In the patch stage of MF a combination of architectural and cytological features is used to make the diagnosis. This includes changes in the epidermis and dermis. In the earliest stage there is a relatively sparse infiltrate of lymphocytes spread along the slightly expanded papillary dermis with little tendency to aggregate around vessels of the superficial plexus. Within the epidermis, lymphocytes are typically confined to the basal layers of the epidermis, either as single cells in a ‘string of beads’ arrangement or as small groups of cells (Fig. 41.1). These cells are often surrounded by a clear ‘halo’, a ‘meaningful artifact’ not caused by mucin accumulation. ²³⁴ There is usually little or no spongiosis.^{235. and 236.} The Pautrier microabscess (sharply marginated discrete clusters of lymphocytes in close apposition with one another, within the epidermis) is, when strictly defined, highly characteristic of MF (Fig. 41.2). They are uncommon in the patch stage, however, and if this feature is given undue importance many cases of MF will be missed.^{237. and 238.} Occasionally the histological appearance mimics a melanocytic lesion. Atypia of cells may be minimal in earlier stages of MF but in some cases, the epidermal lymphocytes are larger that those in the dermis. ⁹² Cytological abnormalities may be seen in thin plastic sections that are not apparent in conventional sections. ¹⁰³ There is frequently, but not invariably fibrosis in the papillary dermis in the form of haphazardly arranged wiry collagen bundles.^{92. and 103.} The epidermis may show mild acanthosis; in poikilodermatous and atrophic lesions the epidermis is thin. ²³⁵ Necrosis of the epidermis is occasionally present. ²²⁸ Basal vacuolar change and pigment incontinence are present in these lesions. ²³⁹ In some cases there is a marked lichenoid reaction with histology resembling lichen planus. Unlike lichen planus there may be plasma cells and eosinophils in the dermal infiltrate as well as atypical lymphocytes. Infiltration of eccrine sweat duct structures may be seen in patch and other stages of MF and may remain after therapy.^{240. and 241.}

The diagnostic changes in large plaque parapsoriasis may be subtle with epidermal changes of mild psoriasiform hyperplasia, overlying mild orthokeratosis and spotty parakeratosis, and a sparse dermal cellular infiltrate. The dermal infiltrate may extend upward into and fill the dermal papillae. Pautrier microabscesses are not seen and cellular atypia is often minimal. ²⁴² The atrophic lesions have identical changes to other atrophic lesions of MF. ²⁴³

The pigmented purpuric dermatoses (PPD) and pigmented purpuric dermatitis-like lesions of MF may have similar histological appearances. In PPD-like MF there may be features typical of MF, but some cases have similar features to PPD. Both PPD and PPD-like MF may have lymphocytes aligned in the basal layer of the epidermis and occasional Civatte bodies. Dermal edema is more common in PPD. Atypia of lymphocytes is seen in MF-like PPD and lymphocytes may be seen in upper levels of the epidermis. ¹⁸⁰ PCR studies may show clonal rearrangements of TCR genes in lichenoid forms of PPD, making distinction of some cases from MF difficult. ¹⁸⁰

Patch and plaque stages are part of a progression. In plaques of MF, the infiltrate is more dense and atypical lymphocytes are more common. The lymphocytes measure 10–30 µm in diameter and their nuclei are often obviously indented, ‘prune-like’ or cerebriform. Prominent convolutions are best appreciated in thin sections. Nuclear morphometry of CD3⁺ T cells has been used as a diagnostic technique to distinguish neoplastic T lymphocytes from reactive ones. ²⁴⁴ It has been suggested that it is not possible to distinguish MF from spongiotic dermatitis based on identifying lymphocyte atypia alone in routine sections. ²⁴⁵

Small collections of cells may aggregate around vessels of the superficial plexus and less often the deep plexus. They also extend round adnexae, particularly pilosebaceous follicles. ²⁴⁶

In addition to lymphocytes, the infiltrate usually contains a small number of eosinophils and sometimes plasma cells. ²³⁷ Epidermotropism is still a conspicuous feature. Pautrier microabscesses are seen in more than 50% of biopsies; this proportion increases if step sections are examined. Epidermal changes include parakeratosis, mild psoriasiform hyperplasia, and epidermal mucinosis.^{237. and 247.} Mild spongiosis does not exclude the diagnosis of MF as sometimes claimed, ²³⁵ but spongiotic microvesiculation is rare. ²⁴⁸ Spongiotic foci resembling Pautrier microabscesses (pseudo-Pautrier microabscesses) are sometimes seen in spongiotic processes such as allergic contact dermatitis and pityriasis rosea. ²⁴⁹ These foci contain monocyte-like cells, Langerhans cells, and rare lymphocytes. Cell nuclei are pale and less complex than in the component cells of true Pautrier microabscesses. These structures are often vase-shaped and appear to open onto the epidermal surface. They may also be seen in MF.^{250. and 251.} Biopsies of apparently normal skin in patients with plaque stage MF sometimes show a mild superficial perivascular infiltrate of lymphocytes, often with epidermotropism. ²⁵²

In the tumor stage, the infiltrate has a more monomorphic appearance and is dominated by atypical cells. ²⁵³ The proportion of tumor cells relative to reactive cells increases. Mitotic figures are easily seen. The entire dermis is often involved and extension into subcutis may occur. Deep dermal and subcutaneous nodules are particularly likely to occur if electron beam therapy has been given to a pre-existing lesion in the same region. ²⁵⁴ Epidermotropism and Pautrier microabscesses are uncommon in the tumor stage. Transformation to a diffuse large cell lymphoma may occur. This was defined in one series by the presence of lymphocytes >4 times the size of small lymphocytes in more than 25% of the infiltrate or microscopic nodules of the same. ²²² The cells can have variable features and resemble the cells seen in anaplastic large cell lymphoma, or resemble immunoblasts or large pleomorphic cells.

