1, they form a heterogeneous group of CD30-positive T cell non-Hodgkin lymphomas according to their site of onset (systemic, cutaneous or breast implantassociated) and their genetic features, with several groups according to the presence of ALK rearrangement and subsequent ALK expression defining ALK-positive or ALK-negative ALCLs [2][3][4]. ALK-negative ALCL may carry DUSP22 rearrangements and/or TP63 rearrangements. ALK-positive and DUSP22-rearranged ALCLs have a better prognosis than triple-negative ALCL and TP63-rearranged ALCL, which have the worst outcome [2,3,[5][6][7]. The site of origin is critical as illustrated by cutaneous ALCL (C-ALCL), which is commonly ALK-negative [8][9][10]. C-ALCL has a favorable prognosis with a 5-year disease specific survival of 90%, even in the presence of regional lymph node involvement or putative adverse genetic lesions such as the rare TP63 rearrangement [3,11,12].

Systemic ALK+ ALCL
Systemic ALK+ ALCL mainly occurs in children and young adults with a male predominance, affecting the lymph nodes as well as extranodal sites (Table 1). ALK+ ALCL presents with several morphological variants sharing the hallmark anaplastic cells (common, small cell, lymphohistiocytic, Hodgkin-like or composite), extensively reviewed elsewhere [4,13]. Tumor cells strongly express CD30, EMA, CD25, BCL6 and cytotoxic

Systemic ALK-ALCL
Systemic ALK-ALCL frequently occurs in older patients and involves the lymph nodes and extranodal tissues, with a more aggressive course, B symptoms and advanced stages [21]. Absence of ALK expression combined with various morphologies may constitute a diagnostic challenge for differential diagnosis with either CD30+ PTCL or classic Hodgkin lymphoma (CHL) [1,4,14]. The key features are the cohesive pattern with sinus infiltration, the presence of hallmark cells, abundant cytoplasm, strong and uniform expression of CD30, frequent loss of T cell markers and expression of cytotoxic molecules (TIA1, granzyme B and perforin) or EMA. In contrast to CHL, ALK-ALCL does not express PAX5, EBV transcripts or LMP1 [4,18].
Genomic profiling of 32 clinical samples and 5 ALCL cell lines by microarray analysis identified overexpression of the BCL6, PTPN12, CEBPB and SERPINA1 genes in ALKpositive ALCL, whereas the CCR7, CNTFR, IL-22 and IL-21 genes were overexpressed in ALK-negative ALCL. High levels of interferon regulatory factor 4 (IRF4), which induces MYC expression, were observed in all ALCL subtypes [22]. Decreased expression of MYCassociated factor X (MAX), identified in both ALK+ and ALK-ALCLs, was associated with an uncommon morphology and expression of MYC and cytotoxic molecules in patients with adverse prognosis [23]. Others detected a high expression of MYC in 37% of ALK+ ALCLs, with a common morphology in older patients with a shorter survival [24].
Genomic profiling showed that ALK+ ALCL displays a homogeneous cytotoxic/Th1 signature in contrast to ALK-ALCL that has either a cytotoxic profile or a Th2-associated signature. The TH2 group was enriched in cases displaying a DUSP22 rearrangement [25]. The presence of TP63 rearrangement in 12% of ALK-ALCLs defines a third ALCL category [2]. The different rearrangements such as inv (3) TBL1XR1-TP63 or TP63-ATXN1 fusion transcripts, leading to expression of a dominant negative oncogenic p63 isoform [26]. In systemic ALK-ALCL, TP63 rearrangement is associated with a poorer outcome. Exceptional C-ALCLs displaying both DUSP22 and TP63 rearrangements have been reported [2,3,7]. The absence of either ALK, DUSP22 or TP63 rearrangement defines a "triple-negative" subset of ALK-ALCL with a 5-year OS rate of 42% [2].

