American Registry of Pathology Expert Opinions: Recommendations for the diagnostic workup of mature T cell neoplasms

The diagnosis of T-cell lymphomas is highly challenging and requires an integrated approach in which clinical, morphologic, immunophenotypic and molecular data are incorporated into the diagnosis. Under the auspices of the American Registry of Pathology, the authors met to discuss this topic with the goal to provide practical and useful recommendations for pathologists when evaluating T-cell lymphomas. In this review, we discuss the diagnostic findings and workup for the various types of nodal T-cell lymphoma including anaplastic large cell lymphoma, nodal peripheral T-cell lymphoma not otherwise specified (PTCL-NOS), and PTCL with a T follicular helper (TFH) phenotype. We review clinicopathologic and immunophenotypic features (including flow cytometry panels) helpful in the differential diagnosis of mature T-cell lymphomas presenting in the peripheral blood and bone marrow, and we discuss some of the more common extranodal-based T-cell lymphomas including extranodal natural killer/T-cell lymphoma of nasal and non-nasal type, gamma delta T cell lymphomas, and aggressive and indolent T- and NK-lymphoproliferative disorders involving the gastrointestinal tract. Mycosis fungoides and most other cutaneous T-cell lymphomas are not the focus of this review, although the differential diagnosis of Sezary syndrome from mycosis fungoides is covered. We do not intend for these recommendations to be anything other than suggestions that will hopefully spur on additional discussion, and perhaps eventually evolve into a consensus approach for the workup of T-cell lymphomas.     

Galectin-1-mediated apoptosis in mycosis fungoides: the roles of CD7 and cell surface glycosylation.

Sezary cells, the malignant T cells in mycosis fungoides/Sezary syndrome, resist a variety of apoptosis-inducing agents, a feature that contributes to the poor response to therapy in mycosis fungoides. Galectin-1 is a mammalian lectin that triggers T cell apoptosis. For T cells to be susceptible to galectin-1-induced apoptosis, the T cells must express specific glycoprotein receptors, such as CD7, that bear the specific oligosaccharides recognized by galectin-1. Because Sezary cells are characteristically CD7(-), lack of CD7 expression has been proposed to render Sezary cells resistant to galectin-1-induced death. However, the role played by aberrant cell surface glycosylation in resistance of Sezary cells to galectin-1 has not been examined. In this study, we demonstrated abundant galectin-1 in mycosis fungoides skin lesions, indicating that Sezary cells are exposed to galectin-1 in vivo. To determine specific characteristics of Sezary cells that contribute to galectin-1 resistance, we assessed CD7 expression and cell surface glycosylation of Sezary cells in mycosis fungoides lesions and of four Sezary T cell lines. Sezary cells in primary lesions and Sezary T cell lines demonstrated a characteristic "glycotype" with sialylated core 1 O-glycans that promote galectin-1 resistance. Expression of CD7 was necessary but not sufficient for galectin-1-induced death of Sezary cell lines. In addition, CD7(-) Sezary cell lines, and Sezary cells within mycosis fungoides lesions, expressed galectin-1, whereas CD7-positive Sezary cell lines did not express galectin-1. We propose that both loss of CD7 expression and altered cellular glycosylation contribute to apoptosis resistance of malignant T cells in mycosis fungoides.     

Classification of non-Hodgkin malignant lymphomas of the skin

The authors subdivide the primary non-Hodgkin cutaneous malignant lymphomas into "proper" and "non-proper" types. "Proper" lymphomas are those which have in the skin their proper site of localization, and include mycosis fungoides, Pagetoid reticulosis, Baccaredda-Sézary syndrome and possibly lymphomatoid papulosis. They are T-cell lymphomas arising in the papillary dermis, characterized by epidermotropism, having a specific clinical feature in that they are unlikely to be simulated by other cutaneous malignant lymphomas. "Non-proper" lymphomas are those which do not usually arise in the skin, but in various other organs. They are B-, T- or null-cell lymphomas, arising in the middle dermis, infrequently epidermotropic, having a papular-nodular-tumoural clinical feature, which are indistinguishable clinically from other neoplastic types such as plasmacytoma and Hodgkin's disease. The three classifications of non-Hodgkin lymphomas most followed are not directly applicable to cutaneous lymphomas because some of the former are not primarily sited in the skin, and because a follicular morphology is infrequently seen in the latter. Whereas the first classification reported for cutaneous lymphomas utilized malignancy as a criterion, the present classification here proposed utilizes the propriety of the site of localization as the criterion for subdivision into "proper" and "non-proper" types.     