Syringotropism may be the predominant pattern of infiltration with invasion of components of the eccrine coil and duct sometimes associated with proliferation of the epithelial structures. ¹⁶⁰

The separation of MF from certain inflammatory dermatoses is not as clear cut as has been suggested. ⁹² There is often disagreement between pathologists in individual cases. Various attempts have been made to provide diagnostic principles, to weigh the importance of particular features and to standardize reporting of cases.^{255.256.257.258. and 259.} The important histological features in most studies appear to be:

Histology is not useful in predicting disease course. ²⁶⁰

Granulomas are a rare finding in mycosis fungoides (granulomatous MF).^{248.261.262.263.264.265.266.267.268.269.270. and 271.} They are usually small and tuberculoid in type but they may be palisaded and mimic granuloma annulare. ²⁷² At other times the granulomas are poorly formed and consist of small collections of multinucleate histiocytes. The granulomas are more localized than in granulomatous slack skin. Granulomas in MF should be distinguished from small collections of lipidized macrophages (dystrophic xanthomatosis) which are rare findings in the dermis in MF.^{262.273. and 274.} A variant with interstitial lymphoid infiltrates superficially resembling granuloma annulare or inflammatory morphea has been reported,^{275. and 276.} as has generalized granuloma annulare associated with granulomatous MF. ²⁷⁷ Rarely there is extensive fibrosis and mucin in the dermis (fibromucinous variant).^{248. and 278.}

Other rare histological changes include the presence of vasculitis, ²⁷⁹ and the formation of bullae. In the bullous lesions the split may occur at any level. The majority of cases have been subepidermal in location with negative immunofluorescence. ¹³² Signet-ring cells constituted the infiltrate in one case. ²⁸⁰ In hypopigmented lesions of MF, there is a reduction in melanin in the basal cell layer and, sometimes, melanin incontinence. ¹²⁴

Epidermal changes such as mild epidermal hyperplasia, dyskeratotic cells, and atypical keratinocytes with large nuclei may be seen following topical treatment with nitrogen mustard. ²⁸¹

Histological mimics of MF have been reviewed. ²⁸²

The tumor cells are typically CD3⁺, CD4⁺, CD45R0⁺, and usually CD8⁻, CD30⁻.^{283.284.285. and 286.} The cells have the characteristics of mature memory cells of the Th2 subset. ²⁸⁷ There is variable expression of the T-cell markers CD2, CD5, and CD7.

Tumor cells frequently lack expression of CD7.^{288. and 289.} Although this aberrant feature was thought to be specific for a lymphomatous T-cell process and a useful feature in the differential diagnosis of reactive from neoplastic processes, it has been reported in inflammatory conditions.^{290. and 291.} Loss of several T-cell markers is unusual in reactive processes and would favor lymphoma.^{292. and 293.} Loss of the T-cell marker CD62L has also been used as evidence for a neoplastic T-cell proliferation, ²⁹⁴ as has a T-cell proliferation which is CD45RB⁺, CD45RO⁻. ²⁹⁵ In the tumor stage there is commonly an aberrant phenotype with loss of T-cell antigens.^{296.297.298. and 299.} Rare cases with similar behavior to classic MF have been reported with a CD4⁻, CD8⁺ phenotype or CD4⁻, CD8^–.^{300.301.302. and 303.} Phenotypic shift from CD4⁺ to CD8⁺ has been reported. Cases of hypopigmented MF are particularly likely to have a CD8⁺ phenotype.^{128. and 129.}

Rare CD4⁻, CD8⁺ cases are CD56⁺. ³⁰⁴ Reactive CD8⁺ cells can also be found in the lesions of MF. ³⁰⁵ Cytotoxic proteins (TIA-1, granzyme B) are sometimes expressed by the CD4⁺ cells, particularly in late stages of the disease. ²⁹² With progression of the disease some cases may transform to a large cell lymphoma which may be CD30⁺ (secondary large cell anaplastic lymphoma) or CD30^–.^{219.222.306. and 307.} Such CD30⁺ tumors are not associated with the t(2;5) translocation. ³⁰⁸ Rarely CD30⁺ cells are found in appreciable numbers in the patch stage of MF. ³⁰⁹ MUM1, a marker for plasma cells, late B cells, and activated T cells, is expressed in transformed CD30⁺ cells. ³¹⁰ Expression of certain CD44 splice variants may be linked to systemic spread.^{311. and 312.} In addition to lymphocytes, the dermal infiltrate in MF includes numerous dendritic cells that express CD1a or factor XIIIa.^{313.314. and 315.} The T cells within the epidermis usually express proliferating cell nuclear antigen (PCNA). Small B lymphocytes are sometimes present in the dermis in small numbers. Rarely neoplastic T cells co-express CD20. ³¹⁶ Malignant T cells express CCR4 (CC chemokine receptor 4).^{225. and 271.}

