Peripheral T-Cell Lymphomas

Microscopic Findings

The nodal architecture is usually effaced by a diffuse lymphoproliferation that may extend beyond the capsule into the pericapsular soft tissue ( Fig. 9.1 ). Follicular centers are generally absent and contain an abnormal proliferation of follicular dendritic cells (FDCs). One of the most striking features of AITL is the proliferation of branching high endothelial venules (HEVs) that are accentuated by periodic acid–Schiff (PAS) with hematoxylin staining. The lymphoid infiltrate may appear hypocellular and consists of a mixture of small lymphocytes and immunoblasts, the latter having clear cytoplasm and clustering around vessels. Plasma cells, eosinophils, and histiocytes are often present in the background. Occasionally, clusters of epithelioid histiocytes impart a granulomatous appearance. This combination of morphologic features is most commonly seen in AITL and is regarded as pattern III. Pattern I has partially preserved architecture, hyperplastic follicles with poorly preserved mantles, and inconspicuous neoplastic cells distributed around the follicles. Pattern II has depleted or “burned-out” lymphoid follicles. FDCs are normal to only slightly increased in patterns I and II.

Immunophenotype

The neoplastic “clear cells” of AITL express the pan T-cell antigens CD2, CD3, CD5, and CD7 ( Fig. 9.2 ); however, aberrant loss of one or more of these antigens is common. The tumor cells demonstrate a CD4 ⁺ (T-helper cell) phenotype and should express two or more TFH cell–associated antigens, such as CD10, CD279 (programmed cell death–1 [PD1]), BCL6, CXC chemokine ligand 13 (CXCL13), CX chemokine receptor 5 (CXCR5), and inducible T-cell costimulator (ICOS). Disrupted and expanded FDC networks can be recognized using antibodies to CD21, CD23, CD35, and clusterin, a feature that helps to distinguish AITL from other PTCLs. Their coexpression of TFH cell–associated antigens and their association with expanded FDC networks are consistent with an origin from TFH cells. Most AITLs are derived from noncytotoxic αβ T cells.

Molecular and Cytogenetic Findings

Approximately 75% of cases of AITL will demonstrate clonal rearrangement of TCR genes. Many cases will also show an oligoclonal or small clonal population of B cells. Indeed, dominant B-cell clones in the absence of obvious diffuse large B-cell lymphoma (DLBCL) (see later discussion) have been reported in AITL, reflecting the immune dysregulation present in these patients. Trisomies 3 and 5, an additional chromosome X, and 1p alterations are the karyotypic abnormalities found most frequently; however, no single translocation is associated with most cases. Gene expression profiling has shown a strong contribution by non-neoplastic cell constituents with overexpression of B-cell–related and FDC-related genes and genes related to extracellular matrix and vascular biology. The tumor cell signature appears enriched in genes normally expressed by TFH cells, which has further established a TFH-cell derivation for AITL.

An increased number of Epstein-Barr virus (EBV)-positive B-cell immunoblasts is detected in many cases by in situ hybridization for EBV-encoded RNA (EBER) (see Fig. 9.2 ). A specific role for EBV in the pathogenesis of AITL remains unproven.

Mutations in epigenetic regulators— TET2, DNMT3A, and IDH2 —are common in AITL but are not specific for this disease. Mutations in the Ras homolog gene family, member A ( RHOA G17V) appear to be more disease specific, occur in approximately 70% of AITLs, and may require a concurrent TET2 mutation for the development of AITL. A highly recurrent novel missense mutation in CD28 ( CD28 T195P) has been reported in approximately 10% of AITL cases and appears relatively specific for AITL. A CTLA4-CD28 fusion also has been found in 58% of AITL but does not appear specific, being found in 23% of peripheral T-cell lymphomas, not otherwise specified (PTCL, NOS) and 29% of NK/T-cell lymphomas. Of note, within PTCL, NOS, those cases with a TFH phenotype more frequently had the fusion (56%) compared to those without a TFH phenotype (31%). Both the mutation and the fusion appear to result in activation of TCR signaling pathways (such as AKT, ERK1/2), and nuclear factor kappa B (NFκB) pathways, presumably providing a growth or survival advantage. These alterations in CD28 remain to be independently confirmed.

