Evaluation of lymphangiogenic markers in Sézary syndrome

Estimation of the tissue and serum levels of IL-35 in Mycosis fungoides: a case-control study

Mycosis fungoides (MF) is the most common primary cutaneous T-cell lymphoma (CTCL) with its etiology not yet fully understood. Interleukin (IL)-35 is an inhibitory cytokine that belongs to the IL-12 family. Elevated IL-35 in the plasma and the tumor microenvironment increases tumorigenesis and indicates poor prognosis in different types of malignancies. The objective of this study is to estimate the expression levels of IL-35 in tissue and serum of MF patients versus healthy controls. This case-control study included 35 patients with patch, plaque, and tumor MF as well as 30 healthy controls. Patients were fully assessed, and serum samples and lesional skin biopsies were taken prior to starting treatment. The IL-35 levels were measured in both serum and tissue biopsies by ELISA technique. Both tissue and serum IL-35 levels were significantly higher in MF patients than in controls (P < 0.001) and tissue IL-35 was significantly higher than serum IL-35 in MF patients (P < 0.001). Tissue IL-35 was significantly higher in female patients and patients with recurrent MF compared to male patients and those without recurrent disease (P < 0.001). Since both tissue and serum IL-35 levels are increased in MF, IL-35 is suggested to have a possible role in MF pathogenesis. IL-35 can be a useful diagnostic marker for MF. Tissue IL-35 can also be an indicator of disease recurrence.

Keratinocytes Present Staphylococcus aureus Enterotoxins and Promote Malignant and Nonmalignant T Cell Proliferation in Cutaneous T-Cell Lymphoma

Cutaneous T-cell lymphoma is characterized by malignant T cells proliferating in a unique tumor microenvironment dominated by keratinocytes (KCs). Skin colonization and infection by Staphylococcus aureus are a common cause of morbidity and are suspected of fueling disease activity. In this study, we show that expression of HLA-DRs, high-affinity receptors for staphylococcal enterotoxins (SEs), by KCs correlates with IFN-γ expression in the tumor microenvironment. Importantly, IFN-γ induces HLA-DR, SE binding, and SE presentation by KCs to malignant T cells from patients with Sézary syndrome and malignant and nonmalignant T-cell lines derived from patients with Sézary syndrome and mycosis fungoides. Likewise, preincubation of KCs with supernatant from patient-derived SE-producing S aureus triggers proliferation in malignant T cells and cytokine release (including IL-2), when cultured with nonmalignant T cells. This is inhibited by pretreatment with engineered bacteriophage S aureus-specific endolysins. Furthermore, alteration in the HLA-DR-binding sites of SE type A and small interfering RNA-mediated knockdown of Jak3 and IL-2Rγ block induction of malignant T-cell proliferation. In conclusion, we show that upon exposure to patient-derived S aureus and SE, KCs stimulate IL-2Rγ/Jak3-dependent proliferation of malignant and nonmalignant T cells in an environment with nonmalignant T cells. These findings suggest that KCs in the tumor microenvironment play a key role in S aureus-mediated disease activity in cutaneous T-cell lymphoma.

TOX Expression in Mycosis Fungoides and Sezary Syndrome

Mycosis fungoides (MF) and Sezary syndrome (SS) are the two most common type of cutaneous T-cell lymphoma (CTCL). Currently, no markers can be clearly related to prognosis or to differential diagnosis between early stages and inflammatory benign diseases (IBD). The thymocyte selection-associated high mobility group box factor (TOX), has been proposed as a possible marker in differential diagnosis between early CTCL stages and IBD. Recently TOX has been related to prognosis. We aimed to investigate whether TOX may be a diagnostic or prognostic marker. MF and SS biopsies between 2010 and 2020 were retrieved. New tissues slides were stained with an anti-TOX antibody, (Clone NAN448B). On each slide, 5 fields were examined at high magnification (400×), to evaluate the percentage of marker-positivity in a quantitative way. Thirty-six patients (12 females and 24 males) and 48 biopsies were collected. Nine patients had multiple biopsies. TOX expression in MF/SS cases showed an increase from early to advanced phases. TOX was not regarded as a prognostic marker due to the absence of significant changes by comparing early MF cases with reactive conditions. TOX statistical significance increased in patients alive with disease and in those dead of disease (p = 0.013 and = 0.0005, respectively) as compared with patients in complete remission. Our results show that TOX should be regarded more as a prognostic than a diagnostic marker.