In most cases, neoplastic lymphocytes of MF express the αβ TCR but rare cases have been reported in which the γδ receptors are expressed.^{317. and 318.} There are many studies in which the detection of clonal TCR gene rearrangements has been used in the diagnosis of MF, particularly in the early, histologically equivocal lesions of MF.^{34.288.319.320.321.322.323. and 324.} The significance of demonstrable clonal TCR gene rearrangements in suspect lesions remains controversial to a certain extent but most authors agree that such a finding warrants clinical monitoring. Many such cases have progressed to frank MF. ³⁴ TCR analysis from two different sites has high specificity in distinguishing MF from inflammatory dermatoses. ³²⁵ Unfortunately, early MF is not uncommonly negative for clonal TCR rearrangements. ²³⁰

Expression of the Fli-1 transcription factor is associated with progression to tumor stage MF. ³²⁶

Complex chromosomal abnormalities have been identified in MF, particularly in the tumor stage but specific chromosomal translocations have not been identified. Chromosomal loss at 10q, and abnormalities in tumor suppressor genes p15, p16, and p53 are common.^{327.328.329.330.331. and 332.}

There is a great diversity in the morphology of the atypical lymphocytes in MF. The characteristic cell has a highly convoluted (cerebriform) nucleus with heterochromatin located predominantly beneath the nuclear membrane (Fig. 41.3). The cytoplasm contains multivesicular bodies and mitochondria, which are sometimes clumped. Some authors use the term ‘Sézary cell’ for a small to medium-sized lymphocyte with a highly convoluted nucleus and the term ‘mycosis cell’ for a cell which is slightly larger with fewer nuclear indentations.^{333. and 334.} Most authors, however, use the terms, ‘Sézary cell’, ‘mycosis cell’, and ‘Lutzner cell’ interchangeably for the various atypical cells, recognizing that intermediate forms also exist.^{108. and 335.}

A much larger cell, sometimes called the ‘pleomorphic cell’, is seen in the tumor stage. It has a vesicular nucleus and a conspicuous nucleolus. ³³⁴ The nucleus of the pleomorphic cell may be variably convoluted.

Quantitative electron microscopy has been used to characterize the atypical lymphocytes in MF.^{191. and 336.}

The atypical lymphocytes have CD3⁺, CD4⁺, CD8⁻ phenotype. Scattered large atypical CD30⁺ or CD30⁻ cells are commonly seen and they may become more confluent in large cell transformation. It has been suggested that the involvement of hair follicles is related to the overexpression of intercellular adhesion molecule-1(ICAM-1) by keratinocytes in the hair follicle. ³⁵⁸

Tumor lymphocytes have a CD3⁺, CD4⁺, CD8⁻ or CD3⁺, CD4⁻, CD8⁺ immunophenotype. CD30 is sometimes expressed.^{362.370.371. and 372.} Clonal rearrangements of TCR genes have been demonstrated. This is usually an αβ TCR gene rearrangement but rarely may be an γδ rearrangement.^{371.373. and 374.} Neoplastic cells have been reported to express cutaneous lymphocytic antigen (HECA 452), a skin homing receptor, which might explain the exquisite epidermotropism of this lesion. ³⁷¹

Although it is not made clear in most studies, there appear to be two cell populations in the epidermis – cells having features of histiocytes and stimulated T lymphocytes.^{375. and 376.} The latter are the major component. ³⁶¹

The small lymphoid cells are CD3⁺, CD4⁺ T cells. The multinucleate cells express histiocyte markers such as CD68.^{387. and 388.} Rarely, large CD30⁺ cells are part of the infiltrate. ³⁸⁹ The neoplastic cells are generally αβ T lymphocytes and clonal TCR rearrangements of the β-chain have been demonstrated.^{379.386. and 390.} Occasional cases demonstrate a clonal γ-chain rearrangement, ³⁸⁵ or do not demonstrate a clonal rearrangement. ³⁹¹ A translocation t(3;9)(q12;24) has been demonstrated in one case. ³⁹²

The neoplastic lymphocytes in the skin and blood are CD3⁺, CD4⁺, CD45RO⁺, CD8⁻, CD30⁻, and frequently CD2⁻, CD7⁻. Studies employing double staining for CD4/CD8 ratios >10 in skin biopsies have been deemed unhelpful in making a diagnosis. ⁴³⁰ Circulating clonal CD4⁺ T cells have been shown to express the skin and lymph node-homing chemokine receptors CCR4, CCR10, and CCR7. Clonal TCR gene rearrangements are present in most cases and are critical if histology and immunohistochemistry are unhelpful. Analysis for TCR gene rearrangements either by PCR⁴²⁶ or Southern blot techniques may be difficult to perform in skin biopsies because of the small number of cells in the infiltrate and problems in extracting cells from skin. Demonstration of clonality in circulating T cells in the peripheral blood aids in the differentiation of SS from other non-neoplastic forms of erythroderma. ⁴³¹ The presence of the same T-cell clone in skin and blood is strong evidence of SS in the absence of diagnostic histological changes.^{432. and 433.} Flow cytometric counts of CD4⁺/CD7⁻ cells in peripheral blood have shown a significant correlation with the number of circulating Sézary cells. ⁴³⁴ In some cases, however, the cells are CD4⁺/CD7⁺ which obscures the phenotypic distinction between neoplastic and normal T cells.^{406. and 435.} Diminished expression of CD3 and lack of CD26 expression have been used to detect and enumerate atypical cells in the blood in SS.^{435.436. and 437.}