Cutaneous malignant lymphomas. A clinicopathologic study of thirty-seven cases

Clinical and histological findings in 37 cases of cutaneous lymphomas other than mycosis fungoides and Sezary syndrome were investigated; there were 31 in adults and 6 in children. Cutaneous lesions were the first manifestations of the diseases in all cases, and they appeared mostly as tumors or nodules. Cytomorphologically, about a half of the cases showed proliferations of large cleaved, non-cleaved cells or immunoblasts (Group I). Eight cases showed a polymorphous appearance containing convoluted cells of various size (Group II). Five cases in children demonstrated monomorphous proliferation of uniform-sized lymphoblasts (Group III). The cytologic findings in 6 cases did not fit into any lymphoid groups (Group IV). The clinical findings observed in each group were reviewed and compared. Follow-up study revealed that the prognosis of Group I was the poorest among the four groups.     

CUTANEOUS MALIGNANT LYMPHOMAS

Clinical and histological findings in 37 cases of cutaneous lymphomas other than mycosis fungoides and Sezary syndrome were investigated; there were 31 in adults and 6 in children. Cutaneous lesions were the first manifestations of the diseases in all cases, and they appeared mostly as tumors or nodules. Cytomorphologically, about a half of the cases showed proliferations of large cleaved, non-cleaved cells or immunoblasts (Group I). Eight cases showed a polymorphous appearance containing convoluted cells of various size (Group II). Five cases in children demonstrated monomorphous proliferation of uniform-sized lymphoblasts (Group III). The cytologic findings in 6 cases did not fit into any lymphoid groups (Group IV). The clinical findings observed in each group were reviewed and compared. Follow-up study revealed that the prognosis of Group I was the poorest among the four groups.     

Is it possible to cure a patient with mycosis fungoides? A case report

Sezary syndrome (SS) and mycosis fungoides (MF) are a group of non Hodgkin lymphomas that originate from T-lymphocytes and involve mostly the skin. These entities are generally non treatable and patient prognosis remains poor even with the advent of current treatment schedules. Complete remissions are seldom observed. For this reason, bone marrow transplant has been used as a treatment option. The high mortality associated with this procedure has turned reduced intensity conditioning stem cell transplant into a treatment option. This case study illustrates how stem cell transplant offers complete remission of this type of lymphomas.     

Primary cutaneous lymphoma: An overview based on the WHO–EORTC classification

Primary cutaneous lymphomas, defined by their presentation in the skin in the absence of extracutaneous disease at diagnosis, are a heterogenous group of T- and B-cell neoplasms. The commonest, mycosis fungoides, has no nodal counterpart but others show morphological, immunophenotypic and genotypic similarities to primary extracutaneous lymphomas and yet display very different clinical behaviour. This review discusses the salient features of the major entities in the context of the recent World Health Organization/European Organization for the Research and Treatment of Cancer (WHO/EORTC) classification of primary cutaneous lymphoma. In particular, it emphasizes the importance of close clinicopathological correlation and detailed immunophenotyping in their diagnosis.     

CD69 expression correlates with expression of other markers of Th1 T cell differentiation in peripheral T cell lymphomas

CD69, a marker of early T cell activation, is associated with Th1 T cell differentiation. Previously we found that peripheral T cell lymphomas could be subdivided based on the expression of markers of Th1 versus Th2 differentiation, including CXCR3, CD134/OX40, CCR4, and CD30. Here we report immunohistochemical staining for CD69 in frozen and paraffin sections of peripheral T cell lymphomas that exhibit immunoreactivity for markers of Th1 or Th2 differentiation. CD69 expression correlated with immunoreactivity for other Th1 differentiation markers in 18 of 19 frozen specimens of peripheral T cell lymphomas (P = 0.0005). In 10 of these cases in which paraffin-embedded tissue was available for study, CD69 immunohistochemical staining of paraffin sections correlated with frozen section expression. CD69 immunostaining was performed on paraffin sections from 53 additional cases of peripheral T cell lymphoma and correlated with immunoreactivity for other Th1 differentiation markers (P < 0.0001) and was associated with specific subtypes of peripheral T cell lymphoma, including angioimmunoblastic lymphoma, Lennert's lymphoma, and mycosis fungoides/Sezary syndrome, previously noted to express Th1 differentiation-associated markers. Anaplastic large cell lymphoma, both systemic and cutaneous, which typically exhibits immunoreactivity for markers of Th2 expression, was negative for CD69 immunostaining in 22 of 24 cases. CD69 immunostaining results support previous findings that a subset of T cell lymphomas exhibits immunophenotypic features of either Th1 or Th2 T cell differentiation. In addition, CD69 is a useful immunohistochemical marker for specific T cell lymphomas in frozen and paraffin-embedded tissue.     