The Role of Podoplanin in Skin Diseases

Podoplanin is a sialomucin-like type I transmembrane receptor glycoprotein that is expressed specifically in lymphatic vessels, sebaceous glands, and hair follicles in normal skin. However, under pathological conditions podoplanin expression is upregulated in various cells, such as keratinocytes, fibroblasts, tumor cells, and inflammatory cells, and plays pivotal roles in different diseases. In psoriasis, podoplanin expression is induced in basal keratinocytes via the JAK-STAT pathway and contributes toward epidermal hyperproliferation. Podoplanin expression on keratinocytes can also promote IL-17 secretion from lymphocytes, promoting chronic inflammation. During wound healing, the podoplanin/CLEC-2 interaction between keratinocytes and platelets regulates re-epithelialization at the wound edge. In skin cancers, podoplanin expresses on tumor cells and promotes their migration and epithelial-mesenchymal transition, thereby accelerating invasion and metastasis. Podoplanin is also expressed in normal peritumoral cells, such as cancer-associated fibroblasts in melanoma and keratinocytes in extramammary Paget's disease, which promote tumor progression and predict aggressive behavior and poor prognosis. This review provides an overview of our current understanding of the mechanisms via which podoplanin mediates these pathological skin conditions.

The Role of Tumor Microenvironment in the Pathogenesis of Sézary Syndrome

Sézary syndrome is an aggressive leukemic variant of cutaneous T-cell lymphomas, characterized by erythroderma, lymphadenopathy, and peripheral blood involvement by CD4+ malignant T-cells. The pathogenesis of Sézary syndrome is not fully understood. However, the course of the disease is strongly influenced by the tumor microenvironment, which is altered by a combination of cytokines, chemokines, and growth factors. The crosstalk between malignant and reactive cells affects the immunologic response against tumor cells causing immune dysregulation. This review focuses on the interaction of malignant Sézary cells and the tumor microenvironment.

Pathogenesis of cutaneous T cell lymphoma: Involvement of Staphylococcus aureus

Mycosis fungoides (MF) and Sézary syndrome (SS) are representative cutaneous lymphomas. In their early stage, a small number of tumor cells and a large number of non-malignant cells form a Th1-dominant tumor microenvironment. Increase in malignant T cells is accompanied by a decrease in CD8-positive T cells, with a shift toward a Th2-dominant milieu in advanced-stage lesions. The etiologies of MF/SS are diverse, and the underlying pathogenetic mechanisms are yet to be elucidated. Advanced MF/SS is known to be highly sensitive to Staphylococcus aureus, and the majority of deaths are caused by severe infections. The susceptibility to infection is associated with barrier dysfunction and immunosuppression, which are the main symptoms of MF. In recent years, skin-colonizing S. aureus has been identified to not only cause severe infections but also play an important role in the pathogenesis of MF/SS. Staphylococcal superantigens activate the proliferation of tumor cells and induce CD25 upregulation, FOXP3 expression, IL-17 expression, and miR-155 expression. Alpha-toxin eliminates non-neoplastic CD4-positive cells and CD8-positive cells and plays a major role in tumor cell selection. Lipoprotein may also be associated with the induction of Th2-dominant milieu. Antibiotic therapy for S. aureus eradication has been reported to cause considerable clinical improvement in the majority of individuals with advanced cutaneous T-cell lymphoma. Therefore, S. aureus may be a novel target for the treatment of advanced-stage MF/SS in the future.

The Microenvironment's Role in Mycosis Fungoides and Sézary Syndrome: From Progression to Therapeutic Implications

BACKGROUND

Mycosis fungoides (MF) and Sezary Syndrome (SS) are the most common cutaneous T-cell lymphomas. It has been hypothesized that the interaction between the immune system, cutaneous cells, and neoplastic elements may play a role in MF/SS pathogenesis and progression.

METHODS

This paper aims to revise in a narrative way our current knowledge of the microenvironment's role in MF/SS.

RESULTS AND CONCLUSIONS

Literature data support a possible implication of microenvironment cells in MF/SS pathogenesis and progression, opening up new therapeutic avenues.