A case of Sézary syndrome-like erythrodermic cutaneous T-cell lymphoma with a CD8⁺, CD56⁺ type phenotype has been described. ⁴³⁸

Cytogenetic studies show that complex karyotypes are common in SS.^{327. and 439.} Amplification of the JUNB gene involved in helper T-cell function has been identified. ⁴⁴⁰

The Sézary cells have a convoluted nucleus with deep and narrow indentations. The cytoplasm has a number of fibrils. Ultrastructural morphometry has been used to distinguish Sézary cells in the blood from normal and reactive lymphocytes although this technique is rarely used in routine practice. ⁴⁴¹

The neoplastic cells are T lymphocytes exhibiting clonal TCR rearrangements. They are usually CD2⁺, CD3⁺, and CD4⁺. Rarely, the cells express a CD4⁺, CD8⁺⁴⁶⁶ or CD4⁻, CD8⁻ phenotype. ⁴⁵⁹ They also express CD25, which is a distinguishing feature between ATLL and Sézary syndrome.^{467. and 468.} HTLV-1 is clonally integrated in T cells in all cases and can be demonstrated in paraffin-embedded tissue. ⁴⁶⁹

No distinctive karyotype or molecular abnormality has been identified in ATLL. Cytogenetic studies have shown a complex karyotype.^{449. and 470.}

The tumor cells show slight to marked nuclear irregularity with a convoluted shape and a speckled chromatin pattern. The circulating lymphocytes have been termed ‘flower cells’. ⁴⁴⁹ The degree of nuclear indentation is much less than in the cells of MF and SS. The cells in ATLL have lysosomal granules and glycogen in their cytoplasm.

More than 75% of the neoplastic cells are CD30⁺. ⁴⁷⁸ The majority are CD3⁺, CD4⁺, CD8⁻. A small number of cases are CD8⁺.^{15. and 514.} Most but not all are EMA⁻ and ALK⁻. ^220,515 The presence of a positive reaction for ALK generally indicates cutaneous spread of primary systemic ALCL rather than C-ALCL. ⁵¹⁶ Rare cases showing a positive cytoplasmic reaction for ALK without the usual t(2;5) translocation or variant translocations have been reported. ⁵¹⁷ Positive reactions for CD15 in a cytoplasmic and Golgi zone pattern have been reported in a small number of cases. ⁵⁰⁵ Cells may express cytotoxic markers TIA-1, and granzyme B as well as CD56.^{518.519. and 520.} Neoplastic cells may express the cutaneous lymphocyte antigen HECA 452 which is not usually expressed in primary systemic ALCL. ⁵²¹ They also express MUM1. ³¹⁰ In the vast majority of cases, the cells do not express EBV by PCR or in-situ hybridization.^{483.515. and 522.}

The majority of cases are of T-cell type and exhibit clonal β or γ TCR gene rearrangements. ⁵²³ Atypical cells lack the t(2;5) translocation except in rare cases.^{524. and 525.}

Clusterin expression, a marker for systemic anaplastic large cell lymphomas, can also be expressed by C-ALCL.^{526. and 527.} Cells also express the cytokines CCR3 and CCR4,^{490. and 528.} and the cell surface adhesion molecule CD44.^{529. and 530.} CD95 (APO-1/Fas) is strongly expressed in C-ALCL and LyP and may play a role in regression of lesions. ⁵³¹

The large atypical cells in LyP types A and C are CD30⁺, CD3⁺, CD2⁺/⁻, CD4⁺/⁻, CD8⁻ similar to the large cells in C-ALCL. Rare cases are CD8⁺.^{599. and 600.} Cells are generally negative for CD15 and EMA but positive reactions have been reported in a few cases, the former having a cytoplasmic and Golgi zone pattern. ⁵⁰⁵ As in C-ALCL, the CD30⁺ cells may express cytotoxic markers TIA-1, perforin and granzyme B, and CD56;^{518.600.601.602. and 603.} they are ALK⁻.^{518. and 601.}

The atypical cells in the type B lesions are CD3⁺, CD4⁺, CD8⁻. Despite some reports, CD30⁺ cells may be present in this subtype and overlap cases with type A histology occur.^{15. and 591.} The CD30⁺ cells may co-express CD134. ⁶⁰⁴

MUM1 (multiple myeloma oncogene 1) is expressed in the large cells in types A and C. ³¹⁰ In one study, it was found that there was positive staining for MUM1 in 85% of cases of LyP but only in 20% of cases of primary C-ALCL. It was suggested that this was a useful marker is distinguishing between these two conditions. ⁶⁰⁵ Tumor necrosis factor receptor (TNFR)-associated factor 1 (TRAF1) has similarly been shown to be useful in distinguishing between LyP and C-ALCL, being expressed in 84% of the former and only in 7% of cases of anaplastic large cell lymphoma, primary and secondary. ⁶⁰⁶

Clonal rearrangement of αβTCR genes has been identified in approximately 60% of lesions.^{587. and 607.} No specific genetic abnormalities have been identified in LyP and the t(2;5) translocation is not present.

