The Pathology and Pathogenesis of Nodal Peripheral T-Cell Lymphomas

Peripheral T-cell lymphomas (PTCLs) represent about 10% to 15% of all non-Hodgkin lymphomas (NHLs). PTCLs are a heterogeneous and often clinically aggressive group of neoplasms. The current World Health Organization (WHO) classification of lymphomas recognizes more than 30 established and provisional entities of PTCLs, including nodal, extranodal, cutaneous, and leukemic forms. These neoplasms, which are derived from mature post-thymic T-lymphocytes, are rare and usually diagnostically challenging. As the understanding of the pathogenesis of PTCL is increasing, this improved knowledge will lead to better molecular characterization, more objective and accurate diagnostic criteria, more effective risk assessment, and potentially better treatments.

Nodal PTCLs

Nodal PTCLs account for more than 50% of all mature TCLs and include peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS); systemic ALK-positive and ALK-negative anaplastic large cell lymphoma (ALCL); and 3 related lymphomas expressing immunophenotypic signatures typical for T follicular helper (TFH) cells (Figure 1).

PTCL-NOS

Nodal PTCL-NOS are mature TCLs that do not correspond to any other specifically WHO-defined entity. PTCL-NOS often occurs at a median age of 60 years and has a slight male predominance. Patients typically present with B symptoms, lymph node enlargement, and advanced disease.

Paraneoplastic features such as eosinophilia, pruritus, or rarely hemophagocytic syndrome may be seen. Extranodal site involvement such as skin is less frequent compared to other nodal TCLs.

The morphologic, immunophenotypic, cytogenetic, and molecular features of PTCL-NOS are variable. Most cases are composed of a mixture of small and large cells. Immunophenotypically, PTCL-NOS have an alpha/beta phenotype, and most cases are positive for CD4 with a subset (~20%) being CD8-positive.

The 2017 WHO classification recognizes two variants of PTCL-NOS:

• Lennert lymphoma is characterized by small neoplastic cells admixed with numerous clusters of epithelioid histiocytes. In microRNA expression profiling of Lennert lymphoma, the PI3K/AKT/mTOR pathway emerged as the most relevant pathway and a novel potential therapeutic target in this variant (Figure 2).
• EBV-positive nodal T-cell or natural killer (NK)–cell lymphoma is characterized by a monotonous infiltration of atypical lymphocytes, cytotoxic phenotype (CD8-positive/CD56-negative) and aggressive clinical course. Gene expression profiling studies of primary EBV-positive nodal T-cell or NK-cell lymphomas showed significant enrichment of immune response genes associated with cytotoxic activation.

Gene expression profiling of PTCL-NOS has delineated two major molecular subtypes with biologic and prognostic importance. One subgroup is characterized by high expression of GATA3 and its target genes (Th2 phenotype) and the other by high expression of T-box 21 (TBX21) and its target genes (Th1 and cytotoxic T cells). These 2 groups depend on distinct genetic pathways; PTCL-GATA3 are significantly associated with PI3K/mTOR activation and PTCL-TBX21 with NF-KB activation.

Given findings from recent profiling studies, PTCL-NOS may be subdivided by the biologic subgroups (PTCL-TBX21 and PTCL-GATA3), avoiding the “NOS” term in future classifications.

Nodal Lymphomas of TFH Cell Origin

Three nodal lymphomas express signatures typical for TFH cells: angioimmunoblastic T-cell lymphoma (AITL), nodal PTCL with a TFH phenotype, and follicular T-cell lymphoma (FTCL).

Angioimmunoblastic T-Cell Lymphoma

AITL is the second most common subtype of PTCLs, representing 20 to 30% of all TCLs and accounting for 2% of all NHLs in Western countries. AITL commonly affects middle-aged patients and is characterized by generalized lymphadenopathy (typically <3 cm), hepatosplenomegaly, constitutional symptoms, and skin rashes secondary to either neoplastic T-cell infiltration or as an autoimmune paraneoplastic manifestation.

Distinctive pathologic features of AITL include expansion of B cells, proliferation of arborizing high endothelial venules (HEV) surrounded by expanded networks of follicular dendritic cells (FDCs), plasma cells and scattered EBV-positive activated B cells (approximately 80% of the cases).