Immune Check Point Inhibitors in Primary Cutaneous T-Cell Lymphomas: Biologic Rationale, Clinical Results and Future Perspectives

Primary cutaneous T-cell lymphomas (PCTCL) are the most common types of cutaneous lymphomas, with Mycosis fungoides as the most frequent subtype. Besides early stages which usually have a good prognosis, advanced stages remain a great therapeutic challenge with low survival rates. To date, none of the currently available therapeutic options have significantly improved the outcomes of advanced cutaneous lymphomas. Recent studies have demonstrated that immune-checkpoint molecules, such as PD-1 and CTLA-4, play part in the proliferation pathways of neoplastic T-cells, as well as in other tumors. Hence, the potential role of immune-checkpoint-inhibitors in treating cutaneous lymphomas has been investigated in the last years. Herein, we outline the current knowledge regarding the role of immune-checkpoint molecules in PCTCL, their signaling pathways, microenvironment and therapeutic inhibition rationale. Moreover, we review the published data on immunotherapies in PCTCL and summarize the currently ongoing clinical trials in this field.

Phenotypical Markers, Molecular Mutations, and Immune Microenvironment as Targets for New Treatments in Patients with Mycosis Fungoides and/or Sézary Syndrome

Primary cutaneous lymphomas encompass a wide spectrum of rare lymphoproliferative disorders originating in the skin, among which, mycosis fungoides (MF) is the most common subtype. The treatment of this disease is based on skin-directed therapies eventually in association with biologic response modifiers in the early phases, whereas in patients with the advanced stages, several therapeutic strategies can be used including mono and/or polychemotherapy and bone marrow transplantation. In recent years, the identification of specific markers (phenotypical, immunological, and molecular) has led to the development of several studies (including two randomized phase III trials). The results of these studies are modifying our therapeutic strategy toward a personalized treatment approach in which the clinical characteristics of the patients and tumor-node-metastasis-blood stage are considered together with the expression of specific markers (i.e., a CD30-positive expression for the use of brentuximab vedotin). This review will provide a comprehensive scenario of the main phenotypical, molecular, and immunological markers related to MF pathogenesis and disease evolution, which could represent the target for the development of innovative effective treatments in this disease.

Podoplanin as a Predictive Marker for Identification of High-Risk Mycosis Fungoides Patients: An Immunohistochemical Study

Background

Podoplanin, an important protein, has been implicated in various cellular processes, including lymphangiogenesis. Podoplanin is a mucin-type transmembrane glycoprotein that is accepted as a novel marker of lymphatic endothelial cells.

Objectives

To study the immunohistochemical expression of podoplanin in the different stages of mycosis fungoides (MF) in comparison to control and to correlate their expression with disease severity and progression.

Materials and Methods

The study included 50 patients of MF, clinically diagnosed and assessed by World Health Organization/European Organization for Research And Treatment Of Cancer Consensus and 20 normal persons as control. Skin biopsy specimens were taken from all and examined for expression of podoplanin immunohistochemically.

Results

Significant upregulation of podoplanin expression was detected in all studied patients of MF in comparison to control group. Podoplanin expression in malignant lymphocytes and also lymph vessel density showed significant upregulation in the aggressive clinical presentations as well as the highest stages regarding TNMB staging of MF.

Conclusions

Evaluation of podoplanin expression may be taken into consideration in the future as a useful tool to identify high-risk MF patients. Furthermore, it may open new therapeutic options for the clinical management of those patients.

Cutaneous T-cell lymphomas: modern data of pathogenesis, clinics and therapy

Cutaneous T-cell lymphomas (CTCL) are a heterogeneous group of extranodal non-Hodgkin’s lymphomas that are characterized by skin infiltration with malignant monoclonal T lymphocytes. More common in adults aged 55 to 60 years, the annual incidence is about 0.5 per 100 000 people. Mycosis fungoides, Sézary syndrome and CD30+ lymphoproliferative diseases are the main subtypes of CTCL. To date, CTCL have a complex concept of etiopathogenesis, diagnosis, therapy and prognosis. The article presented summary data on these issues.

Malignant T cells express lymphotoxin α and drive endothelial activation in cutaneous T cell lymphoma

Lymphotoxin α (LTα) plays a key role in the formation of lymphatic vasculature and secondary lymphoid structures. Cutaneous T cell lymphoma (CTCL) is the most common primary lymphoma of the skin and in advanced stages, malignant T cells spreads through the lymphatic to regional lymph nodes to internal organs and blood. Yet, little is known about the mechanism of the CTCL dissemination. Here, we show that CTCL cells express LTα in situ and that LTα expression is driven by aberrantly activated JAK3/STAT5 pathway. Importantly, via TNF receptor 2, LTα functions as an autocrine factor by stimulating expression of IL-6 in the malignant cells. LTα and IL-6, together with VEGF promote angiogenesis by inducing endothelial cell sprouting and tube formation. Thus, we propose that LTα plays a role in malignant angiogenesis and disease progression in CTCL and may serve as a therapeutic target in this disease.