The neoplastic cells by definition have a TCR α/β, T-cell phenotype and are CD3⁺, CD4^–, and CD8⁺. They also express cytotoxic markers TIA1 and granzyme B. CD30 and CD56 are not expressed. ¹⁵ They do not show evidence of EBV infection.

Recent genomic studies have shown that SPTCL is a uniform entity with characteristic gains of 5q and 13q as well as other abnormalities observed in cutaneous T-cell lymphoma types. ⁶¹⁹

The tumor lymphocytes are CD56⁺, CD2⁺, and surface CD3⁻. They express cytoplasmic CD3ε and the cytotoxic proteins TIA-1, perforin, and granzyme B. CD2, CD7, and CD8 are occasionally expressed.^{623. and 644.} Small numbers of CD30⁺ cells are seen in some cases.^{625. and 645.} EBV can be detected in most cases by in-situ hybridization for EBER (EBV-encoded small RNA) and occasionally by immunohistochemistry (LMP-1).

There is no clonal rearrangement of the TCR (germline) in almost all cases. There can be clonal TCR gene rearrangement in rare cases with a cytotoxic T-cell phenotype.^{15. and 646.} A related CD8⁺, CD30⁺, CD56⁺, CD3ε⁺, EBV⁺ aggressive neoplasm with clonal TCR gene rearrangement has been described.

A variety of cytogenetic abnormalities have been reported; deletions of 6q are the most common. ⁶³⁹

The cells are CD56⁺. The cells are variably reported as CD3⁺/⁻, CD4⁺/⁻, CD8⁺/⁻, and CD30⁺/⁻. ⁶⁵⁰ EBV is demonstrable in cells by EBER in-situ hybridization.

There may be clonal TCR gene rearrangement or the cells may be germline.^{650. and 651.}

The neoplastic lymphocytes have a CD3⁺, CD8⁺, CD7⁺, CD45RA⁺ phenotype and also express the cytotoxic markers TIA-1, granzyme B, and perforin. There is a high Ki-67 proliferation index. Unlike pagetoid reticulosis, these cases do not express HECA 452. Clonal rearrangement of the TCRγ gene has been demonstrated in all cases.

This lymphoma is a monoclonal proliferation of T lymphocytes that express the γδ phenotype. This can be demonstrated by PCR on paraffin sections. Staining for TCRδ (TCRδ1) by immunoperoxidase techniques can only be carried out on frozen section material. ⁶⁵⁶ The cells are negative for βF1 in paraffin sections, a marker for the αβ phenotype. The atypical lymphocytes are CD3⁺, CD2⁺, CD56⁺, and also express cytotoxic markers such as TIA1 and granzyme B. Most are CD4⁻ and CD8⁻ but occasional cases are CD8⁺. ⁶⁵⁶ They are generally negative for EBV but rare EBV⁺ cases have been reported from Asia.^{617. and 658.}

The atypical T cells have a CD3⁺, CD4⁺, CD8⁻, CD30⁻ phenotype and there may be loss of CD7 and CD2. There is clonal rearrangement of TCR genes. This latter feature is useful in distinguishing this lymphoma from benign simulants or cutaneous B-cell lymphomas.

All variants of the small neoplastic cells express CD20 and CD79a. The latter is also expressed by plasma cells which also express CD38 and CD138. The lymphocytes also express bcl-2 but not CD5, CD10, CD43, cyclin D1, or bcl-6. ⁶⁸¹ Plasma cells and lymphoplasmacytoid cells show cytoplasmic immunoglobulin light chain restriction. The scattered large blastic cells can be CD30⁺. Aberrant nuclear bcl-10 expression may be observed.^{676. and 697.}

Immunoglobulin heavy chain (IgH) genes are clonally rearranged. Rare cases have shown a dual B- and T-cell phenotype. ⁶⁹⁸ A proportion of cases exhibit trisomy 3, ⁶⁹⁹ the t(14;18)(9q32;q21) translocation or the t(3;14)(p14.1;q32) translocation which involve the immunoglobulin heavy chain locus (IGH) and MALT1 gene and the FOXP1 gene, respectively.^{700. and 701.} The translocation involving IGH and MALT1 is reported to be uncommon, however. ⁷⁰² The t(14;18)(q32;q21) IGH/BCL2 translocation has also recently been reported in PCMZL. ⁷⁰³ Frequent inactivation of p15 and p16 genes has been observed, most commonly as a result of promoter hypermethylation. ⁷⁰⁴

The follicular pattern is emphasized by CD21 staining of follicular dendritic cells. Only ragged remnants of these cells are present as the pattern of infiltration becomes more diffuse. The neoplastic cells are CD10⁺, CD20⁺, CD43⁺, CD79a⁺, and bcl-6⁺. CD10 and bcl-6 also demonstrate small groups of cells outside the germinal centers. Reactive T cells are present in follicles and interfollicular areas. MIB-1 usually reveals a low proliferation fraction in the follicle centers. Plasma cells are polyclonal. There is variable staining of the follicle center cells for bcl-2 but it is absent in most cases and if present may be only in a small proportion of cells, unlike follicular lymphoma of lymph nodes.38.^{679.680.722.723.724.725. and 726.} A strong positive reaction raises the possibility of a secondary follicular lymphoma. A small number of cases are positive for MUM1. ⁷²⁷