The neoplastic T cells are positive for alpha/beta TCR and CD4 and typically express pan–T-cell markers including CD2, CD3, and CD5, although aberrant loss or downregulation of one or more T-cell markers is frequently observed. As suggested by the 2016 WHO classification a TFH immunophenotype is established when the tumor cells are positive for at least 2, but ideally 3, of these TFH markers: PD1, CXCL13, CXCR5, ICOS (inducible T-cell costimulator molecule; CD278), CD57, BCL6, CD10, and SAP (signaling lymphocyte activation molecule–associated protein). A follicular dendritic cell marker (CD21 or CD23) is helpful to assess for perivascular expansion of FDCs. Partial expression of CD30 by the tumor cells is not unusual, and aberrant expression of CD20 by the lymphoma cells has been reported.

A monotypic B-cell population or plasma cell population may be identified in some cases. A prominent proliferation of B cells is detected in about 20% of the cases. The B cells are usually large and positive for EBER and may be polytypic or monotypic. In a subset of cases, these large B-cell proliferations meet the criteria for EBV-positive diffuse large B-cell lymphoma (DLBCL), qualified for a composite lymphoma. Awareness of the association of EBV infection and clonal B-cell proliferation in AITL may be helpful to avoid misdiagnosis of AITL as DLBCL.

AITL frequently shows a distinctive mutational profile with mutations involving RHOA G17V (50-70%), TET2 (40-80%), IDH2 R172 (20-45%) and DNMT3A (20-30%). Mutations in RHOA G17V and IDH2 R172 have been shown to correlate with certain histologic features associated with AITL. For example, AITL with RHOA-G17V mutation shows higher microvessel density and more FDC proliferation than wild-type cases and has been associated with a poorer prognosis.

Other recurrent mutations frequently seen in AITL include mutations involving TCR signaling, such as VAV1, PLCG1, CD28, and FYN . CD28-mutated AITL patients have inferior survival compared to patients with wild-type CD28. Alterations in RHOA and VAV1 are mutually exclusive. IDH2 mutations in AITL are a secondary event and might refine the differentiation of the premalignant clones towards a TFH signature.

Nodal PTCL With a TFH Phenotype

Nodal PTCLs with a TFH immunophenotype are those PTCL-NOS that lack the morphologic features of AITL and a follicular growth pattern. The minimum criteria for assigning a TFH phenotype is 2 (but ideally 3) TFH markers, in addition to CD4.

The mutational profile of nodal PTCL with a TFH phenotype seems similar to AITL, with mutations in TET2, DNMT3A, and RHOA G17V. Like AITL, mutations in genes of the TCR signaling pathway (including CD28) are often seen in TFH PTCL.

Follicular T-Cell Lymphoma

FTCL is a very rare nodal T-cell lymphoma with a TFH immunophenotype. The clinical syndrome resembles that of AITL and other PTCLs, characterized by lymphadenopathy, splenomegaly, B symptoms, and skin rash.

In its classic pathology description, FTCL has a follicular growth pattern, mimicking follicular lymphoma, with follicles populated by aberrant T cells that express TFH markers. In FTCL, residual B cells can be seen to be pushed to the periphery of the follicles by the neoplastic T cells. Hodgkin and Reed-Sternberg (HRS)-like cells are also frequently noted. These HRS-like cells are of B-cell lineage, positive for CD30, and, in a subset of cases, positive for CD15 and EBER, raising a concern for classic Hodgkin lymphoma.

The mutational profile of FTCL is similar to AITL and nodal TFH PTCL, with mutations in TET2, IDH2 R172 , DNMT3A , and RHOA G17V. TET2 mutations seem to be more frequent in FTCL than in AITL and nodal PTCL with a TFH phenotype. FTCLs harbor a characteristic t(5;9)(q33;q22) resulting in an ITK-SYK fusion in approximately 40% of cases.

Nodal Anaplastic Large Cell Lymphoma

Four anaplastic large cell lymphoma (ALCL) entities are recognized under the 2016 WHO classification: 2 systemic or nodal forms (ALK-positive and ALK-negative ALCL) and 2 localized forms (primary cutaneous ALCL and breast implant–associated ALCL).