Lymphatic Vessels, Inflammation, and Immunity in Skin Cancer

Skin is a highly ordered immune organ that coordinates rapid responses to external insult while maintaining self-tolerance. In healthy tissue, lymphatic vessels drain fluid and coordinate local immune responses; however, environmental factors induce lymphatic vessel dysfunction, leading to lymph stasis and perturbed regional immunity. These same environmental factors drive the formation of local malignancies, which are also influenced by local inflammation. Herein, we discuss clinical and experimental evidence supporting the tenet that lymphatic vessels participate in regulation of cutaneous inflammation and immunity, and are important contributors to malignancy and potential biomarkers and targets for immunotherapy.

SIGNIFICANCE

The tumor microenvironment and tumor-associated inflammation are now appreciated not only for their role in cancer progression but also for their response to therapy. The lymphatic vasculature is a less-appreciated component of this microenvironment that coordinates local inflammation and immunity and thereby critically shapes local responses. A mechanistic understanding of the complexities of lymphatic vessel function in the unique context of skin provides a model to understand how regional immune dysfunction drives cutaneous malignancies, and as such lymphatic vessels represent a biomarker of cutaneous immunity that may provide insight into cancer prognosis and effective therapy.

Aggressive primary cutaneous B-cell lymphomas show increased Angiopoietin-2-induced angiogenesis

Primary cutaneous large B-cell lymphomas, leg type (PCLBCL/LT) are primary cutaneous B-cell lymphoma (PCBCL) with an intermediate prognosis. Therefore, antracycline-based polychemotherapy combined with rituximab has been recommended as first-line treatment. Yet, despite this regimen, the 5-year survival rate remains 50-66% only. Angiogenesis, the formation of a vascular network, is essential for the pathogenesis of nodal lymphomas. So far, no study has analysed angiogenesis and its key factors in PCLBCL/LT. The present study was aimed at characterizing angiogenesis in PCLBCL/LT to identify the angiogenic molecules as potential therapeutic targets. The intra-tumoral microvessel density (MVD) was assessed by immunohistochemical studies of CD20 and CD31. The MVD was higher in PCLBCL/LT compared with indolent PCBCL. Analyses of open-source microarray data showed correlation between the angiogenic molecule angiopoietin-2 (Ang-2) and pan-endothelial cell markers. ELISA studies determined a shift between Ang-2 and Ang-1 towards Ang-2 in the peripheral blood of PCLBCL/LT patients. Immunofluorescence costainings against the Ang receptor Tie2/angiogenic integrins/CD34 revealed that the vasculature in both aggressive and indolent PCBCL tumors harbours an endothelial cell subpopulation with reduced expression of Tie2. In contrast, the alternative Ang-2 binding partners, angiogenic integrins, are strongly expressed in PCBCL. In line with these findings, downstream targets of Ang-2-integrin signalling, that is phosphorylation of focal adhesion kinase at Tyr397, and sprouting angiogenesis are enhanced in PCLBCL/LT. Our data present Ang-2 as a promising therapeutic target and anti-angiogenic therapy as a new line in treatment of PCLBCL/LT as a hitherto intractable disease.

Vascular endothelial growth factor A (VEGFA) expression in mycosis fungoides

AIMS

High levels of vascular endothelial growth factor A (VEGFA) seem to herald a worse prognosis in mycosis fungoides (MF). In this study, we aimed to characterize more clearly VEGFA gene and protein expression in MF.

METHODS AND RESULTS

First, we compared VEGFA mRNA levels in MF and in normal T lymphocyte samples; significantly higher VEGFA levels were found in MF. We then studied VEGFA expression in different normal T cell subsets, focusing on CD4(+) , CD8(+) , resting and activated T lymphocytes. We applied the gene signatures of the normal T cell subsets to MF samples and found that activated T lymphocytes represented the closest normal counterpart of the tumour. However, VEGFA mRNA levels were significantly higher in MF than in activated normal T cells, suggesting that VEGFA overexpression in MF represents an attribute acquired during neoplastic transformation: no significant VEGFA expression differences were recorded between early and advanced stages. Gene expression profile results were supported by immunohistochemistry in routine sections from 27 MF cases.