Peripheral T-cell lymphomas: a clinicopathological and immunological study of 10 cases

Diagnosis and classification of T-cell lymphomas is notoriously difficult. Existing classification schemes are insufficient. Some clinicopathologically well defined T-cell lymphomas exist (mycosis fungoides, Sézary's syndrome, and T-lymphoblastic lymphomas) but the remaining tumours, frequently called peripheral T-cell lymphomas, are a heterogeneous group, clinically, morphologically and immunologically. The data on 10 peripheral T-cell lymphomas are presented and compared to data from the literature. Patients were elderly, had a high frequency of extranodal localizations (notably the skin 75%) and had a poor prognosis: five of 10 patients have died, median survival 22 months. Morphologically and immunophenotypically the group is very heterogeneous. The variety of blast cell morphology is emphasized. No correlations were found between immunophenotype and prognosis, or immunophenotype and morphology.     

Full clinical recovery after topical acyclovir treatment of Epstein-Barr virus associated cutaneous B-cell lymphoma in patient with mycosis fungoides

Primary cutaneous T- and B-cell lymphomas are a heterogeneous group of diseases with varied clinical presentations and prognosis. The use of new molecular, histological, and clinical criteria has improved their recognition. Cutaneous B-cell and T-cell lymphomas are seldom found together in the same patient. Here we report a rare case of mycosis fungoides variant of a cutaneous T-cell lymphoma (CTCL) which later developed Epstein-Barr virus (EBV) associated cutaneous B-cell lymphoproliferative disorder. The patient initially presented with generalized erythroderma, extensive plaques, and axillary lymphadenopathy. Histopathology and immunophenotyping of her tumor from the right breast nodule revealed a T-cell lymphoma consistent with mycosis fungoides. She was initially treated with pentostatin, followed by topical mechlorethamine and topical steroids. After progression of her mycosis fungoides with worsening diffuse skin lesions on this regimen, her treatments were changed to oral bexarotene with an initial partial response followed by stable disease. Three years from her initial presentation, she developed ulcerated cauliflower-like nodules on her forehead. Biopsy of these lesions revealed EBV-positive large- and medium-sized pleomorphic B-cells consistent with EBV-driven B-cell lymphoproliferative disorder. She was treated with topical acyclovir cream on the involved skin areas while continuing with oral bexarotene for mycosis fungoides. Skin lesions gradually diminished and totally disappeared after four weeks of topical acyclovir treatment. Bexarotene treatment was continued for another year until the mycosis fungoides progressed and became wide spread causing her death four and a half years after the initial diagnosis. The coexistence of two cutaneous non-Hodgkin lymphomas of different lineage in the same patient and the complete clinical response of EBV-related B-cell cutaneous component to topical acyclovir makes this rare case particularly interesting.     

Expression of the bcl-2 protein in B cell lymphomas arising from mucosa associated lymphoid tissue.

AIM--To determine whether lymphomas arising from mucosa associated lymphoid tissue (MALT) express the bcl-2 protein. METHODS--Forty two cases of MALT B cell lymphomas, 20 low grade neoplasms and 22 high grade tumours, were studied. Immunohistological staining was performed on paraffin wax embedded tissue using a monoclonal antibody specific for the bcl-2 protein. RESULTS--All of the low grade lymphomas gave positive results on staining, with clear cytoplasmic labelling for bcl-2 protein in the small neoplastic cells, some of which formed characteristic lympho-epithelial lesions. A striking feature was that larger bcl-2 negative cells were observed in nine of these tumours. They were either scattered singly among the small neoplastic cells or formed small clusters, suggesting that they could represent early areas of transformation to high grade neoplasia. Germinal centres in the vicinity of the tumours lacked bcl-2 protein and hence contrasted clearly with the neoplastic cells. In some cases this permitted germinal centres, which were not obvious on conventional histological staining, to be recognised. In 20 of the 22 cases of high grade B cell lymphoma the large neoplastic cells were bcl-2 negative; the remaining two cases, however, contained a proportion of large neoplastic bcl-2 positive cells. In four of the 22 cases of high grade tumours a low grade component was found which expressed bcl-2 in all cases. CONCLUSION--Bcl-2 protein is expressed in low grade, but not in most high grade, MALT lymphomas. In view of recent data indicating that most high grade nodal lymphomas express bcl-2, these findings suggest that MALT lymphomas may regulate bcl-2 gene expression differently to nodal lymphomas.     