Microscopic Findings

Histopathologic features are distinctive. The large cells, which may appear cohesive, extend from the subcapsular sinuses into the paracortical region, often sparing germinal centers ( Fig. 9.3 ). The neoplastic cells are sometimes largely confined to the nodal sinuses, although more often complete architectural effacement is noted. The “hallmark” tumor cell is a large transformed cell with irregular nuclear contours, a prominent nucleolus, and abundant eosinophilic cytoplasm. Multinucleated tumor cells may have nuclei arranged in a circular pattern (i.e., wreath cells) or in a horseshoe shape. Several morphologic subtypes have been described, subdivided according to tumor cell size, nuclear pleomorphism, types of reactive cells, and degree of fibrosis. Histologic types that have been described for ALK-positive ALCL include the common type with pleomorphic or monomorphic large tumor cells (approximately 70% of ALCLs), lymphohistiocytic variant (10%), small-cell variant (5% to 10%), and Hodgkin-like variant (3%). In the lymphohistiocytic and small-cell variants, large tumor cells are a relatively minor population in a background of small, irregular lymphocytes. Occasional cases have abundant histiocytes, fibrosis, and increased neutrophils. In the small-cell variant, the large tumor cells can often be found around vessels. The Hodgkin-like variant has morphologic features resembling nodular sclerosis classical Hodgkin lymphoma. ALK-negative, DUSP22 rearranged cases typically show sheets of “hallmark” cells, like the common type of ALK-positive ALCL, and few large pleomorphic cells, which tend to be more numerous in the ALK-negative, DUSP22 -negative tumors.

Unlike other large-cell lymphomas, bone marrow involvement might not be obvious on routine hematoxylin and eosin–stained biopsy sections because of the propensity for single tumor–cell infiltration in ALCL. Studies have shown that with routine hematoxylin and eosin sections alone, approximately 10% of cases will show detectable involvement. However, if bone marrow evaluation is done in combination with staining for CD30 or ALK, or both, the percentage of cases with detectable involvement is closer to 30%.

Immunophenotype

The tumor cells of ALCL are always CD30 ⁺ , usually with both cytoplasmic membrane and Golgi region staining by immunohistochemistry ( Fig. 9.4 ). The majority of ALCLs have a CD4 ⁺ T-cell phenotype, but CD3 is absent in approximately 75% of cases. Approximately 10% to 20% of cases do not express T-cell antigens and are regarded as null cell type, but they usually can be shown to be of T-cell origin if T-cell gene rearrangement studies are performed. Although B-cell cases with an anaplastic morphology and CD30 positivity were accepted in the original definition of ALCL, they are now included among the diffuse large B-cell lymphoma category in the WHO classification. Most ALCLs are CD45 ⁺ ; however, 20% to 40% of cases may lack or have only weak staining. Approximately 60% of cases are epithelial membrane antigen positive. Approximately 70% to 80% of ALCLs are ALK-positive, with the majority of these demonstrating both diffuse cytoplasmic and nuclear staining. Other ALK-positive ALCLs have cytoplasmic or, more rarely, membranous staining only. When this occurs, a variant ALK translocation is present other than NPM1-ALK . ALCLs are derived from cytotoxic T cells, as the tumor cells routinely stain for TIA-1, granzyme B, or perforin. ALK-negative, DUSP22 rearranged ALCLs tend to lack cytotoxic proteins. A small subset (15% to 25%) of ALCL cases is also CD15 ⁺ .