Cutaneous ALCL
Cutaneous ALCL belongs to primary cutaneous CD30+ lymphoproliferative disorders together with lymphomatoid papulosis and borderline cases. C-ALCL is defined by the presence of more than 75% of large cells expressing CD30+ in patients without evidence or history of epidermotropic T cell lymphoma [12,27]. The disease is limited to the skin, affecting the trunk, face and extremities ( Figure 1). Regional lymph node involvement may exist but does not affect prognosis [11]. Most cases express a CD4+ T cell phenotype with loss of CD2, CD3 or CD5 and frequent expression of cytotoxic proteins [28]. However, some cases are CD4-/CD8+ or CD4+/CD8+. Unlike systemic ALCL, C-ALCLs usually express CLA and CD158k but do not express EMA. C-ALCLs also express skin-homing molecules such as CCR4, CCR10 and CCR8. MUM1 encoded by the IRF4 gene is strongly positive, and CD15 is expressed in about half of the cases [10,27].
C-ALCLs exhibit some privileged cytogenetic features with a DUSP22 rearrangement in up to 30% of cases [5,6]. DUSP22-rearranged ALCLs not only arise at extranodal sites but also present with sheets of large hallmark cells admixed with smaller cells having both nuclear indentations sometimes giving rise to doughnut cells [2,5,29,30]. The phenotype is non-cytotoxic with absence of EMA expression [5,30]. Interestingly, expression of the lymphoid enhancing transcription factor (LEF1) predicts DUSP22 rearrangement with high specificity and sensitivity [31]. Some C-ALCLs and other CD30+ lymphoproliferations were shown to harbor an NPM1-TYK2 fusion gene, leading to STAT1/3/5 activation [32]. However, common TYK2 expression in ALCL cannot serve to identify such cases. Very few C-ALCLs were found to display a TP63 rearrangement that does not affect their good prognosis [3,7].

Breast Implant-Associated Anaplastic Large Cell Lymphoma (BIA-ALCL)
This rare disease, generally occurring 8-10 years after implantation, presents with either a peri-implant seroma elicited by chronic inflammation or a capsular invasion, sometimes as a tumor. It shares many features with ALK-ALCL including expression of EMA, cytotoxic proteins and phospho-STAT3 [33]. CD30+ immunostaining and T cell monoclonality are diagnostic criteria. In late seromas generated by infection or implant rupture, high levels of IL-10, IL-13 and Eotaxin and an elevated IL10/IL-6 ratio are features of the effusion milieu [34]. BIA-ALCL exhibits Th-2 differentiation as neoplastic cells express GATA3 and FOXP3 [34] Nevertheless, bacterial biofilm infection of implants has been implicated in provoking chronic inflammation as a trigger for BIA-ALCL [35].

ALCL Cell of Origin
The detection of NPM-ALK transcripts in neonatal cord blood suggested that ALK+ cells may originate from either stem cells or early thymic progenitors (ETP) [36]. A transgenic mouse model with expression of NPM-ALK under the CD4+ promoter indicated that ALK+ ALCL could emerge from ETP. CD4/NPM-ALK expression functioning as a surrogate TCR permits cells to bypass thymic selection through upregulation of Notch1 expression [37]. However, malignant transformation of CD4+ lymphocytes by NPM-ALK also suggests that ALK+ ALCL may derive from rare peripheral mature CD4+CD30+ T cells [38]. Transduction of NPM-ALK into mature CD4+ peripheral T cells after CD3/CD28 costimulation produced NPM-ALK-transformed lymphocytes displaying an early thymic precursor gene expression signature [39]. NPM-ALK induced lymphocyte survival and spreading but also reversed the mature T cell phenotype into an ETP phenotype associated with expression of pluripotency-associated transcription factors such as OCT4, SOX2 and NANOG under the control of HIF2A [39]. HIF2A silencing abrogated NPM-ALK and STAT3 activity, suppressing the cell growth of NPM-ALK-transformed CD4+ lymphocytes. Whether the above transgenic mice or preclinical models reflect a real difference between pediatric and adult ALCL is still a matter of debate. In a subset of ALCL (14%), a germline TCR also supports lack of thymic maturation [4,16,37].