CONCLUSIONS

For the first time, we demonstrate VEGFA expression in MF cells, suggesting that the VEGF pathway may be implicated in MF pathogenesis and can represent a novel therapeutic target.

Expression of CD31/PECAM-1 (platelet endothelial cell adhesion molecule 1) by blastic plasmacytoid dendritic cell neoplasms

IMPORTANCE

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare malignant neoplasm with cutaneous manifestations and a rapidly progressive clinical course. The diagnosis relies on characteristic clinicopathologic and immunopathologic features. However, the overlap of immunophenotypic features with other cancers, as well as newly discovered interpersonal and intrapersonal phenotypic variations, renders the identification of BPDCN challenging. A greater understanding of the proteins expressed by BPDCN might facilitate its recognition and provide insights into its clinical behavior.

OBSERVATIONS

In 7 of 9 patients at 4 tertiary care institutions, immunohistochemical analysis demonstrated strong CD31/PECAM-1 (platelet endothelial cell adhesion molecule 1) expression by neoplastic cells. Combined with similar findings observed in 1 former patient, 8 of 10 cases of BPDCN were CD31/PECAM-1 positive.

CONCLUSIONS AND RELEVANCE

Expression of CD31/PECAM-1 by BPDCN adds new information about the antigenic profile of this unusual neoplasm. CD31/PECAM-1 influences multiple cell functions including adhesion, apoptosis, coagulation, host response, and protein synthesis that might affect clinical features of BPDCN such as hemorrhage, aggressive tumor growth, and resistance to therapy. Therefore, the potential role of this molecule in the tumor formation and progression of BPDCN warrants additional exploration.

IL-15 and IL-17F are differentially regulated and expressed in mycosis fungoides (MF)

Skin lesions from mycosis fungoides (MF) patients display an increased expression of interleukin-15 (IL-15), IL-17F, and other cytokines implicated in inflammation and malignant cell proliferation in cutaneous T-cell lymphoma (CTCL). In the leukemic variant of CTCL, Sézary syndrome (SS), IL-2 and IL-15 trigger activation of the Jak-3/STAT3 pathway and transcription of IL17A gene, whereas it is unknown what causes IL-15 expression, Jak3/STAT3 activation, and production of IL-17F in MF. Here, we studied the expression and regulation of IL-15 and its relation to IL-17F in MF cell lines and skin lesions from 60 MF patients. We show that: (1) the spontaneous IL-15 mRNA expression is resistant to Jak3 and STAT3 inhibitors at concentrations that profoundly inhibit STAT3 activation and IL-17F mRNA expression; (2) anti-IL-15 antibody blocks STAT3 activation induced by exogenous IL-15 in non-malignant MF T cells, whereas the spontaneous STAT3 activation and IL-17F expression in malignant T cells is not inhibited; (3) patients display heterogeneous IL-15/IL-17F mRNA expression patterns in skin lesions; and (4) IL-15 expression (in contrast to IL-17F) is not associated with progressive disease. Taken together, these findings indicate that IL-15 and IL-17F are differentially regulated and expressed in MF. We propose that IL-15 and IL-17F are markers for different inflammatory environments and play distinct roles in the development and progression of MF.

Expression of miR-155 and miR-126 in situ in cutaneous T-cell lymphoma

Recently, miR-155 has been implicated in cutaneous T-cell lymphoma (CTCL). Thus, elevated levels of miR-155 were observed in skin lesions from CTCL patients as judged from qPCR and micro-array analysis and aberrant, high miR-155 expression was associated with severe disease. Moreover, miR-155 promoted proliferation of malignant T cells in vitro. Little is, however, known about which cell types express miR-155 in vivo in CTCL skin lesions. Here, we study miR-155 expression using in situ hybridization (ISH) with a miR-155 probe, a negative control (scrambled), and a miR-126 probe as a positive control in nine patients with mycosis fungoides, the most frequent subtype of CTCL. We provide evidence that both malignant and non-malignant T cells stain weakly to moderately positive with the miR-155 probe, but generally negative with the miR-126 and negative control probes. Reversely, endothelial cells stain positive for miR-126 and negative for miR-155 and the control probe. Solitary T cells with a malignant morphology display brighter staining with the miR-155 probe. Taken together, our findings suggest that both malignant and non-malignant T cells express miR-155 in situ in CTCL. Moreover, they indicate heterogeneity in miR-155 expression among malignant T cells.