Autopsy case of CD4/CD8 cutaneous T-cell lymphoma presenting disseminated pagetoid reticulosis with aggressive granulomatous invasion to the lungs and pancreas

Pagetoid reticulosis is a rare cutaneous T-cell lymphoma with striking epidermotropism similar to that present in Paget's disease. There are two forms of pagetoid reticulosis: localized and disseminated. Reported herein is an autopsy case of disseminated pagetoid reticulosis with CD4(-)/CD8(-) phenotype T cells and massive invasion of the lungs and pancreas. The abnormal cells in the epidermis expressed a protein derived from a rearranged T-cell receptor beta gene, and this feature was used to confirm the monoclonality of these cells by polymerase chain reaction. At present, the World Health Organization (WHO) classification system considers pagetoid reticulosis to be an indolent form of primary cutaneous T-cell lymphoma and a variant of mycosis fungoides/Sezary syndrome with prominent epidermotropism. Some differences have been observed between pagetoid reticulosis and mycosis fungoides in terms of clinical course, tumor cell phenotype, and genetic findings; and these differences are highlighted in the present case. The relation between disseminated pagetoid reticulosis, CD4(-)/CD8(-) cutaneous T-cell lymphoma, and gammadelta T-cell lymphoma, including whether pagetoid reticulosis is a variant of mycosis fungoides, remains unclear.     

Primary cutaneous CD4-positive small/medium-sized pleomorphic T-cell lymphoma: a clonal T-cell lymphoproliferative disorder with indolent behavior.

Primary cutaneous CD4-positive small/medium-sized pleomorphic T-cell lymphoma, a provisional entity in the 2005 WHO-EORTC classification for cutaneous lymphomas, is not well characterized. Fifteen cases meeting the definition of this entity were identified. Fourteen represented solitary lesions on the head/neck (n=9), upper extremity (n=4), or trunk (n=1). One patient presented with multiple lesions on the trunk and extremities. Histologically, the infiltrate showed a nodular pattern in the dermis and subcutis without epidermotropism, and had a polymorphous composition with a predominance of small to medium-sized CD4-positive T cells. Most cases showed normal T-cell antigen expression; diminished/absent expression of CD7 was seen in three cases and CD2 expression was absent in one case. All cases showed a notable reactive infiltrate including frequent B cells, plasma cells, and histiocytes. Clonal TCR gene rearrangements were detected in each case. No clonal Ig gene rearrangements were detected. Out of the 11 patients with follow-up, none showed systemic disease. The majority resolved without relapse, one without treatment, four with excision, and four with radiation therapy. One patient developed local recurrence. The patient with multiple lesions had disease progression despite chemotherapy and stem cell transplant. These cases highlight the polymorphous histology and prominent reactive B-cell component of this entity. Diagnosis requires molecular genetic analysis, as prominent cytologic atypia and immunophenotypic aberrancy are rare. The differential diagnosis includes reactive lymphoid hyperplasia, mycosis fungoides and cutaneous B-cell lymphomas. In patients with isolated cutaneous lesions, the indolent behavior of this rare T-cell neoplasm should be recognized to avoid unnecessary treatment.     

Macrophages in non-hodgkin lymphomas and hairy cell leukaemia

Methods used in the study of human mononuclear phagocytes in vitro were applied to surgical specimens from 49 patients with non-Hodgkin lymphomas and eight patients with hairy cell leukaemia. Two of the tumours (both classified as “true histiocytic” neoplasms by the Kiel criteria) were distinguished by the presence of atypical macrophages in the in vitro system. In one the atypical cells were adherent; In the other example they were non-adherent. These tumours were the only examples of mononuclear phagocyte neoplasia identified in this series. All the remaining 47 cases of non-Hodgkin lymphoma were judged to be of lymphoid origin. While initial observations on hairy cell leukaemia-derived spleen cells suggested macrophage neoplasia, this impression does not stand up to more detailed analysis. The findings are more in keeping with a B lymphoid cell lineage. In hairy cell leukaemias and low grade lymphoma the proportion of macrophages per gram weight of tissue is diminished. This suggests a deficiency of macrophage functional activity compared with normal; the nature of this defect is not clear.     