Clonal rearrangement of IgH can be demonstrated in over 50% of cases. ⁷²⁸ Unlike follicular lymphoma of lymph nodes the t(14;18) translocation is not found in most cases.^{729. and 730.} A recent study has shown that in 41% of cases the translocation t(14;18) was demonstrated by FISH techniques but in no cases by PCR. ⁷²⁸ Mutations of the BCL-6 gene have been detected.^{723. and 728.}

The main differential diagnosis for both primary cutaneous marginal zone B-cell lymphoma (CMZL) and cutaneous follicle center cell lymphoma (FCL) is cutaneous lymphoid hyperplasia (LH). All have a follicular growth pattern in part. Separation of these conditions still remains difficult but can be achieved in most cases by a combination of histopathology, immunohistochemistry, and B-cell gene rearrangement studies.

Some histological features previously considered to be helpful in separating lymphoma from lymphoid hyperplasia are now known to be unreliable, e.g. ‘bottom heavy’ versus ‘top heavy’ cellular infiltrates. ⁷¹⁶ Although malignant infiltrates are more likely to be ‘bottom heavy’ and deep, often involving the subcutis, this pattern can also be seen in benign infiltrates.

In florid reactive lymphoid hyperplasia, the lymphoid follicles may have a poorly formed or absent mantle zone. Follicle centers in LH and CMZL tend to show zonation, have obvious tingible body macrophages, and more centroblasts and immunoblasts. Follicle centers in FCL lack zonation, have a predominance of large cleaved centrocytes and no, or only rare tingible body macrophages.

A useful immunohistochemical feature for distinguishing FCL from LH is the presence of small clusters of CD10⁺/bcl-6⁺ cells in the interfollicular zones in the former. There is a low MIB-1 proliferation fraction in FCL compared with LH. The follicle centers are predominantly negative for bcl-2 in all three conditions. Rare cases of FCL are positive for bcl-2 but must be distinguished from secondary nodal follicular lymphoma. ⁷³¹

The margins of the follicle centers, delineated by follicular dendritic cells demonstrated by CD21, tend to be well defined in LH and CMZL and more irregular in FCL. Plasma cells in CMZL are monoclonal, and polyclonal in FCL and LH.

The presence or otherwise of other inflammatory cells such as histiocytes, plasma cells, and eosinophils is not helpful in separating these conditions.

The cells are CD20⁺, CD79a⁺, and bcl-6⁺. Most express bcl-2 and MUM1/IRF4⁷⁴⁴ unlike cutaneous follicle center cell lymphoma. Some cases also express FOX-P1 (a member of the FOX-P transcription factors). ⁷²⁷ A minority of cases express CD10. ⁷²⁷ Rare cases are CD30⁺.^{727. and 749.}

The tumor is not associated with EBV or HHV-8.^{727. and 750.}

Treatment of B-cell lymphomas including large B-cell lymphoma, leg type with rituximab (an anti-CD20 monoclonal antibody) therapy may result in recurrences failing to stain for CD20, a potential pitfall for dematopathologists.^{751.752.753.754.755.756.757. and 758.} In this situation staining for Pax5, a pan B cell marker, may be useful. ⁷⁵⁹

The tumor cells exhibit hypermutations of the immunoglobulin genes and mutations of the BCL-6 gene. ⁶⁸⁴ The t(14;18) translocation is not present.^{760. and 761.} A t(8;14) translocation has been identified in some cases. ⁷⁶²

The atypical lymphocytes are CD20⁺, CD79a⁺, and CD3⁻. There is variable staining for CD5 (38%), CD10 (13%), bcl-6 (26%), MUM1 (95%), and bcl-2 (91%). ⁷⁹²

Monoclonal IgH rearrangements have been demonstrated by PCR on paraffin-embedded tissue. ⁷⁹³ Rare cases are EBV positive. ⁷⁹⁴

Diagnosis requires demonstration of a monoclonal population of B cells by PCR for IgH rearrangements or other techniques. The neoplastic B cells are CD20⁺, CD79a⁺, and positive for other pan B-cell markers. Some are bcl-2⁺. ⁸⁰⁹ The background lymphocytes are predominantly small T cells which are CD3⁺.

The neoplastic cells are B cells and are CD20⁺, sometimes CD43⁺, and CD3⁻. They are variably positive for CD30. The atypical B cells are EBV positive. EBV intranuclear early RNA (EBER) can be demonstrated by in-situ hybridization techniques. ⁸¹¹ EBV⁺ cells, like atypical large B cells, are few or absent in grade 1 lesions, which may make the diagnosis difficult. EBV⁺ cells are said to be less frequently identified in skin lesions than lung lesions. Clonal rearrangement of immunoglobulin heavy-chain gene can be demonstrated in some cases. ⁸¹¹ The T cells are CD3⁺, and CD4⁺ cells predominate over CD8⁺cells.