Elucidating the role of interleukin-17F in cutaneous T-cell lymphoma

Inappropriately regulated expression of interleukin (IL)-17A is associated with the development of inflammatory diseases and cancer. However, little is known about the role of other IL-17 family members in carcinogenesis. Here, we show that a set of malignant T-cell lines established from patients with cutaneous T-cell lymphoma (CTCL) spontaneously secrete IL-17F and that inhibitors of Janus kinases and Signal transducer and activator of transcription 3 are able to block that secretion. Other malignant T-cell lines produce IL-17A but not IL-17F. Upon activation, however, some of the malignant T-cell lines are able to coexpress IL-17A and IL-17F, leading to formation of IL-17A/F heterodimers. Clinically, we demonstrate that IL-17F messenger RNA expression is significantly increased in CTCL skin lesions compared with healthy donors and patients with chronic dermatitis. IL-17A expression is also increased and a significant number of patients express high levels of both IL-17A and IL-17F. Concomitantly, we observed that the expression of the IL-17 receptor is significantly increased in CTCL skin lesions compared with control subjects. Importantly, analysis of a historic cohort of 60 CTCL patients indicates that IL-17F expression is associated with progressive disease. These findings implicate IL-17F in the pathogenesis of CTCL and suggest that IL-17 cytokines and their receptors may serve as therapeutic targets.

New molecular insights into peripheral T cell lymphomas

Peripheral T cell lymphomas (PTCLs) are heterogeneous neoplasms and represent about 12% of all lymphoid malignancies. They are often regarded as "orphan diseases," a designation that does not reflect their real incidence but rather signifies the difficulties encountered in their classification, diagnosis, and treatment. Here we revise the current understanding of the pathobiological characteristics of the most common nodal PTCLs by focusing on the contribution given by high-throughput technologies and the identification of potential therapeutic targets proposed by translational studies.

Vascular endothelial growth factor receptor-3 expression in mycosis fungoides

Here, we have studied vascular endothelial growth factor receptor-3 (VEGFR-3) expression in mycosis fungoides (MF), the most common type of cutaneous T-cell lymphoma (CTCL). Immunohistochemistry revealed that in two-thirds of 34 patients, VEGFR-3 was expressed in situ by both tumor and stromal cells irrespective of the disease stage. The natural VEGFR-3 ligand, VEGF-C, partially protected malignant T-cell lines from growth inhibition by the histone deacetylase inhibitor, suberoylanilide hydroxamic acid (SAHA). Whereas the malignant T cells did not produce VEGF-C in vitro, its expression was induced during tumor formation in vivo in a xenograft mouse model of MF. In conclusion, malignant and stromal cells express high levels of VEGFR-3 in all stages of MF. Moreover, malignant T cells trigger enhanced VEGF-C expression in fibroblasts, suggesting that cross-talk between tumor and stromal cells plays a role in lymphangiogenesis and possibly disease progression.

Mast cells play a protumorigenic role in primary cutaneous lymphoma

Primary cutaneous lymphomas (PCLs) are clonal T- or B-cell neoplasms, which originate in the skin. In recent years, mast cells were described as regulators of the tumor microenvironment in different human malignancies. Here, we investigated the role of mast cells in the tumor microenvironment of PCL. We found significantly increased numbers of mast cells in skin biopsies from patients with cutaneous T-cell lymphoma (CTCL) and cutaneous B-cell lymphoma (CBCL). Mast cell infiltration was particularly prominent in the periphery, at lymphoma rims. Interestingly, CTCL and CBCL patients with a progressive course showed higher mast cell counts than stable patients, and mast cell numbers in different stages of CTCL correlated positively with disease progression. In addition, mast cell numbers positively correlated with microvessel density. Incubating primary CTCL cells with mast cell supernatant, we observed enhanced proliferation and production of cytokines. In line with our in vitro experiments, in a mouse model of cutaneous lymphoma, tumor growth in mast cell-deficient transgenic mice was significantly decreased. Taken together, these experiments show that mast cells play a protumorigenic role in CTCL and CBCL. Our data provide a rationale for exploiting tumor-associated mast cells as a prognostic marker and therapeutic target in PCL.