Lymph node histopathologic findings in cutaneous T-cell lymphoma. A prognostic classification system based on morphologic assessment.

The histopathologic features of 251 lymph nodes obtained from 200 patients with various clinical expressions of cutaneous T-cell lymphoma (mycosis fungoides and Sezary syndrome) were reviewed retrospectively. Lymphomatous involvement, defined as partial or complete effacement of lymph node architecture by malignant cells, was identified in 89 specimens (35%) and was characterized by morphologic variability from case to case. The involved specimens were classified into four major histologic subtypes according to the morphologic appearance of the malignant cells in a manner analogous to a modified Rappaport classification of diffuse non-Hodgkin's lymphomas. Although lymph node involvement was associated with a poor prognosis regardless of histologic subtype, the survival of patients with small cell (cerebriform) subtype was found to be significantly better (median survival time, 40 months) than other subtypes (median survival time, 20 months), possibly because this type of involvement sometimes preceded the development of the more aggressive mixed and large cell subtypes. Dermatopathic lymphadenopathy compared to other reactive patterns had no special prognostic importance other than its more frequent occurrence in black patients and in patients with more extensive skin involvement.     

Remission of refractory Sezary syndrome after bone marrow transplantation from a matched unrelated donor.

Sezary syndrome is a leukemic variant of mycosis fungoides (MF)/cutaneous T-cell lymphoma (CTCL). Bone marrow transplantation (BMT) from a matched unrelated donor was performed in a 22-year-old woman with a 10-year history of Sezary syndrome who had failed treatment with corticosteroids, methotrexate, photochemotherapy, photopheresis, hydroxyurea, interferon-alpha, and cladarabine. At the time of BMT, she had persistent erythrodermic skin disease, adenopathy, circulating Sezary cells and bone marrow (BM) involvement. The patient underwent BMT from a 6/6 HLA-matched unrelated male donor in August 1996. A BM biopsy obtained on day 30 after BMT showed no evidence of lymphoma and complete male donor engraftment. Her skin lesions resolved within 100 days after transplant. Complete staging studies, including T-cell receptor gene rearrangement studies performed at 36 months post-BMT, showed no evidence of recurrent Sezary syndrome. This represents her first durable remission since the initial diagnosis more than 12 years ago. To our knowledge, this is the first patient with refractory Sezary syndrome who has been successfully treated with allogeneic unrelated donor BMT. Our results indicate that this modality may be effective in inducing remission in refractory MF/CTCL, including Sezary syndrome.     

Distribution of extracellular matrix components and their receptors in human lymphoid tissue and B-cell non-Hodgkin lymphomas

In this study the distribution patterns of various extracellular matrix components and their receptors (i.e. β1 integrins) in B-cell non-Hodgkin lymphomas were examined and compared to those in reactive lymphoid tissue. Neoplastic follicles within follicular lymphomas showed similar patterns to that observed in reactive follicles, which appeared to be strongly associated with the presence of follicular dendritic cells. Diffuse lymphomas of low and intermediate malignancy grade revealed features comparable to those of interfollicular areas of reactive lymphoid tissue, irrespective to which compartment the tumour cells were related. Highly malignant lymphomas, however, displayed unique extracellular matrix configurations, resulting from active matrix degradation by macrophages; this may support rapid tumour growth. Extranodal lymphomas showed virtually the same matrix patterns as their nodal counterparts, suggesting that (malignant) lymphoid cells generate (at least partly) their own specific microenvironment. In reactive lymphoid tissue β1 integrins were mainly found on resident cells and except for α4, α5 (and β1) the lymphoid cells expressed very little, if any, β1 integrins. In comparison, expression of these integrins on lymphoma cells was reduced (follicular lymphomas) or could not be detected at all (diffusely growing lymphomas); this might contribute to the growth pattern and metastatic properties of the tumours.     

A unique phenotype of skin-associated lymphocytes in humans. Preferential expression of the HECA-452 epitope by benign and malignant T cells at cutaneous sites.