This lesion was initially defined by the co-expression of CD4 and CD56 but this neoplasm can express markers of other lineages. The cells usually have a CD4⁺, CD56⁺, CD8^–, CD2⁺/⁻, CD7⁺/^–, CD45RA⁺ phenotype and are negative for CD3 and cytotoxic markers.^{834. and 848.} The cells also express CD123 (the IL-3R alpha chain), a marker of plasmacytoid dendritic cells,^{849. and 850.} TLC1 (lymphoid proto-oncogene), ⁸⁵¹ and BDCA-2 (an immature dendritic cell marker). ⁸⁵² CD68, TdT, and CD43^{836.851. and 853.} are sometimes expressed. ⁶³⁶ CD56-negative cases have been reported. ⁸⁴⁸ Weak staining for CD33, a myeloid marker has also been described. ⁸⁴⁸ A rare, related condition, acute myeloid (or type 1) dendritic cell leukemia may present in the skin. In this condition myeloid markers are expressed as well as CD4, CD56, and CD123. ⁸⁵⁴

The TCR genes are usually germline but recently a rare variant with a T-cell gene rearrangement has been described. ⁸⁵⁵ Tumor cells are negative for EBV.

Cytogenetic studies have revealed an abnormal complex tumor cell karyotype with frequent deletions of 5q.^{837. and 848.}

In T-cell prolymphocytic leukemia the cells are CD2⁺, CD3⁺, CD5⁺, and CD7⁺. Approximately two-thirds of cases are CD4⁺ but they may be CD4⁺/CD8⁺ or CD4⁻/CD8⁺.^{1. and 862.} Cells also stain in a nuclear and cytoplasmic pattern for TCL-1 (T-cell leukemia oncoprotein). ⁸⁶²

There is clonal TCR gene rearrangement. Chromosome 14 inversion is characteristic.^{861. and 862.}

The atypical neoplastic lymphocytes express T-cell antigens CD2, CD3, CD4, and CD45RO. Recently neoplastic T cells have been reported to show aberrant expression of CD10 in nodes and at extranodal sites.^{873. and 874.} These cells also frequently express CXCL13, a chemokine normally expressed by follicular helper-T cells; it was demonstrated in 80% of skin biopsies in one series. ⁸⁷⁵

Overexpression of c-Maf (a member of the basic leucine zipper transcription factors belonging to the AP1 superfamily) has been demonstrated in the neoplastic T cells in frozen sections. ⁸⁷⁶

There are also CD20⁺ B cells in the background population of cells. EBV is detected in lymph nodes in the majority of cases of AITL but only in a few isolated lymphocytes. It has been reported only rarely in skin lesions.^{865.869. and 877.} Clonal TCR gene rearrangements have been reported in 75% of lymph nodes and have also been reported in the skin, even in non-specific infiltrates. DNA microassay analysis has been performed on a limited number of cases. ⁸⁷⁸

Tumor cells by definition are predominantly CD30⁺ with staining of the cell membrane and Golgi zone. ALK-1 is positive in 60% of cases. The majority are EMA⁺. CD45 is expressed in 60% of cases. T-cell antigens CD3, CD2, and CD45RO are seen in approximately 50% of cases. A minority are CD4⁺ or CD8⁺. CD56, TIA-1, granzyme B, and perforin may be expressed.^{220.883. and 895.}

ALK protein is absent in normal tissues except the brain, and expression in other tissues indicates anomalous ALK expression usually in the form of nucleophosmin (NPM–ALK) fusion protein associated with a t(2;5)(p23;q35) translocation. Other translocations such as t(1;2)(q21;p23), t(2;3)(p23;q21) and inversion² (p23;q35) have also been reported. Immunohistochemistry, FISH, and ALK gene rearrangement studies are equally effective in identifying ALK⁺ cases.

As stated above, the skin lesions in primary cutaneous ALCL are almost invariably ALK negative and do not have the t(2;5) translocation, although rare ALK⁺ cases with or without the t(2;5) translocation have been reported.^{505.896.897.898. and 899.} CD44 (a cell surface adhesion molecule) is highly expressed in both systemic and cutaneous forms of ALCL whereas the variant form CD44v6 is expressed in 90% of the systemic form but in only 50% of the cutaneous form. ⁵²⁹

Approximately 90% of systemic ALCL exhibit clonal TCR gene rearrangements whether or not they express T-cell antigens. The remainder show no rearrangements of TCR or Ig genes. ⁹⁰⁰

EBV is not expressed in cells. ⁹⁰¹

The phenotype is variable and can be of a NK- or T-cell type. NK cell tumors may have a phenotype similar to the NK/T-cell lymphoma, nasal type. T-cell tumors may express cytotoxic markers. ⁹⁰⁴ T-cell tumors have clonal rearrangement of TCR genes; in NK lymphoma TCR are germline. Cases may have an anaplastic large cell phenotype (CD30⁺). EBV is present in more than half the cases reported. ⁷⁹⁴

The cells are CD2⁺, surface CD3^– (cytoplasmic CD3ε⁺), CD56⁺, CD4⁻, and CD8⁻. TCR receptors are germline. Cells are positive for EBV-ISH.^{626.632. and 906.}

The neoplastic cells express the B-cell marker CD79a and less commonly CD20. They are CD10⁺, TdT⁺ (terminal deoxynucleotidyl transferase), CD99⁺, and bcl-2⁺ in most cases, but not always. ⁹⁰⁹ CD99 is particularly useful in distinguishing this lymphoma from other B-cell lymphomas. Cells are Pax5⁺ whereas the cells in T-lymphoblastic leukemia/lymphoma are negative. ⁷⁵⁹ The cytogenetics of this condition are complex and are prognostically important. Rearrangement of the MLL gene and hyperdiploidy have been reported in cutaneous lesions. ⁹¹⁶

The small lymphocytes are CD20⁺, CD5⁺, CD43⁺, and CD10⁻.^{1. and 920.} PCR demonstrates clonal IgH gene rearrangements.