STAT3 a Pivotal Transcription Factor in Most ALCL Subtypes
Many of the biological features of ALK+ ALCL result from epigenetic deregulation triggered by STAT3 activation. This applies to both ALK+ and ALK-ALCL, except for the ALK-ALCL subtype bearing DUSP22 rearrangement [5,45]. NPM-ALK triggers STAT1 phosphorylation and degradation and promotes STAT3 upregulation. STAT3 also induces the expression of DNA methyl transferase 1 that promotes epigenetic reprogramming of ALK+ cells that frequently lack expression of CD3+, TCRs and related molecules, including CD3ε, zeta-chain-associated protein kinase 70 (ZAP70), linker for activation of T cells (LAT) and lymphocyte cytosolic protein 2 (LCP2) [46]. Then, STAT3 induces immune escape mediated by expression of TGF-beta, IL-10, ICOS and PDL1 ( Figure 2). NPM-ALK, through binding of STAT3, was shown to promote Notch1 deregulation which can be inhibited by γ-secretase inhibitors (GSIs), leading to apoptosis [47]. In ALK+ ALCL, several miRNAs including three members of the miR-17-92 clusters are aberrantly overexpressed, while miR-155 is >10-fold overexpressed in ALK-ALCL [48]. Alternatively, other miRNAs such as miR-101, miR-29c and miR-26 are down-regulated in ALK+ and ALK-ALCL cell lines and primary human samples [48]. MiR-29a down-regulation is also driven by NPM-ALK activity and contributes to apoptosis blockade through epigenetic deregulation of MCL-1 expression [49]. A recent exome sequencing study also underscored the interplay between the STAT3 and Notch pathways in both ALK+ and ALK-ALCLs. A point mutation of T349P NOTCH was detected in 12% of ALK+ and ALK-ALCL patient samples [47]. STAT3 activation occurs in ALK-ALCLs through different mechanisms ( Figure 2). Activating point mutations of STAT3 and/or JAK1 have been identified in 18% of nodal ALK-ALCLs and in 5% of C-ALCLs [50]. Another study of PTCL identified in ALK-ALCLs the highest rate of STAT3 mutations (38%) that may combine with JAK mutations (15%), while some ALK+ ALCLs (13%) presented STAT3 mutation alone [51]. The highest pY-STAT3 phosphorylation level is present in ALK-ALCL displaying a typical CD3-CD5-CD7-CD30+ phenotype [51]. Whatever the JAK gene status, ALK-cells are addicted to cytokine receptor signaling, and JAK inhibitor sensitivity correlates with STAT3 phosphorylation [52]. About 60% of BIA-ALCLs have mutations in at least one member of the JAK/STAT pathway including STAT3, JAK1 and STAT5B and in negative regulators such as SOCS3, SOCS1 and PTPN1 [53]. In addition, a majority of BIA-ALCL cases (74%) displayed recurrent mutations of epigenetic modifiers such as KMT2C, KMT2D, CHD2 and CREBBP [53]. In rare ALK-ALCL subsets, chromosomal rearrangements creating chimeras combining a transcription factor (NFkB2 or NCOR2) with a tyrosine kinase (ROS1 or TYK2) were found to elicit STAT3 phosphorylation independent of JAK1 or STAT3 mutations [50].

STAT3-Independent ALK-ALCLs
ALK-ALCLs with 6p25.3 rearrangement are associated with DUSP22 silencing and conserved IRF4 expression, while inactivation of the second allele by DUSP22 mutation is uncommon [54]. Interestingly, DUSP22-rearranged ALCL is characterized by a unique global DNA demethylation profile associated with lack of STAT3 activation and overexpression of CCR8, HAND1, a developmental transcription factor and a group of cancer/testis-associated proteins. This profile includes upregulation of different costimulatory CD58 and HLA class II molecules and down-regulation of PD-1 that may contribute to the good prognosis of DUSP22-rearranged ALCL [55]. Among PTCL, recurrent mutations of the musculin gene (MSC E116K ) encoding a basic helix-loop-helix transcription factor were found to be specific for 35% of DUSP22-rearranged ALCLs [45]. The dominant negative MSC E116K protein promotes the growth of normal and neoplastic T cells by blocking the expression of the cell cycle inhibitor E2F2. This leads to upregulation of the CD30-IRF4-MYC axis in an autocrine feedback loop and confers susceptibility to the BET inhibitor JQ1 [45]. About 24% of ALK-ALCLs are characterized by ectopic co-expression of truncated ERBB4 transcripts and COL29A1 transcripts [56]. Such ERBB4-positive ALCLs frequently display a Hodgkin-like morphology and express MMP9. Two oncogenic truncated ERBB4 transcripts arise from an intronic transcription start site and promote tumorigenesis partially blocked by the pan-HER inhibitor neratinib in experimental models [56].