Molecular and Cytogenetic Findings

Most ALCLs (90%) have clonal TCR gene rearrangements. Tumor cells are negative for the EBV by in situ hybridization for EBER. A t(2;5)(p23;q35) chromosomal abnormality is present in most ALCLs, which translocates a novel ALK gene on chromosome 2p23 next to the nucleolar organizing gene, nucleophosmin ( NPM ), on chromosome 5q35. This translocation results in a fusion protein that can be detected using antibodies that recognize ALK, which produces a cytoplasmic, nuclear, and nucleolar staining pattern. In addition, a number of variant translocations have been described that result in translocation of ALK to other gene partners, including some on chromosomes 1q21, 1q25, 3q21, 4q33 Xq11-12, 17q23, 17q25, 19p13.1, and 22q11.2 and in inversions of 2p23-2q35. These variant translocations can be seen in 20% or more of ALK-positive cases and are often associated with a cytoplasmic-only staining pattern with antibodies to ALK. These ALK translocations are seen mostly in young patients and in primary nodal cases. DUSP22(IRF4) rearrangement at 6p25.3 or TP63 rearrangement at 3q28 can be detected in 30% and 8% of ALK-negative ALCLs, respectively.

Finally, gene-expression profiling can show molecular separation of ALK-positive ALCL from ALK-negative ALCL, PTCL, NOS, and adult T-cell leukemia–lymphoma. This finding supports the separate classification of these lymphomas.

Microscopic Findings

Tissue architecture is generally effaced by a diffuse lymphoproliferation that consists of tumor cells of varying size, often within a polymorphic reactive cell background. Large tumor cells may have hyperlobated nuclei or resemble Reed-Sternberg cells ( Fig. 9.5 ). PTCLs, NOS, that have more than 70% neoplastic large cells tend to behave poorly.

Lymphoepithelioid lymphoma has a proclivity to involve Waldeyer's ring and cervical lymph nodes. Tissues are effaced, and there are numerous evenly dispersed clusters of epithelioid histiocytes between which are infiltrates of mostly small tumor cells (see Fig. 9.5 ).

Immunophenotype

Given that PTCL, NOS, is likely a heterogeneous category, there is no specific diagnostic immunophenotype. Most cases of PTCL, NOS, have an aberrant T-cell phenotype, with loss of CD3 or CD7 most frequently observed. Most PTCL, NOS, cases are CD4 ⁺ αβ T-cell lymphomas, although some are CD8 ⁺ and a few have γδ TCRs. Some of these lymphomas have a cytotoxic T-cell phenotype or express NK-cell–associated antigens such as CD56.

Molecular and Cytogenetic Findings

Most of these lymphomas have clonal TCR gene rearrangements, and a few may have EBV in the tumor cells or in non-neoplastic B-cell immunoblasts. No consistent specific cytogenetic abnormality has been identified for most PTCLs, NOS, but two cytotoxic PTCLs with skin and bone marrow involvement and a t(6;14)(p25;q11.2) involving IRF4 and TCRA have been described. Gene expression profiling has identified two major molecular subgroups of PTCL, NOS, that express GATA3 or TBX21 ( T-BET ). GATA3 expression is present in approximately 33% to 45% of PTCL, NOS, is associated with the production of Th2-associated cytokines, and has a poor prognosis (19% 5-year OS rate). TBX21 expression can be detected in up to 49% of PTCL, NOS, is associated with the production of Th1-associated cytokines, often has tumor cells with a cytotoxic phenotype, and appears to have a better prognosis (38% 5-year OS rate) than the GATA3 subgroup.

Mutations in DNMT3A (27%) and TET2 (48%) appear to be relatively frequent and often occur together but are not specific for PTCL, NOS. Likewise, as noted in the section on AITL, CTLA4-CD28 gene fusion has been seen in 23% of cases but is also not specific. RHOA G17V mutations may occur in PTCL, NOS, but appear to segregate with those cases that have a TFH phenotype.

Microscopic Findings

Regardless of site, the lymphoma will often have an angiocentric, angioinvasive, and angiodestructive infiltrate of cytologically atypical lymphocytes of varying size ( Fig. 9.6 ). Many cases will have a diffuse growth of tumor cells. Necrosis is often present and may be extensive. Occasionally there is a polymorphic inflammatory cell background.