The neoplastic cells are CD20⁺, CD5⁺, CD43⁺, CD10⁻, CD23⁻, and bcl-6⁻. Some cases are CD5⁻. Rare cases have aberrant bcl-6 expression. Most have nuclear staining for cyclin D1.⁹³⁸ The expression of this nuclear protein is due to overexpression of the PRAD1 gene as a result of the translocation t(11;14)(q13;q32) involving the immunoglobulin heavy-chain locus and the bcl-1 locus. The translocation is characteristic of MCL^{938. and 939.} and can be demonstrated by FISH, PCR, or conventional cytogenetics.^{937. and 940.}

Reed–Sternberg cells and mononuclear variants are CD30⁺, CD15⁺ in most cases, and are CD45⁻, CD45RO⁻, CD43⁻, and EMA⁻. Occasional cases are CD15⁻. ⁹⁵⁵ Cells may occasionally express B-cell markers CD20, Oct-2, and Bob.1 but in a patchy fashion. ⁹⁸¹ They are usually Pax5⁺, whereas the cells of anaplastic large cell lymphoma do not stain. ⁷⁵⁹ EBV may be demonstrated in cells by immunohistochemistry for LMP-1 or by in-situ hybridization for EBER. ⁹⁸² The background population of small lymphocytes are predominantly CD3⁺, CD4⁺ T cells.

Large cells in anaplastic large cell lymphoma and lymphomatoid papulosis are usually CD30⁺, CD15⁻, and CD45 may be negative in some cases. ³⁷ They may be EMA⁺ more commonly in the primary systemic form than in the cutaneous form and lymphomatoid papulosis. ALK-1 may be positive in the systemic form, rarely in the cutaneous form and never in HL. ²²⁰

No cytogenetic studies have been performed on cutaneous disease. Studies on HL in lymph nodes have shown clonal rearrangements of immunoglobulin genes in tissue occasionally, but in 98% of isolated Reed–Sternberg cells. ⁹⁸¹

The immunoprofile is determined by the type of myeloid infiltrate. Overall CD68 and lysozyme are the most sensitive immunostains for the detection of myeloid leukemic infiltrates, but they are not lineage specific. ⁹⁹¹

Myeloperoxidase, CD45, CD74, HLA-DR, CD43, and Mac387 are expressed in a large percentage of cases. Myeloperoxidase is less useful in cases with monocytic differentiation. Other markers that may be present in a smaller percentage of cases include CD56, CD34, CD117, and CD15.989.^{990.991. and 1012.} CD30 is expressed rarely. ¹⁰³¹

The B cells, T cells, and histiocytes mark with the appropriate markers for their lineage. The germinal centers stain for CD10 and are negative for bcl-2. Plasma cells are polyclonal when stained for immunoglobulin light chains. Dendritic cells positive for CD1a and S100 are also present in the infiltrate. ¹⁰⁶⁷ Preliminary data suggest that the evaluation of B-cell clonality using the BIOMED-2 PCR system may be useful in distinguishing true cutaneous lymphomas from simulants. ¹⁰⁷³

The infiltrate consists predominantly of T cells, but rarely B cells predominate. This occurs with some antihistamines¹⁰⁸⁰ and thioridazine. ¹⁰⁸⁷ Most T cells are CD4⁺ and there is usually no loss of pan T-cell markers (CD2, CD3, CD5). ¹⁰⁸⁸ Loss of expression of CD7 and CD62K has been described in some cases. ¹⁰⁸⁵

PCR analysis of TCR and immunoglobulin heavy-chain (IgH) genes shows polyclonal infiltrates in most cases.^{1089. and 1090.} In some cases B-cell or T-cell monoclonality has been identified.^{689.1054. and 1085.}

There is a mixture of B and T cells identifiable by conventional markers (CD20, CD79a, CD3, CD45RO). Either B or T cells may predominate. Plasma cells are polyclonal. In one case CD8⁺ cells predominated over CD4⁺ T cells in the follicular epithelium. Aggregates of perifollicular dendritic cells which are CD1a⁺ and S100⁺ are characteristic. Cells are negative for EBER-1, and neither B- nor T-cell clonality has been identified by PCR.^{246. and 1097.}

There is predominantly a T-cell population with a smaller component of B cells. Clear distinction between cutaneous lupus and Jessner’s lymphocytic infiltrate cannot be made based on relative proportions of B and T cells in lesions. ¹¹¹¹ The cells of CLL are B cells which are CD20⁺, CD5⁺.

The dermal infiltrate consists of a mix of T and B lymphocytes with characteristic markers: T cells predominate. The T cells are a mix of CD4⁺ and CD8⁺ cells and the epidermal cells are of similar lineage. In most cases PCR for TCR and IgH have not demonstrated clonality of T or B cells, ¹¹¹⁶ except in one report in which a clonal IgH chain rearrangement was demonstrated. ¹¹²¹

The cellular infiltrate consists predominantly of T cells which are CD2⁺, CD3⁺, CD5⁺, and CD8⁺. CD7 and CD4 are expressed by some cells. Clonal TCR gene rearrangements are not present.