Detection and Monitoring of Cytokines
CD30/TNFRSF8 is a co-stimulatory molecule expressed on activated T and B cells commonly used as a marker for neoplastic cells of cHL, systemic ALCL and CD30+ CLPD [41,57]. Serum levels of soluble CD30 (sCD30) correlate with tumor burden and normalize following successful treatment of cHL and ALCL [58,59]. Similarly, serum levels of the soluble truncated γ-chain of the IL-2 receptor (sCD25) expressed by activated immune cells correlate with disease activity and prognosis in HL [58][59][60] and mycosis fungoides (MF) [61]. Hanson et al. adapted a commercially available enzyme-linked immunoassay (ELISA) (R&D Systems, Minneapolis, MN, USA) to measure sCD30 in malignant and benign seromas and plasma of patients with breast implants [62]. sCD30 could be detected at concentrations of >1800 pg/mL in seromas of nine patients with BIA-ALCL but not in their plasma or serum and in none of the seven patients with non-neoplastic effusions [62].

Signaling through IL-2R Activates the JAK/STAT Pathway in Cutaneous ALCL
Activation of JAK/STAT proteins was found to be involved in the signal transduction pathway mediated by the receptor for interleukin 2 in malignant T lymphocytes derived from cutaneous ALCL and Sézary syndrome [67]. Interaction of cytokine receptors such as IL-2R with their ligands induces activation of intracellular tyrosine kinases [68]. The high-affinity IL-2R is composed of three chains: α which is specific for IL-2, β and common γ. The cytoplasmic domain of the γ chain is associated with the tyrosine kinase Jak3, whereas the β chain is associated with Jak1 [69]. Binding of IL-2 to IL-2R results in the tyrosine phosphorylation of several substrates, including Jak3 and Jak l themselves, as well as the IL-2R β and γ chains [69]. Soluble IL-2 receptor (sIL-2R) also correlated with tumor burden in SCID mice xenografted with the ALCL line JB6 and was detected in the urine of JB6-transplanted mice [70].

Cytokines in BIA-ALCL
In contrast to ALK+ ALCL, neither IL-17A nor IL-17F was detected in 48 h cell cultures of established BIA-ALCL lines nor in any of the eight malignant effusions (seromas) around breast implants, although one malignant effusion contained the Th17/Th22 cytokine IL-22 [71]. Instead, four out of the eight malignant effusions contained the Th2 cytokine IL-13, and one contained IL-5. More importantly, seven out of the eight malignant effusions contained >500 pg/mL IL-9 (attributed to Th2 or, more specifically, to Th9 cells), and six out of the eight malignant effusions contained IL-10. IL-9 was not detected in any benign effusions. In contrast, IL-6 was detected in both malignant and benign peri-implant effusions [71].

BIA-ALCL
A pathologic feature of infiltrative BIA-ALCL that distinguishes it from most systemic ALCLs is the presence of numerous eosinophils [33]. Influx of eosinophils into tissues is promoted by IL-13, Eotaxin and IL-9, which are produced by anaplastic cells in BIA-ALCL [34,71,72]. IL-13, independently and in coordination with IL-4, promotes immunoglobulin heavy chain class switching of B cells to become IgE-producing plasma cells [73]. Numerous plasma cells are often observed in capsules affected by BI-ALCL [72]. IgE binding to high-affinity receptors of mast cells (FcεR1) triggers the release of vasoactive and chemotactic factors including histamine and prostaglandin D2 (PGD2) that recruits Th2 cells, basophils and eosinophils. Indeed, one of the authors of this review (MEK) has found that anaplastic cells in BIA-ALCL often express the PGD 2 receptor, CRTH 2 ( Figure 3). Another feature of BIA-ALCL is thickening of involved capsules due to fibrosis which can be mediated by IL-13 signaling to stimulate fibroblasts through their PDGFR [74,75].