ALK-Positive ALCL

ALK-positive ALCL is most commonly seen in children and young adults with a male predominance and most patients present with advanced-stage disease and systemic symptoms. Extranodal involvement is frequent, and the most involved sites include skin, subcutaneous tissue, bone, lung, and liver. Bone marrow involvement occurs in 10-30% cases. Lactate dehydrogenase (LDH) level is usually normal at diagnosis.

Morphologically, ALK-positive ALCL is characterized by the presence of variable proportions of hallmark cells. Hallmark cells are usually large with an eccentric kidney-shaped nucleus and an eosinophilic perinuclear clearing corresponding to the Golgi area. ALK-positive ALCL includes a number of morphologic variants (common or classic pattern; small cell, lymphohistiocytic, Hodgkin-like, and composite). In up to 20% of cases, more than one morphologic pattern is observed in a single lymph node and relapses may reveal morphologic features different from those seen initially.

Immunophenotypically, these neoplasms can be identified by their characteristic strong and uniform expression of CD30 (membranous and Golgi-staining) and ALK1. ALK-positive ALCL is a neoplasm of T-cell or null-cell lineage. In cases of T-cell lineage, aberrant T-cell immunophenotype is common and many tumors do not express CD3, ZAP-70 or T-cell receptors (TCR) indicating defective T-cell signaling. CD4 and CD43 are the most frequently expressed T-cell markers and most cases have a cytotoxic immunophenotype.

ALK-positive ALCLs are characterized by translocations of the ALK proto-oncogene leading to ALK fusion proteins (the most frequent being ALK- NPM1). Other translocation partners include TPM3, ATIC, TFG, and CLTC. The pattern of ALK staining by IHC in ALK-positive ALCL is determined by the partner of ALK in the fusion protein, for example nuclear and cytoplasmic expression of ALK indicates the presence of the t(2;5)/NPM1-ALK. FISH is generally not needed for diagnosis but can be performed if ALK immunostaining is equivocal.

ALK-Negative ALCL

ALK-negative ALCL is more commonly seen in older individuals, with extranodal involvement in up to half of the cases, and is associated with a worse prognosis. By definition, ALK-negative ALCL shows morphologic and immunophenotypic features similar to the common pattern of ALK-positive ALCL.

The diagnosis requires the presence of hallmark cells. These neoplasms strongly express CD30, but they are negative for ALK1. There is frequent loss of pan–T-cell markers and expression of EMA and cytotoxic markers are less often positive than in ALK-positive ALCL. Some cases have a null-cell phenotype.

Although ALK-negative ALCL is considered an entity in the WHO classification, recent progress suggests that this category is heterogeneous. Rearrangements of DUSP22 and TP63 genes are usually mutually exclusive, and research demonstrates that patients with DUSP22 rearrangements have better survival rates than those with TP63 rearrangement.

DUSP22-rearranged ALCLs tend to have classical morphology with sheets of hallmark cells that often have nuclear pseudoinclusions (doughnut cells) and usually lack cytotoxic markers and EMA expression. FISH studies are recommended to evaluate ALK-negative ALCL for chromosomal rearrangements involving DUSP22 and TP63. In addition to their prognostic significance, these rearrangements may play a role in guiding treatment.

Conclusions and Future Directions

Novel subtypes of PTCLs are being better characterized, and some are known to harbor specific genetic alterations that can be exploited to test novel treatments. It is expected that PTCL classification will evolve with the incorporation of novel entities based on new biomarkers.

Some of the recent incorporations include 2 major molecular subtypes of PTCL-NOS (PTCL-GATA3 and PTCL-TBX21) with biological and prognostic implications and the aggressive cytotoxic PTCL-TBX21 subgroup. Although these PTCL-NOS subtypes can be recognized by immunostaining, currently GATA3/TBX21 profiling does not have therapeutic implications in the front-line treatment setting. This stratification could be used for clinical trials in the near future.

Performing a consistent set of TFH immunomarkers by IHC and mutational studies in PTCLs is important in identifying nodal PTCL with a TFH phenotype and AITL, both for the purpose of establishing a more specific diagnosis and as an opportunity to test novel treatments and clinical trials using histone deacetylase inhibitors in this group of neoplasms.