It has been proposed that the skin is a functionally unique compartment of the immune system, although little direct evidence supporting this hypothesis has been presented. Here we show that lymphocyte populations at cutaneous sites can be differentiated from otherwise similar populations at noncutaneous sites by their preferential expression of an epitope defined by the MAb HECA-452. This MAb recognizes a predominantly 200-kd cell-surface glycoprotein present on about 16% of peripheral blood T cells, including both CD4+ and CD8+ T cells (17% and 11% HECA-452+, respectively), as well as TCR-delta-bearing T cells (32%+). Most thymocytes (99%) lacked HECA-452 antigen expression, and essentially all the HECA-452+ peripheral blood T cells were found in the adhesion molecule high, CD45R low putative memory cell subset, findings suggesting that HECA-452 expression develops peripherally as a consequence of antigenic stimulation. However, the HECA-452 antigen is not a conventional activation antigen because it was not upregulated with mitogen stimulation of peripheral blood T cells. Most significantly, among 54 diverse specimens of normal/reactive lymphoid tissues and sites of chronic inflammation, there was a clear association of lymphocyte HECA-452 expression and cutaneous location. In extracutaneous sites (n = 38) only about 5% of lymphocytes within the T-cell areas of these tissues expressed this antigen, whereas in inflammatory skin lesions (n = 16), 85% were HECA-452+. The association of HECA-452 expression and cutaneous location was also seen in a series of T-cell lymphomas. The malignant cells of 16 of 18 cases of epidermotropic (patch/plaque) stage mycosis fungoides were HECA-452+, as well as 2 of 7 nonmycosis fungoides peripheral T-cell lymphomas in skin. In contrast, this antigen was not expressed in thymic (lymphoblastic) lymphomas (n = 14), nonepidermotropic (tumor) stage mycosis fungoides (n = 5), and noncutaneous peripheral T-cell lymphomas (n = 15). Among lymphocytes, the preferential expression of the HECA-452 determinant by cutaneous T cells supports the hypothesis that the skin constitutes a immunologically unique lymphoid tissue and suggests that this molecule may play a role in either lymphocyte homing to skin or in lymphocyte interactions with the epidermis.     

Exclusive detection of the t(11;18)(q21;q21) in extranodal marginal zone B cell lymphomas (MZBL) of MALT type in contrast to other MZBL and extranodal large B cell lymphomas

Extranodal mucosa-associated lymphoid tissue (MALT)-type lymphomas and nodal and splenic marginal zone B cell lymphomas (MZBL) share morphological and immunophenotypic features with marginal zone B cells of reactive lymphoid tissues. Although displaying a similar immunophenotype, recent investigations suggest fundamental genetic differences among these subgroups. To determine the prevalence of the t(11;18) in a larger series of MALT-type lymphomas and to investigate a possible occurrence in other lymphomas, we screened 106 non-Hodgkin's lymphomas (NHL) by interphase cytogenetics using yeast artificial chromosome (YAC) probes flanking the breakpoint at 11q21. A signal constellation indicating a disruption in 11q21 and thus pointing to the presence of the t(11;18) was observed in 9 of 33 (27%) low-grade lymphomas of MALT type. The complete absence of t(11;18)-positive cells in 32 primary and secondary extranodal high-grade lymphomas suggests that low-grade lymphomas of MALT type characterized by the t(11;18) are unlikely to transform into high-grade tumors. The absence of tumor cells carrying the t(11;18) in nodal MZBL challenges the assumption that most, if not all, of these tumors represent the nodal manifestation of a so far undetected low-grade lymphoma of MALT type. The t(11;18) was not detected in a single case of 29 splenic MZBL investigated. This observation strengthens the view that splenic MZBL are biologically different from extranodal MZBL of MALT type.     

Value of the CD8-CD3 ratio for the diagnosis of mycosis fungoides.

Histopathological diagnosis of mycosis fungoides is difficult, especially in early lesions that may be indistinguishable from inflammatory dermatoses. Mycosis fungoides is a clonal proliferation of mature epidermotropic CD4+ lymphocytes. The aim of this study was to determine the contribution of the CD8-CD3 ratio to the diagnosis of mycosis fungoides. We retrospectively compared the immunophenotypic characteristics of 30 mycosis fungoides with 28 inflammatory dermatoses. The diagnosis of mycosis fungoides was reinforced in all cases by the presence of a cutaneous dominant T-cell clonal population. To analyze exclusively the lymphocytic infiltrates, CD4, which is also expressed by histiocytes, was not considered. The CD8-CD3 ratio was determined separately in the epidermis and the dermis using two methods, one quantitative and the other semiquantitative. Concordance rates between the two methods were higher in epidermal than dermal infiltrates. The mean CD8-CD3 ratio was significantly lower for mycosis fungoides than control cases, with the difference being greater in the epidermal than the dermal component. Although not absolutely specific, a low CD8-CD3 ratio in the epidermal component of a lymphocytic infiltrate supports the diagnosis of mycosis fungoides. It can be evaluated in routine practice using a semiquantitative approach.