C-ALCL and CD30+ CLPD
Cytokines released by anaplastic cells in cutaneous lymphomas can dramatically affect the growth of keratinocytes. A noteworthy feature of CD30+ CLPD is pseudoepitheliomatous hyperplasia (PEH) of keratinocytes which can be mistaken for squamous carcinoma (Figure 1). In a study of 25 patients, two patterns of PEH were noted: (1) a follicular pattern was observed in 14 cases, commonly associated with a neutrophilic-rich infiltrate (p = 0.21), and (2) an epidermal pattern was observed in 11 cases and commonly associated with eosinophil-rich infiltrates (p = 0.03) [76]. PEH in CD30+CLPD was associated with Th17/Th22 cytokine expression detected in tumor cells in 81% of cases tested. All 14 cases tested had a strong expression of cytokeratin 17 (CK17), a myoepithelial keratin not found in a healthy epidermis, but this was induced in a dose-dependent manner by IL-17 through the signal transducer and activators of transcription STAT1 and STAT3 [76,77]. IL-22 inhibits the maturation of keratinocytes and stimulates their migration, causing epidermal remodeling and often leading to psoriasiform/PEH-like lesions [78][79][80]. Transgenic overexpression of IL-22 in mice results in psoriasis-like skin alterations including hyperpro-liferation, acanthosis and hypogranularity. IL-22 is produced by Th17 cells and, in a more restricted manner, by Th22 cells. While PEH lesions were associated with spontaneous regression and an indolent course, some patients developed a generalized process with tumor progression [76]. cells [73]. Numerous plasma cells are often observed in capsules affected by BI-ALC IgE binding to high-affinity receptors of mast cells (FcεR1) triggers the release of tive and chemotactic factors including histamine and prostaglandin D2 (PGD2) cruits Th2 cells, basophils and eosinophils. Indeed, one of the authors of this review has found that anaplastic cells in BIA-ALCL often express the PGD2 receptor, CRT ure 3). Another feature of BIA-ALCL is thickening of involved capsules due to which can be mediated by IL-13 signaling to stimulate fibroblasts through their P [74,75].  SATB1, a thymocyte-specific chromatin organizer, helps to classify CD30+ CLPDs with different clinicopathological behaviors. SATB1 expression was identified in CD30+ anaplastic T cells in 11 of 12 (91.7%) lymphomatoid papulosis cases and in 16 of 42 (38.1%) C-ALCL cases. SATB1+ cases showed Th17 polarization, together with more prominent epidermal hyperplasia and granulocytic infiltration, consistent with the above-mentioned PEH [81]. SATB1+ lesions responded better to combined treatment with low-dose (5-20 mg/weekly) methotrexate and interferon γ2b. In clinical samples of C-ALCLs, genes of the IL-13 signaling pathway, including IL13, IL13Rα1, IL13Rα2 and IL4Rα, were enriched in SATB1+ cases, compared to SATB1-cases [82]. All SATB1+ CD30+ CLPDs were found to highly express pSTAT6, and most were IL-13+. A specific inhibitor of pSTAT6 (AS1517499) reduced cell viability in Mac1/2A cutaneous ALCL cell lines in a dose-dependent manner, as described previously for Sézary cells [82], indicating that blocking the IL-13/STAT6 signaling pathway may be a potential therapeutic regimen for SATB1+ CALCLs. In systemic ALCLs, ALK+ ALCLs strongly express SATB1, while ALK-negative cases lack SATB1, and STAT3, RORC and IL17A were highly expressed in SATB1+ cases [83]. Matsuyama et al. demonstrated that NPM-ALK promoted expression of miR-135b and its host gene LEMD1 through the action of STAT3. Further, miR-135b suppressed Th2 regulators GATA3 and STAT6, and miR-153b blockade attenuated IL-17 production, leading to an ALCL phenotype overlapping with Th17 cells [85]. ALK+ ALCL cell lines express Th17-associated signature genes, including IL-17F, IL-22, IL-26, AHR and RORC [64].
Eckerle et al. performed gene expression profiling of microdissected lymphoma cells of five ALK(+) and four ALK(-) systemic ALCLs, seven cALCLs and sixteen cHLs, and of eight subsets of normal T and NK cells [83]. All ALCL types showed significant expression of NFkappaB target genes and upregulation of genes involved in oncogenesis (e.g., EZH2).
Knörr et al. asked whether a Th subset-specific serum cytokine pattern could be identified in childhood ALCL patients [65]. Although some patients showed a pattern of elevated IFN-γ, IP-10 and MIG (both produced upon stimulation with IFN-γ), and levels of IL-17 and IL-23 suggested activation of Th17 cells, ALCLs of most patients did not show a conclusive Th subset pattern.

BIA-ALCL
The cell lineage or differentiation stage of BIA-ALCL appears to be variable. In cell lines derived from malignant seromas, two of four co-expressed the Th2 cytokine IL-13 and the Th1 cytokine IFNγ [71]. None expressed Th17 cytokines characteristic of ALK+ ALCL. Three of four cell lines secreted IL-9, which was detected in eight out of nine malignant effusions. IL-9 was originally assigned to Th2 cells but more recently to Th9 cells, which also produce IL-10, detected together with IL-9 in four out of eight malignant effusions. Such evidence points to a Th2-type cytokine profile with expression of IL-10, IL-13, IL-9 and Eotaxin and frequent expression of the GATA3 and FoxP3 transcription factors. Breast implants may elicit a Th2-type response with accumulation of T cells, mast cells and eosinophils, and, due to activation of the STAT3 pathway, IL-10-producing T regulatory cells are recruited or induced [34]. The plasticity between Th2 cells and iTregs is in accordance with such observation [86]. Di Napoli used gene expression profiling and immunohistochemical data to suggest either activation-induced FoxP3 expression or a T helper-like regulatory T cell status in a proportion of BIA-ALCLs with upregulation of the RORC and IL17A genes and of the FOXP3 protein [87].
Some CD30+ CLPDs are derived from a novel subset of CD4+Th2 cells that produce inflammatory Th17 cytokines [88]. This subset of Th2 cells was reported to promote exacerbation of chronic allergic asthma [89]. These results are consistent with our recent report linking atopy to the pathogenesis of lymphomatoid papulosis [90].

Front-Line Treatments
In adult systemic ALCL, several combined chemotherapy regimens obtain a high event-free survival rate, especially for local or regional disease [19,21,91]. The addition of etoposide improved prognosis, with a 3-year survival of 100% [92]. However, relapse occurred in 20-30% of patients, requiring reinduction by high-dose chemotherapies before autologous or allogenic hematopoietic stem cell transplantation [21,91].
In pediatric patients, adapted protocols with reduced cumulative doses of toxic drugs obtained a 92% overall survival at 2 years. A risk-stratified strategy restricted autologous HCST for pediatric ALCL high-risk relapses and vinblastine for intermediate or low-risk relapses [19,93].
C-ALCL can be cured by complete surgical excision and/or local radiotherapy as first-line treatment [94]. For multifocal lesions, low-dose methotrexate is the first-line treatment, and vinblastine was also proposed at relapse [95,96].
About 80% of BIA-ALCL patients are cured by surgical resection of implants and the surrounding capsule. Adjuvant localized radiation is performed when excision is incomplete. For patients with tumors and/or regional lymph node involvement, adjuvant chemotherapy or targeted therapy is available [97].

Targeted Therapies
Patients with advanced or refractory/relapsing ALCL, BI-ALCL or CALCL obtain their first benefit from targeted therapies directed against CD30, ALK, NOTCH1 or JAK (Tables 1 and 2). Some of them are now also used in combination therapy in front-line treatment [97][98][99].
The anti-CD30 brentuximab-vedotin is a monoclonal anti-CD30 antibody fused to a microtubule inhibitor. The latter is delivered after endocytosis and lysosome fusion ( Figure 2) [99,100]. It has been approved by US and European agencies for the treatment of relapsing systemic ALCL or C-ALCL either as a single-agent therapy or in combination with chemotherapy [99,101]. The limited durability of response together with significant toxicity including neuropathy and cytopenia has led to considering other therapies such as anti-CD30 CAR T cells [102].
A new anti-CD158k antibody primarily developed for the treatment of Sézary syndrome can also be employed in C-ALCL [103,104].
ALK inhibition by first-generation inhibitor crizotinib provided promising results, especially in pediatric ALK+ ALCL, with an objective response rate ranging from 54% to 90% [19]. Resistance to ALK inhibition may be acquired by the emergence of specific ALK mutations that may be counteracted by new-generation inhibitors such as alectinib or ceritinib [105]. IL-10 autocrine synthesis and aberrant upregulation of the IL-10 receptor subunit also bypass NPM-ALK inhibition and contribute to single-ALK inhibitor resistance [66].
Given the resistance mechanisms arising after single-agent therapies, combination therapy associating chemotherapy with either anti-CD30 targeting or ALK inhibition is proposed for patients with relapsing disease [106].
Blocking the IL-9/Jak3 pathway has potential for treatment of both ALK+ ALCL and BIA-ALCL where tumor cells express IL-9 in an autocrine loop [71,107].
In a model of alternative activation of NF-kB in ALCL revealed by CRISPR screening, Wang et al found that in NF-kB-inducing kinase (NIK)-positive ALK-ALCL cells, common JAK/STAT3 mutations promote transcriptional activity of STAT3 which directly regulates NFKB2 and CD30 expression [108]. Endogenous expression of CD30 induced constitutive NF-κB activation through binding and degrading of TRAF3. In ALK+ ALCL, the CD30 pathway is blocked by NPM-ALK oncoprotein, but STAT3 activity and resultant NFKB2 expression can still be induced by NPM-ALK, leading to minimal alternative NF-κB activation. The study suggests combined NIK and JAK inhibitor therapy could benefit patients with NIK-positive ALK-ALCL carrying JAK/STAT3 somatic mutations [108]. ALK+ ALCL expresses a high level of PD-L1 because of the constitutive activation of multiple oncogenic signaling pathways downstream of ALK activity. In a novel model using CRISPR screening, Zhang et al discovered that PD-L1 induction was dependent on NPM-ALK activation of STAT3, as well as a signalosome containing GRB2/SOS1, which activates the MEK-ERK and PI3K-AKT signaling pathways [109]. These signaling networks, through STAT3 and the GRB2/SOS1, induce PD-L1 expression through the action of transcription factors IRF4 and BATF3 on the enhancer region of the PD-L1 gene. IRF4 and BATF3 are essential for PD-L1 upregulation, and IRF4 expression was correlated with PD-L1 levels in primary ALK+ ALCL tissues. Targeting this oncogenic signaling pathway in ALK+ ALCL largely inhibited the ability of PD-L1-mediated tumor immune escape when cocultured with PD-1-positive T cells and natural killer cells.
In preclinical models of ALK-ALCL, JAK 1/2 inhibition by ruxolitinib proved to be more efficient than STAT3 inhibitors [52]. Targeting the Th2 signaling pathway with a specific pSTAT6 inhibitor, AS1517499, also has potential for the treatment of C-ALCL and other CD30+ CTCLs [82,88]. Recent whole-genome sequencing of 12 C-ALCLs also underscored the potential value of PI3K-AKT inhibition for patients who are resistant to skin-directed therapy and/or with extracutaneous progression [110].

Conclusions
The four main types of ALCL differ according to the age of patients, site of onset, prognosis, tumor cell phenotype and genetic lesions.
Translational studies have revealed key cytokine pathways and epigenetic modification regulating tumor cell growth.
Besides directly targeting tumor cells by cytotoxic drugs or antibodies, cytokine pathways may represent potential targets for personalized therapy according to the specific profile of each ALCL case.  Institutional Review Board Statement: Ethical review and approval were waived for this study as no individual patient was enrolled for this literature review.
Informed Consent Statement: Patient consent was waived.